GEORGE R.
WHEREAS our trufty and well-beloved William Lewis, of Kingfton, in our County of Surry, M. B. has, by his Petition, humbly reprefented unto Us, that he hath been, for upwards of Fourteen Years, engaged in making Experiments, and collecting Materials, for a Work, entitled " Commercium Philosophico-T echnicum, or, The " PHILOSOPHICAL COMMERCE of ARTS ; defigned as an Attempt to " promote ufeful Knowledge •," and hath at length brought it to great Forward- nefs, fo as to be fit for putting to the Prei's ; and whereas he hath been at great Labour, Pains, and Expence, in proiecuting the faid Work, which he humbly prefumes will be of Service to Our Subjects concerned in Arts, Trades and Manufactures, as tending to promote that Kind of Knowledge on which they depend •, He therefore mod humbly prays Us to grant him Our Royal Licence and Privilege for the fole Printing, Publifhing, and Vending the faid Work, for the Term of Fourteen Years, agreeable to the Statute in that Cafe made and provided : We are gracioufiy pleafed to condefcend to his Requeft, and do accordingly, by thefe Prefents, as far as may be agreeable to the Statute in that Cafe made and provided, grant unto him the faid William Lewis, his Executors, Adminifhators, and Affigns, Our Royal Licence for the fole Printing, Publifhing, and Vending the faid Work, for the Term of Fourteen Years from the Date hereof ; ftrictly forbidding all our Sub- jects, within Our Kingdoms and Dominions, to Reprint, Abridge, or Publifh the fame, either in the like, or any other Volume, or Volumes, whatfoever, or to Import, Buy, Vend, Utter, or Diftribute any Copies thereof Reprinted beyond the Seas, during the aforefaid Term of Fourteen Years, without the Confent and Approbation of the faid William Lewis, his Heirs, Executors, Adminiilrators, and Affigns, under their Elands and Seals, firtl had and obtained, as they will anfwer the contrary at their Peril ; whereof the Com- miffioners, and other Officers of Our Cufloms, the Mafter, Wardens, and Company of Stationers, are to take Notice, and that the fame may be entered in the Regifter of the laid Company, and that due Obedience be rendered to Our Will and Plcafure herein declared.
Given at Our Court at St. James's, the Eighth Day of February, 1762, in the Second Year of Our Reign,
By His Mr.jeJIy's Command.
EGREMONT.
f- r~
COMMERCIUM
Philofophko-Technicurn ;
OR, THE
Philosophical Commerce
O F
ARTS:
DESIGNED AS
AN ATTEMPT TO IMPROVE
Arts, Trades, and Manufactures.
By W. LEWIS, M. B. and F. R. S.
Fiat Experimentum. Bacon.
LONDON,
Printed by H. Baldwin, for the AUTHOR; And Sold by R. Whiock, at Sir Ifaac Newton's Head in Cornhil!.
MDCCLXIII.
T O T H E
KING.
S I R,
TH E advancement of arts, trades, and ma- nufactures, and the extenfion of commerce, are now become the more immediate objects of Your Royal care ; as being the moft certain means of attaining Your darling wifhes, the rendering Your people powerful and happy, and perpetuating the bleilmgs of peace. I have therefore pre- sumed, with all humility, to offer to Your Majesty's protection, a work deiigned to improve and enlarge many of the arts of peace, and to pro- mote that kind of knowledge on which they de- pend. The importance of the fubjedt, and the honour, the never to be forgotten honour, which
Your"
[ H]
Your Majesty was pleafed to do me, by Your at- tention to fome lectures and experiments, made by Your command at Kew, for mewing the applica- tion of chemiftry to the improvement of practical arts as well as of philofophy, will, I hope, excuie this ambition in
Your Majesty's
Moft humble and devoted
Subject, and Servant,
Kingfton on Thames Williim T PWIS
7thofAy.il, 1763. vviiiiam i>ewis.
THE
Philosophical Commerce
O F
ARTS:
DESIGNED AS
AN ATTEMPT TO IMPROVE
Arts, Trades, and Manufactures.
By W. LEWIS, M. B. and F. R. S.
Fiat Experimentum. Bacon.
«-
LONDON,
Printed for the AUTHOR;
And Sold by R. W I l l o c k, at Sir Ifaac Newton'i Head in Cornhill.
M.DCC.LXV.
PREFACE.
AS all the arts, by which matter is diverfly modified and accommodated to human ufes, have a neceffary dependence upon the properties or qualities of the bodies on which they are exercifed; enquiries into the pro- perties of different bodies, and the effects refulting from various applications of them to one another, become ap- parently of primary importance, as well for the illuftration and improvement of the prefent arts and the difcovery of new ones, as for the advancement of ufeful knowledge.
The properties of bodies make the objedV. of two fciences, natural fhilofophy and chemijlry ; which, though in many cafes fo clofely interwoven, and fo nearly allied, that per- haps no boundaries can be eftablifhed between them, ap- pear in others to have effential and important differences. In the introduction to a work, of which enquiries into the properties and relations of bodies make a principal part, the neceffary precifion, in regard to matters of fact, as well as of fcience, requires that we mould endeavour to diftinr guifh them.
a Natural
iv PREFACE.
Natural or mechanical philofophy feems to confider bodies chiefly as being entire aggregates or manes ; as be- ing divifible into parts, each of the fame general properties with the whole; as being of certain magnitudes or figures,, known or invefrigable ; gravitating, moving, refilling, &c„ with determinate forces, fubjed to mechanic laws, and re- ducible to mathematical calculation.
Chemistry confiders bodies as being compofed of fuch a particular fpecies of matter ; difioluble, liquefia-ble, vitrefcible, combuilible, fermentable, ccc. impregnated with colour, fmell, tafle, . &c. or confifling of diffimihr parts, which may be feparated from one anotber, or tranf- ferred into other bodies. The properties of this kind are not fubject to any known mechanifm, and feem to be governed by laws of another order.
To the grand active power, called attraction, in the mechanical philofophy, what correfponds in the chemical is generally diflinguifhed by another name, affinity.
The mechanical attraction obtains between bodies con- fidered each as one whole, and between bodies of the fame a$ well as of different kinds. It obtains while the bodies are at fenfible distances ; and the comparative forces, with which they tend together at different diflances, are objects of calculation. When the attracting bodies have come into the clofeil contact we can conceive, they frill continue two diftinct bodies, cohering only fuperficially, and. fepa= rable by a determinate mechanic force. .
The
PREFACE. r
The chemical attraction, or affinity, obtains between bodies as being compofed of parts, and as being of a diffe- rent fpecies of matter from one another. It never takes place while the two bodies are at any fenfible diftance > and when they are brought into the clofeft contact, there is frequently neceffary fome other power, as fire, to excite their action upon one another. In proportion as this action happens, they are no longer two bodies, but one $ the affinity confifting in the intimate coalition of the parts of one body with thofe of the other. The properties of this new compound are not in any kind of ratio of thofe of the compounding bodies, nor difcoverable by any ma- thematical investigation : two bodies, each by itfelf very eafily fufible in the fire, as lead and fulphur, (hall form a compound very difficult of fufion ; and two which cannot feparately be made to melt at all, as pure clay and chalk, mall melt with eafe when joined together.
As the chemical union, and the properties thence re- fulting, are exempt from all known mechanifm, fo neither can the bodies be feparated again by mechanic force. But a third body may have a ftronger affinity to either of the component matters than they have to one another, in which cafe, on prefenting to the compound this third body, the former union is broken, and one of the firft bodies coalefces with the third, while the other is detached and feparated.
Thus, when quicklime is diflblved in water, if we add to the tranfparent fluid a little vitriolic acid, the acid par- ticles unite with the diflblved particles of the. lime into a
a 2 new
li P R E F" A C &
new compound ; which, notwithflanding the pungent tafle of the one ingredient, the corrofive acidity of the other, and the lblubility of both, proves infipid and indiflbluble> and which therefore, feparating from the water, renders it at firff. milky, and on ftanding fettles to the bottom, in form of powder or fmall cryftals, of the fame general properties with the. native gypfums or plafler-of-paris ftones.
If this powder be ground wkh inflammable matter, as powdered charcoal, no action happens between them, how exquilitely foever they be mixed : the two powders cor.r tinue gypfum and charcoal,, and may be in great meafurs parted from one another by means of water, the charcoal powder remaining for a time fuipendcd in the fluid, while, the heavier gypfum fettles. On expoiing the mixture to a proper degree of heat, a ftrong chemical affinity begins to take place : the acid quits the lime, and unites with the inflammable principle of the coal, forming therewith another new compound, common brimftone, which, like the former, proves infipid, and indiflbluble in. watery li- quors, though in other properties remarkably different i melting in a fmall degree of heat into a red fluid; in a fome- what greater heat, if air is excluded, rifing into the upper- part of the veffel unaltered; on the admiflion of air chang-. ing into a blue flame, with a fuffocating volatile acid fume, which by air and moifture returns into the original,, inodo-* jfous, ponderous, vitriolic acid.
By mixing the brimftone with iron filings, a frefh tranf- pofition is produced; and as in the preceding cafe the action, is excited by fire, fo in this it is excited by water. Th^
mixture^.
PREFACE. vii
mixture, kept perfectly dry, continues unaltered for years : on being moillened with water, it grows fpontancoully hot in a few hours, and if the quantity is large, it even burfts into flame, with fuch commotion, as has induced many to afcribe earthquakes and vulcanoes to this caufe. During this action, the acid is transferred to the iron ; and the inflammable matter, before combined with it, efcapes into the air. The combination of the acid with the iron forms the green vitriol or copperas of the fliops ; a fait of a flrong tafte, and of eafy folution in water, though the quantity of iron in it is very far greater than that of the inflammable matter by which, in the form of brimflone, the mifcibility of the acid with water was deftroyed.
To the green folution of the vitriol, if fome vegetable allies, or the earth called magnefia, be added, the iron falls to the bottom, confiderably altered, in form of ochre or rufr, deprived of its attractive power to the magnet, and of all its metallic properties, which however are eafily reftored by expofing it to the fire in mixture with a little charcoal powder. In room of the iron thus thrown out from the liquor, the acid attacks the vegetable earth or magnefia; and though with one kind of earth, as we have feen above, it forms an infipid and indifibluble concrete ; with both thefe earths it compofes a bitterifh fait which difiolves eafily, and which, at leaft when magnefia is made ufe ofj is the fame with that of the purging mineral waters.
If to the folution of this fait we add a volatile alcaline fait, the penetrating fmell of the alcali is fupprefied in an inftant, the acid uniting with the alcali into a new com-
poundj.
Vtii PREFACE.
pound, and depofiting the earth which it had taken up before.
From this compound, fixt alcaline falts abforb the acid, and fet at liberty the volatile alcali with all its original properties. Though the acid and fixt alcali, fepaxately, are very pungent and corrofive, and fo ftrongly difpofed to unite with water that they imbibe it from the air, yet the combination of the two has only a mild bitterifh tafle, and difiolves in water very difficultly and fparingly.
After all thefe tranfpofitions, the acid may ftill be reco- vered pure, and made to pafs again through the fame and through a multiplicity of other combinations. From al- moft all its combinations it may be transferred to inflam- mable matter, and from the inflammable matter to iron : from the brimftone, which it forms with the one, the acid may be obtained by burning with a proper apparatus ; and from the vitriol, which it forms with the other, by diftil- lation.
It is obvious, that in all thefe cafes, the action is not between bodies coniidered as aggregates or mafles, but be- tween the.infenfible and diflimilar parts of which they are compofed; that the feveral effects can be regarded no otherwife than as fimple facts, not reducible to any known mechanifm, not invefligable from any principles, and each difcoverable by obfervation only ; and that the powers, on which they depend, are, fo far as can be judged in the prefent flate of knowledge, of a different kind from thofe, by which bodies tend to approach or cohere with forces proportionate to their diftances, or to refift or propel accord- ing
PREFACE. ix
ing to their quantities of matter and velocities. It feems ef importance, that thefe two orders of the affections of bodies be kept diftincl:, as many errors have arifen from applying to one fuch laws as obtain only in the other.
When Archimedes obferved, in the bath, that the bulk- of a body, however irregular, might be found, by plung-- ing it into a veilel of water, and meafuring. the water which run over, or the fpace which the water occupied in the veffel more than it did before ; and that gold has near twice the weight of lilver under an equal bulk ; he con- cluded, that if gold and filver were mixed together, the quantity of each metal in the mixture might be found by calculation from the bulk of the mafs compared with its weight ; and on this foundation, he is faid to have difco— vered a fraudulent addition of filver made by the workman in Hieros golden crown, at a time when the chemical methods of analyfmg and allaying metallic compofitions appear to have been unknown.
The mechanical philofophy has extended this way of inveftigation to many different mixtures, and computed tables for facilitating the operation; not aware, that though the method is demonftrably juft if the two bodies were joined only fuperficially, the cafe is otherwife when they are intimately combined together. The ad of com- bination, whether in bodies brought into fufion by fire, or in fuch as are naturally fluid, is truly chemical, and the laws of the mechanical philofophy have no place in it. There are inftances, fome of which will appear in the prefent volume, of bodies being dilated .on mixture into a
larger
x PREFACE.
larger bulk than they had before ; and, contrariwife, of two being contracted into lefs bulk than even one of them occupied by itfelf.
To render the procefs anywife to be depended on, ac- tual mixtures of the refpective bodies ought firft to be made, in different proportions, and examined hydroftati- cally, that the quantity of contraction or dilatation in particular cafes may be known and allowed for. By thus borrowing from both fciences, we are furnifhed with means of difcovering the proportions of the ingredients in many mixture?, provided the ingredients themfelves are known, with tolerable certainty : in fome mixtures, as of lead and tin, this method is more commodious, and perhaps more exact, than any which pure chemiftry has afforded.
In this manner the mechanical and chemical fciences concur, and require the amftance of one another, in their own operations, and in almofl all the manual arts. In the greater number of the arts the chemical prevails, and many are no other than direct branches of practical che- miftry, as the arts of dying and flaming, the running down of ores, the refining and compounding of metals, the making and colouring of glafs, enamel, porcelain, &c. making wines, vinegars, fpirits, ccc. preparing indigo, fmalt, Pruffian blue, vermilion, lakes, and other colours for the painter. It is in thofe arts, and in thofe branches of arts, which are flrictly chemical, that the moft impor- tant and moft numerous difcoveries are to be expected : xhemiftry having hitherto been the leaft cultivated, though
not
PREFACE. xi
not the leail fruitful; and producing daily, not barely new applications of principles already known, but new fads or principles to be further applied.
Having for feveral years employed myfelf at times in experiments relative chiefly to the chemical arts, and made proper difpofltions for continuing fuch enquiries, I pub- lished in 1748 propofals for a very exteniive work, con- futing principally of thofe experiments, and of informations received from workmen and others. The feveral articles were to be printed in a mifcellaneous manner, without re- gard to any one being connected with that which preceded or followed it. As nothing was to be admitted but ufe- ful or interefting facts, it feemed of little importance in what order the facts mould be difpofed, provided, by means of proper indexes, the reader could readily have recourfe to fuch particulars as might occaiionally be wanted.
Some friends advifed an alteration in this plan, judging it would be of more utility to the puWick if the facts were methodized; and the mod convenient method was thought to be, to give a complete hiftory of each art by itfelf in all its branches. The difficulty of fuch an attempt, and the imporTibility of executing it to any good purpofe by one hand, were apparent : nor would a fimple detail of the manual operations of different workmen be any wife agree- able, either to the views, or the materials, with which I had engaged in the undertaking.
Another way occurred of procuring fome degree of regularity, without departing from the original views, any otherwife than by rendering them more comprehenfiw;.
b Many
PREFACE.
Many of the arts have natural and ftrong connexions wuh one another ; working upon the fame materials, for purpofes either different or nearly the fame; or producing iimilar effe&s upon different or fimilar fubjedls. One property of fuch materials, or the production of one effect, may therefore influence feveral arts : a colour, which cart. be eafily fixed in animal and vegetable fibres, is equally of benefit to the woollen dyer, the filk dyer, the dyer of linen and cotton thread, and the callico printer: a colour which will bear fire, and unite with vitreous bodies in fulion, concerns equally the glafs maker, the enameller, and the painter on porcelain.
The difcoveries and improvements made in one art, and even its common proceffes, are generally little known tp thofe who are employed in another, fo that the work- man can feldom avail himielf of the advantages which he might receive from the correlative arts, and an effect wanting to the perfection of his own art may be actually produced in another. Thus, though the dyer of linen cloth, and of linen and cotton thread, wants means of communicating to them a black dye that fhall endure wearing, the callico printer fixes both on linen and on cotton a black as durable as can be wiflied for.
To enquire therefore by experiment into the different means of producing one effect, and trace it through all the arts in which fuch aji effect is required* to examine the chemical properties of one fubjedl-matter, and confider its ufes and applications in all the arts in which it is con- cerned i to proceed in this manner with the capital effects, , and. materials, fo far as my own experiments, and my op- portunities
PREFACE.
portunities of information, mould enable me; appeared to be the moft rational and direct means, not only of efta- blifhing folid principles of the feveral arts as now exer- cifed, but of procuring an ufeful intercourfe and communi- cation of knowledge, of fupplying many of their defects, of multiplying their refources, of improving their pro- duces, of facilitating and Amplifying fome complex ope- rations, and rejecting ufelefs ingredients in fundry com- pofitions, of enriching one art with the practices, mate- rials, and fometimes even with the refufe-matters of another.
Such therefore is the plan which I have chofen to fol- low, and of this alteration I gave notice in an advertife- ment in 1761.
I have the fatisfadlion to find that the French academy ■of fciences, who, with the advantage of penfions from the fovereign, and with the affiftance of experienced artifts in different profefTions, have been engaged for near a century in a hiftory of arts which has but lately been begun to be published, exprefs exadtly the fame fentiments with thofe on which I have proceeded. In the memoirs for 1763, the hiftorian of the academy, in giving notice of the pub- lication of that work, obferves that " an inconvenience to be feared is, the want of that knowledge, and of thofe general principles, which bind arts as it were together, and eftablifh between them a reciprocal communication of light. All the arts, for example, that employ iron, have common principles, but it would be in vain to expecl: the knowledge thereof from thofe who exercife thefe arts,
b 2 each
xiv PREFACE.
each of whom knows only the application of thofe prin- ciples to his own art. The farrier, the lockfmith, the cutler, know how to work iron; but each of them knows only the manner of working which he has learnt, and is perfectly ignorant that the art of working iron has general principles, which would be infinitely ufeful to him in a great number of unforefeen cafes, to which his common practice cannot be applied.— -'Tis only by bringing the arts as it were to approach to one another, that we can make advances towards their perfection : we mall thus put them in a condition of mutually illuftrating one ano- ther, and perhaps of producing a great number of ufeful difcoveries : 'tis only by this means that we can know effectually their true principles, and enable them to receive- afliftance from theory."
It were to be wiihed, that convenience had permitted thefe reflections to have had their full influence in the ex- ecution of the work. Thehiftory is pubKfhed in detached- and independent pieces, each containing a minute detail of the whole feries of operations of one art, with defcrip- tions and plates of all the inftruments made uie of: it is defigned not only to fupply the philofopher with the know- ledge otherwife obtainable only among workmen, and to entertain the mind with the hiftory of human inventions, but likewife to enable perfons to exercife the refpective trades in places where workmen are wanting.
It is obvious that this plan does not at all interfere with mine, and that the views of the two undertakings are eflen- tially different. It is not my defign to dwell upon de- fcriptions of common and merely manual practices, to give
particular
PREFACE. xv
particular inffruftions for the fetting up and conducing of common manufactures, or hiftories of eftablifhments whicli mud vary with times and circumffances. The articles which make the main objects of my enquiries are founded on the invariable properties of matter ; and befides the confideration of arts in their prefent ffate, experiments for improving them, or refearches in that branch of knowledge from which their more valuable improvements muff, arife, make a principal part of the work..
With the advantages that refult from the abovemen- tioned alteration in the plan, the execution becomes far more difficult. What is fact or otherwife in regard to the properties of bodies, or the effects refulting from different operations on them, is to be determined by experiment. In the mifcellaneous method, we have no occafion to enter upon any points where the determination of experiment has not been clearly obtained. But in a regular hiffory the cafe is otherwife : we fhall often be led into fubjects with which we are not fufficiently acquainted, and though we thought we had materials in abundance, we fhall find fome deficiency, greater or lefs, in almoff every page: there are numerous particulars, which are not miffed in the de- tached fragments of knowledge, but whofc want is ftriking when thefe fragments come to be joined and methodifed into one whole. From this caufe, and from the difficulty and tedioufnefs, in fome cafes, of obtaining the neceffary informations among different workmen, unexpected delays, if we aim at making the hiftory tolerably complete, muff: frequently happen in the publication ; nor will it be eafyy
on
xvi PREFACE.
on certain occalions, even to avoid errors : indeed In di- rect experimental enquiries, the effect of the operation is fometimes fo much influenced by circumftances which are apt to pafs unheeded, that it is not to be wondered if errors are to be found in the writings even of the mofl ac- curate experimental chemifts.
The hiftory of each fubjeft I have made as complete, as my prefent means of information, and the experiments I have hitherto made, will permit; but much remains ftill undetermined and unexamined : if the work goes on, and the author mould be able to execute it to the utmoft of his hopes, the publick is not to expect that any article will be perfect : refearches in chemical knowledge we can ne- ver hope to make complete, every new acquintion fhewing new paths for our further progrefs in a province of un- bounded extent. Such material deficiences as occurred upon a review of the volume, and fome miftakes which I had been led into either by my own inadvertence or the authority of others, are taken notice of in the appendix. It is hoped that the fame regard for the publick; which influences the author, will prevail on thofe who may dis- cover any other miftakes of moment, to give him an ac- count of them, that they may be rectified in fome future publication. It is hoped alfo, that from the communica- tion of friends and artifts, much of what is wanting in one publication may be fupplied in the next.
One of the principal obftacles to the profecution of chemical enquiries has hitherto been the want of a proper
apparatus.
PREFACE. xvii
apparatus. I have therefore begun the work with an at- tempt to remove this impediment; to procure, at a fmall expence, a commodious and eafily manageable fet of fur- naces, 6cc. which may be all worked under a common chimney, and fome in the middle of a room without offence, and with which moil of the operations, that require the affiflance of fire, may be performed, in the way of experi- ment, with great eafe, expedition, and fafety : if thofe, who have been accuftomed to the common larger and more expenfive furnaces, mould at firft. be at any lofs in the life of thefe fmall ones, a very little practice and attention will remove all difficulties, and convince the operator of the convenience which I have long found from them in expe- rimental purfuits, for which alone they are defigned. The ftructure and management of the large furnaces, kilns, &c. ufed in different bulineffes, are intended alfo to make part of this work, and mechanical contrivances of other kinds are likewife occafionally coniidered, with a view chiefly to render them more fimple, convenient, or effectual. In the prefent volume I have given an entire eflay on the im- provement of the machines for blowing air into large furnaces, &c. by a fall of water, without moveable bellows, in virtue of the water carrying down air with it in falling through pipes ; and I have the pleafure of being informed, by a foreign correfpondent, of a machine which he has conftructed on the principles there eftablifhed, which an- fwers as well as can be defired.
It would be needlefs to fpecify in this place the feveral matters contained in the volume : a lift, of them may be
feeri:
xviii PREFACE.
feen in the table of contents. It is fufficient here to have 'explained the principles, and the views, with which the author has engaged in the work. How far thefe principles and thefe views have a juft foundation, or may tend to the advancement of arts and ufeful knowledge, and whether this laborious and expenfive undertaking mall be dropt or profecuted, is left to the determination of the publick.
THE
THE
CONTENTS.
!• Page
T\ESCRIPTI0 N of a portable furnace for making
*~* experiments ■ — — . — j
II.
Hi/lory of gold, and of the various arts and buftnejfes depending
thereon.
Sed. I. Of the colour of gold, and the methods of refloring
its lujlre when fullied • 38
Se<ft. II. Of the gravity of' gold • — 41
Seft. III. Of the ductility of gold, and the arts depending on
this property — — • 44
1. Preparation of gold leaf < 45
2. Preparation of gold or gilt wire 51
3 . The degree of extenfon of gold in wire and leaf 5 9
4. Application of gold wire and leaf on other bodies 61 Sett. IV ". Of the effects off 're on gold 66
1. Of the melting of gold ■ 66
2. Of the alterations J aid to be producible in gold by fire - 70
Se<ft. V. Of the mixture of gold with other metals 74
1 . Of the mixture of gold with mercury : gold powder, water gilding, &c. - 75
2. Of the mixture of gold with fiver, copper, &c. the alterations produced by different proportions of different metals, and the effects of Jlrong or con- tinued fire on the mixtures 82
Sed. VI. Of the action of acid and fulphureous bodies on gold ; various folutiovs of it, and their properties 90
1. Gold with the nitrous acid • 90
2. Gold with the marine acid 94
3. Gold with the vitriolic acid 95
A a 2 4. Gold
ii The C O N T E N T S.
Page
4. Gold with compound menjlrua 95
5. General properties of Jolutions of gold 97
6. Separation of gold from acids by inflammable liquors • 101
7. Precipitation of gold by alkaline falts 104
8. Precipitation of gold by metallic bodies 108
9. Gold with fulphureous bodies • — 11 1 Se&. VII. Of the alloy of gold ; and the methods of judging
of the quantity of alloy it contains, from the co- lour and weight • — 114
1 . Of the alloy of gold 1 1 4
2. Method of eflimating the finenefs of gold from its colour 1 1 9
3. Of eflimating the finenefs of gold from its gravity — 125
?eft. VIII. Of the affaying of gold • 1 27
1 . Cup e Hat ion with lead 129
2. Parting with aqua fort is — 135 Seft. IX. Of the reflning of gold, and the feparation of fmall
portions of it from other metals — 144
1 . Separation of gold from bafe metals by tefling with lead — 1 44
2. Separation, of gold from fllver by aquafortis 147
3 . Purification of' gold from fiver and bafe metals by cementation • 1 54
4. Reflning of gold from Jiher and bafe metals by antimony — 156
5 . Purification of gold from platina, fllver, and bafe metals, by aqua regia 159
6. Extraction of a fmall portion of gold from a large quantity of fllver ■ 1 6 1
7. Extraclion of gold from copper ■ ■ 166
8. Separation of gold from gilt works — 168 Se<5t. X. Of tinging glafls and enamel by preparations cf
gold ' -• 170
Sedt. XI. The mineral hiflory of gold 1 8 1
j. Of the matrices of gold, and its dijfemination
through different minerals < 1 8 1
2. Separation
The C O N T E N T S. Hi
Page
t. Separation of go/d from earthy and Jlony bcj.ies by water • • 190
3 . Separation of gold from earths and fiones by mercury 193
4. Extraction of gold intimately combined in the compofition offands' • 195
5. Extraction of gold from' the ores of other metals 199
Sett. XII. Of the alchemical hijlory of gold 199
1 . Of the producibility of gold by art 200
2. Of the dejlrutlibility of gold 204
Sed:. XIII. Imitations of gold 213
1. Gold coloured metal • 213
2. Gold 'coloured pigments. 22 r
3. Gold coloured varni/h or lacker 223
Addition to the hijlory of gold : of gold flowers on linen : of ex- tending the gold on brocades by pafjing them between rolls 226
in.
Experiments of the converfion of glafs vefffels into porcelain, and
ffor ejlabliflnng the principles off the art 230
Sedt. I. Experiments of the fuccefjive changes produced in
green glafs by baking ■ 233
Sedl. II. Experiments of the quality of the fubflance into which
green glafs is converted by baking 236
Seft. III. Experiments of comparing the effetls of different kinds
of materials on green glafs by baking 241
Sett. IV. Experiments of the baking oj different forts of glafs,
and of bodies approaching to a vitreous nature 245
1. Vitreous and Jemi-vitreous bodies compofed off earths, without metallic or f aline additions ■ 246
2. Metallic glqffes 247
3. Giafffcs- prepared with faliue additions ■ 248
Sett. V. Objb-vations on the cauj'e of the change which green.
glafs undergoes from baking 251
IV.
Off the expanfion or contraction off certain bod'h s at the time off- t heir pajjing from a jluid to a folid Jlate • ■ ■ 256
v:.
IV
The C O N T E N T S.
V- Page
Of the blowing of air into furnaces by a fall of water 267
Sett. I. Account of the principal machines ufed for blowing air
into furnaces by a fall of water • 270
1. A fimple pipe • 270
2. A pipe with air holes, infer ted into an air veffel 271
3 . A funnel and pipe without air holes, infer ted into an air veffel 272
4. A funnel and pipe with air boles, inferted into art air vefjel — — — — — — 273
At Lead-hills in Scotland ■ 273
In Dauphiny in France ■ • 274
In Foix in France ■ 276
hi Languedoc in France • 277
Sett. II. Experiments and obfervations for the improvement of the foregoing machines, and for efiablijhing their principles of action • 279
1. Of the quantity of water they require, and the quan-
tity and force of air they afford 279
2. Obfervations on the air veffel — — 286
3 . Experiments of air pqffing down with water through
pipes ' 2gi
Water running through a crane 291
Water defending through an oblique pipe with
lateral apertures 293
Water falling through a funnel • 294
Water falling from a conjiderable height into a
funnel with a pipe . 295
Water falling from a funnel through a pipe with
air holes 296
4. Experiments and obfervations for regulating the
JlruSlure of the funnel and pipe 298
Experiments with funnels and pipes of different
heights 298
Experiments of the difpofition of the air holes 301 Experiments of the proportional bores of the funnel
and pipe ■ 303
Experiments of dividing the fream fo as to increafe its eff'ecls, and render lefs water jufficient 304
VI.
The C O N T E N T S. v
VI. Page
Hi/lory of Colours. Parti. Of Black 315
Seel. I. General ob/ervations on black colours — — 317
Sect. II. Native black colours — 320
1. Black chalk 323
2. Pit coal 324
3. Black finds 325
4. Black lead 325
5. Black vegetable juices 329
6. Cuttle f ft ink 333
Seel. III. Black produced by firs 334
1. Charcoal blacks 335
2. Scot blacks 339
3. Black metallic calces 343
Seel:. IV. Black produced by mixture 344
1 . Black from iron 344
2. Black from fiver 350
3. Black from lead — ; — 352
4. Black from the combination of 'other colours 354 Seel:. V. Black paints, varnijhes, &c. 356
1. Black paint with oil 356
2. Black paint with water 357
3. Compq/ition for marking Jheep 3^9
4. Compojition 'for preferving wood, &c. — 362
5. Compojitions for blacking leather ^65
6. Spirit van: 366
7. Amber van; ftes for papier mache, &£. 367
S. Varnijh for metals 369
9. Sealing wax 370-
10. Printing ink 371
ix. Roiling prefs ink 376
Sedl. VI. Of the preparation of common writing ink — ^77
1. Experiments for determining the bejl preparation, of ink with vitriol and afiringents 378
2 . Compq/ition of ink deduced from the experiments ^91
3. Of the preparation of the paper for durable writing 393
Bb 4. Attempts
vi The C O N T E N T S.
Pjge 4. Attempts to prepare an ink from more durable
materials 396
Se&.VII. Of the dying of woollen black 401
1. General obfervations on the black dye 401
2. Black with galls, logwood, and vitriol 410
3. Black dye with verdegris 411
4. Method of dying cloth grey 413
5. The dying of wool black 415
6. Black dye without galls 416
.7. Black dye from a combination of colours — 420
$e€t.VlT1.0fibe dying off Ik black — - 42s:
SeCt. IX. The dying of hats black — 428
Se<£t. X. The dying of linen and cotton black 429
Sedt. XI. The Jlaining of wood, irory, fortes, &c. black 434
1. Wood 434
2. Ivory, horn, hone, &C 435
3. Marble 436
4. Agate 437
Se&.XII. Black glafi and enamel 441
VJI.
Hi/lory ofPlatina -— — — 443
i>edt. I. Of the general properties of platina conjidered by itfelf
or independent of its difpofition to unite or not unite
with -other bodies 449
.1. Defcription rf platina 449
.2. Su/fances mixed with native platina — 450
3. Specific gravity of platina 452
4. Malleability of platina 455
5. Platina expofed to the fire in vefj'els — 456
6. Platina expofed to the fire in contacl with the burning fuel 464
~7. Platina expofed to a burning glafs — 465
-Se&. II. Of the aclion of acids on platina 469
1. Platina with the vitriolic acid 469
2. Platina with the marine acid 470
3. Platina with the nitrous acid 473
4. Additional
The CONTENTS. vii
P»ge
4 . Additional exp crimen ts with the foregoing acids, &c. on platina 473
5. Platina with aqua regia 475
Sett. III. Experiments onfolution of platina 478
1. Colour of the folution, and trials of it for jlaining\7(&
2. Cryjlallization of platina 479
3. Volatilization of platina 480
4. Solution of platina, with vitriolic acid — 480
5. Solution of platina, with volatile alcali — 481
6. Solution of platina, with vegetable fixt alcali 481
7. Solution of platina, with mineral alcali — 483
8. Solution of platina, with P ruffian alcali — 483
9. Solution of platina, with compound falts — 484
10. Solution of platina, with vinous fpirits — 485
11. Solution of platina, with effential oils — 486
12. Solution of platina, with (ether 486
13. Solution oj platina, with tin 486
1 4 . Precipitate of platina expofed to a burning glafs 488 Sett. IV. Platina expofed to J? rongf res, with f aline, inflammable,
fulphureous, vitreous and earthy bodies — 489
1 . Platina with borax 490
2. Platina with alcali 490
3. Platina with nitre 491
4. Platina with common fait 495
5. Platina with vitriolic falts 495
6. Platina with the effential falts of urine — 496
7. Platina with phojphorine acid 497
8. Platina with black flux, &c. 498
9. Platina with fulphur 498
10. Platina with fulpburated alcali 499
1 1. Platina with earthy bodies ro2
12. Platina with vitreous bodies ro?
Sett. V. Of the mixture of platina with metals 507
1. Platina with quickJUver qo8
2. Platina with bifmuth 509
3. Platina with tin rio
4. Platina with lead — - r\z
5. Platina with arfenic — - — ijir
6. Platina
viii The C O N T E N T S.
Page
6. Platina with zinc — — — — » 519
7. Platina with regulus of antimony ■ 521
8. Platina with Jiher ■ ■ • 522
9. Platina with gold ■ ■ 525
10. Platina with copper — — • ■ ■ ■ 529
1 1. Platina with copper and zinc ■ 532
1 2 . Platina with copper and tin - 533
13. Platina with iron ■ ■ . ■ 534
14. Platina with metallic glaffes - > 537
15. General ob/ervations on the mixture of platina •with other metals ■ ■- 539
Sedt. VI. Of the Jpecific gravities of mixtures of platina with
different met ids 541
Sedt. VII. Of the effecl of fire and air on mixtures of platina with
certain metals $$j
1. Calcination of tin with platina 557
2. Separation of mercury from platina —— - 55S
3. Separation of ' arfenic from platina — 559
4. Separation of regulus of antimony from platina 559
5. Separation of zinc from platina 561
6. Cupel/at ion of platina with lead — 561
7. Cupellation of platina with bifmuth 573.
St&.V 111. Of the affinities of 'platina $yy
1. Mercury: Platina: Lead ■ ^j
2. Mercury: Gold: Platina 579
3. Platina: Lead: Iron » ■- ■ 580
4. Aqua regia: Zinc : Platina 581.
5. Aqua regia: Iron: Platina — — - 5S2
6. Platina: Aqua regia and Joint ion of iron vitriol:. Gold " — 582
7. Aqua regia: Copper: Platina > 582
8. Aqua regia: Tin: Platina 583
9. Aqua regia: Mercury: Platina — — 584.
10. Aqua regia: Nickel: Platina — — 586
11. Platina, gold, and aqua regia — — 587
12. Platina, fiver, and acids "■ ■ 588
13. Platina, lead, and acids - 389
14. Platina, regulus of antimony, and aqua regia 589
15. Platina, bi/muth, and acids — — 5^0
The CONTENTS. be
Pige
Se<5t. IX. Of dijlinguijlfmg and purifying gold mixed faith pla- tina - 590
1. Amalgamation with quickfilver 591
2. Precipitation by vegetable.fixt alcali 593
3. Precipitation by mineral fix t alcali 595
4. Precipitation by fal am?noniac —. — 597
5. Separation by inflammable liquors 598
6. Precipitation by green vitriol 599
Se_t. X. Experiments on. the yellow, part Lies mixed with pla-
tina 600
Sedl. XL Of the mineral hijlory of platina 603
S e Qi . X 1 1 . General obfervations 609
APPENDIX.
Obfervations on portable furnaces 613
Gilding of glafes on the edges 614
Gilding on the covers of books 615
Of the melting of gold \ and preferving it's malleability — 61 6-
Fufibility of mixtures of gold with copper 617
Calcination, &c. of tin with gold — • — 617
Gold with fal microcofmicus- 618
Gold plates for enamelling 610,620
Touchflone, to what genus ofjlones it belongs 620
Method of preparing ruby ghifs and enamel with gold — 621
Quantity of gold collected in rivers 622
Source of the gold found in rivers 623
Gold coloured metal from copper and tin 624.-
Gold coloured varnijh 624.
Rectification of fpirit for varniflies 625
Gold coloured glafs with metallic fubjlances 626
Yellow glafs with earths — ■ — 627
Yellow, &c. glafs with inflammable matter 628
Of the converfion of gre^n glafs into porcelain 630
Of the machines for blowing air by a fall of water 631
Of conjlrucling blowing machines with falls of water of great
height ----- _____ 632
C c. Of
x The C O N T E N T S.
Page
Of ' conjl rutting blowing machines for low falls — — 635
Explanation of the flate — -- 637
Black Diamond — — » 637
Indian ink from lamp-bLick and glue 638
Black varni/hfor metals 639
Separation of the alcali of fea fait 639
1. Purification of fea fait — — 639
2. Preparation of cubic nitre 642
3. Separation of the mineral alcali from cubic nitre 645
C O M<
ERRATA.
Page 89, lint 10, 1 1 . for two parts of tin and three of gold, read two pats of gold mi
three of tin. Page 556, line ic from tie bottom, for 87 parts rw/97 parts.
COMMERCIUM
Philosophico-technicum.
I. Defcription of a Portable Furnace for making experiments.
PORTABLE furnaces have been generally contrived, not only with a view to the purpofes of experi- ment, but likewife for anfwering in fome degree the demands of bufinefs. As the fize, which procures them this laft advantage, renders them lefs fit for the firfl and prin- cipal intention j I have long endeavoured to contrive a more manageable, as well as lefs expenfive furnace, for experimental enquiries; and to bring thefe kinds of inflru- ments as near as may be, in regard to the facility and the conveniency of their ufe, to the fame footing with the air-pump, the condenfing engine, and other like inflru- ments employed in philofcphical refearches.
The firfl hint of the apparatus here defcribed was taken from the practice of an ingenious workman in gold ; who employed occafionally, as a melting furnace, two of the larger kind of black lead pots, or blue pots as they are fometimes called, one inverted over the other. Four ox five fmall iron bars, palled acrofs the under-
B moll
[ 2 ]
moft pot, in its lower part, through oppofite holes made for that pnrpofe, ferved for a grate: the fuel, and the crucible containing the metal, were put in at the mouth of the pot : a hole in the fide, made under the grate, admitted air to the fuel : and the upper inverted pot, having a hole cut in its top, fupplied the place both of a dome and chimney. By this means he could melt, with eafe and expedition, feveral ounces of gold at a time; and conveniently examine the metal, during the fuiion, by railing or lifting off the upper pot.
A judicious variation of this fimple contrivance promifed to afford furnaces of more extenfive utility. Black lead pots appeared peculiarly well fitted for this ufe; common experience having (hewn, that they bear vehement fires without melting, repeated or continued fires without being fo liable to fail as any of the other kinds of cru- cibles, and .quick viciffitudes of heat and cold without being fo liable to crack : on thefe properties their excel- lence as crucibles depends. They have another quality, ineffential to them as crucibles, but which particularly adapts them to the making of furnaces: they admit freely of being ground, drilled, fawed with a common toothed faw, and cut with any kind of edged or pointed tool; fo that the neceffary doors, groovings, &c. may be readily made in them with common inftruments, and ftoppers for the doors formed out of broken pieces.
Having feen two of thefe crucibles form a convenient melting furnace where only a moderate fire was required: it was plain that a ftronger fire might be excited in them by the means ufed for that purpofe in other furnaces, viz. a bellows or a chimney; that one crucible would ferve as a furnace for thofe ufes in which no dome is wanted; and that if the upper crucible was fitted up in the fame manner as the lower, it would in fuch cafe be a diftincTk furnace of itfelf. It remained therefore to deter- mine
[ 3 J mine the moil: proper conftrudtion of the two crucibles, for adapting them to different purpofes, feparatelyor combined.
The black lead crucibles of that fize, which has the number 60 marked on the bottom, I have found to be the beft fuited for the general ufes of furnaces: their perpen- dicular height, in the infide, is about twelve inches; their width at the mouth fomewhat lefs than eight inches, and about the middle of their height fix and a half. Thefe veflels I have fitted up in different methods, and found the following conftrudtion to be in general the moft commodious.
Each crucible has its mouth ground fmooth and flat, upon a Hone, with a little fand ; and a round hole fawed in its bottom with the common compafs faw of the carpenters. In the fide, a little above the bottom, another round hole is made; and oppoiite to this, a fquare one ; above which is cut a larger fquare one. The places of thefe apertures, and their comparative fizes, are reprefented in the annexed plates; in which all the figures are drawn to one fcale, to prevent the neceflity of embarafling the defcription with an account of the particular dimenfions of the feveral parts.
All the apertures are fitted with floppers, cut out of pieces of broken crucibles, which are eafily procurable at thofe places where the pots themfelves are fold. The fquare apertures are made, at each of their fides, a little narrower internally than externally, by which means the floppers, though their furface lies equal with that of the outfide of the pot, are prevented from being pufiied inwards : this flope is made on the fides, and not on the top or bottom, that the floppers may not be liable to fall out when the pot ftands on either end. The round ones are in little danger of falling out, being made to fit clofe, by
B 2 grinding:
[ 4 ] grinding them into the apertures. The {toppers are con- veniently taken out and put in by means of a kind of fork, each ftopper being furnifhed with two fmall holes for introducing the point'; : thefe holes are made at the fame diftance from one another, in all the ftoppen of both the crucibles, fo that the fame inftrument ferves for them all : a fpringinefs in its legs accommodates it to fmall inequalities. The bottom ftopper is better managed by the hand or tongs ; a circular cavity being cut in it, fo as to leave a knob for that purpofe in the middle.
The grate confifts of an iron ring, with crofs bars fixed in it: the thicknefs or depth of the bars is confiderably greater than their width, that they may have fufficient ftrength, and that the fpaces between them may be as great as poffible : the ring is formed of a bar, of the fame dimenfions, turned round. Three of thefe grates, of different widths, are required for different ufes : one, of fuch a fize, that it may reft againft the con- verging fides of the pot, in the lower narrow part, juft above the lowermoft fquare hole; another, fo large that it may enter no further than nearly to the top of the uppermoft hole; and the third, of the fame width with the outfide of the mouth of the crucible. One grate of the fmaller fize is neceffary for each of the furnaces ; but one of each of the other two lizes is fufficient for both furnaces; thofe operations which require either of thefe grates, requiring at the fame time both the pots. For more effe&ually keeping the lower and middle grates in their places, either grooves are cut for their edges to reft upon, all round the pot; or three notches are made, for each, at equal diftances, in the pot, and correfponding knobs or pins on the circumference of the grate; which pins are rivetted into the ring. This laft method is the moft
eligible,
[ 5 ]
eligible, as it admits of mod: interfaces for the air to pafs through, and the afhes to fall down j for here, as great a fpace may be left between the rim of the grate and the fides of the pot, as between the bars ; and this vacuity round the fides is the more ufeful, as the afb.es are there moil YvJ.Ae to be accumulated, and the fire to be languid. Jn whatever manner the grate is fupported, care muft be taken that it have fufficient freedom, and that neither the grate itfelf, nor its knobs, bear hard againft the fides; left the expanfion of the metal, when heated, fhould not only make the grate difficult to be got out, but like- wife damage the furnace.
To render the furnaces fufficiently durable, they are bound round, in three or four different parts, with copper wire, fo as not to interfere with the doors or holes. The wire is about the fize of a crow quill, or fomewhat larger, and is foftened and made pliable by nealing or heating it on live coals : it is prevented from flipping by a flight groove made for it round the furnace; and its ends are drawn together, and twifted tight, with pincers. The mouth is moft effectually fecured by a thin copper hoop, which prevents the edges from being broken or worn off: the flexibility of the thin copper admits of its accommodating itfelf to the figure of the furnace ; and what fmall fpace remains between the furnace and its upper edge, is filled with a little moift loam, or with clay mixed with fome powdered pots. The crucibles thus armed, continue fer- viceable, after they have been fo much cracked that they would otherwife fall in pieces.
Thefe furnaces are conveniently lifted or carried by means of a moveable handle, made of an iron rod, or a piece of ftrong iron wire, about three feet long, bent, like the bale of a pail, to the width of the furnace,
with
[ 6 ] with the two cuds turned inwards, fo as to enter into two (mail oppofite holes made in the furnace, through the copper hoop : the fpringinefs of the iron rod admits of the extremities being ealily drawn afunder fufficiently for the introducing or withdrawing of the hooks.
Besides the black lead crucibles which make the body of the furnace, there are required, a foot for them to ftand upon, a chimney, and an iron hoop.
The beft fort of foot is a flat, heavy, iron ring, with three legs live or fix inches high : in one of the legs of this trevet is a fcrew, by which it may be occafionally railed or lowered, fo that the furnace is made to ftand level though the floor be uneven. A foot may be formed alfo of the lower part of another black lead crucible inverted; by making a fuitable aperture in the bottom, and fawing three arches, at equal diftances, in the fides, fo as to leave between them legs of fufficient ftrength. One of the furnaces, inverted, makes likewife a convenient foot for the other. Where either of thefe feet is ufed, the aflies, that fall down, are received in an iron pan, fuch as a frying pan with the handle cut off, placed underneath. If the furnace is fet on a ftone, an iron plate, or any other folid fupport, the aflies, accumulated in long operations, are raked out at the lower aperture in the fide, by means of a piece of narrow iron plate conveniently bent at one end.
The chimney is compofed of three pipes of forged iron plate, which fliould not be thinner than one eighth of an inch, that they may not foon be bent or deftroyed by the fire. Each pipe is a foot and a half or two feet long. The undermofr, that it may ftand fteady, has a broad heavy ring round it, about an inch above the lower 5 erjd
t 7 3 end, which lower part enters into the hole in the top of the inverted pot : the upper end of the pipe is received into the lower end of the next, and the end of this is in like manner received by the third j fo that the chimney is nearly of the fame width, or only infenfibly converging from the bottom to the top. It is convenient to have the upper end made fquare, that it may fit into the larger door of the furnace, and thus ferve occafionally as a lateral chimney.
The hoop is formed of a forged iron plate, not lefs than one fixth of an inch thick, turned round, and welded together at the ends. It is about fix inches deep; and of the fame width externally with the top of the furnaces, but its thicknefs being lefs than that of the furnaces, it is internally wider. One end of this hollow cylinder has an iron ring pafling round it withinfide: this is the end on which it moft commonly ftands, upon the mouth of one of the pots ; and the ring contributes to make it ftand fteady, as well as to ftrengthen it. Near to this end is a femicircular aperture whofe door is rivetted on a large iron plate, which opens downwards on hinges, and drops no lower than to an horizontal fituation. There is no occafion for the hoop being luted; for if made of good hammered iron, of the thicknefs above directed, it will be fufficiently durable, without any defence, in the °reateft degrees of heat which it is intended to fupport.
One of the black lead crucibles, prepared as above defcribed, with the lower fmall grate introduced into it, is a furnace for open fire: the lower fquare aperture, immediately under the grate, is the door of the adi-pit -, and the upper one, above the grate, is a door to the fire- place : which laft, in the intentions this furnace is defigned for, is kept fhut. The fuel, which muft in all
cafes
[ 8 ]
cafes be charcoal, and of which the confumptlon is in thefe kinds of furnaces inconfiderable, is put in at the top, and is fupplied with air through all or any of the apertures beneath the grate: by more or lefs clofing or opening thefe apertures, the fire is diminifhed or increafed.
This open furnace, befides its ufe for keeping fuel ready lighted to be employed in others, affords the con- veniency of nealing metals when grown hard or rigid by hammering or rolling ; of letting any fmall vefiel occafionally upon the coals, as a crucible or iron ladle for the melting of the more fufible metals, and ferves for many other like purpofes that occur in practice.
By introducing into the open furnace an iron pot, empty or containing fand ; it becomes a furnace for a capella vacua as it is called, or a fand furnace; in which the only variation from the preceding is, that the mouth of the furnace being occupied by the iron pot, the fuel is put in through the fire-place door or the aperture above the grate.
An iron ladle, with its handle cut fhort, ferves extremely well for the capella or fand pot. It is fupported over the fire by means of a flat iron ring, into which the ladle is inferted fo as to bear againft the ring by its upper wide part. It is necefTary to have feveral of thefe rings, of different internal diameters for receiving ladles and other vefTels of different fizes, but all of them wide enough externally to reft upon the top of the furnace. Between the furnace and the ring are inferted, at equal diflances, three iron fupporters, about a quarter of an inch thick, an inch long, and equal in breadth to the thicknefs of the fides of the furnace. Through the fpace thus procured beneath the ring, the burnt air paffes off; and being permitted to iifue freely on all fides, the heat is diftributed,
and
[9] and the veflel heated all round : whereas, in thofe furnaces, where the air pafles off by an elbow or chimney at one fide, the action of the fire is chiefly upon one fide of the veffel, and confequently, befides an inequality of the heat, a greater quantity of fuel becomes neceflary for producing in the vef- fel the fame degree of heat. The admiflion of the air by .' 5 bottom hole, perpendicularly under the grate, has likewife fome advantage in this refpect above the lateral admiflion of it by the door.
Instead of the foregoing kinds of veflels, narrower than the furnace fo as to be received into it, a much broader one may be placed upon the top, with the three iron fup- porters under it to procure a fpace for the paflage of the air. The flat iron pan, which on other occafions is fet underneath the furnace for receiving the aflies, may be ufed in this manner as a veflel for calcinations, for the evapora- tion of folutions of lixivial falts, &c.
For veflels of a deeper kind, as a copper ftill, the capacity of the furnace is increafed by placing over it the iron hoop ; by which it is enabled to receive the body of a ftill, of a fize fufficient for the purpofes of an experi- mental elaboratory. In other refpecls, there is no vari- ation from the preceding form : the fuel is put in through the door above the grate ; and the ftill or other veflel hangs, like the capella or fand-pot, in an iron ring, which refls upon the three iron fupporters placed upon the hoop.
With regard to the diflilling veflels, their ftruclure differs from that of the large ones in common ufe. The body of the ftill is a wide copper pan ; and, for diftillation in a water bath, another veflel of the fame figure is received into it almoft to the top, as reprefented in the firft plate, the fpace between them being nearly filled with water. Both
C thefe
[ *° 3
thefc veflels arc of the fame width at the mouth, fo that the fame brcaft fits them both, and either may be ufed as a ftill equally with the other : either of them ferves alfo, on other occafions, as an evaporating pan, a boiler, for experiments in dying, and other like purpofes. All the parts are made of thin copper plate, and well tinned on the infide with pure tin : in confequence of their thin- nefs, they admit of fome alteration of their figure about the edges, fo that, though they ihould not be perfectly round, they are readily accommodated to one another, and fit dole : the juncture is ealily made perfectly tight by applying round it narrow flips of moiftened bladder; which are more convenient than luting, as being readily ftript off when the operation is finimed. A fliort pewter pipe, with a pewter ftopper fitted to it, for returning the diftilled liquor, or pouring frefh liquor occafionally into the ftill, without the trouble of tinluting and feparatirig the veffels, is foldered into the top of the head ; which, in thefe kinds of instruments, is the molt convenient place for it. For feparatirig, by diftillation, fpirituous from watery liquors, or the rectification of fpirit of wine, the head is raifed, by inferting, between it and the breaft, a thin copper pipe about two feet long. A worm and refrigeratory are neceffary, as for the common ftill : and a glafs head is requifite for fome ufes, particularly for the diftillation of vinegar, and fuch other liquors, as would corrode a copper one, and impregnate themfelves with the metal ; in which cafe, the ufe of the metalline worm alio is to be avoided, and the glafs or ftone-ware receiver joined to the pipe of the head.
The above apparatus is as commodious, as can be wifhed, for diftillation in the way of experiment. Con- trivances for expediting and accelerating the procefs do not belong to the prefent defign : but as this is an affair
of
[ II ]
of great importance in fun dry cafes, particularly for pro- curing frefh water eafily at fea, I was engaged by the late Dr. Hales, whofe extenfive philanthropy will ever render him dear to mankind, and who had this falutary object greatly at heart, to undertake a fet of experiments directly with this view ; the refult of which mall be communi- cated in a feparate article of fome of our future publica- tions.
A Wind-furnace, for the fufion of metals, the aflaying of ores, and other like ufes, is formed by invert- ing one pot over the other ; and placing on the top either the entire chimney, or two or one of its pipes, according as the fire is wanted to be more or lefs ftrong. The fecond grate, in the middle of the undermoft pot, is generally, in thefe intentions, more proper than the lower fmall one made ufe of in the former cafes, on account of its having more interflices for admitting air to the fuel. The crucible, containing the fubject- matters, is placed upon a circular flip of brick, or of a broken pot, a little wider than its bottom, laid upon the middle of the grate, to prevent the cold air from ftriking on it. The charcoal is put in through the fire-place door, or larger aperture, of the dome or upper pot, which fliould always be clofed after each fupply of fuel. The furnace flands on its trevet or open foot ; with the flat iron pan underneath, not only for receiving the aflb.es, but that, if the crucible fliould happen to fail during the fufion, its fluid contents may be preferved. The two oppofite holes in the upper part of the dome afford the conveniency of pafling an iron rod through, for fafely and commodioufly lifting it when intenfely heated.
C 2 The
[ 12]
The round hole in the fides of thefe furnaces gives an opportunity of introducing the nofe of a pair of double bellows ; fo that one pot fingly, or two combined together as in the foregoing article, may be readily converted into a blaft furnace; in which cafe all the other apertures beneath the grate mufl be clofed while the bellows acts, to prevent the air forced in by it from efcaping. It is of great advantage, in fundry operations, to be thus able, at certain periods, to fuddenly animate the fire.
One pot inverted over the ocher, with the iron hoop placed between them, make a furnace for cupellation, calcination under a muffle, and the other purpofes of what is called the affay furnace ; as alfo for experiments of enamelling, of baking colours on earthen ware and glafs, &c. The chimney, or a part of it, is occafionally fet upon the top for raifing the fire when it proves too languid.
In want of the muffle made for operations in the afTay furnace, its office may be fupplied by a common crucible laid upon its fide, with a bed of loam in it to form a flat furface for the cupels or other veflels to fland on. The largeff. of the three grates is fet upon the lower pot, which it wholly covers ; and the iron hoop, which of courfe bears upon the rim of the grate, is placed with its door lowermofi:. The muffle, or its fubftitute, is laid upon a flip of brick on the grate, with its mouth fronting the opening in the hoop, through which the cupels, &c. are introduced. In proceffes which require frequent infpeclion of the fubject-matters, and the admiffion of heated air ; the plate, on which the door of this aperture is rivetted, is let down to an horizontal fituation, and fome pieces of lighted charcoal, with fufficient interflices between them for the cavity of the muffle to be feen, are laid upon the
plate,
[ '3 1 plate, which, for this purpofe, is made about two inches wider than the door, and as long as the height of the hoop will admit it to be. The fuel is put in, as before, through the fire-place door of the dome. As the part of the fur- nace above" the grate widens downwards, the coals gene- rally of themfelves fall properly round the muffle : if this fhould not happen, they are eafily pufhed down by means of a crooked iron wire introduced through the door. The dome is lifted off, as in the wind-furnace, by an iron rod pafTed through its oppofite apertures.
The foregoing combination, of the two pots and the iron hoop, with or without the chimney or a part of it according as a greater or lefs degree of heat is required, ferves alfo as a reverberatory, for diftillation in coated glafs retorts, earthen retorts, or longnecks ; with only this variation in their difpofition, that inftead of the large grate, the middle one is introduced into the undermoft pot, and in its upper part two iron bars are laid acrofs : the bottom of the difiilling vefTel refts upon thefe bars, and its neck comes out at the door of the hoop, which is accommodated to vefTels of greater or lefs height by placing it with the door uppermoft or lowermoft. Both the re- verberatory and the afTay furnace are, in effect, no other than the wind-furnace ; with a muffle or a retort fet in the fire inftead of a crucible.
These furnaces may be ufed likewife as a common ffove, for keeping a room warm with a little quantity of fuel. There are three general intentions which have been purfued in contrivances for this purpofe ; (i) making the fuel take fire by degrees, and confume flowly ; (2) conducting its heat, or the air warmed by it, through a number of pafTages or circumvolutions, that the heat,
inftead
[ Hi inflead of being carried up the chimney, and thus loft, may be detained in thefe paflages, and thence communi- cated to the air of the room to which they lie cxpofed ; and (3) applying to the fire a quantity of folid matter, which, being once heated, preferves its heat long. Some ingenious furnaces, on thefe principles, are defcribed in the Tran factions of the Swedifh Academy, and in the fecond edition of Reaumur's Art of hatching Birds. All thefe contrivances are united in the following combination of the two pots and the hoop.
The undermoft pot has the fmall grate introduced into its lower part, the fire-place door doled, and the afh-pit door or the bottom hole open for admitting air. Being then charged with fmall pieces of charcoal, and fome lighted coals thrown above them, its top is covered by the largeft of the grates, and on this is placed the hoop and dome, filled with balls of baked earth, or with pieces of bricks, fo difpofed as to leave fmall vacuities between them. If the flove is placed in the middle of a room, its injurious burnt air may be carried off, by a pipe inferted laterally into the larger door of the dome, and communi- cating at the other end, which lhould be raifed eight or ten inches, with the chimney of the room ; all the other apertures of the dome being clofed.
The furnace, thus charged, will keep up a moderate and nearly equal warmth for many hours, without injury or offence ; the charcoal burning down exceeding flowly ; and the heated balls or bricks continuing the warmth for a considerable time after the fuel is confumed. Frelh char- coal may be occafionally fupplied through the door above the grate ; the check, which the balls give to the motion of the air through the furnace, renders the confumption of this alio flow, and it may ftill be made more fo, at pleafure, by flopping a part of the aperture which
I admits
[ i5] admits the air, or of the pipe or chimney which car- ries it off.
It appears from the foregoing account, that two black lead crucibles, fitted up in a manner which any common workman can eafily execute, with a few additional parts as eafily procurable, are Sufficient for forming almoft any kind of furnace which experimental enquiries have occafion for; and that they not only unite, like the different parts of other portable furnaces, into one furnace, but likewife, for fundry ufes, form two diftinct ones. It will be ex- tremely convenient to have a third crucible prepared in the fame manner : in which cafe, as no operation requires more than two ; whatever kind of furnace is required for one operation, there will always be at lead one left at liberty for another ; fo that two very different kinds of experiments may thus be going on at the fame time, without hindrance, and without interfering one with another.
For enquiries of any confiderable extent or multiplicity, it is neceffary to be provided with feveral of thefe crucibles ; and though, for general ufe, we have given the preference to thofe marked 60 on the bottoms, yet other fizes may be occafionally employed, and have their advantages for particular purpofes : much fmaller ones, down as far as number 20, will make, for fome ufes, very convenient furnaces. Thofe of number 60 may be considered as the mean fize : they are as large as are wanted, in experimen- tal practice, for a capella or bath ; and they are the fmalleft, and mod manageable, that will atifwer effectually as a wind furnace or blaff. furnace for Strong fire. The largeft black lead crucibles are marked ioo : their internal height is about thirteen inches and a half, and their internal width at the mouth ten inches and a half. The next
fizes-
[ 16 ] fizes are marked 90, 80, 6cc. without any intermediate numbers between the tens. All thefe are half an inch or more narrower one than the other, though not with any exact regularity in the diminution of the fizes, or in the dimenfions of the pots of one number. Thofe of 90 or 100, with a hoop and rings adapted to them, will receive fand-pots, ftills, £tc. large enough for the purpofes of the apothecary : but furnaces for experimental purfuits being the prefent object, it is fufficient here to have given a hint of this application of them.
It is expedient likewife, in a well-appointed elaboratory, to have fome furnaces of a different construction as well as of different magnitudes, fet apart for particular ufes, efpecially for thofe which demand great vehemence or continuance of fire.
In all cafes where vehement fire is to be continued for any confiderable length of time, the furnace may be Strengthened by inferting one of the crucibles into another of a larger fize. Some of the thick bottom part of the inner crucible is to be fawed off, and the remainder rounded with a rafp, that its furface may in fome meafure correfpond to the cavity of the outer one; and the mouth of this laft is to be widened, if it does not furficiently admit the wide part of the other, by rafping off a little from its infide all round the converging edge. Any cru- cible may thus be made to fit conveniently into the fecond fize above it ; that of number 60 into 80, 70 into 90, and 80 into 100. Such vacuity, as may remain between the two, is to be filled with dry fand dropt in at the fides ; or rather with flaked lime, or fine fifted wood-afhes, diluted with fo much water as will render them of a due confidence for being poured in. This mixture foon fets, without Shrinking considerably, and without growing hard
in
[ 17]
in the fire : thus it uniformly fills the interfaces, and joins the pots fufhciently together, yet not fo firmly but that upon occafion they may be ealily feparated again.
The fuel, for producing or continuing vehement fire, mould be the moft ponderous and compact kind of char- coal, as that of the oak or beech, free from bark, and in pieces about the lize of hens eggs. It mould be kept ready lighted, for continual fupplies, in another veffel, fiich as the open furnace formerly defcribed ; and freih quantities of the burning charcoal thrown in every feven or eight minutes, or in proportion to the quicknefs of the confumption, that the crucible may always continue covered. Where a dome is ufed, a fupply of lighted fuel is commodioufly obtained by means of an iron plate, turned up at the fides into a kind of fquare trough, and hooked on the fire-place door : the trough being filled with charcoal, the inner part of the coals is kindled by the vicinity of the fire, and this is moved into the furnace by fucceflively pufhing forward the unlighted part. Some have imagined that the fupplying of fuel previoufly made, red hot could affect only thofe kinds of operations, in which the veffel is fet over the fire, and the frefli fuel interpofed between the fire and its bottom ; and that this practice could be of no advantage in the melting furnace, where the crucible is placed in the middle of the fire. But though, in this lad circumftance, the previous kind- ling of the fuel has much lefs influence than in the other, it is by no means to be disregarded : the upper part of the crucible may fometimes be nearly bare, and fome of the pieces thrown in may drop down through vacuities about the fides ; in which cafes it is obvious, that the injection of cold or unlighted coals, muft necef- farily, for a time, diminim the heat about the veffel, and likewife endanger its cracking.
D if
[ i8 ]
If a pot is to be fitted up on purpofe for a blaft fur- nace, no other aperture is required than a round one in the fide for the nofe of the bellows to enter, the fuel being here thrown in at the top : all other apertures need- lefsly weaken the furnace. The pot fliould be of the largeft. fize : if a cover or dome, or an additional part for enlarging its capacity, is wanted, another pot of the fame fize, with a portion of its lower narrow part fawed off, makes a very convenient one ; and this narrow part, as we (ball fee hereafter, will be of ufe for another purpofe. A round plate or flip, fawed from the folid part of the bottom, ferves very commodioufly both for a grate, and for a fupport to the crucible : eight or nine holes, about three quarters of an inch in diameter, are bored round the outer part of the plate, for the tranfmiffion of the air forced in by the bellows ; and four or five fmall crucibles may be placed together in the middle : the holes are made to widen downwards, to prevent their being choaked up by pieces of the fuel.
The bellows I have chiefly employed is the organ bellows ; with this variation in its ftruclure, that the upper board, inftead of rifing obliquely on hinges at one fide, rifes equally on all fides, and continues always hori- zontal. On account of the flexibility or pliablenefs of the leather of this kind of bellows, it is not only moved far eafier than the common ones, whofe leathers are fliff,, but is likewife free from their inconvenience of refitting unequally according to the greater or lefs extenfion of the folds. The board is preffed down, and the air forced out, by a weight on the top : in the common bellows this weight, preffing on an inclined plane, whole incli- nation is continually changing, has its force changing in
like
f *9 ]
like manner, and thus produces an irregular blaft, and an unfteady heat : in ours, the weight, acting always per- pendicularly, has always an equal power, and the air i» propelled in one unvaried current. The fame advantage may be procured alfo to bellows of the common conftruc- tion, by an alteration only in the difpofition of the weight; which, inftead of being placed on the top, mould be hung at the extremity, upon an arc, furniihed with a groove for receiving the cord, and whofe center is the point on which the board moves : the weight being thus made to acl always in a perpendicular direction, and at an equal diftance from the center of motion, an equality of its power is effectually fecured. By thefe means, the heat is kept up uniform ; and may be eafily increafed at pleafure, by increaling the weights, to the greater!: degree that can be excited in furnaces ; in which intention it will be of advantage to have the nozzle of the bellows wider than ufual, that the air may be the mere freely difcharged from it. The bellows is of no incumbrance in the elaboratory, being inclofed in a wooden cafe, whole cover does the office of a common table : to the nozzle, which juft comes through the crofs bar at the bottom of the table, is occafionally fitted a pipe reaching to the furnace.
The bellows thus difpofed ferves likewife for impelling and concentrating the flame of a lamp upon bodies ex- pofed to it. For this purpoie, an upright tin pipe is fitted on the nozzle by a ihort elbow at its lower end ; and at its upper end is a moveable elbow, into which is inferted a lefler pipe, having a very fmall aperture in its extremity : this aperture being applied to the flame of a lamp placed upon the table, and the pipe blown through ; the flame lengthens in the direction of the blaft, and
D 2 converges
[ 20]
converges Into a fmall fpace, producing there a very in- tenfe heat. This application of a lamp is common among fun dry artifls, for melting or foftening glafs or metalline bodies in any particular part, without affecting the reft of the mafs ; as in foldering metals, making balls for thermometers, &c. It is likewife very commodious for the expeditious performance of many kinds of experi- ments, where only a fmall quantity of matter is to be acted upon by the heat : a little gold or filver, laid in a cavity made in a folid piece of charcoal, and expofed to the concentrated flame, melts 'almofl inftantaneoufly j and a little lead, placed in a cupel, may thus be quickly worked off or turned to fcoria, fo as to difcover whether it contains any confiderable proportion of the noble metals.
A bellows of the above conflruclion is worked with very little labour. It is made flill lighter and more com- modious, by ufing a ftiff rod, for moving the lower board, inftead of the flexible cord or chain commonly employed. By the rod, its motion is made to follow that of the hand : whereas, with the cord or chain, it cannot link faff, enough without a confiderable additional weight, and, in railing it again, the hand has this weight to overcome at the fame time with the weight on the upper board by which the air is forced out.
The trouble of ufing bellows has by fome been pro- pofed to be difpenfed with, by fubftituting the aeolipile. This inflrument is a flrong copper veffel, with a neck turned to one fide, and terminating in a very fmall aperture. The veffel being about half filled with water, and fet on fome burning charcoal, in any convenient little furnace, fuch as one of our pots, with its neck di- rected to, and placed at a fmall difiance from, the fire to be excited .; as foon as the water begins to boil, an
i elaflic
[ « ]
elaftic vapour id'uee with vehemence through the fmall orifice, and the fire is animated in the fame manner as by air impelled by bellows ; whence this inftrument has been called the asolian bellows and philofophers bel- lows. From this effect of the aeolipile on an open fire, it has been imagined that it would perform the fame office when its neck was inferted, like the nofe of a common bellows, into the cavity beneath the grate of a furnace, and accord- ingly fome practical writers have given figures of it as employed in this ufe. But on trial, I have conftantly found it, when thus applied, inftead of exciting, to ex- tinguish the fire ; and the event was the fame, in all other cafes, where the vapour did not pafs through a portion of the atmofphere before its admifiion to the burning fuel. From this obfervation it may be prefumed, that it is not the included matter, or any particular element in it, that animates fire, but the common air of the atmofphere which the watery vapour imbibes or propels before it. This mention is here made of the aeolipile, to prevent others from being put to the expence and difappointment of fuch an apparatus as gave rife to thefe obfervations.
A CLOSE-bottomed pot, fuch as that u fed for the fore- going blaft furnace, but without a grate, makes a furnace for the fuiion of metals, the revival of metallic calces, and the fmelting or affaying of ores, trans carbones as it is called, or in contact with the burning fuel, as pradtifed in the large works. The furnace being intenfely heated, and almoft filled with fuel,, fome of the fubjedl-matter is fprinkled upon the coals, chiefly about the middle, and towards the fide oppofite to the bellows, but with care that it no where touch the fide : more charcoal is thrown over it ; the fire, according to the nature of the fubject,. is either kept up ftrong, or abated a little, by leffenin*
the
[ 22 ]
the weight upon the bellows ; and the alternate injection of the fubjedt-matter and of charcoal continued. The metal and (lag, melting and dropping down through the coals, are collected in the bottom : when they are of fuch a kind as to melt with difficulty, it is neceffary, in order to their being continued in thin fufion, to direct, the pipe of the bellows downwards, toward the oppofite fide of the bottom : the aperture is made to admit of the bellows being readily thus directed, without widening it in the middle, by doping off a little, from its upper edge on the outfide, and from the lower edge within.
When iron is thus to be melted (an intention for which this furnace is extremely well adapted) or copper to be purified, among the coals, the black lead pot performs the office both of a crucible and of a furnace. In this cafe it is neceffary to have its bottom furrounded on the out- fide with burning fuel : the liftings and fmall fragments of charcoal, unfit for other ufes, anfwer fufficiently for this purpofe, for they are foon fet on fire by the heat of the pot, and ferve, as well as the larger pieces, to main- tain and augment its heat : they may be placed in a cavity made in the ground, or in the bottom of another veffel. When the procefs is finished, the melted metal may be poured out, by inclining the pot, through the hole where the pipe of the bellows entered.
But when litharge is to be revived, or the ores of lead or of the other more fufible and deitrudtible kinds of me- tals to be fmelted, the metal, in proportion as it is col- lected in the bottom, muft be fuffered to run off from the vehement heat and blafl of air ; for which purpofe a paffage is to be made for it in the moit depending part, and a bafon filled with coals placed conveniently on the outfide for receiving if. The lower part of a black lead pot, fa wed off at a proper height, as three or four inches above
the
[23 ] the bottom of its internal cavity, makes a commodious bafon for this ufe ; and is eafily made to join to the Hoping canal in the furnace, by rafping off a little at one fide, and forming a channel in the lip correfponding to the hole in the furnace : the juncture is fecured by the inter- pofition of a little foftened loam or clay.
The more fulphureous ores are commonly freed, by roafting, from great part of their fulphur, before they are fubmitted to this operation : for, by this procefs alone, the fulphur would not be completely feparated ; and the metal, after the fufion, would prove impure and brittle, or be retained in great part among the flag. With our apparatus, the roafting is more particularly neceffary, for the fake of the furnace as well as of the ore : for black lead crucibles, though they long fuftain the action of vehement fire and of metals made fluid by it, are foon preyed upon and deflroyed by fulphureous bo- dies in fufion.
The pot, employed as a furnace for thefe ufes, fhould, like the preceding blafi: furnace, be of the largeft fize : and its height may be increafed, by inverting over it a ring fawed from the upper part of another pot of the fame fize. By this additiori to its height, the fuel thrown on the top will be kindled, and the fubjecT: heated, before they fink down into the body of the fur- nace ; and the convergency of the upper part of the ring prevents the heat from fpreading and annoying the operator, as it does from furnaces of a diverging mouth.
As the blaft furnace, defigned for intenfe fire, is made fironger than the pots of the general conftruclion, by having no other aperture than that which receives the bellows pipe ; it is in like manner expedient, for fome particular uics, to have a ftronger wind-furnace, with
only
{ 24]
only fuch holes as are eflentially neceflary, that it may- be better able to fupport a long continuance of vehe- ment fire.
The furnace for this intention confifts of two large pots : the lowermoft of which has only a round hole in the bottom for admitting air ; and the upper one, or dome, a fimilar hole correfponding to the chimney, with a door in the fide through which the fuel is put in. The furnace is placed upon an inverted pot, which has a hole in its top anfwering to that in the bottom of the furnace, another large one in its fide, and its mouth ground fmooth that it may apply itfelf every where clofe upon the flat flone or iron plate which ferves as a fland for it. Into the fide aperture of this lower pot, which is both the foot and afh-pit of the furnace, an iron pipe is inferted, fomewhat wider than the widefl part of the chimney, and two or three feet long, on the end of which may be fitted a wooden one of more confiderable length. The whole of this pipe may be laid horizontal, fo as to reach into an adjoining apartment ; or rather, if there is a conveniency, the wooden pipe may be funk perpendicularly through the floor into a room underneath, and the horizontal iron one ioined laterally to it at the top : its ufe is for conveying into the furnace, inftead of the adjacent air rarefied by the heat, the colder and denfer air at a diftance ; and its effedt in animating the fire will be in proportion to the coldnefs and denfenefs of the air to which it reaches.
In the above construction, I have endeavoured to give the wind furnace all the advantages it appears capable of receiving ; and to fupply, with black lead pots, the wind furnace, contrived by Mr. Pott, of the academy of fci- ences at Berlin, on purpofe for experiments that require the utmoft vehemence of fire, as the vitrification of
earthy
[ Ml
earthy and ftony bodies. I neverthelefs apprehend, that furnaces on this principle, with all the advantages that can be given to them, will not be found equal, in regard to the intenfity of the heat, to the blaft furnace above defcribed ; and that air may be fupplied more effectually by a well contrived wide nofed bellows, than by any other means whatever. The preffure of the atmofphere, which actuates the wind furnace, is variable, and fub- ject to many irregularities in its effect : but the power which animates the blaft furnace, is entirely in the opera- tors hands, and its effect may be increafed or diminifhed, with certainty and regularity, by increafing or diminish- ing the force artificially applied. The wind furnace however has its conveniencies ; as the fire may be raifed in it to a degree fufficient for moft purpofes that com- monly occur, and continued without any other trouble than that of fupplying fuel.
The ftrudture of our general furnace, already defcribed, unites in fome meafure the advantages of both kinds ; by affording an opportunity, when the wind furnace is at work, of occafionally animating its fire by the blaft. The fame convenience may be procured alio in the wind furnace above defcribed for vehement fire : a hole, made in the fide of the foot or afh-pit, ferves for ad- mitting the bellows-pipe ; and the air-pipe is at the fame time ftopt by means of a moveable regifter in the end next the furnace. This regifter is a circular iron plate, fixt on an axis, which is placed acrofs the pipe : the ex- tremity of the axis projecting on the outride, the plate is thereby readily turned, fo as either to allow a free paifage for the air, or to clofe the whole bore of the pipe.
It is, in many cafes, a very defirable point, to be able to collect the heat, diffufed through a furnace, into one
E particular
[ 26 ] particular part, or to concentrate its force upon the fubject. Some have thought to effect this, by making the fides and dome of an elliptical or parabolic figure ; expecting, from the mathematical properties of thofe figures, that the rays of fire, finking all over the inter- nal furface, would be reflected back into a fmall focal ipace, in the fame manner as the folar rays are concen- trated by burning concaves. On the authority of thofe, who had recommended thefe kinds of furnaces, I pre- pared, many years ago, an elliptical one ; which was figured with great care, not only in a moid ftate, but after it had been dried and burnt, by the revolution of a femi-elliptical plane about its axis. In the effects of this furnace, I was greatly difappointed : for it could not be obferved, that any focal reflexion obtained in it -, or that any advantage refulted from the exaclnefs of its figuration, or from the precife fpecies of its curvature. And indeed many caufes, which it is foreign to the prefent purpofe to enter into a difcuilion of, concur in preventing both the regular reflection of the rays of heat in furnaces, and the collection and union of thofe which have been fo reflected. Others have attempted this concentration of fire upon a different principle. By blowing a ft ream of air through the flame of a lamp or candle, the flame, as already ob- ferved, is made to converge into a kind of focal point, and ads there with a great increafe of its force : by mul- tiplying thefe ftreams of flame, or impelling a number of flames into one point, the heat may be augmented to a very high degree. In furnaces alfo, the flame, and the moft confiderable part of the heat, follow the direction of the current of air : and hence it has been propOfed to impel ftreams of air, from different parts of the cir- cumference of the fuel, to the middle, by means of feveral bellowfes placed round the furnace. Though the princi- ple,
[ 27 1
pie, however, appears to be a juft one, the application of a number of bellowles is too incommodious to be put in practice ; and by their difpofition in one plane, round the furnace, great part of the fuel lies without their reach. I have therefore endeavoured to improve the contrivance ; to multiply the itreams of air, to throw them in from al- moft all parts of the furface, to fupply them all from one bellows, and in fuch a manner that they may not obftruct one another, but confpire as it were into one ftream about the crucible.
The pot, which ferves as a furnace for this purpofe, has a number of holes, bored at fmall diftances, in fpiral lines, all over it, from the bottom, up to fuch a height as the fuel is defigned to reach to. The crucible is placed, upon a proper fupport, in the bottom ; and the hoks are made, not in a perpendicular direction to it, but oblique, that the ftreams of air forced in through them may but juft touch it : by this means, the crucible ftands out of danger of being cracked by the blaft, and the impelled heat plays in a kind of fpiral upon its furface. The pot, which ferved before for the blaft-furnace, with an iron ring on its top, receives this perforated pot fo far, that all its holes hang in the cavity ; which cavity having no other outlet than the round aperture for the bellows, the air, blown in through this aperture, neceffarily diftributes it- felf through the perforations of the inner pot. The inner pot may be of the largeft fize, as well as the outer one, the lower narrow part of the former falling into the upper wide one of the latter : it wants no addition to increafe its height, but on the contrary will be more commodious, in regard to the infpection and taking out of the crucible, if all the part above where the fuel reaches to is fawed away : the moil convenient cover for it is an iron plate,
E 2 with
[ 28 ] with a round hole in the middle, and a handle projecting at one fide for lifting it by.
The force of the fire being thus, in great meafure, con- centrated upon the crucible in the middle of the fuel j the crucible is heated, expeditioufly, and with a little quantity of fuel, to a very intenfe degree, while the exte- riour parts of the fuel are of no great heat, and permit the operator to approach without incommoding him.
In the ufe of the furnaces hitherto defcribed, the at- tendance of the operator is necefiary, both for infpecting the proceffes, and for fupplying and animating the fuel. There are fome operations of a flower kind ; which re- quire a gentle heat to be continued for a length of time i which demand little attendance in regard to the operations themfelves ; and in which, of confequence, it is extremely convenient to have the attendance in regard to the fire, as much as poflible, difpenfed with. This end is anfwered by the furnace called the athanor ; an account of which is referred to another part of this work, as it cannot well be formed out of black lead pots, and does not fall in with the fimplicity of the prefent contrivances.
Sundry attempts have been made for keeping up a con*- tinued heat, with as little trouble as in the athanor, by the flame of a lamp ; but the common lamp-furnaces have not anfwered fo well as could be wiihed. The lamps require frequent 'fnufhng, and fmoke much; and the foot, accumulated on the bottom of the veflel. placed over them, is apt, at times, to fall down and put out the flame. The largenefs of the wick, the irregular fupply of oil from the refervoir by jets, and the oil being fuffered to fink confiderably in the lamp, fo that the upper part -of the wick burns to a coal, appeared to be the principal
caufes
[ 29]
caufes of thefe inconveniences ; which, accordingly, were found to be in great meafure remedied by the following conffructicn.
The lamp confifts of a brafs pipe, ten or twelve inches long, and about a quarter of an inch wide, inferted at one end into the refervoir of the oil, and turned up at the Other to an elbow, like the bole of a tobacco-pipe, the aperture of which is extended to the width of near two inches. On this aperture is fitted a round plate, having five, fix or feven fmall holes, at equal diftances, round its outer part, into which are inferted as many pipes about an inch long : into thefe pipes are drawn threads of cotton, all together not exceeding what in the common lamps form one wick : by this divifion of the wick, the flame expofes a larger furface to the a&ion of the air, the fuliginous matter is confumed and carried off", and the lamp burns clear and vivid.
The refervoir is a cylindric veffel, eight or ten inches wide, compofed of three parts, with a cover on the top. The middle partition communicates, by the lateral pipe, with the wicks ; and has an upright open pipe foldered into its bottom, whofe top reaches as high as the level of the wicks ; fo that, when this part is charged, with oil, till the oil rifes up to the wicks in the other end of the lamp, any further addition of oil will run- down through the upright pipe into the lower divifion of the refervoir. The upper divifion is defigned for fupplying oil to the middle one ; and, for that purpofe, is furnilhed with . a cock in the bottom, which is turned more or lefs, by a key on the outfide, that the oil may drop faff enough to fupply the confumption, or rather fafter, for the overplus is of no inconvenience, being carried off" by the upright pipe ; fo that the oil is always, by this means, kept ex- aftly at the fame height in the lamp. For common ufes,
the
[ 3° 1 the middle divifion alone may be made to fuffice -, for, on account of its width, the finking of the oil will not be considerable in feveral hours burning. In either cafe, however, it is expedient to renew the wicks every two or three days ; oftener or feldomer, according as the oil is more or lefs foul ; for its impure matter, gradually left in the wicks, occafions the flame to become more and more dull. For the more convenient renewing of them, there fhould be two of the perforated plates ; that, when one is removed, another, with wicks fitted to it, may be ready to fupply its place.
One of the black lead pots, defcribed at the beginning of this paper, makes a proper furnace for the lamp. If one is to be fitted up on purpofe for this ufe, it requires no other aperture than one in the bottom for admitting air, and one in the fide for the introduction of the elbow of the lamp : the refervoir ftands on any convenient fup- port without the furnace. The firopper of the fide aper- ture confifts of two pieces, that it may be conveniently put in after the lamp is introduced ; and has a round hole at its bottom fitting the pipe of the lamp. By thefe means, the furnace being let upon a trevet or open foot, the air enters only underneath, and fpreads equally all round, without coming in ftreams, whence the flame burns fteady. It is not advifable to attempt raifing the heat higher than about the 450th degree of Fahrenheit's thermometer, a heat fomewhat more than fufAcient for keeping tin in perfect fufion. Some have propofed giving a much greater degree of heat in lamp furnaces, by ufing a number of large wicks ; but when the furnace is fo heated, the oil emits copious fumes, and its whole quantity takes fire. The balneum, or other veflel in- cluding the fubjett-matters, is fupported over the flame by an iron ring, as already defcribed in the fand bath
and
[ 3* ] and ftill : a bath is here particularly neceffary, as the fub- ject would otherwife be very unequally heated, only n fmall part of the veflel being expofed to the flame.
The ufe of baths in general is for defending the glafs or other vefTels, placed in them, from the immediate action of the fire, and for preventing their being fuddenly affected by variations of the heat. There is one imper- fection in fand and other folid intermedia ; that their heat is by no means uniform, but different in their differ- ent parts, decreafing gradually from the bottom to the top ; this is always the cafe, even when the fire is uni- formly diftributed round the veflel by the contrivances formerly defcribed. In thofe circumflances therefore, wherein it is expedient to fecure with certainty an equality of the heat, recourfe muft be had to fluid intermedia. The water bath, commonly employed in this intention, is confined to low degrees of heat j a boiling heat being the utmoft that water is fufceptible of. I have therefore, on fome occafions, made ufe of another fluid, quick- filver j which bears a degree of heat exceeding that of boiling water, above twice as much, as the heat of boiling water does that of freezing water.
The mercurial bath is prepared with two iron pots or ladles, of fuch fizes, that one may be received into the other, fo as to leave a vacuity between them all round : this fpace is filled with quickfilver, the two vefTels being properly fecured togethei by pins pafTed through the edges, to prevent the inner one from being buoyed up by the quickfilver. The vacuity may be fo fmall, that a little quantity of mercury {hall fuffice ; and the expence of this article may be further lefTened, by ufing, infiead of pure mercury, a composition of it with about half its weight of lead, or with fb much as it will bear without loling 5 iw
[ 32 ]
its fluidity in a moderate warmth. In this kind of bath, all the parts of the inner veflel will be equally heated, how unequally foever the fire be applied underneath ; and the heat may be increafed nearly as far as that in which lead begins to melt, without any danger of the mercury evaporating. The mercurial thermometer of Fahrenheit, whofe 3 2d diviiion is the point at which water freezes, and the 212th that at which water boils, is raifed by the heat in which lead melts to about the 550th divifion, and by the heat of quickfilver boiling and evaporating to the 600th.
Though the heat, of which mercury is fufceptible, is great in companion with that of boiling water, it is far too little for many purpofes for which baths are wanted. Some curious workmen, for communicating thefe greater degrees of heat equally to different fubjects, as where a number of lmall fteel inflruments is to be equally tempered, employ melted lead as an intermedium. A plate of iron floats upon the melted lead, and receives therefrom, in all its parts, an equal heat : the pieces of fteel, laid upon this plate, acquire all at once the fame degree of heat, and are at once quenched in water ; the blue or other colours, which they fucceflively aflume, affording fure marks of the proper points of heat at which they are to be quenched, according to the different degrees of hardnefs required in them.
From this practice I took the hint of another metallic bath, which fupplies at once both the mercurial and the lead baths. As the imperfection of mercury confvfts in its not bearing fo great a heat, and that of lead in its not becoming fluid with fo fmall a one, as many purpofes require ; I have fubftituted one of the fufible metallic mixtures, mentioned by Sir Ifaac Newton in the Philofo- phical Tranfactions, compofed of two parts of lead, three
of
[33 3 of tin, and five of bifmuth, melted together : one pot or ladle being fixed into another, in the fame manner as for the mercurial bath, the fpace between them is filled with the melted compound. This mixture proves fluid in a heat very little greater than that in which water boils ; and confequently ferves as an intermedium for all degrees of heat above this period, up to that in which the metal itlelf grows red hot and boils ; a heat greater than baths are ever wanted for in practice.
The foregoing furnaces I have ufed with pleafure for many years ; and experienced their commodioufnefs, in public as well as private operations, for continued and extenfive enquiries, as well as for occafional experiments. Eight or ten pots have flood at work together, under a common chimney ; and others, upon a fland, in the middle of the room, with a copper dome, of a conical figure, over them, which communicated with the chim- ney by a pipe bent at right angles : by this means the pro- cerTes could be freely infpected, without any danger of injury or offence from burnt air or fumes, which, as foon as the chimney became fomewhat warm, were com- pletely carried off. It is convenient to have the dome made to flide eafily up and down the perpendicular part of the pipe ; which may be effected by fufpending it to two chains palled over pullies fixed to the ceiling and loaded with a counterpoife.
We flatter ourfelves, that the publication of a fimplc apparatus, eafy of conflruction, of little expence, com- modious in its ufe, and eafily manageable in all its forms and combinations, will contribute to remove one of the chief obstacles to chemical refearches, and to pro- mote thofe kinds of experimental purfuits in which fur- naces are a principal inftrument.
F Exph-
[ 3* 1
Explanation of Plate I.
TH E upper figure on the right fide of the plate is a perpendicular fe&ion of the iron hoop, fee page 7, with the aperture fhewn in its back part. Under it is a plan of the door of the aperture, rivetted on an iron plate.
Under this, is a plan of the middle grate, with three pins projecting from its circumference. The lower grate differs only in being fmaller ; and the upper one, in being larger, and having no pins.
The middle figure in the upper part of the plate is a perpendicular lection, with a perfpective view of the back part, of one of the black lead pots, of the fize marked 60, as fitted for the general purpofes of furnaces ; with the fmall- er grate, cut acrofs the bars, in its lower part ; and an iron pot hung in its mouth, fee page 8. It is drawn, as all the other figures in this and the following plates, to about one fifth of the real fize. The notches for the grates, and all the apertures, are here expreffed : but in the other figures, only thofe apertures are fhewn, which are neceffary for the re- fpeclive ufes to which the furnaces are applied. They are all bound, in three places, with copper wire ; and round the mouth is a thin copper hoop, which preferves the outer edge from wearing off.
On the left fide of the plate is reprcfented the apparatus for diftillation, defcribed in page 9 : and on the right is the lamp, communicating with its refervoir (fee page 29) by a lateral pipe, which is here made a little too fhort for want of room on the plate. Over the lamp furnace is a broad pan ; which is here employed as a water-bath, and contains a lons-neckcd matras or bolthead ; a more commodious
o
veffel of the fame kind compofed of a glafs receiver with a long pipe inferted into its mouth ; and a cucurbit or body with a glafs head for diftillation.
S2%att /
{ 35 1
Explanation of Plate II.
IN this plate are fhewn different combinations of the pots for different ufes, with and without the iron hoop between them. In the back part of the upper pot or dome is expreffed the round hole, through which, and the oppofite afh-pit door, an iron rod is pafTed for conveni- ently lifting it off when hot.
On the right hand is the wind melting furnace (page 1 1) with a feclion of the grate and crucible. It flands upon a portion of another pot, in the fides of which three arches are fawed (page 6).
In the middle is the afTay furnace (page 12) with a fection of the muffle above the grate. This furnace flands upon another entire pot inverted, that it may be raifed to a proper height for the inflection of the cupels, &c. under the muffle.
On the left hand is a reverberatory, with a coated glafs retort for diflillation. It flands on the fame kind of foot as the wind furnace on the right fide •, and in its back part is fhewn one of the two bars on which the bottom of the retort is fupported.
[ 3^ ]
Explanation of P 'l at ■ e III.
TH E three furnaces in this plate are different kinds of blafl furnaces ; with holes bored in the fides for receiving the nofe of the bellows.
The uppermofl confifls of a pot of the fize marked 80, fitted into one of 100 ; with a portion of another of 100 inverted over it for a dome. So much of the bottom of this lafl is fawed off as to procure a Sufficient aperture ; and fo much is fawed from the mouth, that the remainder may be of a proper width to fit upon the mouth of the leffer pot. A round flip, fawed from the bottom, ferves both for a grate and for a fupport to the crucible (page 18). A fection of this grate is fhewn in the furnace, and a plan of it on the right hand.
The right fide furnace at the bottom of the plate is for fufion without a grate or among the fuel. The pot is of the largefl fize, and its height may be occasionally enlarged by the ring cut off from the dome of the foregoing furnace. To the Hoping canal in its bottom is fitted the lower part of another pot. Seepage 21.
The lowermofl furnace on the left hand is that defcribed in page 27, in which flreams of air are impelled from dif- ferent parts of the fuel upon the crucible in the middle. The lower pot is that which ferves for the blafl furnace in the upper part of the plate ; and the perforated pot is of the fize 90, with its bottom rounded, and a part of the top cut off for the greater convenience of infpec~ting or taking out the crucible. The circular plane over it is an iron plate, with a hole in the middle, which ferves as a cover for the pot j and over this is a tranfverfe fection of the perforated pot and crucible, to fhew the direction of the ftreams of air.
_/£//,• 3
[ 37 J
Explanation of Plate IV.
TH E furnace on the right fide of the plate is the wind-furnace defcribed in page 24 ; compofed of the largefl-fized black lead pots ; and conftrucled on the model of that ufed by Mr. Pott in his experiments of the vitrification of earthy bodies. A portion of the air pipe is fhewn inferted into the foot, and five little crucibles upon the grate. To the door is fitted an iron plate, turned up at the fides, for receiving the fuel, which is kindled faft enough by the vicinity of the fire, to afford a conftant fupply of burning charcoal : the kindled part at the door is moved into the furnace by pufhing the reft forward with an iron rod, and more unlighted charcoal is fuccef- fively fupplied behind.
The figure on the left fide of the plate is that of the flove for warming a room, page 14. Jt flands on the iron trevet, with the flat iron pan underneath for receiving the afhes. The hoop is filled with balls of baked earth, which are fupported by the large grate on the top of the lower pot. Into the door of the dome is inferted the fquare end of the iron pipe, the other end of which communicates with the chimney of the room. 4
II. History of GOLD,
And of the various Arts and Businesses depending thereon.
SECT. I.
Of the colour of Gold, and the methods of refloring its lujirc, when fullied.
TH E bright deep yellow colour of gold, com- monly diflinguiflied by its name, is one of the mofl obvious characters of this metal. Its colour and beauty are of great durability, being injured neither by air nor moiflure, nor by any kind of exhalations that ufually float in the atmofphere ; as may be obferved in the gildings of fome public edifices, which have refilled the weather, and the vapours of London, and other popu- lous cities, for half a century or more. In this property confifts great part of the excellence of this metal for orna- mental and fome mechanic ufes : there is no other malle- able metallic body, fo little fufceptible of tarnifii or dis- coloration, or fo little difpofed to communicate any ftain to the matters which it lies in contact with.
As inftruments or ornaments of pure gold are liable to be fullied only from the fimple adhefion of extraneous fubftances ; their beauty may be recovered, without any injury to the metal, however exquifitely figured, or with- out any abrafion of its furface, however thin and delicate,
by
[ 39 1
by means of certain liquids which diffblve the adhe- ring foulnefs ; as folution of foap, folution of fixt alkaline falts or alkaline ley, volatile alkaline fpirits, and rectified fpirit of wine.
In the ufe of the alkaline liquors, fome caution is ne- ceffary in regard to the veffels; thofe of fome metals being, in certain circumftances, corroded by them, fo as remark- ably to difcolour the gold. A gilt fnuff-box, boiled with foap-boilers ley in a tin pot, to clean it from fuch foulnefs as might adhere in the graved figures, and to prevent any deception which might hence arife in a hydroftatic exami- nation of it, became foon of an ill colour, and at length ap- peared all over white as if it had been tinned : fome pieces of ffandard gold, treated in the fame manner, underwent the fame change : and on trying volatile alkaline fpirits, prepared with quick-lime, the fame effect was produced more fpeedily. On boiling the pieces, thus whitened, with fome of the fame kind of alkaline liquors, in a copper veffel, the extraneous coat difappeared, and the gold recovered its proper colour.
For laces, embroideries, and gold thread woven in filks, the alkaline liquors are in no ihape to be ufed ; for, while they clean the gold, they corrode the filk, and change or difcharge its colour. Soap alfo alters the fhade, and even the fpecies of certain colours. But fpirit of wine may be ufed without any danger of its injuring either the colour or quality of the fubject, and in many cafes proves as effectual, for reftoring the luftre of the gold, as the corrofive detergents. A rich brocade, flowered with a variety of colours, after being difagreeably tarnilhed, had the luftre of the gold perfectly reffored by waffling it with a foft brufh dipt in warm fpirit of wine; and fome of the colours of the filk, which were likewife foiled, became at the fame time remarkably bright and lively. Spirit of wine
feems
[40]
fecms to be the only material adapted to this intention, and probably the boafted fecret of certain artifls is no other than this fpirit difguifed : among liquids, I do not know of any other, that is of fufficient activity to difcharge the Joul matter, without being hurtful to the filk : as to pow- ders, however fine, and however cautioufly ufed, they fcratch and wear the gold, which here is only fuperficial and of extreme tenuity.
But though fpirit of wine is the moil innocent material that can be employed for this purpofe, it is not in all cafes proper. The golden covering may be in fome parts worn off; or the bafe metal, with which it had been iniquitoufly alloyed, may be corroded by the air, fo as to leave the par- ticles of the gold difunited ; while the filver underneath, tarnifhed to a yellow hue, may continue a tolerable colour to the whole : in which cafes it is apparent, that the removal of the tarnifli would be prejudicial to the colour, and make the lace or embroidery lefs like gold than it was before. A piece of old tarnifhed gold lace, cleaned by fpirit of wine, was deprived, with its tarnifli, of greatefl part of its golden hue, and looked now almoft like filver lace.
Though no one of the other metallic bodies, fingly, has any degree of the beautiful yellow colour which glows in gold, the true gold yellow may, neverthelefs, be pretty nearly imitated, by certain combinations of other metals, particularly of copper with zinc. But how nearly foever thefe compofitions approach to gold in degree or fpecies of colour, they differ greatly in its durability; and their differences in other refpects are flill more ftrongly marked, and of more eafy difcovery, as will appear in the fequel of this treatife.
SECT.
U< ]
SECT. II. Of the gravity of Gold.
FINE gold, immerfed in water, weighs near one nine- teenth part lefs than in air, and confequently it is up- wards of nineteen times heavier than equal its own volume of water. All the other kinds of matter, that have been known till of late years, are of remarkably lefs gravity; mercury, the next in weight, being only about fourteen times heavier than water, and lead, the next of the folid bodies, little more than eleven times. Hence the gravity of gold has been univerfally reckoned one of its moil certain and inimitable characteristics : and accordingly it has been laid down as an axiom, that whatever body exceeded the weight of water above fourteen times, how unpromifing foever in appearance, muit neceffarily contain gold. The difcovery of the ponderous metal platina has now afforded an exception to this axiom, and fhewn that weight alone is no certain criterion of gold ; for pure platina, perfectly void of gold, is nearly as ponderous as the precious metal itfelf.
The fpecific gravity of gold, or its comparative weight to an equal volume of water, is by fome reported to be 19,640, and in a paper in the Swedifh tranfactions it is made no lefs than 20,000; that of water being 1,000. But in the experiments of Mr. Ellicott, whofe accuracy and fkill are unqueftionable, made upon gold fuppofed to be fine, it did not exceed 19,207; and of different maffes of gold, which I had refined to the greatefl degree of purity that I apprehend gold capable of being brought to, and well hammered, I found the gravity, on many different trials, between 19,300 and 19,400. A mafs of fine gold, weighing in air 13447, being weighed in di-
G ftilled
[42]
ftilled water of the temperature of fifty three degrees of Fahrenheit's thermometer, or twenty one iSoths of the interval between freezing and boiling ; the lofs in water was 694, whence the gravity turns out 19,376 ; the balance, thus loaded, turned fenfibly with half of one of the weights, lb that the true lofs in water could not be half a weight more or lefs than the apparent, and the gravity, of confequence, could not be lb little as 19,362, or fo much as 19,390. It were to be wifhed that thofe, who have examined metals hydroftatically, had fpecified the fenfibility of the balance, and the quality and warmth of the water. An increafe of heat rarefying water much more than it does gold, the gold muft turn out propor- tionably heavier than an equal volume of the expanded fluid ; and this difference is perhaps more confiderable than it has generally been fuppofed. From freezing to boiling water, or by an augmentation of heat equivalent to one hundred and eighty degrees of Fahrenheit's ther- mometer, a rod of gold appears to be lengthened about one part in 700, and confequently its volume is increafed about one part in 233, while the volume of water is in- creafed one 26th or more : hence by an augmentation of forty degrees of the thermometer, or from a little above freezing to the fummer heat, the volume of gold, if its expanfion be uniform, is increafed one part in 1048, and that of water one in 117; and the gravity of gold, weighed in the water fo warmed and expanded, fhould be greater than when the gold and water are forty degrees colder, in the proportion of about 19,265 to 19,400." This calculation gives a difference, in the gravity, of 0,034 for every ten degrees of the thermometer, but fome trials feemed to make it greater. A piece of gold being weighed in water of fifty degrees, and afterwards in the lame water made eighty eight degrees hotter, in which
it
C 43 ]
it was kept immerfed for fome time to acquire its warmth, the gravities turned out 19,372 and 19,769 ; whence the difference for every ten degrees comes to be 0,045 — If the mean gravity of gold be reckoned 19,300; as a cubic inch of water weighs about 254 grains, a cubic inch of gold will weigh of confequence about 4902 grains or ten ounces and a hundred and two grains.
As air refills the defcent of bodies more or lefs in proportion to its own gravity and the furface of the defcending body, and as the brafs, of which weights are made, is more than double in volume to an equal weight of gold ; it follows, that if gold be counterpoifed with brafs weights in light air, the gold will preponderate when the air grows heavier, the addition made to the air's gravity refifting the brafs above twice as much as it does the gold. It has hence been imagined, that the com- parative gravity of gold to brafs weightt 'muft be fo far influenced by the variable gravity of the atmofphere, that there muft be an advantage in buying gold by weight when the air is lighteft. The difference appears however too inconfiderable to be regarded in a commercial view. For the lofs of weight of the two metals in the air being as much lefs than their lofs in water, as air is lighter than water ; and air, as appears from an experiment of Mr. Hawkfbee, being in its lighteft ftate about a 937th, and in its heavieft about an 848th part of the weight of water; it will be found on calculation, that the gold preponderates above the brafs, in the heavieft more than in the lighteft air, only by one part in 145000, or one grain in about three hundred and two ounces : a difference too minute to be fenfible in the tendereft ba- lance.
Notwithstanding the great denfity of gold, and its con- taining, under an equal volume, the greateft quantity of folid parts of all known bodies ; it is faid, that it not
G 2 only
[44] only freely tranfmits the magnetic effluvia, but that even Avater, by ftrong preffure, may be fqueezed through its pores. A hollow fphere of gold being filled with water, foldered up, and prefTed with great force, the water was found to tranfude in multitudes of fmall drops, which covered the outfide of the fphere like dew. This experi- ment was made by the Florentine academy, and is men- tioned by Sir Ifaac Newton on the teflimony of an eye- witnefs. It may be queffioned however, whether the interflices, through which the water ifTued, were the pores proper to the gold in its natural ftate ; or whether they were not rather an enlargement of its natural pores, occafioned by the parts of the metal having been forced afunder by the incompreffibility of the water, and the violence of the preifure.
SECT. III. Of the duSlility of Gold, and the arts depending on this property: Gold-beating, wire-drawing, gilding with gold-leaf on different fubjects.
FINE gold is a foft metal ; eafily chifieled, cut, or graved ; very flexible ; and fo tough, that when at length made to break by repeated bendings back- wards and forwards, the fracture, on each of the pieces, appears drawn out in the middle like a wedge. It takes impreflions from dyes in great perfection ; does not file freely, but flicks in the teeth ; has little elasticity or fo- noroufnefs ; receives great fplendor from the burnifher, but does not appear fo bright from the polifhing flone. It yields freely to the hammer, both when hot and cold, and admits of being ftretched to a vafh extent.
The great value which has at all times been fixed on gold, its beautiful colour, incorruptibility, and compact- nefs, render its dudility an object of primary importance :
on
[45] on this depend fundry arts and manufactures, in which we fee it extended to an amazing tenuity, and varioufly applied on the furface of other bodies, both for their orna- ment and prefervation.
Preparation of Gold leaf.
The gold is melted in a black lead crucible, with fome borax', in a wind furnace, called by the workmen a wind- hole : as foon as it appears in perfect fufion, it is poured out into an iron ingot mould, fix or eight inches long, and three quarters of an inch wide, previoufly greafed, and heated, fo as to make the tallow run and fmoke, but not to take flame. The bar of gold is made red hot, to burn off the unctuous matter, and forged on an anvil into a long plate, which is further extended, by being paffed repeat- edly between polifhed fleel rollers, till it becomes a ribbon, as thin as paper. Formerly the whole of this extenfion was procured by means of the hammer, ario! fome of the French workmen are flill faid to follow the fame practice : but the ufe of the flatting mill both abridges the operation, and renders the plate of more uniform thicknefs. The ribbon is divided by compafles, and cut with fheers into equal pieces, which confequently are of equal weights : thefe are forged on an anvil till they are an inch fquare, and afterwards well nealed, to correct the rigidity which the metal has contracted in the ham- mering and flatting. Two ounces of gold, or 960 grains, the quantity which tne workmen ufually melt at a time, make an hundred and fifty of thefe fquares, whence each of them weighs fix grains and two fifths ; and as 4902 grains of gold make a cubic inch, the thicknefs of the fquare plates is about the 766th part of an inch.
In order to the further extenfion of thefe pieces into fine leaves, it is neceffary to interpofe fome fmooth body
between
[46 ] between them and the hammer, for foftening its blow, and defending them from the rudenefs of its immediate action : as alfo to place between every two of the pieces fome proper intermedium, which, while it prevents their uniting together, or injuring one another, may fuffer them freely to extend. Both thefe ends are anfwered by certain animal membranes.
The gold-beaters ufe three kinds of membranes ; for the outride cover, common parchment, made of fbeep- fkin ; for interlaying with the gold, firfl the fmoothefl and clofeft vellum, made of calves-fkin ; and afterwards the much finer fkins of ox-gut, ftript off from the large ftreight gut flit open, curioufly prepared on purpofe for this ufe, and hence called gold-beaters fkin. The prepa- ration of thefe laft is a diflindl bufinefs, prac"lifed by only two or three perfons in the kingdom, fome of the parti- culars of which I have not fatisfactorily learnt. The ge- neral procefs is faid to confift, in applying one upon ano- ther, by the fmooth fides, in a moifl flate, in which they readily cohere and unite infeparably ; flretching them on a frame, and carefully fcraping off the fat and rough matter, fo as to leave only the fine exteriour membrane of the gut ; beating them between double leaves of paper, to force out what undtuofity may remain in them ; moift- ening them once or twice with an infufion of warm fpices j and Tartly drying and prefiing them. It is faid, that fome calcined gypfum, or plafler-of-paris, is rubbed with a hares-foot both on the vellum and the ox-gut fkins, which fills up fuch minute holes as may happen in them, and prevents the gold leaf from flicking, as it would do to the fimple animal membrane. It is obfervable, that notwith- standing the vafl extent to which the gold is beaten be- tween thefe fkins, and the great tenuity of the fkins themfelves, yet they fuflain continual repetitions of the 4 prccefs
[47] procefs for feveral months, without extending or growing thinner. Our workmen find that after feventy or eighty repetitions, the {kins, though they contract no flaw, will no longer permit the gold to extend between them ; but that they may be again rendered fit for ufe by impreg- nating them with the virtue which they have loft, and that even holes in them may be repaired by the dextrous appli- cation of frefh pieces of fkin : a microlcopical examination of fome fkins that had been long ufed plainly fhewed thefe repairs. The method of reftoring their virtue is faid in the Encyclopedie to be, by interlaying them with leaves of paper moiftened with vinegar or white wine, beating them for a whole day, and afterwards rubbing them over as at firft with plafter-of-paris. The gold is faid to extend between them more eafily, after they have been ufed a little, than when they are new.
The beating of the gold is performed on a fmooth block of black marble, weighing, from two hundred to fix hun- dred pounds, the heavier the better, about nine inches fquare on the upper furface, and fometimes lefs, fitted into the middle of a wooden frame, about two feet fquare, fo as that the furface of the marble and the frame form one continuous plane. Three of the fides are furniflied with a high ledge ; and the front, which is open, has a leather flap faftened to it, which the gold-beater takes before him as an apron, for preferving the fragments of gold that fall off. Three hammers are employed, all of them with two round and fomewhat convex faces, though com- monly the workman ufes only one of the faces : the firft, called the cutch hammer, is about four inches in diameter, and weighs fifteen or fixteen pounds, and fometimes twenty, tho' few workmen can manage thofe of this laft lize : the fecond, called the ftiodering hammer, weighs about twelve . pounds, and is about the fame diameter : the third, called
the
[48]
the gold hammer, or finishing hammer, weighs ten or eleven pounds, and is near of the fame width. The French ufe four hammers, differing both in fize and fhape from thofe of our workmen : they have only one face, being in figure truncated cones : the firfl: has very little convexity, is near five inches in diameter, and weighs fourteen or fifteen pounds : the fecond is more convex than the firfl:, about an inch narrower, and fcarcely half its weight : the third, flill more convex, is only about two inches wide, and four or five pounds in weight : the fourth or finishing hammer is near as heavy as the firfl, but narrower by an inch, and the moil convex of all. As thefe hammers differ fo remarkably from ours, I thought proper to infert them, leaving the workmen to judge what advantage one fet may have above the other.
A hundred and fifty of the pieces of gold are interlaid with leaves of vellum, three or four inches fquare, one vellum leaf being placed between every two of the pieces, and about twenty more of the vellum leaves on the outfides ; over thefe is drawn a parchment cafe, open at both ends, and over this another in a contrary direction, fo that the affemblage of gold and vellum leaves is kept tight and clofe on all fides. The whole is beaten with the heavieffc hammer, and every now and then turned upfide down, till the gold is flxetched to the extent of the vellum ; the cafe being from time to time opened for difcovering how the extenfion goes on, and the packet, at times, bent and rolled as it were between the hands, for procuring fufneient freedom to the gold, or, as the workmen fay, to make the gold work. The pieces, taken out from between the vellum leaves, are cut in four with a fleel knife ; and the fix hundred divifions, hence refulting, are interlaid, in the fame manner, with pieces of the ox-gut fkins, five inches fquare. The beating being repeated, with
a
[ 49 ] a lighter hammer, till the golden plates have again ac- quired the extent of the fkins, they are a fecond time divided in four : the inflrument ufed for this divifion is a piece of cane cut to an edge, the leaves being now {o light, that the moiflure of the air or breath, condenfing on a metalline knife, would occafion them to flick to it. Thefe Iaft divifions being fo numerous, that the fkins neceffarv for interpofing between them would make the packet too thick to be beaten at once, they are parted into three parcels, which are beaten feparately, with the fmalleft hammer, till they are flretched for the third time to the fize of the fkins : they are now found to be redu- ced to the greateft thinnefs they will admit of, and indeed many of them, before this period, break or fail. The French workmen, according to the minute detail of this procefs given in the Encyclopedic, repeat the divifion and the beating once more ; but as the fquares of gold, taken for the firfl operation, have four times the area of thofe ufed among us, the number of leaves from an equal area is the fame in both methods, to wit, fixteen from a fquare inch. In the beating, however fimple the procefs appears to be, a good deal of addrefs is requifite, for applying the hammers fo as to extend the metal uniformly from the middle to the fides : one improper blow is apt not only to break the gold leaves, but to cut the fkins. . After the laft beating, the leaves are taken up by the end of a cane inflrument, and being blown flat on a leather cufhion, are cut to a fize, one by one, with a fquare frame of cane made of a proper fharpnefs, or with a frame of wood edged with cane : they are then fitted into books of twenty five leaves each, the paper of which is well fmoothed, and rubbed with red bole to prevent their flicking to it. The French, for fizing the leaves, ufe only the cane knife ; cutting them firfl flreight on one fide, fitting them into the book by the flreight fide, and
H then
[ 50 ] then paring off the fuperfluous parts of the gold about the edges of the book. The fize of the French gold leaves is from fomewhat lels than three inches to three and three quarters fquare ; that of ours, from three inches to three and three eighths.
The procefs of gold-beating is considerably influenced by the weather. In wet weather, the fkins grow fome- what damp, and in this ftate make the extenlion of the gold more tedious : the French are faid to dry and prefs them at every time of ufing ; with care not to over-dry them, which would render them unfit for further fervice. Our workmen complain more of froft, which appears to affect the metalline leaves themfelves : in froft, a gold leaf cannot eafily be blown flat, but breaks, wrinkles, or runs together.
Gold leaf ought to be prepared from the fineft gold ; as the admixture of other metals, though in too fmall a proportion to fenfibly affect the colour of the leaf, would difpofe it to lofe of its beauty in. the air. And indeed there is little temptation to the workman to ufe any other ; the greater hardnefs of alloyed gold occafioning as much to be loft in point of time and labour, and in the greater number of leaves that break, as can be gained by any quantity of alloy that would not be at once difcover- able by the eye. All metals render gold harder and more difficult of extenfion : even filver, which in this refpect feems to alter its quality lefs than any other metal, pro- duces with gold a mixture fenfibly harder than either of them feparately, and this hardnefs is in no art more felt than in the gold-beaters. The French are faid to prepare what is called green gold leaf, from a compolition of one part of copper and two of filver with eighty of gold; but this is probably a miftake, for fiich an admixture gives no greennefs to gold, and I have been informed by our workmen, that this kind of leaf is made from the fame
fine
[5« ]
fine gold as the higheft gold-coloured fort, the greenilh hue being only a fuperficial teint induced upon the gold in fome part of the procefs : this greenilh leaf is little otherwife ufed than for the gilding of certain books.
But though the gold-beater cannot advantageoufly di- minish the quantity of gold in the leaf by the admixture of any other fubftance with the gold, yet means have been contrived, for fome particular purpofes, of faving the precious metal, by producing a kind of leaf called party- gold, whofe balls is filver, and which has only a Super- ficial coat of gold upon one fide : a thick leaf of filver and a thinner one of gold, laid flat on one another, heated and prefTed together, unite and cohere ; and being then beaten into fine leaves, as in the foregoing procefs, the gold, though its quantity is only about one fourth of that of the filver, continues every where to cover it, the extenfion of the former keeping pace with that of the latter.
Preparation of gold or gilt ivirc.
There is very little wire made entirely of gold -, and this chiefly for one particular purpofe, that of filligree work. What is commonly called gold wire has only an exteriour covering of gold/die internal part being filver. A rod of filver, above an inch thick, two feet in length, and weighing about twenty pounds, is coated with gold j and then reduced into wire, by drawing it Successively through a number of holes, made in metalline plates, diminishing infenfibly in a regular gradation.
The purity of the gold, employed for this ufe, is a point of the utmofl confequence ; for on this principally depends the beauty, and durability of the colour, of the laces, brocades, and other commodities prepared from it ; and unhappily there is more room for abufe here than in gold leaf, the extenfion of the metal in this form
H 2 being
[ 52 ]
being lefs affe&ed by an addition of alloy. The boafted fuperiority of the French laces to the generality of thofe made in England, for which various caufes have been faifely affigned, appears to be wholly owing to a difference in the finenefs of the gold : our workmen have of late years had finer gold put into their hands than formerly, and the product has been judged not inferiour to that of France ; nor is it to be doubted that the Engliih artift, acknowledged not to be wanting in manual dexterity, will, with equal or fuperiour materials, produce an equal or fuperiour commodity. It mould feem therefore necef- fary, for the purpofes of lb important a manufacture, where fo much depends upon the purity of the gold, not only to employ it in the purefr. ftate to which it can be brought by the common methods of refining, but even to feek for means of purifying it to a greater degree than any of the common proceffes are capable of doing : fuch means the fequel of this effay will afford.
With regard to the filver, which makes the internal body of the wire, its finenefs is of lefs importance. I have been informed by fome experienced workmen, that there is an advantage in its being alloyed ; that fine filver, on being nealed in the fire, becomes fo loft, as to fuffer the golden coat in fome meaiure to link into it ; and that the admixture of a little copper communicates a fufficient hardnefs, for preventing this inconvenience. Accordingly the French filver for gilding is laid to be al- loyed with five or fix pennyweight, and ours with twelve pennyweight, of copper, in the pound troy. Some have thought, that this over-foftening of the filver might be equally prevented, by ufing lefs heat ; and that fine filver, receiving a fmoother furface than fuch as is al- loyed, muff, fhew the golden covering to better advantage. How far thefe prefumptions are well founded, or how
far
[ 53 1 far the manufacture is affected by the above differences in the quantity of alloy, I cannot ta^e upon me to de- termine.
The gold is employed in thick leaves, prepared on purpofe for this ufe ; which are applied all over the filver rod, and preffed down fmooth with a fteel bur- nilher. Several of the gold leaves are laid over one ano- ther, according as the gilding is required more or lefs thick. The fmallefl proportion allowed by act of parlia- ment is i oo grains of gold to a pound or 5760 grains of fil- ver. The largeft proportion, for the beft double-gilt wire, as Dr. Halley was informed by the workmen, is 120 grains to a pound ; though I am told, that of late the proportion of gold has been increafed.
The firft part of the drawing procefs, as well as the preparation and gilding of the filver rod, is performed by the refiner ; who ufes plates of hardened fteel, with a piece of tough iron welded on the back to prevent the fteel from breaking. In this back part, the holes arc much wider than the correfponding ones in the fteel, and of a conical ftiape ; partly, that the rod may not be fcratched againft the outer edge ; and partly, for receiving fome bees-wax, which makes the rod pafs more freely, and preferves the gold from being rubbed off. The plate being properly fecured, one end of the rod, made fome- what fmaller than the reft, is pufhed through fuch a hole as will admit it, and laid hold of by ftrong pincers called clamps, whofe chaps are toothed, fomewhat like a file, to keep the rod from flipping out by the violence neceffary for drawing it : the handles or branches of the clamps are bent upwards, and an oval iron loop put over the curvature, fo that the force, which pulls them hori- zontally by the loop, ferves at the fame time to prefs them together : to the loop is faftened a rope, whofe
further
[54]
further end goes round a capftan, or upright cylinder, with crofs bars, which requires the flrength of feveral men to turn it. The rod, thus drawn through, is well nealed, then paffed in the fame manner through the next hole, and the nealing and drawing repeated, lefs and lefs force fufficing as it diminifhes in thicknefs : when reduced to about the fize of a large quill, it is delivered in coils to the wire-drawer.
The remainder of the procefs requires plates of a differ- ent quality ; thofe of fteel, whether in a hard or a foft ftate, being now found to fret the wire, or to raife a bur upon its furface, and ftrip off the gold. The plates for this part of the work are brought from Lyons in France : the holes are drilled in them here. They are formed of a metallic mafs, whofe compofition is kept a fecret, but whofe prevailing ingredient is plainly iron : I have begun an examination of this metal, and fhall communicate the refult of the experiments in one of the future numbers of this work. There are two forts of thefe plates ; one of confiderable thicknefs, for the wire in its larger flate ; the other, only about half as thick, for the finer wire, where lefs force is fufficient in the drawing. There are confiderable differences alfo in the quality of the metal it- felf, not to be diftinguifhed by the eye, or any otherwife than by repeated trials : fuch of the thicker plates, as are found good, are valued at a high price. The Lyons plates, though brittle, have fufficient toughnefs to admit of the holes being beaten up, or contracted, by a few blows of a hammer ; fo that when any of them have been widened by a length of wire being drawn through, they are thus reduced again to the proper dimenfions for preferving the gradation : the holes, after each beating up, are opened by a long (lender inllxument, called a point, made of re- fined fteel ; one end of which, to the length of about five
inches,
[ 5S 1 inches, is round, and ferves as a handle ; the reft, about twice as long, is fquare, and tapered to a fine point. The firfl holes being fooneft gulled, or fo far worn, as to be unfit for bearing further reductions ; the next to them, grown likewife wider, fupply their places, and are themfelves fucceflively fupplied by thofe which follow ; whence, as each plate is furniihed with feveral more of the fmall holes than are wanted at firft, it continues to afford a complete feries after a considerable number of the larger has become unferviceable. Great part of the dexterity of the workman ccnlifts in adapting the hole to the wire ; that the wire may not pafs fo eafily, as not to receive fufficient extenfion, or fo difficultly as to be broken in the drawing. For determining this point with greater certainty than could be done from the mere refinance of the wire, he ufes a brafs plate called a fize, on which is meafured, by means of notches like fteps cut at one end, the increafe which a certain length of wire mould gain in pafling through a frefh hole : if the wire is found to ft retch too much or too little, the hole is widened or contracted. As the exten- fion is adjufted by this inftrument, there are others for meafuring the degree of finenefs of the wire itfelf : flits of different widths, made in thick polifhed iron rings, ferve as gages for this ufe.
The wire-drawers procefs begins with nealing the large wire received from the refiner : this is performed by placing it, coiled up, on fome lighted charcoal, in a cylindrical cavity, called the pit, made for this purpofe, under a chimney, about fix inches deep, and throwing more burning charcoal over it : the pit having no aper- ture at bottom to admit air, the fuel burns languidly, affording only fufficient heat to make the metal red-hot, without endangering its melting. Being then quenched in water for the fake of expedition in cooling it, though
the
[ 56 ] the metal would doubtlefs be foftencd more effectually if flittered to cool leifurely, one end of it is patted through the firft hole in the thick plate, and fattened to an upright wooden cylinder fix or eight inches in diameter : in the top of the cylinder are fixed two ftaples, and through thcfe is patted the long arm of a handle, by which the cy- linder is turned on its axis by feveral men. In the con- tinuation of this part of the procefs, called degrotting, the wire is frequently nealed and quenched, after every hole or every other hole, till it is brought to about the iize of the fmall end of a tobacco-pipe : and in this ttate it is cut into portions for the fine wire-drawer.
In this laft part of the wire-drawing procefs, nealing is not needful ; but it is ftill as neceffary as before to wax the wire at every hole. Much lefs force being now fufficient for drawing it through the plate, a different inftrument is ufed : a kind of wheel, or circular piece of wood, much wider than the foregoing cylinder, is placed horizontally : in its upper furface are fome fmall holes, at different dittances from the axis, and into one or another of thefe, according to the force required, is occafionally inferted the point of an upright handle, whofe upper end is received in a hole made in a crofs bar above. From this the wire is wound off upon a fmaller cylinder, called a rochett, placed on the fpindle of a fpinning- wheel ; and this laft cylinder being fixed on its axis behind the plate, the wire is again drawn through upon the firft ; and being at length brought to the proper finenefs, it is nealed to fit it for the flatting-mill. This nealing is per- formed in a different manner from the foregoing ones, and with much lefs heat ; for if the wire wis now made red hot, it would wholly lofe its golden colour, and become black, bluifh, or white, as I have often experienced in different parcels of gilt wire. Being wound upon a large hollow
copper
[ 57 1 copper bobln, the bobin is fet upright, fome lighted char- coal or fmall-coal placed round it and brought gradually nearer and nearer, and fome more fmall-coal put in the civity of the bobin ; the wire being carefully watched, that as foon as it appears of the proper colour, it may be immediately removed from the heat. This is an operation of great nicety, and is generally performed by the matter himfelf. The wire, though it in good meafure retains the fpringinefs which it had acquired in the drawing, and does not prove near fo foft as it might be made by a greater heat, is neverthelefs found to be fufficiently fo for yielding with eafe to the flatting mill.
The flatting-mill confifts of two rolls, turned in a lathe to perfedl roundnefs, exquifitely polifhed, placed with their axes parallel one over another, fet by fcrews till their circumferences come almoft into contact, and both made to go round by one handle : the lowermoft is about ten inches in diameter ; the upper commonly little more than two, though fome make it considerably lar- ger, and indeed it would be more convenient if made as large, or nearly fo, as the lower : their width or thick- nefs is about an inch and a quarter. The wire, unwind- ing from a bobin, and pafiing firfl between the leaves of an old book, prefled by a fmall weight, which keep itfomewhat tight, and then through a narrow flit, in an upright piece of wood called a ketch, which gives notice of any knot or doubling, is directed by means of a fmall conical hole in a piece of iron, called a guide, to any particular part of the width of the rolls ; that if there mould be any imperfection or inequality of the furface, the wire may be kept from thofe parts ; and that when one part is foiled by the paffage of a length of wire, the wire may be fhifted, till the whole width of the rolls is foiled, fo as to require being cleaned and polifhed anew with the
I tine
[ 58 ] fine powder, called putty, prepared by calcining a mix- ture of lead and tin : the workmen value the rolls from the number of threads they will receive, that is from the number of places which the wire can thus be fhifted to : good rolls will receive forty threads. The wire, flatted between the rolls, is wound again, as it comes through, on a bobin; which is turned by a wheel, fixt on the axis of one of the rolls, and fo proportioned, that the motion of the bobin juft keeps pace with that of the rolls.
The rolls, as well as the drawing plates, have been
often procured from France ; and it has been thought that
the wire received from the French rolls an additional
beauty and luftre ; though it does not appear that the
French have any durable advantage in this refpect above
the Englifh, or that the gloffinefs communicated by either
is of any real advantage to the manufacture j for it quickly
goes off. The moft important point in their preparation
is, the giving them that perfect truth and equability of
furface, which the flatting of fo fine wire demands. The
internal part is formed of iron, and a plate of refined
fteel is lapped round and welded over the iron : where
the two ends of the fteel plate meet there is frequently
an imperfection, the juncture being generally vifible
acrofs the face of the roll. In rolls of great width, fome
curious artifts have obviated the inconveniences ariiing
from this caufe, by ufing, inftead of a broad plate, a long
narrow bar of the fteel, and twifting it round the roll
in feveral circumvolutions, that the little inequalities, in
hardnefs and folidity, happening 'at the junctures, might
be in the direction of the ribbon that paries between the
rolls, and not tranfverfe to it. In the narrow rolls ufed
for the flatting of wire, a practice of this kind would be
very difficult ; but the fame end might perhaps be an-
fwered, and even more effectually, by carting the fteel,
inftead
[ 59]
inftead of a Jlreight bar, into the form of a hoop or ring, of a fomewhat lefs diameter than the fizc of the intended roll ; then forging the hoop, on the round beak and flat of the anvil alternately, to procure it the requifite uniformity of its parts and the due extenlion ; afterwards placing it in a proper mould, fixing the axis in its due pofition, and running into the intermediate fpace fome caft iron, which, from its known property of expanding, as it fets or becomes folid, will continue every where to fill the cavity, and irremoveably fix itfelf both to the hoop and to the axis.
The degree of ex ten/ion of gold in wire and leaf The vafl extent, to which gold is apparently ltretched in the foregoing operations, has induced feveral perfons to make experiments for determining its exact degree by menfuration and weight. In an experiment of Reau- mur's, forty-two fquare inches and three tenths of gold leaf weighed one grain troy -, and Mr. Boyle found that fifty and feven-tenths weighed but a grain. As a cubic inch of fine gold weighs 4902 grains, the thicknefs of the gold leaf examined by the one was the 207355th, and of that by the other only the 248532nd part of an inch.
Dr. Halley found, that of fuperfine gilt wire fix feet weighed a grain : M. de Reaumur makes about four inches more go to the fame weight ; and Mr. Boyle is faid, if there be no error in the numbers, to have had gilt wire much finer than any of thefe. Allowing fix feet to make, a grain, and the proportion of gold to be that commonly ufed by our wire-drawers ; the length to which a grain of gold is extended on the wire, comes to be near 352 feet.
In flatting, the wire is extended, according to M. de Reaumur, one feventh part of its length, and to the width
I 2 of
[ 6o]
of one nincty-fixth of an inch : in fome trials I have feen made by the workmen, the extenfion in length ap- peared lefs, but that in breadth fo much greater, that the fquare extenfion was at leaft equal to that affigned by Reaumur. Hence one grain of gold is ftretched on the flatted wire, to the length of above 401 feet, to a furface of above 100 fquare inches, and to the thinnefs of the 492090th part of an inch.
M. de Reaumur carries the extenfion of gold to a much greater degree. He, fays the wire continues gilded when only one part of gold is ufed to 360 of filver ; and that it may be ftretched, in flatting, one fourth of its length, and to the width of one forty-eighth of an inch. In this cafe, a grain of gold mud be extended to 2900 feet, or upwards of half a mile, and cover an area of more than 1400 fquare inches. He computes the thicknefs of the golden coat, in the thinneft. parts of fome gilt wire, to be no more than the fourteen millionth part of an inch, fo that it is only about a hundredth part of the thicknefs of gold leaf.
Yet notwithstanding this amazing tenuity, if a piece of the gilt wire be immerfed in warm aqua fortis, which will gradually diflblve and eat out the filver, the remain- ing golden coat will flill hang together, and form, while the fluid prevents it from collapfing, a continuous opaque tube. To fucceed in this experiment, the aqua fortis muft not be very ftrong, nor the heat great; for then the acid, a&ing haftily and impetuoufly upon the filver, would difunite the particles of the gold.
Whether any other metal can be extended to an equal degree is not as yet clear ; for as it is the great value of gold which engages the workmen to endeavour as much as pofhble to ftretch it to the largeft furface, the fame efforts have not been made in regard to the lefs valuable
4 metals :
[6i ]
metals : to make a fair comparifon, trial mould be made of extending filver upon the furface of gold in the fame manner as gold is extended upon filver. It may be ob- ferved alfo, that as gold is near as heavy again as filver, or contains near double the quantity of matter under an equal volume ; fo, if equal weights of the two metals be ft retched to equal extents, the filver will be little more than half the thinnefs of the gold ; and converfely, if filver could be brought to equal tenuity with gold in regard to bulk, it would, in regard to quantity of matter, be near of double extenfibility.
Application of gold leaf and wire on other bodies.
There are various methods of applying the gold, thus extended, to cover the furface of other bodies. For laces and brocades, the flatted gilt wire is fpun on threads of yellow filk approaching as near as may be to the co- lour of gold itfelf. The wire, winding off from a bobin, twifls about the thread, as it fpins round ; and, by means of curious machinery, too complex to be defcribed here, a number of threads is thus twifled at once by the turning of one wheel. The principal art confifts, in fo regulating the motion, that the feveral circumvolutions of the flatted wire on each thread may juft touch one ano- ther, and form, as it were, one continued covering.
It is faid that, at Milan, there is made a fort of flatted wire gilt only on one fide, which is wound upon the thread, fo that only the gilt fide appears ; and that the preparation of this wire is kept a fecret, and has been attempted in other places with little fuccefs. There is alfo a gilt copper wire, made in the fame manner as the gilt filver : Savary obferves, that this kind of wire, called falfe gold, is prepared chiefly at Nuremberg -, and that the ordinances of France require it to be fpun, for its
diitin&ion
[ 62 ] diilinctien from the gilt filver, on flaxen or hempen threads. One of our writers takes notice, that the Chinefe, inftead of flatted gilt wire, ufe flips of gilt paper, which they both interweave in their fluffs, and twill upon filk threads : this practice he inconfiderately propofes as a hint to the Britifh weaver. Whatever be the pretended beauty of the fluffs of this kind of manufacture, it is obvious that they mufl want durability : the Chinefe themfelves, according to Du Halde's account, fenflble of this imperfec- tion, fcarcely ufe them any otherwife, than in tapeflries, and fuch other ornaments, as are not intended to be much worn, or expofed to moifture.
Paper, wood, and other like fubjects, are gilded, by fpreading upon them fome adhefive fubflances, and when almoft dry, lb as but jufl to make the gold flick, applying gold or gilt leaf, and prefling it down with a bunch of cotton, or the bottom of a hare's foot : when grown thoroughly dry, the fuperfluous or loofe gold is wiped off", and the fixed golden coat burnifhed with a dog's tooth, or with a fmooth piece of agate or pebble. Different kinds of adhefive matters are employed for this ufe : where refiflance to rain or moiflure is required, oil paints ; in moil other cafes, a lize, made from cuttings of parch- ment or white leather, by boiling them in water.
The compofition commonly ufed for oil gilding confifls of yellow ochre, finely powdered, and a fuitable quantity of drying oil, ground together till they unite into an uni- form mixture, of fuch a confiflence, that it may be freely laid on with the pencil, without fpreading beyond the part on which it is applied, and that it may fettle fmooth with a gloffy furface.
For gilding on wood, &c. with what is called water- fize, the parchment or leather fize above-mentioned is mixed with whiting, and feveral layers of the mixture
fpread
[63 ]
fpread upon the piece, one after another is dry, fo as to cover the grain of the wood, and the imperfections left by the tool, and form a perfectly fmooth furface for applying the gold upon : over this is commonly fpread fome of the fame fize mixed with yellow ochre. Thefe compofitions do not well admit of the* gold being bur- nished ; and therefore, where burnifhed gilding is re- quired, another mixture, called gold-fize, is either laid above thefe, or applied on the wood at firft. The gold- fize is compofed of tobacco-pipe clay, or bolar earths, ground with a fmaller proportion of ruddle and black lead, and tempered with a little tallow and oil olive. In thefe points there is little uniformity among the work- men, the fame end being obtainable by different means, among which we cannot perhaps diftinguilh any fuperio- rity in the effect of one to that of another, and of which fancy or prejudice have often chofen the more com- pounded in preference to the more fimple. The principal caution, in regard to the gold-fize, feems to be, to ufe no more of the unctuous materials than is' neceffary for procuring the due confidence ; and to make a trial of the preparation previous to its being employed in any work of confequence.
For fome purpofes, the gold is ufed in powder, which, from its being kept in mells, is called fhell gold. This is prepared by grinding gold leaves, or gold-beaters frag- ments, with a little honey ; and afterwards feparating the honey from the powdered gold by means of water. Gold may be reduced alfo, by diffolving it in mercury, and evaporating the mercury in the fire, or by diffolving it in aqua regis, and precipitating with certain additions, of which hereafter, into a powder, more fubtile than can eafily be obtained by mechanical comminution.
Gilt
[ 64 ]
Gilt letters or figures on paper may be formed of (hell gold, tempered with gum water : or the characters may be drawn with a milky folution of gum ammoniacum, made in water, and gold leaf applied upon them when almofl dry : if they have become quite dry, they may again be fufficiently moiftened for receiving the gold by breathing on them. For raifed letters, fuch as are feen in fome ancient manufcripts, whiting, yellow ochre, or other earthy powders, are tempered with ilrong gum wa- ter, and the letters formed of this compofition, by a pen, or more commodioufly, by means of a type or ftamp, pre- vioufly oiled, as hinted in a pamphlet on drawing and painting in water-colours, publiflied in 173 1 ; when dried to a due degree of tenacity, the gold leaf is laid on. If the characters are formed of hard bodies, as powdered glafs or cryftal, they may be covered with a burniflied golden coat, by carefully rubbing them with a piece of folid gold.
On the covers of books, the gildings are deprefTed beneath the furface, and cemented with whites of eggs. The part being rubbed with this liquid, the gold leaf is applied all over it, and the letters or figures made after- wards by heated fiamps or rollers, which, at the fame time that they form the cavities, prefs down and fix the gold in them ; while the gold,' on the prominent or fmooth furface, adheres fo loofely as to be eafily wiped off.
In the pofthumous papers of Mr. Hooke, a method is defcribed of gilding live craw-fifh, carps, &c. without injuring the fifh. The cement for this purpofe is prepared, by putting fome Burgundy pitch into a new earthen pot, and warming the vefTel till it receives fo much of the pitch as will flick round it ; then ftrewing fome finely powdered amber over the pitch when growing cold, add-
ing
f 65 ]
inp- a mixture of three pounds of lirtfeed oil, and one of oil of turpentine, covering the veffel, and boiling them for an hour over a gentle fire, and grinding the mixture, as it is wanted, with fo much pumice ftone in fine pow- der as will reduce it to the confidence of paint. The fifh being wiped dry, the mixture is fpread upon it, and the gold leaf being then laid on, and gently preffl-d down, the fifh may be immediately put into water again without any danger of the gold coming off, for the matter quickly grows firm in water. As the qualities of this cement excellently fit it for fome other purpofes, it was thought worth while to infert the whole procefs.
Drinking-glafies, gilt on the edges, have of late been much admired : the belt, of thefe are brought from Ger- many; thofe hitherto made in England, though equal in beauty to the foreign, being greatly inferiour in the durability of the gilding. It is fuppofed that the German glaffes are gilt by fire : and it is certain that gold leaf may be made to adhere firmly to glafs foftened by heat, and that the effect may be promoted by the interposition of fome vitrefcent bodies more fufible than the glafs itfelf : a piece of glafs pipe being moiftened with a weak folution of borax, then covered with gold leaf, dried, and heated to a full red heat, the gold was found cemented more ftrongly than that on the German glaffes, fo as fcarce at all to be fcraped off" with a knife ; though in fome parts it appeared fpecky or full of fmall holes, probably from want of fufheient addrefs in the application of it. But how firmly foever the gold may be thus cemented, it would be very difficult to gild the edges of a glafs in this method without damaging the reft ; and a careful exami- nation of fome of the German glaffes ffiewed pretty plainly that the gold had been fixed on them by other means. The glaffes had evidently been ground and poliflhed ; yet
K the
[ 66]
the polifh even of the part under the gold had not fuffeced any injury, which it doubtlefs would have done from a degree of fire fufficient to lbften its furface, or from any vitreous intermedium melted to it. The gold could be fcraped off pretty eafily with a knife ; and by fleeping for a little time in heated fpirit of wine or oil, particularly in the latter, it became more eafily feparable. One fide being thus cleared from the gold, there appeared a fmear upon the glafs under it ; and this being cleaned off, there ap-. peared a like fmearinefs between the gold and the glafs on the oppofite fide ; whereas, on viewing in the fame man- ner the glafs which I had gilt by fire, the furface of the gold next to it looked remarkably bright, without theleafl cloudinefs upon the glafs. From thefe obfervations it may be prefumed, that the gold is cemented to the German glaffes on the fame principle with the foregoing gildings ; and that the only fccret confifts in finding a matter, which will adhere to glafs, fo as not to be eafily rubbed off. I have tried maflich and fome other refinous bodies rubbed warm upon the glafs, and feveral fpirituous varnifhes ; but nil of them were attended with fome inconveniencies, par- ticularly with the grand one of not adhering fufficiently to the glafs. I recommend to the trial of the artifts concerned in this affair the harder oil varnifhes ; and fhall myfelf profecute the enquiry, and when fuccefsful, communicate the refult.
SECT. IV.
Of the cffcSls of Fire on gold, I. Of the melting of gold.
GOLD melts in a low white heat, and, when in fufion, appears on the furface of a luminous bluifh green colour. Though its expanfion by fmall degrees of heat, as from freezing to boiling water, is lefs than
that
[67] that of mofi of the other metals, yet in fufion it feem* to expand more than any of the others; rifing up with a more convex or elevated furface, as it becomes fluid , and fubfiding, and growing more concave or deprefled, as it fets again or returns to folidity. From this property it follows, that gold is lefs fit for receiving (harp and perfect figures' when cart; into moulds, than filver, cop- per, lead, or tin, which do not fhrink fo much, and far lefs fo than iron or bifmuth, which expand in their paffage from a fluid to a folid flate.
The workmen, for the melting of gold, chufe gene- rally a black lead crucible, on account of its bein^ fmoother than the Heflian or other common forts, and cenfequently lefs apt to retain any particles of the coflly metal : it is likewife much lefs liable to crack, may be ufed for feveral fufions, and does not require the precau- tions neceiTary to be obferved where the others are em- ployed.
When the gold is divided into fmall parts, as filings, though all the particles be brought to perfect fluidity,* they do not eaiily reunite into one mafs, many of them continuing frequently in diftinct drops. This repugnance is judged to proceed from fmall atoms of duft, or other extraneous matters, adhering to the furfaces of the par- ticles, and preventing their clofe contact : the addition of certain fufible faline fubftances, which diffolve and vitrefy earthy bodies in the fire, is found to remove the impediment, and to collect and unite the gold how- ever divided. The ufe of fluxes is abfolutely necefTary in thefe circumflances ; and from their apparent utility here, it has been thought that they were needful in other cafes, and hence they are often employed where they feem to be little wanted.
K 2 Borax,
[68 ]
Borax, one of the mod powerful diflblvents of earthy
matters, is, in this refpect, one of the bed fluxes for
gold ; but the gold melted with it, however fine, is
commonly obferved to have its colour made fomewhat
paler. From what caufe this flight diminution of the
colour proceeds, I have not been able to difcover : nor
do the workmen find the diminution considerable enough
to prevent their ufing borax more generally than any
other kind of flux. Nitre, added to the borax, prevents
this effect ; and gold previoufly made pale by borax has
its colour reftored by melting it with an addition of
nitre : hence this fait is ufefully employed where the
gold is defigned for the high coloured fort of leaf, for
gilding, or other purpofes where the high colour of the
metal is a principal object. When gold is alloyed with
copper, and the full proportion of the copper is to be
preferved, nitre is never to be ufed, the bale metals being
icorified or destroyed by it : in this cafe it will be advife-
able to add to the borax a little charcoal in fine powder,
which will preferve the copper from being fcorified by
the heat.
There is another material point, in melting gold, the prefervation of its malleability ; which is very liable to be injured, either from an excefs, or deficiency, or too iudden an abatement of the heat, occafioning an undue arrangement of its parts at the time of its becoming iolid. When the gold is made exceflively hot, and the mould, into which it is to be poured, is warmed but little or not at all, the metal almofr. always contracts a degree of hardnefs and rigidity ; whereas by duly pro- portioning the heat of the mould to that of the metal, its foftnefs and toughnefs may generally be fecured. The gold-beaters, to whom thefe qualities are of more im- portance than in any other art, heat the mould, as already
obferved,
[69 J
obferved, till the tallow, which it is rubbed with, runs and fmokes, without taking flame ; and pour out the gold as foon as its furface appears of a bright green colour : the clearnefs of the colour ferves them as a mark both of the gold being of a proper degree of heat, and of its be- ing fine. Thofe who work in alloyed gold judge alio from the appearance of the furface, whether the metal is of fuch a heat, or fuch a difpofition, as to prove tou^h or eager when cold ; taught, by ufe, marks which cannot eafily be defcribed. It is fuppofed by fome, that gently making or finking the crucible, fo as to communicate a kind of undulatory motion to the fluid metal juft before it is poured out, contributes to its toughnefs.
It is a general opinion among metallurgic writers, that fine gold, in fufion, is made brittle by the contact of vegetable coals not thoroughly burnt, or by their fume ; and what is pretty fingular, that gold alloyed with copper is not fo fubject to receive this injury. But it is probable that the brittlenefs, afcribed to this caufe, depended ra- ther upon others : for the gold-beaters, who leave their crucible open, do not find, that the toughnefs of the gold is at ail diminished, either by the vapour of the charcoal, or by a coal in fubftance failing in ; though, if any fuch diminution happened, it could not be fup- pofed to efcape their notice. There appears to be little danger to the malleability of gold from any kind of fumes but metallic ones.
When gold is made brittle by a fmall admixture of bale metals, or by their fume?, its malleability may be reftored by melting it with a little nitre, which fcorifies and difiblves all the other metals exxept filver and pla- tina. The nitre fhould be thrown upon the gold jult as it is going to melt ; and the metal poured out as foon as it flows thin. A long continuance of the fufion is apt
to
[7° 1 to deAroy the effect of the nitre, and render the gold as brittle as it was before : for fo much of the nitre, as ha* acted upon the bafe admixtures of the gold, is changed by that action into an alkaline fait ; and the flighted: accefs of any inflammable matter is fufricient to revive the fcorified metallic particles from the alkali, and ren- der them again mifcible with the gold. Corroiive mer- cury-lublimate, thrown by a little at a time upon gold in fulion, with care to avoid its noxious fumes, anfwers the fame end with nitre, and is commonly preferred to it by the workmen : on what foundation the effect of fublim.ite depends, will appear hereafter.
II. Of the alterations faid to be producible in gold by jirc. The greatefl degrees of artificial lire, continued for a length of time, have not been obferved to make any alteration in gold. Gafto Claveus, in an apology for the alchemifts, printed in the fecond volume of the Theatrum cbymicum, relates, that he put an ounce of pure gold, in an earthen veflel, into that part of a glafs-houfe furnace where the glafs is kept conflantly melted, and con- tinued it there in fulion for two months together ; and Kunckel mentions a like experiment, made in the glafs furnace of the duke of Hollatia, in which the gold was expofed to the fire for almoft thirty weeks. Thefe ve- hement and continued degrees of heat it was found to fupport, without differing any fenflble alteration of its quality, or diminution of its weight ; whereas the other metals, platina and filv'er excepted, are foon deprived by fire of their metallic alpecl, and either diflipated in fumes, or changed to an earthy or glaffy form.
What common fire effects in the bafe metals has been faid to be effected in gold by the more intenfe heat col- lected in the focus of large burning-glaffes. Mr. Hom- i berg
IV ]
berg reports, in the memoirs of the French academy for the year 1702, that he expofed gold, on a piece of charcoal, to a burning lens, about thirty-three inches in diameter, whofe activity was further increafed by the interpofition of a fmaller lens placed at a proper diftance for contracting the focus into a lefs compafs : that this vehement heat produced a powdery matter on the furface of the gold, which, gathering together, formed a vitreous drop in the middle, and then run off to the fides } that the furface, now bright, became again gradually covered with a like duft, which in like manner vitrefied and run off j that frefh drops of glafs continued to be thus pro- duced ; and that at the fame time great part of the gold evaporated in fumes.
This experiment, as Cramer very jufily remarks, does not feem to have been made with fufficient care, or carried to a fufficient length, to warrant the confcquences that have been drawn from it. The purity of the gold ought to have been fcrupuloufly examined, which it does not appear to have been at all ; and fuch part of it as remained unaltered after the operation, ought to have been further fubmitted to the fame treatment ; for if any part of the gold was really changed, the whole would doubt- lefs have fuffered the fame change from a continuance of the fame caufe. The author, fenfible of this, fays indeed, that If the gold be expofed long to the heat, it will at length be totally vitrefied or evaporated : but he does not affirm that this actually did happen, and feems only to have judged from the firil appearances that it would hap- pen. To attempt the revival of the glafs into gold again, which Cramer and Macquer require for the fatisfactory proof of its having been produced from gold, was not perhaps to be expected from him •, fince, according to- his theory, the glafs confified only of the earthy part of
the
[ r- J
the gold, the fpecificating principles of the metal being fuppofed to have evaporated in the intenfe heat : yet, if he really believed that it proceeded from the gold at all, it is extremely flrange, that fo curious a chemift mould pafs over a product lb extraordinary, and fo interefling to his favourite purfuits, without the lealt examination, and even neglect to repeat and verify the lingular experiment by which it was obtained; efpecially as he had all poilible opportunities of profecuting the enquiry, the apparatus being at his command, and the materials provided for him by royal munificence. From one of his fublequent papers publifhed in the lame memoirs for the year 1707, it appears, that this vitrification of the gold was called in queftion by one who had been witnefs to the experi- ment ; who took, notice that fome allies, flying oft* from the charcoal on which the gold was placed, fell from time to time upon the furface of the metal ; and hence judged, that the little quantity of glafs obtained was no other than a vitrification of thefe allies. An objection fo well founded required furely a repetition of the procefs, and a more attentive obfervation of the phenomena ; but the author anfwers only by another experiment, which appears alfo to be fingle in its kind, that when filver was expofed on a piece of charcoal in the fame manner, no vitrification happened ; as if the allies could not be cafu- ally accumulated on the metal, fo as to produce a fenlible vitrification, in one inftance, without being fo in another. I have been the more particular in the account of this experiment, becaufe it has been relied on by many as an indubitable proof of gold being alterable in its nature ; and becaufe a due attention to what the author himfelf has thought fit to communicate, fhews it to be at befl: too imperfect for any ftrefs at all to be laid upon it. The want of a proper apparatus here is an infuperable obftacle
to
[73] to my repeating the experiment : but it is faid that others have repeated it, and found ftrong reafons to be- lieve that Homberg was deceived. M. Macquer relates, that feveral perfons have expofed gold to the focus of the fame burning lens, and even of others ftill ftronger, with- out ever being able to vitrefy it : and that, though the gold did indeed decreafe in weight, yet the diminution appeared to happen, not from any of the principles of the metal being feparated, but from minute globules of it forced off in fubftance ; many of which were catched on a piece of paper placed underneath, and found to be per- fect gold unchanged. It is probable, that thefe globules were forced off from the liquefied gold, not by the action of the heat upon the metal itfelf, but upon its vefTel or fupport ; for all the common veffels, or other fubftances that can be employed for this purpofe, on being expofed fuddenly to a great degree even of artificial fire, crackle or fume, and throw up a part of their contents.
The earlier chemifts, finding gold to be proof againft the vehemence of their fires, thought milder means might be more effectual, for loofening the clofe union of its component parts, and producing changes in it fimilar to thofe producible in the bafe metals. Accordingly they expofed it, for feveral weeks or months together, to the immediate action of a gentle fire or flame not much greater than that in which lead melts : by this me- thod it is faid to have been notably altered in its pro- perties, and to have aflumed feveral new ones : Kunckel, in his haboratorhim Cbymicum, affirms that he has fucceeded in this experiment, and fays that the gold fwells up into a fpongy fubftance, like iron treated in the fame manner. The obfcure and imperfect accounts given of the procefs prevent our being able to repeat it fo as to determine with
L certainty
[ 74 ] certainty its true efrecT: : but there appears as little reafon to believe, in this as in the other cafe, that the gold fuf- fered any permanent change. The gold is required to be previoufly prepared : if this preparation confifts, as it molt probably does, in combining it with any other kind of matter that will abide with it in the fire, and reducing it into fubtile powder, a heat of no long continuance will occafion a remarkable alteration in its afpedt., though its other properties remain entire. If gold leaf be divided by grinding it with an admixture of earthy powders, as calcined hartihorn or chalk, or with faline ones of the more fixed and lefs fuiible kind, as vitriolated tartar, and expofed, for fixteen or twenty hours, to a moderate heat, fcarce fufficient to keep the vefTel red hot ; the gold wholly lofes its metallic brightnefs, and changes its yellow co- lour to a red or purple. On feparating, by means of wa- ter or acids, the foluble fait or earth, the remaining golden powder recovers by fimple fufion its proper metallic form ; a flrong heat diverting it of thofe fuperficial appearances which a weaker one had induced.
SECT. V.
Of the Mixture of gold with other metals.
THE repugnancy or contrariety, which obtains in fundry instances, between different metallic bodies made fluid by fire, and which is no lefs ftrongly marked than that betwixt oil and water, is no where obferved in regard to gold ; this metal uniting readily with all the other known metallic bodies, and feeming to have a ftrong, though not equal, affinity to them all.
I. Of
17S 1
I. Of the mixture of gold with mercury : Gold powder, water-gilding, ccc. Mercury, in the greateft cold that obtains in our at- mofphere, adheres readily to gold, totally conceals its colour, communicating a filver whitenefs to every part it touches, and by degrees penetrates and diflblves it. Some of the chemifts fpeak of an animation of mercury, by which its activity on gold is greatly increafed ; and Mr. Boyle relates, that he had himfelf prepared mercury, fo as to dhTolve half or even equal its weight of gold leaf, and to produce, during the diflolution, a fenfible heat, fometimes confiderable enough to be offenfive to the hand : but an enquiry into this point belongs rather to the hiflory of mercury than of gold.
In order to obtain a fmooth amalgam, or uniform mix- ture, of gold and common mercury, the union is expe- dited, by reducing the gold into thin plates or grains -, which are heated red hot, and in this ftate thrown into as much mercury, as will cover them, previoufly heated in another crucible, till it begins to fmoke : on flirrino: them together with an iron rod, the gold foon diflblves and difappears. If the amalgam is defigned for any nice ufes, it lhould be cleanfed, from any filth it may have contracted, by grinding it in a glafs, flone, or wooden mortar, with fome common fait and water, and occafion- ally renewing the water, till the amalgam ceafes to dif- colour it, and appears of a pure vivid brightnefs.
When the proportion of mercury is large, fo that the mixture continues fluid when cold, a confiderable part may be feparated by preffing it through foft leather, as the thicker kind of wafh-leather or doe-fkin : fo much of the quickfilver may be fqueezed out, as to leave a butyra- ceous or confident mafs, containing little more than one
L 2 part
[76]
part of mercury to two of gold, but ftill of a filver white- nefs, as if there was no gold in it. The confiftent amal- gam grows foft on being warmed or worked between the fingers, and hardens on lying in the cold, whence it has been propofed as a proper material for making feals from impreffions in wax : the amalgam of gold appears however to have no advantage in this refpect above thofe of the inferiour metals, as is well known to fome impoftors, who have fold amalgams of bafe metals, for this ufe, as curious preparations of gold. The mercury, /trained off from the amalgam, ihould be referved for the like pur- pofes again, as the leather, though no vifible imperfection happens in it, may have its pores fo far dilated by the prerTure, as to fuffer fome fmall particles of the gold to pafs through with the mercury : this may be difcovered by evaporating a little of the quickfilver over the fire, which in this cafe will leave a yellow fpot on the bottom of the veffel.
Mercury, one of the moft volatile of the metallic bodies, is expelled from gold by a fire not fufficient to make the mixt red hot. If the amalgam is expofed haftily to this degree of heat, it is apt to fwell up and leap about, and part of it to be thrown over the veflel : if the fire is gentle at firft, and increafed by degrees, the mercury exhales quietly. The impalpable atoms, into which the gold had been divided by its diflblution in the quick- filver, continue difunited after the quickfilver has exhaled ; provided due care is taken in the regulation of the fire, and in ftirring and rubbing the matter, towards the end of the procefs, fo as to expofe it equally to the heat, and prevent its running into lumps. By this method a powder of gold may be obtained, much finer than that prepared by the grinding of gold leaf, and which has likewife this advantage, for the purpofes of painting, that
it
[77] it admits better of being burnifhed. It is obvious, that for ufes of this kind, the mercury ought to be pure, as well as the gold : for the lead or other bafe metals, with which mercury is too frequently impregnated, will be left behind, and difcolour the gold.
If an amalgam of gold be fpread upon copper, and the mercury evaporated by fire, the gold will remain fixed all over the furface of the metal, and thus afford a firm and durable gilding. The workmen rarely chufe pure copper for gilding upon in this manner, but gene- rally mix with it about a feventh part of brafs, that is, of a compofition of copper and zinc : they fuppofe that this addition renders the copper lefs porous, and makes a lefs quantity of gold to ferve : whatever there may be in this notion, the brafs is plainly of ufe on another account, to facilitate the adhefion of the mercury ; for mercury unites exceeding difficultly with pure copper, and much more eafily, as I have often found in experiments of amalgama- tion, with copper divided by zinc.
The piece to be gilt being well cleaned, fome mer- cury, (haken with a little aqua fortis, is fpread upon it, till the furface appears all over white as filver : being then heated, and re-touched in thofe parts which have efcaped the mixture, the amalgam of gold is laid on : the heat, foftening the amalgam, makes it fpread the more freely ; and the intervention of the mercury and aqua fortis occafions it to adhere more uniformly. The piece, thus covered with the amalgam, is placed on a convenient fupport, over a charcoal fire ; and examined from time to time, as the mercury evaporates, that if any deficiencies appear, they may be fupplied with a little more of the amalgam before the procefs is completed. If a thicker gilding is required, than can refult from fo much of the amalgam as is applied at once, the piece,.
after
[?8] after the firft quantity has left its gold fixed upon the fur- face, is rubbed afrefh with the mixture of mercury and aqua fortis, and more of the amalgam fpread upon it : after the evaporation of the mercury from this, another and another quantity may be applied in the fame manner. The golden coat, left after thefe operations, is fometimes of a pale dead colour ; occafioned perhaps, partly, by im- purities in the mercury, and partly, by a little of the mercury itfelf left unevaporated. Whatever be the caufe, the workmen find a remedy in rubbing upon the piece, while warm from the fire, (after the loofe particles of gold have been wiped off with a clean fcratch brufh, made of very fine brafs wire bound together,) a compofi- tion, called gilding wax, which being burnt off, fome more of it is rubbed on, and this application repeated till the gold appears of a proper colour. The gilding wax is compofed of bees-wax, red ochre or ruddle, verdegris, vitriol or alum, and fometimes other additions : the acid of the falts and the cupreous part of the verdegris feem to be the materials on which the effect of the compound chiefly depends. I have been informed by an ingenious artift, that he has employed for many years a faline com- pofition without wax, and found it to anfwer extremely well : equal quantities of nitre, fal ammoniac, green vitriol, and verdegris in fine powder, are mixed together, moiftened with water, and applied upon the piece ; which is then heated till the mixture fmokes, and quenched in urine.
There are two principal inconveniencies in this bufinefs : one, that the workmen are expofed to the fumes of the mercury, and generally, fooner or later, have their health greatly impaired by them : the other, the lofs of the mercury j for though part of it is faid to be detained in cavities made in the chimney for that purpofe, yet the greateft part of it is loft. From fome trials I have made
5 it
[ 79 ] it appeared that both thefe inconveniencies, particularly the firft and moft confiderable one, might in good meafure be avoided, by means of a furnace of a due conftruction. If the communication of a furnace with its chimney, in- ftead of being over the fire, is made under the grate, the afh-pit door or other apertures beneath the grate clofed, and the mouth of the furnace left open ; the current of air, which otherwife would have entered beneath, enters now at the top, and palling down through the grate to the chimney, carries with it completely both the vapour of the fuel, and the fumes of fuch matters as are placed upon it : the back part of the furnace, mould be railed a little higher above the fire than the fore part , and an iron plate laid over it, that the air may enter only at the front, where the workman Hands, who will thus be effectually fecured from the fumes, and from being incommoded by the heat, and at the fame time have full liberty of introducing, in- fpedling and removing the work. If fuch a furnace is made of fhrong forged (not milled) iron plate, it will be fufficiently durable : the upper end of the chimney may reach about a foot and a half higher than the level of the fire : over this is to be placed a larger tube, leaving an interval of an inch or more all round between it and the chimney, and reaching to the height of ten or twelve feet, the higher the better. The external air, paffing up between the chimney and the outer pipe, prevents the latter from being much heated, fo that the mercurial fumes will condenfe againfl its fides into running quick- filver, which falling down to the bottom, is there catched in a hollow rim formed by turning inwards a portion of the lower part, and conveyed, by a pipe at one fide, into a proper receiver.
Mr. Hellot communicates, in the Memoirs of the French academy for the year 1745J a method of making raifed
figures
[ 8o]
figures of gold on works of gold or filver, found among the papers of Mr. du Fay, and of which Mr. du Fay him- felf had feen feveral trials. Fine gold in powder (fuch as refults from the parting of gold and filver by aqua for- tis, as defcribed hereafter) is directed to be laid in a heap on a levigating ftone, a cavity made in the middle of the heap, and half its weight of pure mercury put into the cavity : fome of the fetid fpirit, obtained from garlick root by diflillation in a retort, is then to be added, and the whole immediately mingled and ground with a muller, till the mixture is reduced into an uniform grey powder. The powder is to be ground with lemon juice to the confidence of paint, and applied on the piece previously well cleaned and rubbed over with the fame acid juice : the figures drawn with it may be raifed to any degree by repeating the application. The piece is expofed to a gentle fire till the mercury is evaporated fo as to leave the gold yellow, which is then to be preffed down, and rubbed with the finger and a little fand, which makes it appear folid and brilliant: after this it may be cut and embellifhed. The author obferves, that being of a fpongy texture, it is more advifable to cut it with a chifel than to raife it with a graver ; that it has an imperfection of being always pale, and that it would be a defirable thing to find means of giving it colour, as by this method ornaments might be made of exquifite beauty and with great facility. As the palenefs appears to proceed from a part of the mercury retained by the gold, I apprehend it might be remedied by the prudent application of a little warm aqua fortis, which difiblving the mercury from the exteriour part, would give at leaft a Superficial high colour : if the piece is filver, it muit be defended from the aqua fortis by cover- ing it with wax. Instruments or ornaments of gold, flamed by mercury, where the gold is connected with
fubftances
[ Si ] fubflances incapable of bearing fire, may be reftored to their colour by the fame means.
The foregoing procefs is given entirely on the authority of the French writer. I have had no experience of it myfelf, but have feen very elegant figures of gold raifed upon filver, on the fame principle, by a different procedure. Some cinnabar was ground, not with the diddled fpirit, but with the expreifed juice of garlick, a fluid remarkably tenacious : this mixture was fpread all over the polifhed iilver; and when the firft layer was dry, a fecond, and after this a third was applied. Over thefe were fpread as many layers of another mixture, compoled chiefly of afphaltum and linfeed oil boiled down to a due confidence. The whole being dried, with a gentle heat, on a kind of wire grate, the figures were traced and cut down to the filver fo as to make its furface rough : the incifions were filled with an amalgam of gold, raifed to different heights in different parts according to the nature of the defign ; after which a gentle fire, at the fame time that it evapo- rated the mercury, deftroyed the tenacity of the gummy juice, fo that the coating, which ferved to confine the amalgam and as a guide in the application of it, was now eafily got off. The gold was then prelfed down and em- bellifhed as in the former method, and had this advantage, that the furface of the filver under it having been made rough, it adhered more firmly, fo as not to be in danger of coming off, as M. du Fay fays the gold applied in his way fometimes did. The artifl however found the procefs fo troublefome, that though he purchafed the receipt for a confiderable fum, he has laid the practice afide.
Mercury and amalgams rubbed on iron do not at all adhere to it : there are however means of applying the mercurial gilding on this metal, as well as on copper and iilver. If the iron be dipt in a folution of the blue vitriol of copper, or rubbed with the vitriol itfelf fomewhat
M moilfened,
[ 82 J moiftened, it becomes immediately covered with a cupre- ous coat, and now receives the gilding in the fame man- ner as folid copper..
II. Of the mixture of gold with Ji/ver, copper and other metals ; the alterations produced by different proportions of different metals, and the effecJs of Jlrong or continued f re on the mixtures.
All the metals, which melt eafier than gold, diflblve it in a lefs heat than the gold would melt in j and gold,, brought into fufion, diflblves in like manner thofe which are more difficultly fufible. It is particularly difpofed to unite with iron, of which, if the iron be pure, it diflblves twice or thrice its own weight in a degree of heat very far lefs than that in which iron melts : if ftirred in fufion with an iron rod, it corrodes a part of the iron, and a large portion of the gold adheres to the inftrument : hence Cramer, Schl utter, and other writers on aflaying, prudently caution againft. the ufe of an iron rod for the /lining of melted gold. In virtue of this property, gold proves an excellent folder for the finer kinds of iron and ileel inftruments : a fmall thin plate of gold being wrap- ped round the parts to be joined, the gold is foon made to melt by a blow-pipe, and ftrongly unites the pieces to- gether, without any injury to the inflrument, however, delicate.
On copper, its apparent aclion is much lefs confiderable ; yet, when once it is united with this metal, the increafe of fufibility is more ftrongly marked, mixtures of gold with a little copper being found to melt with lefs heat than pure gold itfelf. Hence mixtures of this kind ferve •as folders for gold : two pieces of fine gold are foldered by gold that has a fmall admixture of copper ; and gold ■; cd with copper is foldered by fuch as is alloyed with
more
.[ 83 ] more copper : the workmen add a little filver as well as copper, and vary the proportions of the two to one ano- ther, fo as to make the colour of the folder correfpond, as nearly as may be, to that of the piece : copper alone, in the quantity requitite to procure the due fufibility, would incline the mafs too much to its own colour.
Silver, mixed with gold, dilutes its yellow colour more or lefs according to its quantity. One twentieth or lefs of filver renders gold very fenlibly paler ; and the ad- dition of a twentieth more makes it fenlibly paler than the former proportion : but when the filver is increafed to a tenth or an eighth of the gold, fo fmall differences in the quantity fcarce occafion fenfible variations in the colour, and flill lefs fo when the filver exceeds the gold ; a little gold not near fo much affecting the colour of filyer, as a little filver does that of gold. All the mixtures are very malleable, though fomewhat harder, firmer, and more fonorous, than either of the metals feparately : in this tefpect, as in the colour, a little filver affects gold more than a little gold does filver.
Copper, in fmall quantity, renders the gold fomewhat harder than filver does, and fomewhat heightens the co- lour, by fuperadding its own reddimnefs to the gold yel- low ; but if the quantity of copper is confiderable, the coppery hue prevails : a little gold mixed with copper makes no remarkable alteration either in its colour er ductility. The high colour which a fmall proportion of copper communicates to gold, has been obferved in differ- ent circumftances, and given rife to fundry proceffes for the exaltation of the colour of the noble metal. Some recommend for this purpofe the fuperficial application of verdegris, blue vitriol, or other preparations of copper ; which indeed are often ufed by the workmen, but vvhofe
M 2 effect
r*4 j
effect feems to be, not the heightening of the colour of the metal itfelf, but the removal of the fuperficial tarnim or difcoloration which alloyed gold is apt to receive from, the fire j and this effect appears to proceed, not from, the copper, but from the acid which thefe preparations contain. Others, for communicating a high colour to- the whole mafs, direct the gold to be melted with three or four times its weight of the higheft coloured copper, the mixture to be granulated or flatted into plates, then boiled in weak aqua fortis, in order to feparate as much of the copper as the acid will. extract, the remaining gold to be melted with frefh copper, and this procefs repeated feveral times. It is apprehended, that by this method only a fmall portion of the copper will be left in the gold, and that this little will be fo intimately commixt with it as to refift the action of acids and of the air ; and that the gold will thus receive the admired colour, with- out being made much more liable to tarnifh, or to change its colour on warning or boiling, than fine gold.
Platina, next to the two foregoing metals, injures the malleability of gold the leaft. Mixtures of gold with one twentieth of its weight of platina I haye drawn into- moderately fine wire : mixtures of it with one fourth its weight were forged into pretty thin plates : and a mixture of equal parts (which is- as large a proportion of platina as can be eafily united with gold) was indeed brittle, but bore feveral ftrokes, and ftretched confiderably under the hammer, before, it began to crack about the edges. With regard to the colour, fmall proportions of the platina, as one fixtieth, make little alteration : in larger proportions, as one twelfth, it communicates, not its own whitenefs,. but a particular and remarkable dull hue, the compound approaching more to the colour of bad copper than of
gold :
[85] gold : in the quantity of one fourth and upwards, it gives a dull whitifhnefs.
Iron or fteel, in very fmall proportion, render gold hard and eager, and on increasing the quantity of the iron, the mixt continues brittle : fome of thefe mixtures are of fuch a degree of hardnefs and clofenefs, as adapts them for receiving a fine edge, and it is faid that they have been formed into razors. The colour of the gold is made pale by a fmall quantity of the iron : equal parts of the two- form a grey mafs : if the quantity of the iron is three or four times greater than that of the gold, the mixt proves of a white colour, approaching to that of lilver.
All the reft of the metallic bodies give palenefs, dullnefs,. and brittlenefs, in different degrees, fome more than others in the fame quantities. Of tin and lead the moft minute proportions, even the vapours which rife from them in the fire, though not furficient to add to the gold any weight fenfible in the tendereft balance, make it fo brittle that it flies in pieces under the hammer ; though gold, contrari- wife, mixed with a fmall proportion of the lead or tin, does not appear to injure their malleability. Something ©f the fame kind feems to happen in the mixtures of gold with the metals which of themfelves are brittle, as zinc,, bifmuth, and reguhis of antimony ; a fmall proportion of thefe metals rendering gold extremely brittle, whereas, when the brittle metal is in large proportion, its fragility- is diminifhed by the gold : thus Mr. Hellot obferves, in a paper on zinc published in the French memoirs for the year 173 5, that a mixture of three pans of zinc and one of gold does not break fo eafily as a mixture of equal parts of the two. Some of thefe mixtures, particularly one of equal parts of gold and zinc, bear a fine polifli, and pro- bably, as the above mentioned author takes notice, would- be-
[ 86 ] be excellent for making fpecula, being lefs fubjedt to tarnifli in the air than the com poll ti on s of which copper is the bafis.
It is fuppofed by many, that gold, melted with other metals, is always diffufed equally through their whole vo- lume, infomuch, that the quantity of gold, obtainable from any part of the mixt, mall bear exactly the fame pro- portion to that part, as the whole of the gold does to the whole mafs. There appears, however, in many cafes, a fenfible inequality in the distribution, M. Hellot, in his French tranflation of Schlutter's German work on the fmelting and affaying of ores and metals, gives an account of an experiment which clearly (hewed this inequality : a quantity of lilver, amounting to upwards of twenty pounds, containing about a fifty-fixth part of gold, was melted in a crucible, and poured into cold water, in order to its being reduced into grains : by dipping at different times an iron ladle into the water, under the ftream of metal, he received a part of the firft running, a part of the middle, and a part of the laft : the three parcels, aflayed feparately, were all found to differ in their content of gold.
There is a curious experiment of Mr. Homberg's, re- lated in the French memoirs for the year 1713, which, though I have not yet tried it, I fhall venture to infert on account of its Angularity. Equal parts of gold and lilver, melted together and reduced into fine grains, were put into a crucible, with a mixture of about equal parts of decrepitated fea fait and rough nitre under them : the crucible being kept in a fmall fire, in a wind-furnace, for about a quarter of an hour, and then fuffered to cool and broken, the gold was found in one lump at the bottom, and the filver above it in two pieces, with fome grains,
wrapped
[ »7]
wrapped up in the falts, which had not entirely melted : the filver was perfectly pure, and without the lead: mixture of gold, but the gold retained about a fixth part of lilver. He repeated the experiment with different mixtures of the two metals, and found the filver to be always pure from gold, but that the gold retained a little of the filver, except in two inftances, in which this alfo was pure. He obferves, that unlefs the gold and filver are nearly in equal quantities, the feparation does not fucceed ; and that the only nicety in the procefs confifts in hitting the due point of fufion, for if the fire is too long continued, or the mixt made to flow thin, the two metals, after they have parted from one another, mingle again together.
We have already feen that mercury may be evaporated from gold by a gentle heat : there are fome other metallic bodies alfo, which may be totally difiipated from gold by fire, but with considerable differences in regard to the circumstances of the feparation. Arfenic, though of itfelf very volatile, adheres fo Strongly to gold as not to be eafily expelled : if the mixture is urged hafiily with a violent fire, a part of the gold is carried off by the arfenical fumes. Zinc, in open veffels, burns, changes into white flowers, and throws up along with its own fumes a fmall portion of the gold, which tinges a part of the flowers of a yellowifh colour inclining to purple : thefe flowers do not rife high, part of them forms about the furface of the mafs, and when once they are formed, they refiSt the fire ; fo that though the whole of the zinc, by fre- quent Stirring and Strong fire, mould be thus changed, yet, unlefs the proportion of gold be large, the noble metal remains divided and interfperfed among the flowers. In clofe veffels, or where the external air has no accefs, the zinc, by force of fire, may be totally made to fublime : it may be expelled alio in an open crucible, by keeping the
mixt
[ 38 ] mixt covered with powdered charcoal, which, lb far as it reaches, prevents the above change of the zinc.
The dilTipation of regulus of antimony from gold re- quires, on the contrary, not only an open veflel and free accefs of air, but the artificial impulfe of a blafl of air upon the furface : if the fire is vehement, the crucible (hallow, and the air flrongly impelled, the gold is volatilized by this metal more conliderably than by either zinc or ar- fenic ; but with proper care, the regulus may be blown off without any fenfible lofs of the gold. Some have pro- pofed regulus of antimony inftead of mercury for gilding on copper j as-alfo on earthen ware and glafs, on which the mercurial gilding cannot be applied : the regulus and gold, firfl melted together, are directed to be ground into fine powder, which being lpread upon the piece, the whole is ex poled to a flrong red heat, fo as to evaporate the regulus. The inconveniencies of this method are, that the powder does not of itfelf adhere to the fubject, and can fcarce be equably fpread, and that part of the gold is wafted : that glafs melts in the heat neceffary for the exhalation of regulus of antimony, and that copper is liable to be corroded by the regulus, and have its furface rendered uneven.
The bafe metals in general, which calcine or change to an earthy form in the fire, fuffer the fame change when mixed with gold, though with fome differences in the degree of facility. If gold, mixt with a fmall proportion of thefe metals, be kept for a confiderable time in fufion, the bafe metal, gradually fcori lying, riles to the furface, no longer mifcible with the gold. If the gold is in lefs quantity, and the fire infufficient to bring the mafs into fufion, the whole mixt lofes by degrees its metallic afpect, and the gold remains intermingled among the calx of the bafe metal, in a more attenuated llate than it can perhaps
i be
[ 89 3
be brought to by other means : by long continuance of a moderate fire, the calx acquires more or lefsof a purple hue, according to the quantity of the gold and the natural colour of the calx of the metal it is mixed with.
Tin, which when calcined by itfelf is neither vitreicible nor fufible in the fire, and which cannot be perfectly vi- trefied by the mod active fubftances commonly made ufe of in this intention, is remarkably affected by the admix- ture of gold. Dr. Brandt relates, in the tranfadtions of the Swedifh academy for the year 1753, that two parts of tin, and three of gold, being melted together, reduced into fine powder, and calcined only to an am grey colour, the calx melted with eafe into a yellow glafs, at the bot- tom of which was found a metallic regulus. I (hall exa- mine this curious experiment on another occafion.
Though gold, in the highly attenuated flate into which it is thus reduced by calcination with bale metals, is by fome bodies otherwife a&ed upon than in its grolfer form, as appears from its Habitude to tin in the preceding paragraph, and to the marine acid in the following fection, it is by no means diverted of its metallic properties, or changed into a calx. Mercury, which does not diffolve metallic calces, any more than unmetallic earths and flones, on being triturated with the compound powder, imbibes the gold; and on this foundation, gold, blended with the bafe metals, may in fome cafes be advantageoufly extracted from them .
If mixtures of gold and lead be continued in a fire fufti- cient to keep them in perfect fufion, the lead, calcining and rifing to the furface, changes into a fluid fcoria, eafily feparable from the gold by means to be defcribed here- after. Bifmuth alio fcorifies and feparates in the fame manner ; and both thefe metals, promoting the fcorifica-
N tion,
[90]
tion, or fufion of the calces, of the other bafe metallic bodies, promote their feparation from gold in the fire.
SECT. VI.
Of the aBion of acid and fulpbureons bodies on gold ; 'various folutions of it, and their properties.
I. Gold with the nitrous acid.
TH E acid fpirit extracted from nitre, whether in its concentrated flate, or in the more dilute one, in which it is commonly called aqua fortis, has not been found to have any action on pure gold. Hence gold is freed, by this acid, from filver, copper, lead, 2inc, mer- cury, and fuch other metallic bodies as the acid diffolves : but that this feparation may fucceed, the quantity of the inferiour metal muft be considerably greater than that of the gold, for otherwife its particles will be enveloped by the gold fo as to be entirely defended from the acid.
When nitre in fubftance is mixed with certain bodies containing the vitriolic acid, as calcined vitriol, and the mixture made red hot, the acid of the nitre is extricated in yellowifh red fumes. If the impure gold is interlaid with fuch a mixture, and expofed to the fire along with it, in a clofe veffel that the fumes may be confined j the bafe metal will be in part corroded, though its quan- tity is far lefs than would be acted upon by the acid in its liquid flate, but in this cafe the acid penetrates only a little way into the mafs. Hence, for the purification of gold by this method, the operation muft be two or three times repeated, the metal being each time melted and reduced into thin plates, that frefh furfaces may be ex- pofed to the fumes : and in the procefs by aqua fortis, if the bafe metal does not amount to a certain quantity, more bafe metal mud be added. The method of con- ducting
f 9i ]
dueling the operations will be defcribed in the eighth and ninth Sections.
As pure gold has been always found to refift the nitrous acid, and as gold divided by filver or other metals has not been obferved to be acted upon by that acid in the common proceffes of afTaying or refining ; it has been univerfally laid down as an axiom, that the pure nitrous acid can in no cafe have any action on gold, and that, in whatever manner it be applied to mixtures of gold with other metals, it can diffolve only the inferiour metal, and will always leave behind the full quantity of gold which the mixt contained. Here it may be obferved, once for all, that as the mutual relations of bodies are multifarioufly modified by the circumflances in which the fubjects are applied to one another, fundry bodies dif- covering ftrong repugnancies in fome circumflances, and flrong affinities in others ; we never can infer, from the conftancy and uniformity of the action or inaction of two bodies on each other in all the circumftances in which they have been applied, that their relations will be the fame in any other circumftances ; and consequently, un- lefs all poffible means of application were known and ex- perienced, no axiom, in regard to the chemical affections of bodies, ought to be admitted as univerfal. Though the afTayer and refiner depend upon the abfolute indiffolu- bility of gold by the nitrous acid, yet there are circum- ftances, in which gold is diffolved by this acid in con- fiderable quantity.
This curious and important difcovery was made by Dr. Brandt, and publifhed in the Swedifh tranfaclions for the year 1748. In order to part. a mixture of gold and filver, amounting to about fifteen pounds, in which the propor- tion of the filver to the gold was as fixteen to three (in- cluding with the filver a little copper which it contained)
N 2 he
[ 92 ] he boiled it with frefh portions of ftronger and ftronger aqua fortis, in a glafs body, to which was fitted a head and recipient for collecting the acid vapours that arofe : this method fliould feem at firft to be a notable improve- ment on the common procefs, in which the vapours, that iffue plentifully during the action of the acid, exhale and are loft. Nearly all the filver and copper being diflblved, and the folution poured off from the gold, the next por- tion of aqua fortis was boiled down till the matter at the bottom looked like a dry fait ; which being judged to have been fo much deprived of the acid, that there was not enough left to render the little remaining filver diffoluble by water, he added more aqua fortis; which, after boil- ing for fome time, appeared yellow, and was poured off into a feparate glafs, its yellownefs being looked upon as a mark of its having become exceeding ilrong by the lofs of its watery parts in the procefs.
This yellow aqua fortis he ufed afterwards for diffolv- ing fome filver, when, to his aftonithment, a confiderable quantity of gold was found at the bottom of the glafs, though the filver had before been very carefully purified from gold. This experiment was many times repeated, in the prefence of feveral affayers, and at a meeting of the Swedifh academy, and always with the fame event -, pure filver, which gave no mark of gold with common aqua fortis, precipitating from the above yellow aqua fortis a fpongy lump of gold. In keeping, a part of the gold fepa- rated fpontaneouily, in form of a brown powder : after it had been long kept, and depofited much of its gold, it was found on an affay to contain more gold than lilver, in the proportion of 19 to 12 : in this ftate, a quantity of it fufficient to diffolve four parts of filver yielded during the diflblution one part of gold ; fo that the nitrous acid is capable of diflblving above one fourth part as much gold
5 as
[93 1
as it is of filver. The nitrous fpirit made ufe of in this operation had been prepared from pure nitre, and the experiment itfelf affords a convincing proof, that it was by the pure nitrous acid that the gold was difTolved ; for if the diffolution of this metal had been produced, as might be fufpected, by means of an admixture of marine acid, the menftruum could not, in the above method of application, have difTolved the filver.
The foregoing procefs differs from that commonly fol- lowed for the parting of gold and filver, in the veffel being clofe fo as to exclude the external air, and in the heat being continued at lafl till the matter became dry, lb that as the watery parts of aqua fortis rife firft in diftil- lation, the acid mufl in this cafe have been greatly con- centrated. Though the applying a head upon the veffel may feem to be a very immaterial circumflance in regard to the diffolution of the metal, it is perhaps one of the mofl effential, for both diffolution and precipitation are in many cafes remarkably influenced by the admiffion or exclufion of air : after the gold has been diffolved, if the veffel be well fhaken, fo that air may be copioufly in- troduced and mingled with the liquor, the gold, as Mr. Scheffer obferves, falls quickly to the bottom.
The importance of this experiment, in the way of cau- tion to thofe concerned in the parting of gold and filver by aqua fortis, is apparent. It is probable, that gold has been often difTolved in aqua fortis, without being known to be fo ; and that this was the true caufe of the de- ception of Becher and other chemifls, who report that they had feen filver tranfmuted into gold by diffolution in fome particular kinds of aqua fortis. Had Dr. Brandt's folution paffed into other hands than his own, it might poflibly have been looked upon as another inftance of thefe pretended graduating or tranfmuting menftruums.
II. Gold
[94]
II. Gold •with the marine acid.
The pure acid of fea fait has no a&ion on gold, fo long as the gold retains its metallic form ; whether the metal be boiled with it in open or in clofe vefTels, or ex- pofed in the fire to its fumes ; in which lad circumftance, this acid diflblves or corrodes all the other known metallic bodies, except platina. Hence, though there are feveral metallic bodies, as filver, which the marine acid in its liquid Irate does not diffolve or extract from gold, yet gold may be purified from thofe metals by the fumes of this as well as of the nitrous acid. On this foundation, the brittlenefs, which a fmall admixture of lead or tin produces in gold, is remedied, by repeatedly injecting upon it in fufion a little corrofive mercury-fublimate ; the marine acid of the fublimate uniting with the lead or tin, and either volatilizing, or changing them into a fcoria, which is thrown off to the fides of the veffel. Small pro- portions of moft of the other metals are in like manner feparated from gold by fublimate ; the acid having lefs affinity to the mercury of the fublimate than it has to the others, and accordingly parting from the former to join itlelf to the latter.
When gold is changed to the appearance of a calx, by precipitation from aqua regia with volatile or fixt alkalies, of which hereafter -, or by calcination in mixture with tin or bifmuth, as mentioned towards the end of the pre- ceding fection, the pure marine acid, by the affiflance of a moderate heat, perfectly diffolves it. I have found that even a weak fpirit of fait will take up gold fb pre- pared, though in no great quantity ; and that the gold does not precipitate from this as from the nitrous acid, but continues durably fufpended.
III. Gold
[95 ]
III. Gold with the Vitriolic acid.
The vitriolic acid, in whatever manner applied, has not been obferved to have any action on gold, or to pro- mote the adlion of other acids. Hence, as oil of vitriol diffolves filver by a boiling heat, filver and gold may be parted from one another by this acid, as effectually, though not fo commodioufly, as by the nitrous. If the compound be reduced into grains or thin plates, and boiled in about twice its weight of oil of vitriol to drynefs, the filver will be fo far corroded, as to be eafily warned off by a little more of the acid ; or if the mafs, after the corrofion, be melted in a crucible, the gold will feparate and fubfide, the filver forming a fcoria above it. Gold may thus alfo be purified from feveral other metallic bodies : Mr. Scheffer fays, that this is the moft direct way of fepa- rating tin from gold.
IV. Gold with compound ?nenjlrua.
Gold is faid to be diffolved by the marine acid mixed with a fmall proportion of fpirit of urine ; by a mixture of the vitriolic acid with the fame urinous fpirit ; by a mixture of the vitriolic acid wkh a little fixt alkaline fait; by the vapour, which arifes during the effervefcence of the vitriolic acid with fixt alkaline fait, collected by di- ftillation ; in a fpirit, prepared by faturating the vitriolic acid with volatile alkaline fait, exficcating the mixture, diffolving it in twice or thrice its quantity of aqua fortis, and diftilling the folution. In my experiments, not one of thefe liquors appeared to have any action on gold.
The moft effectual menftruum of gold is a mixture of the nitrous and marine acids, called aqua regia ; which, in a moderate heat, readily and totally diliblves it into a
tranfparent
[96 ]
tranfparent yellow liquor. As this compound does not at all dillblve filver, the gold may be extracted by it from a mixture of gold and filver, in the fame manner as the filver is extracted by aqua fortis j and as the extraction of the filver by aqua fortis requires the quantity of filver in the mixt to exceed that of the gold, fo the extraction of the gold by aqua regia requires the gold to exceed the filver : the two metals may be fo proportioned, that nei- ther aqua fortis nor aqua regia fhall be able to diffolve either, till an addition is made to the quantity of one or the other metal.
When the quantity of gold in the mixt amounts to fo much as a third part of the filver, aqua fortis leaves always a fmall portion of the filver undiffolved along with the gold ; and in like manner, when the quantity of gold amounts to fo much as a third of the filver, aqua regia leaves a little of the gold undiffolved along with the fil- ver : when either metal is in fmall proportion, the other appears to be completely extracted by its proper menftru- ura. If the gold, remaining after the parting with aqua fortis, be diffolved in aqua regia, the filver it had retained will feparate and be left undiffolved : and if the filver, remaining after the parting with aqua regia, be diffolved in aqua fortis, the gold it had retained will in like man- ner feparate. This experiment affords a method of deter- mining the precife quantity of either metal retained by the other, and a proof of the erroneoufnefs of the opinion of fome writers, that fo much filver, as gold retains in parting, is actually tranfmuted into gold.
Aqua regia may be prepared, by diffolving powdered fea fait or lal ammoniac in four times their weight of aqua fortis, or by diffolving nitre in four times its weight of fpirit of fait, or by mixing the pure fpirits of nitre and fea fait together. The firft is the method mod commonly
followed.
[97] followed. Kunckel obferves, that by putting the gold into the aqua fortis firft, and then adding the fait by little and little at a time, lefs of the menftruum will fuffice than if the fait was previoufly diffolved in the acid ; the conflict, excited by each addition of the fait, promoting the diffolution of the gold ; this method appeared upon comparifon to have a fenfible advantage above the other, whether the fal ammoniac directed by Kunckel, or com- mon fait was ufed : the common fait is to be preferred ; for fal ammoniac, efpecially when a ftrong heat is called in aid to haften the folntion, is apt to occafion fome fmall part of the gold to be diffipated during the erFervefcence.
A folution in water of common fait, nitre and alum, boiled with leaf gold to drynefs ; or the falts in fubftance, mixed with the gold leaf, and urged with a flight red heat for fome hours in a clofe vefTel ; corrode a confiderable quantity of the gold into a laline form fo as to be diflblved upon adding water. The mixture of thefe falts, from its acting infenfibly and without erFervefcence, has been com- monly called menjiruum fine Jlrepitu : it can be confidered no otherwife than as an impure aqua regia, acting only by virtue of the acids of the nitre and marine fait, which are extricated from their bafes by the acid of the alum.
V. General properties of Joint ions of gold.
Solution of gold, whether made in fpirit of fait, or in any of the foregoing aqua? regiae, is of a bright yellow colour, refembling that of gold itfelf. It (tains the flcin of a deep purple colour, which cannot be warned out; and gives a like durable ftain, though with fome varia- tions in the fpecies of the colour, to fundry animal and vegetable fubftances, as dreifed leather, ivory and bones, feathers, woollen cloth, filk, linen, cotton, wood :
O to
[98] to marble it imparts a violet or purplifli colour, which penetrates to a confiderable depth, but on the harder Hones, as agates, it makes little imprcflion, communica- ting only a fuperficial brown tinge. The folution for thele purpofes, mould be prepared in Kunckel's method, that the acid may be fully faturated with the metal, and have as little admixture as may be of the faline matter : it fliould be diluted with three or four times its quantity of water, and if the colour is required deep, the piece, when dry, is to be repeatedly moiflened with it. Animal fubftances fliould be previously well cleanfed from their uncluofity, and foaked for fome time in water : the others require no preparation of this kind. The colour does not take place till a confiderable time, fometimes feveral days, after the liquor has been applied, and on fome fubjedls it is more flow than on others : to haften its appearance, the fubjeft fliould be expofed to the fun and free air, and occafionally removed into a moift place, or moiflened with water.
. When folution of gold in aqua regia is foaked up in linen cloths, and the cloths dried and burnt, the particles of gold remain blended in the brown coaly powder, which, being moiflened with a little water, and rubbed on filver well cleaned from any unctuous matter, gilds ir, without the application of heat, or the intervention of any other body : this is a ready but not a frugal way of applying gold on filver.
If the menftruum has been prepared with an addition of fea fait, nitre, or fal ammoniac, and the folution is fet in a warm place, in a veflel flightly covered, fo as to keep out duft, without preventing the evaporation of the wa- tery part of the liquor ; the gold, combined with the fa- line matter, flioots into yellow cryftals, commonly fmall and irregular. Solutions in the pure marine acid, and
in
[99] in mixtures of the pure acids of nitre and fea fait, are very difficultly made to cryftallize : in order to the cryftal- lization of thefe, the liquor mould be evaporated till only about one half of it remains, and then fet by in the cold with the addition of a few drops of pure fpirit of wine. The cryftals obtained from high coloured faturated folu- tions are generally of a red colour, and fometimes, as is faid, of a deep ruby red.
On diftilling with a gradual fire a folution of gold made in ftrong aqua regia, an acid fpirit comes over, which, from its riling in red fumes, and from its diflbl- ving filver, appears to be the nitrous acid. On continuing the diftillation, whitiih fumes fucceed, a mark that part of the marine acid begins to rife ; though, after the operation has been protracted till the reiiduum becomes dry, the gold flill retains fo much of the acid as to be difloluble in water : it appears to be chiefly, if not folely, the marine acid which thus remains combined with the gold ; on which foundation, the nitrous acid, employed for the diflblution of the gold, may be nearly all recover- ed, and its place fupplied by an equal quantity of common water ; the marine fpirit, though ineffectual for procuring the diflblution of the metal in its common form, being fufficient for keeping it diflblved. When the matter has juft become dry, it appears of a deep red colour : on further increafing the fire, the acid is totally diffipated, and the gold remains in powder, extremely fubtile, and of its proper hue. This is a convenient method of obtain- ing a fine powder of gold : if the aqua regia has been made with an addition of nitre or fea fait in fubftance, the faline matter, left with the gold, may be feparated by water. The mod: eligible aqua regia, for the above pur- pofe, is a mixture of the pure acids, or of the nitrous acid and fal ammoniac -, for thefe will be wholly diffipated by
O 2 fire>
[ »°° ] fire, and the gold alone left. If the powder is moiftened with a folution of borax, it may be applied with a pencil- on glafs or pcrcelane, an>. by a fuitable heat durably fixed upon them.
On inf/iiflating nearly to -.Irynefs a folution of gold made
in an aqua regia prepared with common fal ammoniac,.
abstracting from the refidu'mi feveral freih quantities of
the fame kind of aqua regia, and at length increafing the
fire fomewh.it haftily towards the end of the diftillation ;
the acid carries over with it a portion of the gold, fuffi-
cient to communicate a yellow or a red tinge ; and a more
considerable quantity of the gold, united with the more
concentrated acid, fublimes, of a deep red colour, into
the neck of the retort, concreting partly into long flender
cryftals, and partly into a firm fubftance clofely applied
on the glafs : the cryftals lie fo loofe, that they are apt
to fall down again on moving the veffel ; though, if this
mould happen, after the matter has become cold, they
may be eafily feparated again, the refiduum growing firm
as it cools : both the cryftals and the compact fublimate
diffolve eafily in water, deliquiate in the air, and melt
with a fmall heat. By adding to the refiduum more aqua
regia, and repeating the diftillation feveral times, the
whole of the gold may thus be made to rife.
Common aqua regia, prepared with rough fal ammoniac, appears to volatilize the gold as effectually, as any of the more operofe compositions recommended for this purpofe by the chemical writers. The rough fal ammoniac muft neceffarily be ufed, not fuch as has been purified, as it is called, by fublimation ; for Dr. Brandt obferves, that if the fal ammoniac be firft fublimed with a fufficiently ftrong heat, and then diffolved in fpirit of nitre, the aqua regia thus prepared will not make gold volatile. He finds, that when the gold has been diffolved, and the menftruum i diftiJled
[ IOI ]
diftilled off, there remains in the retort a faline mafs, containing the gold ; that on every frefh folution and diflillation with the fame kind of aqua regia, the matter increafes more and more in its weight, and looks like a foul dark brown fait very hard of fufion ( that the liquor which diftils is clear as water, and that nothing of the gold fublimes. He obferves alfo, that an aqua regia made with nitre in fubftance and the acid fpirit of fea fait, and with fea fait in fubftance and the acid of nitre, have lefs effect in volatilizing gold than that with rough fal ammo- niac above mentioned.
Though many have expected, from this volatilization of gold, a refolution of it into diffimilar parts, it is not found to have fuffered any real change. If the diftilled liquor, or the cryftals, or the fublimate, be expofed to a heat gradually increafed, the acid rifes, without carrying with it any part of the metal, the gold being left entire behind. The menftruum is lefs difpofed to elevate the gold a fecond time, than it was at firft.
VI. Separation of gold from acids by inflammable liquors.
The very fubtile inflammable fluid, obtained from a mixture of vitriolic acid with vinous fpirits, commonly called aether, or aethereal fpirit of wine, poured into a folution of gold made in aqua regia or in fpirit of fait, floats diftinct upon the furface, being far lighter than the acid liquor and Dot at all mifcible with it. The asther, of itfelf colourlefs, quickly becomes yellow, and the acid underneath lofes proportionably of its yellownefs ; the jether imbibing the gold, keeping it permanently dif- folved, and, when loaded with the ponderous metal, con- tinuing flill to float upon the acid. Gold is the only one, of the known metals, which the aether takes up from acids, and hence this fluid affords a ready method of
diftin-
[ 102 ]
diftinguifhing gold contained in acid folutions : whether a ftnall quantity of fome other metals may not, in certain circumftances, accompany the gold in this feparation from the acid, or whether very large quantities of fome metals will not defend a minute portion of gold from the action of the aether, may deferve further enquiry ; though fuch experiments, as we have hitherto made, incline us to think that they will not. The aether imbibes the gold, though it lies only on the furface of the acid folution : neverthelefs, to haften the effect, and to fecure againll any particles of the gold efcaping its action, it is expedient to make them lightly together, the veflel being clofely ftopt to prevent the evaporation of this very volatile fluid. If the folution in aether, poured off from the acid, be expofed to the open air, the aether exhales in a few mi- nutes, leaving the gold behind ; if kept for fome months in a flender glafs ftopt fo as that the aether may exhale exceeding flowly, the gold does not refume its proper form, but (hoots, as is faid in the Swedifh tranfactions, into cryftals, of a tranfparent yellow colour, a long prif- matic figure, and an auftere tafte.
Effential oils, fhaken with folution of gold, imbibe the gold in like manner, and carry it up to the furface, but keep it diflblved only for a little time : the metal gradually feparates, and is thrown off to the fides of the glafs in bright yellow films, which on making the veflel fall to the bottom. The oil, though of itfelf colourlefs, continues coloured after the gold has parted from it, effential oils receiving from the pure acid, firfr, a yellow, and afterwards a reddiih hue. Hence where thefe oils are employed as a tefl: of gold in folutions, it is not the colour which the oil acquires, but the fepa- ration of the golden films, that is to be regarded. The
oils
[ io3 ] cils appear to be more fluggifh than the aether in taking up the gold, and hence require to be well fhaken with the folution.
Rectified fpirit of wine mingles uniformly with the acid folution, and does not, for a time, occafion any other apparent change than rendering its colour more dilute. When the folution of gold has been infpiflated to drynefs, the metal, with the acid that remains com- bined with it, diflblves in fpirit of wine : if the men- flruum was either the pure marine acid, or a mixture of the pure nitrous and marine acids, or a mixture of the nitrous acid and fal ammoniac, the infpiflated matter dif- folves totally in the vinous fpirit : if the aqua regia was made by diflblving fea fait in aqua fortis, or by diflbl- ving nitre in fpirit of fait, the neutral faline com- pounds contained in thefe menftrua not being diflb- luble in vinous fpirits, remain perfectly white after the extraction of the gold. From all thefe mixtures, as from eflential oils, the gold feparates by degrees, though lefs fpeedily. On {landing for fome days, efpecially if the glafs is but lightly covered, the metal is feen floating in fine bright yellow pellicles upon the furface. The addition of a little eflential oil to the fpirit haftens the feparation of the gold.
Here it may be obferved, that many of thofe, who have bufied themfelves in the purfuit of medicinal pre- parations from gold, have been greatly deceived in the refult of their operations, from not being acquainted with the above properties of the metal. Finding that ef- fential oils imbibe gold from aqua regia, and receive with the gold a high colour, and that rectified fpirit of wine, by digeftion with the oil, diflblves it, and becomes im- pregnated with its colour ; they imagined they had thus obtained an aurum potabik, or true tincture of the gold,
which:
[ i°4 ] which they fuppofed to be endowed with extraordinary medicinal powers ; not aware, that the gold constantly feparated in the procefs, and that the colour of the pre- paration was no other than that which concentrated acids produce with eflential oils however pale or colourlefs.
Liquors containing a groffer inflammable matter, as wine, vinegar, folution of tartar, are likewife found to extricate gold from aqua regia in its metallic form ; with this difference from the preceding, that the gold, inftead of floating on the furface, falls here generally to the bottom.
VII. Precipitation of gold by alkaline fait s.
On adding to folution of gold a folution of any fixt alkaline fait or a volatile alkaline fpirit, in fufficient quan- tity to fatiate the acid ; the mixture becomes turbid, and on (landing for fome hours, the gold falls to the bottom, in form of a brownifh yellow muddy fubftance, retaining fome of the faline matter, great part of which may be feparated by repeated warning with hot water. That the gold may precipitate the more freely, the folution fhould be diluted with three or four times its quantity of water, or more. The alkaline liquor fhould be added by degrees, in little quantities at a time, till the mixture, after the gold has fettled, appears colourlefs, and a frefh addition of the alkali occafions no further precipitation or turbidnefs.
When gold has been thus totally precipitated by volatile alkaline fpirits, as fpirit of fal ammoniac, the addition of more of the fpirit renders the liquor again yellow, occafioning a part of the gold to be rediflblved : by adding a large quantity of the alkaline fpirit, almoft all the pre- cipitate is taken up; and even when the precipitated gold has been wafhed from as much of the adhering faline
matter
C 105]
matter as water will eafily extract, a confiderable part of it will flill diflolve in pure volatile fpirits, but not fo much as before the ablution : I have not obferved the whole of the gold to be taken up in either cafe, though fome report that they have found it to be fo in both. Pure fixt alkalies, added in large quantity after the preci- pitation, do not appear to rediffolve any of the gold.
If the aqua regia has been prepared with fal ammoniac, or if the precipitation is performed with a volatile alkali, the unwarned precipitate explodes, on being heated, v/ith a bright flafh and a fmart noife j whence its name aurum fulminans. If the aqua regia has been made without fal ammoniac, and the precipitation is performed with a fixt alkali, the precipitated gold makes no explofion : gradu- ally heated, it changes its dull yellowim to a bright pur- ple or purple-violet colour, and on further increafing the heat refumes its metallic afpecl. A volatile alkaline fait, either in the diffolvent or in the precipitant, feems to be efTentially necefTary to the fulmination.
Aurum fulminans weighs about one fourth part more than the gold employed, three parts of gold yielding four of the fulminating powder : this I relate on the authority of Lemery, Kunckel, and other practical writers, for though I have often made the preparation myfelf, I have never examined the increafe of its weight. Part of the increafe proceeds from the volatile alkali ; for on adding to the aurum fulminans a little vitriolic acid, the volatile fait rifes in fublimation, fatiated with the acid : the remaining powder is found to be diverted of its fulmina- ting power. From the coalition of the volatile alkali with the nitrous acid in the menftruum refults an ammo- niacal nitre, a fait which of itfelf detonates on being heated : by what power or mechanifm its detonating quality is fo remarkably increafed in the aurum fulminans, is unknown.
P The
[ io6 ] The explofion of aurum fulminans is more vehement than that of any other known kind of matter : it goes off in a lefs degree of heat than any of the other explofive compofitions ; and even grinding it fomewhat fmartly in a mortar is fufficient for making it explode. Some inftan- ces are mentioned in the Breflau collections, and the ephemerides natura curioforum, of a very fmall quantity burlling in pieces the marble mortar in which it was rubbed; and an accident of the fame kind happened fome years ago to a fkilful chemift here. The operator cannot be too much on his guard in the management of fo dan- gerous a preparation.
It has been reckoned, that a few grains of aurum ful- minans act with as much force as feveral ounces of gun- powder : but the actions of the two are of fo different kinds, that I cannot apprehend in what manner their ftrength can be compared. The report of aurum fulminans is of extreme acutenefs, offending the ear far more than that of a much larger quantity of gunpowder, but does not extend to fo great a diftance ; feeming to differ from it as the found of a fhort or tenfe mufical ftring from that of a long one or of one which is lefs ftretched. In fome experiments made before the royal fociety and mentioned in the firft volume of Dr. Birch's hiftory, aurum fulminans doled up in a ftrong hollow iron ball and heated in the fire, did not appear to explode at all ; while gunpowder treated in the fame manner burn: the ball. On the other hand a little aurum fulminans, exploded on a metalline plate in the open air, makes an impreffion or perforation in the plate ; an effect which gunpowder could fcarcely produce in any quantity.
This remarkable effect of aurum fulminans on the body which ferves for its fupport, has induced fome to believe that its action is exerted chiefly or folely downwards.
It
[ 107 1
It appears however to act in all directions : for a weight, laid upon it, either receives a like imprefiion, or is *hro\vn off; and in the collections above mentioned an account is given of a large quantity (fome ounces) which exploding from too great heat ufed in the drying of it, broke open the doors and mattered the windows in pieces. Mr. Hellot found, that when a few grains of the powder were placed between two leaves of paper, and cemented to one of them by gum water, only the leaf which touched the powder was torn by the explolion, and the other fwelled out ; and that when both were brought into clofe contact with it, by premng them together, it tore them both ; from whence he concludes, that the action of the aurum is greater!: on the bodies which it immediately touches. Both this property, and the acutenefs of the report, may pombly depend upon one caufe, the celerity of the expanfion : experiments have fhewn, that the refiftance of the air to bodies in motion increafes with the velocity of the body in a very high ratio ; and per- haps the velocity with which aurum fulminans explodes may be fo great, that it is refitted by the air as by a folid mafs.
The explofion of this preparation does not appear to make any change in the gold. When the powder is fpread exceeding thin between leaves of paper, and flowly heated, the detonation, as Mr. Hellot obferves, is flight and fuccemve, the powder becomes purple, and appears of the fame quality with the precipitate above mentioned which has no fulminating power. When a quantity is made to explode at once, in a large veflel, or under a proper cover, for confining the particles violently di- fperfed, the gold is found in fine dull:, partly purplifh and partly of its proper yellow colour : it is laid that when
P 2 the
[ xo8] the explofion is performed between filver or copper plates, the revived gold adheres to and gilds fome part of their fur face.
If aurum fulminans be warned with frefh portions of hot water, that as much as poffible of the faline matter may be extracted, its fulminating quality will be greatly diminished. If ground with oil of vitriol, which expels the nitrous acid, and unites with the volatile alkali, or boiled in a folution of fixt alkaline fait, which expels the volatile alkali, and unites with the nitrous acid, it no longer makes the leaft explofion, and the gold may be re- covered by fimple fufion. When mixed with fulphur, and expofed to a gentle fire, the fulphur gradually burns off", and leaves the gold in like manner recoverable without danger of fulmination. In all thefe cafes, if treated with a flow fire, it generally affumes a purple colour before it returns to its metallic form.
VIII. Precipitation of gold by metallic bodies.
All the metallic bodies that diflblve in aqua regia, platina excepted, precipitate gold from it ; the acid part- ing from the gold, and difiblving a portion of the others in its room. Some of them precipitate it alfo when they are previouily diffblved in other acids, and even in aqua regia itfelf.
Iron, in certain circumftances, becomes covered with the gold which it extricates from the acid, particularly where vinous fpirits have been mixed with the folution. A liquor prepared by boiling gold leaf in water with nitre, fea fait, and alum, till the matter becomes dry, and then digefting the mixt in rectified fpirit of wine, is faid to anfwer the beft for the gilding of iron in this way ; tho' it does not appear to have any different effects from thofe of other mixtures of fpirit of wine with folution of gold.
A folu-
[ 109 ]
A folution of gold in common aqua regia being largely diluted with fpirit of wine, a polimed iron, dipt in the mixture, became immediately coated with a fine golden pellicle : the gold folution without the fpirit of wine corroded the iron, and raifed a fcurf upon the furface. Thefe mixtures mould be prepared only as they are wanted, for on ftanding for a day or two the gold begins to feparate.
Iron diffolved in the vitriolic acid,- or common green vitriol diffolved in water, precipitates gold in form of a dufky brown-red powder. As the vitriolic folutions of iron do not precipitate from aqua regia any known metal- lic body befides gold, this experiment affords a commo- dious method of purifying gold from the fmalleft admix- ture of other metals : the particular way of managing the procefs will be given under the head of refining gold, in the ninth feclion.
On adding copper to a folution of gold in aqua regia diluted with water, the copper became inftantiy of a blackifh red colour ; and on ftanding, the gold fell in fubtile powder, of its proper metallic afpect, and of a high reddifh colour, which probably proceeds from fome cu- preous atoms intermixed : it is remarkable in this experi- ment, that the liquor, after the precipitation of the gold, appears colourlefs as water, a proof that the quantity of copper, taken up in the place of the gold, muft be ex- tremely minute. Solutions of copper in the vitriolic acid, or of blue vitriol in water, produced no precipitation or turbidnefs in folution of gold. Copper or verdegris dii- folved in vinegar occafioned the gold to feparate in bright films, which covered the fides of the glafs, forming an- al- moft continuous golden pellicle : this feparation however feems to depend, not fo much upon the copper, as on the inflammable matter of the vinegar.
i A plate
[ no ]
A plate of pure tin, put into a folution of gold largely diluted with water, changes the yellowith colour of the liquor to a beautiful purple or red : by degrees, a powder of the fame colour flowly fubfides, and leaves the mcn- ilruuni colourlefs. Solutions of tin, made in aqua regia, have the fame effect with tin itfelf, in regard both to the precipitation and the colour ; and hence characters, drawn on paper with a diluted folution of gold, not vi- sible when dry, become immediately red or purple on pafiing over them a diluted folution of tin. With the undiluted folutions, no rednefs is produced : after the red powder has fallen from the diluted liquor, if the whole be fet in a moderate warmth till the water has ex- haled, the gold is taken up again, the liquor becomes yellow as at firft, and only a white powder remains, which appears to be a calx of tin. The red liquor, fet to evaporate before the gold has fallen, yields only a yellow mafs ; from which rectified fpirit of wine extracts the gold combined with the acid, leaving, as in the other cafe, a white calx of tin.
Mercury, difiolved in the vitriolic, nitrous, or marine acids, is a precipitant for gold, as well as in its metallic form ; and in all cafes, a part of the mercury is apt to fall down along with the gold. When mercury in fub- ftance is ufed, and the folution of gold largely diluted, the undiflblved mercury gradually imbibes the gold.
On dropping a folution of filver into one of gold, both metals precipitate : the filver, parting from the nitrous, unites and falls with the marine acid, and the gold falls for want of it : the matter which feparates firft is white, then the liquor grows opake and a dark coloured powder fubfides, which leaves the menftruum clear and capable ,of diffolving filver. The fame double precipitation hap- pens,
[ III ]
pens, and on the fame principle, on mixing folution of gold with folution of lead in aqua fortis.
IX. Gold with fulphureoiis bodies.
Pure fulphur, whofe fumes corrode, and which in. fufion diffolves and fcorifies moll metallic bodies, has no action on gold. Hence the ufe of gold for fome mechanic purpofes, where other metals are in time deftroyed by fulphureous fumes ; as in the touch-holes of guns. And hence by fufion with this concrete, gold may be feparated from moil of the other metals. From filver and copper it may be extracted, on this principle, where the propor- tion of gold is too fmall to bear the expences of the other common methods of feparation : fome particular manage- ments and additions, however, are requifite, to render the procefs fuccefsful; fee fection the ninth.
Though gold refifts pure fulphur, it unites perfectly with a mixture of fulphur and fixt alkaline fait, com- monly called hepar fulphur is. As foon as the hepar melts, it begins to diffolve the gold, with a lucid ebullition : two or three parts of fulphur, and three of the alkaline fait, are fufficient for one of gold. Great part of the compound diifolves in water, fo as to pafs through a filter without any feparation of the metal : Stahl obferves, that this folution is lefs offenfive in fmell than that of the hepar itfelf, but of a more bitter naufeous tafle.
The addition of any acid to this folution, abforbino- the alkaline fait, precipitates the gold united with the fulphur ; which laft may be diffipated by fire, or more readily feparated by adding a little copper for abforbing the fulphur. A like feparation may be obtained by adding copper or iron to the mixture of gold and hepar in fufion ; thefe metals precipitating the gold, and uniting with the hepar in its place. Mr. Hellot recommends detonation
with
[ "«]
with nitre as the eafieft method of recovering the gold from the fulphurcous mixture : the bed way of mana- ging this proccfs appears to be, by making the matter red hot in a deep crucible, and dropping in the nitre, which fhould be previoufly well dried and heated, by a very little at a time, as the addition of any considerable quantity at once would occafion the deflagration to be fo flrong as to force off fome particles of the gold : this is the only in- convenience in the procefs, and it is not to be wholly avoided without great precaution ; for in many experi- ments of melting gold with nitre, when inflammable bodies had been mixed with the gold, I have almofl: always obferved numerous globules of the metal thrown up about the fides of the crucible : when a frefh addi- tion of the nitre produces no further deflagration, the fire is to be increafed fo as to bring the whole into fuiion ; and the crucible being then fuffered to cool, the gold is found at the bottom of the faline mafs, pure and of a high colour.
A neutral fait, compofed of fixt alkaline fait faturated with the vitriolic acid, being brought into fufion in a clofe crucible, with the addition of a little foot or pow- dered charcoal j the vitriolic acid and inflammable prin- ciple unite together, and form fulphur, the fame with common brimitone, which remaining combined with the alkali, the compound proves a true hepar fulphuris : and accordingly gold, melted with thefe ingredients, is diflbl- ved by them in the fame manner as by a hepar already made.
Dr. Brandt gives an account of an experiment, from which he concludes, that gold, by being diflblved in the above mixture, and afterwards recovered from it, fuffers a confiderable change. About a grain of gold and two hundred grains of filver were melted with the mixture,
and
[ "3 ] and precipitated by adding twice as much copper : the? fcoria, containing the copper, was melted with calx of lead, and the lead revived from the compound, that if any of the gold and filver fhould have remained in the fcoria, they might be imbibed by the lead : the precipi- tated mafs was cupelled with the revived lead, and then parted by aqua fortis : the gold powder, which the aqua fortis left undiffolved, differed fomewhat in appearance from that which commonly remains in parting, and being melted with a pure white fixt alkaline fait, the gold turned out pale and almoft like filver. I have not yet repeated this experiment, and .do not apprehend that it will bear any great ftrefs to be laid upon it. It is more probable that the gold retained a part of the extraneous matter, than that it fufFered itfelf any real change. The author obferves that the crucible, in which the gold pow- der was melted, had a green tinge round its edge, and that the alkaline fait was coloured yellow, but that the gold after the fufion was found to be of its full weight ; lb that a part of the gold might have been diffolved and retained by the fait, and an equal quantity of other matter remained blended with the reft of the gold.
The phofphorus of urine has been faid by fome to re- duce gold into a red mucilage. By digeftion or diftilla- tion in clofe veffels, as a retort and receiver, the phof- phorus appears to have no action on gold : this I relate on the authority of Mr. Margraff, whole experiments, in the Mifcellanea Berolinenjia for the year 1740, have'faved me the trouble of this examination : gold filings were digefted with thrice their weight of phofphorus for four weeks, and the fire being then increafed, part of the phofphorus fublimed, and part remained above the gold, in appearance like fine glafs : this laft grew moift on the admifTion of air, and diffolved in water, leaving the gold
Q^_ unchanged,
[ "4] unchanged. Nor does gold appear to be affected by the fumes of phofphorus fet on fire. But the flowers or faline acid matter remaining after the inflammable principle of the phofphorus has been confumed, and the microcofmic fait or effential fait of urine, which contains this acid, being melted along with gold in a moderately ftrong fire, manifeflly corrode the metal, and receive from it a purple
tinge.
SECT. VII.
Of the alloy of gold; and the methods of judging of the quan- tity of alloy it contains,, from the colour and weight.
I. Of the alloy of gold.
GOLD, in its pure ftate, is reckoned too foft and flexible for the common purpofes of coins and utenfils; and hence, to increafe its hardnefs, and render it better adapted to thefe ufes, it is allowed to be mixed with a certain quantity of inferiour metals ; which, in refpect to the gold, are called alloy, and whofe propor- tion is fettled by law. That thefe admixtures are of fo much advantage, in regard to the ufe of the metal, as has been commonly thought, may perhaps be questioned : for though fine gold may be fcratched or bent eafier than fuch as is alloyed, yet (as is obferved in a judicious EfTay on money and coins published in 1758) the alloyed ap- pears to be diminimed more by wearing than the fine.
There are cafes in which an admixture of alloy appears abfolutely neceflary, as particularly in gold plates for be- ing enamelled. If the plates are made of fine gold, they bend, and change their figure, in the heat requifite for making the enamel melt : the workmen find, that the quantity of alloy, permitted in coins, prevents this incon- venience y
[ "5 ]
venience ; and that a greater quantity cannot be employed, as it would occalion the gold to melt.
From, the account given in the fifth fection, of the effects of different metals upon gold, it appears, that filver and copper are the only ones fit for ferving as its alloy ; all the others debating its beauty, and greatly injuring or deftroying its malleability. Happily alfo thefe are the two metals which are oftenefl naturally blended with it in the mines, fo that the trouble and expence of refining it are thus greatly leffened. As the natural alloy is fre- quently in a fmaller proportion than the flandard quan- tity, as well as in a greater, it is plain that gold below the flandard finenefs may frequently be brought to the flan- dard, without any refining at all, by melting it with a due proportion of fuch as is above the flandard. In this view, the admitting of alloy, for all thofe intentions in which it is not injurious, is of manifeft advantage.
The degree of finenefs of gold, or the proportion of alloy it contains, is accounted by imaginary weights called carats. The whole mafs is conceived to be divided into twenty-four carats ; and fo many twenty-fourth parts as it contains of pure gold, it is called gold of fo many carats, or fo many carats fine. Thus gold of eighteen carats is a mixt, of which eighteen parts iu twenty-four are pure gold, and the other fix parts an inferiour metal ; and in like manner gold of twenty carats contains twenty parts of pure gold to four of the alloy. This is the common way of reckoning in Europe, and at the gold mines in the Spanifli weft: Indies, but with fome variation in the fubdivifion of the carat : among us, it is divided into four grains ; among the Germans, as appears from the treatifes of Ercker, Cramer, and other German aflayers, into twelve parts ; and by the French, according to Mr. Hellot, into thirty-two. The Chinefe reckon
Q_2 by
[ ii6 ]
by a different divlfion, called touches, of which the highefl number, or that which denotes pure gold, is one hun- dred ; fo that a hundred touches correfpond to our twenty- four carats, feventy-five touches to eighteen carats, fifty touches to twelve carats, and twenty-live to fix ; from whence any number of the one divifion may be eafily re- duced to the other.
The flandard gold of this kingdom is of twenty-two carats, that is, it confiils of twenty-two parts of fine gold and two of alloy : the alloy is more commonly a mixture of filver and copper, than either of them alone ; filver alone, in fo confiderable a quantity, giving too great a palencfs to the gold, and copper alone too great a rednefs. It is difficult for the aflayer, as we (hall fee hereafter, to determine with minute exactnefs the finenefs of a given mafs of gold ; and it is not to be expected that the workman, in every piece intended for llandard gold, mould be able to attain to the exact llandard proportion of alloy. In the Englilh coinage, which all pofiible pre- cautions are taken to keep as near as may be to the flandard, a certain latitude is allowed in this refpect, called the remedy for the mailer of the mint. Out of every fifteen pounds of gold coined at the mint, (accord- ing to the account publifhed by the learned Mr. Folkes, late prefident of the royal fociety, in his curious tables of Engiilh filver coins) fome pieces are taken at random, and depofited in a llrong box called the pix : at certain intervals, fometimes of one year and fometimes feveral years, the pix is opened at Weftminfter, in the prefence of the lord chancellor, the lords commifiioners of the treafury and others ; portions taken from the pieces of each coinage are melted together, and an allay made of the colledive mafs by a jury of the goldfmiths company. At this trial the mint-mailer is held excufable, though
the
[ »7]
the moneys be either too bafe or too light, provided the imperfection and deficiency together are lefs than the fixth part of a carat, which amounts to forty grains of fine gold on the pound of ftandard, or the one hundred and thirty-fecond part of the value. It is faid that this remedy is contained within as narrow limits, as any workers can reafonably be fuppofed to make themfelves anfwerable for.
The proportion of alloy in other nations is various. According to the aftays of fundry foreign coins, made at the Tower by the direction of Sir Ifaac Newton, and publifhed in Arbuthnot's tables of coins, the moidores of Portugal and their fubdivifions, and the old piftoles and doublons of Spain and Italy, are a little worfe than our ftandard, but within the latitude allowed to our own mint-mafter : the new louis d'or of France is about a fifth of a carat below that latitude. The ducats of Ger- many, Holland, Sweden and Denmark are a carat and a half better than ftandard ; and the fequin of Venice, the fineft of all the modern European coins, is a carat and ieven eighths better, or only an eighth of a carat worfe than fine gold.
The flandard gold of England was formerly of the fame finenefs with the Venetian fequin, to wit, twenty- three carats three grains and a half. Our prefent ftandard of twenty-two carats, was introduced in i C27, (about 270 years after the commencement of our gold coinage) for a particular fort of coin called crowns, of equal value with thofe of the fame name which have fince been formed of filver, and hence this kind of gold has been frequently diflinguifhed by the name of crown gold. Both the old and the new ftandard were continued to 1642, fince which period only the latter has been ufed. The remedy for the mafter of the mint has been almoft always
an
[ "8 ]
an eighth part of a carat for the old ftandard, and a fixth of a carat for the new.
A pound of ftandard gold, in the Englifh coinage, is cut into forty-four guineas and a half; fo that the mint price of fine gold is four pounds four fhillings and eleven pence halfpenny an ounce nearly. Lower than this the price of gold bullion cannot fall, the mint being always ready to exchange it on that footing for coin ■, but there are fundry caufes which may render it higher, and which it does not belong to the prefent purpofe to examine : the reader may confult on this fubject the eflay on money and coins already quoted, where he will meet with abun- dant fatisfadtion.
A pound of ftandard filver, containing eleven ounces two pennyweights of fine filver, is cut into fixty-two (hillings : whence the proportional value of fine gold to fine filver is, in our coinage, as fifteen and one fifth to one. Sir Ifaac Newton oblerves, in a reprefentation to the lords of the treafury in the year 17 17, that in the mints of Spain and Portugal, the value of gold is fixteen times that of filver ; but that in thofe countries, pay- ments in filver bearing generally a premium of fix per cent, the proportion may be looked upon as fixed by commerce at fifteen and one twenty-fifth to one : that in the .other parts of Europe, the value of gold is at moft fifteen, and in China and Japan but nine or ten times, that of filver : fo that gold is rated higher in England than in any other part of Europe, and higher in Europe than in the eaftern countries. Hence, in great meafure, arife the profits of exchanging gold for filver in one place, and re-exchanging them in another ; and hence the greater difparity between the relative quantities of gold and filver in one commercial nation than in another, that metal being brought in moft abundance which is
rated
[H9] rated higheft in proportion to the other, and that which is rated lowefl being drained away.
The alloy of gold, though it confifts of filver, and though its quantity be greater than that of the flandard proportion, down to certain limits, is reckoned as of no value : the value of the mats is eflimated only from the quantity of fine gold it contains ; and from this, for every carat that it is below the flandard finenefs, there is com- monly a deduction made, of four pence an ounce, for the charges of refining. A certain quantity of gold, mixed with filver, lofes alfo its own value, and is reckoned only as a part of the filver. There can be no fixt limits for the proportions in which the value of one metal is thus abforbed by the other ; as they mud depend on the ex- pence of feparating the two metals, in different places, by the operations there commonly practifed. The author of the effay on money and coins above mentioned fays he has been informed, that a pennyweight of gold in a pound of filver, or one part in two hundred and eighty- eight, is reckoned among us the leaft proportion of gold that will pay for refining, and that in this there is a profit only of about one farthing on the ounce.
II. Method of judging of the finenefs of gold from its colour,
Those who are accuftomed to the infpedtion of gold varioufly alloyed, can judge nearly, from the colour of any given mafs, the proportion of alloy it contains, pro- vided the fpecies of alloy is known. Different compofi- tions of gold with different proportions of the metals which it is commonly alloyed with, are formed into ob- long pieces, called needles, and kept in readinefs, for affifling in this examination, as ftandards of companion.
The proportions, in the compofition of the feveral
needles, are adjufled, in a regular feries, according to the
5 carat
[ 120 ]
carat weights as explained in the preceding article. The firfl: needle confifls of fine gold, or of twenty-four carats ; the fecond, of twenty-three carats and a half of fine gold and half a carat of alloy ; the third, of twenty-three carats of fine gold to one carat of alloy, and fo on, the gold diminifhing, and the alloy increafing, by half a carat in each needle, down to the twentieth carat : all below this are made at differences of whole carats, half a carat being fcarcely diftinguifhable by the colour of the mafs when the proportion of alloy is fo considerable. Some make the needles no lower than to twelve carats, that is, a mixture of equal parts of gold and alloy : others go as low as one carat, or one part of gold to twenty- three of alloy.
Four fets of thefe needles are commonly directed ; one, in which pure filver is ufed for the alloy ; another with a mixture of two parts of filver and one of copper; the third with a mixture of two parts of copper to one of filver ; and the fourth with equal parts of the two : to which fome add a fifth fet, with copper only, an alloy which fometimes occurs, though much more rarely than the others. If needles fo low as three or four carats can be of any ufe, it fhould feem to be only in the firfl fet : for in the others, the proportion of copper being large, the differences in colour of different forts of copper itfelf will be as great, as thofe which refult from very confiderable differences in the quantity of gold. When the copper is nearly equal in quantity to the gold, very little can be judged from the colour of the mafs.
In melting thefe compofitions, the utmoft care muff be taken, that no lofs may happen to any of the ingredients, fo as to alter the proportions of the mixtures. - The crucibles fhould be of the fmoothefl kind, that no particle of the metal may lodge about the fides. The copper
fhould
[ 121 ]
/hould be taken in one round lump, that its furface being as fmall as pofiible, it may be the lefs difpofed to be fcorified : that this may be the more effectually guarded againft, fome inflammable matter, as pitch, refin, or a little charcoal in fine powder, Should be added to the borax ufed as a flux ; and the fufion Should be expedi- tioufly performed, fo as that the copper may be no longer cxpofed to the fire, than is absolutely neceffarv for its due union with the others. The flux being previously melted in the crucible, and brought to a thong heat fuch as is fufficient for the melting of copper, the metals are to be dropt in : as foon as they appear perfectly fluid, the crucible, after being gently jogged or Shaken to promote the collection and fettling of the metal, is to be taken out of the fire, and fet on fome warm fupport, that the mixture may not cool too haftily. The fufion may be commodioufly performed alfo, the quantity of the metals employed for this ufe being commonly fmall, by placing them in a cavity made in a piece of charcoal, and direct- ing upon them, by a blow-pipe, the flame of a lamp : thofe who are accuftomed to the ufe of the blow-pipe, will find this method rather more fecure than that by the crucible, as well as more convenient and expeditious. In whatever manner the procefs is performed, the feveral maffes muSt be weighed after the fufion ; and if the leaft diminution has happened in any, freih mixtures mufc be prepared in their room.
The colours are bell examined by means of Strokes drawn with the metals on a particular kind of Stone, brought chiefly from Germany, and called from this ufe a touchftone ; the beSl fort of which is of a deep black colour, moderately hard, and of a Smooth but not poliShed furface. If it is too Smooth, foft gold will not eafily leave a mark upon it ; and if rough, the mark proves imper- il fe<5t
E *2* ]
feci. If very hard, the frequent cleaning of it from the marks, by rubbing it with tripoli or a piece of charcoal wetted with water, gives the furface too great a fmooth- nefs ; and if very foft, it is liable to be fcratched in the cleaning. In want of the proper kind of ftone, mode- rately fmooth pieces of flint are the beft fubftitutes : the more thefe approach in colour to the other, the better.
The piece of gold, to be examined, being well cleaned in fome convenient part of its furface, a ftroke is to be made with it on the ftone ; and another, clofe by it, with fuch of the touch-needles as appears to come the neareft to it in colour. If the colour of both, upon the flone, is exactly the fame, it is judged that the given mafs is of the fame finenefs with the needle : if different, another and another needle muft be tried, till fuch a one is found as exaftly correfponds to it. To do this readily, practice only can teach.
In making the flrokes, both the given piece, and the needle of comparifon, are to be rubbed feveral times back- wards and forwards upon the ftone, that the marks may be ftrong and full, not lefs than a quarter of an inch long, and about a tenth or an eighth of an inch broad : both marks are to be wetted before the examination of them, their colours being thus rendered more diftindT:. A ftroke, which has been drawn fome days, is never to be com- pared with a frefh one, as the colour may have fuffered an alteration from the air; the fine atoms, left upon the touchftone, being much more fufceptible of fuch altera- tions than the metal in the mafs. If the piece is fuppofed to be fuperficially heightened by art in its colour, that part of it, which the ftroke is deiigned to be made with, fhould be previoufly rubbed on another part of the ftone, or rather on a rougher kind of ftone than the common touchftones, that a frefh furface of the metal may be
expofed.
[ 1*3 ]
expofed. If it is fufpe&ed to be gilt with a thick coat of metal finer than the internal part, it mould be raifed with a graver, to fome depth, that the exteriour coat may be broken through : cutting the piece in two is a lefs certain way of difcovering this abufe ; the outer coat being frequently drawn along by the fheers or chifel, fo as to cover the divided parts.
The metallic compofitions, made to refemble gold in colour, are readily known by means of a drop or two of aqua fortis, which has no effect upon gold, but diffolves or difcharges the marks made by all its known imitations. That the touchftone may be able to fupport this trial, it becomes a neceffary character of it not to be corrofible by acids ; a character which fhews it to be effentially differ- ent from the marbles, whereof it is by many writers reckoned a fpecies. If gold is debafed by an admixture of any confiderable quantity of thefe compofitions, aqua fortis will in this cafe alfo difcharge fo much of the mark as was made by the bafe metal, and leave only that of the gold, which will now appear difcontinued or in fpecks. Silver and copper are in like manner eaten out from gold on the touchftone, and hence fome judgement may thus be formed of the finenefs of the metal from the proportion of the remaining gold to the vacuities.
Ercker obferves that hard gold appears on the touch- ftone lefs fine than it really is. It may be prefumed that this difference does not proceed from the fimple hard- nefs : but from the hardnefs being occafioned by an ad- mixture of fuch metallic bodies, as debafe the colour in a greater degree than an equal quantity of the common alloy. Silver and copper are the only metals ufually found mixed with gold whether in bullion or in coins ; and the only ones, whofe quantity is attempted to be judged of by this method of trial.
II 2 The
[ i24] The Chinefe are faid to be extremely expert in the ufe of the touchftone, To as to diftinguifh by it fo fmall a difference in the finenefs as half a touch, or a two hun- dredth part of the mixt. The touchftone, as I am in- formed, is the only teft, by which they regulate the fale of their gold to the European merchants ; and in thofe countries it is fubject to fewer difficulties than among us, on account of the uniformity of the alloy, which there is almoiT. always fiiver ; the leaft appearance of copper being ufed in the alloy gives a fufpicion of fraud. As an aflny of the gold is rarely permitted in that commerce, it behoves the European trader to be well pradtifed in this way of examination : by carefully attending to the above directions, and by accuftoming himfelf to compare the colours of a good fet of touch-needles, it is prefumed he will be able to avoid being impofed on, either in the touch itfelf, or by the abufes, faid to be fometimes committed, of covering the bar or ingot with a thick coat of finer metal than the interiour part, or of including maffes of bafe metal within it. A fet of needles may be prepared, for this ufe, with fiiver alloy, in the feries of the Chinefe touches ; .or the needles of the European account may be eafily accommodated to the Chinefe, by means of a table formed for that purpofe on the principles already 'explain- ed. It may be obierved, that the gold fhoes of China have a depreffion in the middle, from the fhrinking of the metal in its cooling, with a number of circular rings, like thofe on the balls of the fingers, but larger : I have been told, that when any other metallic mafs is included within, the fraud is difcoverable at fight, by the middle being elevated inflcad of depreffed, and the fides being uneven and knob- by ; but that the fame kind of fraud is fometimes praclifed in the gold bars, where it is not difcoverable by any ex- ternal mark.
III. Of
[ 1*5 ]
III. Of ejiimating thejinenefs of gold from its gravity.
The great excefs of the weight of gold, above that of the metals ufed for its alloy, affords another method of judging of the quantity of alloy or debafement, in any given mixture where the fpecies of alloy is known.
It may here be proper to caution the reader againft an error which has fometimes been fallen into, in computing the fpecific gravities of mixts from thofe of their ingredi- ents. If the gravity of one metal was nine, and that of another eighteen, it has been inadvertently reckoned that the gravity of a mixture of equal parts of the two would be the medium between nine and eighteen, or thirteen and a half. If by equal parts were meant equal bulks, this would indeed be the cafe ; but when the parts are taken by weight, as they are always underfiood to be in mix- tures of this kind, it is otherwife. For eighteen weights of the one metal, on being immerfed in water, will lofe two, and eighteen of the other will lofe one ; fo that thirty-fix of the mixt will lofe three : whence the fpecific gravity (which is found by dividing the weight in air by the lofs in water) inftead of being thirteen and a half, turns out but twelve.
Fine gold, as we have feen before, lofes in water one grain in every nineteen and three tenths nearly -, whereas fine filver lofes one grain in about eleven : from whence it is eafy to find the lofs of any number of grains of each, and confequently of any aflignable mixture of the two metals. Thus fifty grains of gold will lofe above two and a half, and fifty grains of (liver ibmewhat more than four and a half ; fo that a mixture of equal parts of the two will lofe above feven in a hundred, or one in fourteen. In like manner, a mixture of gold with half its weight of filver will be found to lofe one part in
fifteen
[ **° ]
fifteen and four tenths ; with a third of filver, one in fix- teen and two tenths ; with a fourth, one in fixteen and fzven tenths ; and with an eleventh of filver, which is the ftandard proportion of alloy, one in eighteen and one tenth. On this principle, the fpecific gravity, or propor- tional lofs in water, of gold alloyed with different quanti- ties of filver, copper, and mixtures of both, may be com- puted, and formed into tables, for abridging the trouble of calculation in the trial of given mafTes.
A perfon, faid to have made large profits in the purchase of gold from the Chinefe, mads ufe of this method for eftimating the finenefs of the gold. With the affiftance of tables, now in my hands, he could readily determine by the balance the quality of the whole compound, or the quantity of fine gold it contained ; without any danger of being impofed on by a fuperficial coat, however thick, or by any bafer materials, that were there known, being included within the mafs. The Chinefe alloy being, as' already obferved, almoft always filver, contributed not a little both to the facility and accuracy of the examination.
The above method of calculation fuppofes, that when the two metals are melted together, each of them frill retains its own proper gravity, as if they were joined only by fimple appofition. In mixtures of gold with filver, this appears to be the cafe, but in mixtures of it with other metals there are fome variations. Gold and copper, melted together, prove fpecifically lighter, or lofe a greater proportion of their weight in water, than if they were weighed feparately : Mr. Gellert, in a treatife of metallur- gic chemiftry published in 1750, obferves, that the fame thing happens in mixtures of gold with zinc, tin, and iron j but the reverfe in mixtures of it with lead and bifmuth. In fome of the compofitions of gold with platina, a dilatation of the volume (whence necefTarily
refi>^
[ 127 ]
rcfults a diminution of the fpecific gravity, or of the weight under an equal volume) is apparent to the eye ; the mixture, in its return from a fluid to a folid ftate, inflead of mrinking and hecoming concave, expanding and becoming convex. Platina, purified by folution in aqua regis and precipitation with quickfilver, being melted with twice its weight of fine gold, and the fufion repeated upwards of twelve times fucceffively, the furface of the mafs, when cold, was every time convex : the gold being gradually increafed, the convexity continued fenfible till the quantity of gold was upwards of ten times greater than that of the platina ; but when the gold was in very large proportion, the mixture fhrank and became con- cave like pure gold.
From thefe dilatations and contractions of volume, which happen in different mixtures, it may be prefumed, that the hydroftatic balance cannot difcover, with certainty, the exadt finenefs of gold, unlefs when- filver is the metal mixed with it. When the alloy is copper, fome allowance muft be made, not only for the diminution of gravity arifing from mixture, but likewife for the differences in the gravity of copper itfelf, that of fome forts being about nine, and that of others, though feemingly of equal fine- nefs, fcarce eight and three fourths. When gold is alloyed with both copper and filver, though the foregoing caufes had no influence, the quantity of gold could not be found to any exactnefs unlefs the proportions of copper and filver to one another were known.
SECT. VIIL. Of the ajfaying of geld.
TH E quantity of gold allowed for an affay is among us fix grains -, in France, as we learn from Hellot, nearly the fame ; in Germany, according to Schlutter,
about
[ 128 ]
about three times as much. It is evident that great nicety is requisite, in regard both to the weights, and the con- duct of every part of the operation, where the value of a large mafs of gold is to be determined by an experiment on lb fmall a quantity. Care mull: be taken alfo that the portion to be allayed is of equal finenefs with the reft of the mafs : we have already feen that the alloy may in fome cafes be unequally diflributed in fulion, and the upper and lower parts of the mixt prove different in richnefs : in large ingots or pieces of caft gold, a little fhould therefore be collected from the bottom, and a little from the top, fo as to obtain a mixt correfponding as nearly as may be to the quality of the whole mafs.
The allaying of gold confifts of two proceffes ; one for feparating it from filver, the other from bafe metals. The feparation of filver from gold is effected by aqua fortis, which diflblves the filver, and leaves the gold entire behind : but that this feparation may fucceed, it is neceffary that the mixt contain confiderably more filver than it does gold ; for otherwife the particles of filver are enveloped by the gold, and defended from the action of the acid. Some judgement mufr, therefore be previoufly formed of the contents of the mafs, from its colour on the touchflone or by the hydroflatic balance : if it appears to be about the flandard finenefs, it is melted with about twice its weight of filver : if it is finer, a little more filver is added, and if coarfer lefs ; fo that the alloy and addi- tional filver together may always amount to fomewhat more than twice the quantity of the gold. The writers on affaying in general direct three parts of filver to one of gold : but a lefs proportion is found to be fufficient, and more than is fufficient fhould never be ufed, for reafons which will appear in the fequel of the procefs.
i The
[ u9 ]
The leparation of bafe metals is effected, by keeping the mixt in fufion for fome time upon a cupel with the addition of lead. The lead by degrees turns to a fcoria or drofs, which rifing to the furface and liquefying, looks like oil, and is no longer mifcible with any metallic body- in its perfect metallic ftate : all the metals, filver and platina excepted, change into drofs, and feparate from the gold, along with the lead. As filver (lands this ope- ration, equally with gold itfelf, the gold and the addi- tional filver are fubmitted to it together : and indeed though there was no bafe metal to be feparated, the little quantities of gold and filver employed for an affay, are more commodioully mixed, form a neater bead, and with lefs danger of lofs, upon a cupel with a little lead, than by fufion in a crucible. It is obvious that both the filver and lead ought to be pure from any admixture of gold.
I. C up e Hat ion with lead.
The cupel is a fmall veffel, which abforbs metallic bodies when changed by fire into a fluid fcoria, but re- tains them fo long as they continue in their metallic ftate. One of the moft proper materials, for making a veffel of this quality, is the afhes of animal bones : there is fcarcely any other fubftance, which fo ftrongly refifts ve- hement fire, which fo readily imbibes metallic fcoria?, and which is fo little difpofed to be vitrefied by them. In want of thefe, fome make ufe of vegetable afhes, freed, by boiling in water, from their faline matter, which would occalion them to melt in the fire.
The bones, burnt to perfect whitenefs, fo as that no particle of coaly or inflammable matter may remain in them, and well wafhed from filth, are ground into mo- derately fine powder, which, in order to its being formed
S into
[ »3° 1
into cupels, is moiftened with juft fo much water as is fufficient to make it hold together when flrongly prefTed between the fingers ; fome direct glutinous liquids, as whites of eggs or gum water, in order to give the powder a greater tenacity : but the inflammable matter, however fmall in quantity, which accompanies thefe fluids, and which cannot eafily be burnt out from the internal part of the mafs, is apt to revive a part of the metallic fcoria that has been abforbed, and to occafion the vefTel to burfl or crack. The cupel is formed in a brafs ring, from three quarters of an inch to two inches in diameter, and not quite fo deep, placed upon fome fmooth fupport : the ring being filled with the moiftened powder, which is prefTed clofe with the fingers, a round-faced peftle, called a monk, is ftruck down into it with a few blows of a mallet, by which the mafs is made to cohere and rendered fufficiently compact, and a {hallow cavity formed in the middle : the figure of the cavity is nearly that of a portion of a fphere, that a fmall quantity of metal, melted in it, may run together into one bead. To make the cavity the fmoother, a little of the fame kind of afhes, levigated into an impalpable powder, and not moiftened, is commonly fprinkled on the furface, through a fmall fine fieve made for this ufe, and the monk again ftruck down upon it. The ring or mould is a little narrower at bottom than at top, fo that by preffing it down on fome of the dry pow- der fpread upon a table, the cupel is loofened and forced upwards a little, after which it is eafily pufhed out with the finger, and is then let to dry in a warm place free from duft.
Another kind of vefTel is required in cupellation, called
a muffle, formed of any clayey earth that will bear a ftrong
fire, with a flat bottom, arched at top, and open in the
front : it is made nearly of a femicylindrical figure, its
i length
[ «3« 1
length about double to its height, and the height fome- what lefs than the width of the bottom. This is placed upon a grate, in a proper furnace, fuch as that defcribed in page 12 and 35, with its mouth facing the door, and fitting as clofe to it as may be. The fur- nace being filled up with fuel, fome lighted charcoal is thrown on the top, and what fuel is afterwards neceffary is fupplied through a door above. One or more cupels are fet in the muffle, and being gradually heated by the fucceffive kindling of the fuel, they are kept red hot for fome time, that the moifture, which they ftrongly retain, may be completely diffipated ; for if any vapours fhould iffue from them after the metal is put in, they would occafion it to fputter, and a part of it to be thrown off in little drops. In the fides of the muffle are fome perpendicular flits, with a knob over the top of each to prevent any fmall pieces of coals or afhes from falling in. The door, or fome apertures made in it, being kept open, for the infpedlion of the cupels, frefh air enters into the muffle, and paffes off through thefe flits : by laying fome burning charcoal on an iron plate before the door, the air is heated before its admifllon ; and by re- moving the charcoal, or fupplying more, the heat in the cavity of the muffle may be fomewhat diminifhed or increafed, more fpeedily than can be effected by fup- prefling or exciting the fire in the furnace on the outfide of the muffle. This renewal of the air is neceffary alfo for promoting the fcorification of the lead.
The cupel being of a full red heat, the lead cart into a fmooth bullet that it may not fcratch or injure the furface, is laid lightly in the cavity : it immediately melts, and then the gold and filver are cautioufly introduced, either by means of a fmall iron ladle, or by wrapping them in paper and dropping them on the lead with a tongs. The
S 2 quantity
[ «i* I
quantity of lead (hould be at lead: three or four times that of the gold : if the gold is very impure, ten or twelve times its quantity will be neceflary. It is reckoned that copper requires for its fcorification about ten times its weight of lead ; that when copper and gold are mixed in equal quantities, the copper is lb much defended by the gold, as not to be feparable with lefs than twenty times its quantity of lead j and that when the copper is in very fmall proportion, as a twentieth or thirtieth part of the- gold and filver, upwards of fixty parts of lead are necef- fary for one of the copper. The cupel mud: always weigh at lead half as much as the lead and copper, for otherwife it will not be fufficient for receiving all the fcoria : there is little danger however of cupels being made too fmall for the quantity of a gold aflay.
The mixture being brought into thin fufion, the heat is to be regulated according to the appearances, and ia this confifts the principal nicety in the operation. If a various-coloured fkin rifes to the top, which liquefying runs off to the fides, and is there abforbed by the cupel, vifibly ftaining the parts it enters > if a frefti fcoria con- tinually fucceeds, and is abforbed nearly as fart as it is formed, only a fine circle of it remaining round the edge of the metal ; if the lead appears in gentle motion, and throws up a fume a little way from its furface ; the fire is of the proper degree, and the procefs goes on fuccefs- fully.
Such a fiery brightnefs of the cupel as prevents its coloured parts from being diftinguiflied, and the fumes ©f the lead rifing almoil up to the arch of the muffle,, are marks of too fhong a heat ; though it mud: be ob- fsrved, that the elevation of the fumes is not always in proportion to the degree of heat, for if the heat greatly exceeds the due limits, both the fumes and ebullition will:
entirely
[ T33 ]
entirely ceafe. In thefe circumftances, the fire muft ne- ceSTarily be diminished : for while the lead boils and fmokes vehemently, its fumes are apt to carry off fome part of the gold, the cupel is liable to crack from the hafty abforption of the fcoria, and part of the gold and Silver is divided into globules, which lying discontinued on the cupel after the procefs is finished, cannot eafily be collected : if there is no ebullition or fumes, the fcorifi- cation does not appear to go on. Too weak a heat is known by the dull rednefs of the cupel, by the fume not rifing from the furface of the lead, and the fcoria like bright drops in languid motion, or accumulated and grow- ing conliStent all over the metal. The form of the fur- face affords alfo an ufeful mark of the degree of heat ; the Stronger the fire the more convex is the furface, and the weaker the more flat : in this point however regard muft be had to the quantity of the metal, a large quantity being always flatter than a fmall one in an equal fire.
Towards the end of the procefs, the fire muft be in- creafed ; for greateft part of the fufible metal lead being now worked off, the gold and Silver will not continue melted in the heat that was fuSficient before. As the laft remains of the lead are feparating, the rainbow colours on the furface become more vivid, and varioufly interfeft one another with quick motions : foon after, difappearing all at once, a fudden luminous brightnefs of the button of gold and Silver Shews the procefs to be finished. The cupel is then drawn forwards, towards the mouth of the mufHe j and the button, as foon as grown fully folid, taken out.
It is obfervable, that when fine gold is thus cupelled with lead, it retains always a portion of the lead, very minute indeed, but Sufficient to render it pale and brittle. Ercker endeavours to prevent this inconvenience, by pat- fcing die cupel with the tongs, fo as to produce a tremu- lous
[ ^34] lous motion in the gold jufl: before it hardens ; but though this practice may be of ufe in fome cafes, it cannot pro- cure a total feparation of the lead, when the gold has no other admixture. Mr. SchefFer obferves, in the tranfac- tions of the Swedifh academy for the year 1752, that if the gold is mixed with a little copper, as one twenty- fourth of its weight, it parts in cupellation with all the lead, and retains nearly all the copper ; that if a fmall proportion of filver be fuperadded, greater than that of the copper, it contrariwife parts with the copper, and retains a little of the lead ; but that if the quantity of filver is nearly equal to, or greater than, that of the gold, as in the prefent procefs, both the copper and lead may be completely worked off, and only the gold and filver left.
The metal principally intended to be feparated by cu- pellation is copper. If the gold contained any tin, the procefs does not fucceed well, the tin calcining with a part of the lead, and rifing up in a powdery or fpongy mafs, which is apt to retain a part of the gold, and which cannot eafily be made to melt, the calx of tin being ex- tremely refractory. In this cafe, which rarely occurs to the affayer, the addition of a little iron filings is of ufe ; the tin having a flrong affinity to iron, and forming with it a new compound, which works off pretty freely with the lead.
Though the lead continues to emit fumes during the cupellation, yet little of its fubftance is diffipated. The cupel, after it has abibrbed the fcoria of the lead, weighs as much as the cupel and lead did at firfl: ; and even more, metallic bodies being found to gain weight in their fcori- fication. Several experiments of this kind, made at the Tower by the direction of lord Brouncker, are inferted in Sprat's hiftory of the royal fociety : when lead, or a mix- ture of lead and copper, were worked off in a cupel,
there
[ 135] there was always an increafe of weight, though not quite fo great as lead commonly acquires in the procefs of flow calcination.
II. Parting, with aquafortis.
Aqua fortis is an acid fpirit prepared from nitre by the intervention of other bodies. The principle, on which the extraction of the acid depends, has been but lately underftood 5 and hence in the earlier writers on tbele fubjects, as Ercker and Agricola, we meet with many in- congruous compositions ; fome containing powdered rlint, fand, and other ingredients which ferve only to take up room in the distilling veffel ; fome, quicklime, which can do no more, than to leffen the produce of acid, by ab- forbing and detaining a part of it ; and fome> common fait, whole acid, mingling with the nitrous, forms with it a menftruum of a quite different nature from that here required. What is wanted is the pure acid of nitre ; and the extrication of this, from the alkaline bafis of the nitre, is effected by the acid of vitriol.
Thofe, who prepare aqua fortis in quantity, ufe fre- quently green vitriol uncalcined or undried. This method is accompanied with two capital inconveniences: the wa- tery parts, which the vitriol abounds with, being expelled firft by the heat, together with a portion of the acid, this part of the vitriolic acid is thus fo far diluted, as not to ad fufficiently upon the nitre, and rifing over into the receiver, fouls the aqua fortis that fucceeds : at the fame time the vitriol, which at firft: liquefies in the veffel along with the nitre, concretes, on the diffipation of its watery moifture, into a hard mafs, from which the full quantity of acid cannot be forced out bv any violence of fire.
The more judicious workmen calcine the vitriol, before its mixture with the nitre, till it is freed from its phlegm,
and
[ 136 1 and will no longer liquefy in the fire. For this purpofe, a quantity of the vitriol may be put into an iron pot, Such as one of thofe which are ufed as land-pots for the por- table furnaces already deicribed : The vefTel is fet over a gentle fire, which is gradually increafed when the vitriol melts, till the matter thickens again, and acquires an aSh grey colour : the vitriol is to be constantly ftirred, till it becomes dry and powdery, and is then to be taken out whilst hot; for if Suffered to cool in the vefTel, with- out Stirring, it concretes fo hard, as Scarce to be beaten off with a hammer. Some calcine the vitriol in an earthen pan : the pan is at firSt about half Silled, and when this has funk down, and incruStated about the tides, more is added, till the vefTel is full, which muft afterwards be broken for getting the matter out.
Eight pounds of vitriol thus calcined to about four, and three pounds of nitre made likewife very dry, are to be reduced Separately into very Sine powder, and thoroughly mixed together. The mixture is to be put into the lame iron pot in which the vitriol was calcined, a Stone-ware head with a large glafs receiver fitted to it, and the junc- tures luted with Windfor loam, or a mixture of clay and fand, beaten up with fome cut tow, and moiStened with a Solution of fixt alkaline fait. In the receiver may be placed a pint of water, which will promote the condensa- tion of the nitrous fumes, without rendering the acid too dilute for the purpofes which it is here defigned for. During the distillation, there ariles a quantity of elaStic vapour, which muSt be Suffered to efcape, as it would otherwifc either force the luting, or burSt the receiver. The moSt convenient way of procuring an outlet for it, without endangering any lofs of the acid, appears to be, by making a hole in that part of the receiver which is to be placed uppermost, and inferting into it a Slender glafs
pipe,
[ 111]
pipe, four feet long or more, which is to be fecured by the fame lute as the juncture of the head and receiver : the pipe allows a free paflage to the air or unconfinable vapours, while little or nothing of the more fluggifh acid fumes will arife fo high. The hole in the receiver may be made, by parting on it a piece of thick leather, having a hole of the intended fize cut in it, then filling the cavity with emery, and turning round in it a fteel inftrument, with a hollow in the point for retaining the emery, till the glafs is worn through.
A gentle fire being made under the pot, ' the receiver foon grows warm, and appears covered with dewy drops, which are the more watery part of the mixture. The re- ceiver beginning to grow cool again, the fire is to be gra- dually increafed, till yellow or reddifh fumes appear, and when thefe ceafe, it is to be further urged by degrees, till the pot becomes red hot, and nothing more can be forced over.
This procefs is nearly the fame with that commonly followed in the way of bufinefs j differing little othervvile than in the fize of the veffels, and the quantity of the materials ufed at once. But as the efFedt of the vitriol depends wholly upon its acid, and as the acid of ful- phur is the fame, and is now to be procured at a very cheap rate, the moft advantageous way of making fpirit of nitre or aqua fortis is, to ufe the acid fpirit infiead of vitriol. Two pounds of oil of vitriol are to be mixed with an equal quantity of water, in a flone-ware vefiel, by a little at a time ; for if the acid is added all at once to the water, the mixture becomes fo hot, as to be apt to make the vefiel crack. Three pounds of nitre being put into a glafs retort, the mixture is to be poured on it through a long-necked funnel, that none of the vitriolic acid may adhere to the neck, and foul the nitrous fpirit as it di-
T ftil
[ 138 3 fills. The retort being placed in an iron pot on a little fand, and a receiver with its upright pipe luted on, the fire is to be gradually increafed, fo long as any red fumes arife, or any drops fall from the neck of the retort.
In either of thefe methods, a portion of the vitriolic acid frequently rifes along with the nitrous ; and frequently alio, as nitre has often an admixture of fea fait, the dift- illed fpirit partakes of marine acid. If a piece of lilvcr be put into this impure aqua fortis, fome part of the filver will be diffolved by the nitrous acid, but the other acids will immediately feize it, and form with it an indif- foluble white powder. For this ufe therefore, the aqua fortis muft be previoufly purified from thefe extraneous acids : and their property of uniting with and precipi- tating diffolved lilver affords a commodious and effectual means of its purification. A little folution of filver, al- ready made, is dropt at intervals into a quantity of the aqua fortis ; which, if it contains any marine or vitriolic acid, becomes inftantly milky : when the addition of a frefh drop or two of the folution occafions no further milkinefs or cloudinefs, we may be fure that thofe acids are completely abforbed by the iilver : the whole is fuf- fered to ftand till the white matter has perfectly fettled to the bottom, and the clear liquor is then poured off. The folution of filver, from its carrying down, and fixing as it were, the heterogene acids, is called by the workmen fixes.
Care muft be taken alio that the common water, made ufe of in the procefs of parting, have no impregnation that would impede the dilfolution of filver, or precipitate it when diffolved. Spring waters have generally fuch an impregnation, moft of them producing a ftrong milkinefs with folution of filver : rain water, collected with proper care, is for the mofl part fufficiently pure, as is likevvife
that
[ *39 ] that of moil rivers, though the preference is always to be given to fuch as has been diftilled. Thofe waters, which turn milky with folution of filver, may be made fit for this ufe, in the fame manner as the impure aqua fortis, by dropping in a little of the folution, till all the matter, that is capable of precipitating the filver, is fepa- rated : in this cafe, great care muft be taken, not to ufe more than is neceffary of the folution ; for fo much of the diffolved filver, as is added after the marine and vitri- olic acids have fatiated themfelves, will continue diffolved in the water ; and as the gold is at laft to be wafhed in the water, the moifture, that hence adheres to the gold, containing a proportionable part of the diffolved filver, will on drying leave it in the gold.
Befides the purity of the aqua fortis, a good deal of caution is requiiite in regard to its ftrength. The only fure mark of its due ftrength, for the parting affay, is its effect in the procefs itfelf ; and the manner of adjufting it will be the more intelligible after the procefs has been defcribed.
The little bead of gold and filver, remaining after the cupellation, is carefully hammered a little, and paffed feveral times between polifhed fteel rollers, fcrewed gra- dually clofer and clofer, till it is extended into a very thin plate, which is coiled up into a fpiral form, fo as that the feveral circumvolutions may not touch one ano- ther : by this means it lies in a fmall compafs, fo as to be covered by a quantity of aqua fortis fufficient for dif- folving the filver, and yet expofes a large furface to the aftion of the diffolvent. The metal is now and then nealed during the flatting ; and after this part of the procefs is finifhed, it is again made red hot, both to burn off any unctuous matter that may have adhered to it, and to foften the filver, which in this ftate is fuppofed
T 2 to
[ i4o ] to yield more eafily to the menftruum. The coiled plate is put into a fmall glafs vefiel, called a parting glafs, broad at the bottom and tapering upwards ; with twice its weight or more of the prepared aqua fortis. The veiTcl is fet in a fand-bath or other moderate heat, not exceeding that of boiling water; and its mouth flopt lightly with paper, or covered with a plate of glafs, fo as to keep out duft, without preventing the efcape of the elaftic vapours which rife during the diffolution. So long as the acid continues to acl:, the metal appears every where encompaffed with minute bubbles, which iffue from it in jets : the difappearance of thefe, or their uni- ting into a few large ones, is a mark that the acid is fa- tiated.
The coiled plate, after the filvcr is thus eaten out from it, fhould ftill retain its original form : for if the gold falls into powder, it can fcarcely be collected without the lofs of fome particles, which, though fmall in bulk, may amount to a confiderable proportion of the little quantity of metal made ufe of. This cohefion of the gold depends, partly, upon the quantity of filver not being fo large as to leave the golden particles difcontinued ; and partly on the action of the acid not being fo violent, as to divide and difunite the gold by its impetuous extraction of the filver. The fhength of the acid is to be afcertained by previous trials on gold and filver mixed together in the afiay pro- portions : if it is found to difunite the gold, it muft be lowered with water till it leaves the plates entire. Thefe trials are to be made exactly in the fame manner as the afiay procefs itfelf.
The liquor is poured off whilft hot, left fome of the diflblved filver fhould cryftallize in cooling upon the re- remaining gold. To the golden plate, which appears fpongy asid of a dark reddiih brown colour, a little frefh
aqua
[ r4i ]
aqua fortis is added, and heated more considerably than before, to extract what filver may ftill be left in it ; this may be repeated a fecond or a third time ; after which fome water is poured on, and renewed two or three times, to warn off the faline matter. The parting glafs being then full of water, a fmall gold vefTel (a filver one will do) is applied clofely on its top ; and both being nimbly inverted, and the parting glafs carefully raifed a little at one fide, the golden plate is wafhed down into the lower vefTel : if this laft is infufficient for receiving all the water, the glafs is to be lifted up a little, fo as that the thumb or a piece of ftifT paper can be applied to its orifice under the water, after which it may be removed without disturbing the liquor, or damaging the brittle plate. The water be- ing poured off, the plate is dried, and gradually heated till the gold refumes its proper colour, which happens foon after its becoming red hot. Some make ufe of an earthen crucible ; but in this cafe, fmall particles of the earth are apt to adhere imperceptibly to the gold, whence the allay becomes lefs certain.
If the gold, after having paffed through thefe opera- tions, is found to be of the fame weight as at firit, it is reputed nearly fine, but not entirely fo : for the aqua fortis leaves always in the gold a fmall portion of the filver, amounting commonly to above a three hundredth, and fometimes to a hundredth part of its weight ; whence, if the gold was at firfl fine, it will in this procefs receive an increafe. If it is required to determine exactly the pro- portion of this increafe, it may be done by fubmitting to the fame operation an equal quantity of gold known to be fine, mixed with the fame proportion of filver. The dif- ferences in the quantity of filver, thus left in the gold, are fuppofed to proceed from unheeded differences in the quality of the aqua fortis, particularly in its Strength ; fo
that
[ M*.]
that the aflayer ought to examine in this view each parcel of aqua fortis he employs", and deduct, from the weight of the gold remaining in the aflay, the proportion of filver which that particular aqua fortis is found to leave.
The affayer's report, of the finenefs of the gold which he has examined, expreffes the number of carats, with the odd grains or fourths of a carat, and quarters of thefe, by which it is finer or coarfer than the ftandard. Thus flan- dard gold being of twenty-two carats, that is, twenty- four parts of it lofing two in the purification ; if the mafs allayed lofe one lefs, it is reported B. i car. or one carat better ; and if it lofes one more, it is reported IVo. i car. or one carat worfe.
By thefe procefles gold is feparated from all the known metallic bodies, platina excepted : if any of this was mixed with it, nearly the whole of the platina will /till remain, not defiructible by the lead, and not difibluble by the aqua fortis. If the quantity of platina is confiderable, it may be diflinguifhed by the brittlenefs and ill colour of the mixt : but there are proportions of it, not fufficient to fenlibly affect the gold in thefe rcfpects, though they may neverthelefs deferve regard. If the gold is fufpected to be thus debafed, the abufe may be difcovered by the following means.
After the golden plate has been weighed, and its fine- nefs determined in the common method, a part of it is to be diifolved in a little aqua regia, and a filtered colourlefs iblution of any fixt alkaline fait gradually dropt into the liquor, fo long as it occafions any turbidnefs or precipita- tion : all the gold will fall to the bottom, with a part of the platina, but fo much of the platina will continue dif- folved, as to difcover itfelf by communicating a yellow tinge. This intention may be anfwered fiill more effec- tually by the aether, which imbibing the gold, and carry-
5 ing
[ 143 1 ing it up to the furface, leaves the full quantity of the pla- tina to fhew its colour in the acid liquor. By this me- thod, a moft minute proportion of platina may be diftin- guithed, a little of this metal giving a high colour to a fur- prizingly large quantity of the menftruum.
The method of preparing the aether is defcribed by many chemical writers, but the mod fafe, eafy, and certain procefs I have met with, is that which Dr. Morris has favoured the publick with in the medical obfervations and inquiries of a fociety of phyficians in London. Nine parts by weight of oil of vitriol are poured, by two ounces at a time, at intervals of a quarter of an hour, into eight parts by weight of rectified fpirit of wine, in a large ftone bottle : after {landing for a night, the mixture is decanted from one vefTel to another three or four times, and then conveyed, through a long-necked funnel, into a retort capable of containing three times the quantity. The retort is fet on a little fand in an iron pot, and more fand put round it up to the height of the mixture : a larger receiver being luted on, with fome flrips of wet bladder, a fmall hole is made in the luting with a pin : the fire is raifed fomewhat haftily, till an ebullition ac- companied with large bubbles is obferved in the mixture; after which the fire is to be entirely removed, the heat of the fand being fumcient for completing the diftillation. The diftilled liquor is put into a clean retort, with two or three ounces of fixt alkaline fait : about half the liquor is drawn off, by a very gentle heat, into a large receiver ; and this being fhaken with an equal quantity of pum^ water, the pure asther rifes immediately to the top.
SECT.
[ 1 44 3
SECT. IX.
Of the refining of gold ; and the feparatlon of final! portions of gold from other metals.
I. Separation of gold from bafe metals by tefling 'with lead.
TH E proceffes defcribed in the preceding fection, for the allaying of gold, are ufed alfo for refining it in the way of buiinefs ; with fuch variations, in the manner of conducting them, as the greater quantities operated upon, and the requilite cheapnefs and difpatch, render neceffary.
The teft is a large kind of cupel, formed of the fame materials with the fmall ones. Some of the German writers recommend, both for tefts and cupels, a fort of friable opake flone, called white fpath, which appears to be a fpecies of gypfum, or of the (tones from which plafter- of-paris is prepared. The fpath is directed to be calcined with a gentle fire, in a covered veffel, till the flight crack- ling, which happens at firft, has ceafed, and the flone has fallen in part into powder : the whole is then reduced into fubtile powder, which is paffed through a fine titve, and moiftened with fo much of a weak folution of green vitriol, as is fufficient for making it hold together : Gellert how- ever finds, that if the flone is of the proper kind, which can be known only by trials, calcination is not neceffary. Scheffer obferves, that thefe kinds of tefts are liable to foften or fall afunder in the fire, and that this inconvenience may be remedied, by mixing with the uncalcined flone fome- what lefs than equal its weight, as eight ninths of fuch as has been already ufed and is penetrated by the fcoria of the lead, taking only that part of the old teft which appears of a green-grey colour, and rejecting the red cruft on the top. Tefts or cupels made of the fpath are faid not to 5 require
f H5 1
require fo much caution, in nealing and heating them, as the common ones : it appears however from Scheffer's account, that they are lefs durable than thofe made of the allies of bones, though greatly fuperiour to thofe of wood afhes. Vegetable afhes, which ftand pretty well the teft- ingof filver, can fcarcely bear any great quantity of gold, this metal requiring a confiderably ftronger fire than the other : but bone afhes anfwer fo effectually, and are among us fo eafily procurable, that it is not needful for the re- finer to fearch for any other materials ; though thofe who work off large quantities of lead, in order to gain a little filver or gold contained in it, may poffibly, in places re- mote from populous cities, avail themfelves of fubflances fimilar to the fpath above mentioned.
The ten;, for its greater fecurity, is kept fixed in the mould in which it was formed j which is fometimes a fhallow veffel made of crucible earth or caft iron, more commonly an iron hoop, with three bars arched down- wards acrofs the bottom, about two inches deep, and of different widths, from three or four inches to fifteen or more, according to the quantity of metal to be tefted at once. The afhes or earthy powder, moiftened as for making cupels, are preffed down in the mould fo as to completely fill it or rife a little above the fides -, with care to make the mafs equally folid, and to put in at once, or at leaft after the bottom has been preffed clofe, as much of the matter as will be fufficient for the whole, for any additional quantity will not unite thoroughly with the reft, but be apt to part from it in the fire. The edges are pared fmooth, and a portion cut out from the middle with a bent knife, fo as to leave a proper cavity, which is fmoothed by ftrewing fome dry powder on the furface, and rolling on it a wooden or rather a glafs ball.
U The
[ 146 ] The procefs of tefling is often performed in the fame manner as that of cupellation : but where great quantities of bafe metal are to be worked off from a little gold, re- courfe is had to a more expeditious method, that of tefling before the bellows.
An oval teft is placed in a cavity, made in a hearth of a convenient height, and fome moillened fand or allies prefTed round it to keep it fteady : the nofe of a bellows is directed along its furface, in fuch a manner, that if allies are fprinkled in the cavity of the tell, the bellows may blow them completely out : fome have an iron plate fixed before the bellows, to direct the blaft downwards. To keep the furface of the tell from being injured in put- ting in the metal, fome clothes or pieces of paper are in- terpofed. The fuel confifls of billets of barked oak, laid on the fides of the tefl, with others laid crofswife on thefe : the bellows impels the flame on the metal, clears the fur- face of afh.es or fparks of coal, haftens the fcorification of the lead, and blows off the fcoria, as fall as it forms, to one end of the tell, where it runs out through a notch made for that purpofe. About two thirds of the fcorified lead may thus be collected, the reft being partly abforbed by the teft, and partly diflipated by the action of the bellows. Care mull be taken not to urge the blall too flrongly, left fome portion of the gold fliould be carried away by the fumes impetuoully forced off from the lead, and fome minute particles of it entangled and blown off with the fcorias.
In the hiftory of the French academy of fciences for the year 1727, a procefs is given for purifying a particular kind of debafed gold, which is faid to be quite brittle and intra&able, not to flow thin enough to be poured com- pletely out of the crucible, to appear on the furface of a livid hue, and which is fuppofed by du Fay and Hellot to
receive
[ 147 ] receive thefe imperfections from an admixture of emery. The gold is to be melted with equal its weight of bifmuth, and fo much poured out as is fluid enough to run : to the remainder is to be added equal its weight more of bif- muth, and this procedure repeated till the whole of the gold has run thin from the crucible. The mixt is put into a large thick cupel or teft, included in a mould of crucible earth ; by a fuitable fire, the bifmuth works off as lead does, leaving the gold fUll impure and covered with a livid fkin. For every eight ounces of gold, two or three ounces of lead are then to be added, and the fire continued till the lead is worked off: the gold is frill found not fufficiently fine, though lefs brittle and lefs livid than it was before. It is now to be melted in a forge or blaft furnace, and the flame impelled by the bellows upon the furface of the metal, till it begins to grow clear ; after which the repeated injection of fome mercury-fublimate, with a little borax towards the end, completes the purification.
I have not had an opportunity of examining this procefs, having never met with any gold that had the characters of impurity above defcribed, except fuch as was mixed with platina, which I did not find to be benefited by this treatment.
II. Separation of gold from Jiher by aqua for tis.
Parting with aqua fortis is one of the mofr. common operations, both for purifying gold from a little filver, and for extracting a little gold from a large proportion of filver. Frequently both intentions are anfwered at once : for when gold is thus to be purified, it requires, as we have already feen, an addition of filver, and fuch filver as contains gold is always preferred for this ufe ; fo that
U 2 the
[ i4« ]
the gold is got out from the filver, without any additional expence, in the fame operation by which the other gold is refined.
The moft defifable proportions of the two metals are, one part of gold to three of filver, or one part of gold in four of the mixt, whence the procefs is fometimes called quartation. When filver is added to gold, merely with a view to the purifying of the gold, thefe proportions fhould be kept to, as nearly as may be : for if the quan- tity of filver is lefs, the diflblution of it will not go on with fufficient difpatch ; and if greater, it will occafion an unneceffary expence of acid. Silver containing only a fmall portion of gold is frequently fubmitted to this ope- ration : but in fuch cafes, there are lefs expenfive me- thods, which will be defcribed in the fequel of this fec- ticn, for feparating great part of the filver, fo as to leave only a moderate quantity to be diflblved by the aqua fortis.
The metal, inftead of being flatted into plates, as for the parting affay, is reduced, with lefs trouble, into fmall grains, by melting it in a crucible, and pouring it into cold water. Some interpofe a number of twigs, or a birch broom, wetted, between it and the water, to divide the fluid metal into flender ftreams : Cramer defcribes a ma- chine for this purpofe, compofed of a wooden roller, laid acrofs the veflel of water, with its lower furface touching the water, covered all over with twigs, and made to turn by a handle. The granulation may be performed very fuccefsfully without any contrivance of this kind, by nimbly ftirring the water round, fo as to communicate to it a rapid circular motion, and pouring in the metal at one fide.
The granulated metal, with a fuitable quantity of aqua
fortis, is put into parting glaffes, which are commonly
about twelve inches high, feven inches wide at the bot-
5 torn,
[ '49 ] torn, and tapering upwards : feveral of thefe veflels are placed along an iron range covered with fand to the thick- nefs of about two inches. Great care fhould be taken that the glafs be well nealed, as equal as may be in thicknefs, and free from blebs, for otherwife it generally cracks in the procefs. The aqua fortis mutt be purified as for the parting afTay, though it is not needful to be fo curious in adjufling its ftrength : it ought to be flrong enough to begin to act fenfibly on filver in the cold, and not fo flrong as to act with violence.
A gentle fire is made under the fand bath, which is increafed or diminifhed, according as the difTolution ap- pears to proceed flowly or haftily. Care mufl be had not to apply too much heat at the beginning, the liquor being very liable to fwell up and run over the veflel ; but towards the end, when moil of the filver is diflbived, and the acid nearly fatiated, there is no danger of this accident. When the menflruum has ceafed to act, which is known by its growing clear, and no more air bubbles rifing in it, the folution is poured off"; and if, on flirring the remaining matter, any grains are perceived in it, a little more aqua fortis is added, to complete the ex- traction of the filver : fome ufe a fmooth wooden rod for the flirring, and what diflbived filver the wood imbibes, they recover by burning it. The blackifh mud, into which the gold is reduced by the difTolution of the filver from it, is wafhed five or fix times with water, and after- wards melted.
One of the principal inconveniencies, attending this operation, is, that the parting glafles are extremely liable to crack, from the contact not only of a cold body, but even of the hand. Schlutter reports, that in the Hunga- rian refineries, where great quantities*of gold-holding filver are parted, the glafles are fecured by a flrong coating, up
to
[ *5° ] to fuch a height, as not to hinder the operator from obferving how the diffolution goes on : fome quicklime, flaked with beer, and mixed with whites of eggs, is fpread on a linen cloth, which being wrapped round the glafs, a compofition of clay and hair is applied over it. He gives alfo a contrivance of his own, which he feems to have introduced into the works of the lower Hartz, for pre- ferving the diffolved filver, as well as the gold, when the glaffes happen to break, or the liquor to run over. His parting glaffes are fifteen inches high, ten or twelve inches wide at the bottom, and at the top about as wide as the mouth of a common bottle : for each of thefe he has a copper pan, twelve inches wide at bottom, fifteen at top, and ten in height, which ftands on a trevet, with fome charcoal under it : fome water is put into the pan, and two pieces of wood placed crofswife in its bottom, as a fupport for the glafs, to prevent it from hitting againft the copper. Into one of thefe glaffes he puts about eighty ounces of gold-holding filver, with twice as much aqua fortis, without danger of any lofs though the glafs fhould break : the heat may likewife be fpeedily diminifhed, if the acid fhould acT: too impetuoufly, by pouring cold water into the pan. Great care muft be taken in the addition of the cold water : it fhould be poured againft the fides of the pan, and ftirred with the reft, that it may be equally mixed before it reaches the glafs.
The filver is recovered from its folution by means of copper. The folution, diluted with water, being put into a copper veffel, or into a glafs one along with copper plates (the refiners ufe commonly a wooden bowl-difh lined with copper) the filver begins immediately to fepa- rate from the liquor in form of fine grey fcales or powder; a part of the copper "being diffolved in its place, fo as to tinge the fluid more and more of a blue colour. The
plates
[ '5* 1 plates are now and then fhaken, that luch part of the filver as is depofited upon them, may fall off and fettle to the bottom,' for otherwife the copper would be defended by it from the acid, and the precipitation of the filver would not go on. The digeftion is continued, till a frefh bright copper plate, kept for fome time in the hot liquor, is no longer obferved to contract any powdery matter upon the furface : the liquor is now poured off, the precipitated filver wafhed with frefh portions of boiling water, and afterwards melted with nitre to fcorify fuch particles of the copper as have fallen with it. Without the affiftance of heat, the precipitation is fcarcely completed in feven or eight days : Schlutter obferves, that the difpatch requi- fite for bufinefs, can fcarcely be obtained without a boilin°- heat. Great part of the filver indeed foon feparates, but in proportion as the acid loads itfelf with the copper, its action becomes more and more languid, and is at len<nh fo weak, that fome fmall portion of the filver is frequently at laft retained : this may be difcovered by adding to a portion of the folution a drop or two of a folution of com- mon fait : if the liquor contained any filver, it will grow turbid on this addition, and depofite the filver combined with the acid of the common fait. I have been fome- times furprized to find copper plates produce no precipita- tion at all in folution of filver : this happened when the menftruum was loaded with as much filver as it could be made to diflblve ; and on adding a drop or two of frefh acid, the precipitation went on as ufual.
From the folution of copper is prepared a blue pigment called verditer, by which the expence of refining is lef- fened. According to Dr. Merret's account, a quantity of whiting is put into a tub, the copper folution poured on it,, and the mixture flirred every day for fome hours together, till the liquor lofes its colour, the copper being depofited 5 in
[ '5* 3
in the whiting, and a part of the whiting taken up in its room. The liquor is then poured off, more of the copper folution added, and this repeated, till the matter appears of the due colour, after which it is lpread on large pieces of chalk, and laid in the fun to dry. Boyle obferves that the procefs often mifcarried, and that heating the liquor before it is poured on the whiting, has been found to con- tribute to its fuccefs. It is ftill however, as I am inform- ed, very apt to fail, in the hands of the moft fkilful work- men, the preparation, inflead of a fine blue, turning out of a dirty green.
From the liquor poured off in making verditer, confift- ino- of the nitrous acid faturated with the whiting, great part of the acid is recovered, by evaporating the watery part, and adding the remaining thick matter in the diftillation of the next quantity of aqua fortis. The acid may be extracted alfo either from the folution of copper or filver, and the metals recovered, the filver by fuiion without any addition, the copper by the addition of inflammable matter. The following procefs is given for this purpofein the French memoirs for the year 1728, as the communication of an experienced artift.
The copper folution is put into a copper veffel placed in a furnace, and evaporated to about half : the veffel is then filled up with more of the liquor, and the evapora- tion continued till the fumes begin to fmell of aqua fortis. The acid, being already fatiated with copper, does not adl on the veffel, or fo little, that du Fay fays he has leen one veffel bear almofl conftant work for near a year : the veffel fhould be raifed out of one plate, not formed of pieces, for if it is rivetted or foldered, the liquor will loon make its way through the junctures, as I have often obferved in this and other folutions of the fame kind. On decanting off the liquor, a portion of filver is found at
the
[ 153 J the bottom, which the acid had before retained, and which the long boiling has difengaged. Stone ware cu- curbits, coated with lute, are charged with the liquor to about two thirds of their height : the French have a kind of ware (pots de gra'uj which is faid to anfwer ex- tremely well for this ufe : fuch of our common ftone wares as I have formerly tried, frequently failed. Five or fix of thefe veffels are let into one furnace, up to the height of the liquor, and the bottoms made to reft on iron bars : the furnace is long and narrow, with a door at one end for putting in the fuel, and the chim- ney at the other. On each jar is luted a fione ware head with two fpouts and receivers. The fire is raifed to fuch a degree as to make the diftilktion go on with fufficient difpatch, with care only not to increafe it fo far, as to endanger the matter fwelling up into the head. When about three fourths have come over, the fire is fufFered to decay, and the veffels to cool, that the heads may be unluted, and more of the copper fo- lution poured in. This is repeated three or four times, till it is judged that the calx of copper in each jar rifes to about a fourth of its height, after which the fire is ftrongly urged, till the bottoms of the jars become red hot, and nothing more will diftil. This troublefome pro- cefs might be improved by fubftituting to the cucurbits the copper pan in which the evaporation is performed ; which may be converted into a diftilling veffel, by fitting to it a ftone-ware bread: and head, "in the fame manner as the copper breaft and head are fitted to the ftill in the firft plate : there is no occafion for two fpouts, as one, of a proper width, will fupply the place of the two : the cop- per pan fhould be let into the furnace almoft to its upper edge ; and the breaft fhould enter the pan nearly to the furface of the liquor. The aqua fortis thus ob-
X tained
[ <54 1 tained is perfectly free from any admixture of the vitri- olic or marine acids, fo as not to require the purifica- tion neceflary for the common forts : it is generally too itrong for the common purpofes of aqua fortis, and is therefore to be diluted with a proper quantity of pure water. The calx of copper may be revived, without much lofs, by melting it, in a fuitable furnace, in contact with the burning charcoal.
I'll. Purification of gold from fiver and bafe metals by
cementation*
Though the nitrous acid in its liquid ftate does not extract filver from gold, unlefs the quantity of filver greatly exceeds the gold ; yet in cementation, where the acid, refolved into fumes, is applied to the metal at the fame time flrongly heated, it attacks and corrodes a part of the filver though its proportion be very minute.
For this purpofe, nitre in fubftance is mixed with equal its weight of common green vitriol calcined, or dried as for the making of aqua fortis, and with twice its weight of powdered bricks ; the one to extricate its acid when fufiiciently heated, and the other to prevent the mixture from growing fluid in the fire. The metal is flatted into thin plates, which are furrounded and inter- laid with this powder, in a crucible, or in an earthen vclTel made on purpofe for this ufe called a cementing pot : the vefiel is clofely covered, and the juncture fecured with a mixture of foft clay and fand, or other proper clayey compofitions ; and being placed in any convenient furnace, a moderate heat is kept up for twelve or fixteen hours. The filver, and moll of the bafe metals along with it, are corroded by the nitrous vapour into a faline concrete, which partly adheres in the pores of the gold,.
ajid
1 '55 )
and is partly difperfed through the mixture. From the gold, the corroded iilver may be boiled out with water: and afterwards recovered from the liquor in the fame manner as from its folution in aqua fortis : from the mixture, it is much more difficultly extracted, by boiling the matter in melted lead, and afterwards working off the lead, into which the filver has thus transferred itfelf, upon a cupel or tefl. The quantity of filver however, which cementation is employed for feparating from gold, is commonly fo fmall as to be entirely disregarded.
The acid of fea fait, applied in the fame manner, corrodes all the metallic bodies except gold and platina. Hence either fea fait or nitre may be ufed in this procefs indifferently ; but they mufi: never be taken together, as fome have directed them to be, for the two acids in con- junction would diffolve the gold itfelf. The mixture of fea fait with the calcined vitriol and brick duff has been commonly called the regal cement, from gold, before the difcovery of platina, having been the only known metallic body that was capable of refilling it.
The gold plates cannot be wholly freed from their alloy by one operation either with the nitrous or marine cements, the vapours penetrating but a very little way into their fubflance. Hence for the effectual purification of gold by this method, the metal is to be remelted, flatted into plates, and again expofed to the fumes. The procefs indeed appears upon the whole to be incommodious, whether confidered as a method of purifying gold or of afcertaining its purity ; and accordingly, though once in much eiteem, it is now rarely pra&ifed. Its principal ufe is for extra&ing filver or bafe metals from the furface of gold, and thus giving fuperficial purity and high colour to alloyed or pale gold.
X 2 IV. Re-
[ 156 ]
IV. Refining of gold from fiver and bafe metals by
antimony.
Antimony confifts of a metallic fubftance united with fulphur. Sulphur has lefs affinity to the antimonial metal than to filver, copper, or the other metals com- monly mixed with gold : and accordingly when antimony and alloyed gold are melted together, the fulphur of the antimony unites with and fcorines the alloy of the gold, while the gold falls to the bottom blended with the metal of the antimony, which laft may be afterwards diffi- pated from it by fire.
One of the greatefl: difficulties in this procefs regards the crucibles, which are very liable to crack, and to be cor- roded by the fulphureous matter. SchefFer relates, that the crucibles he has found to be moil durable are thofe which had been fteeped feveral days in linfeed oil ; then cleared from the oil, fo as to remain only of fuch a degree of moiftnefs, that fome borax in fine powder may adhere and be fpread all over the inner furface ; and afterwards let by to dry flowly : in a crucible thus prepared he fays he can perform two or three hundred fufions. It is neverthelefs advifable, in a cafe where the vefTel is fo apt to fail, to take precautions for preferving its contents in cafe of fuch an accident ; as inferting it into another cru- cible, or placing a bafon underneath.
The gold being melted in the crucible, about twice its weight of powdered antimony is thrown upon it, in dif- ferent parcels, one parcel after another is melted j with tare to prevent the falling in of any pieces of charcoal,, which would occafion the antimony to fwell and froth up, fo as to be apt to run over the veflel : as this fwelling up cannot be wholly avoided, the crucible ought to be lar^e. If the gold is- very impure, or contains above one
fourth
f *S7 I
fourth its weight of alloy, the antimony is previoufly mixed with about a fourth part of common fulphur ; becaufe if antimony itfelf was ufed in fufficient quantity to fcorify all the alloy, the quantity of metal afforded by the anti- mony would be fo large as to render its diffipation from the gold extremely tedious. As foon as the mixture fparkles upon the furface, and appears perfectly fluid, it is poured into a conical brafs or iron mould, greafcd, and equably heated all over till it fmokes ; and the fupport, which the mould ftands on, is gently {truck or jogged fo as to produce a tremulous motion of the fluid matter, by which the fettling of the gold is promoted. When the matter is fixed or grown folid, it is eafdy got out, by inverting the mould and flriking a few blows on it with a hammer : the metallic mafs, which had fubfided into the lower part of the cone, if it does not feparate, in coming out, from the fulphureous fcoria, is beaten off by a flight blow.
This metallic mafs confifts of the gold, mixed, inflead of its former alloy, with the metal of the antimony. But as part of its alloy may have ftill efcaped the action of the fulphur ;. if the gold is required to be of a high purity, it muft be melted in the fame manner with the fame quan- tity of frefh antimony, and the proccfs repeated a third or even a fourth time. The gold does not receive much ad- dition from the antimony in thefe hit fufions; the antimo- nial metal uniting with the gold only in proportion as the fulphur of the antimony is abforbed from its proper metal by the alloy of the gold.
In order to leparate th« antimonial metal from the gold, the mixt is put into a flrong crucible, which being placed in a proper furnace, a fire is kept up, juft fufhcient to make the matter flow thin with a clear furflice. A blafl •of air being directed upon the mixt, by means of a beri: i c0PPec
[ ^ ] topper pipe applied to the nofe of a pair of double hand- bellows, the antimonial matter gradually exhales in copious white fumes, which ceafe on difcontinuing the blaft, and reappear upon renewing it. The fire muft from time to time be increafed ; for the mafs being cooled by the air impelled upon it, a hard fkin forms on the furface, and in this ftate the evaporation does not fucceed. When frefh fuel is to be fupplied, the crucible fhould be covered, that no pieces of the charcoal may fall in ; and as foon as the metal has come again into fufficient fufion, the blaft on its furface is to be repeated, till no more fumes can be made to rife from it, and the gold remains of a clear bright green colour without the lead cloudinefs.
If the procefs is continued, as it ought to be, to this point, there will be. no occafion for remelting the gold with nitre and borax, as the writers on thefe fubjefts ge- nerally direct. Where a little of the antimonial matter left in the gold, difcoverable by itspalenefs and brittlenefs, renders this laft operation neceffary, the nitre and borax mould be added by a little at a time, the matter being extremely apt to fwell up and run over the veffel : it is the more difpofed to fwell up as the antimonial remains are the more confiderable.
This procefs has been commonly fuppofed to afford the higheft purification of gold ; and hence antimony has been diftinguimed by the title of balneum Jolius foils, or the bath which gold alone can fupport, and by which it is warned from all its impurities. But befides that platina cannot thus be feparated from it; if gold containing filver be highly refined by antimony, it will ftill, on being dif- folved in aqua regia, difcover a little of the filver, which .had been defended by the gold from the action of the antimony. It may be obferved in general, that gold /cannot be fo effectually purified by fubftances which ope- rate
[ 159]
rate upon the alloy and not upon the gold, as by thofe which acT: on the gold itfelf and not on the alloy.
A fmall portion of the gold is commonly retained in the fulphureous fcorias, along with the filver or other metals with which it had been debafed. The fcoriae of the laft fufions, in which the fulphur and metal of the antimony have fuffered little feparation, are therefore to be referved for the fame purpofes again. From thofe of the firft, both the gold and filver may be recovered, by keeping them in fufion in a crucible, and blowing off the antimonial matter, in the manner above directed for diffi- pating it from gold.
V. Purification of gold from platina, Jilver and bafe met ah
by aqua regia.
Aqua regia, in diffolving gold, leaves behind what filver the gold had been mixed with ; and certain bodies, added to the folution, feparate the gold from it, without being able to feparate any metal befides ; fo that on this principle gold may be brought with eafe to its ultimate purity.
The gold, flatted into thin plates or reduced into grains, is to be put into about thrice its weight of moderately ftrong aqua fortis, and the veffel being fet in a gentle heat, a little fea fait is to be added : the dirTolution will immediately begin, with a confiderable effervefcence, and when the action ceafes, the addition of a little more fea fait will renew it : the injection of fea fait is to be con- tinued, by a little at a time, till the whole of the gold appears to be diflblved ; the quantity of fait requifite is generally about a third of the aqua fortis. The clear part of the folution is to be poured off, and the remainder palled through a double filter of paper i the undiffolved i matter
[ 160 ]
matter is to be wafhed two or three times with water in the filter, and this liquor poured to the reft.
For recovering the gold from the folution, Cramer directs two methods, diftilling off" the menftruum, and precipitating the gold by mercury. But in either of thefe ways we cannot be certain of having the gold pure. For though it has been previoufly cupelled with lead, yet, if it contained any platina, it will retain the whole of the piatina after the cupellation, and in fome circumftances, as we have already feen, it will retain alfo a little copper : both the platina and copper will difiblve with it in aqua regia ; mercury will precipitate the platina along with the gold; and the abftraction of the menftruum will leave with it both the platina and copper.
The purity of the gold is fecured by precipitation with common green vitriol. The vitriol is to be diftolved in cold water, the folution pafted through a filter, and added' in large quantity to the folution of gold : the quantity of vitriol, before its diflblution, fliould be ten or twelve times greater than that of the gold. As the precipitate falls flowly, the mixture is to be fet by for twenty four hours or more : the liquor, then become clear, though of a deep colour, is to be poured off"; the browniftt powder at the bottom, boiled in a little aqua fortis, then wafhed with water, and melted with the addition of a little nitre.
Gold, thus purified, appears to be perfectly fine; a point not obtainable by any other known means that can be praclifed in the way of bufinefs. Nor does the procefs feem to be fo expeniive as the imperfect one by aqua fortis ; for there, three parts or more of filver being added to one of gold, at leaft fix parts of aqua fortis are required for diflolving the filver ; whereas the gold, in the above procefs, may be difiblved by half that quantity of the menftruum: great part of the acid may like- wife
[ i6i ]
be recovered by diftillation from the liquor which remains after the gold has fallen.
Kunckel is the firft who has taken notice of this pre- cipitation by vitriol : but having ufed a vitriol which partook of copper as well as iron, he feems to have thought that the effect depended on the copper, and recommends the bluer!: and moll: venereal of the common forts of vitriol as the beft : accordingly moft of thofe, who have mentioned this procefs, direct blue vitriol or vitriol of copper. I have not found that blue vitriol pro- duces the leaft precipitation in folution of gold ; fo that, by this mifapprehenfion in regard to the nature of the precipitant, Kunckel's difcovery was rendered ufelefs, till Brandt happily obferved, that green vitriol produces the effect which had been afcribed to the blue.
VI. Extraction of a fmall portion of gold from a large quantity of fiver.
T H E moft advantageous method of feparating a fmall proportion of gold from a large one of filver appears to be by means of fulphur, which unites with and fcorifies the filver, without affecting the gold, But as fulphurated filver does not flow thin enough to fuffer the fmall particles of gold, diffufed through it, to reunite and fettle to the bottom, fome addition is neceflary for collecting and car^ rying them down.
In order to the commixture with the fulphur, fifty or lixty pounds of the mixt metal, or as much as a large crucible will receive, are melted at once, and reduced into grains by lading out the fluid matter, with a fmall crucible made red hot, and pouring it into cold water ftirred with a rapid circular motion. From an eighth to a fifth of the granulated metal, according as it is richer or poorer in gold, is referved -, and the reft well mingled with an
Y eighth
[ i&a ]
eighth of powdered fulphur -, which eafily adheres to the moifl grains. The grains, enveloped with the fulphur, are put again into the crucible, and the fire kept gentle for fome time, that the lilver, before it melts, may be thoroughly penetrated by the fulphur : if the fire was haftily urged, great part of the fulphur would be diffipated, without acting upon the metal.
If to fulphurated filver in fufion, pure filver be added, the latter falls to the bottom, and forms there a diftinct fluid, not mifcible with the other any more than water is with oil. The particles of gold, having no affinity to the fulphurated filver, join themlelves to the pure filver, where- ever they come in contact with it, and are thus trani- ferred from the former into the latter, more or lefs per- fectly according as the pure filver was more or lefs tho- roughly diffuied through the mixt. It is for this ufer that a part of the granulated matter was referved.
The fulphurated mafs being brought into perfect fufion,. and kept melted for near an hour, in a clofe covered crucible, one third of the referved grains is thrown in, and as foon as this is melted, the whole is well ftirred, that the frefli filver may be diftributed through the mixt to collect the gold from it : the ftirring is performed with a wooden rod : an iron one would be corroded by the fulphur, fo as to deprive the mixt of its due quantity of the fulphur, and likewife render thefubfequent purification of the filver more troublefome. The fufion being con- tinued an hour longer, another third of the unfulphurated grains is added, and an hour after this, the remainder ; after which the fufion is further continued for fome time,, the matter being ftirred at leaft every half hour from the beginning to the end, and the crucible kept clofely covered in the intervals.
The
[ i63 ] The fulphurated filver appears in fufion of a dark brownifh colour. After it has been kept melted for a certain time, a part of the fulphur having efcaped from the top, the furface becomes white, and fome bright drops of filver, about the fize of peas, are perceived on it. When this happens, which is commonly in about three hours after the laft addition of the referved grains, fooner or later according as the crucible has been lefs or more clofely covered, and the matter more or lefs ftirred, the fire muft be immediately difcontinued j for otherwife more and more of the filver, thus lofing its fulphur, would fubfide, and mingle with the part at the bottom in which the gold is collected. The whole is poured out into an iron mortar greafed and duly heated ; or if the quantity is too large to be fafely lifted at once, a part is firft laded out from the top with a fmall crucible, and the reft poured into the mortar. The gold, diffufed at firft through the whole mafs, is now found collected into a part of it at the bot- tom, amounting only to about as much as was referved unfulphurated. This part may be feparated from the ful- phurated filver above it by a chifel and hammer ■, or more perfectly, the furface of the lower mafs being generally rugged and unequal, by placing the whole mafs, with its bottom upwards, in a crucible : the fulphurated part quickly melts, leaving unmelted that which contains the gold, which may thus be completely feparated from the other. The fulphurated filver is affayed, by keeping a portion of it in fufion in an open crucible, till the fulphur is difii- pated, and then diffolving it in aqua fortis : if it fhould ftill be found to contain any gold, it is melted again, as much more unfulphurated filver added, as was employed in each of the former injections, and the fufion continued about an hour and a half.
Y 2 The
[ i64 J
The gold, thus collected into a part of the filver, nlay be further concentrated into a fmaller part, by granulating the mafs, and repeating the whole procefs. The opera- tion may be again and again repeated, till fo much of the filver is feparated, that the remainder may be parted by aqua fortis without too much expence.
The foregoing procefs, according to Schlutter, is prac- tifed at Rammeliberg in the lower Hartz. The prevail- ing metal in the ore of Rammeliberg is lead : the quan- tity of lead is at moll: forty pounds on a quintal or hundred pounds of the ore : the lead, worked off on a tell: or concave hearth, yields about a hundred and ten grains of filver, and the filver contains only a three hundred and eighty fourth part of gold : yet this little quantity of gold, amounting fcarcely to a third of a grain in a hundred pounds of the ore, is thus collected with profit. The author above mentioned confines this method of feparation to fuch filver as is poor in gold, and reckons parting with aqua fortis more advantageous where the gold amounts to above a fixty fourth of the filver : he advifes alfo not to attempt concentrating the gold too far, as a portion of it will always be taken up again by the filver. Mr. Scheffer however relate^, that he has by this method brought the gold aim oft to perfect finenefs, and that he has likewife collected all the gold which the filver contained ; the filver of the laft operations, which had taken up a por- tion of the gold, being referved to be worked over again with a frefh quantity of gold-holding filver. The fulphu- rated filver is purified by continuing it in fufion for fome time, with a large furface expofed to the air; the fulphur. gradually exhales, and leaves the filver entire : the particu far method of managing this operation will be given here- after, in the hiftory of filver.
Mr.
[ i65 ]
Mr. Eller, in the memoirs of the Berlin academy for the year 1747, defcribes a procefs fomewhat different from the foregoing ; which has been kept a fecret in a few hands ; and from which, he fays, Saxony has for feveral years reaped confiderable profits, by the feparation of gold from gilt laces.
The metal being granulated, a part of the grains is mixed with half their weight of litharge and an eighth of fandiver : this is called the precipitating mix- ture. The reft are mingled as above with powdered ful- phur, and expofed to a gradual fire till they are brought into fufion, which is known to be perfect., when the fur- face, on lifting up the cover of the crucible, appears coloured, chiefly with red and yellow, and the colours come and go, as if fomething attracted them. To every thirty-two ounces of the fulphurate'd metal, one ounce of the precipitating mixture is added, at three different times, at intervals of at leaft five or fix minutes ; after which,, the crucible is covered again, and the fufion continued feven minutes. Part of the matter being now laded out, the reft is poured off, till a metallic mafs mews itfelf at the bottom : this is eafily distinguishable, by its bright fiery afpect, from the fulphurated mixt, which is of a leaden brown colour.
The filver poured off, ftill containing a little gold, is treated in the fame manner a fecond and a third time, ex^ cept that in the third another precipitant muft be ufed ; for that employed in the two firft, being partly compofed of filver not freed from its gold, would add to. the fulphu- rated filver, now almoft entirely purified from gold, more gold. The precipitant is now a mixture of equal parts of pure copper and lead, melted together and reduced into grains. . If aqua fortis fhould ftill difcover a little gold in the filver, which never happens unlefs the filver contained
a large.:
[ 166 ]
a large quantity at firft, the precipitation is repeated a fourth time.
The Several metallic mafles, thus precipitated, are to be granulated, fulphurated, and further concentrated by the fame precipitants as before ; about an eighth part of lead being added, before the granulation, which is faid to ren- der the mixt more fufible, and promote the feparation of the gold. It is probable that bifmuth would anfwer better in this intention, as it forms both with the metals and with the fulphur a much more fufible compound than lead does, and poffeifes alfo all the other properties of lead that appear to be here required.
The matter which now fubfides is again granulated, mixed with a fixteenth of fulphur, kept in fufion about half an hour, the fcoria poured off, and the remaining mafs treated in the fame manner, without any precipitant, a fecond or a third time. The gold being now fo far con- centrated as to exhibit a yellow colour, the mafs is melted with a fixteenth of copper, then granulated, the grains mixed with a fixteenth of fulphur, cemented for fome time in a heat below ignition, after which, the fire being raifed, the matter is kept in fufion about fifteen minutes, and then poured out into a greafed and heated mould: the gold is found at the bottom, commonly of a brafs colour, and about the finenefs of eighteen carats : if too pale, the laft operation is repeated with half the quantity of copper; after which the gold is further refined by antimony as already defcribed.
VII. Extraction of gold from copper.
For feparating gold from large proportions of copper,
as from the gilt clippings left by the button-maker, fome
of our refiners have recourfe to cupellation or tefting with
lead. But the long continuance of fire and great quantity
5 of
[ *7 ] of lead neceffary for fcorifying fo large a proportion of copper, and the difficult revival of the copper from the fcoria, render the procefs too expenfive for the produce of gold.
Some have melted the gold-holding copper with about thrice its quantity of lead, and caft the mixture into cakes ; which being ranged in the higher part of a doping canal, and moderately heated, it was expected that the lead, melting out and running down from the copper, would carry the gold with it. But though this procefs fucceeds for the feparation of filver from copper, it is otherwife in regard to gold : if the copper contains both gold and filver, only the filver melts out with the lead, the gold re- maining behind in the unmelted mafs of copper.
Alonfo Barba gives a method which may in feveral- cafes be practicable to advantage. The copper is calcined with fulphur, till it becomes pulverable, and the powder ground with quickfilver in the fame manner as earthy or ftony bodies containing gold : the mercury imbibes the gold, without acting upon the calcined copper, which may now be warned off with water.
Many proceffes have been given for feparating gold from' copper by precipitating powders, which are compofed of very difcordant materials, as antimony, lead, fulphur, crocus of iron, mercury-fublimate, arfenic, vitriol, ver- degris, alum, nitre, fal ammoniac, wood ames, quick- lime. Though thefe proceffes, a number of which is collected by Swedenborg in the third volume of his regnum fubterraneum, are apparently fo injudicious, that an artift can have no inducement to make trial of them, yet they are not altogether without foundation : lead and fulphur, as Barba intimates, and as an experiment of Mr. Scheffer's has Ihewn more fatisfadtorily, are the ufeful ingredients ; and by means of thefe ingredients, gold may be extracted
from
[ i68 ] from copper more advantageoufly than by any other known method.
The way of procedure is as follows. Some litharge, or any other calx of lead, is melted with about an equal quantity of fulphur, with which it unites into a fparkling mafs, of a femimetallic afpect, and nearly of the fame quality with the common lead ores. The copper being brought into fufion, this mixture is thrown upon it, by a little at a time, till the quantity of lead becomes nearly equal to that of the copper : the copper abforbs the ful- phur from the lead, and the lead, being in a Hate of calx, remains uniformly blended with the fulphurated copper. A little powdered charcoal is then thrown in, and the whole well ftirred with an iron rod : the lead is immedi- ately revived into its metallic form, and finking to the bottom carries the gold with it j neither lead nor gold having any affinity to fulphurated copper. The effect is the fame when the copper is firft fulphurated, and the litharge or calx of lead added to this mixture j and pro- bably gold might be feparated in the fame manner from fulphurated iron.
VIII. Separation of gold from gilt works.
The folubility of gold and the indiffolubility of filver in aqua regia affords a principle on which gold may be feparated from the furface of filver ; and on this founda- tion different proceffes have been contrived, of which the two following appear to be the befl. Some powdered fal ammoniac, moiflened with aqua fortis into the con- fiftence of a parte, is fpread upon the gilt filver, and the piece heated, till the matter fmokes and becomes nearly dry : being then thrown into water, it is rubbed Y'ith a fcratch brufh, compofed of fine brafs wire bound 5 together,
1 169 ]
together, by which the gold eafily comes off. The other way is, by putting the gilt filver into common aqua regia, kept fo hot as nearly to boil, and turning the metal fre- quently till it becomes all over black : it is then to be warned with a little water, and rubbed with the (cratch brufh, to get off what gold the aqua regia may have left. This laft method appears preferable to the other ; as the fame aqua regia may be made to ferve repeatedly till it becomes faturated with the gold, after which the gold may be recovered pure by precipitation with folution of vitriol, as direclcd in the fifth article of this fection.
For feparating gold from gilt copper, fume direct a folution of borax to be applied on the gilt parts, but no where elfe, with a pencil, and a little powdered fulphur to be fprinkled on the places thus moiftened ; the princi- pal ufe of the folution of borax feems to be to make the fulphur adhere j the piece being then made red hot, and quenched in water, the gold is faid to be fo far loofened, as to be wiped off with a bruih. Others mix the fulphur with nitre and tartar, and form the mixture with vine- gar into a pafle, whicy is fpread upon the gilt parts.
Schlutter recommends mechanical means, as being ge- nerally the leaft expeniive, for feparating gold from the iurface both of filver and copper. If the gilt veffel is round, the gold is conveniently got off by turning it in a lathe and applying a proper tool, a fkin being placed underneath for receiving the (havings : he fays it is eafy to collect into two ounces of (havings all the gold of a gilt veffel weighing thrice as many pounds. Where the figure of the piece does not admit of this method, it is to be properly fixed, and fcrapers applied, of different kinds according to its fize and figure, fome large and furniih- ed with two handles, one at each end, others fmall and narrow for penetrating into depreffed parts. If the
Z gold
[ 1 7o ]
gold cannot be got off by either of thefe ways, the file mud be had recourfe to, which takes off more of the me- tal underneath than the turning tool or the fcraper, parti- cularly than the former. The gold fcrapings or filings may be purified from the filver or copper they contain by the methods delcribed in the preceding part of this fection.
The editors of the encyclopedie give a method of recover- ing the gold from wood that has been gilt on a water fize : this account is extracted from a memoir on the fame fubjedt prefented to the academy offciences by M. de Montamy. The gilt wood is deeped for a quarter of an hour, in a quantity of water, fufficient to cover it, made very hot : the fize being thus foftened, the wood is taken out, and fcrubbed, piece by piece, in a little warm water, with fhort ftiff brittle brufhes of different fizes, fome fmall for penetrating into the carvings, and others large for the greater dilpatch in flat pieces. The whole mixture of water, fize, gold, &c. is to be boiled to drynefs, the dry matter made red hot in a crucible to burn off the fize, and the remainder ground with mercury, either in a mortar,, or, where the quantity is large, in a mill, as defcribed hereafter, in the eleventh feiftion : fome clean fand is directed, to be added, which is faid to occafion the gold to be eafier laid hold of by the mercury.
SECT. X. Of tinging glafs and enamel by preparations of gold.
THE tinging of glafs and enamels by preparations of gold appears to have been firft attempted about the beginning of the laffc century, Libavius, whofe works compofe a valuable body of the chemical knowledge of his own time, conjectures, in one of his tracls entitled. Alchymiay printed in 1606, that the colour of the ruby- proceeds
[ i?« 1
proceeds from gold, and that gold diflblved and brought to rednefs might be made to communicate a like colour to factitious gems or glafs. Neri, in his art of glafs dated 1611, gives a procefs on this principle, which he fays was found to fucceed : he directs the gold to be diflblved in aqua regia, the menftruum to be evaporated or drawn off by diftillation, more aqua regia added, and the abftraction repeated five or fix times : the remaining matter is to be calcined till it becomes purple, and then mixed with a proper quantity of the fineft white or cryftal glafs. But though this procefs may be fuppofed to have fometimes ' proved fuccefsful, it doubtlefs very often mifcarried j info- much that the introduction of this defirable colour into glafs was very little known for many years after.
Glauber, in the fecond part of his phiiofophical fur- naces publifhed in 1648, gives another method of pro- ducing a red colour by gold in a matter which is of the vitreous kind, though not perfect glafs. When powdered flint or fand is well ground with four times its weight of fixt alkaline fait, the mixture melts in a moderately ftrong fire, and when cold looks like glafs, but on account of its over-proportion of alkaline fait it runs into a liquid ftate on being expofed to the air : on adding this liquor to folution of gold in aqua regia, the acid, which held the gold diflblved, unites with the alkali which held the flint diflblved, and the gold and flint precipitate together, in form of a yellow powder, which by calcination be- comes purple : this powder being mixed with three or four times its weight of the alkaline folution of flint, the mixture dried, and kept melted in a ftrong fire for an hour, a mafs is obtained, of a tranfparent ruby colour, and of a vitreous appearance, though ftill foluble in water or by the moifture of the air, on account of the redundance of fait.
Z 2 Boyle,
t I?2 ]
Boyle, in his treatife on the porofity of bodies, and in the appendix to his fceptical chymift publiflied in 1680, mentions an experiment, in which a like colour was introduced into glafs without fufion, A mixture of gold and mercury having been kept in digeftion for fome months, the fire was at laft immoderately increafed, info- much that the glafs burfl with a violent explofion : the lower part of the glafs was found tinged throughout of a tranfparent red colour, which feemed, he fays, to emulate that of a not common ruby.
About the fame time Caflius difcovered the precipita- tion of gold by tin, and that glafs might be tinged of a ruby colour by melting it with this precipitate. I can give no further account of his experiments, having never had the good fortune to meet with his treatife.
The procefs was foon after brought to perfection by Kunckel, who fays he prepared the ruby glafs in large quantity, and fold it for about forty millings an ounce ; and that he made a chalice of it for the elector of Cologn, weighing no lefs than twenty four pounds, a full inch thick, and of an uniform fine colour through- out. He has no where communicated the procefs he followed, but fome ufeful obfervations relating to it are difperfed through his writings : he fays, that one part of the precipitate by tin is fufficient to give a ruby colour to twelve hundred and eighty parts of glafs, and a ienfible rednefs to upwards of nineteen hundred parts : that the fuccefs is by no means conftant, and that after long practice, he flill frequently failed : that oftentimes the glafs comes out of the fire colourlefs as cryftal, and receives its ruby colour on being afterwards expofed to a fmoky flame, infomuch that he imagines the difcovery of the ruby glafs did not arife from fimply melting the £old precipitate with glafs, but from the fubfequent foft-
ening
I '73 ] ening and working of the glafs in the flame of a lamp, in the ufe of which Caffius was very converfant : that the addition of nitre and fal ammoniac calls forth the colour, and that the colour produced by fal ammoniac is more beautiful than that by nitre, but 'quickly difappears on a continuance of the fire.
Orfchal, in a treatife entitled fol fine vejle, gives a pro- cefs, by which he fays he obtained a very fine ruby. He directs the purple precipitate, made by tin, to be ground with fix times its quantity of Venice glafs in very fine powder, and this compound to be exquifitely mingled with the fritt or vitreous compofition to be tinged : his fritt confiits of equal parts of borax, nitre, and fixt alkaline fait, and four times as much calcined flint as of each of the falts j but in what proportion the gold precipitate is to be mixed with the fritt, and in what manner the fufion is to be performed, he does not mention. He reports that he had found the muddy matter, obtained in polifhing gold by a pumice flone, to impart likewife a ruby colour to glafs.
Grummet, who had been operator to Kunckel in making the red glafs, published a tract in oppofition both to him and Orfchal, under the title of fol non fine ve/le, in which he obferves, that the furnace ought to be fo con- flructed, that the operator may have full liberty of ex- amining the glafs in the fire, and of removing it as foon as it appears to have acquired the proper colour : he fays the enamellers obtain a ruby colour, by melting, with a large proportion of Venice glafs, the brownifh powder precipitated from folution of gold in aqua regia by fixt alkaline falts. But he imagines that the gold is nowife concerned in the production of the colour. Venice glafs, and moft. of the finer colourlefs kinds of glafs, have an ad- dition of manganefe, without which it would be very diffi- cult
[ i74] cult to render them perfectly void of colour : the man- ganefe communicates at firft a purplifli hue, which on continuing the fire difappears, and at the fame time fup- prefTes or difcharges any other tinge that the glafs may be impregnated with : the* addition of a little nitre revives the purplifli colour of the manganefe, and Grummet is of opinion that the colour with which glafs becomes tinged, by the admixture of preparations of gold, is no other than that of the manganefe extricated by the nitrous fait which the gold has retained in its precipitation. He affirms that the fame purplifh red colour will be obtained on melting Venice glafs with an eighth part of nitre, without any gold ■, that in a hundred repetitions of this experiment, it fcarcely fails once ; and that neither nitre nor the gold- precipitate were found to give any thing of the admired colour to thofe kinds of glafs which have no manganefe in their compolition.
The colours which manganefe imparts to glafs, it be- longs not to this place to examine : but that precipitates of gold will communicate, in certain circumftances, a purplifli red colour, I have feveral times experienced ; having myfelf tinged of this colour fritts compofed of calcined flint, nitre and borax, without the addition of any manganefe or of glafTes containing it. Though gold, diflblved in common aqua regia, exhibits its own yellow colour ; yet, when the menftruum is feparated by fire to a certain point, or when the gold is precipitated by tin, or when it is precipitated by alkaline falts and afterwards moderately heated, or when the gold is barely divided by mechanical means into fubtile powder, and expofed for fome time, in mixture with earthy bodies, to a flight heat, it affumes, in different circumftances, a violet colour, a purple, or a red verging to purple : in a ftrong fire, thefe colours vanifti, and the gold melts into a mafs of its ori-
i ginal
f >7S] ginal appearance. All thefe colours I have introduced into glafs by preparations of gold ; and I have found them to be nearly as perifhable in the fire when the coloured gold powder was thus diffufed through the glafs, as when expofed to the fire by itfelf : when the fire was raifed to any great degree, and the glafs made to flow thin, there was generally a button of revived gold collected at the bottom.
A folution of gold in aqua regia being infpifTated to drynefs in the bottom of a Florence flafk, and the heat further increafed till the gold refumed its proper colour, the lower part of the glafs was by this fimple procefs tinged purplifh : pieces of it being expofed to the flame of a lamp, they became in fome parts violet coloured, in fome of a bright purple, and in others purplifh red ; and the parts which in one pofition looked violet or purplifh, in another appeared red.
A colour nearly of the fame kind is imprefTed on glafs by gold leaf in fome electrical experiments ; a fact which we are obliged to Mr. Franklin for the firft knowledge of. A narrow ftrip of gold leaf being placed between two flips of glafs, with both the ends hanging out a little, and the glafs well tied round with filk thread, a firong electrical explofion is made to pafs through the gold leaf. On ex- amining the glafs, the gold leaf, he obfervt^, will be found miffing in feveral places, and inflead of it a reddifh flain on both the glaffes, exactly limilar on both in the min :efr. ftroke, though fometimes fpread a little wider than he breadth of the leaf : the ftain appears to have penetrated into the fubftance of the glafs, fo as to be protected by it from the action of aqua regia. I have had this experiment feveral times repeated with plate glafs, id found it tinged,, as above defcribed, in fome parts violet, in fome purplifh, and in fome reddifh ; the colours could not be fcraped off,
and
[ i?6 ]
and rcfifted aqua regia and fpirit of fait. If the elc&ric explofion is made very ftrong, the glafs commonly flies in pieces, with fuch force, that it is neceffary for the operator to have his face fkreened from them.
The preparation of gold which has been principally recommended for tinging glafs is Caffius's precipitate by folution of tin. To obtain this precipitate of the due colour, a good deal of care is necefTary both in diffolving the tin, and in diluting the folutions. A mixture of two parts of aqua fortis and one of fpirit of fait is fuppofed to be the beft menftruum for the tin : into this mixture fome fine block tin, granulated, is to be let fall, grain by grain, waiting till one grain is diffolved before another is dropt in, that the diffolution may go on (lowly, with- out any heat or difcharge of fumes. The gold is dif- folved in common aqua regia ; and a few drops of this folution being mixed with fome ounces of pure water, as many drops of the folution of tin are added. If the mixture changes immediately to a clear bright purpliih red colour, the due degree of dilution has been hit ; if the colour appears dull, a greater quantity of water muft be added foi the reft of.the folutions. After the mixture has depofited its red matter, and become clear, a little more of the tin folution is to be dropt in, for difcovering, and precipitating, any gold that may ftill remain in it : the liquor being then poured off, the precipitate is warned and dried.
Kunckel mentions another purple gold-powder, made nearly like that of Neri already mentioned, by infpifTating folution of gold to drynefs, abftracting from it frefli aqua regia three or four times till the matter looks almoft like oil, then precipitating with ftrong alkaline ley, and wafti- ing the precipitate with water. By diffolving this powder
i in
t m ]
in fplrit of fait, and precipitating again, it becomes, he fays, extremely fair, and in this ftate he directs it to be mixed with a due proportion of Venice glafs.
Hellot defcribes a preparation which in mixture with Venice glafs was found to give a beautiful purple ■enamel. Equal parts of folution of gold, and of folution of zinc in aqua regia, are mixed together; and a volatile fpirit, prepared from fal ammoniac by quicklime, added to the mixture in fufficient quantity to precipitate the two metals. The precipitate is to be gradually heated, till it acquires a violet colour : it does not fulminate, makin» only a flight dull decrepitation without any cf its particles flying about.
Though a purple, or a red colour approaching to that of the ruby, may by the foregoing means be baked upon glafs or enamels, and introduced into the mafs by fuiion, the way of equally diftufing fuch a colour through a quan- tity of fluid glafs is ftill a fecret.
I was once, many years ago, fortunate enough to fuc- ceed, at a glafs-houfe, in a fmall pot of glafs, of which a falver was blown of a fine ruby red : the tinging mat- ter was the precipitate of gold by tin ; the particulars of the procefs cannot now be recolledted. I have fince tried the remainder of the fame preparation, with common flint glafs, with green glafs, with various fritts compofed of flint, borax, pure fixt alkaline fait, nitre, fal ammoniac. When flint was ufed, it was feveral times made red hot, and quenched in water, to render it more eafdy pulvera- ble : both the flint and glafl'es were powdered in an iron mortar, and the powders well wafhed with diluted oil of vitriol, to extract fuch particles of iron as they might have worn off in the trituration ; the gold precipitate was ground with the other ingredients, in agate or glafs mor- tars ; its proportion was varied from an eighth part to
A a an
( '73 1
an eight hundredth of the vitreous materials ; and the fire was continued, in a wind furnace, from fix to thirty hours. All the glafTes came out considerably coloured ; fome of a deep dufky yellow ; fome of a fine pale tranfpa- rent yellow ; fome of a brown colour, greatly refem- bling that which the glafs mentioned in the following page acquired under a muffle : fome appeared yellowifh or brownifh when looked down upon, and of a purple- violet or reddifb purple when held between the eye and the light : fome had fpecks and veins of a fine red ; no one was either red or purple throughout. Several of thefe glafTes were melted again and again, by themfelves, and with the addition of more vitreous matter : fome were worked in the flame of a lamp : fome were laid in a mixture of powdered charcoal and foot, and made red hot in a clofe crucible ; and others being laid in the fame manner, the fire was increafed till they melted. The colours were by thefe means altered, but did not be- come uniform, or more approaching to the ruby colour than before : fome pieces, which had at firft very con- siderable fpecks of a ruby luftre, loft them on a repetition of the fufion.
At the fame time that thefe experiments were tried, the fame kinds of vitreous compofitions, mixed with dif- ferent metallic preparations, were expofed to the fire in different parts of the fame furnace, and were all found to receive beautiful and uniform colours, of which an ac- count will be given in their places. To what caufe the mifcarriage of thofe with gold was owing, whether the fuccefs, in regard to this metal, is influenced by the quan- tity of the matter, by the unfteadinefs of the heat in a fmall furnace, by the fufibility of the vitreous compofi- tion, by the metallic matter being ground with the ingre- dients before their expofure to the fire or added to them
in
[ *79}
in fulion, by the continuance of the fire, by the fluid mat- ter being kept unmoved or flirred with an iron rod, by the crucible being covered or open, or other like circum- stances, or whether the admixture of a little manganefe, though gold will certainly give a ruby colour without it, does not contribute to fecure the fuccefs, I have not yet difcovered. The proportion of the gold precipitate to the vitreous matter is perhaps of principal importance. Solution of gold, as we have feen already, produces no rednefs with tin unlefs diluted with a very large quantity of water, in which circumftance the whole mixture ac- quires that beautiful colour which we here want to trans- fer from the watery fluid into fluid glafs : it fhould feem therefore that the quantity of gold precipitate, for com- municating the admired colour to a certain volume of glafs, ought to be the fame with that, which communicated a like colour to an equal volume of water in the preci- pitation : a quantity extremely minute, and much lefs than that employed in any of my experiments.
I have lately been favoured with fome pieces of glafs, in greatefl part colourlefs, with one or two large red fpots, feveral fmall flreaks of violet, and fome of a light brownifh yellow. The perfon from whom I received them informs me that he had " found that in a heat not very Strong, under a muffle, the glafs becomes of an opake brown, and, if then poliihed, appears va- riegated like a fine pebble." I expofed a colourlefs piece to the flame of a lamp, impelled by a blow pipe, and on working it about, fometimes in the fmoke and fometimes in the flame, found it change to a true ruby red, perfectly transparent, and free from veins of any other colour. Another piece, kept for two hours under a clofe muffle, in fuch a heat as made it juft foft enough to bend and receive an impreflion, became on the Surface
A a 2 green,
I t*° 1 gfecn, brown and pale yellow in different parts, greatTy re- fembling the coat of fome pebbles : in this ftate, looked through againft the fun, it appeared of a beautiful ruby colour, and on breaking it, the internal part was found throughout of an uniform dark red when looked down upon, and of the ruby red when placed between the eye and the light. A large piece being continued under the muffle for four hours, its figure was found fcarcely al- tered, the coat was much thicker and beautifully veined with various colours, which were all left in a glorious red when the piece was viewed between the light.
All I have been able to learn in regard to the prepara- tion of this glafs k, that the quantity made at once is about fix hundred weight ; that the tinging matter is mixed with the vitreous materials before they are put into tiie melting pot, the mixture being brought to the glafs- houfe in tubs ; that the matter is not ftirred in fulion ; and that it is kept no longer in the fire than is necefiary for perfe&ing the glafs, which, as foon as fine, is caft into a kind of bricks. Some imagine that this glafs has no mixture of calx of lead, of which a large proportion is ufed in the composition of the common flint glafs,. and that the principal vitrefying ingredient is nitre : others judge it to be compofed of the fame materials as the common fort; its weight feeming to be a proof of its containing lead, for it is found to be nearly of the fame fpecific gravity with flint glafs, which is greater than that of the glafies made without lead in the proportion of above fix to five. This point I have determined in a more Satisfactory manner : four hundred grains of the glafs, made red hot and quenched in water, were reduced into powder, and mingled with about twice as much black flux and a little alkaline fait : the mixture being Vielted in a crucible, and the veftel fufTered to cool, a
i lump
[ i8i ]
lump of metal was found at the bottom, weighing ninety grains. The metal appeared to be fomewhat iliffer than pure lead, and experiments convinced me that it contained fome tin and a little gold.
SECT. XI.
The mineral hiflory of gold.
GO L D is found in its perfect metallic ftate ; fome- times in maffes of confiderable magnitude ; more frequently in dull or minute grains, intermingled among earths and fands ; or in little drops and veins, bedded in different coloured {tones, which ftrike fire with fteel, and are not foluble in aqua fortis. It is never debafed into a true ore, as other metals generally are, by the coalition of arfenical or fulphureous bodies ; though it is often very intimately combined in the compofition of fands and Hones, and blended, in fmall proportion, with the ores of other metals. It is fcarcely ever free from fome admix- ture of other metals, particularly of filver : Cramer ob- ferves, that fuch as is found loofe in earths and fands generally contains more filver than what is lodged in a folid matrix. To fuch an admixture is apparently owing the palenefs of fome kinds of gold : and probably the Malacaffean gold, faid by Flacourt, in his hiflory of Ma- dagafcar, to be not only paler but much more fufible than that of Europe, and which has hence been fuppofed by fome to be in its own nature diilincl, is no other than a mixture of gold with a certain quantity of filver : it is faid to be of considerably lefs value than the European gold, from which circumftance, omitted by Boyle and ethers who have quoted Flacourt's account, it may be prefumed that it is not regarded, upon the fpot, as being pure gold.
The
[ i32 ] The Brafils, the Spanifh Weft Indies, fome parts of the Eaft Indies and the coaft of Africa afford the largeft quan- tities of gold. Some parts of Europe alfo appear to be rich in this metal : the mines of the upper Hungary, which feem to be the moft considerable in this quarter of the world, have continued to yield gold for upwards of ten centuries.
Feru, Mexico, Chili, and other provinces of the Spanifh Weft Indies, abound with gold in a variety of forms. It is found both in the fands of rivers, and in mines ; inter- mixed with loofe earth, lodged in fifTures of rocks, and bedded in hard ftones ; at the furface of the earth, and at great depths ; in duft, in grains like the feeds of apples, called pepitas, and fometimes in maffes of an ex- traordinary fize. Reaumur reports that a piece was fhewn to the French academy, which was faid to weigh fifty- fix marcs, or four hundred and forty-eight ounces ; and Feuillee fays he faw one, in the cabinet of Antonio Por- tocarero, which weighed upwards of fixty-fix marcs or five hundred and twenty-eight ounces. Both thefe pieces were affayed, and found to be of different finenefs in dif- ferent parts of the mafs : the firft was in one part twenty- three carats and a half, in another twenty-three, and in another only twenty-two : the fecond was in one part twenty-two carats, in another twenty-one, and in another but feventeen and a half. It is, however, rare to meet with mafles of the weight even of an ounce : the largeft in the Britifti mufeum weighs but fifteen penny- weight. Notwithftanding the extenfive diffemination of it through thofe provinces, yet the quantity of the gold, in proportion to the earthy and ftony matter mixed with it, appears to be in general exceeding fmall. According to Frezier in his voyage to the fouth fea, and Captain
Bretagh's
[ i83 ]
Bretagh's account printed in Harris's collection, the common yield is no more than five or fix ounces of gold upon the caxon or fifty hundred pounds of the mineral : the richeft mines afford only ten or twelve ounces, and thole vviiich are but juft rich enough to pay the charges of woiking of them, yield only two ounces on that quantity. It may be obferved in general, that the quantity of gold in minerals is more variable than that of other metals in their ores, and the profits of a gold mine more precarious ; this me- tal not being formed into any regular veins, or uniformly diftributed through any particular kind of earth or ftone, but fcattered as it were here and there through different mineral bodies : when united with other metals in their ores, its proportion is by no means conftant, though in this cafe it is fubject to much lefs variation than when it is barely bedded in earths or ftones.
Of the fource of the gold of the Eaft Indies and of Africa, we know but little. From Cape-coafl on the coaft. of Guinea we receive yearly between two and three thoufand ounces of gold duff, which is fuppofed to be collected from the fands of rivers ; and fome European traders are faid to have been witneffes of the richnefs of the fands in certain parts of that coaft. In Hook's poft- humous papers an account is given of a perfon having met with great quantities of gold in the fands of one of the rivers, the fand feeming to grow richer and richer as he advanced further up : in fome places he fays he gained fixty-three grains of gold from five pounds of land, and he feems afterwards to have met with much more profitable fpots. Three or four hundred ounces, as I am informed, are collected yearly from the fands of the Gambia, and caft into bars at James-fort, one of our fettlements on that river. i
It
[ i84 ]
It is laid, that the gold dull from Africa, in its purcft flate, is from twenty-one to upwards of twenty-two carats fine ; but that the natives frequently mix with it filings of brafs. This admixture may be diitinguilhed by the hydroftatic balance, the fpecific gravity of brafs not being half fo great as that of the gold dull : in this way of examination great care muft be taken to make the water penetrate as perfectly as poflible into all the interfiices fo as to come into clofe contact with every particle. A little aqua fortis alio, poured on the mixture, will immedi- ately difcover the fraud, receiving, from the copper in the brafs, a blue tincture. It has been fufpedted, that if the gold be naturally alloyed with a little copper, this trial will prove fallacious, and that the natural copper will tinge the menftruum equally with that which is added by art : of this, however, there is no danger, the natural alloy not being in diftinct particles, but ditfufed through each mafs or particle of the gold, fo as to be covered by the gold, and protected from the action of the menftruum. There are feveral other means by which this abufe may be difcovered : if the dull be fpread thin on a piece of white paper, and moiflened with any volatile alkaline fpirit, as that of hartlhorn, of fal ammoniac, or of urine, the fpirit will in a few minutes diflblve fo much of the copper as to ftain the paper blue : ftale urine itfelf has a like effedl, in an inferiour degree ; and a folution of crude fal ammoniac, applied in the fame manner, produces a greenilh ftain.
There are fundry European rivers which roll particles of gold with their fands, in no great quantity, yet fuch, that the neighbouring inhabitants, at certain feafons, find their account in collecting them. M. de Reaumur, in an elTay in the French memoirs for 171 8, drawn up from materials furnifhed by the intendants of the feveral pro- vinces
[ i«5] vinces in parfuance of the orders of the duke of Orleans, gives an account of ten rivers or rivulets in the territories of France that have gold mixed with their fands in certain parts of their courfe : the Rhine, from Brifac down to Strafburg fparingly, from thence to Philipfburg more abundantly, and mofl of all fo between Fort Louis and Germefheim : the Rhone, in the pais de Gex, from the conflux of the Arve, from which it is fuppofed to receive, its gold, to about five leagues lower down : the rivulets of Ferriet and Benagues, which rife from the heights on the left hand of the defcent from Varilhere to Palmiers : the Ariege, aurigera, about Palmiers, below where it re- ceives the two foregoing rivulets : the Garonne, fome leagues from Touloufe, below where it receives the Ariege : the Salat, which rifes, as the Ariege, in the Pyreneans : the Ceze and the Gardon, which come from the Cevennes ; and the Doux in Franche-Compte. The lafl of thefe rivers is the poorest, the gold having hitherto been collected from it rather in the way of curiosity than in a lucrative view : the greatest quantities are obtained from the Rhine, not that this is really the richest, but on account of its fands being the mofl induflrioufly fearched, for fome of the others, particularly the Ceze and the Gardon, appear to be at leafl equal to it in richnefs. The quantity got from the Rhine, in an extent of near two leagues below Strafburg, is faid to amount to no more than four or five ounces in a year : this is the quantity brought to the ma- gistrates of Strafburg, who farm out the right of collect- ing it on condition of its being fold to them at a price considerably under its value, fo that it may be prefumed that a part is otherwife difpofed of, and that the quantity really obtained is considerably greater.
There are many other rivers reported to yield gold, as the Tagus, the Danube, the Elb, the Oder, the Inn, the
JB b Sala,
[ i86 ] Sala, &c. The Schwartz, in the county of Schwartzburg in upper Saxony, is faid to be rich in this metal, and its lands to be worked with great profit : Stahl mentions a piece found in it as broad as a middling bean, though not fo thick, and fuppofes that the Sala receives all its gold from this river ; the gold of the Sala being found only below where the Schwartz enters it, and being lefs plenti- ful and in fmaller grains. It is generally reckoned that the gold particles, in all auriferous rivers, are warned out by the ftream from fome rich beds, and afterwards reft or fettle where the current is languid, or in places where they efcape its force : from what particular fources the gold in different rivers proceeds, does not feem to have been examined.
The richeft parts of rivers, within the extent in which they roll gold, are thofe where their courfe is flow and interrupted, where they widen or change their direc- tion : the moft favourable feafon is when the water has fubfided after a flood. The appearance of the fand af- fords an ufeful mark for diftinguifhing the richeft fpots ; the gold being always moft plentiful where the fand is reddiih or blackifh, or of a colour fomewhat differ- ent from what is feen elfewhere ; not that the red or black fands have any natural connexion with the gold, but on account of their being more ponderous than the white, fo that the fame caufe, which determines the par- ticles of gold, determines thefe alfo to fublide. The black fand abounds with iron, being vigoroufly attracted by the magnet ; the red, viewed in a microfcope, appears, as Reaumur obferves, a beautiful congeries of coloured cryftals, imitating all the gems known to the jeweller, among which thofe of the ruby and hyacinth teints are the moft common, whence the reddifh colour of the fand to the naked eye ; the particles of gold are of irregular
figures,
[ *87]
figures, but conftantly flat, fmooth, and with the edges
rounded off.
Some gold has been difcovered in Britain, at different times, though hitherto in little quantity. Inflances are mentioned in Houghton's collections, from Camden, Sib- bald, and Gerard de Malines, of gold found in Scotland, about the head of the Clyde, in Crawford-moor ; and Boyle fays he had from fome part of Scotland divers large grains of gold, taken up near the furface of the earth, over a lead mine, one of which, clear from fpar, weighed two hundred and one grains. Mr. Boyle had alfo an Englifh tin ore, wherein there lay, in little cells, a number of fmall leaves or chips of gold : he obferves that though the tinmen, unable to feparate them to profit, ufually melted both metals together, he was affured that one perfon advantageoufly employed his children to pick out the gold from the ore fkilfully broken. Some earlier writers mention alfo gold found in the tin ores of Corn- wall, and about the beginning of the prefent century a patent is faid to have been obtained " for feparating gold " and filver from tin by precipitation in a reverberatory -" furnace with fome peculiar fluxes": what fuccefs this fcheme met with, I have not learnt. Mr. Borlafe, in his natural hiftory of Cornwall, gives an account of fome tin ore abounding with a yellow matter, which was taken by the workmen for mundic : fome bits of the yellow matter, one of which was a vein as large as a goofe-quill included in a flone about the fize of a walnut, produced, on being melted, an ounce of pure gold : he mentions feveral other pieces of confiderable bulk, and gives a figure of one, found in 1756, weighing three hundred and feventy-fix grains. It is fuppofed to be chiefly the flream tin, (that is the tin ore found in detached pieces on the fides of hills) which contains gold; that all of this fort
B b 2 contains
.- . •
[ i8» ]
contains more or lefs of it ; and that both the gold and tin ore have been brought from elfewhere by torrents of water, and depofited where they are now found like the gold fands of rivers.
Boyle conjectures, that befides the grains of gold which lie detached among funds, there may be many particles fo minute and clofely fixed to the fand, as not to be per- ceivable by the eye, or feparable by the common me- thods of wafliing or picking ; that many fmall portions of the metal may be incorporated alfo with the body of the fand, and that by fkilful management they might be ex- tracted. Experiments, he fays, confirmed him in this perfuafion : later experiments have verified it, and (hewn the exiftence of gold in fands to be even more extenfive than he feems to have apprehended. Many of the com- mon fands, particularly the yellow, red, black, and thofe of a black colour inclining to violet, appear to be rich in gold : Becher and Cramer prefume that there is no fand in nature entirely free from it. Hellot relates, that in eleven affays of one kind of fand, made by M. Lieberecht, by a procefs defcribed in the fequel of this fection, the yield of noble metal turned out conftantly from eight hundred and forty to eight hundred and forty-four grains on the quintal or 921600 grains, exclufive of what re- mained in the fcoriae, which were ftill found to be rich ; that different parcels of fand, taken up at no great dift- ance from one another, differed in degree of richnefs, fome having afforded above a thoufand grains, others only three hundred and fifty, and others yielding none by the treatment which had fucceeded fo well for the reft ; and that of the metal thus obtained from fands two thirds are commonly gold, and the reft filver. Yet notwithftanding the great richnefs of thefe fands, no means have hitherto been difcovered of availing ourfelves
of
[ i89] of the metal they contain, or of extracting it to advan- tage in the large way. Becher indeed undertook to obtain gold with profit from the common fea fands, and entered into engagements with the ftates of Holland for eflablifh- ing a mineral work on this foundation : but though expe- riments made on little quantities promifed very confider- able gain, and though one trial in large is likewife faid to have proved fuccefsful, yet, as he communicated the whole procefs to the commimoners appointed to examine the affair, and as he has fhewn that fuch a work might be carried on more advantageoufly in Holland than in other parts of Europe, its never having been profecuted in Hol- land affords a ftrong prefumption of its not being fuffi- ciently lucrative. The exigence of gold in fands is never- thelefs an interefting fact, at leafl to the philofopher, and further examination may perhaps find means of making it turn to account.
Though gold has been but lately difcovered,or expected, in rhefe minerals, in which it is fo common and fo plen- tiful, their unpromifing appearance having given little en- couragement to examine them ; there are others, whofe flattering colour has raifed great expectations, but which have not been found upon experiment to yield any gold. The yellow pyritae or marcafites, and other minerals of a golden colour, or containing gold-coloured fpecks, have by fome been regarded as ores or matrices of gold, and accordingly fubmitted to different operations, as fruitlefs as expenfive : their lofing their colour in the fire, or changing it to a yellow, foon difcovers that their tinging matter is not gold, but lulphur or a ferrugineous calx. There are indeed pyrita? which contain gold, and in quan- tity fufficient to deferve notice: Henckel, in the 12th chapter of his pyritologia, gives an account of fome affays of minerals of this kind brought from the Hungarian
mines*
L 190 1
mines, one of which yielded, on the centner or 1600 ounces, half an ounce of noble metal, of which one fifth was gold and the reft filver : from the fame quantity of ano- ther he obtained no lefs than fixty ounces of filver, and eight ounces and a half of gold : but the gold, as he obferves, is by no means proper to the pyrites, or an effential part of its compofition, having been only cafually intermixed, as it is in ftones and other minerals in the gold mines.
Many have been deceived alfo by fome of the talcs ; of which there is one fpecies, naturally of a grey colour, which in a moderately ftrong fire changes to a gold yel- low.; and another, naturally of a glittering gold colour, which receives no change from moderate fire : both thefe bodies have a further refemblance to gold, in imparting a deep yellow tincture to ftrong aqua regia. By repeated digeftion in frefh aqua regis, all the colouring matter may be extracted, and the earthy part left white : but the fo- lution yields no gold, and is found to be no other than a folution of iron. Reaumur obferves that fpangles of the yellow talc are frequent in the fands of fome rivers, and that they may readily be diftinguifhed from gold, which they have often been taken for, by viewing them with a magnifying glafs ; the gold particles found in rivers be- ing conftantly imooth, with the edges rounded, while the talky ones are rugged and fharp-edged.
II. Separation of gold from earthy and Jiony bodies by water.
Gold intermixed with earthy bodies in fmall particles or duft, is ieparated by waihing with water, which carries off the lighter earth, and leaves the more ponderous metal behind : the great gravity of gold renders it bet- ter adapted to this way of feparation than any of the other metals. There are fundry variations in the manner of
conduct-
r 19M
conducting the procefs, according to the quantity of the matter, the nature of the earthy body, and the conveni- ency of the place ; many of which are minutely defcribed by Agricola in his treatife de re metallic a : it will be fuffi- cient here to give a general idea of the manner of procedure, in an operation merely mechanical, and whofe fuccefs de- pends chiefly on manual dexterity acquired by practice.
Where the quantity of matter is fmall, it is laid by a little at a time in a round mallow dim called a buddle, or in an oblong vefTel like a boat, which being gently fhaken backward and forward, in a tub of water, the lighter part of the earth is taken up and warned off, leaving with the gold fuch fand or fmall ftones as the mixt con- tained. By dextroufly repeating the agitation, that the whole may acquire as it were a kind of fluidity, the me- talline particles fink to the bottom, and the fand or ftones are thrown up to the top, and may be removed by the hand.
At feveral of the gold mines of the Spanifh Weft In- dies, the gold is completely leparated by this fimple ope- ration. According to D' Ulloa's account, in his voyage to thofe parts, the eaxth, as it comes from the mine, is thrown into a refervoir, a ftream of water conducted on it, the whole ftirred- together, and the muddy water let off into another and another refervoir : what the water leaves in the firft refervoir, and what it depofits in the others, is taken up in trays, or buckets with two handles, and agitated in freih waters, with an uniform circular motion, till the gold is collected at the bottom.
M. de Reaumur, in the French memoirs for the year 171 8, gives a particular defcription of the method of warning the gold lands of the Rhine and fome other rivers. A board, five feet long, a foot and a half wide, with a ledge at each fide, and at one end, is laid aflope,
1 with
[ »9* ]
with this lafl end on the ground, and the other raifcd a foot and a half : acrofs the board are nailed three pieces of rough cloth, about a foot wide, and at diflances of a foot -, and at the upper end is placed a kind of baf- ket made of rods. The fand is (hovelled into the bafket : water thrown on it wafhes the fand through, the flones remaining : the earth and lighter parts of the fand are carried down to the bottom of the board, while the par- ticles of gold, and the heavy black and red fands already mentioned, are detained by the rough cloths, which, when they appear covered, fo as not to be able to detain more, are taken off and warned in a tub of water, then nailed on again, and the procefs continued till a proper quantity of this richer fand is obtained. In fome places, inftead of cloths, fkins with the hair or wool on are ufed ; and in others, notches are made acrofs the board. The richer fand thus detained is put into a veffel fome- what like a boat, which is gently maken in water, in the fame manner as the fan in winnowing corn, till the lighter grains rife to the top : thefe being carefully poured off with the water, the agitation is repeated fo long as any grains, of a different colour from the reft, are found to rife. No further reparation is to be expe&ed by this method, and the gold, of which fome particles begin now to be diftinguifhable by the eye, is extracted from the remaining matter by mercury, as defcribed in the fol- lowing article.
Gold bedded in ftones may frequently be feparated from great part of the ftony matter on the fame principle, the ftone being previoufly reduced to powder. In the large way it is beaten in mills, under water, by large wooden {tampers armed at the bottom with iron, an iron grating being commonly fixed at one fide of the trough or pit, through which the finer parts axe continually
wafhed
[ r93 J wafhed off by the water. When a little quantity is to be powdered in a mortar, it mould likewife be done by blows of the peftle, not by grinding : a blow only flat- tening the metalline particles, while triture wears and divides them, in part, to fuch tenuity, as not freely to fubfide in water. Stones of the hard flinty kind are pre- vioufly made red hot, and quenched in water; by which means they are rendered more eafily pulverable, and at the fame time many of the fmall particles of gold, melting in the fire, unite and form larger mafTes.
III. Separation of gold from earths andjiones by mercury.
Where the minutenefs of the particles of gold, and the weight of the matter with which they are intermixed, renders them infeparable by water, quicklilver is called in aid for imbibing and detaining the gold. The gold fands, freed by water, as in the foregoing article, from as much of the lighter matter, as can be warned off" by water with- out endangering a great lofs of the gold, are dried, and a fmall proportion, lefs than a hundredth part of their weight, of mercury poured on them : the whole is well kneaded up together, that the mercury may penetrate, as much as poffible, into all the interfaces between the grains ; it imbibes the atoms of gold it meets with, and the fand is afterwards wafhed off by water.
In the Spanifh Weft Indies, at thofe mines where the gold is bedded in ftones, and requires quickfilver for its feparation, the ftony matter is reduced to as fine pow- der as poffible, that every atom of the gold may be laid open to the mercury. The powder is foaked for fome time in a folution of common fait ; the mercury fqueezed in through a linen cloth, fo as to fall like dew all over the furface ; and the mixture being well ftirred and
C c kneaded,
[ 194 ] kneaded, a gentle heat is applied, by which the activity of the mercury is fo far increafed, that the incorporation of the gold with it, which in the cold requires about thirty days, is faid by J. Hernandez, in an eflay on thefe mines, to be effected by this method in five or fix days.
Alonfo Barba, in the third book of his art of metals* defcribes another method, which he lays he has practifed with great advantage : he puts the powder, with a fuitable quantity of mercury and water, into a deep copper veffel fixed in a furnace, and applies a fire fufficient to keep the water boiling : a fmall wooden mill afTifts the ebullition of the water in giving motion to the earthy powder, which continually rifing and falling down again, is brought into frequent contact with the mercury at the bottom, fo as to give out its gold to the mercury in as many hours, .as the common procefs without heat requires days.
When the gold is judged to be united with the mer- cury, the earthy powder is warned off by water, fo as to leave the amalgam clean. Where the mercury has been kneaded up with the powder, a considerable part of it is always divided into fuch minute globules as to be warned away along with the earth ; an inconvenience which in the other method does not happen or in a far lefs degree.
The gold dufl or filings difperfed through the fweepings of the goldfmiths fliops, are recovered alfo by amalgama- tion with mercury. Two broad iron bars, rounded at the ends, placed crofswife and fixed on an upright axis, are made to turn, by a handle at the top, on an iron plane fitted into the bottom of a tub. The fweepings being put into the tub with a quantity of mercury and water, the powder, pafling fucceflively under the iron bars, is ground and brought into contact with the quickfilver, which by degrees extracts the gold ; while the water, which after a certain time is fuffered to run off through a fmall
quill
[ *95 1 quill in the fide of the tub, carries with it the lighter earthy matter : frefh water is fupplied till all the earth is thus warned off. The operation might doubtlefs be expedited by the ufe of heat, as in the above procefs of Barba.
The gold being by thefe means transferred into the mercury, and the mixture warned clean, as much as may be of the mercury is preffed out through leather, and the remainder forced off by fire. To collect the exhaling mercury, a head and receiver are fitted to the iron pot in which the mafs is expofed to the fire : Barba advifes the pot to be lined with a mixture of clay and fand, to prevent the gold from adhering to or difiblving a part of the iron in cafe the fire mould be railed fo far as to make the gold melt.
It is fcarcely to be expected, that the greatefi: addrefs of the workman can collect, either by water or mercury, the whole of the gold difrufed through a large quantity of other matter ; at leafr, when the mercury is ufed, as it is in the large way, in fo fmall a proportion as a hundredth or a two hundredth part of the weight of the earthy pow- der. Reaumur, after having worked fome gold fand with mercury, in the method commonly praclifed by thofe who warn the fands of rivers, obtained, from the remaining fand, by treating it with a double quantity of mercury, near as much gold as he had done the firfr. time.
IV. Extraction of gold intimately combined in the compofition
of fands.
For extracting gold from the ferrugineous fands men- tioned in the preceding part of this fection, the fand is to be made red hot and quenched in water, and the ignition
C c 2 and
[ '96 ] and extin&ion repeated four times or oftcner. The colour changes, from yellow, red, or black, to a reddifh brown. In the firft and fecond heating, the fand yields a flight fmell, fomevvhat like that of garlick, a mark of its con- taining arfenic : at the third time, the arfenical fmell is fcarcely to be perceived, but on throwing into the crucible a little tallow, or other like inflammable matter, it becomes ftronger than before : this remarkable circumftance is by no means peculiar to thefe fands, for there are feveral ar- fenical minerals which give out little of their arfenic in the fire till fome inflammable matter is added. The fand, thus calcined, is mixed with twice its weight of granu- lated lead, and equal its weight of black flux, the mixture put into a crucible, and covered with fome fca fait dried over the fire till it has ceafed to crackle. The crucible is placed in a good blaft furnace, the fire ftrongly excited, and the matter ftirred from time to time with an iron rod : the fire muft be urged till the fcoria flows thin as water, which is known by the rod coming out almofl: clean, without the leafl: knob flicking to the end of it. The crucible is then fufFered to cool, and broken for getting out its contents : on the top is the common fait, in a diftinct cake; under this, a fhining black, compact, vitre- ous fcoria ; and at the bottom, a lump of lead, eafi'y fepa- rable from the fcoria. The gold, contained in the fand, is now transferred into the lead, and may be feparated by working off the lead upon a cupel or teft. Thefe fand 3 contain alfo filver, which here accompanies the gold, and which may be parted, after the cupellation, either by aqua fortis or aqua regia, according as one or the other metaL appears to prevail in the mixt. This is the procefs fol- lowed by M. Lieberecht in the aflays already given an, account of,
As.
[ *97 ]
As the black flux (which confifls of one part of nitre, and two or three of tartar, mixed together, and burnt in a covered veffel to a black alkaline coal) is apparently too expenfive to be employed for any confiderable quantities of the mineral ; its place may be fupplied by a mixture of potafh, or other fixt alkaline falts, with powdered charcoal. Four parts of potafh, three of charcoal, and thirty-two of lead, are fufficient for iixteen of the calcined fand. This mixture, as Hellot obferves, requires the fire to be con- tinued longer than the preceding to make the matter equally fluid ; but when it is made fo, the yield is found to be in both cafes alike.
The fame end may be obtained alio, by boiling the calcined fand in melted lead, without any inflammable or faline addition. For the vitreous matter, into which the lead is gradually converted, will dilTolve the earth, and the unvitreried part of the lead will receive and collect the gold; but a much larger quantity of lead is requifite in this than in the other ways.
In all the foregoing methods, though the quantity of gold obtained is confiderable, much ftill remains in the fcoria?, from want, probably, of a thorough commixture of them with the lead : for whatever degree of fluidity the fcoria; are brought to, the particles of gold are too minute tO fubfide by their own weight, and the lead can collect them only from thofe parts which it comes in contact with. It may therefore be prefumed, that the feparation will be the more complete, as the commixture of the lead is the more perfect. Lead cannot be mixed perfectly with fands but in a vitreous or femivitreous form. By grinding the calcined fand with litharge or other calces of lead, and expoling the mixture to a moderately ftrong fire, they may be intimately united into an uniform glaffy compound ; the fand being diffolved by the vitrefied leacb, 5 nearly
[ 198 ] nearly as fait is diffolved by water. On adding to this compound a little powdered charcoal, or barely ftirring it with an iron rod, the lead revives and falls to the bottom ; and as the find had thus been in contact with every part of it, the gold will probably be extracted from every part, or at leaft □ i rectually than by the
other methods. In this way it will be proper to mix fome alkaline fait with the litharge and fand ; partly, to pro- mote the diffolution of fuch portion of the fand as floats on the top of the ponderous metallic preparation ; and partly, to continue the fluidity of the fcoria after the re- vival of the lead. The crucible may be made of Stur- bridge clay, which fcems to be one of the beft, of the common materials, for refitting glafs of lead in fuiion. It appears to have been on a procefs of this kind, that Becher's propoiition to the States of Holland was founded, for extracting gold with profit from common fands ; it is plain from his account of this affair in his ?ninera arenaria, that he vitrefied the fand with glafs of lead, or litharge, and he exprefsly mentions in one place the precipitation of the lead from the glafs by iron : he ufed alio an addi- tion of filver, in great proportion, for imbibing the gold, and thus required a vail capital for eflablifhing a work in laree ; but where the lead is to be revived, the filver is rather detrimental than ufeful ; for the gold and filver contained in the fand are imbibed by the lead, and the additional filver occafions an enormous expence of aqua fortis for difiblving it in order to the feparation of the gold. Whether, with this reduction of the expence, of which Becher himfelf fecms to have had fome idea, the procefi might be practicable to advantage, or whether fome earthy bodies might not be an ufeful auxiliary for promoting the fufion of the gold fands, may deferve fur- ther enquiry : one kind of earth is frequently obferved to 5 bring
[ 199 1 bring another into fufion, though both are of themfelves tinfufible ; and the earthy parts of different ores are made fluid in the furnace of the fmelter by the addition of other earths.
V. Extraction of gold from the ores of other metals.
When gold is intimately combined with other metals in their ores, the ore is to be run down in the fame man- ner as the fame kind of ore without gold. The gold commonly melts out with the proper metal of the ore, from which it may be afterwards feparated by different procefTes according to the nature of the metal. There are grounds to believe that moft metallic bodies, as ex- tracted from their ores, contain generally a portion of gold, though rarely fufficient to bear the expences of its fepa- ration.
Zinc, arfenic, and mercury, are obtained from their ores by a kind of fublimation : hence if the ores of thefe contain gold, the gold is to be fought for, not in the me- tallic fubftance feparated, but in the remaining matter. There are fome other cafes alfo, in which the gold, in- ftead of accompanying the metal in its fufion, is thrown oft in the flag : but the ores and flags of this kingdom have been fo feldom examined for gold, that at prefent I can fay little fatisfa&ory on this fubject.
SECT. XII.
Of the alchemical hi/lory of gold.
O M E of the Greek writers, in the fourth and fifth :enturies, fpeak of an art, as being then known, of tranlmuting the bafer metals into gold ; and towards the end of the thirteenth century, when the learning of the eafl had been brought hither by the Arabians, the fame
pretentions
[ 200 ]
pretentions begun to fpread through Europe. It has been fuppofed that this art, called alchemy, was of Egyptian original ; and that, when the ancient Greek philofophcrs travelled into Egypt, they brought back fome of the alle- goric language of this Egyptian art ill underftood, which afterwards paffed into their mythology. This is all that is known with certainty, or can be admitted with any fhew of probability, about the origin of an art, whole hiitory and antiquity have been the object of elaborate re- fearches, and treated with a profufion of erudition.
Alchemy was the earliefl branch of chcmiflry con- fidered as a philofophic fcience. In the other parts of chemical knowledge, facts preceded reafoning or {pecula- tion ; but alchemy was originally fpeculative. Such of the alchemical writers as are reckoned of moft authority, as Geber, Hollandus, and others, declare, that we are not to hope for fuccefs in the practice of this work, without being previoufly well acquainted with the nature, eflence, and principles of metals ; whence they were produced in the mines ; whence they receive their increafe ; how and to what ftate they have a natural difpofition to be brought, and would have been brought if it had not been for fome impediment ; and what thefe impediments are.
The alchemifts fuppofed that nature, in all her works aiming at perfection, in producing metals aimed at gold : that the imperfect or bafe metals failed of being gold, either from a redundancy or deficiency of fome particular element in their compofition, or for want of fufficient coction, maturation, or depuration of their principles ; and that art could correct or remove thefe impediments, fo as to complete the work which nature had begun.
They fuppofed the general principles of metals to be chiefly two fubftances, to which they gave the names of mercury and fulphur ; and that of both thefe there were
different
[ 201 J
different kinds, particularly of the latter ; which they ad- mitted as many varieties of as there are metals ; and which, in gold, they held to be pure, red, fixt and incombuftible, but of different qualities in the other metals. In thefe points there is no perfect, uniformity among the different alchemical philofophers, which indeed could not be ex- peeled in hypothefes on fo abftrufe a fubjecT:, v/here ex- perience had afforded fo little light : fome have added a faline, fome an earthy, and others an arfenical principle.
They fuppofed that the pure mercurial, fulphureous, or other principles of which they imagined gold to be com- pofed, were contained, feparately, in certain other bodies. Thefe principles' therefore they endeavoured to collect, and to concoct and incorporate by long digeflions. In the many volumes written profeffedly to teach the procefs at full length, the fubjects, from which the golden feeds are to be obtained, are wrapt in impenetrable obfeurity : thus much is plain, that the fuppofed adepts in this myfterious fcience do not all make choice of the fame fubjecls, or work upon them in the fame manner, their practice being probably adapted to their particular hypothefes.
By thus conjoining the principles of gold, if they cculd be fo procured and conjoined, it might be ex- pected that gold would be produced. But the alchemifts pretend to a product of a higher order, called the elixir, the medicine for metals, the tincture, the philofophers flone ; which, by being projected on a large quantity of any of the inferiour metals in fufion, fliould change them into fine gold ; which, being laid on a plate of lilvcr, copper, or iron, and moderately heated, fhould fink into the metal, and change into gold all the parts it was applied on ; which, on being properly treated witfi pure gold, fhould change the gold into a fubftance of the fame nature and virtue with itfelf, fo as thus to be
D d fufceptible
[ 202 ]
lufccptible of perpetual multiplication ; and which, by continued coction, mould have its power more and more exalted, fo as to be able to tranfmute greater and greater quantities of the inferiour metals, infomuch that, ac- cording to its different degrees of perfection, one part of it fhall be fuflicicnt for ten parts, a hundred parts, a thou- fand parts, twenty thoufand parts, two hundred and fe- venty two thoufand three hundred and thirty parts of bafe metal.
If thefe pretentions were propofed 33 matter of fpecula- tion only, I believe no one, who has at all confidered the nature of metals, could hefitate in pronouncing them abfurd : they are inconfulent even with the alchemical philofophy itfelf. But they are endeavoured to be fup- ported by arguments of another kind ; by hiflorical rela- tions of the actual tranfmutation of all the common me- tals, ftrongly attefted, not only by the alchemifts them- felves whofe teilimony might be thought fufpicious, but likewife by perfons fuppofed to have been entirely un- prejudiced, who had been carnally favoured with fome quantity of the tranfmuting powder, or who had been witneffes to its aftoniming operation and to the immenfe riches it had procured.
In regard to thefe narratives I fhall only remark, that at a time when the tranfmutability of metals was generally believed, the circumftances of certain princes might render it an advantage to them to be thought to have fuch an inexhauftible refource for wealth : that fome perfons who, by methods which it was their intereft to conceal, had acquired fudden riches, might, in this art, find plaufible means of giving an account for them ; that many of the fuppofed alchemifts have been con- victed, and perhaps many others guilty, of impoflure ; the gold, which they pretended to have made, having
fometimes
[ 2o3 1 fometimes been previoufly concealed in the crucible, or in the materials, or at the end of the rod, with which the matter was (lirred in fufion, and fometimes intro- duced into the crucible by a confederate, when the fur- nace was covered, through an aperture communicating with another apartment. So many frauds and juggling ar- tifices are known to have been praclifed on this occafion, that the evidence of a fpectator can be of no force ; and perhaps thofe, who were more than fpe&ators, were too much interefted to be admitted as evidences.
I am very far, however, from cenfuring as impofiors all thofe who have declared themfelves convinced, from their own experiments, of the tranfmutability of bafe metals into gold. Many experiments have been alledged, in which bafe metals were made to yield fome portion of gold, and in which gold, treated with certain addi- tions, received an increafe : though the quantity ob- tained was rarely fuch as to bear the charges of the pro- cefs, it is reckoned fuflicient at leaft, in a philofophic view, to demonstrate the aclual tranfmutation, into gold, of a fubflance which before was not gold. Moft of thefe experiments are free from fufpicion of any fraudulent de- fign ; but there are ftrong reafons to fufpect that the authors have been themfelves deceived by fallacious ap- pearances.
Gold, as we have already feen, is now known to be far more frequent in metals and other minerals, than it was formerly fuppofed to be ; and there is little wonder, if men of warm imaginations, biaffed by a favourite hy- pothefis, have been led to believe that they produced gold when they extracted it from materials in which it was not imagined to prseexift. We have feen alfo, in a foregoing feclion of this efiay, that the common method of parting filver from gold by aqua fortis does not com-
D d 2 pletely
[ 204 ]
pletely feparate the filver ; and it has often happened, that when fine gold, melted with filver, was fubmitted to cer- tain operations, and afterwards parted, the portion of filver which the aqua fortis left in the gold, has been taken for an augmentation of the gold itfelf. Of this I knew a remarkable inftance in a procefs which was fome years ago referred to my examination, and whofe fuccefs was averred to have been fuch, that it was offered as a very lucrative operation, and a considerable price demanded for the communication of it : the gold, after it had paffed through the tedious procefs that was to enrich it, and had been parted from the filver by weak aqua fortis, ac- cording to the directions, weighed indeed notably more than the pure gold employed : but on reducing it to its former purity by folution in aqua regis, I found it reduced at the fame time to its former quantity. It is probable that many of thofe, who have been moft fanguine in • their expectations of gain from alchemical operations, had no other foundation than thefe mifunderfiood kinds of experiments ; which having once perfuaded them that they could make gold, they might naturally conclude that it could be made in any quantity. I fhall only further remark on this head, that if a part, of the fub- ftance of any metal was by any operation tranfmutable into gold, a part more would be in like manner tranfmu- table by a repetition of the operation, and this fucceffive- ly, fo long as any part of the metal remained entire and perfect, or fo long as it retained the properties which it had at firfr. : a procefs of this kind would be decifive, but fuch a procefs has not. yet been made known.
The deflruction of gold is affirmed by the alchemifls
to be more difficult than its production. This point alio
has been eagerly profecuted, not only on account of its
being interefting as an object of philofophy, but. on ac~
5 count
f Z0S ]
count likewife of fome advantages expected to refu'.i from it, many having perfuaded themfelves that its de- frruction or refolution would afford the fure foundation for its artificial production. Divers experiments have been alledged, in which gold is faid to have been deftroyed, or changed into a matter which was no longer gold, or refolved into diffimilar principles : but in thefe experiments, as in thofe of its production, there was probably fome deception ; and many of them, as related by the authors themfelves,. are apparently incon- clufive.
Mr. Boyle gives an account of a procefs, by which he imagines part of the fubflance of gold to have been tranfmuted into filver. Into rectified butter of antimony, that is a folution of the metallic part of antimony in the marine acid, he poured as much fpirit of nitre as was fufficient to precipitate the metal, and having diililled off all that would come over in a fmart fire, he returned the liquor on the antimonial powder, and abftxacted it again : of this menflruum, which is a kind of aqua regia, he had a great opinion, and gave it the name of menflruum peracutiim. Some gold was melted with three or four times its weight of copper, the copper extracted by aqua fortis, and the remaining gold powder being brought to its due colour by heat, a large proportion of the men- flruum was poured on it : the gold difTolved flowly and quietly, and there remained at the bottom of the glafs a confiderable quantity of white powder. The folution of gold being abftracted, and the gold again reduced to a body and difTolved a fecond time, it yielded more of the white powder, but not fo much as at firft. On melting thefe powders with borax, he obtained a white metal, which yielded to the hammer, and which, on being dii- iblved in aqua fortis, fhewed itfelf, by the odious bitter-
nefs
[ 206 3
nefs it produced, to be true filver. He fays, that even with good aqua regis, he could obtain from the very beft gold fome little quantity of fuch a white powder, but in fo very fmall proportion, that he never had enough at once to make him think it worth while to profecute fuch trials.
It were to be wifhed that the ingenuous author had been more careful in ascertaining the purity of the gold made ufe of in thefe experiments, and noted the exact quantity of filver obtained from it. Gold parted from filver or copper by aqua fortis, is by no means to be looked upon as being pure : nor is there perhaps any other method, as yet known, of perfedly purifying it from filver, than that by which the filver was feparated in the above expe- riments ; the diffolution in aqua regia being in effect no other than a purification of the gold. Even aqua regia, when made with an under proportion of marine acid, will not produce a complete feparation ; this imperfect aqua regia taking up, along with the gold, a little filver, feparable by .a fecond diffolution.
Mr. Boyle has given an account alfo of a very extraor- dinary experiment, under the title of the degradation of gold by an anti-elixir, which was publifhed in his own life time, and fince reprinted in 1739. The known cha- racter of the author, the earnefl defire he has fliewn in all his writings for the difcovery of truth and the expofing of falfe pretences, have not only rendered the fact un- questionable, but likewife induced many to adopt the confequences which he thought might be drawn from it j and to regard it as a proof of the real alterability of gold, and as Strongly favouring the alchemical doctrine of the tranfmutability of metals. I fliall therefore infert the ac- count of the experiment in the author's words, and Sub- join a few remarks; lamenting that it is not in my power 5 to
[ 20/ ] to enquire more fatisfactorily.into Co curious a fact by a repetition of the experiment. The fubflance, by which the apparent degradation was produced, was a powder of unknown compofition, communicated to Mr. Boyle by a ftranger, and its quantity not fufficient for more than a fingle trial. There was fo very little of it, that he could fcarce fee the colour of it, fave that, as far as could be judged, it was of a darkifli red : the quantity was eftimated at an eighth or a tenth part of a grain. The gold had been formerly Englifh coin, and to be fure of its goodnefs, he caufed it to be, by one whom he ufually employed, cupelled with lead, and afterwards quarted with refined filver and purged aqua fortis. Two drams of this gold, thus purified, were weighed out, and put into a new crucible firft carefully nealed, and the gold being brought into fufion, without addition^ he put into the well-melted metal the little parcel of powder with his own hand, continuing the fire about a quarter of an hour, that the powder might have time to difTufe itfelf every way into the metal : the well-melted- gold was then poured out into another crucible, which had been gradually heated to prevent its cracking. But though, from the firft fufion of the metal to the pouring out, it had turned in the crucible like ordinary gold, fave that once, as the afliftant obferved, it looked for two or three minutes almoit like an opal : yet when the mat- ter was grown cold, though it appeared on the balance that it had not loft any thing of its weight ; yet, inflead of fine gold, there was a lump of metal of a dirty colour, and as it were overcaft with a thin coat almoft like half vitrefied litharge : to one fide of the crucible there ftuck a little globule of metal, that looked not at all yellowilh, but like coarfe filver ; and the bottom of the crucible was overlaid with a vitrefied fubftance, whereof part was
of
[ 203 ]
of a tranfparent yellow, and the other of a deep brown inclining to red : in this vitrefied fubftance there were plainly perceived flicking at lead five or fix globules that looked more like impure filver than pure gold. Having rubbed this odd metal upon a good touchftone, whereon there was likewife rubbed a piece of coined filver and a piece of coined gold, the mark left by it on the ftone was notoriously more like that of the filver than of the gold. Having knocked the little lump with a hammer, it was found brittle, and flew in feveral pieces. Even the in fides of thofe pieces looked of a bafe dirty colour, like that of brafs or worfe, for the fragments had a far greater rdcmblance to bell-metal than either to gold or filver. One dram being carefully weighed out, and put on an excellent new and well-nealed cupel, with about half a dozen times its weight of lead ; though it turned very well like good gold, yet it continued in the fire above an hour and a half, which was twice as long as was expected, and yet almoft to the very laft the fumes copioufly af- cended, which fuflicienfly argued the operation to have been well carried on ; and when at laft it was quite ended, the cupel was found very fmooth and entire, but tinged with a fine pnrplifh red ; and, befides the refined gold, there lay upon the cavity of the cupel fome dark coloured recrements, which were concluded to have proceeded from the deteriorated metal, not from the lead. But when the gold was put again into the balance, it was found to weigh only about fifty-three grains, and confe- quently to have loft feven ; which yet was found to be fully made up by the recrements abovementioned, whofe weight and fixity, compared with their unpromifing ap- pearance, did not a little puzzle the ingenious author, efpecially becaufe he had not enough either of them, or cf leifure, to examine their nature. The ill-looked, mafs,
before
[ 209 ] before it was divided for the cupelling, was weighed in water, and inftead of weighing about nineteen times as much as a bulk of water equal to it, its proportion to that liquor was but that of fifteen and about two thirds to one; fo that its fpecific gravity was lefs by about three and a third than it would if it had been pure gold. From this experiment the author concludes, that an ope- ration very near, if not altogether, as ftrange as that which is called projection, and in the difficulteft points much of the fame nature with it, may be fafely ad- mitted : for the experiment plainly /hews that gold, though confeffedly the moft homogeneous and lead mu- table of metals, may in a very ffiort time be exceed- ingly changed, both as to malleablenefs, colour, homo- geneity, and what is more, fpecific gravity ; and all this by an inconfiderable proportion of injected powder, not amounting, on the modefteft effimate, to a nine hundred and fiftieth part of its weight. He adds, that there is a ftill ftranger effect of this admirable powder, which he has not mentioned becaufe he muff not do it.
On this hiftory it may be remarked, i. That little de- pendance can be had on the conjecture of the weight of the powder, as it might poffibly not be all diftinguiffied on the paper it was wrapt in, and as different kinds of bodies have different weights under equal volumes. 2. If no miffake was made in weighing the metal after the fufion, the quantity of powder muff have been greater than was imagined ; for the lump of metal appeared to weigh as much as the gold employed, exclufive of the vitrefied fubftance which the bottom of the crucible was overlaid with, and of the metallic globules that adhered to it. If thefe globules had been picked out, or feparated by pounding and waffling the pieces of the crucible, and weighed with the lump, it is fcarcely to be fuppofed that
E e tjhe
[ 210 ]
the author, fo minute in his details, would have omitted fuch a circumilance ; and befides, repeated experience has convinced me, that when gold, from any admixture, has contracted a vitreous coat, and any fmall grains of it flick about the crucible by means of this coat, the grains can- not by thefe methods be completely collected. 3. That the gold employed was fine, and that its fpecific gravity was nineteen, was only fuppofcd. The author himlelf appears to have fufpected its purity, for he fays that to remove fcruples on this head he caufed a dram and a half of it, which had been purpofely referved, to be melted in his amftant's prefence, and found it fine and well-coloured ; but furely the colour of the mafs cannot be admitted as a fufficient criterion of its purity. Indeed it could not have been entirely pure ; becaufe, though it had been fo at firfl, yet the procefs of parting, however carefully per- formed, would have left fome iilver in it. 4. Admit- ting the gold to have been perfectly fine, and no error to have happened ; it will not follow that the gold was degraded or altered in its nature, or that the experiment gives any countenance to the pretenfions of the alchemifts. Gold is flrongly affected, in many of its properties, by the admixture of very fmall proportions of certain other bodies : a quantity of tin, lets even than that which the foregoing powder is fuppofed to have amounted to, renders gold brittle. It is plain that at leafl fifty-three parts in fixty of the gold in the above experiment were in like manner debafed only by the coalition of another matter with it, and that this extraneous matter was feparable by fimple cupellation with lead. The hiflory affords no rea- fon to believe that the remainder of the gold might not alfo have been recovered, by fuitable management, from the ponderous fixt recrement : for gold has frequently been found debafed and difguifed by fubflances which
refilled
[211 ]
refifted cupellation, and which have afterwards been fe- parated by other proceffes. An inftance of this has been already mentioned in the ninth fedtion, and a more re- markable one is given by Homberg in the memoirs of the French academy for 1693 : a quantity of gold, after cupellation, quartation, fufion with antimony and the difflation of the antimonial metal, and repeated fufions with nitre, continued quite brittle though of a high colour : by treating it again with antimony and lead, and working off the fuperadded metals on a cupel, it loft its colour alfo, and became grey, but by further repetitions of the fufion, both its colour and malleability were at length reflored.
Juncker reports from Borrichius and Ofiander, that on grinding for a length of time, in a glafs mortar, an amalgam of one part of gold and four of mercury, with diflilled water, there feparates daily a black matter, which may be collected by pouring off the water and differing it to fettle : that after the triture has been con- tinued for fome weeks, the water yields, on being evapo- rated, fome granules of a cryftalline fak : that the black powder yields, on fufion, a green glafs ; and that the metal is thus refolved or deftroyed. But mercury alone by con- tinued triture or agitation, is changed into a fimilar powder, of which a part abides fixt in a considerably ftrong fire : the vitrefication probably proceeded from fome particles abraded from the glafs mortar ; and the faline matter, the proportion of which is acknowledged to have been very minute, either praexifled in the water or was extracted from the glafs. Borrichius himfelf affords a ftrong prefumtion that she black powder pro- ceeds rather from the mercury than from the gold : after fome days, he obferves, the amalgam grows ffiff, and the feparation more fparing, and therefore frefh mer-
E e 2 cury
[ 212 ]
cury is dire&ed to be added. I have continued the trituration of an amalgam of gold, almoft inceffantly, for more than a week, and afterwards recovered the gold entire.
. Kunckel imagines, that when glafs is tinged red by Caflius's precipitate or other preparations of gold, the particles of the metal are not barely diffufed through the glafs, but rcfolved into their elementary parts, fo as no longer to be reducible into gold again. He might indeed fail of recovering the gold; but though no means were known of feparating fo final 1 a propotion of it from fo large a quantity of vitreous matter, it furely could not follow that the gold was deflroyed.
Some other procefTes, proposed for the deflruction of gold, have been already taken notice of. It has been fhewn, that the vehement heat, collected in the focus of large burning glalTes, and the long continued action of a gentle heat, do not, fo far as can be judged from the facts as yet known, make any real change in it; and that the much boafled volatilization of gold does certainly not deftroy it, fince the volatilized gold may with eafe be reftored to its fixity and all its former qualities.
Upon the whole, both the producibility and deftructi- bility of gold continue frill problems in chemiftry. I know of no experiment from which the pofTibility of either can be inferred; and to demonstrate their impofli- bility is beyond the reach of experiments.
But though thofe, who have laboured the moft ardently in thefe purfuits, inftead of acquiring, have generally exhaufted riches, and may on good grounds be prefumed to have miffed of their principal aim; yet juftice requires us to add, that their labours have not been altogether ufelefs, and that many valuable difcoveries, relative to different fubje&s, have refulted from their enquiries. It
is
f «3 1
is to be regretted, that their affected myfleribufnefs, and peculiar mode of philofophizing, have rendered their writings fo forbidding, that many ufeful fads, fcattered through them, lie ftill unknown.
SECT. XIII.
Imitations of gold.
I. Gold coloured metal.
FROM the general eflimation of the colour of gold, which has attracted the notice of the moll: barbarous nations, the communication of the fame admired colour to metals of low price, for ufes where the other qualities of gold are not required, becomes an important object. The production of a kindred colour, by artificial compofition, in the common metal brafs, affords a principle for this imitation.
Brafs is prepared by melting copper with the mineral called calamine, from which it receives an increafe of one third or one half its weight. The matter, which the copper imbibes from the calamine,, is found to be zinc, of which that mineral is properly an ore; and accordingly zinc itfelf, melted with copper, communicates a colour of the fame kind. According to the purity of the zinc and copper, the proportions in which they are mixed, and the intimacy of their union, the compound metal proves more or lefs malleable, and approaches more or lefs to the colour of gold.
Thofe who have given receits for making a gold coloured metal, differ greatly in the proportion of the two in- gredients; fome directing the zinc to be taken only in a fifth or a fixth part of the weight of the copper, and others in an equal weight or more. From a fet of ex-
I perimc'
[ 214 ]
periments made to determine this point it appeared, that there is fome foundation on both fides; and that, both with the fmallefl and the largeft of thefe quantities of zinc, the metal proves more like gold than with the intermediate proportions.
One part of zinc and three of copper formed a com- pound of a brafiy yellow colour, but rather brighter than common brafs, and at the fame time more brittle : when broken, its texture appeared partly fibrous, and partly grained.
On increafing the quantity of copper to four, five, fix, eight, and ten times the weight of the zinc, the metal proved more and more tough, of a fine grain without any appearance of fibres, and its yellownefs more and more mixed with a reddifh caft, like that of golJ alloyed with copper. The befl coloured mixture, obtained by an augmentation of the copper, confided of five parts of copper to one of zinc : even this, however, differed greatly in colour from fine gold. A mixture of ten parts of copper and one of zinc looked like gold fomewhat worfe than ftandard, and hammered extremely well.
On diminhhing the proportion of copper, from thrice, to double, equal, and two thirds of that of the zinc, the colour was improved much more than by an augmentation of it, the compounds proved much more brittle, and wholly of a fibrous texture, without any appearance of grains : they broke over fhort on trying to bend them, and fell in pieces under the hammer. Equal parts of copper and zinc, or a little more zinc than copper, feemed to produce the fined colour: thefe mixtures, in the mafs, had a near refemblance to pure gold,- though flrokes drawn with them on a touchftone were remarkably paler, looking almofl white when compared with thofe of the gold. And indeed all the compofitions I have examined,
whether
[2i5 ] whetker made by myfelf or others, how nearly foever they approached to gold in the mafs, were very different from it on the touchftone.
The colour of thefe compounds is improved by a fmall admixture of certain other metallic bodies. Cramer ob- ferves, that when copper is melted with a fourth or a fixth of zinc, and a little pure tin j the compound metal, well cleaned and laid in the air for fome days, acquires on the furface the colour of fine gold : this teint, though merely fuperficial, is not the lefs valuable; for though it mould be difcharged by cleaning, the piece foon recovers it again, every frefh furface tarnifhing as it were to a like colour. Geoffroy relates, in the French memoirs for the year 1725, that on trying different metals, iron feemed to have the befr. effect: equal parts of- copper and zinc being brought into fufion, he threw in fome iron filings, amounting to an eight part of each of the other metals : the mixt turned out of a beautiful yellow colour, and a fine fmooth grain, not at all fibrous, as mixtures of copper and zinc in this proportion by themfelves always are, yet very brittle : on repeating the experiment with a fourth more of zinc, the proportions being ten parts of zinc, eight of copper, and one of iron filings, the metal proved of a grain like the former, but more compact, harder, brighter, and in colour ftill more like gold. He fays the commixture of the iron with the other ingredients requires a particular management, which I cannot find that he has any where communicated.
It has been laid, that the mixtures of zinc and copper may be made tough, by injecting upon them in fufion a little mercury-fublimate, as alio by nitre, fal ammoniac, borax, and different kinds of inflammable bodies: but thefe additions, as I have often found from experience, and as Pott alfo obferves in a differtation de zinco,
1 will
[ 2i6 ] will not anfwer. The great brittlenefs has been generally imputed to the lead, of which the common forts of zinc are fuppofed to partake in a greater or lefs degree; and hence it has. been fuppofed neceflary to previously purify the zinc, by cementation and fufion with fulphur, which abforbs and fcorifies the lead without acling upon the zinc: fome forts of zinc may doubtlefs require a treatment of this kind, but fuch, as has been ufually brought from the Eaft Indies, does not feem, when prepared in this manner, to give lefs brittlenefs than when unprepared.
It is certain, however, that copper impregnated with zinc, by cementation and fufion with calamine, proves more malleable, than when melted directly with as much of common zinc itfelf as it imbibes from the calamine ; on account, perhaps, of the" commixture being in the firfl: cafe more equal and perfect. By the procefs with calamine, copper cannot eafily be made to receive the full quantity of zinc neceflary for producing a good gold colour : by combining the two procefles together, that is by making the copper firftinto brafs, and then melting it with a fuitable quantity of zinc, a metal may be obtained of better quality than by either method fingly. A very ingenious artifl, who now prepares a gold-coloured metal in great perfection, has a fine kind of brafs made on purpofe for this ufe. An enquiry into the preparation and improvement of brafs will make a feparate article in one of the future numbers of this work.
A good deal of addrefs is requifite in melting the copper and zinc together; for the heat neceflary for the fufion of copper occafions the zinc to burn and flame, and a con- fiderable part of it to be diflipated, fo that the remaining copper is defrauded of its due proportion. If the two metals are put into the crucible at firft, and the fire gradually raifed, greateft part of the zinc will be burnt off
before
[ 217 ]
before the copper melts: if the copper he firfr. melted by itfelf, and the zinc heated and plunged into it, a ftrong commotion enfues, though the diffipation is much lefs confiderable than in the other cafe, the zinc being quickly- imbibed by the melted copper and in fome meafure protected and retained by it: if the copper and zinc be brought feparately into fufion, and one poured into the other, an explofion happens, and great part of the mixture, in my experiment above two thirds of it, is thrown about in drops, to the great danger of the bye-ftander. The union appeared to fucceed bed, and with lead: lofs of the zinc, when fluxes, containing inflammable matter, were added : I have generally ufed a mixture of black flux and borax j to which may be fubftituted a cheaper compo- fition of twelve parts of green glafs in fine powder, fix parts of potafh, two of borax, and one of powdered char- coal. The flux is firfl to be brought into fufion in the crucible, and the copper and zinc dropt into it : aflbon as thefe appear perfectly melted, they are to be well ftirred together with an iron rod, and expeditioufly poured out. The fame flux ferves repeatedly for the melting of feveral frefh quantities of the metal.
There are many receipts for making a gold coloured metal, from verdegris a preparation of copper, and tutty a preparation or ore of zinc : the difference, above taken notice of, in the effect of zinc itfelf and of its common ore upon copper, induced me to try, whether, in this form of combination alio, fome ufeful variation might not happen. One of the beft of thefe receipts feems to be that among Hook's papers publifhed by Derham ; in which eight parts of diflilled verdegris (that is, verdegris purified by folution in diflilled vinegar and cryffallization) and four parts of Alexandrian tutty, with two of nitre and one of borax, are directed to be mixed with oil to the confidence
F f of
[ 2l8 ]
of pap, then melted in a crucible, and poured into a flat moulu firlt well warmed. The perfon, who communicated thi receipt to Dr. Ilooke, lays that the metal will not only appear, but work, like coarfe gold ; that he fold it as dear as filver; and that the king of Poland had a fervice of it, only mixing fifteen ounces of gold with a hundred of the compound metal. I tried this procefs with verdegris, which I had myfelf purified, by diilblving it in diftilled vinegar, and evaporating the filtered folution to drynefs : a large proportion of the verdegris remained undiflblved ; and this refiduum, on being melted with black flux, yielded a brittle pale coloured metal alrnoft like bell-metal : from whence it might be prefumed, that the copper, in the infpiflated matter, was rendered purer than ordinary by the fepa- ration of this extraneous metal. On melting it with choice tutty, and the other ingredients, the refult was a very fine metal, which bore the hammer well ; but it was rather a fine brafs than a true gold coloured metal, its colour having lefs refemblance to gold than that of the mixture of equal parts of common copper and zinc already mentioned.
Tutty and calamine contain zinc in a flate of calx ; and hence, in the ufe of thefe, inflammable additions are dfentially neceflary, for reviving the zinc into its me- tallic form. Some of the earlier writers direct for this purpofe fubflances of a yellow colour, as turmeric, rhu- barb, faffron, aloes, which are ftill ufed, as I am inform- ed, by feveral workmen, who do not feem to have con- fidered, that thefe kinds of fubftances can be of fervice- no otherwife than as they furrulh an inflammable matter, and that common charcoal anfwers the fame end.
Two ways have been recommended, for giving a gold colour to copper, and at the fame time preferving it.s malleability, without the addition of any zinc, or of
i fubflances
[ 2I9 1 fubftances containing it ; the one, as is faid, by Horn- berg (for, though given as from him by fome late repu- table writers, I cannot find it among his papers in the French memoirs) the other by Vigani.
In Homberg's method, the copper is to be amalgama- ted with pure quickfilver, the amalgam boiled in river water for two hours, the quickfilver diftilled of? in a re- tort, and again poured back and abftracted once or twice : the remaining copper, being now fufed, is faid to appear of a beautiful gold colour, and to be more ductile than common copper, fo as to be well fitted for the finer machines and utenfils. The great difficulty of amalga- mating copper by the common methods feems to have prevented this procefs from coming to a fair examination. This difficulty I have furmounted in different ways : one of the eafieft and molt expeditious of which was, by diflblving the copper in aqua fortis, and, when the menftruum would take up no more, pouring the folution into an iron mortar, along with fix times as much quick- filver as there was of copper, and fome common fait, and then grinding them well together with an iron peftle : the diflblved copper is extricated from the acid by the iron, in a very fubtile form, and falling in this ftate into the quickfilver, is readily imbibed by it. This amalgam was ground and waflied with water till it became per- fectly bright, and the mercury was then diftilled off: the remaining copper, melted in a crucible, had, as was indeed expected, no degree of yellownefs, and appeared exactly of the fame colour as at firft. As no ienfiblc alteration was thus produced, a repetition of the trouble- fome operation was judged unneceflary.
Vigani's procefs carries with it ftrong marks either of error or referve; yet from the general character of the author, and the favourable reception ht met with in this
F f 2 country,
[ 220 ]
country, I mould not perhaps be held excufed if I did not take fome notice of it. Copper is to be melted in a crucible, an equal weight of powdered fulphur fprinkled on it, the fufion continued till the fulphur is all burnt off, and the metal afterwards flatted into plates. A quantity of orpiment, auripigrnentum, is to be melted and quenched in vinegar, and the fufion and extinction repeated twenty- four times. The materials being thus prepared, fome bean meal is to be placed in the bottom of a crucible, above this nitre and tartar, then fome auripigmentum, on this fome of the copper plates with more auripigmentum over them : in this order of ftratification we are to proceed till the velTel is full, and then to invert into the mouth another crucible having a hole in its bottom. A moderate heat is to be continued fo long as any flame or fumes appear, after which the fire is to be railed fo as to bring the matter into fufion, and continued in this ftate for an hour. It is not to be expected that this procefs can afford the ductile gold coloured metal which the author promifes from it ; for orpiment, in virtue of the arfenic of which it largely partakes, tinges copper, not yellow, but white. As Vigani throws a veil over fome of his preparations, though commonly but a thin one, I have been led to fuf- pect that he has done fo here ; and that by auripigmentum he does not mean the orpiment which makes a gold pig- ment for the painter, but zinc the auripigment for cop- per. If this explication be right, a yellow metal may doubtlefs be obtained, though the troublefome method ©f procedure is not to be recommended. The burning of fulphur upon the copper, and the repeated extinction of zinc in vinegar, do not appear to be of any advantage;, and the gradual augmentation of the fire occalions always, as already obferved, a. great diffipationof the zinc.
I. It
f 221 I
It has been fuppofed by many, that the yellownef?, refulting from mixtures of zinc and copper, was no other than a dilution of the coppery red by the whitenefs of the zinc : if this was the cafe, filver would have a like erfecT:, but filver is not found to give any yellownefs to copper. The yellow colour produced from the combination of copper and zinc, is apparently a new quality ; as much as the brittlenefs produced from the combination of two malleable metals, gold and tin. It has not been obferved that any metal, befides zinc, yields any confiderable yel- lownefs with copper, though tin, in certain proportions, yields a flight one ; or that any metal befides copper forms a yellow compound with zinc.
Silver is tarnifhed fuperficially, by certain vapours, as that of putrefied urine, to a colour fo like that of gold, that abufes are faid to have been often practifed on this foundation, particularly in regard to wire and laces : Savary gives an account of feveral edicls iflued in France for preventing thefe frauds. It is obfervable alfo that fine filver, on being melted with nitre, acquires frequently a yellow fpot on the furface where the fait lay in con- tact with it ; and Stahl affirms that filver, by being treated in a certain manner, with certain fubftances, of which nitre is the principal, may be tinged throughout of a golden colour : he conceals the procefs, for fear of giving occalion to impofition ; though of this there does not appear to be much danger, for he obferves- that the filver acquires none of the other diftinguiihing characters of gold, and that the adventitious colour is very readily deflroyed.
II. Gold coloured pigments.
It* the gilding of wood, fome pigments, approaching as near as may be to the colour of gold itfelf, are both laid
under
[ 222 ]
under the gold, and ufed alfo for the colouring of depreffcd parts where gold leaf cannot be conveniently applied. The iubftance chiefly employed for thispurpofe is yellow ochre j the colour of which may be improved, or brought nearer to the gold teint, by a fmall addition of vermilion or other red powders.
Of the mineral called auripigmentum or orpiment, fome forts are of a beautiful glittering gold colour. This mine- ral confifts of arfenic and fulphur, and on being ground with oil for painting, yields an offenfive fmell, as fulphur always does when united with oils : this is the principal in- convenience it is accompanied with, and renders its ufe lefs frequent than it would otherwife be. Though it is offen- five from the fulphur, the fufpicion of its being poifonous in virtue of the arfenic appears to be without foundation, for the fetid vapour proceeds wholly from the fulphur, and even arfenic in fubflance, if we may judge from trials made on brutes, has its poifonous quality fheathed or destroyed by the combination of fulphur with it.
A beautiful gold coloured preparation, called aurum mo- faicum or mufivum, is obtained from tin. Some fine tin is melted in an iron ladle; and half its quantity of pure quick- filver, previoufly heated in another ladle till it begins to fmoke, is poured into the melted metal, and the mixture fiirred .with an iron rod : when cold, the matter is found friable, and being reduced into fine powder, it is well mixed with half or a third its weight of fal ammoniac and the fame quantity of flowers of fulphur. With regard to the proportion of thefe ingredients, practical writers differ not •a little, and indeed they admit of great latitude, for I have fucceeded equally with very different proportions : very little of any of them is retained by the tin in the fubfe- qnent part of the operation. The powder is put into a icatras, or round glafs with a fhort neck, wheh is placed in
2
[ "3 ] a fand-bath, and the fire increafed by degrees, fo as to keep the land at laft red' hot for fome time. The fire being then fuffered to decay, and the veflel broken when cold, a faline matter, confiding chiefly of fal ammoniac, is found in its upper part : under this is a dark red mafs, which proves to be cinnabar, or a combination of mercury and fulphur: at the bottom is the aurum mofaicum, a fparkling, gold coloured, flaky mafs, weighing about a twelfth part more than the tin employed.
The gold coloured talcs, formerly mentioned, have too much flexibility and elafticity to be reduced into powder of fufficient finenefs for the purpofes of painting : but there is one imitation of gold, for which powders of much fine- nefs are not required, and for which the talcs are better adapted than any other material I know of, on account of their refinance to fire. A kind of glafs, with gold coloured fpangles diffufed through its fubftance, has been much admired, and the preparation of it kept a fecret : this ap- pearance may be communicated by the yellow talcs, by mixing them well with powdered glafs and bringing the. mixture into fufion.
III. Gold coloured varnijh or lacker.
Silver, coated with a tranfparent gold coloured varnifh, is made to refemble gold fo exactly, as wholly to fupply the place of gold in fome of the works called gilt. The bafis of the varnifh, or what gives adhefivenefs and gloflinefs to the colouring matter, is a folution of lac made in fpirit of wine.
Lac or lacca is a fubftance collected by certain infects in the Eaft Indies : it is found incruftated on fticks or branches of trees, in brittle mafles of a dark red colour,, which being reduced into imall grains, and freed from part of the colouring matter by infufion in water, are fold , under the name of feed lac. It is in this ftate that the lac
is
[ 224 ]
is to be ufed for varnifhes : what is called fhell lac, or the grains formed into plates by melting them in boiling water, does not anfwer fo well.
The fpirit muft be highly rectified, or freed as much as poflible from any admixture of phlegm or water, for other- wife it will not diflblve the lac. The mod convenient and expeditious way, of preparing the fpirit for this intention, is by adding fome dry potafh or other fixt alkaline fait : the fait imbibes, and diflblves in, the watery part of the fpirit, and forms therewith a diftintt fluid at the bottom, from which the fpirituous part on the top may be poured off. More or lefs of the fait will be required according as the fpirit is more or lefs phlegmatic : if the firfl: quantity, after ftanding for fome hours and occafionally (baking the vefTel, wholly diflblves, more mult be added and the agitation renewed.
The fpirit being thus dephlegmed, fome feed lac, reduced into fine powder, is added to it, in the proportion of about three ounces to a pint : the veffel being fet in a moderate warmth for twenty-four hours and frequently fliaken, a part of the lac diflblves ; and the fpirit, now tinged of a reddifh brown colour, is ftrained off" from the undiflblved part, and fet by for a day or two to fettle. The digeftion fhould be performed in a wide mouthed veflel, covered Co as to prevent the exhalation of the fpirit : the undiflblved lac foftcns into a vifcous mafs, fo as fcarce to be got out through a narrow aperture.
In different portions of the foregoing folution, poured off* clear after the draining and fettling, fome gamboge and annotto are diflblved feparately. Gamboge is a yellow juice, ifluing from certain trees in the Eaft Indies, and exflccated into maffes by the fun's heat : Annotto is arti- ficially prepared from the red fkins of the feeds of an Ame- rican tree, by fteeping and agitating the feeds with water
till
[ 225 ]
till their colouring matter is transferred into the liquor : on boiling the drained liquor, the colouring matter is faid to be thrown up to the furface in fprm of fcum, which is afterwards exficcated by itfelf, and formed into maffes, which, as brought to us, are moderately hard and dry, of a brown colour on the outfide, and a dull red within. Both thefe fubflances diffolve very readily in the fpirit : the gam- boge communicates a high yellow colour, and the annotto a deep reddifh yellow. The folution of the gamboge is mixed with about half its quantity of that of the annotto, and trial made of the mixture on fome filver leaf: if the colour inclines too much to the yellow or the red, more of the one or the other liquor is added, till the true golden colour is obtained. There are fundry other materials, from a due mixture of which a like colour may be produced, as turmeric, faffron, dragons-blood, 6cc.
The filver leaf being fixed on the fubject, in the fame manner as gold leaf, by the interpofition of proper gluti- nous matters ; the varnifh is fpread upon the piece with a brufh or pencil. The firfl coat being dry, the piece is again and again wafhed over with the varnifh till the colour appears furficiently deep.
What is called gilt leather, and many picture frames, have no other than this counterfeit gilding. Wafhing them with a little rectified fpirit of wine affords a proof of this ; the fpirit difTolving the varnifh, and leaving the filver leaf of its own whitenefs.
For plain frames, thick tin foil may be ufed inflead of filver. The tin leaf, fixed on the piece with glue, is to be burnifhed, then polifhed with emery and a fine linen cloth, and afterwards with putty applied in the fame manner : being then lackered over with the varnifh five or fix times, it looks very nearly like burnifhed gold.
The fame varnifh, made with a lefs proportion of the colouring materials, is applied alfo on works of brafs ; both
G g for
[ 226 ]
for heightening the colour of the metal to a refemhlance with that of gold, and for preferving it from being tar- nilhed or corroded by the air.
Addition to the History of G O L D.
SINCE the foregoing fheets went to tfie prefs, a new manufacture has been fet on foot in London, for embelliihing linen with flowers and other ornaments of gold leaf. The linen looks whiter than mod of the printed linens ; the gold is extremely beautiful, and is faid to bear wafhing well. I have feen a piece, which I was credibly informed had been warned three or four times, with only the fame precautions as are ufed for the finer printed linens, and on which the gold continued entire and of great beauty.
The Venetians have carried on a large trade, to the Levant, in a kind of brocade called damafquete, which, though it has only about half the quantity of gold or filver as that made among us, looks far more beautiful. The flatted wire is neither wound clofe together on the filk threads, nor the threads ftruck clofe in the weaving ; yet, by pafling the fluff betwixt rolls, the difpofition and management of which is kept a fecret, the tiflue or flower is made to appear one entire brilliant plate of gold or filver. The French miniftry, ever vigilant for the ad- vancement of arts and commerce, judged this manufac- ture important enough to deferve their attention ; and accordingly, for contriving the machinery, they engaged the ingenious M. Vaucanfon, known throughout Europe
for
[ 227 1
for his curious pieces of mechanifm, who, in the me- moirs of the academy for the year 1757, lately printed, gives an account of his fuccefs, and of the eftablifhment of luch a manufacture at Lyons.
The lower roll is made of wood, thirty-two inches in length and fourteen in diameter j the upper one of copper, thirty-fix inches long and eight in diameter : this laft is hollow, and open at one end, for introducing iron heaters. For making the rolls cylindrical, he has a particular kind of lathe, wherein the cutting tool, which the moft dex- trous hand could not guide in a ftreight line through fuch a length as thirty-fix inches, is made to Hide, by means of a fcrew, on two large fleel rulers, perfectly ftreight, and capable of being moved at pleafure, nearer, and always exactly parallel, to the axis of the roll.
He firft difpofed the rolls nearly as in the common flatting mill. In this difpofition, ten men were fcarcely fufficient for turning them with force enough to duly extend the gilding -, and the collars, in which the axes of the rolls turned at each end, wore or gulled fo faft, that the preffure continually diminifhed, inibmuch that a piece of fluff of ten ells had the gilding fenfibly lefs ex- tended on the la ft part than on the firft. He endeavoured to obviate this inconvenience by fcrewing the rolls clofer and clofer in proportion as the fluff paffed through, or as the wearing of the collars occafioned more play between them i but this method produced an imperfection in the fluff, every turn of the fcrew making a fenfible bar acrofis it. To leffen the attrition, each end of the axes, inftead of a collar, was made to turn between three iron cylin- ders called friction-wheels : but even this did not anfwer fully, for now another fource of unequal preffure was dif- covered. The wooden roll, being compreffible, had its diameter fenfibly diminifhed : it likewife loft its round-
G g 2 nefs,
[ 223 ]
riefs, Co that the prefTure varied in different points of its revolution. On trying different kinds both of European and Indian woods, all the hard ones fplit, the foft ones warped without fplitting, and, of more than twenty rolls, there was not one which continued round for twenty- four hours even without being worked in the machine.
Thefe failures put him upon contriving another method of preffing the rolls together, fo that the force fliould always accommodate itfelf to whatever inequalities might happen. The axis of the copper roll being made to turn between friction wheels as before, that of the wooden one is preffed upwards by a lever at each end furnifhed with a half collar for receiving the end of the axis. Each lever has the end of its fhort arm fupported on the frame of the machine, and the long arm is drawn upwards by an iron rod communicating with the end of the fhort arm of another lever placed horizontally : to the long arm of this laft lever is hung a weight, and the levers are fo propor- tioned, that a weight of thirty pounds preffes the rolls together with a force equivalent to 17536 pounds, which was found to be the proper force for the fufficient exten- fion of the gilding. By this contrivance four men can turn the rolls with more eafe than ten can turn thofe which are kept together by fcrews ; and the fame weight acting uniformly in every part, the preffure continues always equal, though the wooden roll fliould even become oval, and though the fluff be of unequal thicknefs.
A piece of cloth, of about two ells, is fowed to the beginning and end of the fluff, to keep it out to its width when it enters and parts from the rolls, which could not be done by the hands for fear of burning or bruifing them : as it would take too much time to fow thefe cloths to every fmall piece of an ell or two, a number of thefe is fowed together. The fluff is rolled upon a cylinder, which
is
[ 229 ]
is placed behind the machine, and its axis prefTed down by fprings to keep the fluff tight as it comes off. Four iron bars, made red hot, are introduced into the copper roll, which in half an hour acquires the proper degree of heat, or nearly fuch a one as is ufed for the ironing of linen : the wooden roll is then laid in its place, and the machine fet to work. If more than thirty ells are to be paffed at once, the wooden roll muffc be changed for ano- ther, for it will not bear a longer continuance of the heat without danger of fplitting, and therefore the manufactu- rer mould be provided with feveral of thefe rolls, that when one is removed, another may be ready to fupply its room : as foon as taken off from the machine, it mould be wrapt in a cloth and laid in a moift place.
The principal inconvenience, attending the ufe of this machine, is, that the heat neceffary for extending the gilding, though it improves the brightnefs of white and yellow filks, is injurious to fome colours, as crimfon and green. A double preifure will not fupply the place of heat ; and the only method of preventing this injury, or rendering it as flight as poflible, appeared to be, to pafs the fluff through with great celerity.
III.
III. EXPERIMENTS
Of the conversion of Glass Vessels into Porcelain, and for cftablifhing the prin- ciples of the art.
HAVING many years ago diftilled fome wood foot, with a ftrong fire, in a green glafs retort fet in fand, I obferved great part of the bottom of the retort, after the operation, to be remarkably changed : it was quite opake, of a black colour on the infide where the foot had been in contact with it, and whitifh on the outlide where it refted upon the fand : it had no longer the brittlenefs of glafs, but broke with difficulty like the better kinds of ftone ware : its internal fubftance was white like porcelain j and not of a glaffy fmoothnefs, but of a fine fibrous texture.
This fingular change, in a body fupofed fo little fuf- ceptible of alteration, was attributed to the vapours of the foot having penetrated into the fubftance of the glafs : fundry pieces of the fame kind of glafs were therefore intermixed with another quantity of foot, in an iron pot, to which was adapted a head with a receiver, and the diftillation conducted in the ufual manner, till nothing more could be forced out from the foot in a ftrong fire : on examining the pieces of glafs, fome, in the middle of the matter, feemed fcarcely at all altered ; others, about the fides and bottom of the pot, were changed in part nearly in the fame manner as the bottom of the retort had been.
c This
[ 23!.]
This change of glafs has doubtlefs happened often, without being attended to. Neumann is the firft writer by whom I find it taken notice of: in diftilling milk in a glafs retort, he obferved that the bottom of the vefTel acquired the appearance of porcelain, which he attributes to the fine white earthy matter of the milk forced into the glafs by the heat.
M. de Reaumur was led to the fame difcovery by analo- gical reafoning, as I have already mentioned in the notes on Neumann's chemical works, where a further account of this affair is promifed. Reaumur, having had large experience of the effects of inflammable and earthy bodies on iron, by baking, in the converfion of forged iron into fteel, and in the foftening of caft iron, applied the fame procefs to common glafs, and thus difcovered the new porcelain, which he calls porcelain by tranfmutation, porcelain by revivification, or porcelain of glafs. The glafs was cemented, or baked, in crucibles, firft with the foot, powdered charcoal, and other fubftances employed in the experiments on iron : it became opake, externally dark-coloured or black, but internally of a fine white colour. Other materials were then made trial of, in hopes that fome one might be found, which fhould occafion the furface to be as white as the internal part : among the fubftances tried in this view, of which he gives no parti- cular account, he judged white fand and plafter-of-paris, or rather a mixture of the two, to anfwer the beft. He directs vefTels of common green glafs to be filled and fur- roun-ded with this mixture, in large crucibles or cement- ing pots, fuch as are commonly ufed for the baking of earthen wares ; the crucibles to be covered and luted, and fet in a potter's furnace : the fame fire, which bakes the common wares, changes the glafs veffels into verTels of the new porcelain. He obferves, that this porcelain may
be
[ 232 ] be made at a very cheap rate, as the glafs maker can form veifels more expeditioully than the potter, and as it is happily the very coarfeft green glafs that yields the fined porcelain : That it is ealily diltinguifhed from all the other forts of porcelain by the texture which it exhibits on breaking, as it has nothing of the granulated appear- ance of the other porcelains and earthen wares, any more than the glofly fmoothnefs of glafs and enamels, the fur- face of the fracture being compofed of fine fibres like frlken threads : That in beauty it is inferiour to the Chi- nefe, but equal to many forts that are held in efleem ; that in utility, and every effential quality of porcelain, it is equal to the beft, and that in fome refpecls it is fupe- riour to all that have hitherto been made : That it fuffers no injury from being fuddenly heated or cooled, bears a vehement fire without melting or altering its figure, and hence, befides its ufe for ornamental vafes, promifes to make excellent veffels for the chemift.
The character given of this porcelain by Reaumur, and the valuable qualities he afcribes to it, rendered it an object of more importance than it had appeared at firft, and engaged me in a further examination of it. That the enquiry might be carried on with fome regularity, it was divided into five heads. ( i .) To trace the gradual pro- grefs of the change from the ftate of glafs to that of perfect porcelain, and to difcover whether a continuance of the procefs would be productive of any further changes. (2.) To determine the qualities of this kind of porcelain, and how it differs from other porcelains and from glafs, in thofe properties which regard the application of it to common ufes. (3.) To compare the effects of different cementing materials on the fame glafs, and (4.) the effecls of the fame materials on different kinds of glafs. (5.) To afcertain, as far as might be, the caufe of the change, or 5 the
r 233 j
tire true principle on which it happens. The following is the general refult of the experiments I have hitherto made upon thefe fubjects.
SECT. I.
Experiments of the fuccejjive changes produced in GreenjGlafs
by baking.
N order to determine the progrefs of the vifible change produced in glafs by baking, and the efFe&s of different degrees or a different continuance of the heat; a num- ber of pieces of common quart bottles were furrounded with white fand, in crucibles, which were placed in a wind-furnsce, built on purpofe for experiments of this kind, confining of feveral chambers one over another, with proper apertures in the middle for the alcent of the flame and heated air through each. The crucibles were left open, that fome of the pieces might be taken out from time to time, for difcovering how the change went on : and that the effects of the procefs might be feen in its full extent, the fire was flowry raifed, and continued for upwards of forty hours.
Such pieces as were taken out before they became red hot, did not appear to have fuffered any change, though they had been kept for feveral hours in a heat very little below ignition. In a low red heat, the change did indeed take place, but exceeding flowly ; thofe which had been expofed for feveral hours to fuch a heat being very little altered. In a ftrong red heat, ap- proaching to whitenefs, juft not fufficient to make the glafs melt, the change went on pretty faff. : after an hour's continuance of this degree of heat, the glafs had ac- quired the appearance of porcelain to a confiderable thick- nefs » and in two hours longer, the thickeft pieces, of the
H h bottoms
[ 234 ] bottoms of the bottles, were found fully changed through- out. In thofe pieces which were flowly affected by a weak heat, and thofe which were more hailily acted upon by a moderately flrong one, the progrefs of the change itfclf was, for the mod part, nearly in the fame manner.
The green glafs became firit of a bluifh colour on the furface, and in this flate, when held between the eye and the light, it appeared lefs traniparent than before, and of a yellowim hue. After this it was found changed a little way on both fides into a white fubftance, exter- nally flill bluifh ; and as this change advanced further and further within the glafs, the vitreous part in the mid- dle approached more and more to yellow : the white coat was of a fine fibrous texture, and the fibres difpofed, nearly parallel to one another, not longitudinally as might be ex- pected from the direction given to the parts of the glafs in blowing it into veffels, but crofswife to the thicknefs of the piece. By degrees, the glafs became throughout white and fibrous, the external bluifhnefs at the fame time going off, and being fucceeded by a dull whitiih or dun colour : the fibres were for the moil part regularly and uninterruptedly arranged from each fide to the middle, where the fibres from the two fides, meeting together, formed a kind of partition : along this juncture, there were in fome pieces confiderable cavities here and there; others were perfectly folid.
The pieces which were continued in the fire for any considerable time beyond this period, and thofe which were afterwards returned to it along with frefh fand, fuf- fered a frefh change, which proceeded, like the firff, from the furface to the center. The fibres became divided or cut into grains at the outer ends, and by degrees they were thus fucceffively divided through their whole length ; the whole internal part of the porcelain affuming a gra- nulated
F *$$ J
Fmtated» texture, not ill refcmbling that of the eommoii porcelains.
Thofe which were longer and longer expofed to the fire* received more and more alterations. The grains, at firfl fine and of fome degree of gloffinefs, grew larger and duller ; and at length, through fundry gradations, lels re- markable and lefs uniform, what had once been glafs, and afterwards a compact hard porcelain, became a porous friable fubflance, like a mafs of white fand flightly cohering.
During the change of the glafs into a fibrous porcelain, it generally preferved the fmoothnefs of its furface, and the fand freely parted from it : in the fubfequent changes, part of the fand baked together upon the furface, and flrongly adhered, not to be got off, and not greatly differ-, ing from it in appearance ; I have fbmetimes been at a lofs to diftinguiih the matter which had been glafs from the fand that furrounded it. In fome pieces the fandy coat was parted from the internal matter by a number of fine cavities refembling a dotted line : in others, they were clofely applied together.
Such were the general effefts of continued cementation in many repetitions of the experiment, though not with- out variations in fome particulars. Sundry pieces became throughout white, and almoft opake, and fome blue, before they contracted any fibrous coat, which afterwards proceeded in the fame manner as in the others. Some pieces, being broken in different parts after they had been changed to a fibrous flate, inftead of the uniform tranf- verfe difpofition of the fibres, had feveral prominences on the furfaces of the fractures, from which the fibres iffued as rays in all directions. After the fine granulated flate,. which fucceeded to the fibrous on a continuance of the ce- mentation, fome pieces became porous, while in others the
H h 2 grains
[ 236 ] grains formed a kind of clofe plates, and the mafs proved very compact. In fome, the texture was clofe and even throughout, without any diftinguifhable grains, fibres, or plates. Of fome of thefe variations, the probable caufes will appear in the following fection : others depended per- haps upon the nature of the glafs employed.
Notwithftanding thefe, and other leffer differences, the general appearances, and the gradation of the vifible change proportionably to the degree of baking, are fo flron<»]y marked, that, from the'texture of the porcelain on breaking, we can always judge with certainty of its quality, or of the degree to which it has been baked. Reaumur has alio taken notice of fomething of this kind, finding the porcelain fometimes turn out granulated in- ftead of fibrous : but his experiments do not feem to have been carried far enough to difcover the foundation of this difference ; to difcover, that the different kinds of texture regularly fucceed one another from the continued action of one caufe, that they are all at the command of the workman, and that they are accompanied with remark- able differences in the intrinficqualities of the porcelain.
SECT. II.
Experiments of the quality of the fubjlance into which Green Glafs is converted by baking.
TH E porcelain into which glafs is converted by bak- ing, whatever its fuperficial colour be, is, in its inter- nal fubftance, always white; and its whitenefs is frequently not inferiour to that of the internal part of China ware. Its furface is unhappily the part which is leaft beautiful. All the thick pieces were quite opake : fevoral thin ones had a decree of tranfparency, refembling that of China ware. la this refpect confiderable differences appeared : a very
thin
[ 237 ] thin coat of fibrous or granulated porcelain upon the glafs gave opacity ; of the pieces of a clofe fmooth texture, fome, tho' pretty thick, were femitranfparent, and others, tho' thinner, were opake.
In the fibrous ftate of the porcelain, it is confiderably hard ; much more fo than the glafs it was made from, and than any of the common kinds of porcelain. It freely and plentifully ftrikes fire with fteel, which green glafs does but in a low degree. It cuts common glafs, as indeed one piece of glafs in fome meafure will another ; but nei- ther any of the common kinds of glafs, nor the file which cuts them, make any mark on the fibrous porcelain. Even when the change is fcarcely vilible on the furface of the glafs, the external part is found fenfibly harder to the file than the internal.
It perfectly refifls both acid and alkaline liquors, neither permitting them to tranfude through it, nor being at all corroded by them.
It bears viciffitudes of confiderable degrees of heat and cold, fo that veffels of it may be plunged at once, without any danger of their cracking, from freezing into boiling water. It may likewife be fet on burning coals, with much lefs precaution than any of the porcelains or earthen wares- ufed for containing liquids.
In a moderate white heat, it melts, fo as to be eafily drawn out into long flender firings, which appear femitranf- parent, and, on breaking, prove not fibrous as before, but of a vitreous fmoothnefs like white enamel. Some of the melted pieces were confiderably bright or glofiy, fome had no glofiinefs, and all of them proved fofter than before the fufion, feeming, though very compact, to be little harder than common green glafs.
It does not however melt near fo eafily as the glafs itfelf.
When the cementation has been continued no longer than
4 till
I 238. 1
IfcH only the external part of the glafs was changed, I h often oblervcdr that on haftily railing the fire, the un- changed glafs has melted and run out,, leaving a cavity in. the middle with a eruft of porcelain on each fide.
When the porcelain has been lb far baked, that the fibrous texture has difappeared, and a coarfe granulated one come in its place, it proves far fofter than before : it now neither ftrikes fire with fteel, nor cuts glafs ; but is itfelf cut with eafe both by common glafs and by the file.
With this imperfection, it acquires an advantage of greater reliftance to fire : the longer the cementation was continued, the fulibility fecmed always to be more and more diminished. A piece of the concave bottom of a common green bottle, which had its fibres only in part changed into grains, flood the melting of a 'lump of brafs, of about two ounces, without anywife altering its figure, or firffering any other apparent change, than that the thin edges were rounded off, and covered in fome parts with a green glazing, which feemed to have exuded from the mafs. Some pieces of the fame cemented glafs being put into a fmall crucible, into which another was inverted and clofely luted, and the whole urged for two hours or more in a fea-coal fire vehemently excited by bellows, the pieces melted together into a very fpongy mafs, of an almoft pearly whitenefs and fome brightnefs, intermixed in different parts with a green glafs, exactly refembling the glafs employed, and which probably was no other than a part of it, that had efcaped unchanged in the cementation, though not diftinguifhable by the eye, till thus fpued out from the lefs fulible porcelain, and col- lected in its cavities. Pieces which had acquired through- out a fine bright grain, were likewife in an intenfe fire, made to melt or foften into lumps, which generally proved
fpongy :
[ 239 ] fpongy : but thofe, in which a large coarfe grain had fucceeded to the fine one, could fcarcely be made to fof- ten at all, whether expofed to the fire in crucibles, or in contact with the burning fuel. Thefe unfufible pieces, though a continuance of the baking with a moderate heat would have rendered them more and more porous and friable, on being haftily urged with an intenfe fire became remarkably more compact than they were before -, fome of them feemed fuperiour in folidity to any kind of ware I know of.
It may hence be prefumed that the fmooth texture ob- ferved in feveral pieces after the baking, fo clofe and compact that neither grains nor fibres can be difiunguifhed, proceeds from their having undergone a greater degree of fire than the others. Several of the pieces, which ac- quired this appearance in the baking, had in part begun to melt, and others had not : perhaps the former received their compadtnefs from an augmentation of the heat, in the earlier period of the cementation, and the latter in the fubfequent flages when their fufibility had been greatly diminidied ; and probably the femitranfparency of fome of the compact pieces, and the perfect opacity of others, proceeded from the fame caufe. Jt is plain that the refinance to fire, which Reaumur makes a ee- neral property of this kind of porcelain, belongs to it only in certain fiates ; and that the vefiels, which he found to bear the vehement heat of a forge, could not be of the fibrous porcelain he describes, but fuch as had been baked considerably beyond that period.
From the foregoing general refults of a great number of experiments, a particular detail of which could add little either to the inftrudlion or entertainment of the reader, it follows, that this porcelain, though iittle adapted for ornamental purpofes, on account of its want of ber.sty 4 on.
i 240 J
©n the outiide, is very well fitted for many kinds of use- ful vellels.
Green glafs mortars, or planes for levigation, may be advantageoufiy changed into this hard porcelain, greatly exceeding the hardnefs of, any kind of marble, and no ill fubftitute for. agate or porphyry. Mortars and levi- gating planes of this kind feem for fome purpofes particu- larly excellent, as. for the grinding of enamels, which, though they ihould wear off fome of the matter of the baked glafs, as well as of marble or alabafler, yet cannot be fo much injured by the little they receive from the for- mer, which is analogous to their own compofi'tion, as by the greater quantity they receive from the latter, which is of very different quality.
Glafs veffels, intended for bearing the fire, may, by converting them into porcelain, be made capable of bear- ing it in a much greater degree, and rendered much lefs liable to crack. As veffels of this porcelain have in thefe intentions manifeft advantages above glafs, they have ad- vantages alfo above all the other porcelains and earthen wares. The unglazed earthen wares permit faline liquors to foak through them ; thofe which are glazed have their glazing corroded by acids ; the compact ftone wares, and thofe in general which have clay in their compofition, as all the common wares neceflarily have, are corroded and partly diffolved by certain acids in a boiling ftate, parti- cularly by the vitriolic : while the porcelain from glafs is neither affecfed by thefe caufes, nor fo liable to be in- jured by haftily expofing it to the fire. I know of no material, fo proper, and fo commodious, for evaporating veffels, or others, that fhall be proof againft all kinds of faline liquors.
There are fome veffels alfo, which may be figured more perfectly, and with lefs expence, in this kind of
porcelain
[ 241 ] porcelain than in any other. Long-necked matrafles for inftance could fcarcely at all be formed by the potter, of that equal thicknefs, and internal fmoothnefs, to which they are expeditioufly blown by the glafs-maker.
The above account of the qualities of this porcelain in its different ftates, points out a caution to be obferved in the procefs, efpecially where mortars are to be changed, or in other cafes where great hardnefs is required ; viz. to difcontinue the baking at the period of the greateft hardnefs ; for otherwife the matter foon becomes foft again, and even fofter than the glafs was at firft. It feems to have acquired its full hardnefs as foon as it has become white and fibrous ; and hence perhaps it may for moft purpofes be advifeable to difcontinue the operation as foon as the glafs appears covered with a moderately thick white coat. Fragments of the fame kind of glafs, put in along with the veflel to be changed, and occalionally taken out and broken, will ferve to inform the operator how the change goes on. Of wide-mouthed veflels, feveral may be placed within one another, with fand between them. In all cafes, care mufl be taken to apply the heat, as equally as may be, all over the crucible or pot containing the veffels, that the change may be as uniform as poffible throughout : no particular contrivances however are necef- fary for this ufe, the fame caution and the fame furnace, employed for baking the common fine wares, being fuffi- cient for the baking of glafs.
SECT. III.
Experiments of comparing the effects of different kinds of ma- terials on Green Glafs by baking.
ECES of green glaf: bottles were furrounded with
F
powdered charcoal, foot, and fundry kinds of earthy bodies, in feparate crucibles, which were all covered and
I i luted,
[ -42 ] luted, and placed in a wind-furnace : the fire was gradu- ally raifed, fo as to make the crucibles of a moderately flrong red heat, and continued in this flate for fix or feven hours. The fire being then fuffered to decay, and the crucibles taken out and examined, the glafs was found in all to have become porcelain. In the upper chamber, moff. remote from the action of the fire, the pieces were in general fibrous, and fome of the thick ones not changed throughout : in the lower chamber, more immediately expofed to the fire, where the matters intermixt with the glafs were the fame as in the upper one, the fibres had in moil of the pieces difappeared, and given place to grains. There did not feem to be any differences, that could be afcribed to the quality of the cementing matters, in the internal colour, hardnefs, texture, or the regular fuccefTion of the changes ; though, in external appear- ance, the differences were very confiderable.
All the pieces, which had been furrounded with foot, with charcoal, or with mixtures of the two, were exter- nally of a deep black colour : where fmall proportions of foot or charcoal were mixed with white earths, the por- celain turned out of a brown colour, deeper or lighter, according as the inflammable ingredient was in greater or lefs quantity. Judging that the dark colour, which the foot or charcoal communicate, might be burnt off by fire affifted by the action of the air, I put fome of the black pieces into a crucible, which was placed open in a blafr. furnace, and excited the fire, for above an hour, to as o-reat a degree as the porcelain feemed capable of bearing without beginning to melt : the colour refitted this heat, continuing as deep a black as at firft.
The coloured clays, boles, ochres, powdered red bricks, and the fands which burn red, gave likewife different fhades of brown, inclining more or lefs to blackiih, red-^
2 difh,
[ 243 1 diffi, or yellowifh : thefe colours alfo refilled the joint action of fire and air, equally with the preceding. The browns and blacks were on fome pieces very gloffy and tolerably beautiful.
Different white earths gave different fhades of whitifh, greyifh, or brownifh ; but none of them gave a pure white, nor a whitenefs equal to that of the internal part of the porcelain. It is difficult to diftinguifh precifely the effect of particular earths, in this refpeCt, from that of the degree of fire or other circumftances in the procefs : for of pieces of the fame bottle, which had been furrounded and baked with the the fame earth, fome turned out ma- nifestly whiter than others. White fand, calcined flint, and gypfum, fcemed in general to give the greateft white- nefs, and tobacco-pipe clay the greateft brightnefs or gloffinefs, though this laft, baking together in a lump upon, the porcelain, made the furface in fome parts rough.
In this experiment, and in feveral repetitions of it, the furface of fome of the pieces proved rough like fhagreen, that of fome wrinkled like fhrivelled leather, and of others blirtered or full of blebs. Thefe appearances feem to have depended more upon the fire having been too ftrong cr too haflily raifed, fo as to make the glafs foft or rea'dy to melt, than on any particular quality of the materials with which it was furrounded ; though it appeared alfo that fome materials diipofe to thefe imperfections more than others. Pieces of one and the fame glafs bottle having been baked, fome with tobacco-pipe clay, and others with quicklime, and with bone allies, in the fame degree of fire, and for the fame length of time, the por- celain with the clay proved almoft every where fmooth and polifhed as the glafs was at firft, while thofe with the lime and with the bone am were all over wrinkled.
I i 2 From
[ 244 ]
From the different effects of different materials on the furface of the porcelain, I have endeavoured to add feme embellishment to this kind of ware. By covering the outtide of the glafs veffel to be changed with powdered charcoal, or with a mixture of powdered charcoal and white land, and the infide with white land alone, the por- celain veffel, resulting from the cementation, has proved of a deep black or a bright brown colour without, and of a tolerable whitenefs within. By covering different parts of the glafs with different powders, as charcoal, white fand, white clay, lime, and coloured earths, I have ob- tained, in one piece, variegations, not inelegant, of differ- ent fiiades of black, brown, grey and whitifh, and with gloffy and wrinkled veins. The above colours, with the bluith call which the glafs exhibits before the change is completed, are all that I have obferved glafs to receive by the procefs of baking.
As the action of loot and charcoal on iron, in the con- verfion of iron into Heel by baking, is promoted, and in fome refpedts varied, by the admixture of a little fea fait, and of the faline allies of wood, I made trial of the fame compolition for the baking of glafs ; and as the inflamma- ble ingredients in this mixture could not fail to give a black colour to the porcelain, I tried at the fame time, in another crucible, wood alhes alone, which had been calcined in a itrong fire, to burn out all remains of their inflammable matter, and reduce them to perfect white- nefs.
The fteel-making mixture did not anfwer fo well as the foot or charcoal by themfelves : the glafs did indeed become porcelain, but of a bad quality, all over bliftered, with many cavities, and fome of them very large, in the internal part. The wood allies, inftead of changing the
i glafs
[245 3 glafs into porcelain, melted and united with it into one femivitreous lump.
I tried likewife coleothar, or the red calx of iron, which remains from vitriol after the acid has been expell- ed by fire. Pieces of green glafs being furrounded with this powder, and baked for feveral hours in the upper chamber of the wind-furnace, the glafs and coleothar were all found to have run together into a black mafs, externally rough, internally fomewhat fmooth and caver- nulous, of confiderable hardnefs fo as to ftrike fire freely with fteel. It is pretty remarkable, that a metallic fub- flance fo refractory in the fire, ihould be fo greatly dilpofed to melt with green glafs.
SECT. IV.
Experiments of the baking of different forts of Glafs, and of bodies approaching to a vitreous nature.
C^\ L A S S confifls of earthy or ftony fubflances, or ~J metallic calces, brought into fufion and franfpa- rency in a fhrong fire. Pure unmixed earths cannot be made to vitrefy by any known degree of fire; but fre- quently one kind of earth is made vitreicible by mixin? with it a certain proportion of a different one, which fepa- rately is as unfufible as the other : thus clay and chalk, though each by itfelf is altogether unfufible, yet when mixed together in due proportions, melt and form a truly vitreous compound : in feveral of the experiments I have been giving an account of, the crucibles were found partly vitrefied, not on the outfide which had been immediately expofed to the fire, but on the iniide, which had been in contact with earths of a different kind from thofe of which the crucible was compofed. The feveral forts of glafs in common ufe are prepared however on another
principle ;
[ M* ]
principle; from fond, calcined flint, or pebbles, mixed with certain metallic or faline bodies, by which the earth i brought into fufion more eafily than by the addition of other earths.
Some glaffes of each of thefe kinds were cemented in the lame manner as the green glafs bottles in the preceding experiments; in hopes, that by purfuing the effects of the procefs upon a variety of bodies, though nothing lhould refult of practical utility, the nature of the change, philofophically considered, might at leafl: be illustrated.
J. Vitreous bodies compofed of earths, •without metallic or fa- line additions.
Pieces of crucibles, which from vehemence of fire had melted into a femitranfparent glafly Slate, were fur- rounded with bone am ; which was here made choice of as being the mod indifpofed to vitrefy of all the earthy bodies I know of. As this kind of glafiy matter is very hard of fufion, the crucible containing it was placed in a blafr. furnace, and the fire Strongly excited by the bellows for feveral hours, that the matter might undergo as great a degree of heat as it could bear without melting: the fuel was fea-coal, coaked or charred as for the drying of malt, which I find to be a very convenient fuel where bellows .is ufed, being very durable, and giving a Strong heat, without fmoke. The crucible being grown cold, the pieces were found of their original vitreous appearance, and without any change in their colour or tranfparency. Nor have I obferved that any compositions of mere earths, whether brought to a perfectly vitreous or only to a femi- vitreous flate, received any alteration from this procefs. China ware, which is reckoned a mixture of two different earths femivitrefied, was alfo found to refiSl it : the glazing
of
[ 247 ] of the ware foftened, fo as that the powder it was fur- rounded with partly adhered, but in other refpe&s there was no fenfible alteration, I tried likewife fome of the more fimple tranfparent and femitranfparent {tones, as cryftal and flint ; which, by long cementations with dif- ferent materials, received no other change than the dimi- nution of hardnefs- and tranfparency which fimple heat . produces in them.
II. Metallic Glaffes.
Pure glafs of lead, furrounded with fand, and baked for many hours in a moderate red heat, fuffered no per- ceptible alteration, except where fome of it had melted off and dilTolved a part of the fand. Common flint glafs, in which the flint or fand that makes its bafis is vitrefied chiefly by an addition of calx of lead, proved alfo unfub- duable by cementation : it became rough and brownifh on the outflde, and internally fomewhat cloudy, but gained nothing of the appearance of porcelain by long continued and repeated bakings.
I cemented likewife fome glaffes tinged with metallic bodies, as the common blue glafs tinged with the prepa- ration of cobalt called zaffre, blue and green glaffes with copper, and the ruby glafs -already mentioned in the tenth feclion of the hiflory of gold. All of them retained their vitreous appearance, and fuffered very little change even in their colour: the ruby glafs. grew fomewhat darker, and one of the copper glaffes more dull, but the blue glafs with zaffre did not appear to have received any alteration. The bone allies, with which all thefe glafles were fur- rounded, adhered to them pretty firmly, probably from the furface having been foftened or partly melted by the heat : on the zaffre glafs and the ruby glafs, the earthy cruit remained white as at firfr, : on all the copper glafles
[ 248 ]
it was reddifh ; on that particularly, which had loft con- siderably of its beauty in the procefs, the bone am was tinged of a fine flefh colour. This glafs had been prepared from twenty-four parts of green glafs, four of borax, and one of the powder feparated by agitation from an amal- gam of copper.
III. Glaff'cs prepared with Jaime additions.
All the glaffes that could be changed into porcelain were fuch as had been brought to their vitreous ftate by means of faline bodies ; though fome of this clafs refifted the operation, and in thofe which did become porcelain, there were considerable differences, in regard both to the facility of the change, and the quality of the porcelain itfclf.
Green glafs bottles, compofed of fand and the faline allies of wood, anfwered much the beft ; and the French bottles better, in point of colour, than the Englifh. One of our common quart bottles, and a French quart bottle, being furrounded with the fame fand, and baked in the lame fire, for the fame length of time, the porcelain from the French bottle turned out, in feveral repetitions of the experiment, manifeftly the whiteft, tho' in other refpedls no material difference was obferved. It is probable that the difference in colour proceeded from the French glafs being made with a whiter fand than the Englifh : it is faid that the fand ufed for green glafs in France retains its whitenefs, in great meafure, after ftrong calcination; while that of our glafs-houfcs burns reddifh.
The vials, in which Hungary water is brought from France, are very difficultly converted into porcelain, and the porcelain they afford is lefs white and lefs compact than that of the common bottles. The vials are much more fuiible than the bottles, probably from their having a
larger
[ 249 1
larger admixture of faline matter: they begin to melt nearly as foon as the fire is raifed high enough to change them ; and how carefully foever the procefs is managed, a part of the infide commonly runs out, and the fand they are furrounded with bakes into a hard cruft upon the furface. The lower portion of one of thefe vials having been ce- mented with a mixture of fand and gypfum, a part of it appeared changed throughout into a pretty hard porcelain, a part into a fubftance refembling the mixture baked to- gether, and a part feemed fcarcely changed at all : there were many large cavities, and the glafs, which had run out from them, coated a part of the mixture with a green glazing. In fome other trials the change was more equal, but I have never obtained from thefe vials a porcelain fo uniform, or fo hard, as from the common bottles.
Glafs tubes, of a pale green colour, were affected nearly in the fame manner as the Hungary water vials : they feemed to be fomewhat lefs difpofed to melt, and the fand did not fo flrongly adhere to them : from whence it may be prefumed, that this kind of glafs has a larger propor- tion of faline matter than the common bottles, but lefs than the Hungary vials.
The common pale green glafs retorts and receivers did not anfwer well. A piece cut off from the bottom of a retort, and feveral circular fegments of receivers, were placed within one another in a large pot, with fome bone am between and furrounding them, and cemented in a wind-furnace for feveral hours. They all became brown- ifh, rough and fhrivelled on the furface, in fome parts blif- tered, and in fome extremely thin as if part of the glafs had melted off. They were femitranfparent, nearly in the fame degree as the finer forts of ftone ware. They eafily broke, and appeared internally white, not fibrous or granulated, but of a fmooth glafiy furface.
K k Common
[ 250 ]
Common window glafs appeared to fuffer the fame change as the glafs bottles, becoming opake, and inter- nally both fibrous and granulated according to the con- tinuance of the fire : but it was greatly difpofed to blifler, and part of the glafs generally melted out.
The finer fort of glafs called crown glafs, and looking- glafs plates, did not become porcelain at all. In a mode- rate heat they grew wrinkled and mrivelled on the furface, and lefs tranfparent than before, but ftill remained glafs : on repeating the cementation with a flronger heat, they partly melted and mixed with the furrounding powder, but did not appear to have fuffered any other alteration. A glafs which I had prepared myfelf from calcined flint and pure fixt alkaline fait, remained alfo unchanged, in the fame heat, and furrounded with the fame powders, by which common green bottles had been turned into per- fect porcelain. Green glafs itfelf, melted with an addi- tional quantity of fait, amounting to about a ninth part of its weight, and then cemented with fand, continued like- wife unchanged.
The foundation of thefe remarkable differences may be prefumed, from this laft experiment, to depend chiefly on the different quantities of fait in the feveral forts of glafs. The vitrification of fand with wood-afhes is influenced not a little by the action of the two earths on one another; fo that, though vehement fire reduces the mixture into glafs, yet the quantity of faline matter in the allies is much lefs than would be furficient for the vitrification of the two earths feparately. In the other coarfe forts of glafs, a larger proportion of the afhes, and confequently of faline matter, is ufed, or fome alkaline fait itfelf is added, to render the mafs more fufible. In the fine glaffes, the quantity of fait is ftill larger, the vitrification being effected almofl wholly by this ingredient.
It
[25I ]
It feems to follow, from the whole, that bodies of the glafly kind are changeable into porcelain by baking, only in fo far as their vitrefcent flate has been procured by means of faline fubftancesj that thofe which hold the leafl fait are the moll: eafily changed, and that the porcelain they afford is the moil perfect ; and that thofe, in which the proportion of fait is large, are very difficultly, if at all, made to undergo this change. Agreeably to thefe remarks, M. de Reaumur obferves, that the very worft glafs makes the beft porcelain ; and fuggefls, that in order to the per- fection of this kind of ware, it may be neceffary for the glafs-maker to acquire a habit of blowing veffels from more refractory forts of glafs than thofe which are com- monly worked. Perhaps the fame end might be obtained more advantageoufly on another principle, which the fore- going obfervations point out, viz. by forming the glafs of certain earthy compofitions more difpofed to vitrefy than thofe commonly employed ; fo that a very little fait fhall be fufficient for their vitrification in the furnace of the glafs-houfe, and that the glafs they afford, inftead of being more refractory, fhall be even more fufible than the common green glafs.
The different effects of cementation on different kinds of glafs may perhaps afford fome light into the caufe of the change which coarfe glafs undergoes, and fome ufeful characters and difcriminations of different vitreous and fe- mivitreous bodies.
SECT. V.
Obfervations on the caufe of the changes 'which green Glafs undergoes by baking.
THE mofr. obvious way of accounting for this extra- ordinary change is, to fuppofe the earthy or other unvitrefcible particles of the matters, with which the
K k 2 . glafs
[ 252 ]
glafs is baked, to be forced into its fubftance by the heat : iiich is the idea that firft occurred to me on the difcovery of this change, and fuch is the idea of Neumann and Reaumur. But fpecious as this theory appears to be, there are fome facts which feem to overthrow it.
If the change proceeded from the introduction of any extraneous matter into the glafs, the porcelain would weigh more than the glafs ; as fleel, prepared from forged iron by cementation, is found to weigh considerably more than the iron before the cementation. But pieces of glafs bottles, baked with fand till they had become fibrous throughout, and then wiped clean from the fand, were found, on feveral trials, to have received no increafe of weight.
In fome of the foregoing experiments in which the glafs became perfect porcelain, the cementing material was a very coarfe fand, which had been fifted from the finer grains for other ufes. Is it not improbable, that the large grains of fand fhould be fubtilized by the heat, and driven every w here fo equally into the fubftance of the glafs, as to produce the remarkable finenefs, and regularity of texture, of the fibrous porcelain ? Charcoal, which gives fo permanent a blacknefs to the outer furface, can fcarcely be fuppofed, when introduced into the internal part, to make it white. Nor could the porcelains, produced with different materials, be fo exactly alike in their internal fubftance, if they proceeded from a coalition of the differ- ent bodies with the glafs.
When green glafs is heated till ready to melt, and then Suffered to cool, it frequently contracts bluifli fpecks or veins ; which induced me to fufpect, that the fame change was there beginning, as happens at the beginning of the baking. In purfuance of this obfervation,- 1 placed fome necks of quart bottles upright in large crucibles3
fecuring
[ 253 1 fecuring the lower ends in a bed of luting : the crucibles being clolely covered, and expofed to the fame degrees of heat as in the preceding experiments, the glafs underwent the fame changes as if it had been furrounded with fand ; with only this difference, that the changes happened much more flowly ; and that the pieces of glafs having no folid matter to fupport them at the fides, many of them, made foft by the heat, had failed and bent downwards. It feems manifeft from this experiment, that the fand or other materials are of ufe only for expediting the procefs, and affording a fupport to the veffel ; and that they are entirely ineffential to the porcelain itfelf.
There are other facts which feem to fhew, that the glafs, inftead of receiving any new ingredient in its con- verfion, lofes a part of one of its own, at leaft in the lat- ter ftages of the procefs. In the cementation with fand, the fand near the furface of the glafs, as already obferved in the preceding part of this effay, was commonly found to cake together ; a prefumption that it had imbibed fome of the faline matter of the glafs, for fand of itfelf is- never found to cohere by heat. When frefh pieces of glafs were cemented repeatedly with the fame fand, the fand actually begun to melt, and covered the furface of the glafs, or of the porcelain refulting from it, to a con- fiderable thicknefs, with a femivitreous coat, which ad- hered fo ftrongly as not to be feparated by a blow. When the baking of one piece of glafs was long continued, it became friable and porous, the adjacent fand concretino- at the fame time into a mafs fcarcely to be dftinguifhed from it.
From thefe obfervations I apprehend it may be con- cluded, that a part of the alkaline fait of the glafs exudes by the heat, and is imbibed by the furrounding matters; and that by a long continuance of the baking, fo much 5 of
t 254 ] of the alkali is forced out, that there is fcarcely enough left to make the earthy bafis of the glafs cohere. Thus glafs, whofe production has been commonly fuppofed to be the utmofl limits of the power of fire, has its earth and its fait, which one degree of fire had fo firmly united, almoft wholly disjoined by another.
If thefe principles bejuff, they afford a fatisfactory ex- plication of the moft remarkable phenomena of the pro- cefs; as of thofe glafles only being convertible into por- celain which are prepared with faline matters, of thofe being eafieft changed which have leaft fait, of the fame changes being producible by cementation with very diffi- milar materials, of the glafs becoming lefs and lefs fufi- ble in proportion to the change, of its becoming more and more hard to a certain point, and afterwards more and more brittle.
It has been obferved above, that glafs, in its converfion into a fibrous porcelain, did not receive any increafe of its weight : it mud be added alio, that it fcarcely fuffered any fenfible diminution. I do not however apprehend, that this experiment is altogether irreconcilable with thofe which feem to prove the exudation of the alkaline fait of the glafs. The flrong marks of the exudation do not appear till after the porcelain has considerably paffed its fibrous ftate ; and then the fand, concreting infepa- rably upon its furface, prevents our being able to deter- mine any thing from the weight. Green glafs has as I'm all a proportion of fait, as can be made to vitrefy the' earthy matter in the flrongeft fires of the glafs-houfe : it is probable therefore that the feparation of a very mi- nute portion of the fait may be fufficient for producing the firfl degree of change, or rendering the mixt no longer glafs ; the fufibility of the fibrous porcelain feems to be a proof, that the quantity of alkali feparated at this 5 period
period cannot be confiderable. Perhaps alfo this fir ft degree of change may depend in part on an alteration produced, by the heat, in the glafs itfelf confidered as a compound, or in the nature of its alkaline ingredient. The remarkable differences, in point of brittlenefs, which happen to glafs merely from the quick or flow manner in which it is cooled, are well known : and as to alkaline falts, when expofed for fome time to a moderate fire, a part of them is always found to lofe its faline nature, and become an earth.
IV. Of
[ 2J6 ]
IV. Of the Expansion or Contraction of cer- tain Bodies at the Time of their paiTing from a fluid to a folid State.
TH E expanfion of bodies by heat, and their con- traction by cold, fuppofed to proceed always uni- formly by equal augmentations or diminutions of heat, appear to have fundry irregularities ; which may deferve to be taken notice of, not only in a philolbphical view, but likewife as being productive of fome effects interefting to the workmen.
It has been frequently obferved, that when thermo- meters prepared with different fluids, as quickfilver, fpirit of wine, water, and oil, have two diftant points of heat marked equally on them all, and the fpaces between divi- ded into an equal number of parts ; the heat, which makes the fluid in one expand to any of thefe intermedi- ate points, fhall raife that in another above the corre- fponding divifion, and in another not fo high. It was pro- bably this irregularity in the expanfion of the fluids, that prevented the agreement of the mercurial and fpirit ther- mometers which Boerhaave fays he had made for him by Fahrenheit : the different expanfions of different kinds of glafs, to which the ingenious artift has recourfe in order to account for the variation, appears to be infurfi- cient for producing it ; fince, if the expanfion of the two tubes be always uniform, or in the fame proportions to one another, the quantity of this expanfion cannot influence the apparent proportional expanfions of the fluids. I have feen a mercurial and fpirit thermome- ter very nearly correfpond, at different divifions, from the freezing point to the heat of melted wax : the di- vifions
[ 257 ] virions on the mercurial one were all equal, thofe of the other widened upwards ; as if heated fpirit either ex- panded more, or heated mercury lefs, by a certain addi- tional heat, than the fame fluids do by an equal addi- tion of heat made to them in a colder ftate. Reaumur fays, that water from freezing to temperate expands only one tenth part as much as fpirit does, but that from freezing to boiling it expands half as much as fpirit in the fame interval. Though the difference in the propor- tion at different periods of the heat is doubtlefs very confiderable, I apprehend it does not amount to quite fo much as this ; and that the miftake arofe from fuppo- fing the full heat of boiling water to have been commu- nicated to the fpirit thermometer immerfed in it for a little time ; whereas fpirit cannot bear fo great a heat as that in which water boils, and confequently, in this part of the experiment, the fpirit was lefs heated than the water it was compared with. Thefe variations in the proportional expanfions of different fluids feem to have been little confidered by thofe who have given compari- fons of different thermometers, by reducing the divifions of one to thofe of the other from only two correfponding points on each.
A more remarkable exception from the general law of expanfion is obferved in the freezing of water. Though water fh rinks more and more, as its warmth diminishes, down to the period of its congelation ; yet, at the inftant of its becoming ice, it expands into a larger volume, fo as to burfl the ftrongeft veffels that have been employed for confining it. The floating of ice in water is a necef- fary confequence, and a convincing proof, of ice being lefs denfe, or more expanded, than water in its fluid ftate. M. de Mairan, in a differtation on ice, attributes this increafe of the bulk of the water chiefly to a different arrangement of its parts j the icy fkin on water being com-
L 1 pofed
[ 258 ] pofed of filaments which are found to be joined con- Aantly and regularly at an angle of fixty degrees, and which, by this angular difpofition, occupy a greater vo- lume than if they were parallel. He found the augmen- tation of the volume of water by freezing, in different trials, a fourteenth, an eighteenth, a nineteenth, and when the water was previoufly purged of air, only a twenty-fecond part : that ice, even after its formation, continues to expand by cold -, for after water had been frozen to fome thicknefs, the fluid part being let out by a hole in the bottom of the veffel, a continuance of the cold made the ice convex j and a piece of ice, which was at firft only a fourteenth part fpecifically lighter than water, on being expofed fome days to the froft, be- came a twelfth part lighter. To this caufe he attributes the burffing of ice on ponds.
Wax, refins and animal fats, made fluid by fire, in-
ftead of expanding like watery liquors, mrink in their
return to folidity ; for folid pieces of the fame bodies fink
to the bottom of the refpedlive fluids, a proof that thefe
bodies are more denfe in their folid than in their fluid
ftate. The oils which congeal by cold, as oil olive and
the effential oil of anifeeds, appear alfo to flirink in their
congelation. Hence, the different difpofitions of different
kinds of trees to be burfl by, or to refill, ffrong frofts, are
by fome attributed to the juices, which the tree abounds
with, being in the one cafe watery, and in the other refi-
nous or oily.
The earthy powders that mingle with water into an uniform parte, exhibit differences, not lefs ftrongly marked, in the affections of their volume by drying. The contraction of clay in drying is well known, and allow- ance is made for it by the workmen, in forming models or other works of moifl clay where any exaclnefs is re- quired in the dimeniions. I tried pure clay, and mixtures of it with different proportions of fand, all beaten up
I with
[ 259 ]
with fo much water, as made them juft foft enough to admit of being formed into long narrow plates : A par- ticular account of thefe experiments will be given here- after ; at prefent it is fufficient to obferve, that the plate of pure clay fhortened in drying one part in eighteen, while a mixture of the clay with twice its weight of find, fhortened but one part in thirty. It is not known that any kind of earth fhrinks fo much as clay, and hence the purity of clay may be judged from the degree of its contraction.
Plafter-of-paris on the contrary, diluted with water into the confidence of a foft or thin parte, quickly fets or grows firm, and at the inftant of its letting has its bulk increafed, as appears from the pretty experiment men- tioned fomewhere in Boyle's writings, and which I have often tried : A glafs vefTel being filled with the fluid mixture and clofely ftopt, the glafs burfts while the mix- ture fets, and fometimes a quantity of water iffues through the cracks. This expanlion of plafler-of-paris, in paffing from a foft to a firm ftate, is one of its valuable proper- ties ; rendering it an excellent matter for filling cavities in fundry works, where other earthy mixtures would fhrink and leave vacuities, or entirely feparate from the adjoining parts. It is probable alfo that this expanfion of the plafter might be made to contribute not a little to the elegance of the impreffions which it receives from medals, &cc. by properly confining the foft matter, that its expanfion may force it into the minuteft traces of the figure j the expanfion of the matter doing the fame office as the preflure by which wax is forced into the caviti :. of a feal.
There are grounds to believe, that differences of the fame kind obtain in melted metals at the inftant of their fix- ing or becoming folid ; that at this period they do not ob- ferve the fame laws as before or after it ; and that, while
LI 2 fome
[ 260 ]
fome of them contract like oily or refinous fluids in their congelation, or like clay in drying ; others expand, like congealing water, or like plafter when it fets.
Mr. Smeaton found, in a fet of curious pyrometrical experiments, of which an account is given in the 48th volume of the Philofophical T ran factions, that from the heat of boiling water to freezing, a rod of zinc fhrunk near three times as much as one of regulus of anti- mony ; yet, when the two metals were melted, the regu- lus of antimony feemed to lhrink in fixing confiderably more than the zinc. This difference is the more remark- able, as among all the metallic bodies that have been tried, regulus of antimony in its folid ftate contracts or expands the leaft, and zinc the moft, by equal augmen- tations or diminutions of heat ; whence the excellence of this laft for metalline thermometers, and other inff.ru- ments whofe effect depends on their length varying ac- cording to the degree of heat.
An elegant phenomenon of the contraction of filver in fixing is often obferved at the end of the procefs of cu- pellation. When the filver remains fine on the cupel, if the veflel be drawn forwards from the heat, that it may cool fome what haffily, the furface of the metal fuddenly fixing and contracting, fqueezes out fome of the fluid part within, which iffues in little jets through different parts of the folid cruff, and fometimes fpirts up to a confidera- ble height, hardening in the air as it rifes. M. Morel, refiner of the mint at Paris, made feveral experiments of this vegetation as it is called, of which an abftract is given in the French Memoirs for 1727 : to cool the metal the more haftily, he applied a wet cloth to the furface, and at the fame time dipt the bottom of the cupel in cold wa- ter, by which means he obtained larger and more nume- rous jets, varioufly arranged : he obferves, that the larger
the
[26l ]
the quantity of metal, the finer the vegetations are ; that a mixture of two parts of lead and one of filver gave finer vegetations than pure filver ; that pure lead had its furface perforated too haftily, and that its jets hardened without rifing high ; that copper is not eafily made to vegetate at all, its furface growing fo hard as to afford more refift- ance to the fluid underneath than the bottom of the cu- pel does, which laft accordingly burfts ; and that gold, inftead of jets which continue fixed at the lower end to the furface of the mafs, throws off fmall round grains, fometimes to the diftance of ten inches.
M. de Reaumur, from fome phenomena in the cafting of iron, fufpected that this metal expands in fixing ; and accordingly made feveral experiments, which are related in a paper in the memoirs of the French Academy for the year 1726, for determining whether iron really expands at this period of its cooling, and whether it is the only me- tal poffeffed of that remarkable property.
He obferves, that lead, tin, copper and filver, caft into- ingots, are always concave or depreffed on the upper fur- face, which feems a mark of their having fhrunk in fix- ing. He melted each of thefe metals, feparately, in fmall cylindrical crucibles, which being quite filled with the fluid metals, a plate of iron was paffed over the furface to take off fuch part as might have rifen above the edges : when grown folid, they were all found to have funk con- fiderably in the veffels ; and on melting them again they were found to fill up the fpace which they had forfaken in cooling. Having melted pieces of each of thefe me- tals in feparate crucibles, he dropt into them pieces of the refpective metals unmelted ; they all funk beneath the furface, and fome fell with a thump on the bottom of the crucible : from whence it is plain, that filver, copper, lead and tin, are heavier, or more denfe, in their folid
1 than
[ 262 ]
than in their fluid ftate, and confequently that the melted metals contract in becoming folid. In making the laft of thefe experiments fome caution is neceffary, particu- larly with regard to lead and tin : for if thefe metals are made very hot, the pieces thrown in will melt fo fad, that it cannot be judged whether they fink or fwim : if not heated fufficiently, they prove fo thick, that the ex- cefs of gravity of the folid piece will not be able to over- come the tenacity of the fluid, whence the piece will be fuftained on the furface, more efpecially if it is fmall, and if the fkin which forms on the fluid has not been pre- vioufly taken off.
On trying the fame experiments with iron, the event was different. Ingots of iron appeared, he fays, not hol- low .on the furface, but fenfibly elevated or convex. Ha- ving brought fome iron into thin fufion, and carefully cleared the furface from grofs matter, he threw in folid pieces of caft iron of different kinds: they all fvvum, like ice on water, and when pufhed down beneath the fur- face, they immediately rofe again ; a proof that the folid pieces were lighter or more expanded than the fluid, and confequently that melted iron expands or increafes its bulk in becoming folid. To this property he afcribes the neatnefs with which caft iron receives impreffions from moulds, a neatnefs wondered at by thofe who work only in copper or other metals : he fays he has feen a hundred and a hundred times, that though iron was poured quite thick into the mould, it neverthelefs took the figure well, its expanfion forcing it into the fmalleft cavities : that he has often feen the workmen furprized to find, that it was with the utmoft difficulty they could unferew the moulds in which iron had been caft, while nothing of this kind is ever obferved to happen in the cafting of other metals : that works of lead, copper, gold, and filver, are always
found
[ *63 ] found lefs than the moulds, but thofe of iron equal to them, or a little bigger.
Having found on trial that folid pieces of iron rofe to the furface of melted iron, and being thus convinced of the truth of Reaumur's principal experiment; I thought I had fufficient foundation to fay, in the foregoing part of this work, page 67, that iron expands in its paflage from a fluid to a folid ftate. Some inflances I had Ceen of melted iron having clofely applied and fixed itfelf to folid pieces by which it was confined, confirmed me in this opinion, and induced me in page 59, in propofing a method of obviating the common imperfections of the rolls for flatting gold and filver wire, by cafting a hoop of fteel, and after fufficiently forging the hoop, fixing an iron axis within it, to recommend, as the readieft way that occurred for fecuring the axis, pouring melted iron into the fpace between it and the hoop.
An ingenious correfpondent has fince obferved to me, that melted iron does not expand in fetting, and will not fix itfelf to the hoop, but fhrink from it as other metals would do : that though works of caft iron are indeed ge- nerally larger than the mould, yet this increafe of bulk does not proceed from the expanfion of the metal itfelf, but from its fluxing and drinking into its furface a con- fiderable quantity of the fand of which the mould is com- pofed, which he judges to have been the foundation of Reaumur's miftake : that large iron works caft in open moulds, as forge hammers and anvils, have confiderable hollows on the upper fide : that large works caft in clofe moulds have always a cavity fomewhere in the internal part j that cannon balls cannot be caft without fuch cavi- ties ; of which the workmen are fo fenfible, that in order to avoid as much as poffible the inconvenience of the hol- low being near the fide, they turn the mould upfide down
foon
[ *64 ] foon after the metal has been poured in. It is fuppofed, that as cafl iron begins to fet on the furface almofl imme- diately on its being poured into the mould, lb as to be- come covered with a folid fhell, the cavities mufl proceed from the fluid part within fhrinking while it becomes folid. Several experienced artifls have alfo aflured me that the melted iron will fhrink from the hoop.
I mall always take the earliefl opportunity of acknow- ledging miftakes, and my obligations to thofe who mail point thern out. With regard to the method of making the rolls, it may be obferved, that the difputed property of iron does not affecl the eflential part of the propofal, which may therefore flill deferve attention, whether iron has or has not that property : for though the particular way, recommended for joining the parts, fhould not an- fwer, the artifl cannot be at a lofs to find means of fup- plying the defect, or of fixing an iron axis in a fteel hoop.
To fatisfy myfelf in regard to the fact, and to difco- ver whence any fallacy might have arifen in the confe- quence of an experiment which appeared fo decifive, I made fome further trials.
A rod of iron being placed upright in the middle of a fteel ring, I melted fome cafl iron, and poured it into the fpace between them. When cold, the caft iron firmly embraced the rod, but parted without difficulty from the ring, though it had received very neat impreflions from fome marks on its furface.
I melted a quantity of the iron in a large crucible, and thoroughly cleared it from the grofs matter on the furface. When in perfect fufion, I threw into it a folid piece of the fame kind of iron previoufly heated : the piece dropt to the bottom, but immediately rofe up again to the top, as wood does in water : being pufhed down with an iron rod, it rofe again, and continued to float till it melted
and
[ 265] and united with the reft. I tried different kinds of call iron, with the fame event. Even forged iron, though considerably heavier than caft iron in its folid ftate, was found to be lighter than melted caft iron ; for it floated on the furface, and when pufhed to the bottom, it rofe up again, and this repeatedly till it was diflblved by the melted metal. Had the folid pieces barely fwum on the top, it might have been fufpected that they were kept from finking only by want of fufficient fluidity in the melted iron : bat their conftantly rifing up from the bot- tom, feems a proof of their being lighter than the fluid.
It appears therefore that melted iron is really of greater fpecific gravity, or more denfe, than folid iron, and con- fequently that in fixing or becoming folid, it becomes lighter, or expands into a larger volume j and yet, that when grown cold, it does not prefs againft, or keep dif- tended, the vefTel or cavity it was poured into. Nor do thefe different effects feem to be at all repugnant to one another. It is not pretended that iron expands at any other period of its cooling, than in the inftant of its paf- fage from a fluid to a folid ftate : after this time it con- tracts like the other metals. The internal cavities are agreeable to this account : the outer furface firft expand- ing and fixing, a vacuity would remain under it if the next did not alfo expand : a vacuity muft neceffarily remain at laft, which can be filled only by the fubfequent contrac- tion ; and its not being filled feems to fhew that the expan- fion is greater than the contraction.
To judge in fome meafure of the degree of the con- traction, I melted fome caft iron, and poured it into a long narrow iron ingot mould. The ingot proved in fome parts convex on the furface, and in others a little deprefied : it was fhorter than the mould by nearly three parts in three hundred and thirty-two, or one part in a
M m hundred
[ 266 ]
hundred and ten ; though it had filled the mould in its fluid /late, having taken an impreffion from both the ends. The real contraction mud have been fomewhat greater than this ; becaufe the mould muft necefTarily have acquired a considerable heat at the time of the iron's fixing, and confequently in cooling fhrunk along with it.
I have mentioned above that I had feen inflances, in which melted iron applied itfelf firmly to unmelted pieces by which it was confined. The foregoing obfervations occafioned me to recollect the circumftances in which thofe inflances had happened, and indeed naturally point them out. Caft iron fhrinks from an iron or fteel ring, which it filled and diftended at the time of its fixing : but if this ring be previoufly made very hot, it might be prefumed that its fhrinking would keep pace with that of the caft iron, fo that the latter would ftill continue to fill it.
Accordingly I heated the ring to a ftrong red or rather white heat -, and placing it on a bed of fand, poured into it the melted iron : when cold, the caft iron filled the ring, and was firmly applied to it, fo as to be in no dan- ger of being feparated or moved by any force that the rolls for flatting gold or filver wire are defigned to un- dergo ; though the juncture was not, perhaps, fufficiently ftrong for refilling fo great force as other rolls muft ne- cefTarily bear in the flatting of larger metalline mafles.
This laft experiment is entirely agreeable to, and feems to confirm, the foregoing. For though the ring or mould be fuppofed heated even to the degree in which caft iron fets ; yet, if the melted iron fhrunk in fetting, it would have become lefs than the mould, and continued fo in the fubfequent period of the cooling.
V. Of
[267 ]
V. Of the blowing of Air into Furnaces by a Fall of Water.
TH E earlieft method of animating the large fires of the furnaces for fmelting ores, appears to have been by expofing them to the wind. Such was the practice of the Indians of Peru before the arrival of the Spaniards in that country. Alonfo Barba relates, that their furnaces, called guairas, were built on emi- nences, where the air was freed ; that they were perfo- rated on all fides with holes, through which the air was driven in when the wind blew, which was the only time the work could be carried ©n ; that under each hole was made a projection of the ftone-work on the outfide, and that on thefe projections were laid burn- ing coals, to heat the air before its entrance into the furnace. Some authors fpeak of feveral thoufands of thefe guairas burning at once on the fides and tops of the hills of Potofi.
I have been informed, that feveral remains of a like rude procefs are to be feen in fome parts of our own coun- try. The old blomery hearths, as they are called, for the running down of iron ore, are all on the tops of hills; a fituation which can fcarcely be fuppofed to have been chofen on any other account than for the conveniency of the wind, being, in other reipects, extremely incommo- dious.
M m 2 The
[ 268 ]
The gradual fuccefTion of bellows to this precarious and infufficient way of fupplying air, and the gradual improvements made in the ftruclure and manner of work- ing of the bellows, cannot perhaps be traced. It appears, that at fome of our iron furnaces and others, the bellows were formerly moved by a handle as thofe of the fmith's forge, or by the preflure of the foot upon a treadle, or by other means requiring the ftrength of men : and that, fince the force of water has been called in aid to move them, the quantity of ore run down has not only been far greater, but the feparation of the metal more complete; infomuch, that great part of the iron now prepared at fome confiderable works, particularly in the county of Gloucefter, is no other than what had been formerly left in the flags or cinders for want of fufricient force of air.
The bellows ufed at our furnaces are compofed of two boards joined by leather, nearly in the fame manner as the common bellows. A cheaper kind of bellows, made en- tirely of woood, was introduced at the furnaces of the Hartz forrefl: in Germany, according to Schluter, about the year 1620, and has fince been received in Sweden and fome parts of France. It confifls of two long boxes, of the fame figure with the fmiths bellows, one of which drops over the other, and is of fuch depth, that when raifed up on a hinge as high as it is intended to be, it no where comes entirely off: the air enters by a valve as in other bellows, and is forced out by preffing down the upper box : along the edges of the lower or inner box are placed laths, which Aide horizontally in grooves, and to which are fitted fprings capable of preffing them an inch or two beyond the box, fo as to form a ledge, of variable width, which always accommodates itfelf to the
outer
[ 269 ] outer box, and in great meafure prevents the air from escaping between them.
Though the wooden bellows have an advantage above the leather ones of being lefs expenfive and more durable, they have confiderable defects ; for it is fcarcely pomble to make the junctures fo tight as to allow no exit to the compreffed air, and the friction mud neceflarily be very great. Some have therefore had recourfe to water, for doing the office of the under board of the bellows. A bellows on this principle is defciibed by Mr. Triewald in the Philofophical Tranfactions, and I have been favoured with defcriptions and drawings of two fingular ones, now ufed at fome of the iron works in this king- dom, one for the finery, the other for a large iron furnace, in which the fuel is coak, and which requires the greateft force of air of any known kind of furnace. An account of thefe will be given hereafter.
There is another method of applying water, fo as to produce a ftrong blaft, by means more fimple than any of the foregoing, and at little expence. A ftream of water, falling through a pipe, in certain circumftances, carries air down with it ; and this air, afterwards difen- gaged from the water at the bottom, may be fo colle&ed, as to have no other vent than a pipe which fhall carry it to the furnace.
Machines, conftru&ed on this foundation, though little known among us, are ufed in different countries, inftead of bellows, for animating the large furnaces. But their ftructure and principles of action have hitherto fo little undergone a fcientific examination, that thofe, which have been found to anfwer the beft, may be prefumed to owe their excellence merely to chance, and that the workmen have often laid the greateft ftrefs upon the pro- portions of parts which are ineffential. Thefe machines 4 are
[ 27° ] arc doubtlefs capable of being much improved, fo as to produce greater effects, with a lefs quantity, and what is of more confequence, with a lefs fall of water : and principles may doubtlefs be difcovered, by which their ftrudture may be regulated, and their power afcertained.
The importance of procuring commodious and cheap means of fupplying the vaft quantities of air which the fmelting furnaces require, induced me to examine tho feveral accounts that have been publiihed of thefe fimple fubftitutes to bellows ; and to make fundry experiments for bringing them nearer to perfection, and for eftablim- ing their laws of action.
SECT. I.
Account of the principal machines ufcd for blowing air into furnaces by a fall of water.
I. A fmple pipe.
TH E firft account I have met with of a machine for propelling air into furnaces, by a fall of water carrying down air with it, is of one at the copper or brafs furnaces at Tivoli near Rome, of which a defcription and figure are given in the third number of the Philo- fophical Tranfadtions, and in the Journal desfavans for the year 1666.
A fquare wooden pipe, of confiderable width, and open at both ends, is placed upright. A ftream of water runs in at the top, and is difcharged at the bottom ; and about the middle of the height of the pipe a fmaller horizon- tal one is inferted, which reaches to the furnace, and is faid to convey to it a ftrong blaft of air.
From fo imperfect a defcription, we can learn little of the nature of the machine, or of the manner in which the blaft is produced. It may be prefumed that the wa- ter,
[ 271 ] tcr, running forcibly againfl the fide of the pipe, as it appears to do in the figure, is in great part dafhed into drops ; the intervals between which being filled by air, this air is fucceflively pufhed down by the drops which follow, and afterwards efcapcs as foon as it meets with a vent. There feems, however, to be either fome inaccu- racy in the defcription, or fome effential part omitted : for in fuch trials as I have made, when air, thus conveyed into a perpendicular pipe along with running water, was difcharged by a lateral aperture, part of the water always accompanied it in a flream ; and more of the water feemed to iffue out in proportion as the quantity of air introduced was the greater.
II. A pipe "with air holes, infer ted into an air veffel.
M. B e L I D o R, in his architecture hydraulique, gives a more particular defcription of a water machine ufed in fome parts of France : he fays there are four or five forges on the river Ifere, between Romans and Grenoble, which have no other bellows.
The flream is divided into two channels, and each di- vifion falls into an upright pipe ten or twelve feet high. Near the tops of the pipes are feveral holes, made floping downwards from the outfide to the infide : through thefe holes air enters, and is carried down by the water ; though the experiments in the following fedtion will fhew, that the quantity of air thus introduced is not fo great as in the difpofitions mentioned hereafter.
The effential difference of this inftrument from the fore- going confifts in its having an air veffel, or refervoir for the air, at the bottom. An oval wooden tub, near feven feet high, and three or four feet wide, is inverted, and its lower edge let into the ground five or fix inches. The 4 lower
f 272 ]
lower ends of the two upright pipes enter into the top of the tub, and under each pipe is a kind of fmall ftoot which the water falls on. The water loaded with air, daihing againft the ftool with great velocity, rebounds, and its air is difengaged : a pipe communicating with the top of the tub carries the air to the furnace, while the water runs out at a hole in the lower part ; a fufficient height of water being kept in the tub, above this hole, to prevent any air from efcaping by it.
III. A funnel and pipe without airholes, infer ted into an
air vejfel.
M. Mariotte, in his treatife dn mouvement des eaux, "ives an account of another contrivance for blowing fire by a fall of water, which Belidor fays, from the infor- mation of a friend who travelled in Italy, is ufed in the Tiburtine mountain near Rome, and near Salan on the lac de Guarde.
A wooden or tin pipe, fourteen or fifteen feet high, and one foot in diameter, has its lower end fixed into an air veflel or inverted tub, as in the preceding article, from one fide of which a blafl-pipe goes tapering to the furnace.
The upper end of the large upright pipe is contracted to an aperture of three or four inches, into which is fitted a funnel, whofe neck exactly fills it. Into the funnel there falls a ftream of water, from the height of ten, fif- teen, or twenty feet ; which we may prefume to be dafhed into drops in its fall, and to pufh down air before it on the fame principle as in the machine of Tivoli already mentioned.
This
[ 273 ] This inftrument promifes to be more effectual than ei- ther of the preceding, though irrthis country it can be of little ufe, fo high a fall of water being rarely to be pro- cured, at leaft in thofe places where fmelting furnaces are eftablimed.
IV. A funnel and pipe with air holes., infer ted into an
air vejfel.
At Lead hills in Scotland.
In N°. 576 of the Philofophical Tranfa&ions, in the year 1745, Mr. Stirling defcribes a machine erected in Scot- land, for blowing air into the furnaces in which lead ores are fmelted ; and for conveying frefh air into the works, fo as to fave the trouble and expence of the double drifts and fhafts, and the cutting of communications between them.
A flream of water runs into a wooden funnel, fo as to keep it always nearly full : the height of the funnel is five feet, and the diameter of its throat three inches and a half. The neck of the funnel is inferted into an upright pipe, whofe diameter is five inches and an half, and its length fourteen, fifteen, or fixteen feet : imme- diately under the throat of the funnel, four air holes are made in the pipe, at equal diftances round it, about an inch and a half wide, doping downwards from the outride to the infide.
The lower end of the pipe enters into a wooden tub, clofe at top, but without a bottom, fix feet high and five and a half wide, funk into a pit dug in the ground, and well rammed about with clay : in the middle of the tub, diredtly under the pipe, is a flat ftone about two feet high, for the water to fall upon ; and into the top of the tub is fixed a wooden pipe for carrying off the air, com-
N n municating
[ 274 ] municating at the further end with an iron one which enters the furnace : for'regulating the blaft, a fmall hole is made in fome convenient part of the pipe, which is flopt with a pin, or opened, according as the blaft is re- quired more or lefs ftrong. The hole in the lower part of the tub, by which the wafte water paflesout, is about five inches fquare ; and one fide of the pit, where the water runs off, is a little lower than the furface of the ftone, fo that the water can never rife high enough in the tub to cover the ftone ; though it is fuppofed to continue always a confiderable height above the top of the hole.
Though this machine is faid in the Tranfactions to be fufficient for the fmelting of harder ore than any in Lead- hills where it was erected, I have been informed by a perfon concerned in thofe works, that it has fince been found not to anfwer fo well as could be wifhed, and that accordingly it has been laid afide, and its place fupplied by the common bellows.
In Dauphiny in France.
The blowing machines ufed in Dauphiny for the forges and fmelting furnaces have a great refemblance in their general ftru&ure to the foregoing. They are defcribed by Swedenborg in the fecond volume of his regnum fubter- raneum, but with little exactnefs : a more accurate de- fcription and figures of them, taken from the papers hft by Reaumur, are inferted in the art des forges & fourneaux a jet; published laft year by the direction of the French academy.
The upright pipe is generally between twenty-five and twenty-fix feet high : it is compofed of two pieces of fir, hollowed, and joined together by iron work. In- ftcad of a diftinct refervoir or funnel on the top, a part
4 of
[ *7S ] of the pipe itfelf is hollowed fo as to perform the fame office : at the top it is twelve inches and a fifth in dia- meter (Engliih meafure) from thence it grows narrower to the depth of nearly thirty-four inches, where its width is only about three inches and three quarters : immedi- ately below this part, called the choak, its cavity widens to nearly eight inches and a half, and this width it pre- ferves throughout the reft of its length. Under the choak. are ten air holes, fix of which are in one horizontal plane, at equal diftances from one another, and the reft about three inches and three quarters lower down : all the holes are cylindrical, near two inches in diameter, and cut at fuch an obliquity, that the orifice of the upper ones is on the infide of the pipe eight inches, and on the outfide only five inches, below the choak.
The tub or air veflel, which receives the lower end of the pipe, is five feet and a half, or a little more, in depth, and nearly as much in width : the pipe enters into it about feventeen inches : about the middle of its height is a flat ftone or iron plate, fupported by crofs bars of wood. The air pafles off, as already mentioned, through a pipe inferted into the upper part of the tub, and the water through a hole at the bottom : on the outfide of this hole is fixed a wooden frame, with an upright Aider, by which the aperture for letting out the water may be oc- cafionally increafed or diminished. The blaft is regu- lated, and the air fufFered to efcape when it is not wanted, by a hole in the blowing pipe, to which is fitted a valve or a ftopper.
One of thefe machines is faid to be fufficient for the forge or iron finery, and two or three for the furnace in which the iron ore is run down.
N n 2 In
[ 276]
In Foix in France.
In the county of Foix, the blowing machines, as de- fcribed by Reaumur in the art des forges above quoted, are considerably different from the foregoing. The pipe is rectangular, and the part above the choak divides into three funnel-fhaped partitions. On the top is a refervoir or ciftern of water ; and two of the partitions, clofe on all fides, pals up above the furface of the water, for carrying down air, and thus fupplying the place of the lateral air holes : the water enters into the third partition, which is only the fpace between the two foregoing, and which has but two fides, formed by the two oppofite fides of the others.
The author makes the principal difference of thefe ma- chines from thofe of Danphiny to confiff. in this difpo- fition of the upper part : but the plate, annexed to his defcription, fhews another, which is, perhaps, more ma- terial to the effect of the inftrument. The whole height of the pipe, including that of the water in the refervoir on the top, is, according to the fcale, twenty or twenty- one feet, and the choak or narrow throat is almofl down at the middle of this height ; fo that the water iffues through the choak with a velocity which it acquires from a preffure of about ten feet, which is greater than in the machine of Dauphiny in the proportion of about eleven to fix : the quantity of water feems alio to be much lefs in proportion to the width of the pipe, the great pref- fure probably occasioning it to fpread, fo as to fill a large? bore than it could do when falling with lefs velocity.
Two pipes, divided in the fame manner at the top, are fed by one refervoir : the lower ends of the pipes enter into one large oblong box, from which the air and water pafs out as in the foregoing machines.
4 At
[ 277 1
At St. Pierre in Languedoc.
Mr. Barthes, in a curious paper printed in the third volume of the memoirs of the correfpondents of the French academy, gives a minute defcription, though in fome parts not fo clear as could be wifhed, of a blowing ma- chine at the forge of St. Pierre on the river Obriou, which he looks upon as one of the mod perfect of the inflruments of this kind. Its general ftructure is nearly the fame with that of Foix, but the height of water above the choak much lefs.
The upright pipe is fquare, about nine feet high, and fomewhat more than feven inches wide. Into its top are inferted, at oppofite fides, two pyramidal air pipes, widening upwards, and pafling up obliquely through a bafon of water four feet high. The fpace included be- tween the pipes, at their lower end, under the bafon, is a kind of hopper, into which the water enters through two apertures in the bottom of the bafon : to each of thefe apertures is fitted a pifton or flopper, hung to the end of a lever, by which it is raifed more or lefs, accord- ing as more or lefs water is required. Two of thefe in- flruments are furni£hed with water from one bafon ; and the lower ends of both enter into one air vefTel, which is near five feet high, about fix and a half long, near three and a half wide at one end, and not quite two at the other. The ftones, for the water to fall upon, are fomewhat lefs than four inches and a half diltant from the pipes : the water runs off through two rcctano-ular apertures at the bottom, eu-.h about eight inches and a half wide, and near fix inches high : the pipe which carries off the air, is an inch ii d a quarter in GiUirseter at the fmall end \v it enters the furnace.
The
[ *7* 1
The obfcure part of the defcription relate? to the hop- per, and the aperture, by which the water is difch.irged from it into the perpendicular pipe. The hopper feems to be divided into two upright partitions ; and there are ** two horizontal rectangular openings, through which the water runs into the two hoppers, each of them about feven inches and a half long, and in width five inches and a half, meafured on the level of the bottom of the refervoir, which width is reduced to four and a half at the extremity of the air pipes, where the hopper alio terminates."
The author obferves that in this machine, the water, iffuing from the hopper, is neceflarily reduced into drops. To fatisfy himfelf more fully of this particular, he took a tin veflel, eight inches and a half lquare and fix and a half high : in the middle of the bottom he cut a rectan- gular opening, about an inch and a tenth long, and eight tenths wide : to the two long fides of the flit he foldered two tin plates, inclined to one another, and a third acrofs them. Thefe apertures, he fays, reprefent thofe of the machine when the ftoppers are drawn up ; and water put into this veflel came out always, during the whole time of its running, in ftreams which ftruck againfl and crofied one another, and which, after fpreading, were re- duced into drops.
In this illuftration of the machine, though it feems clear, there muft be fomething which efcapes my appre- henfion. Having cut an aperture of the above dimen- fions in the bottom of a veflel, I fitted to each of the longer fides a plate half the width of the aperture, both of which plates were moveable, and kept at different inclinations by means of the third plate which pafled acrofs the middle of the two. The veflel being filled with water, I could not obferve, as indeed was expected, the
leaft
f 279 ]
Ieaft crofling of the ftreams that run through it : on the contrary greatefl part of the water iffued in two oppofite directions, horizontally, from between the ends of the plates.
SECT. II.
Experiments and obfervations for the improvement of the fore- going machines, and for ejlablifiing their principles of action.
I. Of the quantity of water they require, and the quantity or force of the air they afford.
TH E quantity of water may be estimated with fuffi- cient exadtnefs, from the height of the water in the funnel or bafon on the top, and from the width of the choak or throat of the funnel, through which it is prefTed by the force of a column of that height.
Defaguliers found, by an experiment often repeated, that the quantity of water running through a fquare inch hole, twenty-five inches under the furface, is five tons and a fifth in an hour, the ton containing two hundred and fifty-two gallons. The quantities discharged through equal holes at different depths being as the fquare roots of the depths, and the quantities through different holes at equal depths being as the areas of the holes ; it will appear on calculation, that in the machine at Lead-hills, whofe funnel is five feet high, and its throat three inches and a half in diameter, the expence of water is fomewhat more than feventy-feven tons in an hour, or near three hundred and twenty-four gallons in a minute ; and that in the machine of Dauphiny, where the height of water in the funnel is only about half as great, and the bore of the throat a little wider, the quantity of water is about two hundred and fixty-fix gallons in a minute. Perhaps the real quantity of water may be fomewhat lefs than this
calculation
[ 23o ] calculation gives, as the refinance of the comprefled air may occafion fome retardation of the motion. Of the other machines, the defcriptions are too imperfect or ob- fcure for any computation to be made from them.
The water, iffiiing from the narrow throat of the funnel with great velocity, is laid to fpread fo as to fill the wider bore of the pipe, and to become frothy from the mixture of air with it. The jet thus enlarged may be conceived as confining of a multitude of flender ftreams or drops, the intervals between them being occupied by air, which is continually fupplied through the air holes, and pufhed down by the fucceeding drops or ftreams. It has therefore been reckoned, that the volume of air which paffes down the pipe mull: be as much greater than that of the water, as the tranfverfe area of the jet, when fpread and reduced to drops in the pipe, is greater than when it parted through the throat of the funnel. Circles being to one another as the fquares of their diameters, the area of the pipe of the Lead-hills machine will be to that of the funnels throat as eighteen to twelve and a quarter : the volume of air, according to the above principle, being to that of the water in the fame proportion, and the quan- tity of water nearly 324 gallons in a minute, the quantity of air in a minute mould be about four hundred feventy- five gallons and a half, or 1 34000 cubic inches, or feventy- feven cubic feet and a half. In the fame manner, the machine of Dauphiny will be found to yield about 1080 gallons, or upwards of 304000 cubic inches, or 176 cubic feet, of air in a minute : fo that by this way ot reckon- ing, the Dauphiny machine, with near a fourth lefs water than that of Lead-hills, mould produce more than a double quantity of air.
But tho' this method of computation appears fpecious, it is not perhaps to be much depended on ; air, in differ- ent
[28l ]
cnt circumftances, occupying very different volumes, in virtue of its great compreflibility : nor is it certain that the bores of the pipes are fufiiciently filled, fo as to carry down the full quantity of air. It may be prefumed, that the air, intermingled in the jet, is always in fome degree comprefTed by the water; fo that the interftices between the ftreams or drops contain more air than equal fpaces of the atmofphere. It may be judged however from the above comparifon, that the wider the pipe is, in propor- tion to the funnel's throat, provided the water running through the throat will fpread through the whole extent of the bore of the pipe, the more air will be carried down.
Mr. Barthes, fhe only perfon I know of who has ex- amined thefe machines philofophically, and endeavoured to improve them, gives a method, in the memoir above- quoted, of comparing the proportional quantities or forces of the air in different blowing machines, on another principle. From confiderations too abflracted to be here particularized, he deduces a general rule, that the produce of air will be in all cafes in proportion to the quantity and velocity of the water : fo that the quantity of water and height of the fall being given in two machines, and the volume or force of the air afforded by one of them being meafured by experiment, the volume or force of the air in the other may be determined by the rule. Ac- cordingly he made feveral experiments of this kind in two machines ; meafuring the force of the air, when the water in the bafon was at different heights, by the weight, which the blafr, acting on the arm of a balance, was ca- pable of raifing. Taking one of thefe experiments for a ftandard, he computed by the rule what the refults of the others ought to have been ; but the experiments and calculations agreed ill together. And indeed the rule
O o does
[ 282 ]
does not feem to be applicable but in circumftances,, which can fcarccly be expected to occur ; for it fuppofes. the machines to be all perfect, and every drop of the water to have its utmoft effect, or to carry down with it as much air as it is capable of doing ; which cannot be admitted to be the cafe in any of the blowing machines yet conftructed.
In the art des forges are mentioned fome obfervations of Reaumur of the quantity of air afforded by the wooden bellows. He finds that thofe ufed at the iron furnaces yield 98280 cubic inches, or upwards of five cubic feet of air at every flroke, and* including the two bellows, which act alternately, 240 ftrokes in a quarter of an hour, which, on a reduction of the French meafures to the Englifh, make 130 1896 cubic inches, or upwards of 753 cubic feet, in a minute : this quantity exceeds that which the foregoing calculation gives for the machines of Dauphiny above four times, and therefore four of the machines mould fcarcely be able to fupply the iron furnace with fo much air as the wooden bellows does , whereas two or three are faid to be fufficient. Again, the bellows of the iron finery and forge was found to give two thoufand fifty-one cubic inches and a third at each ftroke, and four hundred and twelve ftrokes in a quarter of an hour j whence the quantity of air in a minute is 458247 cubic inches, or fomewhat more than 265 cubic feet: this is greater than the calculation of the water machine, in the proportion of about three to two, tho' one of the water machines is found to fupply the office of the bellows.
It is not to be fuppofed, that the quantity of air, which furnaces require, is confined to any fuch precife limits, as that two bellows, from their being found to anfwer . futficiently for one kind of furnace, or even for one indi- vidual furnace, can be concluded to yield quantities of
2 air.
L 283 1 •air exactly or nearly equal. The above differences are perhaps as little as can be expected in companions of this kind where the effecls compared are fo indeterminate.
As to the water machines, it is plain, that the quan- tity of air carried down cannot be greater, than the fpaces between the drops or divided ftreams in the pipe can contain ; and that though the air in thefe fpaces muft be coniidered as being comprefTed to a certain degree, yet it cannot be fuppofed comprefTed into two thirds of its na- tural volume, which would be neceffary for making the calculations of the wooden bellows and the blowing ma- chine to agree, becaufe fuch a condenfation would require the weight of a column of water of eleven or twelve feet, or the third part of fuch a column as is equivalent to the preiTure of the atmofphere ; whereas in the Dauphiny machine, though the air was preffed down with the full force of the column of water above the choak, the height of this column is lefs than three feet, and could not con- denfe it more than one twelfth part.
In what manner Reaumur computed the air of the wooden bellows, we have no account : it is probable that he judged, as others have done in the fame cafes, from their capacity ; fuppofing the whole quantity of air they contained to be delivered at every ftroke. If fo, we can lay no ftrefs on the computation, for neither the wooden nor the leather bellows deliver their full contents of air ; a confiderable fpace remaining full of air when the bellows are clofed ; and this fpace containing conliderably more air than an equal volume of the atmofphere, on ac- count of the air being condenfed in it by the prefTure of the bellows. I have been informed by a judicious work- man, that the bellows of the iron finery retains commonly a third, and fometimes half of its air ; and that when lined
Oo 2 Witla
f 2*4 3
with wood, io that as little vacant fpace as portable might be left, he found it to blow much ftronger than before.
The ftrength of bellows is beft judged from the force of the blaft: itfelf ; and this force may be determined, in the method recommended by Mr. Barthes, already men- tioned, by the weight it is capable of raifing. He found that in the blowing machine of St. Pierre, defcribed at the end of the preceding fection, the force of the blaft: ifluing from a hole of an inch and a third in diameter, raifed the arm of a balance loaded with a weight of twenty-five ounces and a half. He gives fome other ex- periments, of comparing the proportional diminution of its force according to the diminution of the height of the water; which I ftiall here infert in the original French meafures, to avoid unneceftary fractions. The above force of twenty-five ounces and a half is the maximum of this machine, produced by the full quantity of water in the bafon, or a height of forty-eight inches above the choak :. with a height of forty-one inches, the weight raifed was twenty-two ounces ; with a height of thirty-two inches, nineteen ounces; with a height of twenty-eight inches and a half, feventeen ounces and a quarter ; with twenty- four inches and a half, fifteen ounces and a quarter; with nineteen inches, twelve ounces and three eighths ; with fixteen inches and two thirds, ten ounces and a quarter ; and with a height of thirteen inches and a half, eight ounces and three quarters.
It may be obferved, that in fome of thefe experiments the water muft have been employed to disadvantage ; and that by increafing the height of the water much further than the above limits, in the fame machine, we could not expedt to produce proportional augmentations of the force of the blaft: : for if a certain quantity of water, run- ning with a certain velocity through the choak, be fup-
pofed
[285] pofed to fill the bore of the pipe ; a lefs quantity, with a lefs velocity, mult leave a vacancy, which will luffer part of the air to efcape ; and a greater quantity, with a greater velocity, mull have fome part of it fpent ineffectually, for want of fufficient room to fpread. Some experiments men- tioned hereafter afford a clear proof of this.
The force of the air may be determined in an eafier and more Ample method, by means of a glafs pipe, open at both ends, with one end fixed in a bafon of water. The bafon may be hung in the upper part of the tub or air-veffel of our water machines, and the glafs pipe let into it through a hole in the top, what fpace may remain between the pipe and the hole being properly clofed : the preffure of the air on the furface of the fluid in the bafon, forces part of it up into the pipe ; and this afcent will always be the meafure of the power or denfity of the air. Water is here greatly preferable to the quickfilver ufed in the fame intention on other occafions, as it dis- covers fmaller variations in the force j for being fourteen times lefs ponderous than quickfilver, an equal preflure forces it fourteen times higher in the pipe : the whole afcent of quickfilver, by the preflure of the air in bellows, is fo fmall, as frequently not to exceed that part of the pipe which is inferted into the tub. Inflead of a glafs pipe, a copper or iron one may be ufed ; and the afcent of the water meafured, either by occafionally dipping a rod in it, or by means of a hollow copper ball, or other floating body, with a Hem {landing out of the pipe, and a proper weight below to keep it upright. It mull be obferved, that the height of the water in the pipe is to be eflimated from the furface of the water in the bafon : whence the pipe ought to be of fmall bore in proportion to the bafon, that the water may not fall confiderably in the bafon by the lofs of that which rifes in the pipe.
Dr.
[ 236 ] Dr. Hales found that a fmith's bellows railed a mercu- rial gage about an inch, fo that it would have raifed a water-gage about fourteen inches. The twenty-five ounces and a half, railed in M. Barthes's experiment by the blaft of the machine of St. Pierre from an aperture of an inch and a quarter bore, Englilh meafure, are equiva- lent to the afcent of water in the gage pipe forty or forty- one, inches. I have been informed, that the pipe by which the air is difcharged into our iron furnaces is at leaft of an inch and a half bore ; and that the air, with this aperture to pafs off by, ought to be of as great denfity as it can be reduced to by the human breath in a con- fined fpace.j which is fuch as to raife the water in the gage about fifty inches ; in which cafe it is comprefled into near an eighth part lefs volume than it commonly occupies in the atmofphere. But the quality of the fuel and other circumftances occafion fuch variations in this refpect, that no general ftandard can be laid down. I have been aflured, that a charcoal fire will be excited as ftrongly by fuch a blaft as raifes the gage thirty-fix inches, as a fire of coaked pitcoal will be by one of fifty inches.
. II. Obfervations on the air veJJ'el.
The ftruclure of the air vefTel, or tub at the bottom, is in great meafure independent of that of the reft of the inftrument j the fame air velTel ferving equally for dif- ferent kinds and fizes of thefe machines, while the per- fection of the other parts confifts in their adjuftment and proportion to one another. The office of this veffel be- ing only to ferve as a refervoir for the air, and to fufFer the wafte water to pafs off, no great care feems to be needful for regulating its dimenfions ; and as the ftone, which is placed in it under the pipe, ferves only to receive
and
and fupport the fall of the water, or to occafion the water- to be dafhed into fmall particles, that the air may be the more effectually extricated, its diftance from the pipe feemsalfo to require no exact adjustment. There are however fome particulars, in regard to the fize of this vefTel, and the difpofition of fome of its parts, -which ap- pear to deferve attention.
The gage, mentioned, in the preceding article, will be an ufeful addition to it j fhewing at all times by infpec- tion the force of the blaft, and thus enabling the work- man to judge whether it is Sufficient for the purpofes intended, and giving him notice of any failings or imper- fections that may have happened in the machine ; as whether any air efcapes through the joints or cracks, or whether the choak or throat of the funnel is obftrudted by ftones or other matters brought by the ftream.
All the writers I have met with, who give any account of thefe kinds of blowing machines, feem to fuppofe the water within and without the air vefTel to be upon a leveL But as the air in the air veffel is fo far compreffed, as to be able to raife the water in the gage to a considerable height, it muft neceffarily act with equal power on the water below it ; and if this water can pafs off freely at the bottom, it muft be depreffed as much as that in the gage pipe is raifed. The water within and without the veffel is exactly in the fame fituation with that in the bafon and pipe of the gage ; excepting only . that the former receives a continual fupply within, which paffes off as faft on the outfide. The excefs of the height of water on the outfide of the veffel, above that of the water within, appears to be the very power by which the air is., compreffed and driven into the furnace.
To be further Satisfied of this depreffion of the water, I.ufed, for the air veffel of a fmall machine, a tall glafs, .
without :
[ 283 ]
without a bottom, feven or eight inches of its lower part being immerfed into a tub full of water. As foon as the machine began to play and the gage to rife, the water within the glafs funk lower than that in the tub on the outiide ; and the depreflion of the water and rifing of the gage were, as nearly as could be judged, equal, and kept pace with one another. In a little time the water was forced quite out of the glafs, and the air following it rofe in bubbles to the top of the tub.
The bottom of the air veffel ought therefore to be funk at leaf! as much below the level where the external water palfes off, as the gage is expected to rife j for otherwife, before the air is fufficiently compreffed to raife the gage to the due height, it will force all the water out below, and in part efcape itfelf by the fame aperture. Hence the depth of the air veffel, in any of thefe machines where the water has a free paffage at the bottom, gives a power which the force of the blaft in that machine can never be made to exceed : thus at Lead-hills, the water being only of the height of two feet from the bottom of the veffel to the level of the bank where it runs off, the air can never be compreffed further, than to be able to fupport a column of two feet of water, or to raife the gage to that height ; whereas in the machine of St. Pierre, the com- preffure is about two thirds greater.
The finking of the water in the air veffel may indeed be prevented, by making the aperture at the bottom, through which the water is difcharged, of fuch a fize, that the preffure of the air may be able to drive through it no more water than is received at the top. But fuch an adjustment would be apparently very difficult -, and tho' it fhould be exactly hit, yet, if the quantity of water received was not always the fame, it would fcarcely be poffible to 2 avoid
[28o]
avoid either a deprefllon or elevation of the water in the air veffel.
Though the depth of water be fufficient to refill: the preffure of the air, it will be eafily conceived, that if there was no folid body to fupport the fall, the great force of the ftream, falling from fuch a height, would pufh down or dam about great part of the water in the bottom, fo that the air would get at the hole, and in part make its efcape with the water. It may be prefumed that even the drops of water, rebounding from the flone, and falling down again, have a like effect, though in a lower degree : for drops falling through the common atmof- phere into water, carry air with them, which afterwards rifes in bubbles, as may often be obferved in heavy rains ; and it is not to be fuppofed, that the drops mould not here alfo carry into the water fome of the compreffed air, which furrounds them and is entangled between them. Though part of the air, which thus paffes into the water, doubtlefs rifes again in bubbles, as appeared in ufing the glafs air veffel above mentioned ; yet part may alfo be pufhed fo low, as to efcape through the hole, and difcover itfelf by bubbles in the water on the outfide of the veffel, which I feveral times obferved before the water was driven entirely out of the glafs.
Mr. Barthes likewife takes notice of air being thus carried down into the water by the drops, or introduced into the cavities which they form in falling. In order to prevent it, he recommends making a partition acrofs the tub, at the level of the flone, with only a hole at one fide, and this in the part mofl remote from the pipe through which the water falls : the rebounding drops are received upon the board, and run off gently through the hole into the water underneath.
Pp The
[ 290 ] The Inconvenience may be prevented alfo, as effectually, and with more advantage in other refpecls, by making the air veffel of a very confiderable depth below the fur- face of the ftone : it may be funk feveral feet into the ground below the level of where the outward water runs off, fo as to have always a column of water in the veffel, of any height required, or of a height which mall fecure againft any air palling down to the bottom. This ftruc- ture would free the workman from any care about in- creafing or diminifhing the aperture, or regulating the height of the water. For if the deep veffel has an aper- ture in its lower part, large enough to difcharge all the water that can fall into it through the pipe in the top, or, for the greater fecurity, a good deal larger, its mag- nitude being of no inconvenience ; if this veffel is funk in a pit of water up to the level of the flone, or to a cer- tain height above it ; and if the pit has a drain fufficient to carry off what more water it may receive : we may be fure that the water will be always high enough in the veffel, becaufe the preffure of the water on the outfide will keep it fo ; and that the preffure of the air within the veffel will always keep it below the furface of the (tone.
The air extricated from the water is always moift : when let off at a little way above the ffone, I have often obferved it to leave drops like dew on any folid body oppofed to it. A fmall degree of moifture may perhaps be of no difadvantage ; but fuch a degree as this muft doubtlefs be injurious, and render the air of lefs effi- cacy for animating the fire.
In the water machines of Dauphiny, inclined plates are faid to be placed at the entrance of the pipe which carries off the air, to keep back the watery drops. M. Barthes propofes letting the air off into another veffel, in
which
[ 29i ] which fponges are to be hung for imbibing its moifture, and in the bottom of which a cock is to be fixed for oc- casionally letting off the water that drops from the fpon- ges. I apprehend the intention may be more effectually anfwered, by making the air veflel of a confiderable height above the furface of the water : for though the air at the bottom is neceffarily loaded with moifture, yet in rifing to the height of four or five feet, fo much of the water feparates and falls down, as to leave the air feemingly of fufficient drynefs. The veflel might be made as high as the pipe itfelf : nor would this large fize be of any incon- venience in regard to the blaft, for as foon as it is filled with air of a certain denfity, the blaft will continue of the fame force as from a fmall veflel. The joints fhould be well fecured to prevent the efcape of any air through them : the ftone for receiving the dafli of water, fhould be placed near as much below the level of where the water runs off, as the gage is expected to rife ; and the pipe fhould reach as low as within five or fix inches of the ftone. It would perhaps be of fome advantage to have the furface of the ftone a little concave, fo as to occafion the watery drops to be rather dafhed backwards towards the ftream, than thrown upwards through the cavity of the veflel.
III. Experiments of air pajfing down through pipes with falling water.
Water running through a crane.
In the running of water through a fiphon or common crane, when the fucking pipe on the long leg of the crane was ftopt, the water, as it iflued from the extremity, filled the bore : on opening the fucking pipe, the column of water appeared lefs than the bore.
Pp 2 Judging
[ 202 ]
Judging that the motion of the water muft be re- tarded in this laft circumftance, I meafured by a pen- dulum the times in which equal quantities of water run through the crane in both cafes ; and found, in many trials, that the quantity which took the time of a hundred fvvings of the pendulum to run in when the fucking pipe was open, run in ninety-three, and fometimes ninety- two, when it was ftopt.
As thefe differences feemed to proceed from air intro- duced into the water through the lateral pipe ; I tried to make this air fenfible, by raifing the veiiel which re- ceived the water from the crane, and keeping the nofe of the crane immerfed in it. As often as the fucking pipe was opened, air bubbles arofe in the water of the receiver, and frefh bubbles fucceeded while it continued open •, but fo long as it was kept ftopt, no air bubbles were feen.
To collect the air, a cafk without a bottom was funk nine or ten inches in a tub of water, and the nofe of the crane inferted into a hole made in the top of the cafk : into another hole in the top was fitted a fmall pipe for giving vent to the air ; and within the cafk was fixed an inverted mortar for the ftream to fall on. So long as water was kept running though the crane with the fucking pipe open, a fenfible blart iffued from the blow- ing pipe of the cafk, and a burning coal expofed to it was excited in the fame manner as by a common bellows : the fucking pipe being ftopt, no blaft was perceived, nor was any motion produced in the flame of a candle ap- plied to the orifice.
It appears therefore that water, running down through an upright pipe, and filling its bore, admits air to enter through a lateral pipe : that after this admiffion, the width of the column of water contracts, the introduced
5 air
[ 293 ]
air occupying part of the cavity of the pipe j and that this air paffes down on the outfide of the water, or in a feparate column, not intermixed with it fo as to ren- der it frothy.
Water defcending through an oblique pipe with lateral
apertures.
I varied the foregoing experiment by taking, inftead of the crane, a leaden pipe, about ten feet long and three quarters of an inch bore. Several holes were made, at intervals, in the length of the pipe, and fmall tubes fixed into them like the fucking pipe of the crane. The pipe being laid aflope, its upper end was turned up perpendi- cularly, and a funnel fitted to it, which was fupplied with water by a cock in the bottom of a refervoir : the other end of the pipe, which the water iffued from, was infer ted into the air veffel ufed in the preceding experiment.
The lateral tubes being ftopt, and the cock fo turned as to let the water run faft enough to keep the funnel always full, no air iffued from the blowing pipe. On opening the tubes, a confiderable blaft was perceived ; the water paffed flower through the pipe, fo that the fame flxeam made the funnel run over ; and on pulling out fome of the tubes, and looking in through the holes, the column of water was very vifibly lefs than the bore of the pipe. The tubes being flopt again, the blaft ceafed, and the ftream did no more than keep the funnel full.
A fmall variation in the circumftances of this experi- ment made a very material difference in the effect. The fupply of water having been diminifhed, fo as to rife only a little way above the throat of the funnel, a pretty ftrong blaft iffued from the blowing pipe though all the lateral tubes were clofely ftopt j and when the tubes were open, inftead of air pafling in by them, a blaft paffed
out
[ 294 ] out from them, the air veflel in this cafe yielding none ; fo that here the air mud have been introduced at the top and pafled down the funnel, and afterwards efcaped where it firft found a vent. To be further fatisfied in this point, I repeated the experiment with a fomewhat different ap- paratus, in the following manner.
Water falling through a funnel.
The glafs receiver of an air pump, about two feet high, open at both ends, had its lower end immerfed about feven inches in a veflel of water, and fupported at a proper diftance above the bottom for the free paffage of the water under the edges. A brafs plate being prefTed clofe on the top, with leather between, a glafs funnel, about twelve inches deep, and above half an inch diame- ter in the throat, was fixed into a hole in the plate ; and into another hole was fitted a fmall blowing pipe.
A ftopper being introduced into the funnel, till the water it was filled with had become perfectly quiet, and then cautioufly removed, the water run in a ftream, which falling into that in the receiver, produced air bubbles : but no blaft iffued from the pipe ; and when the pipe was ftopt, the water in the receiver did not fink lower than the level of that in the outer veflel, whereas, if any air had entered with the water, and been compreffed in the receiver, it muil have forced a proportional quantity of the water out below.
The funnel was then fupplied from a pipe, by which the water was made to dafh againft one fide of it. By this means the fluid received a fpiral motion, and twirling round the funnel, left a large vacuity in the middle, reaching down fometimes to the funnel's throat. The ftream, as it run through, was alfo twirled ; a fenfible
blaft
[ 295 ]
blaft iffued from the air pipe ; when the pipe was ftopr, the water in the receiver was forced lower and lower, and was foon driven entirely out, abundance of air bubbles fol- lowing it into the water in the outer veffel.
When the funnel was kept entirely full: though the ftream was directed as before againft its fide, there were little marks of any air being carried down. And when the funnel was near empty, the effects were alfo incon- fiderable ; the vacuity in the middle of the fpiral circum- volutions of the water feeming to reach to the bottom, fo as to fuffer the air to efcape upwards through the hollow column of water.
Water falling from a confiderable height into a funnel with
a pipe.
A leaden pipe, fix feet high and an inch and a half in diameter, was inferted into an air vefTel, with the water gage already defcribed. Into the top of the pipe was fixed a tin funnel, whofe throat fitted clofe to it ; and into the funnel a ftream of water was let fall, from a refervoir five feet above, in quantity fufficient to keep the funnel run- ning over. This apparatus reprefents Mariotte's blowing machine defcribed in the third article of the preceding feclion.
The water, divided by the fall, pufhed down abund- ance of air with it : a ftrong blaft iffued from the blow- ing pipe, and the gage rofe high. On raifing up the funnel a little, the ftream that iffued from it appeared all frothy : as often as the funnel was lifted up, the gage funk, the air, which had been driven in by the dam of water, efcaping between the funnel and pipe : on letting down the funnel clofe, the gage immediately rofe again.
Inftead
[ 296 ]
Inftead of a fall of five feet, a ftream was dire&ed into the funnel from only about half that height. The gage ftill rofe confiderably, though not fo high as before. "
It is obfervable, that in the circumftances of thefe ex- periments, a twirling motion communicated to the water in the funnel impeded the carrying down of air, the gage always linking on the water receiving fuch a motion ; whereas, in thofe of the preceding article, it feemed to be by the twirling of the water that the air was puflied down.
It appears therefore that there are two ways of making air pafs down with water through a funnel, one by di- recting the ftream againft the fide of the funnel, the other by letting it fall from a great height : that in the one cafe the air enters between the fpiral circumvolutions which the water forms in the funnel, and in the other between the drops into which a confiderable part of it is reduced by the fall ; that we cannot avail ourfelves of both ways at once, the one impeding the effect of the other ; and that in either cafe the air holes under the throat, fo necef- fary in other machines, can have no place, as they give a vent to the air brought down from above.
Water foiling from a funnel through a pipe with air holes.
The fix-foot pipe, ufed in the foregoing experiment, continuing fitted into the air vefTel, its upper orifice was widened, that the fmall end of a funnel-fliaped copper pipe, of the fame bore with the preceding funnel, might hang freely in it, without touching the fides. The funnel- pipe reached up to the refervoir, and was kept always full, that the water might receive little or no air but at the vacuity between the nofe of the funnel and the leaden pipe.
In
[ 297 ] In this fituation, the quantity of air was much lefs than in the preceding : the water fell through the funnel in a ftream not at all frothy, and the gage rofe but a little way. I widened the aperture of the leaden pipe, to let in more air, but flill the gage continued low.
Into the orifice of the funnel I inferted a fmaller pipe, whofe diameter was one inch, and whole area was of confequence lefs than half of that of the leaden pipe. The blafl was now flrong, and the gage rofe higher than when the water fell from an equal height into the low funnel of the foregoing article. I tried funnels conlider- ably fmaller, and found the gage (till to rife high : but at lafl, with one of a quarter of an inch diameter, it did not rife at all, and no blafl could be perceived.
One of the funnels which anfwered beft being properly fixed, with two or three inches of its neck hanging free within the wider pipe, I made feveral variations in the manner of admitting the water and air, with a view to com- pare the effects of different ways of admifhon. The fun- nel being full, and gently fupplied fo as to keep the water in it as fleady as poffible, the height of the gage was marked : on giving a circular motion to the water, or letting it fall from a height, the gage always funk, even a flight twirl or dafh fenlibly affecling its height. The fpace between the nofe of the funnel and the pipe was flopt, fo that no air could enter but at the top : the fun- nel being now full, and the water quiet, the gage fcareely rofe at all ; on twirling the water, it rofe coniiderablv, and when the water fell from a height, it role further, though not fo high as the flandard mark.
It appears therefore that there are two general methods in which water may he maJe to carry down air, one in which it receives the air at the top, and the other through lateral apertures ; and that the circumftances, which con-
Qj tribute
[ *9S J
tribute to the effect in one cafe, impede it in the other . That water, being at reft in a funnel, and then fuffered. to run through, carries little or no air with it j that when made to twirl round in the funnel, it carries a confiderable quantity; and that when it falls from a height, fo as to. be in great part daflied into drops, it pufhes down con- fiderably more : That running through a pipe with lateral apertures, perpendicularly or obliquely, it receives air through the apertures, even when its motion is flow ; that when the pipe is of equal bore throughout, the quan- tity of air thus received is not great ; but that, when the pipe is contracted to a certain degree in the part where the apertures are, the quantity of air is greater than that intro- duced through the funnel without air holes: That air brought down from the top of the pipe or funnel prevents the introduction of frefh air through the lateral holes, which in this cafe, inftead of receiving more air, difcharge. that already received.
Finding that the two general methods, by v/hich air is made to pafs down with a ftream of water, could not be united in one machine; and that the pipe and funnels with apertures for the entrance of air about or under the throat of the funnel, have the greateft effect ; I pro- ceeded to examine the mod proper form and difpofition . of thefe.
IV. Experiments and obfervations for regulating the Jlrutlure of the funnel and pipe.
Experiments with funnels and pipes of different heights*
The water, as already obferved, pafTing through the narrow throat of the funnel, is afterwards en- larged into a jet which fills the bore of a wider pipe. 2 The.
I 299 ]
The quantity of air introduced appears to depend upon the degree of this enlargement, and on the quantity of water that runs through in a given time.
The greater the height of water above the narrow throat, the greater velocity will the jet receive, and the more it will be diipoied to fpread and be enlarged. The length of the pipe does not appear to be of fo much im- portance : it mould feem fufficient if the pipe is of fuch length, that the pre/lure of water in it may be able to refift the comprefled air in the air veflel, and that after part of its power has been fpent in overcoming that force, it may ftill have velocity enough left to run down as fafl as it can be fupplied from the funnel. In order to attain to fome determinate proportions, the following trials were made.
A leaden pipe, feven feet high, and an inch and a half in diameter, being fitted into an air veffel, as in the fore- going experiments, funnel-fhaped pipes of different heights were fupported over it, fo as that the fmall end of the funnel might hang freely in the orifice of the leaden pipe, and leave fpace enough for the entrance of air all round. For the greater fecurity of the throat being of the fame area in all the funnels, one and the fame copper pipe ferved as a throat for them all : the funnels being formed by infert- ing this pipe into larger tapering ones of different heights. The funnels were always kept full, and the water conveyed into them as gently as pofiible, fo as to produce no darn- ing or twirling motion.
A funnel of one foot high had very little effect : the rifing of the gage in the air veflel was inconfiderable, and the ftream of air from the blowing pipe was but jufl: to be felt : on opening fome holes made in the upright leaden pipe under the throat of the funnel, the jet of water appeared not fpread, but rather contracted, and did not fill
Qjj 2 the
[ 3°° 1 the bore. With funnels of two and three feet, the gage rofe more, and the jet fpread, though it did not appear to fill the pipe, till it had reached about half way down to the bottom. Funnels of five and fix feet produced a ftrong blaft, and kept the gage high, the jet filling the pipe be- fore it had fallen a foot below the throat of the funnel.
On many repetitions and variations of thefe experiments, I have not obferved that the jet fpread fufficiently with lefs than a fall of five feet. With a fall of fixry-four inches, the gage rofe more than five times as much as with one of fixteen inches, though the quantity of water which run ia the firft cafe was only double to that in the latter, viz. as the fquare roots of 64 and 16 : from whence it is plain that the above differences do not depend entirely on the different quantities of water which run through funnels of different heights, but in great part on its different velocity. Some other experiments feemed to confirm this point : for having ufed fhort funnels lb much wider than the high ones, that the quantity of water difcharged by the former was equal to or greater than that by the latter, the fhort never produced fo ftrong a blaft, or raifed the gage fo far, as the others.
Being fatisfied of the advantage of having the funnel of very considerable height, I in like manner varied the length of the pipe. Having made a mark at the part where the gage rofe to when the funnel was five feet, and the pipe feven, I added to the pipe about a foot more : the gage fcarcely rofe any further. A foot being cut off from it, the gage fell a little : two feet being cut off, it fell con- fiderably ; and the retrenchment of another foot made the machine of little effect, the gage finking almoft to the bot- tom, and the blowing pipe yielding but a weak current of air. The pipe, thus reduced to four feet, was tried with a funnel of near eight feet : in this cafe there was no blaft
at
[ 301 1 at all. But with funnels lefs than its own height, as of two and three feet, it flill railed the gage confiderably.
It appears from thefe experiments, that in mod of the machines defcribed in the preceding feclion, the lengths of the funnels and pipes are greatly difproportioned to one another, and confequently the water applied to difadvan- tage. Thofe of Dauphiny in France are particularly faulty in this refpecl, the funnel being fcarcely three feet high, and the pipe twenty-five or twenty-fix : with fo fmall a height of water above the choak, I have never been able to make the jet fpread near to fuch a degree as it is faid to do in the machines of Dauphiny, without particular con- trivances for that purpofe, which will be mentioned in the fequel of this, paper. The Foix machine agrees the beft with my experiments : but as the funnels of the others are undoubtedly much too low, that of this feems to be rather too high. The effedt appears to be the greateft, when the funnel is about two thirds of the length of the pipe.
Experiments of the difpofition of the air holes.
In the foregoing, experiments,, the fimpleft and moll ob- vious way of admitting air was chofen, by leaving a fpace between the funnel and the pipe. The air pipes of the machines of Foix and Languedoc anfwer the fame end, car- rying in the air above the furface of the jet of water. As the other machines have the air holes under the jet, I tried what variations would refult from this circumflance, and from making the apertures at different depths under the throat of the funnel.
Into a pipe of fix feet was fitted a funnel of four feet ; a nc fix inches below theonrice of the funnel, four holes were bored round th-j pipe, limping down from without inwards : eight inches lower down, I made another row of holes •„
and
[ So2 } nnd at like diftances under thefe, a third and a fourth. To each hole was fitted a Hopper which exactly clofed it.
All the holes being ftopt, the funnel was firft hung free in the pipe, as in the former trials, and the height to which the water rofe in the gage was marked. The funnel being then let down into the pipe, fo as exactly to clofe it, the upper air holes were opened : the gage did not now rife fo high as before. The upper air holes being ftopt, and the fecond row opened, the gage continued at its laft height. With the third row open, it rofe rather higher than the firft mark ; and with the fourth it fell the loweft of all.
The feveral entrances for the air were then opened by •two and two. With the fpace between the funnel and pipe, and the upper air holes, open, the gage did not rife fo high as with the fpace only ; and with the upper and fecond row of holes it continued at the fame height. With the fecond and third, it rofe confiderably further, though not up to the firft mark ; and with the third and fourth, it fell a little below the preceding height. In all thefe cafes, where two rows of holes were open, the water manifeftly did not fill the bore of the pipe at the upper holes ; but fpread fo as to completely fill it by the time it had reached the lower ones, at which laft, part of the water fpirted out and car- ried fomc of the air with it.
In another pipe of the fame fize I made two fets of air holes, three inches apart, and the uppermoft of them twelve inches from the orifice of the funnel. With the upper row open, and with both rows open, the gage rofe almoft equally, being only a little lower in the-latter circumftance than in the former ; but with only the lower row open, it funk about one half. Thefe being all ftopt, and another fet bored oppotite to the orifice of the funnel, the gage rofe as high as in the firft cafe.
Thefe
**«
[ "'3°3 J Thefe experiments, and lSveral others Ihave made orr the fame fubjecl:, are not lb conclufive as could be wiihed. They fcem to mew that it is more eligible, to have the entrances for the air in one horizontal plain, than in two plains above one another ; and either above, or at fome diftance below the jet, than immediately under it : That they ought to be of greater magnitude than in fome of the machines defcribed in the firft fe&ion, particularly in that of Lead-hills, whofe air-holes, taken all together, are not of half the area of the fpace in the pipe which the air has to fill. They ought atleail to be of an equal, or rather of, a, double, extent, that the air may enter the more freely.
Experiments of the proportional bores of the funnel and pipe.
We have already feen, that unlefs the throat of the funnel is lefs than the pipe, the quantity of air carried down will be incom'iderable j and that by lefiening it further than to a certain point, the effect is alfo diminiihed or destroyed. To hit this precife point is not perhaps poili- ble ; and the point which is the moil perfect proportion for one height of water, cannot be fo for any other, an iocreafe of the prefiure difpofing the jet to fpread more and fill a larger bore.
It appears from fome experiments already mentioned, . that when the whole height of the fall of water is fifteen feet, the height of the pipe ought to be nine feet, and that of the funnel fix. This being as low a fall as thefe kinds of machines have been generally erected for, and as high a one as is generally to be expected in this country, I made feveral trials for adjufiing the proportions to thofe heights ; ufing for the funnel a tapering copper pipe, into the lower end of which were occafionally inferted fmaller pipes of
different bores.
By
[ ?o4 ] By trying fcveral of theft; . Is, we came to certain
fizes, which could not be much increafcd or diminished, without diminifhing the effect of the machine ; but if there is, in this refpect, any cxadr. ftandard, our expe'.iier..3 did not difcover it. There are fo many circumftan ss, as we have already feen, which influence the effect, that it is very difficult to judge, when the differences are fmall, ho v far they depend on any particular one. When the area of ths orifice of the pipe was fi m jur to five times greater th.m that of the funnel, the differences in the heigl t of the gage were not very confiderable : the ue prof • tion eem to lie within thefe bounds, and perhaps n« r o the latter than to the former ; for when the funnel was only about a fixth part of the area of the pipe, the gage flood rather higher than when it was a third part.- from whence the proportions fhould be as one to fomewhat more than iour and a half.
Experiments of dividing the flream fo as to mcreaft its effect, mid render lefs mooter fufficient.
-As the effect of thefe kinds of machines dcj.mds on the water being fpread and divided, and the air, which comes in to fill the interfaces between the little flreams or drops which compofe the jet, being pufhed down with velocity by the fucceeding water j I have endeavoured to divide the flream, more effectually than is done in the common machines, and with little or no diminution of its velocity, by varying the form of the aperture of the funnel.
On the orifice of the funnel I fitted a perforated tin plate, like the nofe of a watering pot, but with the holes larger, and of a triangular figure ; this figure was chofen on ac- count of its great furface, water, palling through a triangu- lar aperture, having about a third part more furface than
through
[ 3°5 ] through a circular one of equal area : fome more holes were made round the fides, in fuch pofitions, that the ltreams iffuing from the higher holes, might no where fall upon or coincide with thofe from the lower ones, but that the water might be uniformly difperfed through the whole cavity of the pipe. By this divifion of the water it was made to fill a much larger bore than other- wife, and to produce as great an effect as the full quan- tity of water which the fame pipe would otherwife have required ; infomuch that quantities of water which had little effect in the common way of application, were by this contrivance made to yield a flrong blafr..
This method is accompanied with an inconvenience, which often fhewed itfelf in the courfe of the experiments, and which mull be more considerable in the continued working of the machine. After it had acted vigoroufly for fome time, its action frequently abated of a fudden : the blaft from the blowing pipe grew weak, and the gage funk : fometimes its force increafed again in a little while, but for the moil part it continued to diminim more and more. The caufe was difcovered to be bits of leaves and other like matters which the water had carried into the funnel, and which had in part ftopt up the fmall apertures. The remedy was obvious, letting the water pafs from the refervoir through a wire fieve whofe holes were much finer than thofe in the nofe of the funnel ; and doubtlefs an expedient of the fame kind would prove effectual for the largeft machines. It is in all cafes advifeable to have the water pafs through a grating before it enters the funnel ; even the common large apertures being fometimes choaked up by matters which the dream brings along with it. Where fcantinefs of water, or want of fo high a fall as is commonly required, perfuade to this contrivance for procuring a more effectual divifion of it,
R r and
[ 3°6 ]
and for augmenting its power with its furface, two or three gratings, or perforated plates, with apertures of different iizes, will be neceffary : one with very fine holes, much fmaller than thofe of the cullender, that nothing may pafs through the former which can be in danger of flicking in the latter : another with larger apertures, for detaining weeds, and fuch other matters as would foon obftrudt the finer ftrainer.
I have tried other methods of procuring this difperfion of the water, by making the throat of the funnel of differ- ent figures i but with little fuccefs. Whether the throat was made converging or diverging, in greater or lefs de- grees, there did not appear to be any material difference in the effedl of the machine. I introduced into the funnel a cylindrical core, which was fixed in the middle, by means of pins projecting from it, fo as to leave a cir- cular aperture all round it ; and this core was fometimes folid, and fometimes a pipe which reached above the fun- nel and carried down air into the middle of the jet below : but no other difference was obferved in either cafe than what arofe from the neceffary diminution of the quantity of water. It is probable indeed, that by duly propor- tioning the core to the funnel, and the width of the pipe to the fheet of water falling round the core, the effedl, by this divifion of the ftream, would be made greater than an equal quantity of water would produce when falling in one column; though the increafe, obtainable by this method, did not promife to be considerable enough to deferve the troublefome invefiigation of the proportions. One trial however, depending partly on this principle, appeared of fome importance to be made.
As the water machine of St. Pierre is faid to#have two apertures in the bottom of the funnel, whofe flreams, as they iffue out, crofs one another and are darned into drops,
I tried
r s°7 ]
I tried to anfwer this intention, by ufing for the funnel a wooden trunk, with two of its fides Hoping downwards fo as. to leave a long narrow aperture between them : in the middle of this aperture, and parallel to the inclined fides, was placed a wedge of the fame Hope with the fides of the funnel, that the water might pafs out in two fheets directed towards one another.
The funnel was at top about eight inches fquare : its width at bottom feven inches and eight tenths by one inch and nine tenths. The wedge, dropt into it, entirely flopt the lower aperture, and had its thin edge hang- ing down confiderably below : flips of wood of different thickneffes fattened on the wedge occafionally, two on each fide, prevented its falling down fo far, and procured lpaces of different widths between it and the fides of the funnel; fo that the water could be reduced at pleafure into two fheets, feven inches and eight tenths wide, and from lefs than a quarter of an inch to three quarters of an inch thick ; the par- tition in the middle reaching in all cafes lower down than that which confined them on the fides, that they might not unite into one upon their difcharge from the throat. Along the floping fides of the funnel were two air pipes, of the fame breadth with them, and about an inch and a half wide; fo that at the bottom there were three oblong rectangular apertures, the middle one, with a wedge in it, for the water, and the two lateral ones for air : the outfides were con- tinued about feven inches and a half below thefe aper- tures, fo as to form a large cavity for the water to fpread in.
The funnel, above the throat, was fomewhat more than three feet high : on the top was fitted a wooden pipe, nearly of the fame width with it, and four feet eight inches high. The top of this pipe paffed up through a rectangular cittern, nearly i63 inches in length and 96 in width, and
R r 2 which
[ 3°$ ] which confequently contained about fifty feven gallons on every inch in depth. For admitting the water, two holes were made in two oppofite fides of the pipe, about ten inches high, with two Aiders fitted to them, for occafionally vary- ing their height and confequently the quantity of water received. On the outfide of each hole was fixed an iron plate, perforated with numerous fmall holes, to keep back fuch matters as might choak up the throat : that the holes might be fufficient to allow water enough to pafs in, the ftrainer was made wider than the aperture in the pipe, . and bent to a femi-cylindrical form.
To the bottom of the funnel, enlarged as above men- tioned, was fitted a pipe fix feet high, and in width four inches by feven and a half. The lower end of this pipe was- inferted into the head of a large calk without a bottom, which was fet in a tub above three feet deep, .with three fupports under the lower edge of the caik to procure a fpace between it and the bottom of the tub for the water to pafs freely off. About eight inches under the orifice of the pipe, a round board, for the water to fall on, was hung by three cords, which parted up through the head of the cafk and were fecured by pegs. At one fide, a tin veflel full of water was fupported in the fame manner ; and through a faucet, over the middle of this vefTel, was inferted a glafs tube thirty four inches long. At the other fide was the blaft: pipe, about three quarters of an inch in diameter,
The machine being thus prepared, we proceeded to the trial of it, expe&ing that the two fheams, from their doping direction towards one another, would crofs and be darned into drops, and carry down abundance of air. . But in the effect we were greatly difappointed : the blaft was weak, and the gage rofe to no confiderable height, whether the wedge was dropt down or drawn up, fo as to fuffer the water to pafs in lefs or greater quantity, in
thin
[ 3°9 ] thin or In thick meets : in continued trials and va- triaions of the apertures for three or four days, the gage was not once obferved to rife fo high as ten inches. A good deal of air indeed efcaped through the junctures of the pipe and of the air veflel, but not near enough to make up the expected quantity.
The wedge anfvvering lb ill, it was laid afide ; and in its place was introduced a leaden veiTel, of the fame ihape with the funnel's throit, and of fuch a fize, as to reft againft the fides of the aperture by its upper edge, and hang fix or ftven inches down in the wilier part of the pipe : in the fides and bottom of this veflel were made feveral holes, about two tenths of an inch in diameter. With this alteration I had the pleafure to find, that though air rulbed out from the joints even more plentifully than before, yet the blafl from the blowing pipe was ftrong, and the water in the gage pipe rofe to the top and run over.
I tried to measure the quantity of water necefiary for pro- ducing this effect for a certain time. The refervoir being filled to the depth of fourteen inches, the gage rofe as before, and continued high for four minutes ; after which it begun to fink fall:, the water in the refervoir havino- then become too low to keep the pipe full, though it con- tinued to run for a confiderable time longer. From the dimenfions of the refervoir already mentioned it will ap pear, that if all the water had run out in the four mi- nutes it would have amounted to near two hundred gallons in one minute ; but at leaft a fourth of it remained after that period, fo that the expence could not exceed a hundred and fifty gallons in a minute. We could not expect any great accuracy in this determination, becaufe as the height of the water continually decreafed in the refervoir, its velocity like wife decreafed, fo that if a due
quantity .
[ 310 ] quantity run in the laft minute, a fuperflaous quantity mufl have run in the firft.
The leaden cullender being taken out, and the whole throat left vacant for the ftream, the gage ftill rofe to the top ; but the expence of water was now more than dou- ble to what it was before.
Thefe trials, though not carried to fuch a length as I could have wifhed, fatisfied me, and thofe who affifted at them, that much more air is to be obtained, by divi- ding the ftream by means of a cullender, than by any other methods that have been tried ; and that with fuch a ma- chine as is above defcribed, a ftream of a hundred and fifty gallons at moft in a minute is fufficient to produce a continued blaft, from a pipe of three quarters of an inch bore, of fuch ftrength as to fupport a column of water of three feet or more.
To afford as much affiftance as poflible to thofe who may be defirous of erecting machines of this kind, I fhall here collect into one view the moft material particulars which my experiments have difcovered with regard to the perfection of their ftru&ure, and form from them a defcrip- tion of fuch a machine as promifes to be the moft effec- tual.
The bottom of the refervoir of the water fhould be about fourteen feet above the level of the ground : we need not be very folicitous about procuring a greater height, for though a greater would be of fome advantage, yet this advantage appears to be much lefs confiderable than has been commonly imagined.. In the channel by which the water is conveyed, are to be placed gratings of different fizes, as already mentioned, and before the aper- ture a finer grating, which may be either a perforated iron plate or a wire fieve, to ferve as ftrainers for keep- ing back fuch matters as would obftruct the apertures
which
[3ii r
which the water is afterwards to pafs through. The Itream fhould enter at one fide, or be fo managed, that the water in the refervoir or funnel may not be agitated by it, or put into a fpiral motion, which our experiments have {hewn to be very injurious.
In the bottom of the refervoir is to be made a round hole, for admitting the upper end of what we have hi- therto called the funnel, but which may here be more conveniently a cylindrical pipe, of copper or of caft iron, five or fix inches in the bore, and (even., feet long. To the end of this pipe is to be fitted a cullender, about a foot long ; with the holes triangular, of half an inch each fide ; and fix or feven ftrips from top to bottom, at equal diftances, preferved without holes, for admitting air to pafs down to the lower ftreams. All the holes fhould be directed downwards, that the ftreams may not be forcibly projected againft the fides of the pipe which is to receive them, fo as to have their velocity too much diminished.
If there are fix of the perforated fpaces in the cullen- der, the number of holes in each may be twenty, fo that the whole number will be one hundred and twenty. The fide of each of the triangular holes being half an inch, the area of each will be the eighth part ofafquare inch, and the fum of their areas will be fifteen fquare inches. The quantity of water running through one aperture of fuch an area* at the depth of feven feet and a half under the furface, comes out on calculation about fix hundred and twenty-two gallons in a minute j but the real quantity will doubtlefs be much lefs than this, on account of the great fri&ion of the water in palfing through a number of fmall holes, and of the refinance of the air, which increafes in a very high ratio accord- ing to the increafe of the velocity and enlargement of the furface : it is in part to make up for thefe retardations,
that
[ 3'2 ] that the pipe is directed to be made fo high. 1h:furface of the water is here above thirteen times greater than if it paffed all through one circular aperture.
Both the pipe and the cullender fhould have a flanch or rim round their orifices, and be fecured to one ano- ther by fcrews palling through the rims of both, with a plate of lead between them to make the juncture tight, as commonly practilld in joining iron pipes for water works. This way of joining them admits the cul- lender to betaken oiT and cleaned, when a diminution of the effect of the machine mews the holes to be ch .!•:- ed up, which however, it is apprehended, will feldom, if ever, happen.
As the holes will permit more water to run through, than may at all times be wanted, it is proper to have fome contrivance for occafionally doling a part of them. This may be effected by means of a thin copper pipe, open at both ends, as high as the cullender, and of fuch width as juft to drop into it. It will be eafily conceived, that when this regifter is let entirely down, the lateral holes will be covered, and the water admitted only to thofe in the bottom ; and that, by raifing it fur- ther and further, more and more of the lateral holes will be uncovered. The regilter is to be hung by a wire to a crofs bar over the refervoir, by which it may be raifed or lowered ; and a fcale or divided board may be adjufted againft the upper part of the wire, for mewing the height of the regifter, or the number of holes doled by it.
The moft commodious and effectual way of admitting air to the water appears to be that of our firil experiments, viz. hanging the throat of the funnel, in this cafe the cul- lender, within the wider receiving pipe, for by this means the air is admitted freely and uniformly all round. This lad pipe (hould likewile be of iron or copper, twelve
inches
r 3*3 ]
Inches in diameter, and fpread oat at top to the wid t" fixteen or eighteen inches, that a large fpace may b<: left round the cullender : this fpace mould reach three or four inches above the uppermc.il perforations of the cullender, to prevent any of the water from being dallied over the top. A pit is to be funk in the ground, not lefs than fix feet deep. In this is to be placed an air vefitl, made of wood lined with lead, without a bottom, three or four feet in width, and ten or eleven high. The vefiel fliould be fup- ported on feet, of a proper ftrength, with furhcient fpaceff between them for the water to pafs freely out : this way is preferable to the common one of placing the lower edge of the veffel on the bottom of the pit, and cutting an aperture in the fide, bocaufe the height of the aperture is fo much taken off" from that of the vefiel. The refer- voir being fourteen feet above the ground, and the upper pipe and cullender reaching down eight feet, only fix feet remain below the cullender ; fo that the air vefiel, having fix feet funk in the ground, will reach nearly up to the cullender, and almoft the whole height of the undermofi: pipe will be included within the vefiel. This pipe may be above nine feet long, three feet or more of it going down into the pit ; which three feet are here an entire gain in the height of the fall, for the pipe in the other machines comes at mod no lower than the level of the ground where the water runs off on the outfide. This height is gained, in virtue of the ctfm- preffed air in the veffel pufhing down the water below, as already (hewn in the fecond article of this feclion : it may be always as great as the height to which the water is intended to rife in the gage. At the diftance of five or fix inches under the orifice of the pipe is to be placed the concave iron plate or flone for the water to fall on. In the top of the air vefiel is to be fixed the gage and the blowing pipe.
S f Such
[ 3H]
Such is the general conftruftion of the blowing ma- chine, which promifes to be particularly ufeful in cafes where water is fcarce, or where the want of a natural fall renders it necelYary to raifc, by very expenfive means, the great quantities requifite for working the common bellows. It is prefumed, that one of thefe ma- chines will be fufficient for the iron forge, and for fundry other purpofes where the quantity of air is not required to be very great ; that it will be lefs expenfive, on account of the durability of its materials, and the fim- plicity of its ftructure, than any kind of bellows now in ufe ; and what is of principal importance, that much lefs water will ferve for working it. In cafes where one of the machines cannot fupply air enough, as for the large iron fmelting furnace, two pipes may be ufed, both fed by one refervoir, and entering into one air veflel, as praclifed in fome of the inftruments defcribed in the firft lection. The ufing of two pipes appears more eligible than enlarging the bore of one ; for air cannot be fo freely introduced into a large body of water, though divided into ftreams by the cullender, as into two fmaller ones of equal quantity.
It may be obferved, that the blaft will be Wronger in a denfe ftate of the atmofphere, than when it is more rare or expanded, a greater quantity of air being then intro- duced nnder an equal volume. If therefore the quantity of water has been adj lifted fo as to raife the gage to a pro- per height when the air was light, it will frequently hap- pen that the fame quantity of water fhall raife it higher, and confequently, if no greater height is required, that a part of the water may be faved. As the gage of our ma- chine difcovers by infpection thefe variations in its effect, the regifter affords convenient means of regulating its power, and increalingor diminifhing the quantity of water.
VI.
C 3'5 J
VI. History of Colours.
part I. Of Black.
BLACK, a colour in many cafes the raoft important, and in its ufe the molt extenfive, of all thole which art is concerned in preparing or applying, is chofen as the firft article of an experimental hiftory of colours; which will be occafionally continued in the profecution of this undertaking.
The practices of the workmen in one branch of colour- ing are generally little known to thofe who are employed in another; the feveral methods of applying even one colour, on different kinds of bodies, being the objects of fo many diftinct arts, each of which has its own rules of working, peculiar to itfelf, and eftablifhed by long cuftom,
Of the arts of communicating a black colour to diffe- rent fubjedts, there are iome which have made great ad- vances towards perfection, whilft others remain far more imperfect, in regard not only to the difpatch and facility of the execution, but like wife to the beauty and duration of the colour. Thus woollen and filk are both dyed of a permanent deep black, but with this difference, that what the woollen dyer effects by three or four dippings of the cloth in his dying liquor, the filk dyer fcarcely obtains from twenty or thirty dips; whereas, on the contrary, the dyer of linen and cotton thread, however he prolongs the operation, or repeats the dippings, is unable to communi- cate to the thread a blacknefs that fhall endure wearing. Thus alfo the printer fixes upon paper an ink which con-
T t tinucs
1 316 ]
tinues unaltered for ages, and which is not perhaps capa- ble of being changed by any natural agent that the paper itfelf can refill:; while the common writing inks foon lofe of their colour both on paper and on parchment, in- fomuch that records, of no very long Handing, have become almoft entirely obliterated.
In the prefent hiftory, I mall endeavour to trace, as far as my opportunities of information will enable me, the preparation, production, and communication of black co- lours, through all the profeffions in which they are con- cerned ; that the artift, confined by his employment to particular views, may be made acquainted with the me- thods, by which fimilar effects to thofe which he pro- duces, or wants to produce, are obtained in other arts, or in arts which in other refpects differ from his own. Ex- periments, while they ferve as a fure tefl for afcertaining the refpective facts, will often contribute at the fame time to enlarge and render thern more extenfive ; and likewise afford means of diflinguifhing, in fome complex opera*- tions, the circumftances or materials effential to fuccefs, from the fuperfluous or injurious ones, which ignorance or chance perhaps at firft introduced, and which prejudice or cuftom have continued.
By thus examining and comparing the different me- thods, by which a fimilar colour is obtained or produced, and by which the feveral tinging materials are applied on different fubjedts, I flatter myfelf that many of thefe arts, however disjoined among different fets of workmen, will be found to have natural and flrong connections, . fo that an effective and ufeful commerce may be eflablifhed among them; that they will not only tend to illuftrate, but mutually to improve one another ; and that in many cafes the practice of one art may be abridged or facilitated, its imperfections remedied, and its deficiences fupplied, by
means? .
[ 3*7 ] means, which could fcarcely ever be thought of by a per- fon converfant in that art fingly, but which a general knowledge of the others may be expected to fuggeft.
SECT. I.
General obfervations on black colours.
OF black, as of other colours, there are many fhades or varieties} different bodies, truly and limply black, or ■which have no fenfible admixture of any of the reft of the colours, as black velvet, fine black cloth, the feathers of the raven, &c. appearing, when placed together, of teints very fenfibly different.
2. One and the fame body alfo affumes different degrees of blacknefs, according to the difpofition of the fenfible parts of its furface ; and in this refpect, there is not, per- haps, any other colour, which is fo much affected by an apparent mechanifm. Thus black velvet, when the pile is raifed, appears intenfely black, much more fo than the filk it was made from ; but on preffing the pile fmooth, it looks pale, and, in certain pofitions, fhews fomewhat even of a whitifh caft.
3. This obfervation is agreeable to the phyfical theory, which afcribes the blacknefs of bodies to the luminous rays, that fall upon them, being in great part abforbed, or ftifled in their pores. When the furface is compofed of a multi- tude of loofe filaments, or fmall points, with the extremi- ties turned towards the eye, much of the light is ftifled in the interftices between them, and the body appears dark : when the filaments are preffed clofe, or the furface fmooth- ed and polifhed, more of the light is reflected from it, and the intenfity of the blacknefs is diminifhed j though the beauty may be improved by the gloffinefs which refults from the fmoothing.
T t 2 4. There
[ 3i8 1
4. There is one cafe however, in which a high polifh may, on the fame principle, produce blacknefs, in bodies ctherwife even white. We find that fpecula of white me- tal or of quickfilvered glafs, which reflect the rays of light to one point or in one direction, look always dark, unlefs when the eye is direttly oppofed to the refle&ed rays.
5. As the abforption of the luminous rays, except in the cafe juft mentioned, makes the phyfical caufe of black- nefs ; it is concluded that black bodies receive heat more freely than others. Black marble or tiles, expofed to the fun, become fenfibly hotter than white ones. Black pa- per is kindled by a burning-glafs much fooner than white, and the difference is ftrongly marked : a burning-glafs, too weak to have any vilible eftett at all upon white paper, ihall readily kindle the fame paper rubbed over with ink. Hence black clothes, when wetted, are faid to dry fader j black habits, and rooms hung with black, to be warmer ; black mould to be a hotter foil for vegetables ; and gar- den walls, painted black, to anfwer better for the ripening of wall fruit ; than thoie of lighter colours.
6. It is not however to be affirmed that the like dif- ferences obtain in the impreffions made by common fire. Black paper, held to the fire, does not feem to be affected fooner, or in a greater degree, than fuch as is white. It may be proper to obferve alfo, that the combuftibility of the paper may be increafed, by impregnating it with fub- ftances of themfelves not combuftible, and which give no colour to it. This is the foundation of one of the fympa- thetic inks, as they are called, made of a ftrong folution of fal ammoniac in water, which, though colourlefs when written with on paper, becomes very legible on expoiing the paper to the fire ; that is, it occafions the parts moift- ened with it to fcorch or burn, before the reft of the paper is hurt, to a brown or black. All the falts I have tried
produced
[ 3'9 ] produced this effect in a greater or lefs degree ; nitre, alum,
tartar, very weakly ; fea fait more ftrongly ; fixed alcaline
falts ftill more fo; fal ammoniac the moil: ftrongly of all.
Metallic folutions, made in acids, and diluted fo as not to
corrode the paper, acted in the fame manner.
7. Beiides the fimple blacks, there are a multitude of compound ones, inclining more or lefs to other colours. Thus the painters have blue-blacks, brown-blacks, &c. which may be made by mixing pigments of the refpective colours with fimple black ones, in greater or lefs quantity, according to the fhade required. The dyers alfo have different blacks, and often darken other colours by flightly palling them through the black dying liquor ; but the term brown-black is in this bufinefs unknown, brown and black being here looked upon as oppofite to one another. In effect, the colour called brown-black is no other than that which ill dyed black clothes change to in wearing : no wonder then that it is excluded from the catalogue of the dyers colours.
8. The true or fimple blacks, mixed with white, form different fhades of grey, lighter or darker according as the white or black ingredient prevails in the mi.\t. The black pigments, fpread thin upon a white ground, have a like effect.
9. Hence the painter, with one true black pigment, can produce on white paper, or on other white bodies, all the fhades of grey and black, from the ilightefr. difcoloration of the paper, up to a full black : and the dyer produces the fame effect on white wool, filk, or cloth, by continu- ing the fubjects for a fhorter or longer time in the black bath, or making the bath itfelf weaker or fironger.
10. Hence alfo the dilution of black pigments with white, or the fpreading of them thin upon a white ground, affords a ready method of judging of the quality or fpecies
of
[ 320 I of the colour ; which, if it be a true black, will in this diluted flate look of a pure or fimple grey, but if it has a tendency to any other colour, that colour will now betray itfelf.
1 1 . All the colours, in a very deep or concentrated (late, approach to blacknefs. Thus the red liquor prepared by boiling or infufing madder root in water, and the yellow decoction or infufion of liquorice root, evaporated in a gentle heat till they become thick, look of a dark black colour, or of a colour approaching to blacknefs; and thefe thick mafies, drawn out into {lender firings, or diluted with water, or rubbed on paper, exhibit again the red and yel- low colours, which the liquors had at firft. Nature affords many black objects, whofe blacknefs depends upon the fame principle, being truly a concentration of fome of the other colours. Thus in black cherries, currants, elderberries, &c. what feems to be black is no other than an opake deep red : their juce appears black when its furface is looked down upon in an opake veffel, but red when diluted or fpread thin. The black flint, as it is called, of the ifland of Afcenlion, held in thin pieces between the eye and the light, appears green ifh ; and one of the deep black flones called black agate, viewed in the fame manner, difcovers its true colour to be a deep red.
SECT. II.
Native black colours.
H E mineral kingdom affords abundance of bodies uniformly tinctured or variegated with black, or with a deep colour approaching to blacknefs : Such are, the black flates, which make an ornamental covering for houfes: the black touchflone, on which pieces of metals being rubbed leave a mark of their own colour; which fhews the
colour
T
[ 3*1 1 colour the more perfectly by virtue of its blacknefs, and which thus enables us to judge and compare the colour and finenefs of metallic compofitions, with much more certainty than could be done by viewing them in the mafs: the black flint fo called, which performs the .fame office with the touchftone, and being harder than the common touchftones, anfwers better for the hard metals : the com- mon black marbles, ufed for many kinds of ornamental works : the more rare black marble, called lapis obfidianus or opjianus, which, in virtue of the very high polifh it re- ceives, was made into mirrours by the Greeks and Romans: the black gallinazo ftone, defcribed by D'Ulloa, which anfwered the fame purpofes among the Indians of Peru before the conqueft of that kingdom by the Spaniards : the black jet, and other fubftances of the fame clafs, which are formed into many elegant toys-: the plain and. variegated black agates, pebbles, cryftals, 6cc. which are cut by the jeweller. The ftone called tourmalin, remarkable for the fingular phenomena it exhibits in fome electrical experi- ments, is in its rough ftate of a full black. colour, though when polifhed it looks rather of a brown black: I have been informed by a fkilful jeweller that he had feen a black diamond, cut and fet in a ring; though perhaps the examination made of it was not fo rigorous as could be wifhed for determining its being truly of the diamond kind. However this may be, a black tinge in bodies of this clafs is looked upon as an imperfection or foulnefs, although, when the diamond is cut, a black foil placed under it im- proves its beauty: for the rofe diamonds, the collet, or focket in which the ftone is fet, is fpecked with ivory black in little dots; and for brilliants it is all over blacked. Among vegetables and animals, blacknefs, though fre- quent, is lefs difTufed* or of more limited extent. The black colour of the ftalks and feeds of certain plants, that
of
[ 322 ] of {hells, feathers, and hair, and that of the (kin of the human lpecies in certain climates, is only a fupcrficial teint. The blacknefs of fruits, as already obferved, is generally rather a concentration of fome other colour than a true black. Some woods, particularly the ebony, are tinged throughout with a true blacknefs, on which great part of their value depends, and which art very happily imitates on more common woods. The bony matter which lines the mouth of certain whales has like wife frequently a pretty deep black tincture, joined to another quality, elafticity, on many occafions more important than its colour.
Among the various fubftances which nature has im- pregnated with a deep and permanent black colour, there are few of which art can avail itfelf for communicating this colour to other bodies. There is not perhaps any in- ftance of a black colour being, as the reft of the colours frequently are, extracted from one body by means of dif- folvents, and thence introduced into another. Nor can the generality of natural blacks be applied in their whole fubftance; fome of thefe bodies being of fuch a texture, as does not admit of their being reduced into fufficiently fine powder, for being mixed with a proper cementitious matter, fo as to be fpread fmooth; and others having their colour deftroyed by the pulverization. Of thefe laft we fee an inftance in the common black flates, which may be fcraped into a white duft, in which the flighteft fcratches look white, and which, when drawn along any other black body, as hard as themfelves and not polifhed, leave a white mark; a property which, while it renders them ut- terly unfit for any purpofes in painting or ftaining, is that on which their ufe depends for occafional writing, or for making pencils for writing on other ftones. It is obvious, that for this intention, black ftones are better adapted than
thofe
[ 323 3 thofe of any other colour; and that the flone fhould be fomewhat harder than the pencil, that the marks may pro- ceed chiefly from the pencil, without fcratching the fub- flance of the flone.
The only native blacks I know of, that have been em- ployed as colouring materials, are the following.
I. Black chalk.
The black chalk or black marking-flone of the mops, fo called from its ufe in drawing black lines on paper, is a light earthy fubflance, of a pretty deep black colour, moderately firm, in texture fomewhat flaky like flate, not of a rough harih furface like common chalk, but rather foft and fmooth to the touch. It flams very freely, and, in virtue of its fmoothnefs, makes very neat marks. It is eafily reduced into an impalpable foft powder, without any apparent diminution of its blacknefs.- In this flate, it mixes eafily with oil into a fmooth pafle; and being dif- fufed through water, it flowly fettles, in a black flimy or muddy form; properties which make its ufe very con- venient to the painter both in oil and water colours. En- tire mafles of it, laid in water, are alfo by degrees pene- trated and difunited, though much more difficultly than thofe of white chalk.
It appears, in effecT:, to be an earth of a quite different nature from common chalk, and feems to be rather of the flaty bituminous kind. In the fire it becomes white with a reddifh cafl, and very friable; retaining its flaky flructure, and looking much like the white flaky mafles which fome forts of piteoal leave in burning. Acid liquors neither dif- folve, nor alter the colour of the black chalk itfelf; nor have they, fo far as I could obferve, any fenfible action upon the white afhes.
U u Our
[ 324] Our colour fhops are faid to be fupplied with this ufeful earth from Italy and Germany ; though fome parts of England afford fubftanccs, nearly, if not entirely, of the fame quality, and which are found to be equally fervico- able, both for marking and as black paints. Such parti- cularly is the black earthy lubftance called Killovv; faid by Dr. Merrett, in his Pinax rerum Britannicarum, to be found in Lancafhire; and by Mr. Da Cofta, in his hiftory of fof- fils, to be plentiful on the fide, near the top, of Cay-Avon, an high hill in Merioneth/hire. The killow has fome- what of a bluifh or purplim cafl: mixed with its blacknefs, as the black chalk likewife has: hence it is named by Merrett blue marking ftone, lapis caruleus killow diclus ducendis lineis idoneus. There is a harder and fofter kind of it, killoia duriufcula et molliufcula of Woodward's me- thod of foflils.
II. Pit coal.
From the deep glofly black colour of fome of the com- mon forts of pitcoal, I was induced to make trial of them as paints : their affinity to oils, in virtue of their bitumi- nous nature, promifed alfo fome advantages, in oil paint- ing, above the iubftances of a more earthy kind. Several of the finer pieces, levigated into an impalpable powder, W ere mixed both with oil and with gum water, and applied on paper and on wood. Both mixtures, when laid on thick, appeared of a pretty good black colour, though much in- ferior to that of the coal at firrr. ; and the oily one feemed to dry fooner than oil paints generally do. Laid on thin, or in a dilute ilate, they looked brown, not of the grey colour which refults from the dilution of a pure black. Pitcoal therefore may be confidered, not as a true black, but as a brown-black; a colour on many occafions wanted in painting, and which, as I have been informed by an in- genious
[ 325 ] genious artift, is often in bufinefs produced with this ma- terial.
As different forts of pitcoal, and different pieces from one pit, differ much from one another in degree and fpecies of colour, fome care mould be taken in the choice of them, according to the purpofe they are intended for. All the forts, at leaft all which I have tried, require long grinding in order to their being reduced into a powder of fufficiant finenefs.
III. Black funds. The black fands, one of the brighteft and moft beautiful of which is found in Virginia, lofe their colour on being ground into powder, and hence cannot be ufed as pig- ments. There are however cafes, in which they may con- tribute to the embellishment of certain works, by being ftrewed upon oil paintings for a fparkling black, in the fame manner as fmalt is ftrewed for blue. In this inten- tion they are ufed on writings, preferably to the white fands, as they do not weaken the colour of the ink, but coincide with its blacknefs, and give an agreeable luftre.
IV. Black-lead.
This mineral is dug in our own country; and is here, as Dr. Woodward obferves, in the preface to his method of fomls, more plentiful, and of a better kind, than in any other part of the world. According to Dr. Plott's account, in the Philofophical Transactions, No. 240, it is found only at Kefwych, in Cumberland, and is there called wadt or kellow, by which lafl name, as we have already taken notice, an earth like the black chalk is diflinguifhed in other places.
The colour of black-lead, rather a deep fhining bhriih grey than a black, may be feen, diluted a little, in the black
U u 2 melting
[ 3^6 ] melting pots when broken or the furface fcraped off, and entire in the genuine fort of black pencils. It differs not a little in goodnefs, fome forts marking paper freely, and others very difficultly or fcarce at all. It is fmooth and as it were unctuous to the touch, and hence is fometimes ufed inftead of oil or foap, for giving flipperi- nefs to the rubbing parts of machines. Acids neither diffolve it, nor alter its colour or unctuofity.
Black-lead has not been found to contain any of the metal from which it receives its name, and its compofition appears to be of a very fingular kind. From its known refiftence to vehement degrees of fire, whether urged by itfelf in clofe veffels, or made with clay into melting-pots and placed among the burning fuel, it fhould feem that it could not partake largely of any volatile fubftance ; and it has been generally fuppofed to confift chiefly of a talky earth. But Mr. Quift relates, in a curious paper of ex- periments on black-lead, publifhed in the Swedifh trans- actions for 1 7 ca, that having expofed many different fpe- cimens of this mineral to a ftrong heat, on a fcorifying difh under a muffle, they all yielded lulphureous fumes and flowers in great abundance; and that there remained be- hind, from one fort, only a fifth part of its weight, and from another no more than a twentieth part, of a yellow or brown calx, which being treated with inflammable fluxes, yielded feven tenths its weight of a metallic mafs, which feemed to be a mixture of iron and tin. Agreeably to thefe experiments, in an effay for a new fyftem of mine- ralogy, publifhed lately in Sweden, afcribed to the cele- brated Mr. Cronfledt, and which bears ftrong- marks of great knowledge and experience in the mineral kingdom, black-lead is claffed among the fulphureous minerals, and cz\\zdi Jitlphur fatiated with iron and tin.
I could
[ 327 ]
I could not perfuade myfelf that the minerals, on which the above experiments were made, could be fuch as are called among us black-lead, till fome of the fineft black- lead of our pencil-makers, weighing one hundred and fixty-eight grains, in three pieces, having been kept of a moderately ftrong red heat on a fcorifying difh for three hours,with the common precaution of covering the veffel for a time, left the matter fhould crackle, and fome par- ticles be thrown off from it in fubftance ; I found it re- duced to about an hundred and twenty grains, and all the pieces changed on the outride to a fparkling rufty brown calx, of which a considerable part was attracted by a mag- netic bar, the internal parts continuing of the fame appear- ance as at firft. Being then broken into fmaller pieces, and expofed to a like heat for two hours, it fuffered the fame change as before, and was reduced to about fixty grains. Being further broken, and calcined with a mo- derate red heat for ten hours, it was diminifhed to thirty grains ; and by a repetition of this operation, to twelve grains, or a fourteenth part of its firft weight.
The remarkable diffipation, in thefe experiments, of a fubftance which in clofe veffels reiifts intenfe fires, may be fomewhat illuftrated by the known property of charcoal, which when excluded from the aftion of the air, whether by being inclofed in a vefTel, or mixed with clay into a mafs, remains unconfumed and unaltered in the fire. MafTes of black-lead feem to calcine and fuffer a diffipation only on the furface ; the internal part remaining long un- changed, unlefs the mafs be broken, or the calx rubbed off, fo as that freih furfaces may be expofed to the air. The common black-lead melting-pots, made of clay and the coarfer kinds of black-lead powdered, like thofe made of clay and charcoal powder, lofe their external blacknefs with part of their weight, and thus have their ftaining
quality
[328 J quality deftroyed, by flrong fire. Hence furnaces made of thefe pots, as defcribed at the beginning of this volume, after they have fuffered ftrong fire, ceafe to difcolour the hands.
Black-lead in fine powder, ftirred into melted fulphur, unites with it fo uniformly, and in fuch quantity, in virtue perhaps of its own abounding with fulphur, that though the compound remains fluid enough to be poured into moulds, it looks nearly like the coarfer forts of black-lead itfelf. Probably the way which prince Rupert is faid to have had, mentioned in the third volume of Dr. Birch's Hiflory of the Royal Society, of making black-lead run like a metal in a mould, lb as to ferve for black-lead again,, confirmed in mixing widi it fulphur or fulphureous bodies.
On this principle the German black-lead pencils are laid to be made -, and many of thole which are hawked about by certain perfons among us, are prepared in the fame manner : their melting or foftening, when held in a candle, or applied to a red hot iron, and yielding a bluifli flame, with a ilrong fmell like that of burning brimllone, betrays their composition; for black-lead itfelf yields no fmell or fume, and fuffers no apparent alteration, in that heat. Pencils made with fuch additions are of a very bad kind : they are hard, brittle, and do not call or make a xnark freely either on paper or wood, rather cutting or fcratching them than leaving a coloured flroke.
The true Englilh pencils (which Vogel in his Mineral Syftem, and fome other foreign writers, imagine to be pre- pared alio by melting the black-lead with fome additional fubftances, and calling it into a mould) are formed of black-lead alone, fawed into flips, which are fitted into a groove made in a piece of wood, and another flip of wood, glued over them : the fofteil wood, as cedar, is made choice of, that the pencil may be the eafier cut -, and a
part
C 329 1
part at one end, too fhort to be conveniently ufed after the reft has been worn and cut away, is left unfilled with the black-lead, that there may be no wafte of fo valuable a commodity. Thefe pencils are greatly preferable to the others, though feldcm fo perfect as could be wiihed, being accompanied with fome degree of the fame inconveni- ences, and being very unequal in their quality, on account of different forts of the mineral being fraudulently joined together in one pencil, the fore part being commonly pretty good, and the reft of an inferior kind. Some, to avoid thefe imperfections, take the finer pieces of black-lead itfelf, which they faw into flips, and fix for ufe in port- crayons : this is doubtlefs the fureft way of obtaining black-lead crayons, whofe goodnefs can be depended on.
V, Black vegetable "Juices.
The excellent black varnifh of China and Japan, which bas hitherto been but imperfectly imitated in Europe, and which was formerly thought to be an artificial compofition of refinous bodies coloured with black pigments, has been difcovercd, by the later travellers in thofe countries, to be a native juice, exuding from inciiions made in the trunks of certain trees. One of thefe trees, according to the ac- count given of it in Kacmpfer's Amcvnitates exotica, is that whofe fruit is fometimes brought to Europe, as a medicinal drug, under the name of anacardium.
The anacardium itfelf, as it comes to us, is remarkable for a black-colouring juice. It is a kind of nut, with a double ihell, containing, in the fpace between the outer and the inner fhell, a fungous fubftance filled with a dark- coloured vifcous fluid, which is eafily forced out, by cut- ing the nut, and fqueezing it between the fingers : a little warmth, by liquefying the thick matter, makes it come out more freely; though the quantity obtained, either with or without heat, is not very confiderable. This juice, rub- bed
[ 33° 3 bed on linen or cotton, gives a reddifh-brown ftain, which foon deepens in the air to a black, and which I have not found to be difcharged by waihing, and boiling, with foap or alcaline ley. Hence the anacardium is Paid to be ufed for marking linen and cotton cloths, and to be known ail over India by the name of marking nut.
The caftiew nut, called by fome the anacardium of the Weft-Indies, and which in feveral refpects has a great re- lemblance to the oriental anacardium, differs from it in ics colouring quality ; the juice lodged between its fhells be- ing much paler, and giving to linen, cotton, or paper, only a brownifh ftain, durable indeed, but which does not change at all towards blacknefs.
There are however trees, natives of our own American colonies, which appear to contain juices of the fame nature with the valuable productions of the Indian. Of this kind are feveral, and perhaps the greater number, of the fpecies of toxicodendron or poifon-tree. Mr.Cateiby,inhishiftory of Carolina, defcribes one, called there the poifon-afh, from whofe trunk flows a liquid, black as ink, and fuppofed to bepoifonous: this reputed poifonous quality,, as I have been informed by fome gentlemen of that country, haj hitherto deterred the inhabitants from attempting to col- lect, or make any ufe of it. The abbe Mazeas, in tht Philofophical Tranfactions, vol. 49, for the year 1755", gives an account of three forts of the toxicodendron, raifed in a botanic garden in France, containing in their leaves a milky juice, which in drying became of a deep black, and communicated the fame colour to the linen it was dropt on : the linen, thus ftained, was boiled with foap, and came out without the leaft diminution of its colour; nor did ftrong ley of wood-afhes make any change in it.
Several of thefe trees have been raifed in the open ground in England: fome of them ftill remain in the
bifhop
[ 331 ] bifhop of London's garden at Fulham, after having been long neglected, and fufFered many fevere winters: fee a catalogue of the exotic trees in this garden, by Dr. Watfon, in the Philofophical Tranfactions, vol. 47, for the years 1 75 1 and 1752. They appear alfo to perfect their co- louring juices, in this, nearly as well as in their native cli- mate. The fpecies called by Mr. Miller the true lac tree, of which I was favoured with a branch by himfelf from Chelfea garden, was found to contain, in its bark, and in the pedicles and ribs of the leaves, a fomewhat milky juice, which foon changed in the air to a reddilh-brown, and in two or three hours to a deep blackifh or brownifh-black colour: wherever the bark was cut or wounded, the in~ ciiion became blackifh} and on feveral parts of the leaves the juice had fpontaneoufly exuded, and flamed them of the fame colour. This juice, dropt on linen, gave at firft little or no colour, looking only like a fpot of oil; but by degrees the. part moiftened with, it darkened in the fame manner as the juice itielf. On wafhing and boiling the linen with foap, the {tain not only was not difcharged, but feemed to have its blacknefs rather improved ; as if a brown matter, with which the black was manifeitly de- bafed, had been in part wafhed out, fo as to leave the black more pure.
It were to be wifhed that fome attempts were made, for collecting the colouring juices of thefe trees, in fufficient plenty, for anfwering the important purpofes to which they promife to be applicable. Perhaps alfo means might be found of introducing into fome parts of the extenfive dominions of Great-Britain, in which all varieties of foil and climate are now to be met with, the oriental trees themfelves, to which fome of the Indian manufactures are fuppofed to owe diftinguifhed advantages. This there are. now fome grounds to hope for, from the patronage of a
- X x fociety,
[ 332 1 fociety, whofe encouragement has already fo greatly pro- moted the culture of many valuable plants and trees.
As the milky juices of fome of our common plants turn dark-coloured or blackifh in drying, I was induced to make trial of feveral of them on linen : The milks of wild pop- pies, garden poppies, dandelion, hawkweed, fowthiftle, gave brown or brownifh-red ftains, which were difcharged by warning with foap : the milks of the fig-tree, of let- tuces, and of different kinds of fpurges, gave no colour at all. The colourlefs juice which ifTues from hop-ftalks when cut, ftains linen of a pale reddifh or brownifh-red, extremely durable : I tried to deepen the colour by re- peated applications of the juice, but could never make any approach to blacknefs. The juice of floes gave likewife a pale brownifh ftain, which, by repeated warnings with foap, and wetting with flrong folution of alcaline fait, was darkened to a deeper brown : on baking the floes, their juice turns red, and the red ftain which it then imparts to linen is, on wafhing with foap, changed to a pale bluifh, which alfo proves durable. The juices both of the raw and baked floes were applied repeatedly on the fame fpots, in order to deepen the refpective colours ; and the brown or reddilh-brown ftain of the raw floe, and the blue of the baked, were applied on one another, on principles here- after explained. In all thefe ways a ftain was obtained, which when flightly wafhed with foap, looked of a pretty deep black ; but by longer wafhing, much of the colour was difcharged, and little more was left than a Angle ap- plication of the juice would have produced. The floes were tried in different ftates of maturity, from the begin- ing of feptember to the middle of december; and the event was always nearly the fame. Though thefe experi- ments, with many others of the fame kind, proved unfuc- cefsful in regard to the production of the colour here inten- ded,
[ 333 ] «kd, they ferve to point out means,which may be convenient and ufeful on fome occafions, of marking linen with a colour, pale indeed, but fufficiently vifible, which foap does not difcharge.
In the fifth volume of the celebrated Linnaeus's Ama- nitates academics, mention is made of a black colour ob- tained from the berries of two plants, which grow wild in fome of the northern parts of England, and which I have not hitherto had an opportunity of trying. One is the atlaa fpicata or chrijiophoriana, herb-chriftopher or bane- berries ; the other empetrum procumbens or erica baccifera nigra, black-berried heath, crow-berries, or crake-berries. The juice of the bane-berries, boiled with alum, is faid to yield a black ink; and the heath-berries, boiled alfo with alum, to dye cloths of a purple-black.
VI. Cuttle fijh ink.
The cuttle fifh, faid to be pretty common in the Me- diterranean, is not wholly a ftranger to our own feas, as appears from its bone found on our fhores. This bone is hard on one fide, but foft and yielding on the other, fo as readily to receive pretty neat imprefiions from medals, &c. and afterwards to ferve as a mould for the cafting of me- tals, which thus take the figure of the original : the bone is frequently employed likewife for polifhing or cleaning filver. xMx. Borlafe, in his natural hiftory of Cornwal, fays that thefe bones, whofe characters are fo obvious and fo fingular that they cannot be miftaken, are found frequently on the fhores of Mounts bay; and likewife gives adefcrip- tion of the fifh itfelf as caught there on the fands in 1756. Dr. Leigh alfo, in his natural hiftory of Lancashire and Chefhire, relates that he has feen the fifh feveral times on the fhores of thofe counties.
Xx 2 This
[ 334 ] This fiih contains, in a certain diftind: veffel, a fluid as black as ink : which it is faid to fhed on being purfued, and thus to conceal itfelf by difcolouring the water. The particular qualities of this black animal liquor I have had no opportunity of examining myfelf, nor have I been able to obtain any fatisfadtory information concerning them from others. Dr. Leigh, in the place before referred to, fays he faw a letter, which had been written with it ten years before, and which ftill continued : it were to be wiflied he had fpecified more particularly the continuance of the colour, whether in its full deepnefs, or much faded. Some report that the ancients made their ink from it, and others that it is the bafis of the Indian or China ink : both thefe ac- counts appear however, from fome experiments and obfer- vations which will be related in the fequel of this eflay, to have little foundation : Pliny, fpeaking of the inks ufed in his time, after obferving that the cuttle fifh is in this refpecl: of a wonderful nature, adds expreflly that ink was not made from it.
SECT. m.
Black produced by Fire.
TH E action of fire properly applied, in the burning of animal and vegetable fubftances, produces, in the coal and in the foot, the two mofl durable and ufeful blacks of the painter and the varnifh-maker. The coal in particu- lar is of extreme permanence, refilling the force of time, and all the known agents of nature, except only that of an open fire, which burns it into white afhes. Some bodies of the metallic kind aflume alfo, in certain circumflances,
a black colour from fire.
■ ■
I. Cbar-
[ 335 ]
I. Charcoal Blacks.
Most of the blacks of this clafs, befides their incorrup- tibility, have' the advantage of a full colour, and work freely in all the jforms in which powdery pigments are applied ; provided they have been carefully prepared, by thoroughly burning the iubjecT: in a cloie veffel, and after- wards grinding the coal into a powder of due finenefs.
Pieces of charcoal are ufed alio in their entire ftate, for tracing the outlines of drawings, 6cc. in which intention they have an excellence, that their mark is eaiily wiped out. For thefe purpofes, either the finer pieces of com- mon charcoal are picked out and cut to a proper ihape ■, or the pencils are formed of wood, and afterwards burnt into charcoal, in a crucible, or other like veflel, covered and luted. When the procefs is fkilfully managed, the coal retains exactly the figure of the wood : fome have been fo dextrous as to char an arrow, without altering the form even of the feather.
The artifts commonly make choice of the fmaller branches of the tree, freed from the bark and the pith ; and fome particular kinds of wood, as the willow and the vine, they generally prefer to others. To difcover the foundation of this preference, and how far the coals of different vegetables differ from one another as colour- ing materials, I made the following experiments.
Small branches of the willow, vine, cherry, apple, pear, peach, plum, fig, birch, oak, elder, alder, yew, floe, hazel, fir and pine trees, were thoroughly dried, and in- clofed in a mafs of luting, made of clay beaten up with fand and horfe-dung : the mafs, dried flowly and gradu- ally heated to prevent its cracking, was kept red hot about three hours. On carefully breaking it, the pieces were all found well charred; but it could not be ob-
ferved
[ 336 ] ferved that they differed greatly from one another, either in degree of colour, or in the freedom of their marking upon paper.
This experiment affording little decifive, I repeated the operation in crucibles, with greater quantities of the ma- terials, that a more exact comparifon might be made of the colour of the coals, by ufing them as paints, both in a concentrated and diluted flare. Two crucibles were filled with vine twigs, cut in fmall bits, freed from the knots, and thoroughly dried : the mouth of one crucible being then fitted into that of the other, the juncture was. well fecured with luting. Small fmooth branches of moll of the other kinds of trees above mentioned, were in like manner incloied, each in two crucibles, and all of them continued about four hours in a ftrong red heat. Cuttings of white paper, beaten with water into a parte, fuch as is called- papier mache, that they might take up lefs room in the crucibles, and have lefs air lodged in their interftices, were dried and treated in the fame manner ; but fome flame appearing to burft out through a fmall crack which the vapour had forced in the luting, it was neceffary to take out thefe crucibles after they had been about ten mi- nutes in a red heat : the paper, neverthelefs, was perfect- ly charred.
The feveral coals were levigated into fine powder, mixed both with gum water and oil, and applied as paints, both thin and thick, by themfelves, and diluted with diffe- rent proportions of white. All of them, when laid on thick, appeared of a Itrong full black ; it could not be judged that one was of a finer colour than another. When fpread thin or diluted, there were indeed fome fenfible differences among them, but neither very considerable, nor of fuch a kind as to be eafily expreffed or defcribed : •they had all fomewhat of a bluifh caft, but different per-
fons,
t 337 ] fons, to whom the comparifons were referred, differed in their iudgements of them, and could not fix on any parti- cular coals as being more bluifh, more truly black, or more beautiful than the reft.
Inftead of the fmall branches, I tried next pieces of dif-> ferent woods, taken from the trunks of the trees. Here alfo the feveral coals appeared alike among themfelves, and fcarcely differed in point of colour from thofe of the twigs j but they feemed in general fomewhat harder, and did not mark quite fo freely on paper when ufed as crayons. Sufpedting from hence that the hardnefs of the coal might be proportional to that of the fubjecl: it was prepared from, I made fome further trials, which feemed to confirm this notion. The coals of the hard woods, box and guaiacum, were very fenfibly harder than thofe of the foft ones : the fhells and (tones of fruits yielded coals ftill harder, which would fcarcely make any mark on paper at all ; while the coals of the kernels of fruits were quite foft and mellow.
It may be judged from thefe experiments, that the preference of one kind of wood to another for making charcoal crayons, does not depend fo much upon any dif- ference of colour in the coals, as on their lbftnefs; in which quality perhaps none of our common woods is equal to the willow. Dr. Grew obferves, in his anatomy of plants, that in this wood the lbftnefs is equal or alike in all parts ; whence the coal, when ufed as a crayon in painting, not only makes the ftroke light, but every where certain, without disturbing the even motion of the hand. Deal or fir is likewife a very foft wood, but of unequal foft- nefs, fo that when cut acrofs, it tears, and will never polifh or work fmooth, whereas the willow works well in all directions.
Horns,
[ 338 ]
Horns, and the bones both of fifties and of land ani- mals, gave coals rather gloffier and deeper coloured than the vegetable coals, and which in general were very hard,, fo as difficultly or not at all to ftain paper. It feemed here, as in vegetables, that the hardnefs of the coal de- pends on that of the fubjecl matter; for lilk, woollen, leather, blocd, and the flefliy parts of animals, yielded foft coals. Some of thefe coals differed from others very fenfibly in degree of colour : that of ivory is fuperior to the reft, and is indifputably the fineft of all the charcoal blacks. Indeed we have no black pigments equal in beauty to ivory black, genuinely prepared, but fome care is requifite in the choice of it, what is generally fold under this name being no other than the coal of common bones.
On comparing the vegetable and animal coals together, in their lighter fhades, on paper, the bluifh caft, obferved in all thofe of the vegetable kingdom, was much lefs con- fpicuous in thofe of the animal, many of which feemed to incline rather to brown than to blue. In the eplcur- lhops a preparation is fold under the name of blue-black, which in this refpedt differs from the animal and agrees with the vegetable coals, feeming to have no greater a. decree of bluenefs than the coals of the woods and twigs above-mentioned, and even than common charcoal. That this preparation is no other than a vegetable coaL appeared from the following experiment. Laid on a red hot iron, it burnt and glowed like powdered charcoal, and turned into white allies; which afhes, thrown into oil of vitriol diluted with water, very readily diffolved into a bitterilh liquor, the characleriftic by which the ve- getable earth is diftinguifhed. From what particular vegetable matter this blue-black is prepared, experiments cannot difcover ; but thofe already mentioned feem fuffi- oienttp mew, that it may be obtained from many, and
that
[ 339 ] that the choice of the vegetable fubject affects rather the foftnefs or hardnefs than the colour of the coal.
After examining the different fubftances of the vege- table and animal kingdoms, I tried a mineral body, pitcoal; of which feveral pieces, of different forts, were charred in clofe crucibles. The charred coals, reduced into fine powder and ufed as paints on paper, fhewed nothing of the brownifh hue which the unburnt pitcoal had when tried in the fame manner, all of them inclining to bluifli, and moft of them having this caft in a greater degree than any of the vegetable coals. The blue-black of the mops cannot however bs of this origin, the afhes of the charred pitcoal not being diffoluble by the vitriolic acid, as thofe of the blue-black were found to be.
II. Soot blacks.
The foot blacks are in general much fofter and of a more yielding texture than thofe of the charcoal kind, and require much lefs grinding for uniting them with oily, watery or fpirituous liquors, into a fmooth mafs : of fome of them a part is diffolved by water or fpirit of wine, while none of the charcoal blacks have been found to contain any thing diffoluble.
This foluble matter of foot is not however black like the indiffoluble parts ; and in this particular, as well as "in the colour of the entire mafs, different forts of foot differ from one another. Thus the foot of pitcoal collected ia common chimneys, of itfelf rather greyifh-black than of a full black,, being infufed feparately in rectified fpirit of wine and in water, tinged the former of a tranfparent reddiih colour, and the latter of a paler reddiih ; while the deeper black foot of wood, both to fpirit and to water, gave an opake dark brown.
Y y From
[ 340 ] From the watery infufion of wood foot is prepared the brown pigment called biftre, for painting in water colours. According to Mr. Landois, in the French encyclopedic the foot is either boiled in water, or ground with a little urine (water will do as well) into a fmooth parte, and then diluted with more water : after ftanding for about half an hour, till the groffer fubftance of the foot has fettled, the liquor is poured off into another veffel, and fet by for two or three days, that the finer parts may fall to the bottom, which fine matter is the biftre. That the biftre of our colour fhops has been prepared by a procefs of this kind, and not, as fome have fufpedted, by evaporating the infu- fion of foot to an extract, may be prefumed both from its appearance and its qualities. It is in little maffes, fuch as are obtained in die common way of drying precipitates, or earthy powders that have been ground with water, by dropping them on a chalk ftone. It readily mingles with water, and continues for a time uniformly diffufed through the fluid : a confiderable part was obferved to fettle in an hour or two, a part more flowly, and after ftanding for many weeks a part remained diffolved in the water, fo as to tinge it of a brownifh-yellow colour, like a weak in- fufion of foot : this tinged liquor pafTed through a filter, without any feparation of the colouring matter. In the preparation of the biftre, when the foot liquor has depoiit- ed all that will fettle, the fediment, however drained, will neceffarily retain fome of the coloured fluid, which, dry- ing, will leave in it fome of the truly diffoluble parts of the foot ; and hence probably proceeds the matter in biftre which we find to continue diffolved in water, the propor- tion of which is inconfiderable, compared to that which precipitates. Different parcels of biftre differ confider- ably in their colour, on account, probably, of the different qualities of the foots which they were made from.
The
[34i ]
The fineft of the foot blacks, and the only one com- monly made ufe of as a black pigment, is that called lamp-black, which is brought chiefly from Germany and Sweden. Its preparation is defcribed in the Swedifh tranfactions for the year 1754, as a procefs dependent on the making of common refin : the impure refinous juice, collected from incifions made in pines and fir trees, is boiled down, with a little water, and (trained, whilft hot, through a bag : the dregs and pieces of bark, left in the (trainer, are burnt in a low oven, from which the fmoke is conveyed, through a long paflage, into a fquare cham- ber, having an opening in the top, on which is fattened a large fack, made of fleafy or thin-woven woollen fluff : the foot, or lamp-black, concretes partly in the chamber, from which it is fwept out once in two or three days ; and partly in the fack, which is now and then gently ftruck upon, both for making down the foot, and for clearing the interftices between the threads, fo as to pro- cure a fufficient draught of air through it. Confiderable quantities of this foot are prepared alfo in fome parts of England, particularly at the turpentine houfes, from the dregs and refufe parts of the refinous matters which are there manufactured.
The foot arifing in common chimneys from the more oily or refinous woods, as the fir and pine,, is obferved to- contain more diflbluble matter than that from other woods : and this diflbluble matter appears, in the former, to be more of an oily or refinous nature than in the latter, fpirit of wine extracting it moil plentifully from the one, and water from the other. The oilinefs and folubility of the foot feeming therefore to depend on thofe of the fub- jedt it is made from, it has been thought that lamp-black muft poflefs thefe qualities in a greater degree than any kind of common foot. Neverthelefs, on examining feveral
Y y 2 parcels
[ 342 ] parcels of lamp-black, procured from different fhops, I could not find that it gave any tincture at all, either to fpirit or to water.
Sufpecting fome miftake or fophiffication, or that the lamp-black had been burnt or charred, as it fometimes is to fit it for fome particular ufes, I prepared myfelf fome foot from linfeed oil, by hanging a large copper pan over the flame of a lamp, to receive its fmoke. In this me- thod the more curious artifts prepare lamp-black for the nicer purpofes, and from this collection of it from the flame of a lamp, the pigment probably received its name. The foot fo prepared gave no tincture either to water or to fpirit, any more than the common lamp-black of the fhops. I tried different kinds of oily and refinous bodies, with the fame event ; even the foots obtained from fifh oil, and tallow, did not appear to differ from thofe of the vegetable oils and refins. They were all of a finer colour than the lamp-black commonly fold.
Some foot was collected in like manner from fir and other woods, by burning fmall pieces of them flowly under a copper pan. All the foots were of a deeper black colour than thofe obtained from the fame kinds of wood in a common chimney, and very little if at all inferior to thofe of the oils : they gave only a jult difcerniblc tincture to water and fpirit, while the foots of the chimney im- parted a flrong deep one to both. The foot of mineral bitumens, in this clofe way of burning, appears to be of the fame qualities with thofe of woods, oils, and re- fins : in fome parts of Germany, as I am informed by a worthy foreign correfpondent, great quantities of good lamp-black are prepared from a fort of pitcoal.
It appears therefore that the differences of foots do not depend altogether on the qualities of the fubjeds, but in great meafure on the manner in which the fubjedT: is
burnt,
[ 343 ] burnt, or the foot caught. The foots produced in com- mon chimneys, from different kinds of wood, refinous and not refinous, dry and green, do not differ near fo much from one another, as thofe which are produced from one kind of wood, in a common chimney and in the more confined way of burning above-mentioned.
III. Black metallic calces.
The mineral cobalt, roaffed till its arfenical parts are dif- fipated, becomes black ; and being then melted with in- flammable fluxes, yields a regulus, which likewife affumes a black colour by calcination. The fame regulus is ob- tained from the artificial zaffrc, whofe bafis is the roafled cobalt, and which is employed for tinging glafs blue ; as alfo from the deep blue glafs itfelf, called, when ground, by the painters Jhialt, and by the laundrefles powder-blue. The cobalt, more valuable for thefe important products than for the property which occafions it to be here taken notice of, and which has hitherto been afforded chiefly by Saxony, has of late, by the encouragement of the Society for pro- moting arts, been difcovered in our own country : in further fearches for it, the property here mentioned may be of great aififtance, thofe minerals, and thofe only, which calcine black, promifing to be ufeful cobalts. Calces of iron, whether red, yellow, or of other colours, on being brought into fufion by the addition of vitreous bodies, give always a black colour to the glafs if the quantity of iron is confiderable. From copper alfo a black colour may be pro- duced by fire, and applied to the flaming and embellifhing of certain ftones, of which an account will be given here- after towards the end of this hiflory. I have not obferved that any of the other metallic bodies are changed black in any circumftances by fimple heat.
SECT.
[ 344 ]
SECT. IV.
Black produced by mixture.
I. Black from Iron.
FROM infufions of certain vegetables, mixed with green vitriol, is produced a deep black liquor, of moff. extenfive ufe for dying and fraining black. To wool- len and filk it gives a permanent colour, although from linen, and other vegetable bodies, its blacknefs is dif- charged by warning.
The fubftances chiefly employed for producing this co- lour with vitriol, are the excrefcences of the oak tree, called galls ; of which there are two principal kinds, one laid to be brought from the Levant, and the other from ibme of the fouthern parts of Europe, particularly Sicily and Romania. The former, called by authors Aleppo galls, and in the Ihops of our dry-falters blue galls, are generally of a bluifh colour, or of a greyifh or blackifh verging to bluenefs, unequal and warty on the furface, hard to break, and of a dole compact texture : the others, commonly called white galls, are of a pale brownifh or whitifh colour, fmooth, round, eafily broken, lefs compact,, and of a much larger fize. The two forts differ in ftrength, but in other refpects they appear to be of the fame quality. The Aleppo or blue galls are the ftrongeft : two parts of thefe are reckoned by the workmen to be equivalent to three of the white -, and fuch comparifons, as I have made of the two, incline me to think, that the difference in their ftrength is rather greater than this proportion..
Thefe excrefcences appear to proceed from the juices of the oak tree iffuing out through fmall wounds made by certain infe&s ; which infefts not being found in this cli- mate, no galls are here produced j though other kinds of
excrefcences
[ 345 3 excrefcences are frequent on our oaks, occalloned perhaps by infects of another kind. It is not, as might be thought, on any particular fpecies of the oak tree that galls are formed ; for Mr. Ray fays, that in his travels abroad, he has feen them on the fame kind of oak with that which is common here.
All the parts of the oak tree feem to contain juices of nearly the fame general virtue with galls -, for the leaves, the acorns, and more particularly the bark and wood, ftrike with vitriol a black or a deep colour approaching to blacknefs. There are many other vegetable fubftances which have a like effect in different degrees ; as the leaves, fmall branches, and flowery clufters of the fumach tree, balauftine flowers, pomegranate peel, alder bark, biftort root, and thofe in general which are auftere, aftringent, or corrugating to the tafle ; infomuch that turning a folution of vitriol black is looked upon as a fure teft of aftringency in vegetables. The power by which they produce this blacknefs, and their aftringency, or that by which they contract an animal fibre, and by which they contribute to the tanning of leather, feem to depend upon one and the fame principle, and to be proportional to one another. Of the other properties of this aftringent and colouring matter, little more is known, than that it is dilfolved and extracted from the fubject both by water and fpirit of wine, and that it does not exhale in the evaporation of the liquors by heat.
Green vitriol commonly called copperas, die other in- gredient in the black mixture, is a preparation of iron, made at Deptford, Blackwall, Newcaftle, and fome other parts of England, by boiling old iron with an acid liquor which runs from certain pyritse on being long expofed to the weather. By diffolving iron in the vitriolic acid, and
cry flail
[ 346 ] cryftallizing the folution, the fame fait is obtained in a purer Hate.
When a decoction or infufion of the galls is dropt into a folution of the vitriol largely diluted with water, the firft drops produce bluiih or purplifh red clouds, which foon mingling with the liquor tinge it uniformly of their own bluiih or reddifh colour. It feems to be on the qua- lity of the water that this difference in the colour depends. With diftilled water, or the common fpring water;-., the mixture is always blue. If we previoufly dillblve in the water the moft minute quantity of any alcaline fait, t<><; imall to be difcoverable by any of the common mean' by which waters are examined, or if the water is in the l'eaft degree putrid, the colour of the mixture proves purple or reddifh. Rain water, caught as it falls from the clouds, in an open held, in clean glafs veffels, gives a blue, but fuch as is collected from the tops of houfes grows purple with the vitriol and galls ; from whence it may be pre- fumed, that this laft has contracted a putrid tendency, or received an alcaline impregnation, though fo flight as not to be fenfible on other ways of trial.
Both the blue and the purple liquors, on adding more of the aftringent infufion, deepen to a black, more or left intenfe according to the degree of dilution : if the raixCura proves of a deep opake blacknefs, it again becomes bhiiili or purplifh when further diluted. If fuffered to ftand in this dilute flate for two or three days, the colouring matter fettles to the bottom in form of a fine black mud, which, by flightly fhaking the veffel, is diffufed again through the liquor, and tinges it of its former colour. When the mix- ture is of a full blacknefs, this feparation does not happen, or in a far lefs degree; for though a part of the black matter precipitates in ftanding, yet fo much remains difu folved. that the liquor continues black. This fufpenfion
of
[ 347 J of the colouring fubftance in the black liquid may be attributed in part to the gummy matter of the aflringent infufion increafing the confidence of the watery fluid, for the feparation is retarded in the diluted mixture by a fmall addition of gum arabic; though another principle appears alfo to concur for part of the effect.
If the mixture, either in its black or diluted ftate, be poured into a filter, the liquor palTes through coloured, only a part of the black matter remaining on the paper. The filtered liquor, to the eye perfectly homogene, on ftanding for fome time becomes turbid and full of fine black flakes : being freed from thefe by a fecond filtration, it again contracts the fame appearance, and this repeatedly, till all the colouring parts are feparated, and the liquor has become colourlefs. It mould feem therefore, that there happens a gradual and flow concretion of the black corpufcles, into particles large enough to fubfide by their own weight, or to be retained on a filter ; and that this concretion is greatly influenced by dilution with water. Perhaps it is affected alfo by the action of the air ; for having once fet fome of the diluted mixture to fettle in a clofe ftopt glafs, the feparation of the black matter was remarkably more flow than in the other experiments, in which the veilel was open.
The colouring matter, thus feparated from the liquor, being drained on a filter and dried, appeared of a deep black, which did not feem to have fuffered any change on lying expofed to the air for upwards of four months. Made red hot, it glowed and burnt, though without flame- ing, and became a rufty brown powder, which was readily attracted by a magnetic bar ; though in its black ftate, the magnet had no adfion on it. The vitriolic acid, di- luted with water and digefled on the black powder, dif- folved greatefl part of it, leaving only a very little quantity
Z z of
[ 348 ] of whitifh matter. Solution of pure fixt alealine fait dif- folved very little of it : the liquor received a reddifh brown colour, and the powder became blackifh brown. This reiiduum was attracted by the magnet after being made red hot, though not before : the alealine tincture, pafled through a filter, and mixed with folution of green vitriol, (truck a deep brownifh-black colour, nearly the fame with that which refults from mixing with the vitriolic folution an alealine tincture of galls.
From thefe experiments it feems to follow, that the colouring matter in the black mixtures is iron, extricated from its acid folvent in a highly attenuated or divided ftate, and combined with a peculiar fpecies of matter con- tained in aftringent vegetables ; which matter, after the watery fluid that the compound floats in has been feparated, is in part extracted from the iron by alealine liquors, and may thence be again transferred into frefh diflblved iron.
The blacknefs is generally attributed to the iron being barely revived from the vitriol in its metallic ftate j the black matter being fuppofed to be of the feme nature with the impalpable black powder, into which fine iron filings are changed by lying for many months under water. But. this black matter differs from that of our mixtures in two very material properties. It is attracted in its black ftate by the magnet -, and when moiftened and expofed to the air, it changes fpeedily into ruff. The refiftance of ours to the magnet and to the air proceeds doubtlefs from the combination of the other matter with the iron ; and there appears fome analogy, in regard to the manner of production, between this black fubftance and Pruflian blue ; one being a precipitation and coalition of diflblved iron with one fpecies of matter, and the other with another : the prin- cipal difference is, that the fubftance combined with the iron in the Pruflian blue defends the metal from the action
of
[ 349 ] of acids, which that in the black compound is unable
to do.
I tried likewife folutions and different foluble prepa- rations of iron, made with the nitrous, marine, and vege- table acids -, as alfo an alcaline folution of it, obtained in Stahls method, by dropping into ftrong alcaline ley a folution of the metal made in the nitrous acid. All the preparations, in which the iron was diffalved by an acid, (truck a black colour with aftringents ; but the alcaline folution gave only a reddifh brown. In this refpedt alfo the tinging fubftance in our mixtures agrees with that of Pruffian blue.
It has been affirmed, that folutions of iron in the ni- trous and marine acids produce no blacknefs with aftrin- gents ; and fome trials, formerly made, led me into the fame opinion. On re-examining this matter, the fallacy appeared to lie here ; that thofe acids do not very readily fatiate themfelves with the iron, and that any confider- able quantity of redundant acid in the folution prevents the blacknefs. The cafe is the fame with the vitriolic acid alfo ; and probably if we had not, in the vitriol itfelf, a faturated combination of this acid with iron, ready pre- pared for other ufes, the vitriolic folutions would as often have been found to refufe linking a black colour, as the nitrous and marine.
After the blacknefs has been produced, the addition of any of thefe acids, and even of the vegetable in fufficient quantity, deftroys it, by rediilblving the ferrugineous mat- ter : hence the ufe of acids or acidulated liquors, for dif- charging ftains made by the black mixtures, fuch as that of common ink. Alcalies alfo deftroy the blacknefs, ap- parently on a different principle, by diflblving the aftrin- gent matter, and precipitating the iron nearly in the fame ochery ftate, as they do from the fimple acid folutions
Zz 2 of
[ .35° ] of the metal. After the blacknefs has been difcharged by an alcali, the addition of any acid, in fuch quantity as to fatiate the alcali, reftores it j and after its difcharge by an acid, it is in like manner reftored again by an alcali.
II. Black from filver .
Solution of filver in aquafortis, of itfelf colourlefs as water, dropt upon white bone or other like animal fub- ftances, produces at firft no ftain. In fome time, fooner or later according as the fubject is more or lefs expofed to the fun and air, the part moiftened with the liquid becomes firft of a reddifh or purplifh colour, which by degrees changes into a brown, and at length deepens to a black. Several kinds of ftones and wood receive, from the fame folution, purplifh, reddifh, bluifh, brown, or black ftains.
On what particular combination the colours here de- pend, has not as yet been explained, nor indeed, fo far as I can find, examined. The following obfervations, which we owe to a paper of Mr. Schulze, in the firft. volume of the aSla nature curioforum, though they do not difcover the caufe of the effect, may contribute to its illuftration.
White chalk, moiftened with a folution of filver, and dried in the lluide or by a fire, receives no colour : dried in the fun, or expofed to the fun after it has been other- wife dried, it becomes on the furface of a purplifh black. When the earth is thoroughly moiftened with the foluti- on, if fo much water be added, as will reduce it into a thin pafte, the matter in this ftate alfo becomes coloured in the fun, though in the fhade it acquires no tinge. The colour is produced only in thofe parts on which the fun mines : a thread applied on the outfide of the glafs, be- tween it and the fun, occafions a correfponding uncoloured vein on the included matter ; and hence diftincl: characters
may
[ 35* ] may be exhibited on the mafs, by intercepting a part of the funs light by threads or cut paper.
I repeated thefe experiments, and obferved that the co- lours produced were nearly the fame with thofe which the folution of filver communicates to bone or ivory, except that they did not deepen to fo true a black. The colour was entirely fuperficial ; for when the matter, by now and then turning the glafs in the fun, had been tinged all over to a reddifh or brownifh black, it appeared white again when fhaken and mixed together. By continuing the expofure for many weeks, and frequently making the mixture that frefh furfaces of it might be fucceffively acted upon by the fun, it became at length coloured, though weakly, throughout. The funs rays in december produced the fame change as in June, and, fo far as can be recol- lected, as fpeedily. The light of candles, and a gentle Warmth from common fire, did not feem to affect the colour. In a confiderable heat, greater than that of the fun in fummer, the matter became brown, but without acquiring the black colour which the fun communicates.
I tried alfo feveral other earthy bodies, and found that thofe which diffolve in acids, the afhes of vegetables and of bones and horns, fuffered the fame changes as chalk and the other mineral calcareous earths. But powdered flint, however moiftened and drenched with the folution, received no colour in half a years expofure to the fun. White clay, plafter-of-paris, and powdered talk remained alfo uncoloured ; and even chalk itfelf previoufly fatiated with the vitriolic acid, fo as not to be adfcd upon by the acid in which the filver was diffolved, fuffered no change.
It mould feem from thefe experiments, that in order to the production of a black ftain from folution of filver, it is neceffary that the fubject moiftened with it be not only expofed to the folar light, but that it contain fome matter
which
[ 352 ] which the nitrous acid may diflblve preferably to the filver which it already holds diflblved. This is plainly the cafe in bones, horns, hair, marble, and feveral other bodies which are ftained by the filver folution ; though there are alio fome ftones which are ftained by it, as agate, in which a fubftance foluble by the acid has not yet difcovered itfelf. It may be obferved, that the production of a dark co- lour from the action of the fun is not peculiar to folution of filver, or to a combination of this folution with foluble earths. When bifmuth is diflblved in the nitrous acid, and afterwards precipitated by dilution with water, the precipitated powder, exceedingly white, foon becomes dark coloured in the fun, fo as to require great care, in drying and keeping it, to preferve the whitenefs, for which that preparation is valued. Mercurius dulcis, a combina- tion of quickfilver with the marine acid, fuffers a like change. The effect, however, is lefs confiderable here than in the filver liquor; for though both. preparations be- come dark, I have not obferved blacknefs produced in either.
III. Black from Lead and Sulphur.
Lead, a metal which of itfelf makes a blackifh mark on paper, yields colours more approaching to blacknefs in fome of its diflblutions and combinations with other bo- dies. Solutions of lead made in acids, dropt upon paper or other white fubjedls, communicate no flain ; but on being expofed to fulphureous vapours, or wafhed over with alcaline folutions of fulphur, the parts moiftened with the folution of lead become immediately yellow, and foon after of a deep brown or black, according as the liquors were more or lefs faturated with the matters diflblved in them.
The production of this colour has not been applied to any important ufe, being regarded chiefly as a matter of
curiofity,
[ 353 ] curiofity, as affording the foundation of one of the writings called invifible or fympathetic. For this purpofe, vinegar is ftrongly impregnated with lead, by boiling with li- tharge, ceruife, or other calces of the metal ; or what amounts to the fame thing, the common preparation called fugar of lead is diffolved in water : folutions of lead in aqua fortis anfwer the fame end, except that, when writ- ten with, they are apt to corrode the paper. The fulphure- ous liquor is commonly prepared by boiling fome orpi- ment, which is a natural mixture of fulphur and arfenic, in water with quicklime, till the water is ftrongly im- pregnated with the orpiment : in the room of this prepa- ration may be ufed a faturated folution of common brim- ftone, made by boiling the brimftone either with quick- lime, or in ftrong alcaline ley. Characters written with the lead folution, which when gently dried in the air are not to be diftinguifhed from the reft of the paper, become of a legible deep colour, on paffing over them a pencil dipt in the fulphureous folution. Thofe who amufe them- felves with this experiment, have black characters in the neighbourhood of the invifible ones ; which black ones are drawn with burnt cork, or other charcoal blacks, mixed only with water : by the wet pencil, thefe are warn- ed off, at the fame time that the others are made to appear.
If any acid be added to the fulphureous folution, a very offenfive fmell arifes ; and a folution of fulphur made in ftrong volatile fpirits, prepared with quicklime, exhales a like fmell. This penetrating diffufive vapour, particularly that of the laft of thefe preparations, gives colour to the invifible writing with the lead folution at a confiderable diftance : though the writing be placed in the middle of a quire of paper, or of a pretty thick book, the vapour will foon reach it, and ftain it brown op black. The colour
is
[ 354] is difcharged by acids, and reflored again by the fulphure- ous vapour or folution.
Calces of lead, melted with fulphur, form a black, or blackilh mafs, which proves an ufeful matter for taking carts from medals, being confiderably more tough than fulphur alone. For this purpofe, equal parts of minium and flowers of fulphur are put in an iron ladle over the fire, till they foften into the confidence of pap, and are then kindled with a piece of lighted paper, and ftirred for fome time : the veffel being afterwards covered clofe and continued on the fire, the mixture becomes fluid in a few minutes, and is then poured upon the medal previously oiled and wiped pretty clean. This procefs, communi- cated to me by a friend, I have often tried with fatisfaction. The calls are very neat ; the colour, fome times a pretty deep black, and fometimes a black grey, according to dif- ferent circumftances in the fufion ; they are very durable, and when foiled may be vvaihcd clean again with fpirit of wine.
There are other metals alfo which produce a black co- lour with fulphureous bodies. When a folution of fdver in aquafortis is added to a folution of fulphur made in alcaline ley, the filver and fulphur unite and precipitate together in the form of a black powder. Quicklilver and fulphur, by being barely rubbed together in a mortar, be- come black, and hence this mixture, commonly made for medicinal ufes, is called the mineral ethiops. But as thele kinds of compoiitions afford nothing of importance for the art of colouring black, it would be needlefs in this place to confider them more particularly.
IV. Black from the combination of other colours. In the three foregoing articles we have feen blacknefs generated from the a&ion of certain bodies on one an- other,
[ 355 J other, and, in the preceding lection, from alterations pro- duced in the bodies themfelves or their component parts. There is another general principle, on which fome or' the moft common colours are obtained, the combination of two or more differently coloured bodies together, whence refults a new colour compounded of thofe of the ingre- dients : thus green is formed from a mixture of blue and yellow, and purple from blue and red. Thefe compound colours are found to fucceed, by grinding together coloured earthy powders, by uniting coloured flames or the funs beams which have pafled coloured through glafles, by mixing dyed wool, threads, &c. where there can be no fufpicion of any intrinfic change made in the fubjects, or of any action of the ingredients on one another.
Mr. le Blon, in his harmony of colours, forms black on the fame foundation, by mixing together the three colours called primitive, blue, red, and yellow; and Mr. Cartel, in his opt hue des coiileurs, published in 1740, fays that this compound black has an advantage, in painting, above the Ample ones, of anfwering better for the dark- ening of other colours. Thus if blue, by the addition of black, is to be darkened into a blue-black, the Ample blacks, according to him, if ufed in fufficient quantity to produce the requilite deepnefs, conceal the blue, while the compound blacks leave it diftinguilhable.
Le Blon does not mention the proportions of the three primitive colours neceflary for producing black. Cartel directs fifteen parts of blue, five of red, and three of yel- low •, but takes notice, that thefe proportions are rather fpeculatively than practically jurt, and that the eye only can be the true judge -, our colours being all very imper- fect, and our pigments or other bodies, of one denomi- nation of colour, being very unequal in its degree or in- • tenfity. He obferves that the colours ihould each be the
A a a deepeft
[ Jjt ]
deepeft and darkeft in its kind -, and that, inftead of taking one pigment for each colour, it is better to take as many as can be got ; for the greater contrail there is of hetero- geneous and dilcordant drugs, the more true and beautiful,. he fays, will the black be, and the more capable of uniting with all other colours, without fupprefling them, and even without making them tawncy.
The trials I have made of mixing different blue, red, and yellow powders, have not fucceeded fo far as to afford a perfect black ; but I have often obtained from them very dark colours, fuch as may be called brown-blacks and grey-blacks, fuch as we commonly fee in the dark parts of paintings, and fuch as the charcoal and foot blacks appear when diluted a little. The ingredients being each of a dark deep colour is a very neceffary condition ; for bright blues, bright reds, and bright yellows, mixed in fuch proportions that neither colour prevailed, produced only a grey. In effedl, all compofitior.s of this kind, phy- fically confidered, can be no other than greys, or of fome of the intermediate teints between whitenefs and blacknefs ; and thefe greys will be fo much the lighter or darker, as the component colours of themfelves are bright or dark. Some further experiments of producing a black by com- pofition, for the purpofes of dying and flaining, will be mentioned in the fequel of this effay.
SECT. V. Black paints, varniftjcs, &c.
I. Black paint with oil.
BLACK oil paint is prepared, by grinding, with a proper quantity of oil, the charcoal or foot blacks, or the natural black earths, or pitcoal, till they are united into a fmooth, uniform, thick compound, which is ocqa-
fionally
[ 357 1 fionally diluted with more oil, to a due confidence for being worked freely with the brum or pencil.
The finefl black colour is made with ivory-black, ground, before the addition of the oil, into an impalpable powder. The material mofl commonly made ufe of is lamp-black, whofe colour is for moll purpofes of fumcient deepnefs and beauty. The undtuofity of lamp-black gives it an advantage above the other pigments, of mixing more eafily and perfectly with the oil ; but from the fame qua- lity it receives a difad vantage, of being too flow in drying for the difpatch requifite in bniinefs. Some deprive it of this imperfection by burning it, that is, by heating it red hot in a clofe vefiel ; but being by this means reduced to the flate of coal, it is deprived alfo of its eafy mifcibility with oil. It may, however, be made to dry as fpeedily as other oil paints generally do, by a due preparation of the oil ; as particularly by fetting it on fire and boiling it, in the manner hereafter defcribed, in the tenth article of this feclion, for making printers ink.
The oil, for all paints, requires fome preparation, to promote its own drying ; and the method here recom- mended appears for this pnrpofe both the mofl expeditious and the mofl effectual : The dark colour, which it com- monly acquires in the procefs, and which renders it unfit for the brighter coloured paints, is of no inconvenience to it for blacks. The oil is made coniiderably thick by the boiling, and being in this ftate well mixed with the black matter, the mixture is diluted for ufe with unboiled oil, to which it communicates a fufficient degree of the drying quality defired.
II. Black paint with water. An opake deep black for water colours is made, by grinding ivory-black with gum water; or with the liquid
Aaa 2 which
[ 353 ] which fettles from whites of eggs, after they have been beaten up and fuffered to ftand a little. Some ufe gum water and the white of eggs together -, and report, that a fmall addition of the latter makes the mixture flow more freely from the pencil, and improves its gloffinefs. .
It may be obferved, that though ivory-black makes die deepeft colour, in water as well as in oil painting, yet it i9 not always, on this account, to be preferred, in either kind, to the other black pigments. A deep jet black colour is leldom wanted in painting; and in the lighter fhades, whether obtained by diluting the black with white bodies, or by applying it thin on a white ground, the par- ticular beauty of ivory-black is in great meafure loft : the fame intentions may be anfwered by pigments of fefs price and more eaiily procurable.
A valuable black for water colours is brought from China and the Eafl-Indies, fometimes in large rolls, more commonly in fmall quadrangular cakes, generally marked with Chinefe characters. By dipping the end of one of the cakes in a little water, and rubbing it about on the bottom or fides of the veflel, a part of its fubftance is taken up by the water, which may thus be readily tinged to any fhade of black or grey, from fuch as will juir. colour paper, to a full black. The compofition of this Indian ink has not hitherto,, fo far as I can learn, been revealed ; and I therefore made fome experiments with a view to difcover it.
Though the Indian ink is readily diffufed through water, it is not truly diifolved : when ths liquid is fuf- fered to ftand for fome time, the black matter fettles ta the bottom in a muddy form, leaving the water on the top colourlefs ; in the fame manner as the common black pigments fettle from diluted gum water. The ink, kept moift, in warm weather, becomes in a few days putrid,
like
[ 359 ] like the fluid or foft parts of animals -, as does likewife the clear water, after the black matter has fettled and been feparated from it. The Indian ink appears there- fore to contain an animal fubflance foluble in water ; and to confift of a black powder mixed with fome animal glue. For the greater certainty in regard to this conglu- tinating ingredient, I boiled one of the China cakes in feveral frefh portions of water, that all its foluble parts might be extracted, and having filtered the liquors through paper, fet them to evaporate in a flone bafon : they fmelt like glue, and left a very confiderable quantity of a tena- cious fubflance, which could not be perceived to differ in any refpect from common glue.
Being thus convinced of the compofition of the mafs,. I tried to imitate it, by mixing fome of the lamp-black, which I had myfelf prepared from oil (fee page 342) with as much melted glue as gave it fufficient tenacity for be- ing formed into cakes. The cakes, when dry, anfwered fully as well as the genuine Indian ink, in regard both to the colour, and the freedom and fmoothnefs of working. Ivory-black and other charcoal blacks, levigated to a great degree of finenefs, which requires no finall pains, had the fame effect with the lamp-black ; but in the flate in which ivory-black is commonly fold, it proved much too gritty, and feparated too haflily from the water.
III. Compofition for marking foeep.
Great quantities of wool are annually made unfer- viceable by the pitch and tar, with which fheep are marked, and which are commonly not laid on with a {pa- ring hand, as they confiderably increafe the weight of the fleece at a trifling expence. With a view to prevent, as much as poffible, this great wafle of fo ufeful a commo- dity, the fociety inflituted in London for the encourage- ment
[ 3*o ] ment of arts, manufactures, and commerce, and who continue vigorously and judicioufly to profecute the im- portant ends of their inftitution, offered a considerable premium for the difcovery of any cheap compofition, that might fupply the place of thofe hurtful materials ; whofe colour mould be ltrong and lafting, which mould bear the weather a proper time, and not damage the wool. Several propofals for this purpofe were laid before the (o- cicty, hut none of them have as yet been thought deferr- ing of the premium. The enquiry having been warmly recommended to me by the late Dr. Hales, as an object of very great importance to the woollen manufactory, I went through a fet of experiments with this view in the year 1759.
It was hoped, that the ill qualities of tar and pitch might be corrected, by mixing with them fome foap or fize, which ihould prevent their too great adhefivenefs, and render them fp far diffoluble in water, as to be dif- chargeable from the wool by the means commonly prac- tifed for cleaning it; or, in failure of tar and pitch, that fome compofition of reiins, oils, or fats might be found, which mould be rendred harmlefs to the wool by the fame correctors, and which mould ferve as furncieiu ce- ments for certain coloured powders, among which black appeared to be the belt, as being the itrongeir. and mofr confpicuous colour. On thele principles many trials were made, but with little fuccefs : for the unctuous and re- finous materials, with the advantage which they received from the foap cr fize, of being eafily waflied out from the wool, received alfo the difadvantage of being too foon dilcharged by the weather.
It was' next coniidered, that as wool has always a natu- ral greaiinefs, which the workmen wafh out with Male urine, foap, or ley, as defcribed in the fequel of this hif-
toryi
[ 3*i ] tory; the common animal fats might probably be difcharged from it by the tame means, fo as not to ftand in need of thofe ingredients, from which the foregoing compofiti- ons had contracted the imperfection of being too eafiiy Jilchargeable. Accordingly I melted ibrrre tallow ; and ftirred into it fo much charcoal in fine powder, as made it of a full black colour, and of a thick coniiflence. This mixture, eafiiy procurable and at fmall expence, being ap- plied warm with a marking iron on pieces of flannel, quickly fixed or hardened, bore moderate rubbing, refilled the fun and rain, and yet could be wafhed out freely with ibap, or ley, or ftale urine. All the good qualities, that can be dciired in a composition for marking (keep, appeared therefore to be united in this fimple preparation.
Though the mixture of tallow and charcoal powder w<3jj found furficiently durable v/hen applied as above upon pieces of flannel ; it occurred, that it might neverthelefs, by the repeated attritions to which it is expofed on the body of the animal, be in. danger of being rubbed off too ibon. If we could add to the compofition a little pitch or tar, we fhould effectually fecure againfl any inconveni- ency of this kind, and it was apprehended that thefe in- gredients might here be added with fafety ; for being perfeftly difiblved by the tallow, it might be prefumed that they would wafh out along with it from the wool. Thus we fee flams of tar got out from clothes by means of oil, which, difiblving the tar, the whole compound is then discharged by the fame detergents that the oil itfelf would be. I therefore melted, fome tallow with an eighth, with a fixth, and with a fourth of its weight of tar, and having thickened the mixtures with charcoal powder, fpread.them while hot upon pieces of flannel. None of the compofitions could be difcharged by any rubbing or wafhing with water. By foap they were all wafhed out
completely j
[ 3** ] completely ; that which had the fmallelt proportion of tar, eafily enough ; that which had the largeft proportion, difficultly. If therefore it fhould be feared, that the tallow will fail in point of durability or adhefivenefs, which, however, 1 do not apprehend that it will ; it is plain, that as much as can be delired of this quality may be commu- nicated, without damaging the wool, by a proper addition of the fubftanccs commonly made ufe of. I do not con- ceive that the nature of the thing can admit of any greater perfection.
There is a material circumftance in this affair, which does not feem to have been fufficiently confidered by thole who propofed the enquiry. If we could difcover, which fome have fruitlefsly endeavoured to do, a ftaining compo- fition in the nature of a dye, polTelling all the good qualities that have been mentioned ; it would fcarcely be poffible, as matters ftanfl at prefent, to induce the farmers to make ufe of it. They require a fubftance that will add weight : and I apprehend it will be no fmall recommendation to the above composition, that in this refpedt, as in all others except its being innocent to the wool, it agrees as nearly as can be expected, with the materials to which they have been long familiarized.
'6
IV. Compofitionfor preferving wood, &c.
The great adhefivenefs, which renders tar unfit as a principal ingredient, and excellent as a fecondary one, for the purpofes of the foregoing article, adapts it to another ufe, on fome occalions not a little important ; the preferv- ing of fome kinds of wood on the outfides of certain build- ings, the covering of fheds, paling, &c. as alfo for coating common tiles, in imitation of the black glazed tiles, which are fold at a much higher price.
Tar
1 363 ]
Tar and pitch of themfelves are too foft for thefe in- tentions ; even the pitch being liable to be melted off by the heat of the fun in fummer, however firm in the cold of winter. Different powdery fubftances, as aihes, ochres and other mineral pigments, have been mixed with them, but without remedying the imperfection fo effectually as could be wifhed. In the Swedifh tranfactions for the years 1742 and 1740, two compofitions are recommended, which are faid to be firm, durable, and gloffy.
One is prepared by melting the tar over a moderate fire, fo as to make it fluid but not to boil, and ftirring in as much coal duft as will render it thick : this mixture, the author fays, is to be laid on with wooden trowels, in a hot day, as thick or as thin as fhall be thought proper. The other is prepared by mixing the melted tar with a furficient quantity of lamp-black : a little of this mixture is fpread upon the upper fide of each tile with a ftiff, fhort-haired, painting brufh : next day, when dry, the tiles are done over with tar alone, and two days after with tar again : this coating being well dried, which in fum- mer, according to the author, is generally in eight or ten days, fome powdered lead ore is ftrewed over it, and well rubbed in, firft with a coarfe and afterwards with a fine linen cloth ; from this it receives a fparkling appearance. I tried both thefe compofitions, and found them of a good black colour : when the bodies coated with them are held before the fire till the furface begins to run, they become glofly. They are not however wholly exempt from the inccnveniencies complained ot in the others. For though the tar was made as thick, both with the coal duft and lamp-black, as was confiftent with its being fpread fmooth even in a hot fun and while warm from the fire, it afterwards foftened in the fun confideraF-
B b b though
[ 364]
though the parts, which the fun did not immediately mine upon, proved fufficiently firm in the hotteft weather.
By coal duff, in the firft compofition, is meant pow- dered charcoal. Sufpecting however that pitcoal, in vir- tue of its bituminous nature, might unite more perfectly with the tar, and be in fome meafure diffolved by it, I made trial of this alfo, enuring the nneft coloured pieces,. of thofe kinds which melt in the fire, and grinding them into impalpable powder. The mixture of this powder with, the melted tar, made of luch confidence as to be freely fpread while warm with a brum, feemed to foften lefs in the funs heat than either of the other two. The durabi- lity of thefe compofitions I cannot yet determine : after having flood, without any apparent alteration, one lum- mer and winter, they continue expofed to the weather, for difcovering what effects longer time and vicilfitudes of feafons may have upon them.
The mixture of tar and lamp-black is found the moft effectual prefervative for the mafls and yards of mips. Such parts of the mart, as the Aiding up and down of the fails requires to be only greafed, and thofe which are covered with turpentine or relin mixed with tallow or oil, generally contract large rents, while the parts coated with tar and lamp-black remain perfectly found. I have been favoured by a gentleman on board of a veffel in the Eaft- Indies, with an account of a violent thunder ftorm, by which the mainmafl was greatly damaged, and whofe effects on the different parts of the maft were pretty re- markable. All the parts which were greafed, or covered with turpentine, were burfl in pieces : thofe above, be- tween, and below the greafed parts, as alfo the yard arms, the round top or fcaffolding, &c. coated with tar and lamp-black, remained all unhurt.
In
1 365 ]
In this place it may be proper to obferve, that the coat- ing or painting of wood does not in all cafes contribute to its prefervation : unlefs the wood be very thoroughly dry, especially thofe kinds of wood whofe juices are not oily or refinous, the painting, by confining the watery fap, haftens the corruption. Several preifes for a paper manu- facture having been made of heart of oak feemingly very dry, fome of them, which with injudicious care had been well painted over, rotted and perifhed in a few years, while the unpainted ones continued for many years per- fectly found.
V. Compofitions for blacking leather.
In the tanning of leather, it is fo much impregnated with the aftringent parts of oak bark, or with that matter which ftrikes a black colour with green vitriol, that rub- ing it over three or four times with a folution of the vitriol, or with a folution of iron made in vegetable acids, is fufficient for ftaining it black. Of this we may be convinced, by dropping a little of the folution on the unblacked fide of common fhoe leather. This operation is performed by the currier, who, after the colouring, gives a glofs to the leather with a folution of gum-arabic and iize made in vinegar. Where the previous aftringent impregnation is infufficient to give a due colour, and for thole forts of leather which have not been tanned, fome galls or other aftringents are added to the folution of iron; and in many cafes, particularly for the finer forts of leather, and for renewing the blacknefs, ivory or lamp- black are ufed. A mixture of either of thefe with lin- feed oil makes the common oil blacking. For a mining blacking, fmall beer or water are taken inftead of oil, in the quantity of about a pint to an ounce of the ivory- black, with the addition of half an ounce of brown fugar
B b b 2 and
[ 366 ] and as much gum-arabic. The white of an egg, fubfli- tuted to the gum, makes the black more mining, but is fuppofed to hurt the leather and make it apt to crack. It is obvious, that all thefe kinds of compoiitions admit of many variations : it is fumcient here to have given a general idea of them.
VI. Spirit varni/h.
Black varnifh, for japanning on wood or leather, is prepared by mixing lamp-black or ivory-black with a proper quantity of a ftrong folution of gum lac in fpirit of wine, fuch as that defcribed in the preceeding part of this work, page 224. The lamp-black is commonly pre- ferred to the ivory-black, on account of its uniting better with the fluid, and working fmoother. The thicker part of the varni/h, which fettles at the bottom, is ufed with the lamp-black for the firft coatings, and die mix- ture applied at different times, in a hot room, one layer after another is dry, till a full body of colour is obtained : after which, the piece is wafhed over in the fame man- ner, feveral times, with the finer part of the varnifh, juft tinged with the black, fo as to make a coating of fufficient, thicknefs to bear poliihing with tripoli.
VII. Amber varnifoes for papier mache, &c.
Papier mache is made of cuttings of white or brown paper, boiled in water, and beaten in a mortar, till they are reduced into a kind of pafte, and then boiled with, folution of gum-arabic or of fize, to give tenacity to the parte, which is afterwards formed into different toys, &c. by preffing it into oiled moulds. When dry, it is done over with a mixture of fize and lamp-black, and after- wards varnifhed. The black varnifh for thefe toys (of which the firft, account I have feen is in a pamphlet on
drawing,
[ 367]
drawing, &c. printed for Mr. Peek in 1732, and faid to be taken chiefly from manufcripts left by Mr. Boyle) is prepared as follows.
Some colophony, or turpentine boiled down till it be- comes black and friable, is melted in a glazed earthen veflel, and thrice as much amber in fine powder fprinkled in by degrees, with the addition of a little ipirit or oil of turpentine now and then : when the amber is melted, fprinkle in the fame quantity of farcocolla, continuing to ftir them, and to add more fpirit of turpentine, till the whole becomes fluid : then {train out the clear through a coarfe hair bag, prefTing it gently between hot boards. This varnifh, mixed with ivory-black in fine powder, is applied, in a hot room, on the dried paper parte ; which is then fet in a gently heated oven, next day in a hotter oven, and the third day in a very hot one, and let ftand each time till the oven is grown cold. The parte thus varnifhed is hard, durable, glofly, and bears liquors hot or cold.
A more fimple amber varnifh, of great ufe for many purpofes, and faid to be the bafls of the fine varnifhes which we fee on coaches, &c. is prepared, by gently melt- ing the amber in a crucible till it becomes black, then re- ducing it into a powder, which looks brown, and boiling the powder in linfeed oil, or in a mixture of linfeed oil and oil of turpentine. Drying oil is commonly made choice of by the workmen; but it feems more eligible here to take the oil unprepared, that the boiling, requifite for giving it the drying quality, may be employed at the fame time in making it act: upon the amber.
By the previous melting of the amber,, its nature is changed, and part of its oily and faline matter expelled, as happens in the common diftillation of it. When the diftillation is not far protracted, the caput mortuum, or
fhining
[ 368 ] Shining black mafs which remains in the retort, anfwers as well as the amber melted on purpofe. Hence fomc of our chemifts, inStead of urging the diftillation to the ut- molt, by which the amber would be reduced to a mere coal, find it more advantageous to difcontinue the procefs when the thinner oil and greater part of the fait have arifen, that the remaining mafs may be in great meafure foluble in oils, fo as to fupply the common demand of the varnifli makers.
It has generally been thought, that amber will not at all diffolve in oils, till it has thus fuffered a degree of de- compofition by fire. Hoffmann relates an experiment, in his obfer-oationes phyjico-cbemicce, which difcovers the folubility of this concrete in its natural ftate. Pow- dered amber, with twice its quantity of oil olive, was put in a wide-mouthed glafs ; and a digeftor, or ftrong copper vefTel, being filled about one-third with water, the glafs was placed in it, the cover of the digeftor fcrewed down tight, and a moderate fire continued an hour or more : when cold, the amber was found diffolved into a gelatinous transparent mafs.
In Dr. Stockars very curious fpec'wicn inaugurate de Jkc- cino, printed at Leyden, in 1760, there are fundry mure important experiments on this fubjecl, made by himfelf, conjointly with my worthy correfpondent Mr. Ziegler of Winterthur. They found that by continuing a Simmer- ing heat twelve hours, and confining the vapour as much as {tone-ware vcflels would bear without burfting (the dan- ger of which was avoided by making a fmall notch in the cork Stoppers) powdered amber diffolved perfectly in ex- preiled oils, in turpentine, and in balSam of copaiba : a Strong copper veffel, with a cover fcrewed on it, feems moil eligible, and for the greater fecurity, a valve may be made in the cover, kept down by a fpring that Shall give way
before
[ 369 ]
before the confined vapour is of fufficient force to be in any danger of burfting the vefTel. Though fuch a heat as converts part of the oil into Strong elaftic vapours, and the forcible comprefTure of the vapour, are expedient for haftening the diffolution, they do not appear to be eflen- tially neceiTary ; for by digeftion for a week in clofe ftopt, glais vefTels, in which the comprefTure could not be very great, folutions equally perfect were obtained.
The folution in rape-feed oil, and in oil of almonds, was of a fine yellowifh colour ; in linfeed-oil, gold coloured; in oil of poppy-feeds, yellowifh red; in oil olive, of a beautiful red ; in oil of nuts, deeper coloured ; and in oil of bays, of a purple red. It is obfervable that this lafr oil, which of itfelf, in the greateft common heat of the atmofphere, proves of a thick butyraceous confif- tence, continued fluid when the amber was diflblved in it. The folutions made with turpentine, and with balfam of copaiba, were of a deep red colour, and on cooling hardened into a brittle mafs of the fame colour. All the folutions mingled perfectly with fpirit of turpentine. Thofe made with the oils of linfeed, bays, poppy-feeds, and nuts, and with balfam of copaiba and turpentine, be- ing diluted with four times their quantity of fpirit of tur- pentine, formed hard, tenacious, glofly varnifhes, which dried furhciently quick, and appeared greatly preferable to* thofe made in the common manner from melted amber,
VIII. Varnijhfor meta/s. Iron fnuff-boxes, mourning buckles, &c. are coloured black, by making them considerably hot, and applying en them in this ftate a thick mixture of lamp-black, with a certain varnifh called gold-fize. There is a gold- fize, formerly mentioned, for gilding, or fixing gold-leaf on wood, &c. The fize here meant is a compofition of a
different
[ 37° 1 different kind, confifting of drying oil, turpentine, and the pigment called Naples yellow ; which laft ingredient is ufed for giving a high gold colour to the mixture, to fit it for fome of the other purpofes for which it is em- ployed. In the prefent intention, the yellow might doubtlefs be omitted, and the varnifh formed at once by mixing lamp-black with a proper quantity of turpentine and drying oil.
IX. Scaling-ivax,
Black fealing wax is compofed of gum lac, melted with one half or one third of its weight of ivory-black in fine powder. The inferior fort of lac, called fhell- lac, anfwers as well for this ufe as the fineft. It is cuf- tomary to mix with it, for the ordinary kinds of fealing wax, a considerable proportion, as two thirds its weight, of the cheaper refmous bodies, particularly Venice turpen- tine, by which the beauty of the mafs is here lefs injured than in the red wax, and of which a fmall addition is in all cafes expedient, to prevent the compound from being too brittle. The ingredients being melted and well Air- red together over a moderate fire, the mixture is poured out upon an oiled ftone or iron plate, and rolled, while fofv, into flicks, which afterwards receive their gloffinefs by being heated till the furface begins to fhine.
The black figures on the dial-plates of clocks and watches, which look like black enamel, are formed of the finer kind of black fealing wax, which is melted into ca- nities made in the plate, and afterwards polifhed. Black enamel or ftones are fome times imitated in the fame manner in other works.
X. Printing
[ 371 1
X. Printing ink.
Printing ink differs from the common oil paint, defcribed at the beginning of this fe<£tion, only in the preparation of the oil, which muft here have its confif- tence and tenacity greatly increafed, and its greafinefs di- minifhed, by means of fire. The fame way of prepa- ration, either not carried to fo great a length, or with a fubfequent addition of frefh oil to dilute the mixture, affords, as already obferved, one of the beft drying oils for the black paint.
The oils of linfeed and nuts are made choice of for this ufe : the nut oil is fuppofed to be the beft, and is accord- ingly preferred for the black ink, though the darker colour which it acquires from the fire makes it lefs fit for the red. It is faid that the other expreffed oils cannot be fufficiently freed from their undtuous quality ; whence the ink made with them dries exceeding flowly, is apt to come off and fmear the paper in the beating and prefling which it undergoes in the book-binders hands, or finks into the iubitance of the paper, beyond the mark of the type, and ftains it yellow.
Ten or twelve gallons of the oil are fet over the fire, in an iron pot, capable of holding at leait half as much more j for the oil fwells up greatly, and its boiling over into the lire would be very dangerous. When it boils, it is kept ftirring with an iron ladle ; and if it does not itfelf take iiame, it is kindled with a piece of lighted paper or burning wood ; for iirnple boiling, without the actual accenfion of the oil, does not communicate a fuffi- cient degree of the drying quality required : it feems to be in the more inflammable parts, which are foonefr. con- fumed by the burning, that the injurious fatnefs or greali- nefs conlilts. The oil is fuffered to burn for half an
C c c hour
[m ]
hour or more, and the flame being then extinguished by covering the veflel clofe, the boiling is afterwards con- tinued, with a gentle heat, till the oil appears of a proper confidence : in which ftate it is called varnifh. It is necefiary to have two kinds of this varniih, a more and lefs boiled, or a thicker and a thinner, which are occa- iionally mixed together as different purpofes may require : that which is of a juft confiftence in warm weather proves too thick in cold ; and that which anfwers well for large characters, proves in the fame feafon rather too thin for fmall ones.
The thickeft varniffi is of fuch confiftence when cold, that it draws into threads between the ringers nearly like weak glue : this is the mark by which the workmen judge of the due boiling, a little of it being from time to time taken out for this trial, and cooled by dropping it on a tile or other cold body. It is very vifcous and tenacious, like the foft refinous juices or thick turpentines. It is not at all diffolved, any more than the oil at firft, by water or fpirit of wine, but mingles readily enough with frefh oil, and unites with mucilages into a mafs which diftblves in water into a milky liquor : by boiling with ftrong alcaline ley it forms a foapy compound ; whence the types, after an impreiTion, are cleaned from the ink, by wafliing, and rubbing them with a brufh, in hot ley- The oil emits, during the whole time of the boiling, very offenfive penetrating fumes : when grown cold, it has an acrid difagreeable tafte, but little ill fmell. The oil is faid to lofe, in being boiled into thick varnifh, from a tenth to an eighth part of its weight, which proportions agree fufficiently with my trials : common linfeed oil, boiled down to a confiftence which appeared fomewhat too thick, loft about one fixth : being further boiled, till it became quite firm when cold, the lofs was near one half. Diffe- rent
t 373 3
rent oils, and perhaps the fame oil in different ftates, differ in this refpect : fifh oil, boiled to thicknefs, loft much more than that of linfeed, the thick matter amounting only to about one fourth of the original weight of the oil.
The workmen are accuftomed to add, in the prepara- tion of ten or twelve gallons of oil, as foon as the burn- ing is over, a pound or two of dry crufts of bread, and a dozen or two of onions, by which they fuppofe the greafi- nefs to be more effectually deftroyed. It may however be queftioned, whether additions of this kind are of much ufe ; for I have prepared the varnifh, feemingly of a very good quality, by fire alone.
There is another fort of additions whofe effect is more apparent. To give a greater body to the varnifh, and in- creafe its drying quality, a proportion of turpentine is thought necelTary ; and with fome artifts, litharge has in this intention been a fecret. It is obferved, in the French encyclopedie, by Mr. le Breton, the printer of that work, that when very old oil is ufed, neither turpentine nor litharge are needful ; but that, when the oil is new, ibme turpentine mult neccifarily be employed, for without it, the (blearing of the paper, by the fpreading or coming off of the ink, cannot be avoided ; that it is much more eli- gible to ufe old oil than to have recourfe to this correction of the new, both turpentine and litharge, particularly the laft, making the mixture adhere fo firmly to the types, that it is lcarce to be got entirely off by the ley, whence the eye of the letter is foon clogged up.
When turpentine is ufed, it is firft boiled by itfelf, untill, on dipping in a piece of paper, it is found to crumble and part §com the paper when cold : the oil be- ing then taken from the fire, the turpentine, while ftill fluid, is poured into it, after which the boUing is peated, and continued till they are fufficiently incorporated.
C c c 2 It
[ 374 ] It is here fomewhat more difficult to hit the due point of boiling, than when the oil is prepared without addition ; the mixture being more apt to grow too thick from con- tinuing the heat too long, and full of little hard grains from not continuing it furficiently ; which grains are pro- bably undiflblved particles of the turpentine. The uie of boiling the turpentine firft by itfelf is to diffipate its moilture or eJTential oil : by the boiling it becomes a re- linous matter, nearly the fame with common refin, which poifibly would anfwer the fame end.
For making the varnilh into ink, lamp-black is the. common material; of which, according to Mr. le Breton, two ounces and a half are fufficient for fixteen ounce- c the varnilh. They are ground together on a ftone wiib a muller, in the lame manner as oil paints.
The paper, for printing, is moiltened with water ; by which it is made more yielding and pliable, fo as not only to be lefs apt to be torn by the types in the prefs, but likewife to be more clofely and evenly applied to them, and confequently to take a neater and more perfect im- preffion. The due moifhire of the paper, and the care and attention of the preffmen in well working the ink . on the types with the balls, are very material points; without which, how excellent foever the ink is, the im- preffion will not be beautiful.
The adheiion of printers ink to wetted paper feems to mew that it is not truly of an oily nature. All exprerfed oils contain probably a gummy or mucilaginous matter ; . and perhaps the tenacity, confidence, drying quality, and the property of adhering to bodies moiftened with water, which the oil acquires in the procefs above defcribed, may be all owing to fome of the purer part of the oil' being deflroyed, fo as to leave the remainder more gum- my. When the oil dries, it proves a tough flexible fub-
flance*.
[ 375 ] fiance, which has little difpofition to unite with frefli oil any more than with water, as if the gummy and oily matter were in fuch proportions, that one defends the other from the menflruum that would ctherwife diffolve it.: efTential oils on the contrary, being free from gum, harden into a merely refinous mafs, brittle like other refins, and which dilfolves, like the oil at firft, in frefh oil or in fpirit of wine. The differences obferved in different exprefled oils, in regard to the drying quality, may de- pend on the different quantities of gummy matter ; and the difference of old oil from new, on the gum being in the latter more intimately combined, ib as not to fepa- rate in the burning and boiling, When thefe oils are firfl preifed out from the fubjects, they abound- with muci- lage, great part of which is only fuperficially mixed, fo as to give a turbidnefs and opacity to the fluid : in keeping, a part of this loofe mucilage is thrown off, and the re- mainder may be prefumed to become at the fame time more intimately incorporated with the oil. The repug- nance which we obferve between oil and gum does not in the leaft invalidate thefe conjectures, any more than the repugnance between- oil and water can be an argument againft the exiflence of water in oils : indeed we have plain proofs of the coalition of oil with gum, in the ana- lyfis of the purefl- gums, gum arabic, fenica, tragacanth, from which an actual oil is obtained by diftillation. The diftillation of expreffed oils themfelves feems to favour the opinion here propofed : from all of them there re- mains in the dift illing veffel a large quantity, though from fome more than from others, of a grofs coaly matter ; and there arifes a fluid oil, which does not dry or grow thick in the air as the oils did at firft ; and which is therefore found to anfwer for certain purpofes, as in the lapidarie9
bufinefs,,
[ 376] bufmefs, for which the thickening of the oils in their na- tural ftate renders them unfit.
XI. Rolling-prefs Ink.
Between the rolling-prefs printing, and that of the prin- ters prefs ftrictly fo called, there is this effential difference ; that in the former the impreflion is received from figures hollowed in a copper plate, but in the latter from promi- nent types. The damping of the paper is equally necef- fary for the rolling-prefs as for the other, in order to foften it, fo that the parts, correfponding to the cavities in the plates, may be forced into them. But the ink is of a fomewhat different quality. For while the printers types require a glutinous or fticky mixture, which fhall adhere upon the prominences of the type, without running into the hollows ; the ink for copper-plates muff run into and fill the hollows, efpecially when the plate is warmed, and be fo little glutinous, as to be eafily wiped off clean from the fmooth parts of the plate, or thofe which are to leave the paper white.
The oil, for this ink, rnufr be boiled, and fet on fire, in the fame manner as for the other, to take off its greafi- nefs and promote its drying : the boiling is continued more or lefs, according to the different confidences which different kinds of plates may require, but never fo far as to communicate to the oil the adhefive gluey quality of the printers varnifh. The block matter muff be of the charcoal kind : the lamp-black gives always a degree of toughnefs, but the charcoal blacks, as they do not unite intimately with the oil, divide its texture, and render it lefs gluey. The coal commonly employed for this ufe is brought in powder from Germany, and called German or Frankfort black : this is fofter, and more free from gritti-
nefs,
C 377 3 nefs, than the ivory or other charcoal blacks as ufually prepared among us.
The Frankfort black is fuppofed by fome to be the coal of vine-twigs ; by others, that of the kernels of fruits and v/ine lees burnt together. The coal of vine-twigs, as we have already feen, does not appear to differ, in any great degree, from that of the fmall branches of other kinds of trees ; but the kernels of fruits yield a coal con- siderably more foft and mellow, eafily crumbling between the ringers into a fine meal. That the Frankfort black is no other than a vegetable coal, appeared from its burning on a red hot iron, like charcoal powder, into white afhes ; and from the allies, like common vegetable allies, being plentifully dilToluble by the vitriolic acid into a bitterilh liquor, while the alhes of animal fubftances are very Spa- ringly affected by that acid, and form with it a compound of a different kind of tafte.
c
SECT. VI.
Of the preparation of common writing ink.
O M M O N writing ink is prepared from galls, or other aftringent vegetables, and green vitriol, fteep- ed or boiled in water or other liquors. The ingredients, in different receipts, are fet down in very different pro- portions : in fome, fix parts of galls are directed to one of vitriol ; and in others, three or four parts of vitriol to one of galls : fome order the weight of the liquor to be equal to that of the vitriol and galls together, others fifteen or iixteen times as much.
Moft of the common inks have a capital imperfection, that though at firft of a pretty good colour, yet in length of time they decay, fome fooner and others later, the writing becoming fcarcely legible, or even entirely difap-
pearing >
[ 378 ] pearing ; of which too many inftances are known to thofe, who have examined records and other writings of any confiderable age. The preparation and improvement of this ufeful fluid, on whofe duration fo much depends, hecomes therefore a very important object.
The ingredients being known, it was hoped, that by a proper fet of experiments, the belt ink they are capable of affording, in regard both to the durability and beauty of the colour, could not efcape difcovery. Though length of time be the proper teft of the abfolute duration of inks, it was prefumed that a few years would be fufHcient for judging of their comparative durability; and that in this comparifon, fome afliftance might be obtained from ex- pofing the writings for fome months to the fun and weather, by which the influence of time on colours is haftened in a remarkable degree : dyed clothes expofed for a month or two to the fun in fummer, lofe more of their colour, than they do in an age when kept clofe from the air. With thefe views, about fifteen years ago, I engaged in a fet of experiments, of which the general refults are as follow.
•I. Experiments for determining the bejl preparation of ink
with vitriol and ajlringents.
When common ink, or a black infufion of galls and
vitriol, is diluted largely with water, and fuffered to fettle,
the black matter, as .already obietved, falls to the bottom,
and the liquor becomes colourlefs. If fuch a mixture
•contained -more vitriol .than the galls could faturate and
decompofe, it was judged that the redundant vitriol would
•remain diflblvcd in the liquor, and ftrike a black colour
with frefh galls; and that on the contrary, if the quantity
-of galls employed at firft. was more than fufficient for the
decompofition of the vitriol, the liquor would in like
manner
[ 379 ] manner retain the redundant impregnation of the galls, and flrike a blacknefs with frelh vitriol ; lb that by try- ing, till fuch proportions of the two mould be found, as that the liquor, after the precipitation, mould produce no blacknefs with a frefh addition of either ingredient, the proportions requifite for the exact faturation might be difcovered.
Many trials were accordingly made on this principle. When the quantity of galls was feveral times greater than that of the vitriol, an addition of more vitriol to the liquor after fettling produced a frefh blacknefs; and when the vi- triol greatly prevailed, an addition of galls had the like effect. But there were feveral intermediate mixtures, with pro- portions confiderably different from one another, in which no blacknefs could be perceived to arife from the addition of one or the other ingredient. By taking a medium be- tween the quantities, in thofe trials where one or the other did produce a fenfible colour, it was reckoned that about equal parts of the two were the mean proportions, which could receive no additional blacknefs from a further in- creafe of either. In thefe and all the other experiments made with galls, the Aleppo or blue galls were the fort employed ; and care was taken, by boiling or long infu- fion, to get out as much of their virtue as can be expected to be done in practice.
Different infulions of the galls and vitriol, in a more and lefs dilute ttate, were tried next on paper. Here it was found, that the proportions, which gave the greateft blacknefs, were not thofe, whofe colours were the moft durable, though in both refpedts there were confiderable latitudes. Equal parts of the two, the quantities which the foregoing way of trial feemed to point out for the beft, gave an ink of good blacknefs at firft ; but on keep- ing for a few weeks, and in a few days when expofed to
Ddd the
[ 38o] the fun and open air, the writing changed to a yellowifh brown. The mixtures, in which the vitriol exceeded the galls, underwent greater and fpeedier changes ; more and more fo, according as the excefs of the vitriol was the greater. Thofe in which the galls exceeded the vitriol were more durable : an infufion of two parts of galls and one of vitriol did not fade fo much in two months expofure, as an infufion of equal parts of the two did in one month ; and three parts of galls to one of the vitriol made an ink which held its colour ftill better. When the galls were increafed to five or fix times the weight of the vitriol, the colour did not prove black enough, though it feemed to be rather of more durability than the others.
The writings which had changed to a brown or yel- low, I wafhed over with an infufion of galls. Where the ink had been well loaded with the ingredients, the cha- racters became of a pretty good black ; and thofe, which had been written with more dilute inks, became, though not black, yet fenfibly more coloured than before, info- much that many, which had grown almoft indiftinguifh- able, were now fufficiently legible. How far this infufion would ferve for the recovery of decayed writings of great age, I have not had an opportunity of trying ; but thus much is clear, that a diftilled water of galls, recommended for this purpofe in Caneparius's collection de atramcntis, cannot anfwer ; the aftringency, or the power of giving blacknefs to iron, refiding in fuch parts of the galls as do not rife in diftillation.
It feems to follow from the above experiments, that the decay of inks is owing chiefly to a deficiency of galls ; that the galls are the raoft perifhable ingredient, the quantity, which gives the greateft blacknefs at firft, being iftfumcient to maintain the colour -, that for a durable
ink,
[3*1 J ink, the quantity of galls cannot be much lefs than three times that of the vitriol; and that it cannot be much greater without fomewhat injuring the ink in point of blacknefs.
The proportion of liquid admits of much greater lati- tude, than that of the vitriol and galls to one another. One ounce of vitriol, three ounces of galls, and a hundred and fifty ounces of water, made an ink, legible indeed, though greatly too pale. With a hundred ounces of wa- ter to the fame quantity of galls and vitriol, the colour was frill too pale. With forty and fifty ounces of water, the ink was of fufficient blacknefs for common ufes : but the fulleft and blackefr. colour of all was produced when the quantity of liquor was little more than enough to cover the powders, as fix, eight, or ten ounces. It was expected that thefe fmall quantities of water, difiolving all the vitriol, without being able to fully extract, the vir- tue of the galls, and thus occafioning a deficiency in the quantity of aftringent matter, would have yielded a perifh- able ink. Neverthelefs, characters written with thefe mixtures have preferved their colour for fifteen years, continuing ftill feniibly blacker than where the menftru- um was in larger quantity. It appears therefore, that though a large portion of fluid may be tinged by the vitriol and galls of a blacknefs fufficient for many pur- pofes, the uiing a little quantity is of advantage both to the deepnefs and durability of the colour ; perhaps only from the liquor being in this cafe more loaded with the colouring matter of the ingredients, fo that a greater body of colour is accumulated upon the ftroke.
I next tried what alteration would refult from the ufing of different waters or other liquors for the menftruum. Diftilled water, rain water, and hard fpring water, em- ployed in the fame proportions, had, lb far as could be
D d d 2 judged,
[ 3^2 ] judged, the fame effects. White wine produced an ink ef a deeper black colour than the waters ; and with vine- gar, the colour was rather dill deeper. Proof fpirit ex- tracted only a reddifh brown tinge, and rectified fpirit a paler brown, the vitriol not being diflbluble in thefe li- quors. Both the fpirituous tinctures funk and fpread upon the paper, that in rectified fpirit more than the other : hence the addition of fpirit to common ink, directed by fome for preferving it from mouldinefs, or from freezing in winter, occalions generally a precipitation of part of the colour, and makes the ink fink, more or lefs ac- cording to the quantity of the fpirit. The coloured juice?- of fruits, as of privet-berries, mulberries, black-cherries ufed as menftruums for the galls and vitriol, made the colour rather fuller on firft writing than water did, but it was lefs black, and foon grew dull or rufty in keeping. A decoction of logwood, ufed inftead of water, fenfibly improved both the beauty and the deepnefs of the black, without at all difpofing it to fade.
Inftead of the galls, I tried other ftrong aftringents, as oak bark, alder bark, floe bark, fumach, tormentil root, biftort root, balauftine flowers, pomegranate peel, &c. but could not find one of equal efficacy with galls. Nor did any but the oak bark feem to give the fame fpecies of blacknefs that galls did, moft of the others having more or lefs of a greenifh hue : the oak bark itfelf however, though it came neareft to the galls, made a very bad ink, and though ufed in eight or ten times their quantity, was far from being equal to them in effect. The faw-duft of oak wood, which has lately been found to anfwer for tanning leather, as well as the bark or rather better, and confequently to have no inconfiderable impregnation of true aftringent matter, produced with vitriol a tincture ibmewhat different from all the other aftringents I have
tried :
1 383 ]
tried : the colour at firft was an opake deep blue : after the liquor had flood for fome days on a frefh quantity of the wood, it approached more to blacknefs ; but ftill re- tained a very confiderable blue tinge. Galls themfelves give indeed a bluifh colour when the liquor is diluted fo as to be tranfparent -, but in an opake ftate, fo far as I have obferved, they exhibit no bluenefs.
I made trial alfo of the juice of iloes, which to the tafre is pretty ftrongly aftringent, and which, as we have formerly feen, page 332, gives of itfelf, to linen, a ftain of remarkable durability. By mixing the juice, whether of raw or of baked floes, with different proportions of folution of vitriol, 1 could not produce the leaft tendency to blacknefs, the vitriol feeming to make little alteration in the colour. Some of the mixtures, however, having been written with on paper, the characters, after flanding for feveral days expofed to the air, changed by degrees to a full black, which appeared to be more durable than that of any of the inks made with galls; their colour hav- ing flood well in the open air from the beginning of no- vember lafl, till the papers were deflroyed by the weather in the end of february. Writings with good common ink, expofed along with them as a flandard, had faded much.
As all the aftringent vegetables communicate of them- felves fome colour to water, galls a brownifh, biftort root a dark brown, logwood a purplifh, tormentil root a reddiih, pomegranate peel a greenifh yellow, &c. I en- deavoured to prepare a compound black from the aftrin- gents alone, on the principles mentioned at the end of the fourth fection, page 355 ; hoping that this additional black in the liquor might coincide with, and heighten, that which the vitriol would produce with the direct aftrin- gent matter. Accordingly, taking a decoction of galls
and.
[384]
and logwood for the bafis, I infufed fucceflively in this liquor feveral different aflxingents, till at laft its colour approached pretty near toblacknefs. By diflblving in the dark blackifh liquor a due quantity of vitriol, I obtained a good ink indeed, but not at all better, fo far as could be judged, than if the decodtion of galls and logwood only had been ufed. Perhaps the vitriol, uniting with the properly aftringent parts, and thus fupprefling fome of the component colours of the black infufion, deftroyed at once all the blacknefs which refulted from the combi- nation of the colours.
Inflead of the vitriol, or vitriolic folution of iron, I tried faturated folutions of the metal in other acids. With the nitrous and marine acids, the ink was too cor- rofive, though made as dilute as it could bear to be con- fiftently with a due colour ; nor was the colour fo true a black as that of the vitriolic ink, the marine acid inclin- ing it to blue, and the nitrous to brownifh green. Though the ufing of vinegar, for diflblving the vitriol, was of ad- vantage to the colour; yet a folution of iron, made in vinegar only, gave a very bad ink. A folution of iron, made by boiling iron filings in water with tartar, and fe- parating the unfatiated tartar by cryftallization, 6cc. pro- duced with galls only a rufty brownifh colour. A folu- tion of iron in lemon juice anfwered better than thofe in vinegar or tartar, but did not feem to come up to the vitriolic folution.
As the iron of the vitriol, and the aftringent matter of the galls, unite together into a new compound, which is the tinging matter of the ink -, it may be prefumed that the acid, which held the iron diflblved, is extricated, at leaft in part, and remains loofe in the liquor. Sufpedting this difengaged acid to be a principal caufe of the change of the inks to a rufty colour, I endeavoured to feparate it,
by
[ 385]
by adding to a black infufion of vitriol and galls a little quicklime ; this earth having the property of imbibing the vitriolic acid, and forming with it a felenitic com- pound, which will not remain diffolved in the liquor. The ink was far from being anywife improved by this addi- tion. A very fmall quantity of the lime made no fenffble change in the colour of the liquor ; but a larger quantity turned it reddifh brown, the lime feeming to have nearly the fame effect as alcaline falts have. After the writings with thefe mixtures had been expofed to the fun and weather about two months, thofe with the larger pro- portions of lime could not be read ; and thofe with the fmalleft quantities had faded more than the ink by itfelf. I then endeavoured to feparate the acid in another manner. Some good ink was diluted with water, that the colouring matter might fettle; and the black fediment was warned repeatedly with frefh portions of water, that what- ever faline matter it contained might be extracted, The black matter thus prepared was mixed with water in which Ibme gum arabic had been diffolved. Writings with this mixture were of great durability : after hanging about four months againft a fouth wall, they had contracted no rufty hue, though they were grown much paler than at firft, and rather grey than black : perhaps even this change pro- ceeded in part from fome of the colouring matter being waihed off by rain. It is the capital imperfection of mixtures of this kind, that after the colouring matter has been feparated from the liquor, it cannot be again incor- porated with watery fluids near fo perfectly as it was be- fore in the ink: it can only be diffufed through the water in a powdery form, as the charcoal powders are, and fettle9 from the liquor, and may be waftied off from paper, almoft as eafHy.
The
[ 386 ]
The moft effectual way of preventing any ill effe&s from this redundant acid feems to be by an addition of iron itfelf; part of which, in proportion as the iron of the vitriol is extricated, will be diflblved in its place, and thus continue both to fatiate the acid, and fupply one of the eflential ingredients of the ink. It mould feem that in this method, a much lefs proportion of acid, that is, a lefs quantity of vitriol, would fuflice, than is otherwife neceflary ; the fame acid ferving to combine with the galls fxefh quantities of the iron : and in fuch cafe, keep- ing for a length of time, as a year or more, would im- prove the ink. Of this I have not yet had full experience; but a friend informs me, that he has feen writings of more than eighty years {landing, which continued of a full black colour, without any tendency to yellow or brown ; that the ink was made in the common manner with vitriol and galls, and long kept with pieces of iron in the veffel. Pombly boiling for a little time might anfwer the fame purpofe as long keeping ; for boiling remarkably promotes the feparation of iron from vitriol, and confequently the action of the acid on frefh iron.
Gum arabic is added to inks, to give a greater con- nivence to the fluid, and enable it better to keep the co- louring matter fufpended: it contributes, perhaps, to pre- vent the black matter from concreting into particles large enough to fettle by their weight, as well as to prevent or retard their fettling after they have fo concreted ; for we have feen in a foregoing part of this eflay, that the coa- lition into fenfible particles is fucceflive, and that before it happens, the black matte,r is in fo fubtile a flate, that it may be confidered as being in actual diflblution.
The gum appears alfo to be ,of another advantage, pre- venting the ink from fpreading upon the paper, fo that a greater quantity of the fluid, and confequently a greater
body
[ 387 ]
body of colour, is eolle&ed on each ftroke. An infuiion of vitriol and galls was mixed with different proportions of common water, and with the fame proportions of a folution of gum arabic in water : all the mixtures with gum water, written with on paper, were at firffc, and ftill con- tinue, very manifeftly deeper coloured than thofe which had b,een equally diluted with plain water. In an ink which had an over proportion of vitriol, and which, when written with, foon loft its blacknefs and turned to a yel- lowifh brown, I diffolved as much gum as it would bear without becoming too thick to run freely from the pen : the colour was not only deepened, but made greatly more durable; partly perhaps, from the greater quantity of colour in the ftrokes, and partly from its being in good meafure defended by the gum from the action of the air.
Gum arabic, gum fenica, and the plum and cherry tree gums diffolve in the ink almoft as eafily as in pure water. But iiinglafs, a glue prepared from a kind of fiih, would not at all mingle with it : when the ifinglafs was previously diffolved by itfelf in water, and the folution poured into an infufion of the galls only, the fifh-glue, immediately on mixture, begun to curdle and feparate. Solutions of common glue or fize feemed however to mingle uniformly enough with die ink, no fenfible coa- gulation or feparation enfuing.
Sugar, fometimes added to inks, is much lefs effectual than gum, either as a coat for defending the colouring matter on the paper, or for preventing its precipitation from the liquor. It even haflens the precipitation of fome part of the colour, and is accompanied with another inconvenience, that of making the ink exceeding flow of drying. The fhining hue, which the fugar communicates, is by no means furffcient to counterbalance its difad-
E e e vantages j
[ 3«8 ] vantages ; and befides, where this quality is defired, an almoft equal gloflinels may be obtained by means of gum.
I tried likewife to prepare an ink, in which the colour- ing parts mould be fecured by a refinous varnifh. Here no water could be ufed, the diffolution of the refm re- quiring ftrong fpirit of wine ; and as this fpirit does not difTolve the vitriol of iron, another preparation of the metal became neceffary. Iron filings were digefted in fpirit of fait, with a moderate heat, till the acid would difTolve no more ; and the folution being let to evaporatt till it became thick, it was in this ftate diluted with rec- tified fpirit of wine : this preparation is the tinElura martii, in Jpiritujhlis of the apothecaries. I made a ftrong tincture of galls in fpirit of wine, and diflblved in it as much maftich as it would take up : with this folution poured off clear, I mixed different proportions of the tincture of iron, and obtained bluifh-black liquors, of a pretty good blacknefs when written with^ and fufficiently durable, but unfit for the common purpofes of ink, on account of their fpreading and finking in the paper, and growing clotty in the pen. Part of the maftich feemed to be pre- cipitated on mixture with the tincture of iron, as refinous bodies generally are with acids ; whereas gums diftbhe in acids without precipitation.
Inftead of the preparation of iron, called green vitriol, fome have recommended the blue vitriol of copper, and others the white vitriol of zinc. The white vitriol, though its principal metallic matter be zinc, generally contains alfo no inconiiderable quantity of iron, and in virtue of this iron it ftrikes a black with galls. Many blue vitriols alfo have a mixture of iron with the copper, and in this cafe they may in like manner ftrike a black with aftringents. To common green vitriol I added dif- ferent proportions of the pure vitriols both of zinc and
copper :
[ 3^9 3
copper : the inks prepared from thefe mixtures were not equal to thofe made with the green vitriol only. I tried alfo another preparation of copper, verdegris : a fmall addition of this made the colour of the ink remarkably- deeper on firft writing, but this additional blacknefs did not fland, and the colour turned rufty much fooner than when no verdegris was ufed. The effecl: of this ingre- dient will be further confidered in the next fection.
In fome receipts for ink, the galls are directed not to be powdered, but only bruifed, or broken into three or four pieces. To fee if this precaution could be of any advan- tage, I cut fome galls into four pieces each, and fome into bits like large pins heads : another parcel was reduced into pretty fine powder. Equal weights of the three were digefted for a fortnight, with vitriol and water in equal proportions : the ink from the large pieces was conliderably paler than the other two, and that from the powdered galls was the deepeft coloured.
A fmall wooden cafk, or a ftone bottle, is commonly chofen for making ink in, and the veffel is generally kept ftopt. As air appears to contribute to the deepening of the colour of ink upon paper, the characters not acquiring their full blacknels till a day or two after writing, it feemed probable, that a free admifTion of air might have a like effecl: upon the ink in its fluid ftate, and confe- quently that a broad mallow open veffel, and frequent ilirring, fo as to expofe frefh furfaces to the air, would contribute to improve the colour, and make the ink flow black from the pen. Accordingly mixtures of galls and vitriol with different proportions of water, were expofed to the air in flat ftone-ware difhes, and fUrred nine or ten times a day for a month. The liquors wrote blacker than thofe made from the fame quantities of the in- gredients in clofe vefiels; but whether the difference
E e e 2 proceeded
[ 39° ] proceeded only from part of the water having evaporated, fo that the quantity of liquid was made lefs, or from the inftrumental efficacy of the air deepening the colour as it does that of ink written with on paper, was not fully apparent from the experiments : probably it de- pended on both caufes conjointly.
For obtaining an ink that mould write black at once, on many occafions a very defirable point, I tried anothe: method, fimilar to that by which the dyer produces ex- peditioufly a deep black on cloth. The dyer firft 1 the aflringent materials in water for a confiderable time, and then adds the vitriol and flackens the heat, fo that the liquor may never fully boil after the vitriol is put in. By this way of management, the ink was made to write tof a pretty deep colour, much more fo than thofe prepared fcy long continued cold maceration.
It may here be proper to give a caution againft the ufe of copper veffels for ink. Mr. Marggraf relates, that when folution of pure vitriol of iron is boiled with copper, part of the iron is precipitated in an ochery form, and the liquor becomes ftrongly impregnated with the copper, which in this dirTolved ftate, as we have al- ready feen, debafes the colour of ink. I have found that copper is dirTolved alfo by mixtures of vitriol with aftringents ; for having ufed a copper pan in experiments of dying black, related in the following fection, the liquor, immediately after the addition of the vitriol to the aflrin- gent decoclion, fhewed plain marks of its having taken up a portion of copper, by giving a coppery flain to an iron knife with which it was ftirred. Mr. Marherr, in an ingenious inaugural difTertation on the chemical affini- ties of bodies, printed at Vienna in 1762, gives an obfer- vation more decifive of the effedls of copper veffels : when the befl inks were kept in a copper ink-ftand, fo
much
[ 39* ] much of the copper was diflclved, that the writings be- came in a fliort time as ill-coloured, as if the ink had been of the worft kind. It is pretty Angular that the vitriolic acid, in its feparation from the iron, mould dif- folve a body, on which otherwife, in fo dilute a ftate, it does not feem to have any aftion. Leaden veflels are alfo obferved to be corroded by ink, and debafe its colour ; and probably, except the materials of which ink itfelf is compofed, all the other bodies, that the vitriolic acid is capable of diflblving, will be found injurious to ink.
II. Compaction of ink, deduced from the experiments.
The foregoing experiments point out, for the beft pro- portions of the ingredients, one part of green vitriol, one of powdered logwood, and three of powdered galls. The beft menftuum appears to be vinegar or white wine, though for common ufe water will fuffice. The quantity of menftruum admits of great latitude : to make an ink of a full body of colour, it mould not exceed a quart, or at moft three pints, to three ounces of the galls, and one ounce of each of the other two ingredients. The pro- portion of gum may be varied at difcretion, according as the ink is wanted to be more or lefs glofly or mining, or as the nature of the paper may require the fluid to be well gummed to prevent its finking : half an ounce to a pint is in moft cafes fufficient ; though the more gum we can employ, confiftently with due free- dom of writing, it is probable that the ink will be the more durable.
The ingredients may be all put together at once, in any convenient veflel, and well fhaken four or five times a day. In ten or twelve days, and fooner if fet in a warm place, the ink will be fit for ufe ; though both its colour and durability will be improved by Handing longer on
the
[ 392 ] the undiffolved ingredients. The ink thus prepared, though it flows pale from the pen, turns to a good black in a day or two after writing.
Or the logwood and galls may be firft boiled in the liquor for half an hour or more, with the addition of a little more liquor to make up for that which evaporates in the boiling. Strain the decodtion while hot, and hav- ing put it into the vefiel which the ink is to be kept in, add to it the vitriol and the gum : as foon as thefe are diffolved, the ink may be ufed. By this way of manage- ing the procefs, we obtain all the advantage of boiling, and the feparation of the grofs feculence, without daub- ing any other veffels or utenfils than the ink-veffel itfelf : the ink is expeditioufly made, and writes of a pretty full colour.
Common pale ink, prepared by cold maceration, may be improved, fo as to write black at once, by evaporation. It may be fet in fuch a heat as will make it vifibly fteam, not greater ; and the heat continued until, on trying the liquor now and then, it is found to be of furricient black- nefs. On the fame principle, when ink is kept in an open ink-ftand, till it begins to grow fomewhat thick from the exhalation of part of the watery fluid, it writes as black as can be wiihed ; and when grown too thick to be conveniently written with, it gives blacknefs to a certain quantity of frefh ink. Hence, when we have pale ink to be thus improved, it will be fufficient, in many cafes, to evaporate to blacknefs only a part of it, and to dilute this occaiionally, as it thickens in the ink-ftand, with fome of the reft, ftirring them well together after each addition, as the thickened and dilute inks do not very readily unite ; if the evaporation was fuffered to continue till the black matter remained dry, it would fcarce diffolve at all in common ink or in water.
There
t 393 1 There is another method of giving blacknefs to inks, by the addition of fome of the black pigments formerly mentioned ; but the ufe of thefe pigments for writing will make the fubjecl: of another article at the end of this fection.
As the galls and logwood ought to be in pretty fine powder, that their virtue may be more readily and effec- tually extracted, it is expedient to have the ink feparated from them, as in the fecond of the above proceffes ; be- caufe otherwife the ink will often be loaded with the finer parts of the powder in fubftance, which being mixed up by making the vefl'el, remain long fufpended in the liquor. It is proper, however, in order to fecure againft any danger of a deficiency in the aftringent materials, to add to the ink feparated from its feculence, fome galls in coarfe powder freed from the fine duft by a fieve. On the fame principle, an oaken cafk is one of the beft vefiels for keeping ink in, this wood having a manifeft aftrin- gency, and anfwering nearly the fame end with the addi- tional galls. Befides the galls, fome pieces of iron may be put into the veflcl, as mentioned in page 386.
III. Of t he preparation of the paper for durable writing.
The dyers, as we (hall fee hereafter, prepare their cloth for receiving a permanent black colour, by boiling it with galls, that it may be thoroughly penetrated by the aftringent parts of the galls before the vitriol is intro- duced j fo that wherever the vitriol can reach, it meets with aftringent matter, to unite and produce a black with.
It is obfervable, that writings firft begin to fade or change their colour on the back of the paper, where the larger ftrokes have funk in or are vifible through it ; as if part of the irony matter of the vitriol was in a more fubtile or diffolved ftate than the reft, and funk further
into .
t 394 ] into the paper, on account of its not being fully difen- gaged from the acid, or furhciently combined with the a/iringent matter of the galls.
Hence it mould feem probable, that if the paper was impregnated with afrxingent matter, the colour of the ink would be more durable ; and that therefore a practice fimilar to that of the dyer, would be a valuable addition to the bufinefs of the paper-maker.
To fee how far this notion was well founded, I dipt fome paper in an infufion of galls, and when dry, repeated the dipping a fecond and a third time. On the paper thus prepared, and on fome of the fame paper unprepared, I wrote with different inks; feveral of which, that the effect might be more fenfible, had an over proportion of vitriol. The writings being expofed to the weather, till the beft of the inks on the unprepared paper had faded and changed their colour, thofe on the prepared •paper were all found to retain their blacknefs.
It is therefore recommended to the confideration of the paper-makers, whether a particular kind of paper might not be prepared for thofe ufes where the long duration •of the writing is of principal importance, by impregnating it with galls, or other aftringents, in fome of the opera- tions which it paffes through before it receives the glaz- ing ; as for inftance, by ufing an aftringent infufion, inffcead of common water, in the laft operation, when the ^matter is reduced into a pulp for being formed into fheets. The brownifh hue, which the paper receives from the galling, would not perhaps be any great obftacle to its ufe ; and if the propofal mould be thought worthy of being carried into execution, further enquiries may pombly difcover means of obviating the imperfection, and communicating aflringency without colour.
An
[ 395 J An aftringent matter might be introduced alio into
parchment and vellum. The common tanned lkins, as already obferved, and not only thofe of the fofter kind but the firm foles of fhoes, have the very impregnation, which we here propofe to communicate to the finer fkins ufed for writing. I fteeped a thick piece of parchment in water, along with fome oak bark, for three or four days, and having then preffed it fmooth and dried it, I found it as effectually penetrated with the matter which makes ink durable, as the paper in the experiment before mentioned. Even when the furface of the parchment was pared off, and the internal part written upon, the characters continued of a good black, while thofe made with the fame ink, on unprepared parchment, were changed to a yellowifh brown.
It may here be obferved, that an impregnation of paper with one or both of the ingredients of ink, has been fometimes already practifed, in a more imperfect manner, and with a view rather to amufement, than to the anfwering of any ufeful purpofe. Galls in fine powder being well rubbed into the paper with a hares foot, a folution of vitriol, made fo dilute as to have little or no colour, writes black upon the paper fo prepared, forming with the galls, in all the parts it touches, an extempo- raneous ink upon the furface of the paper. If powdered vitriol be firfi: rubbed in, the fame blacknefs is produced by infufion of galls ; and if powdered galls and powdered vitriol be mixed and applied together, both in a very dry ftate that they may not act upon one another, plain water makes a black writing.
But though practices of this kind fhould in fome cafes be convenient ; as for making occafional minutes, in want of ink, with common watery fluids ; or for the purpofe mentioned by Boyle, the keeping of the fingers from being
F f f blacked,
[ 396 ] blacked, by uiing colourlefs fluids for writing with ; it is plain, that the inks thus produced muft be in greater danger of fading than thofe made in the common man- ner, as the proportions of the ingredients, which form the ink, cannot be afcertained, and will be different on different parts of the paper. The preparation before recommended depends on a different principle, in regard both to the intention and the means : for here the pre- paration is only fuperficial, while there it is diffufed through the fubftance of the paper : the intention here is only the producing of a black colour on the furface, by applying a fluid which has no blacknefs, while there the paper is impregnated with the material which is moft. perifhable in ink, in order to continue the blacknefs be- yond the period in which that of the ink itfelf would decay.
IV. Attempts to prepare an ink from more durable materials.
To introduce into writing the ink whofe permanence we fee daily in printed books, appeared fo defirable an ob- ject, that though there were fmall hopes of attaining it, its importance feemed to deferve feme trials.
Printers ink, as we have feen in the foregoing feclion, is a thick mixture of lamp black and oil ; and fuch a mix- ture, though diluted with more oil, is evidently unfit for writing. Iaftead of oil, I mixed both lamp black and ivory black with folution of gum arabic, made of fuch confidence as juft to flow fufficiently from the pen. The liquors wrote of a fine black colour, but when dry, part of the colour could be rubbed off, efpecially in moift wea- ther, and a pencil dipt in water warned it away entirely.
I tried folutions of the animal glues, with the fame event. Ifinglafs or fifh-glue being the moft difficultly dif- foluble of thefe kinds of bodies, I mack a decoction of it
in
[ 397 J in Water, of fuch ftrength, that the liquor concreted into a gelly before it was quite cold: with this gelly, kept fluid by fufficient heat, I mixed fome ivory black : character? drawn with this mixture on paper bore rubbing much better than the others, but were difcharged without much difficulty by a wet pencil.
It was now fufpected, that the colour could not be Suf- ficiently fixed on paper without an oily cement. As oils themfelves are made mifcible with watery fluids by the intervention of gum, I mixed fome of the fofter printers varnifh, already defcribed, with about half its weight of a thick mucilage of gum arabic, working them well together in a mortar, till they united into a fmooth uni- form mafs : this was beaten with lamp black, and fome water added by little and little, the rubbing being con- tinued, till the mixture was diluted to a due confidence for writing. It wrote freely, and of a full brownifh-black colour: the characters could not be difcharged by rub- ing, but water wafhed them out, though not near fo readily as any of the foregoing. Inftead of the printers varnifh or boiled oil, I mixed raw linfeed oil in the fame manner with mucilage and lamp black, and on diluting the mixture with water; obtained an ink not greatly dif- ferent from the other.
Though thefe oily mixture's anfwered better than thofe with fimple gums or glues, it was apprehended that their being dischargeable by water would render them unfit for the purpofes intended. The only way of obviating this imperfection appeared to be, by ufing a paper, which ihould admit the black liquid to fink a little into its fub- fiance. Accordingly I took fome of the more finking kinds of paper, and common paper made damp as for printing; and had the fatisfaction to find, that neither the oily nor the fimple gummy mixtures fpread upon them To
F f f 2 much
[ 3^ ] much as might have been expected, and that the cha- racters were as fixed as could be defired, for they could not be warned out without rubbing off part of the fub- ftance of the paper itfelf.
All thefe inks muff be now and then ftirred or fhaken during the time of ufe, to mix up the black powder, which fettles by degrees to the bottom: thofe with oil muft be well ihaken alfo, though not ufed, once a day, or at leaft once in three or four days, to keep the oil united with the water and gum; for if once the oil feparates, . which it is apt to do by {landing at reft for fome days, it can no longer be mixed with the thin fluid by any agi- tation. But though this imperfect union of the ingre- dients renders thefe inks lefs fit for general ufe than thofe commonly employed, I apprehend there are many occa- fions, in which thefe kinds of inconveniences will not be thought to counterbalance the advantage of having writings, which wc may be affured will be as lading as the paper they are written upon. And indeed the incon- venience may be in great meafure obviated by ufing cotton in the ink-ftand, which, imbibing the fluid, prevents the feparation of the black powder diffufed through it.
It has often been remarked, that the inks ufed in former times were far more durable than thofe of later years; many modern records being more decayed than the manufcripts of much greater antiquity. Camillo Paderni, in his letters from Herculaneum publifhed in the PhilofophicalTranfadlions for the years 1753 and 1754, fpeaking of the ancient Roman, and Greek volumes dis- covered there, written on the Egyptian papyrus, com- plains of the paper being fo much decayed and rotten, that they have been able to unroll only a few pieces, but makes no complaint of the ink having anywhere faded, all the parts that have been unrolled feeming, from what
he.
[ 399 ] he fays of them, to be legible enough: in one place he mentions expreflly the characters being of a very black tincture, exceeding that of the coal to which fome parts of the paper were reduced. This obfervation occurring to me on revifing the foregoing experiments, I was in- duced to look into the Greek and Roman writers, who flouriihed before the deftrudtion of that city, to fee if any account could be found in them of the ink they made ufe of.
On this enquiry it plainly appeared, that the ancient inks, whofe great duration we now admire, were no other than fuch as we have been propofing in the prefent article. Pliny and Vitruvius expreflly mention the pre-r paration of foot, or what we now call lamp black, and the compofition of writing ink from lamp black and gum. Diofcorides is more particular, fetting down the pro- portions of the two ingredients, to wit, three ounces of the foot to one ounce, of gum. It feems the mixture was formed into cakes or rolls, which being dried in the fun, were occafionally tempered with water, as the cakes of Indian ink are among us for painting. It may be ob- ferved, that the Indian ink is ftill the writing as well as the painting ink of the Chinefe. The Chinefe writing indeed is performed in the fame manner as painting, with a ftiff hair pencil fixed in the end of a reed: but the Romans ufed a pen, and the inks of this kind are found to anfwer with a pen nearly as well as thofe now com- monly ufed. It might be matter of curiofity at leaft, and perhaps of utility, for thofe who have proper oppor- tunities, to enquire more particularly into the preparation of ink in different nations and different ages, and the legi- bility of the manufcripts of the refpedtive periods.
I have already taken notice, that all the inks, made on the principle we are now fpeaking of, can be difcharged by
wafhing.
[ 4^0 ]
wafhing, unlefs the paper admits them to fink into k> fubftance. The ancients were not infenfible of this im- perfection, and fometimes endeavoured to obviate it, according to Pliny, by ufing vinegar, inftead of water, for tempering the mixture of lamp black and gum. I tried vinegar, and found it to be of fome advantage, not as giving any improvement to the cement, but by promoting the finking of the matter into the paper. As this wafhing out of the ink may be prevented, by ufing a kind of paper eafy enough to be procured, it is fcarcely to be confidered as an imperfection j and indeed, on other kinds of paper, it is an imperfection only fo far as it may give occaiion to fraud, for none of thefe inks are in danger of being otherwife difcharged than by defign. The vitriolic inks themfelves, and thofe of printed books and copper plates, are all difchargeable; nor can it be expected of the ink maker to render writings fecure from frauds.
Our experiments and reflections on inks having thus led us back to the practice of the ancients, a further improve- ment occurred, that of uniting the ancient and modern inks together; or ufing the common vitriolic ink, inftead of water, for tempering the ancient mixture of gum and lamp black. By this method it fhould feem that the writings would have all the durability of thofe of former times, with all the advantage that refults from the vitriolic ink fixing itfelf in the paper. Even where the common vitriolic mixture is depended on for the ink, it may in many cafes be improved by a fmall addition of the ancient compofition, or of the common Indian ink which anfwers the lame purpofe: when the vitriolic ink is dilute, and flows fo pale from the pen, that the fine ftrokes, on firft writing, are fcarcely vifible, the addition of a little Indian ink is the readieft means of giving it the due blacknefs. By this admixture it may be prefumed alfo that the vitriolic
ink
[ 4°* ] ink will be made more durable, the Indian ink in fome meafure covering it, and defending it from the action of the air. In all cafes, where Indian ink or other fimilar compofitions are employed, cotton mould be ufed in the ink-fland, as already mentioned, to prevent the fettling of the black powder.
Though the foregoing enquiries have not attained to the perfection which might be defired, I flatter myfelf that they will not be found unimportant; that even the unfuc- cefsful experiments, if they contribute nothing in a philo- fophic view, will at leaft have this ufe, that they will leflen the labour to others who may engage in the fame purfuit; that a compofition of ink has been given, of as black and durable a colour, as there are grounds to believe the ma- terials to be capable of producing; that an improvement has been propofed in the manufacture of paper, by which the duration of inks will be greatly prolonged; and that means have been pointed out of obtaining, for purpofes where fuch duration is required, writings as lafting as the paper itfelf, with fewer inconveniences, than thofe, which for all occafions of writing, men acquiefced in for ages without complaint.
SECT. VII.
Of the dying of woollen Mack. I. General olfervations on the black dye.
THE ingredients from which common writing ink is prepared, green vitriol and aftringent vegetables, make the bafis of the black dye; the dying of cloth black being no other than the producing of an ink in its pores, or impregnating it with the colouring parts of ink already made. There are, however, fome variations in the com-
pofitiorw
[ 4°z ] pofition of the dying ink if it may be fo called; mixtures; which prove too perifhable when applied fuperficially on paper, being of fumcient durability when introduced into wool or woollen cloth; and mixtures, which make a good black ink on paper, making only a brown in the dyers bufinefs.
2. Cloth is generally fuppofed to be weakened by the black dye, more than by any other; on account of the cor- rofive quality of the vitriol, which is increafed by the heat made ufe for making it thoroughly penetrate the fubjedt: though the vitriol of iron is much lefs corrofive than the folutions of the metal made in the nitrous and marine acids, it is reckoned much more fo than the alum and tartar employed in moft of the other dyes. The finer the black, the more it is thought to weaken the cloth; info- much that fome writers look upon the beauty of the co- lour, and its durability or innocence to the cloth, as being incompatible with one another, and hence think it ad- vifable to abate a little in both points, and to be fatisfied with a colour of moderate finenefs that the cloth may be moderately lading. A German writer on dying, diftin- guifhed by the approbation of the celebrated Stahl, places this affair in a fomewhat different light. He obferves that the vitriol proves corroiive only fo far as it is not faturated with the galls, and that by ufing a proper quantity of galls, it will be mortified, fo as to be incapable of doing any injury to the cloth: to determine the quantity fumcient for this complete faturation, he diredls a decodlion of the galls, and a folution of the vitriol, to be mixed together in different proportions, and dropt upon white paper, the li- quors being made very dilute that their colours may be the better judged of: the proportions, which give the deepeft black colour, are thofe which ought to be followed by the dyer, and by which, according to him, the vitriol is
made
[ 4°3 ] made harmlefs. The experiments in the foregoing fection have mewn, that about equal parts of galls and vitriol produce the full blacknefs on paper ; and our dyers, fo far as I can find, have generally employed the galls in a proportion not lefs than this, or at leaft fupplied their deficiency by a quantity of other aftringents equivalent in virtue ; from whence it mould follow, that the common black dye cannot hurt the cloth. In this point I have not myfelf had any fair experience, but am allured by a fkil- ful and judicious dyer, that black, properly dyed, has by no means the corrofive quality generally attributed to it ; and that the rottennefs or perifhablenefs, often complained of in black cloths, &c. proceeds only from the cloth have- ing been damaged before the dying, for black is the dye commonly had recourfe to for damaged and unfaleable pieces, and fuch as have been fpoilt in other dyes. Though vitriol, however mortified, be admitted to weaken the cloth, it is pretty clear that black is not the dye which weakens it moft -, for vitriol is ufed for fome coffee colours, not indeed with quite fo great a heat but in greater quan- tity than for the black dye itfelf ; and the aquafortis em- ployed in fcarlets, oranges, and fome other colours, is certainly more corrofive.
3. For dying black, efpecially on fuperfine cloths, it is cuftomary to give a previous ground of fome other deep colour ; and blue is preferred for this ground, as being one of the moft innocent dyes in regard to the cloth, and as being of all colours that which has the neareft affinity to black : common black ink, and the black liquor of the dyer, when diluted largely with fpring water, appear blue, as if their blacknefs was no other than a concentrated blue. The ufe affigned for this blue ground by the wri- ters on dying is, that the cloth, having already a confider- able body of colour, may require lefs of the blackening
G g g materials,
[ 4©4 ]
materials, and confequently be lefs weakened, than if it was dyed directly from white to black. But there is another more important ufe of it, the blue being eflential to the production of the black dye ; for without either a blue ground, or a blue fuperadded to the vitriol and galls, no other than brown dyes are obtained. There are means, (fee hereafter N°. 7.) of introducing this neceffary blue- nefs along with the vitriol and aftringents ; but the colour proves more perilhable than when dyed upon a blue ground of indigo or woad.
4, The dyers commonly leave fome blue marks at the ends of the cloth, by fixing pieces of lead on them, by which they are fecured from the action of the black liquor, to fhew that the piece has been regularly dyed on a blue ground, and confequently that the colour may be expected to be durable. This may be difcovered, with greater certainty, by fteeping a fmall bit of the black cloth, for a day or two, in water acidulated with a little oil of vitriol j or more expeditioufly, by boiling it about a quar- ter of an hour, in a folution of alum and tartar, made in the proportion of an ounce of each of the falts to a pint of water. Great part of the black matter being deflroyed or diffolved by the faline liquors, the cloth will remain of a bluifh black colour if it has had a previous blue ground j but if it has been dyed directly from white, it will now look of a muddy reddifh brown. The folution of alum and tartar is the effay liquor for black cloths,, directed in the new French regulations, which were drawn up from the experiments of Dufay, and published at the end of Hellots Art de teindre.
5. Stuffs, whofe price will not admit of the blue dye, ace faid, by the French and German writers, to be grounded with a deep brown, by boiling them with wal- nut peels, or walnut-tree roots. This practice, as I am
informed,
t 4°S 1 informed, is never followed by our dyers, who look upon brown as a colour oppofite to black, and therefore very unfit to ferve as a ground for it. Whether a brown ground is ufeful or otherwife I cannot take upon me ab- folutely to determine j but thus much I can affirm, that I have known brown fluffs dyed to a black, which was reckoned, by good judges, to look, and to hold its colour in wearing, remarkably well. It mould feem that any deep colour, which does not hurt the cloth, would be preferable to white ; and it may here be proper to obferve, that all colours whatever receive a black dye, though black will not receive any other j whence black, as already mentioned, is the laft refou'ree for cloths that have been damaged or had their colour flained or impaired by diffe- rent accidents.
6. The excellent regulations for the French dyers, drawn up and published by the order of Mr. Colbert, require the cloth, after it has been blued, to be maddered. In order to fix the colour of madder, the cloth muft be firft boiled with alum and tartar ; and as thefe falts muff neceffarily contribute to augment the ill qualities that were fuppofed to refult from the black dye itfelf, and which were endeavoured as much as poflible to be avoid- ed, it might be thought that the madder was accompanied with fome confiderable advantage, fufficient to counter- balance that inconvenience and the addition which it makes to the expence. It has not been found however, on fair trials, to contribute any thing either to the beauty or duration of the black. Mr. Hellot relates, that have- ing dyed a piece of cloth of a deep blue, he maddered one of the halfs, and then dyed both the maddered and unmaddered halfs black in one copper : both turned out of a good black, but the unmaddered, he fays, was plainly the beft, the maddered piece having fomewhat of a rufty
Ggg 2 hue,
[ 4o6 ] hue. The beft way of comparifon is, by placing famples of the dyed pieces flat, againft a full light, that is, with their edges towards the light, and then going back a little, fo as to look partly down upon them, and partly over the furface : this is the way in which the dyers judge of colours. On viewing in this manner feveral famples of blacks dyed on blue cloth maddered and unmaddered, I could not perceive that they differed greatly from one another, but was convinced, that if the maddered ones are not inferior to the others, they certainly have no advantage above them. In fome of the old receipts, madder is directed as an ingredient in the black dye itfelf, along with the vitriol and galls ; but here it is evidently fuperfluous, its colour not fixing itfelf in the cloth. Among the reafons alledged for the ufe of maddering the cloth, there is only one which appears to have any plau- fibility, viz. that it prevents the black cloth from flaming the fkin or linen; but all that the madder can do in this refpect, as Mr. Hellot juftly obferves, is, to difcharge the fuperfluous blue, and this not in virtue of the madder itfelf, but of the boiling with alum and tartar preparatory to the madder dye. The fame advantage may be ob- tained by fufficiently fcowering the cloth in the fulling mill after the dye. This is evident from the fuperfine cloths dyed by our dyers, among whom the injudicious and unfrugal practice of maddering, from fuch informa- tion as I have received, appears to be unknown. They have indeed a colour called madder black, dyed on baize, (a kind of coarfe cloth fluff) for Portugal and Spain ; but this depends on another principle, as will appear here- after.
7. Logwood, which as we have feen in the foregoing feciion is a very ufeful ingredient in writing ink, is frill more fo in the black dye. Vitriol and galls, in whatever
pro-
[ 407 ] proportions they are ufed, produce no other than browns of different fhades : I have often been furprifed, that with thefe capital materials of the black dye I never could obtain any true blacknefs in white cloth, and attributed the failure to fome unheeded mifmanagement in the pro- cefs, till I found it to be a known fadt among the dyers. Logwood is the material which adds blacknefs to the vitriol and gall brown j and this black dye, though not of the moil durable kind, is the moft common. On blue cloth, a good black may be dyed by vitriol and galls alone ; but even here, an addition of logwood contributes not a little to improve the colour.
8. The addition of verdegris, which deepens the colour of the inky liquor, is found alfo to deepen the dye on cloth; and this improved blacknefs, very perifhabie in the ink applied on paper, appears in cloth to be more durable, though not entirely fo much as could be wifhed. The effedt of the verdegris feems to proceed from its adtion on the logwood : for with galls, and with green vitriol, feparately, it produced no tendency to black- nefs ; but with decodtion of logwood it ftruck immediately a deep black, which when diluted appeared of a fine blue. This experiment reconciles two obfervations I have lately met with, one by Mr. Scheffer in the Swedifh Trans- actions, the other by Mr. Hoffmann in a German treatife of oeconomical chemifbry, &c. the former of whom re- lates that logwood with verdegris gives a blue dye, and the latter that it gives a black. Blue is the proper colour of the mixture, and the black is a concentration of the blue. Part of the colouring matter of the mixture con- cretes very fpeedily into fenlible particles, fo as to look like a black powder diffufed through the liquor : the liquor is found to pafs blue through a filter, and the black matter, which remains on the filter, appears like- wife
[ 4°8 ] wife merely blue, when fpread thin on paper, or diluted with white powders.
9. Inftead of the verdegris, I tried a cheaper preparation of copper, blue vitriol; This had fomewhat of a like effect, but in a lefs degree : the colour on mixture was lefs black, and the concretion of the colouring parts lefs remarkable : the black or bluifh-black matter being fepa- nited by nitration, the liquor proved not at all blue, but purplifh or reddifh, much like a decoction of logwood by itfelf ; it foon turned to a blue colour when dropt on paper and expofed to the air, but both the blue and the black were greatly more perifhable than thofe produced by verdegris.
10. Some have preferred vitriols impregnated with a little copper, as that of Dantzick, to the more purely ferrugineous Englifh vitriol ; not indeed fufpecting that the copper would add any thing to the colour, as in the foregoing experiments -, but from an opinion of its ren- dering the vitriol more penetrating or corrolive, fo as to enable the colouring matter to fink better into the fubjedt. With regard to its adding colour, if the vitriol of copper was even as effectual in this intention as verdegris, which it is very far from being, yet the very fmall quantity, contained in the vitriols recommended, could be of no material ad- vantage ; and as to the penetration, I believe it will be admitted, that vitriol of iron without any copper is pene- trating and corrofive rather more than enough. The Dantzick vitriol appears however to have one advantage, not depending on its coppery part, but on the manner of its preparation : greateft part of the Englifh vitriol, by hafty cryftallization, is run into large irregular maffes, abounding with loofe ochery matter and with watery moifture, if not with foreign fubftances of another kind ; while the Dantzick, more flowly cryftallized, is more
pure,
[ 4^9 ]
pure, lefs watery and confequently ftronger. The moft perfect vitriol of iron is that which is in the mod folid regular cryftals, of the deepeft green colour ; not rufty or yellowifh, from its containing an ochre unfatiated with acid; nor pale, from its being too watery, or holding aluminous or other foreign matter.
ii. For producing a black dye on cloth, the cloth is firft impregnated with the aftringent matter, and after- wards pafled through a folution of vitriol mixed alfo with aftringents. If it was firft charged with the vitriolic folution, the colour would not fucceed fo well, and the cloth would be more damaged : if the aftringent and vitriolic liquors were mixed together at firft in one cop- per, the operation would be prolonged, and feveral re- peated dippings would be neceflary for introducing into the fubjett a due body of colour. In the dying of great lengths of cloth, where fometimes there is an interval of a quarter of an hour between the pafling of the two ends into the liquor, a little tartar is often added, which does not affedt the colour itfelf, but is fuppofed to make the dye take more uniformly, and prevent the cloth from being what the workmen call bloted.
12. If after the cloth has acquired a full black colour, k be again and again pafled through the dying liquor, its colour by no means receives any improvement, but is rather debafed and inclined to brownifh. An over- quantity of the ingredients, employed at firft, has a like effedl:. The lefs quantity of the blackening materials we make ufe of on blue cloth, provided they are fufficient to give full blacknefs, the more durable will the colour be in wearing.
13. The proportions of the ingredients to one another, are regulated on quite other principles than in inks. Equal parts of vitriol and galls feem to be the beft pro- portions,
[ 4*o ] portions. If the galls are much increafed, which it is neceffiry they fhould be for ink, they make the dye incline to brown ; but an increafe of the vitriol, by which inks are made fo perifhable, does not appear at all to affect the dye : even the largeft additions of vitriol, however they may weaken the cloth, do not feem to injure the colour.
14. In the dying of black, as of moft other colours, there are considerable variations in the practices of diffe- rent workmen, which it would be difficult and even ufe- lefs to collect. I fhall here defcribe two proceffes, which I have often tried in fmall, and which appeared to me to be the beft.
II. Black with galls, logwood, and vitriol.
A hundred pounds of woollen cloth, dyed firft to a deep blue, require, for the black dye, about five pounds of vitriol, five of galls, and thirty of logwood. Thefe, as I am informed by an experienced artift, are the quantities generally allowed by our dyers.
The galls, beaten into moderately fine powder and tied up in a bag, are boiled for a little time in a copper of water fufficient for working the cloth in. The blued cloth, after being fteeped in river water and drained, that it may be every where thoroughly moift, but not fo as to drip, is in this ftate put into the boiling decoction of the galls, and kept turning therein for two hours or more, the bag of galls being now and then fqueezed, that the virtue of this drug may be more effectually extracted and communicated to the cloth.
The logwood, rafped or fhaved into fmall chips, or rather ground into powder, is boiled in another copper for feveral hours, this wood giving out its colour exceed- ing difficultly. The logwood liquor is moft commonly
prepared
[4u ] prepared a considerable time before it is ufed, its colour being found to improve in keeping.
The logwood decoction being made of a fcalding heat, but not quite boiling, the vitriol is thrown into it, and as foon as this is diffolved, the galled cloth is put in. A boiling heat mould never be ufed after the addition of the vitriol, not only as it would needlefsly augment the corrofive power of the fait, but like wife as it would injure the beauty of the colour, by haftily extricating part of the ferrugineous matter of the vitriol in an ochery form, before it can come fufficiently in contact with the aftringent fubftance with which the cloth is impregnated. The cloth is inceflantly turned in the liquor that it may receive the colour uniformly, and now and then taken out and aired for a moment, which contributes to fecure the colour, and at the fame time affords an opportunity of judging of its deepnefs.
After about two hours continuance in the dye, the cloth is found to have received a good black, and is then taken out, warned with cold water, and paffed through the fulling mill. The fuperfine cloth is three times fulled, with warm folution of foap, which not only difcharges the fuperfluous colour that would otherwife ftain the fkin or linen, but contributes alfo to foften the cloth itfelf by mortifying the acid.
III. Black dye with verdegris.
For fome of the fuperfine black cloths, a little verdegris is ufed by our dyers, and this addition appears among the French to be more frequent. Mr. Hellot, after trial of fundry proceffes, gives the following as being the beft, or as that which produces the fineft velvet black on cloth, and which accordingly is followed in the beft dye-houfes in France^
Hhh For
[ 4I2 ]
For a hundred pounds of blue cloth ; ten pounds of logwood chips, and the fame quantity of Aleppo galls in powder, are tied up together in a bag, and boiled in a middling copper, with a fuitable quantity of water, for twelve hours.
One third of this decoction is taken out into another copper, and two pounds of powdered verdegris added to it. In this mixture, kept gently boiling; or rather only fcalding hot, the cloth is dipt, and turned without ceafe- ing, for two hours ; after which it is taken out and aired .
Another third of the decoction is laded out into the fame copper, eight pounds of green vitriol added, and the fire flackened about half an hour. The vitriol being now all diiTolved, the cloth is put in and worked for an hour, and then taken out and aired again.
The remaining third of the decoction in the firfr. cop- per is then put to the other two in the fecond, the bag of galls and logwood being well prefled out. Fifteen or twenty pounds of fumach are now added ; and as foon as the copper begins to boil, two pounds more of vitriol are thrown in, with fome cold water to flacken the heat. The cloth is kept in for an hour, then taken out and aired, dipt a fecond time, and kept turning for an hour longer.
The cloth, now compleatly dyed, is warned in a river, a«d fcowered in the fulling mill till the water comes from it colourlefs. It is then paffed through a copper of weld or woold, prepared as for dying yellow, which is- fuppofed to foften the cloth and confirm the colour. - This procefs affords a very fine black, but it is too expenfive to be followed by our dyers, the fire, and manual labour of the black dye as here defcribed amounting to more, as I am informed by a perfon converfant in this bufmefs, than the dyer is paid for the whole dye of the
above
[4i3 J
above quantity of fuperfine cloth, including the blue ground. The quantities of vitriol and galls may be di- minished, and the time of boding greatly Shortened. The pafling through weld liquor, after fcowering with foap, is entirely unneceffary ; though probably it may be of ule where the fcowering is not complied with ; not however in virtue of the weld itfelf, but of the alcaline fait with which the decoction of it is generally prepared by the dyers, fo that the weld liquor does no more than fupply the place of foap.
Both in this and the foregoing procefs, the liquor re- mains black after the dying of the cloth is finifhed, and communicates a dilute black, that is a grey colour, to as much frefh cloth as can be conveniently worked in it.
IV. Method of dying cloth grey.
The fimple greys, which are all no other than Shades of black, are dyed nearly in the fame manner as the full blacks ; only by uiing a lefs proportion of the dying in- gredients, or continuing the cloth in the liquor for a Shorter time>
A decoction of galls and folution of vitriol being pre- pared feparately, a little of each .of them may be put to- gether at once into a copper of water made fcalding hot : the liquor becomes black ; and cloth, dipt and worked in it, acquires a lighter or deeper grey according to the quantity of the decoction and folution employed : By adding more of the liquors with the next parcel of cloth, and thus proceeding fucceffively, a Series of fhades may be obtained, from the lighted to the darkeft grey. Or the cloth may be firft boiled with a proper quantity oi galls, and afterwards worked in the fame liquor, with the addition of more and more vitriol according to the in- tended depth of colour. The liquor remaining after
H h h 2 the
£ 4»4 ] the dying of full black may be ufed alfo for the dying of
greys.
For the quantities of the ingredients, and the time of the cloths continuance in the liquor, no general rule can be given : as they muft depend upon the degree of colour required, the eye only can be the judge. If the colour happens to be too deep, it may be remedied, in fome meafure, by pairing the cloth through hot water mixed with a little decoction of galls, by which a part of the colour will be carried off. A weak folution of alum, tartar, or foap, are in this intention much more effectual, but at the fame time very liable, particularly the two firft, to exceed in their operation, difcharging fo much- of the colour, unlefs due care is taken, as to occalion a neceffity for re-dying the cloth, which is thus needlefsly weakened by the repeated action of the corrofive liquor The too great deepnefs of colour may be eafily prevented... by examining the cloth from time to time, and taking it out as foon as it has acquired the due fhade. It mould be immediately warned with a large quantity of water, and the very dark fhades fhould be fcowered with foap in the fame manner as the full blacks, to fetclv out the fuperfiuous colour, or fuch as is. not fixed in the cloth.
The fimple greys are dyed from white cloth without any previous ground of blue or other colours. There are alfo a multitude of compound greys and browns, produced from cloth dyed blue, red, yellow, brown, or of colours compounded of thefe, by darkening them with the black dye. The diftinctions of thefe various fhades, and the manner of hitting any particular one, practice only can teach,
V. The
[4isJ
V. The dying of wool black.
The natural grcafe of wool, of great advantage to it in the warehoufe, as being a fure prefervative againfl: the moth, muft necefiarily be removed, before it is attempted to be dyed of any kind of colour : the more perfectly it is cleanfed, the better it will be difpofed to receive the dye.
The liquor commonly ufed for the fcowering of fleece wool is faid to be a mixture of flale urine with twice or thrice its quantity of water. This mixture being made fcalding hot, but not boiling, for a boiling heat would felt the wool, or make it run into lumps, fo much wool; as the copper will conveniently receive, is dipt in it, and turned from time to time with wooden poles, for a quar- ter of an hour ormore: it is then carried in a large bafket into running water, where it is worked by two men, backwards and forwards, one drawing it from under the Others pole, till it ceafes to render the water turbid. The volatile alcaline fait, produced in urine by putrefaction, unites with the greafy matter into a foapy compound,which, diflblving imperfectly in water, continues to give the tur- bid appearance till it is totally warned out. The wool is faid to lofe in this procefs between one fifth and one fourth of its weight.
The wool thus cleanfed is dyed blue, then fimmered with galls, and the black dye finiflied with logwood and vitriol ; or for a finer black, which however is feldom wanted on wool, the above method with verdegris may be followed. The manner of procedure is in all refpects the fame with that for dying woollen cloth ; and all the obfervations, mentioned under the foregoing articles, are equally applicable here. It is only to be added, that the operations, which wool has to undergo, render the pre- venting
[ 4*6 ]
venting of harfhnefs of more importance here than in cloth.
VI. Black dye without galls.
O f the galls, directed in the foregoing proceffes, a part is commonly omitted in bufinefs, and fupplied by cheaper aftringents, which, being weaker in virtue, are taken in quantity proportionably larger. From the pre- fent high price of galls I was induced to try whether this expenfive article could not be entirely fuperfeded. I pro- ceeded exactly according to the French procels with ver- degris, page 411, only inftead of the galls taking fix times as much oak bark, fuch as the tanners ufe : the cloth, well warned with foap after the dye, appeared of a black colour, not indeed quite fo beautiful as that dyed in the fame manner with galls,' yet not a bad one. I tried fu- mach alfo, with the fame event. It appears therefore, that though no effectual fubftitute to galls could be found for the purpofes of making ink, yet cheaper fubftances may often be made to fuffice in the dying bufinefs, where the great confumption of aftringent materials ren- ders the reduction of the price of more importance.
In the Swediih Tranfactions for the year 1753, a fine black is faid to be dyed without galls or logwood j the place of both which is fupplied by a plant common in Sweden, called there mj&lon or ?njcelon-ris, which is ga- thered in autumn while the leaves continue green, and carefully dried that they may retain their green colour. A hundred pounds of woollen cloth are directed to be boiled with fixteen pounds of green vitriol and eight •pounds of white tartar, for two hours 5 and the cloth next day to be rinfed out as after the common alum boiling. A hundred and fifty pounds of the dried mjeelon cut a little, or a fomewhat greater quantity if the plant
has
[4i7] has been long kept, are boiled in water for two hours j and the mjcelon being then taken out, a little madder is put into the liquor. The cloth is put in along with the madder, boiled for an hour and a half or an hour and three quarters, and afterwards rinfed in water. This dye is faid to be ufed chiefly for fine cloth, and to give lefs harfhnefs than the common black.
What the mjcelon is, we learn from a paper by Linnaeus in the fame Tranfactions for the year 1743. He obferves, that about a year before, a leaf called jackafoapuck was brought into England from North-America, and mixed with tobacco for fmoaking. Mr. Collinfon favoured him with large fpecimens of it, entitled " the plant Jackafha- " puck which is mixed with tobacco, gathered on " Churchill river in Hudfons bay." This plant, he fays, was eafily known by a Swede, as it grows in Sweden in abundance, on uncultivated gravelly fandy hills. He gives its Swedifh names mjcelon, mjcelon-ris, mjcelbcers-ris ; and likewife the latin names under which it is defcribed by different botanic writers, from which it is clear, that the mjcelon is the fame with the uva urfi that has lately come into efteem in Germany for medicinal ufe. Some quantity of the uva urji has been brought from Germany, to be tried as a medicine in this country : the plant is raifed alfo in fome of our botanic gardens, and if the pro- pagation of it mould be found of any importance, it would doubtlefs thrive on many of our now barren hills.
I have been informed by a foreign correfpondent, that the uva urfi is faid to be ufed in England for dying black, and that it is imported for this purpofe from Hudfons bay. I cannot find that this plant, or any other from Hudfons bay, is known among our dyers or dry-falters ; but the two foregoing quotations account fufficiently for the report,
I. made
[ 4i»]
1 made trial of the German uva urfi both on white and on blue cloth, exactly according to the Swedifh di- rections ; boiling the cloth firft with vitriol and tartar, and afterwards with a decoction of the uva urfi: on the blue cloth I obtained a tolerably good black, but on the white cloth, as with other aftringents, the colour was only a dark brown. I repeated the experiment without the madder, and with a variation in the order of applying the other ingredients, boiling the cloth firft in a decoftion of the uva urfi, and then adding the vitriol and tartar : by this method I obtained, as before, a pretty good black on the blue cloth, but only a brown on the white. I afterwards omitted the tartar alio, and did not obferve that the want of it occafioned any difference in the colour produced. All the famples dyed brown with uva urfi and vitriol, became black on being palled through log- wood liquor ; but without either logwood or a blue ground, no true blacknefs could be obtained. A dyer, whom I confulted on this head, made fome trials for me, on the uva urfi, with the fame event ; this plant giving no black dye with vitriol alone, any more than the other aftringents.
On adding green vitriol to a ftrong decodtion of uva urfi, I took notice of a phenomenon which did not hap- pen at all with galls, and which I do not remember to jiave obferved, in lb remarkable a degree, with any of the other ftrong aftringents. The liquor, inftead of the uniform appearance of the common black mixtures of this kind, looked like a black powder diffufed through water ; and being written with on paper, the ftrokes ap- peared everywhere unequal and fpecky, as if made with charcoal powder and water, though they were of a deep and durable black where the colouring matter lay thick. This hafty concretion of the black matter from the liquor,
while
[ 419 3 white it renders the uva urfi entirely unfit for the puf- pofes of making ink, may poflibly be of fome advantage to it for the black dye ; as the largenefs of the colouring particles, which concrete in the pores of the cloth, may render them more fixed, fo that lefs of the colouring matter is wafted in the liquor, and lefs of it can be dis- charged from the cloth. To this caufe may perhaps be afcribed a quality of the uva urfi dye mentioned by the Swedifh author, that the cloth is cleaner than after the other black dyes, or requires lefs wafhing to free it from the loofe colour.
Among many aftringents I have tried, oak wood came the neareft to the uva urfi in this concretion of the colouring matter. A piece of white flannel was boiled firft with oak faw-duft, and afterwards with an addition of vitriol as in the foregoing procefles. The liquor, as foon as the vitriol was put in, became bluifh-black, though with much lefs bluenefs than the cold infufion of oak-duft and vitriol, page 383 : fome of it being poured off into a glafs, it appeared full of powdery matter, which foon fettled to the bottom, leaving the liquor of a pale bluiih. From the blue colour of this mixture it was hoped, that a black dye might be obtained from it without logwood or a blue ground; and in effect the piece of flannel, though it did not acquire a true black, approached more to black nefs than I remember to have obferved with other aftringents : its colour was a dark grey, without any mixture of blue or brown, like a pure black diluted with a little white. This wood leems therefore to deferve the attention of the dyers : there are grounds to believe that oak faw-duft, or the heart of oak reduced to powder in mills, will be found an aftringent of fufficient efficacy, and fupply with ad- vantage the place of galls : the oak tree doubtlefs con-
I i i tains
I 420 ]
tain6 a matter fimilar to the galls which are produced from it. Pofhbly by fome preparation of the oak-duft, it might be brought nearer to the nature of galls : does not it6 difference from galls depend on fome particular juice, more foluble than the dirett aftringent matter, and feparable by flight infufion in cold water ?
VII. Black dye from a combination of colours.
In the firfl article of this feftion it has been fhewn, that the madder dye, required by the French regulations to be applied upon blue cloth as a ground for black, is rather injurious to the colour than of any real advantage. In the experiments which the determination of that point required, a fomewhat unexpected phenomenon occurred, an account of which was referved for this place. A piece of deep blue cloth was boiled in water with alum and tartar, as cuftomary for preparing cloth to receive the madder dye. The cloth being taken out and fqueezed a little, fome powdered madder was boiled in water, in fuch quantity as to communicate a dark red colour to the liquor. The cloth, ftill moift, was put into this decoction, and a boiling heat continued about half an hour. Being then taken out and wafhed with foap, it looked of a very dark colour, fuch as any perfon would call a black, though not a fine black. Thus we have a kind of black dye, very durable, without any vitriol or other preparation of iron, from a combination of the blue dye with the mad- der red.
This effect: of madder upon blue cloth is well known to the dyers, among whom the colour hence produced is called madder-black. Our black cloths for home con- fumption are all dyed with vitriol and aftringents, either on a ground of woad, which makes the true black, or with an addition of logwood only, in which cafe the
colour
[ 4*1 ]
colour being more perifhable is called falfe : but the black baize, which we export to Spain and Portugal, are dyed chiefly of the madder black, a fpecies of blacknefs which there, it feems, is in eftimation.
If, inftead of madder, the purer red of cochineal be ap- plied on blue cloth, the colour hence refulting, is not at all black, but purple. Cochineal, independently of its too great expenfivenefs for purpofes of this kind, is too bright a colour to have a place in the compofition of blacknefs : to change the purple into a colour approach- ing to black, the addition of other colours is neceflary, for it is not to be expected that a mixture of fimple blue and red fhould produce a black (fee page 355.) But madder is both a dark and a compound colour, in which an admixture of brown or tawny with the red is very manifefl. If the madder be (lightly infufed in warm water, and afterwards boiled in a frelh quantity of water, the firft liquor will appear of a pretty good red colour, the other remarkably more dark and brownifh. Hence for dying a good madder red, a boiling heat fhould be avoid- ed ; but for the black dye the madder ought to be well boiled, that the brown as well as the red parts may be extracted.
The madder black might probably be deepened by making it ftill more compounded, as particularly by the addition of a dark yellow ; but any improvement of this kind would be of little advantage to the dyer, who find- ing the dye already too expenfive, endeavours to imitate it with the cheaper vitriolic black. And indeed, indepen- dently of confederations of this kind, he is here rather confined to a particular fhade or fpecies of colour, which fafhion has brought into efteem, than folicitous about deepening the dye or making it more perfectly black.
ni 2 sect:
[ 422 ]
SECT. VIII.
Of the dying ofjilk black.
RAW filk, in the ftate in which it is wound off from the cocons, has a harfhnefs which renders it unfit for being fpun, and for the moft part a pretty deep yellowifh or reddifh-yellow colour, from both which it is cleanfed, by boiling it with foap, and afterwards thoroughly warning it with foft water : when woven, it is again warned with foap, to free it from the greaiinefs it may have contracted, which would occafion it to be fpotted in dying. The iilk lofes in the boiling generally about a fourth part of its weight : this proportion is affigned by the writers on dying, and on enquiry among the workmen, I find it univerfally allowed to be the near- eft calculation. In being dyed black, this lofs is fully made up, the weight of the dyed filk being commonly even greater than that of the raw fdk. There is no dye which adds fo much to the weight as black : the increafe is coniiderable in woollen as well as in filk, though moft taken notice of in the latter on account of its great price. Mr. Macquer obferves, in his art de la teinture enfiie, publifhed in 1763, that the fineft oil foap is required for this cleanfing of filk ; that there is nothing faved by ufing the inferior kinds, a proportionably greater quantity of them being neceffary ; that fome forts of foap curdle with the matter which they extract from the filk, info a fubftance almoft of the confiftence of wax ; that thofe, which are made with animal fats, prevent the filk from having the proper drynefs and luftre, and difpofe it to grow reddifh in keeping ; that even the beft foaps are accompanied with fome imperfections in this refpec~t, and that the fuperiority in luftre, of the Chinefe filks to the
European,
[ 423 ] European, is owing to the former being cleanfed without
any foap. In a French difTertation on this fubjecTr, to which a premium was adjudged by the academy of Lyons, in 1 76 1, the ill qualities of foap are attributed to its oil, and a folution of fimple alcaline fait, made fo dilute as not to corrode the filk itfelf, is recommended in its place : the fait of foude or bariglia, as being the mildeft of the alcaline falts, is for this purpofe juftly preferred to the common more corrofive alcalies. Alcaline falts, either in their pure ftate or made into foap with oils, are the only known menftrua that extract the matter which gives harihnefs and colour to raw filk.
What this matter is, has not been fufficiently ex- amined. As it is not diifolved by water, fpirit of wine, or by acids fo far diluted as not to deflroy the filk itfelf, Mr. Macquer fuppofes it to be either a concrete oily fubftance, whofe oil is of the nature of expreffed oils ; or a compound of oily and gummy matter, fo proportioned and combined, as to protect one another from the action of their refpedtive diffolvents. Whatever can be faid of the compofition of this matter, may perhaps be faid equally of that of filk itfelf, which is not an organifed fibre like wool, but is in its whole fubftance a concrete animal juice : naturalifts obferve, that on opening the filk-worm at a proper feafon, the yellow filky juice may be readily diftinguifhed, and drawn out into fine flexible filaments. Alcaline falts, which when diluted with water, or fheathed with oil, to a certain degree, are found the proper menftrua of the harfh and tinging part of raw filk, in a purer or lefs dilute ftate, or by longer boiling, diffolve alfo the matter on which the tenacity or cohefion of the filk depends. Some of the fpun filk called in the fhops raw filk, but which has been boiled with foap pre- vious to the fpinning, and fuffered the diminution of
weight
[ 424 ] weight before-mentioned, on being boiled in a folution of alcaline fait, received a further diminution of two- thirds : another quantity of the fame filk being boiled longer with the alcali, about four-fifths of its weight were taken up by the liquor, which became reddifh, and the remaining fifth was an incoherent friable mafs, not ill refembling papier mache. It mould feem from thefe experiments, that even the common procefs of cleanfing filk, in which a fourth of its weight is difiolved, cannot be entirely innocent, but muft contribute in fome degree to diminish the ftrength of the filk ; and accordingly I find it allowed by the workmen, that a thread of filk boiled is not fo ftrong as when raw. Some further ex- periments of the effects of different fubftances on raw filk are now in hand : if any thing of importance refults from them, they fhall be communicated in the appendix to this volume.
Silk is rarely or never dyed of a blue as a preparatory ground for the black dye. The regulations of the French filk dyers expreffly order its being dyed directly from white to black, and this, as I am informed, is the general practice among us, though fome report that the German filk dyers give a brown ground for their black filks, by boiling them with the root or bark of the walnut-tree. The only reafon I have heard afiigned for the omiflion of the blue ground on filk is, its adding to the expence of a procefs, which is otherwife, as commonly managed, con- fiderably more expenfive and troublefome than the dying of woollen.
Mr. Macquer reckons black a difficult colour to dye on filk: and indeed, if all the circumftances, and materials, of the complex procefs, which he defcribes as being fol- lowed in many of the good dye-houfes of France, were necefiary for fucceeding in the colour, a multitude of
trials
[ 425 ] trials muft undoubtedly have been made, before fuccefs could have been attained to. But experience has abun- dantly fhewn that the cafe is otherwife; that the fenugreek feed, fleawort feed, cummin feed, coloquintida, cocculus indus, buckthorn berries, agaric, nitre, fal ammoniac, fal gem, litharge, antimony, black-lead, orpiment, corrofive fublimate, white arfenic, realgar, feveral of which are added again and again in different parts of the operation, are entirely ineffential to the dye, and contribute rather to do harm than good. Mr. Macquer himfelf fufpecls that fome of thefe ingredients are unneceffary; and he has fubjoined a procefs followed in the manufactures of Tours and Genes, from which we may fairly conclude that they are all fo; and that a fine black may be dyed on filk in as iimple a method as on wool or woollen cloth, the filk re- quiring only a greater quantity of the ingredients, and a greater number of dippings in the black liquor. The procefs is as follows.
The filk, wafhed with foap as above dire&ed, is fteeped in a decoction of one third its weight of Aleppo or blue galls, or half its weight of the weaker white galls of Sicily and Romania, and afterwards warned with water: every twelve ounces are reduced by the cleanfing to nine, which ought to be increafed by the galling to eleven and not more. The dying liquor, for a hundred pounds of filk, is prepared by boiling twenty pounds of galls in a fufficient quantity of water (about a hundred and twenty fix gallons) and adding to this decoction, after being fettled and drawn off from the fediment, two pounds and a half of Englifh vitriol, twelve pounds of iron filings, and twenty pounds of the gum of the cherry or plum tree : that the gum may difiolve the more readily, it is put into a large copper cullender, immerfed in the hot liquor, and ftirred and worked from time to time with a
wooden
[426 ] wooden rod till it is all palled through. This mixture is kept for fix or feven days or more, a circumftance fuppofed to be neceflary to its perfection; and being then made as hot as the hand can bear, frefh parcels of the galled filk are dipt in it fucceffively, and kept in about ten minutes each ; and all of them, after being aired, are dipt over again, feveral times, with the addition of more vitriol and iron filings, till they have acquired the requifite blacknefs, after which they are well warned in water. It may be obferved that while rive or fix pounds of galls are furficient for a hundred pounds of wool, upwards of fifty pounds of galls are here allowed to the fame quantity of filk ; and that logwood, an eflential ingredient in the black dye on white woollen, is not at all required for filk. The quantity of vitriol is not fpe- cified.
I tried this procefs in fmall, with the exact proportions of each of the articles above fet down ; and by adding more and more of the vitriol, and repeating the dippings thirty times or more, I obtained at laft a good black. After lefs than half this number of dippings, the filk ap- peared of a beautiful black when taken out of the liquor, but by warning it became pale, and in drying it turned always paler. The quantity of vitriol ufed in all was about eight times that prefcribed above to be added at one time, or one fifth of the weight of the filk ; but the iron filings put in at firft remaining undiifolved, it was not thought needful to add any more of this ingredient. I repeated the operation without any iron filings, and could not obferve that the two blacks differed from one another. I tried it alfo without the gum : there was here a very confiderable difference in the filk as taken out of the dye, that which had been dyed with gum have- ing a fine glofiinefs, which the other wanted : the fub-
fequent
t 427 ]
fequent warning, however, deftroyed, as was expected, the glofiinefs of the gummed filk, and reduced them both to the fame appearance, fo that the gum feemed to be of no manner of advantage : perhaps it is rather of dif- fervice than otherwife, by thickening the liquor, and making it more difficultly penetrate into the filk, in the fame manner as it renders ink indifpofed to link into paper. I likewife dyed fome filk by the two proceffes defcribed in the foregoing feftion for woollen cloth, (page 410 and 41 1) and obtained by both of them a rufty black upon white filk, and a very good black upon blue : fo deep a blue as is allowed for the true black on fine woollen cloth, did not appear neceflary for filk ; a very flight blue ground being here fufficient to make the black both deep and durable.
It fhould feem therefore that filk is not, in any parti- cular manner, more averfe than wool to the receiving of the black dye •, and that a good black may be dyed on filk, with the fame materials, in the fame method, and with the fame difpatch, as on wool and woollen cloth ; of which a further confirmation will appear at the end of the following fedlion. It may be obferved, that though filk takes a fufficiently good black dye from the method pradlifed for fine woollens, yet woollen does not take a black from the procefs that has been appropriated to filk ; for fome pieces of white flannel having been put in along with the white filk in one of the trials of the French procefs above defcribed, the flannels became only brown, while the filk turned out black. Though a black may be dyed on white filk without logwood or verdegris, the firft of which is a neceflary material for white woollen ; yet an addition of both contributes not a little to improve the colour on one as well as on the other.
Kkk SECT.
[4*8] SECT. IX.
The dying of hats black.
TH E Inftruftions of Mr. Colbert dired hats to be ftrft ftrongly galled, by boiling them a long time in a decottion of galls with a little logwood, that the dye may penetrate the better into their fubftance -, after which a proper quantity of vitriol and decoction of log- wood, with a little verdegris, are added, and the hats continued in this mixture alfo for a confiderable time. They are afterwards to be infufed in a frefh liquor of logwood, galls, vitriol, and verdegris ; and where the hats are of great price, or of a hair which difficultly takes the dye, the fame procefs is to be repeated a third time. For obtaining a colour of the utmoft perfection, the hair or wool is ordered to be dyed blue previously to its being formed into hats. The prefent practice is more com- pendious, and affords, as we may daily fee, a very good black. The method of our hatters, as I have been in- formed, does not differ materially from that of the French, defcribed in the encyclopedie, which is as follows. An hundred pounds of logwood, twelve pounds of gum, and fix pounds of galls, are boiled in a proper quantity of water, for fome hours ; after which, about fix pounds of verdegris and ten of green vitriol are added, and the liquor kept jufl fimmering, or of a heat a little below boiling. Ten or twelve dozen of hats are immediately put in, each on its block, and kept down by crofs-bars for about an hour and a half : they are then taken out and aired, and the fame number of others put in their room. The two fets of hats are thus dipt and aired alternately, eight times each ; the liquor being refrefhed each time
with
[ 429 3 with more of the ingredients but in lefs quantity than at
firft.
This procefs affords a very good black on woollen and (ilk fluffs as well as on hats, as we may fee in the fmall pieces of both kinds which are fometimes dyed by the hatters. The workmen lay great flrefs upon the verde- gris, and affirm that they cannot dye a hat black without it : it were to be wifhed that the ufe of this ingredient was more common in the other branches of the black dye ; for the hatters dye, both on lilk and woollen, is reckoned a finer black, than what is commonly produced by the woollen or the lilk dyer.
SECT. X.
Of the dying of linen and cotton black.
T~^ H E black vitriolic dye, though very durable on the fubftances hitherto mentioned, is perifhable on linen and cotton. Pieces of linen and cotton cloth, and fkains of thread, boiled firft with galls, and afterwards infufed and dipt repeatedly in a decoftion of logwood with vitriol, received a good black colour ; but both the brownifh ftain which the galls communicate, and the blacknefs iuperinduced by the vitriol, were in great mea- fure difcharged by wafhing with loap ; even the rufty colour, which the vitriol of iron gives by itfelf, feeming, . in this way of application, to be lefs fixed than if it had been employed without the galls. Steeping the linen for a month, previous to the dye, with galls, and with oak bark, by which method fifhing nets receive from the aftringents a pretty durable ftain, was here of no fervice, the black dye proving equally perifhable.
The dyers of thread follow a procefs fomewhat different from the above. They firft fteep the thread in alum
K k k 2 water
[ 430 ] water for feveral days ; and then dip it repeatedly in the dying liquor, cold, or only lukewarm. The dying liquor confifts of the irony and aftringent matters mixed to- gether ; and in the room of, or along with, vitriol, they life either filings of iron, or the muddy matter by fome called flipp, found in the troughs of grindftones where iron tools are ground. The woollen dyers are lbmetimes required to dye certain pieces of linen black, and in fuch cafes they practife a method of the fame kind ; fteeping the piece firfl in alum water for two or three days, and then dying it in their mixed black liquor. By this means the colour is made to hold fomewhat better ; but how perilhable it ftill is, we may fee in all black thread.
As the ftain produced by folutions of iron is very fixed on linen and cotton -, and as the perifhablenefs of the black dye feemed to be owing to the aftringent matter of the galls not fufticiently penetrating or uniting with the vegetable fibre, and therefore too eafily coming off, and carrying the iuperinduced vitriol with it ; I boiled pieces of linen and cotton, firft in folution of vitriol, and after- wards with galls, hoping that the vitriol, fixing itfelf firft in the cloth, would make the aftringent matter ap- plied upon it likewife fixed. But the event was other- wile : the colour did not prove lb black as when the contrary method of application was followed, and the blacknefs was rather more deftruclible.
The colour of indigo and madder being very durable on linen, it was hoped that a ground of thtle might con- tribute to fix the black. I therefore made trial of lundry pieces of red and blue linen, dying them black by the methods already defcribed. They appeared to have no advantage above thofe which had been dyed directly from white : the black was as eafily wafhed out, the blue
pieces
t43« ] pieces remaining nearly of their original colour, and tin.
red ones a little darker coloured than at firft.
After many other fruitlefs attempts, with different fo- lutions of iron and different intermedia, no probability of fuccefs appeared to remain, unlefs the vegetable fub- ject could be changed as it were in its nature, or im- pregnated with an animal principle. Accordingly I boiled linen and cotton, previous to the galling, with weak fo- lutions of animal glues, but the fuccefs was no better than before.
In the fourth volume, lately publifhed, of the Memoirs of the correfpondents of the French Academy of Sciences, M. l'Abbe Mazeas gives a curious differtation on the red printed cottons of the Eaft-Indies ; in which he defcribes a method, practifed by the Indians, of impregnating their cotton with animal matter in order to its receiving a red flain. A ley is made from the afhes of a certain kind of wood, and with this is mixed fome fheeps dung and a quantity of the oil of fefamum, in want of which oil, they ufe hogs lard : thefe ingredients ftirred together, are laid to unite into a milky liquid. The cotton is fteeped in this liquor during the night, and expofed to the hotteft fun during the day for a fortnight. The author above-mentioned fays he tried this procefs with the com- mon exprelfed oils, without fuccefs ; but that with hogs lard it fucceeded perfectly-.
On reading the Abbe Mazeas's paper, I immediately let about trying, what effect a like preparation would have in regard to the black dye. Here a confiderable difficulty occurred in making the mixture ; for with a ilrong ley of wood afhes, or with a folution of purified alcaline fait, the lard could not be made at all to unite by ftirring, or even by boiling ; the liquor acquired no milkinefs, and the lard floated diflinct on the furface ?
and
[ 432 ] and indeed it was not expected, that a perfect union of
this ingredient could be procured without the ufe of the cauftic ley of the foap-boilers prepared with quicklime. The intention, however, being only to obtain a foap made with animal fat, and the common foft foap being fuch a one ; I mixed foft foap and fheeps dung well to- gether, three parts of the former to two of the latter, and diluted the mixture with warm water. Some pieces of linen and cotton cloths, and fome fkains of linen thread, were fteeped in this liquid every night, and hung out in the day-time, not indeed in a hot fun, but in all that the month of december laff. afforded. The fubjects were then all dyed black, by the fecond of the proceffes de- fcnbed for woollen cloth, page 41 1 ; and fome of the fame kind unprepared, were put into the dye along with them. All the pieces being taken out and warned, the prepared ones appeared to hold their colour better than the unprepared, though not in fuch a degree as to make the procefs interefting to the workman. From this fhew of fuccefs however, in an unfavourable feafon, the ex- periment feems worthy of being tried again in more ad- vantageous circumftances.
We have feen in the fecond fection, that linen and cotton are ftained of a lafting black colour by certain vegetable juices; and that thefe juices might probably be obtained in quantity, if not in our own country, yet in certain parts of the Britifh dominions, fome of the trees which afford them being natives of our American fettlements. Till this branch of vegetable curation fhall be eftabliihed, the Britifh artift can receive little benefit from knowing the materials, with which the deep black (lain on the Indian cottons is faid to be fixed.
We have {een alfo, in page 420, that a black colour, or a colour approaching to blacknefs, may be produced
on
[ 433 ] $n woollen cloth, from a combination of two other dyes, viz. by applying a full madder red upon a deep blue ground. Both the blue and the madder red can be fixed upon linen as well as on woollen ; and accordingly I tried compounding them on linen in different ways, fome- times applying the red upon the blue, and fometimes the blue upon the red. In feveral of thefe experiments the linen, as it came out of the dye, appeared of a good black colour, but on warning it, fo much of the colour was difcharged, that only a kind of dark purplifh remained.
Some printed linens and cottons have a durable black ftain, which, as I am allured by a fkilful and ingenious artift, is made with madder and a folution of iron. A quantity of iron is put into four ftrong beer; and to pro- mote the diffolution of the metal, the whole is occafionally well fUrred, the liquor at times drawn off, the ruft beaten off from the iron, and the liquor poured on again: a length of time is required for making the impregnation perfect, the folution being reckoned unfit for ufe till it has flood at leafl a twelvemonth. This folution ftains linen yellow, and of different lhades of buff colour, and is the only known material by which thefe colours can be fixed on linen. The cloth, ftained deep with the iron liquor, being afterwards boiled with madder, without any other addition, becomes of the dark colour which we fee on printed linens and cottons, which, if not a perfect black, has a very near refemblance to it. It is fubiritted to the confederation of thofe whom it may concern, whether this fixt colour would not be preferable, on linen thread, to the perifhable black with which thread has hitherto been dyed. It is probable, that even a better black might thus be dyed on thread, than that which the printer on linen produces: for in this laft bufinefs, while fome parts of the linen are ftained deep with the iron liquor, in order to their
being
[ 434 ] teing made black; others are flamed paler, with the fame liquor diluted with water, for making purple; and others, defigned to be red, are prepared with a folution of alum and fugar of lead : all thefe colours are dyed in one and the fame copper of madder, with a heat a little below boil- ing : a boiling heat would give a dark tawney or blackifh hue to the red, and therefore in this procefs muft neceflarily be avoided; but for the fame reafon it would contribute to deepen the black, and therefore ought always to be called in aid where thread, or entire pieces of linen or cotton, are to be dyed of this colour.
SECT. XI.
The Jlaining of Wood, Ivory, Stones, &c. black..
I. Wood:
TH E ftaining of wood black, for pidlure frames, &c« depends on the fame principle with the black dye in the foregoing fedtions. For a deep black, the wood is brufhed over four or five times with a warm decoction of logwood, and afterwards as often with a deco£tion of galls, being fuffered to dry thoroughly between the feveral applications of the liquors : thus prepared, it receives a fine deep black colour, from being warned over with fo- lution of vitriol ; in the room of which, fome ufe a folu- tion of iron in vinegar, keeping the vinegar for this purpofe upon a quantity of the filings of the metal, and pouring off a little as it is wanted. A pretty good black is obtained alio, more expeditiouily, by brufhing over the wood, firfi; with the logwood liquor, and afterwards with common ink.
Plumier, in his Art de tournir, directs the wood to be previoufly wafhed twice with the fecond parting water of
the
[ 435 I
the refiners (aqua fecunda jorti Jcparatorla, eau forte feconde) by which I fuppofe he means, not aquafortis itfelf, but the folution of copper in aquafortis remaining after the filver has been precipitated. Waihing with aquafortis was found to prevent the production of any black colour on the application of vitriol and aflringents, as indeed was expected, this acid liquor deflroying the colour of ink already made : a faturated folution of copper in aquafortis appeared*to be of no immediate injury, but it appeared alfo to be of no advantage.
II. Ivory, bone, horn, &c.
Ivory, bone, horn, and other folid parts of animals, may be flamed black in the fame manner as wood. They likewife receive a deep black flain from folution of filver," which fliould be diluted with water to fuch a degree, as not fenfibly to corrode the fubjecl:, and applied two or three time*, if neceffary, at confiderable intervals, the matter being expofed as much as poflible to the fun, to h alien the appearance and deepening of the colour: fee page 3 50. Hair alfo, made perfecllv clean, and moiflened with the fame folution, is changed from a red, grey, or other difagreeafole colours, to a brown or deep black: the ids commonly fold under the name of hair- waters are at bottom no more than folutions of iilver, diluted largely with water, with the addition perhaps of other ingredients, which contribute nothing to their ethcacy. The folution fhould be fully faturated with the filver, that there may be no more acid in it than is neceflary for holding the metal diflblved; and belides dilution with water, it will be proper to add a little rectified fpirit of wine for the further dulcification of the acid. It mull be obferved, that for diluting the folution, diililled water or pure rain water mull always be ufed; the common fpring waters turning
Lll it
[ 436 ] it milky, and precipitating a part of the diflblved filver It is to be obferved alfo, that if the liquor touches the fkin, it has the lame effect thereon as on the matter to be flamed, changing the part moiftened with it to an indeli- ble black.
III. Marble.
It is difficult to introduce into marble a true black colour. Solution of lilver finks 'deep into the ftone, lbmetimes an inch or more ; but the colour it communi- cates, at firif reddiih or purpliih, deepens only to a brown. Mr. du Fay, in the Memoirs of the French Academy for the years 1728 and 1732, gives two methods of ftaining marble of a blue colour, approaching more or lefs to black according to its deepnefs, and not ill refembling thofe which are naturally found in fome marbles : one is with effential oil of thyme digefted in volatile fpirit of fal ammoniac, the other with tincture of archel. When the oil of thyme is digefted with the volatile fpirit, it be- comes firft yellow, then red, then violet, and at . laft of a deep blue. In fix weeks digeftion it had acquired a pale- blue, and in this ftate gave little colour to marble : after (landing for fix months, it was deepened almoft to a black blue, and being now applied on warm marble, gave the ftain deiired.
With regard to archel, a tincture of it in water is ap- plied on cold marble, and renewed as it evaporates, till, the colour is fufficiently deep. Though the colour of archel is very perifhable on cloth, yet in marble it appears to be more durable. Mr. du Fay fays he faw pieces of marble ftained with it, which in two years were not fen- iibly changed. The colour however, though made very deep, is far from being a true black, being rather a dark purplifh blue,
The
[ 437 1
The porous marbles, which admit water to fink into them, I have ftained of a full black colour with common ink ; either by applying on the warm marble an ink already made, or by the alternate application of aftringent liquors and folutions of iron. With the more compacl marbles, this did not fucceed, though they were heated (o far as to make the liquors boil upon them : in fome parts the colouring matter fcarcely penetrated at all ; and where it did fink a little into the ftone, it was lb dilute as to appear only purplifh. The fpirituous tinctures, defcribed in page 388, made without the maftich, feemed to pene- trate better than the watery infufions.
On marbles which would not receive the inky matter, I tried the alternate application of folutions of lead and of fulphureous folutions, applying fometimes the one firft, and fometimes the other; but could not find that they produced in the ftone any degree of the black or dark colour which they do on paper. By folution of copper, managed as at the end of the following article, and by a folution of the metallic part of cobalt in aqua regia, em- ployed in the fame manner, the moft compacl; pieces were ftained black; though this procefs requires too great a heat to be practifed on marble without danger of injuring the llone. The colour which folutions of gold commu- nicate to marble, in its deep lhades obtained by repeated applications of the folution, approaches very near to black.
IV. Agate* &c.
Several of the hard ftones, which ftrike fire with lleel, receive a dark ftain inclining to black from folution of lilver. Mr. du Fay relates, in the French Memoirs for 1728, that to chalcedony, this folution gave a reddilh brown colour ; to oriental agate, a blacker ftain ; to an agate fpotted yellow, a purple ; to the jade ftone, a pale
L 1 1 2 brown ;
[ 438 ] brown ; to the common emerald, an opake black j to the white parts of the common granite, a violet unequally deep,- to Terpentine ftone, an olive colour ; while the much fofter flates, talks, and amianthus received from it no colour at all. The experiments formerly mentioned, page 350, afford room to fufpedfc, that the folution of filver ftains ftones only in virtue of their containing a calcareous earth, or fuch an earth as the acid is capable of diffolving : if this be the cafe, there is little wonder, that fome of the hard ltones fhould be ftained, and fome of the foft unaf- fected by it.
Among the hard (tones that have been tried, the agates feem to be thofe which are acted upon moft readily :. (hey are thofe alfo which have ofteneft been attempted- to be ftained. The folution mould be made in ftrong' aquafortis or fpirit of nitre, and fully fatiated with the metal. The ilone, after the fluid is- applied, fliould be expofed to the fun for two days or more j and if, when dry, it be removed into a moift place, and afterwards ex- pofed again to the fun, the production of the colour will be the more fpeedy. After the (tone has acquired the full colour which the firft quantity of the folution can communicate, it may be moiftened with more, and this repeated two or three times, by which the colour will be deepened, and made to penetrate further : Mr. du Fay found that an agate about a iixth part of an inch in thick- nefs, by applying the folution on both fides, may be Itained throughout its whole fubftance. The tincture, however, is rarely uniform, on thefe or other ftones; moft of them having veins, which, though indifcernable in the natural ftone, are in this procefs made apparent, being more eafily or more difficultly penetrable than the reft of. the mafs, and fometimes forming not inelegant varieties: in the itained ftone.
Mr*
[ 439 ] Mr. du Fay obferves, that though ftones may without much difficulty be ftained by folution of filver, yet it is fcarcely poffible to form very neat defigns on them, on account of the fpreading of the liquid ; and that this im- perfection appears to be the lefs, according as the folution is the more faturated, fo as to dry or cryftallize the more fpeedily. An eafy method of obviating this inconvenience is fuggefted by the practice of the engraver j for the means, by which he confines the aquafortis on his copper plates to the minuteft ftrokes, would doubtlefs anfwer the fame intention here. The furface of the ftone being coated with a proper tenacious fubftance which the acid cannot ait upon, as the compofition called etching wax, which confifts of refinous fubftances melted with wax or boiled with oil to a due confidence, and the drawing being made on this ground, fo that each ftroke may reach down to the ftone, it may be prefumed that the folution of filver, afterwards applied, will nowhere fpread further than the parts thus laid bare..
The Hones thus coloured by art differ from the natural in two remarkable properties of the colouring matter. The natural colours refift moderate heat, by which the artificial are in great part deftroyed. The natural ftones, fteeped for feveral hours in aquafortis, fuffer no apparent change ; whilft thofe, which have been coloured by art, almoft entirely lofe their colour. It is obfervable that the colour deftroyed by aquafortis is reftored again by ex- pofing the ftone to the fun : but that the colour deftroyed by fire cannot be recovered without a freih application of the. colouring folution.
There is another method of ftaining ftones, of a colour more truly black than that which the folution of filver communicates to moft of them, and with this further difference, that the colour being produced by fire, I have
not
[ 44° ] not found that either moderate fire or aquafortis will deftroy it. Pieces of different {tones, marbles, pebbles, flint, &c. were warned over with a faturated folution of copper made in aquafortis : when dry they were put into a crucible, and kept for a little time in a fire juft fuf- ficient to make the veffel almoft red hot. All of them were ftained, in the parts which had been moiftened with the folution, of a black colour, durable and pretty deep, though it had penetrated only a very little way into the fubftance of the flones.
When the fmooth furface of an agate, or other itones not diflblvible in aquafortis, is moiftened with the copper folution ; if a fmall iron nail be fet upright on its head in the middle, the iron abforbs the acid from the copper, and the copper, now feparating from the fluid, moots into fine ramifications like the branches of trees or fhrubs, generally of a very elegant appearance. If the nail be then removed, and the corroded iron carefully warned off by dipping the flone in water, the vegetations may be changed by heat to the fame black colour as the fimple folution of copper in the foregoing experiments, fo as greatly to refemble the figures naturally found in certain ilenes, as that called the Mocho flone. The colour is not indeed fixed on the ftone, like that refulting from the folution of copper alone ; but a plate of cryftal laid over it in the manner of a doublet, conceals this imperfection . The only difficulty in this operation confifis in the warn- ing, in which great dexterity is requifite, to feparate the corroded iron, which would give a rufty ftain, without warning off or difordering the fine vegetations of the copper.
SECT-
T
[ 44» ]
SECT. XII.
Black glafs and enamel.
HERE is a fpecies of blacknefs, as we have for- merly feen, which refults, in certain circumftances, from the limple deepnefs or concentration of other colours. Thus many vegetable juices and infufions, yellow, reddifh, blue, &c. on being evaporated to the thick confidence of an extract, look black ; and thefe black maffes, when fpread thin or diluted with water, exhibit again the ori- ginal colours of the liquors. Something of the fame kind leems to happen in glafs and enamel. Smalt or zaffre, which in a certain proportion give a blue colour to vitre- ous bodies, if employed in a larger quantity make them black. Manganefe, which in a little quantity gives a purpliih tinge, in a large one gives a black. Preparations of iron, whofe colour in glafs, in a dilute ftate, is fome- times yellow and fometimes greenifh or bluiih, are always of a dark brown or black when the glafs is over-dofed with them : hence many of the ferrugineous earths and {tones melt into a black glafs, as the coloured clays, feveral flates, and the {tone called whynn Hone, with which fome of the ftreets of London have been lately paved. Black glaffes or enamels made on this principle have however, like the concentrated vegetable liquids, one imperfection ; that though of a deep black colour when in malfes of any confiderable thicknefs, yet when fpread thin they always betray fome of the original colour, or of the particular hue which they would have if the colouring matter was in lefs quantity. The mofr. perfect black is obtained by adding a mixture of two or more of the above darkening materials : inftead of taking colourlefs glafs or enamel for the bafis, it will be of advantage to ufe fragments of dif- ferent
[ 442 ] ferent coloured pieces; and compofitions which have been fpoilt, in trying to tinge them of other colours, anfwer as well for this purpofe as any.
The common black glafs, of which beads are made for necklaces, &c. is coloured, as I am informed, with man- ganefe only ; hence when powdered it looks of a dirty purple colour. The manganefe perhaps increafes the fu- iibility of the glafs, for an ingenious friend obferves, that in making impreffions in different kinds of glafs, he has found this black fort to be by far the moft fufible of any. That there is a ftrong action between the manganefe and the glafs may be prefumed from the great effervefcence which happens on melting them together. One part of manganefe is fufficient to give a black colour to near twenty of glafs.
The enamellers require a black more perfect than that which manganefe alone can produce, and employ, as I am informed by an experienced artift, a mixture of man- ganefe, zaffre, and fcales of iron. Thefe ingredients may be mixed together in equal quantities, and one part of the mixture added to fifteen or twenty of the bafis of enamels; which bafis is prepared by calcining a mixture of about equal parts of lead and tin, and melting this calx with equal its quantity of fritt or powdered glafs.
VII.
[ 443 ]
VII. History of PLATINA.
IN the beginning of the year 1749, there was brought into England, from Jamaica, a quantity of a white metallic fubftance in grains, lcarcely known before to Europe, faid to be the produce of the Spanifh Weft- Indies, and there called Platina, Platina di Pinto, or del Pinto, and 'Juan bianco.
The name Platina feems to be a diminutive of plat a, filver, and confequently to exprefs the moil obvious ap- pearance of this body, that of a filver-coloured metal in fmall grains. From its being called platina of Ptnto, it may be fuppofed that Pinto is the name of fome particular ipot or diftricl: which affords it : I have not met with this name in any accounts I have feen of Spanifh America, but Mr. Cronftedt, in an eflay for a new mineral Syftem, lately publiihed in Sweden, fpeaking of platina in the courfe of his fyftem, calls the place it is brought from Rio di Pinto. Its other appellation, Juan bianco, aroie perhaps from fome frauds which had been pradtifed with it, from the difficulty of feparating the gold naturally intermingled with it, or from its refraftorinefs in the hands of the workman ; for as in our own country a duilty coloured mock-ore, that is, a mineral which has the appearance of a metallic ore, but does not in the uiual ways of trial yield any metal, is commonly called bhick-jack ; the Spaniards may in like manner have given the name white jack, 10 hit e rogue, white mock metal, to this lingular metallic body, which though of the true metallic afpecf and weight, and in fome degree malleable, had eluded all their attempts for fmelting or running it down.
M m m Mr.
[ 444 ]
Mr. Charles Wood, aflay-mafter in Jamaica, had feen fome platina in that ifland eight or nine years before it was imported here. He fays it was brought thither from Carthagena ; that the Spaniards have a way of cafting it into different kinds of toys; that thefe toys are very common in the Spanifh Weft-Indies j that fome pounds of the metal were bought at Carthagena for lefs than an equal weight of filver, and that it was formerly fold at a much lower price. He gave fome fpecimens of it to Dr. Brownrigg, who in 1750 prefented them to the Royal. Society.
The feeming inconfiftency between this account and the foregoing, in regard to the fufibility of platina, was eafily reconciled by examining Mr. Woods fpecimens.. Some of them were of the true platina in grains, called native or mineral platina, which we have very good grounds to believe the Spaniards have never been able to melt. But there was one of an actual caft metal, a piece of the pummel of a fword. A part of this was fent to me for trial ; and I was afterwards favoured with a large piece of an ingot of the fame kind of metal, by the right honourable the earl of Macclesfield, the late worthy prefident of the Royal Society. This metal was found, to melt with great eafe, and was apparently not true platina, but a compofition of it with fome other metallic bodies. As the compound metal has been frequently confounded with the platina itfelf, and called by the fame name, fome confiderable errors have hence arifen in regard to the properties of the platina, which will be occafionally taken notice of in the courfe of our experiments. It is fufficient here to have obferved, that the caft metal differs materially from the true platina which makes the object of the prefent hiflory.
The
[445 J The platina foon engaged the attention both of phi- losophers and metallurgies, on account of its agreement, in fome remarkable particulars, with gold. From this relation to gold, it was called by fome white gold. Hence alfo many people were induced to think, that it was at bottom no other than gold, difguifed by a coat of fome extraneous matter ; and it was hoped that means might be difcovered of diverting it of this coat, and laying bare the gold which it was fuppofed to conceal. But the more it was examined, the more did this notion feem impro- bable ; and the more grounds were found for believing, that platina is a metal of a peculiar kind, diflincT: in na- ture from gold, as well as from the other metals, though endowed with fuch properties, as had hitherto been fup- pofed to be characteristic of gold, or to be pofTeffed by gold alone ; infomuch that this new metal was reported to have been fometimes fraudulently mixed with gold, in confiderable quantity, without being either feparable, or distinguishable, by any of the common methods in which gold is aSTayed or refined.
A full examination of fuch a body appeared of the utmoft importance, as regarding not only the difcovery of the various properties of the platina itfelf, an object fuf- ficiently interefling, but likewife, what is much more fo, the preventing the abufes which it was liable to give occafion to, and the fecuring the finenefs and value of the precious metal ; fo that if the platina fhould not be rendered an ufeful commodity, it might at leaft be no longer a dangerous one.
I had begun this examination in the year 1749, but could not then procure enough of the platina for carrying the experiments to fuch a length as I aimed at; for a metal fo extraordinary, entirely new, at leaft to this part of the world, of which only a few general properties were
Mmm 2 known,
[ 446 ] known, and thefe but partially and imperfe&ly, deferved to be fubmitted to all the kinds of operations that are pradtifed on the other metals, and to all the agents by which other metals are found to be affected. In the be- ginning of the year 1754, his excellency general Wall,, at that time ambafiador from Spain, enabled me to pro- ceed in the experiments, by fending me about an hundred ounces; and I was afterwards favoured with confiderable quantities more by fome other gentlemen. The moil ingenious and experienced chemifts in Europe, as foon as they could obtain any of the new metal,, entered into the fame purluits; and (Several of thefe enquiries have from, time to time been made publick.
The firft publication I have feen on this fubject is that of Mr. Wood, in the 44th volume of the Philofophical Tranfactions, for the years 1749 and 1750. To the hiftorical obfervations, of which an abftradt has been given above, Mr. Wood fubjoins a few experiments, made partly, as may be prefumed from their event, on the true platina in grains, and partly on the caft metal ; one of which experiments, the cupellation of the caft metal with lead, was afterwards repeated, more circumfpeclly, by Dr. Brownrigg.
In the 48th volume of the Tranfactions, part 2d, for the year 1754* is inferted an account of the principal experiments which had been then made on the platina by me. They are divided into four papers, which are followed in the next volume by two papers more.
On the publication of the firft four, I' was informed that Mr. Scheffer alfo had given an examination of this metal in the Handlingar of the Swedifh academy of. fciences for the year 1752. Thofe books being difficultly procurable in this country, and written in a language which I did not underftand, it was fome time before I
could
[ 447 ] could avail myfelf of his enquiries, which I found to be curious and interefting, and carried, though not fo far as could be wifhed, yet much further than could have been expected, considering that for his principal experiments he had only a hundred grains of the crude mineral, from which he could pick out but forty grainj of the platina to work upon, and that he had no previous notice of its porTerling any remarkable properties, but looked upon it at firft as being only an iron mineral ; he afterwards in- deed obtained feme more, but it was only fuch another Uttle quantity. Thefe experiments were made by the encouragement of Mr. afleflbr Rudenfchceld, who has lately informed me,, in a letter from Stockholm, that he brought the platina from Spain in the year 1745, nearly four years before it was known in England. In one of the following volumes of the Swediih Handlingar, there is another paper by the fame gentleman, containing ob- fervations on fome parts of mine, concerning the fpecific gravities of mixtures of platina with other metallic bodies.
A French tranflation of all the papers above-mentioned,, except the laft of Mr Scheffers and the two laft of mine, which had not come to the tranflators knowledge, was published at Paris in 1758, under the title of la platine, I'or blanc, ou I'buitie/ne metal : to this treatife is added an extract of a letter from Venice, relating to what may be called the alchemical hiftory of platina, not containing any new facts, but fome reflections drawn from mine.
Profeflbr Marggraf, of the academy of fciences at Ber- lin, having obtained a quantity of platina from London, made a large fet of experiments upon it, repeating and further profecuting feveral of mine, and adding many new ones. Thefe appeared firft in a French tranflation, among the Memoir es of the Berlin academy for the year 1757, printed in 1759 : they have lince been published,,
more
[ 44« ] more correctly, in the original German, in the firft volume of a collection of his chemical writings, the continuation of which is earnestly wifhed for.
In the Memoir es of the academy of Paris for 1758, printed in 1763, there is a paper on this metal by Mr. Macquer and Mr. Baume conjointly ; who, befides re- peating and varying fome of my experiments, and draw- ing from them fome new confequences, have expofed the platina to an agent which the other enquirers have not had opportunities of doing, a large burning concave. Their platina, in quantity a pound, was fent to them from Madrid.
The foregoing are the only writers I know of, who have treated exprefsly and experimentally on platina. Some others have mentioned it occafionally, as particu- larly Mr. Cronftedt and Mr. Vogel, in their new mineral fyftems. The former has in general given a very juft account of it ; but the latter appears to me to be a little miftaken in fome points, which will be further taken notice of in their places.
Since the publication of my experiments in the Tranf- actions, I have at times been adding others, and en- deavouring to afcertain fome properties of platina which before had been too {lightly examined. Nothing now is fo much wanted, in regard to this extraordinary metal, as a regular hiftory of what has already been done, or a connected view of the experiments that have been made upon it. Such a hiftory I fhall here attempt, quoting every where the authors of fuch facts as are not taken from my own diaries, and, where any doubts arife on comparing the different accounts, making new trials.
SECT.
[ 449 ]
SECT. I.
Of the general properties of platina confidered by it f elf, or independent of its difpo/ition to unite or not unite with other bodies.
I. Defcription of Platina.
PLATINA in grains, as brought into England, is of a mining whitifh colour, fomewhat approaching to that of filver, but lefs white : from this refemblance, which becomes much greater when the platina has pafTed through- certain operations, it probably, as already taken notice, received its name. Mr. Macquer refembles its colour to that of coarfe iron filings unrufted, but all I have feen was a good deal whiter than any iron filings : this difference from iron is mentioned alfo exprefsly by Mr. Scheffer, for while he had no fufpicion of the platina being a new diftindf. metal, he fays it feemed to be iron which by fome accident had been made externally white, Mr. Marggraf calls the colour white inclining a little to that of lead.
The colour of platina is not tarnifhed or altered, fo far as I have obferved, by air or moifture, or by any exha- lations that are commonly diffufed through the atmo- fphere : it refifts vapours which difcolour filver, and ap- pears equally permanent with that of pure gold.
The grains are of various fizes : fome few are as large as linfeed, but moft. of them a good deal imaller. Their figure alfo is various and irregular : fome approach to a triangular, others rather to a circular form : moff. of them are flat, none globular, and few of any great convexity : the furface is fmooth, with the edges and angles generally rounded off, On viewing them with a microfcope, the
furface
[ 45° ] furface appeared in fome parts uneven : the prominencies looked bright and polifhed ; the cavities dark coloured and roughifh, as if they were fprinkled with a powdery matter. A few of the grains were attracted, though very weakly, by a magnetic bar.
II. Subflances mixed ivith the native platina.
With the grains of platina, above defcribed, feveral heterogeneous matters are intermingled ; fome of which are in fmall particles or duft, feparable by a fine fieve ; others larger, fo as to be diftinguifhed by the eye and picked out. Thefe fubftances, in the different parcels of platina which I examined, were the following.
i. A considerable quantity of blackifli duft, which ap- peared to confift of two difiimilar fubftances; a part of it being attracted vigoroufly by a magnetic bar, and a part not attracted at all. The part attracted is of a deep fparkling black colour, much refembling the black fand from Virginia : the reft is of a brownifh hue, and has feveral bright particles intermixed, which appear to be fragments of the grains of platina itfelf. It is probable that the roughnefs and dark colour of the cavities of the grains of platina, and the magnetic quality of fome of the grains, proceed from fome portion of thefe extraneous powders adhering in them.
2. Among the larger grains of platina, feparatcd by means of a coarfe fieve, were obferved fundry irregul.ir dark-coloured particles, fome blackifli, others with a caft of brownifh-red, in appearance refembling fragments of emery or loadftone. Some of thefe were attracted by the magnet, very weakly, and others not at all. The un- magnetic duft of the preceding paragraph feems to be only fmaller fragments of this laft kind of matter.
3. There
[ 45* 3
3- There were fome rough yellow particles, very mal- leable, which appeared to be gold, though not free from a mixture of platina. A further examination of thefe golden particles will be given hereafter. Their quantity differs in different parcels of the mineral : twelve ounces of the richeft that has come to my hands being diligently picked, with the afliftance of a magnifying glafs, the grains partly or entirely yellow amounted to about two pennyweights, or one part on a hundred and twenty of the mixt.
4. A few globules of quickfilver containing gold, with fome particles of platina intermixed and pretty ftxongly adhering. Mr. Marggraf likewife obferved fome quick- filver among the platina which he examined, having been induced to look for it with attention, by finding, that when an ounce of platina had been urged with a flrong fire in a glafs retort, a little true running mercury came over into the receiver. The yielding of quickfilver and containing fome magnetic parts, the former of which is particularly mentioned in the firfi: of my papers in the Philofophical Tranfactions, and the latter not only there, but by all thofe I know of who have given any experimental account » if platina, are ranked byVogel among the new properties of this mineral difcovered by Marggraf.
5. Some fine colourlefs tranfparent particles, which were hard to break under the hammer, and were not fenfibly acted upon by aquafortis. Thefe are probably fragments of the hard kind of flone, which frequently inverts ores in mines, and in which native gold is oftcneft found lodged, called by the Germans quartz, but which has not, that I know of, received any diftinctive Englifh name.
6. A very few irregular particles of a jet black colour. Thefe broke very eafily, and looked like the finer forts of
N n n pitcoal :
[ 45* ] pitcoal : laid on a red hot iron, they emitted a yellowilh £inoke, and fmclt like burning coal.
The foregoing obfervations afford fome room to fufpect, that this mineral has not come to us in its native form, but has probably been ground in mills, and worked with quick- ulver, in order to cxiracl the particles of gold intermixed with it. But its mineral hiitory will be confidered more particularly after we have gone through the hiftory of the experiments, as fome points cannot till then be Efficiently underftood. It is here only to be well attended to, th.it all thefe matters are entirely adventitious to the platina; that their quantities are very variable, and that one or more of them, in fome parcels, feem to be altogether wanting the magnetic or ferrugineous matter being always the molt conuderable, and polTibly the only conflant admixture.
III. Specific Gravity of Platina.
The mineral called platina being, as we have befon feen, a mixture of very difhmilar matters not uniformly blended, I weighed hydroilatically feveral different parcels, . taking fometimes four or five ounces for one experiment, and in one twelve ounces. In moll of the trials, the gravity turned out, to that of water, very nearly as 17 to i : it was never lefs than 16,500, nor greater than 17,200,. The gravity of platina was examined alfo by Dr. Pember- ton and Mr. Ellicott,who both reported it to be about 1 7 The late Mr. Sparkes informed me, that a fpecimen which he made trial of turned out. but 16; and Dr. Davies, tha> he weighed a parcel whole gravity was found to be 17,233. .
To come as near as might be to the fpecific weight of the pure platina, I feparated a quantity of the larger grains by a coarfe fieve, and endeavoured . to cleanle them from the duff that might adhere, by boiling them in aquafortis., . mixing them with fal ammoniac and forcing off the fait
I 453 1 by fire, and afterwards warning them with water. The gravity of thefe was found on many trials to be upwards of 1 8, though the microfcope ftill difcovered a portion of blackifh matter in their cavities. Fahrenheits thermo- meter {landing at the fortieth degree, a quantity of thefe grains which weighed 642 in air, weighed in diftilled wa- ter 606 J, whence the fpecific gravity comes out 18,213. ^ was doubtlefs the larger and purer grains that Mr. Marg- graf examined, when he makes the gravity of platina to that of gold as 187- to 19.
The remarkable weight of platina appears to have been the principal inducement for believing that it is rich in gold, and is fliil innfted on by many as a proof of its be- ing fo, agreeably to the general axiom already taken notice of in the hiftory of gold, which, having long been univer- fally received, men cannot eafily think to be erroneous, that as mercury, among the bodies hitherto known, is the next in weight to gold, all bodies heavier than mercurv, whofe gravity is about 1 4, muft. therefore necellarily par- take of gold. Accordingly it has been affirmed that a twentieth, a tenth, and fome have gone fo far as to pretend that a fourth part of platina is true gold, the reft being a ferrugineous matter enveloping the gold.
But if we fuppofeplatir.a to contain even this laff. quan- tity of gold, I apprehend that the fame difficulty will Mill remain, and that the axiom will be as effectually overturned as if we fuppofe it to contain none. If the matter mixed with the gold in platina is ferrugineous, its fpecific gravity cannot be admitted to be more than 8, for pure iron itfelf does not come up to that weight. Now if 8 parts of this matter lofe 1 in water, 3,0000 parts will lofe ,37^05 and 1,0000 parts of gold, the gravity of this metal being about 19,300, will lofe ,0518; fo that 4,0000 parts of - the compound will lofe ,4268 ; whence, dividing 4,0000
N n n 2 by
[ 454 ] by ,4268,. we have 9,372 for the gravity of the compound: The gravity of platina fhould be no more than this, if its compofition was fuch as is fuppofed ; fo that one part of gold, wrapt up in three of ferrugineous matter, fs very far from accounting for the great weight of the mineral. To make the gravity 17, the quantity of gold ought to be 10 parts in 1 1 of the mafs.
If it be fuppofed that the matter mixed with the gold is not iron, but fome thing of a heavier kind, let us inveftigate what its weight mud be. If gold be blended with three times its weight of another matter, and the gravity of the mixt be \j ; then 4 J parts of gold, and 12$ of the other matter, will together lofe 1 in water : the 4^ or 4,25 of gold lofe ,22 in water, fo that the 12,75 of the other matter muft lofe ,78, whence the gravity, of this laft comes out above 16. If platina therefore be fuppofed to contain gold becaufe it approaches to gold in fpecific weight, we mufl ftill admit that there is a fubftance which does, the fame though it contains no gold.
To this way of reafoning the degraded gold of Mr. Boyle- has been objected ; which however does not feem to me at all to afFeft the argument. . For in Boyles procefs, of which an account has been already given in the hiftory of gold, page 206 of this volume, the gravity of the gold, by the mixture of an inconfiderable quantity of foreign mat- ter, was diminilhed between a fifth and a fixth part, proba- bly from accidental cavities in the mafs j whereas here, according to the fuppofition we have been fpeaking of, the gravity of the, compound, inftead of being diminifhed, is increafed almoft to double of what it ought to be. There may indeed be fome variation of gravity from the mixture of two bodies with one another, but of fuch an increafe as this I believe it will not be pretended that there is any, inftance. The great, weight of platina therefore, inftead
of
[455] of being a proof of its containing gold, affords rather a
prefumptipn of its being a ponderous body diftinc~t from gold.
IV. Malleability of Platina.
Some of the purer grains of platina, by gentle ilrokes of a flat hammer upon a fmooth anvil, bore to be extended into thin plates, without breaking or cracking about the edges : fome cracked before they had been much flattened, and difcovered internally a clofe granulated texture : others were fo brittle as to be reduced, without much difficulty, into powder. Even the tougher ones foon broke from rude blows in an iron mortar; and they feemed all to be more brittle when red hot than when cold.
Mr. Scheffer, in his little quantity of platina, did not take notice that the grains differed in toughnefs : the par- ticles he tried having been of the more malleable kind, he makes platina in. general to be as> malleable a metal as the beft iron. Mr. Macquer feems alfo to have tried only a Angle grain : he fays he took one of the largeft of the grains, and having beaten it with moderate ftrokes on a lteel anvil, he found that it fuffered itfelf to be flattened into a pretty thin plate, which however cracked upon con- tinuing the beating. But Mr. Marggraf examined feveral grains, and obferved the fame difference in their malleabi- lity as I had. done :. fome flretched confiderably ; others but a little, breaking from a few blows ; whilft others bore to be extended into pretty thin plates : he takes notice that thefe laft were moftly the convex grains.
Upon the whole, as many of the grains are apparently of confiderable malleability, and as the brittlenefs of the others proceeds doubtlefs from fome accidental caufe, we can by no means refufe platina the title of a malleable me- tal ; though little advantage can refult from this property, unlefs means fhould be found of uniting the grains into i larger mafles. V. Platina-
[456 ]
V. Platina expofed to the fire in vejfels.
i . An ounce of platina, containing its ufual admixture of magnetic duft, was kept for fome time of a moderate red heat in an iron ladle. The white grains became dark co- loured, and almoft loft their metallic brightnefs ; and the magnet feemed no longer to attract any part of the mixt : in other refpedts no alteration was obferved.
2. Several ounces of platina, freed from the black duft, and in which no yellow particles could be feen, were heated to a ftrong red heat, and quenched in urine. The platina, as before, loft its brightnefs: many of the grains looked blackifh, others of a rufty or reddifh brown, and fome of a high yellow colour; which laft proved more malleable than platina, and appeared to be in great part gold. Sur- prifed at this event, and imagining at firft, agreeably to the common opinion, that the platina had fuffered a de- compofition, or been diverted of its coat, I repeated the ignition and extinction upwards of thirty times, quenching -the matter fometimes in urine, and fometimes in folution offal ammoniac and other faline liquors: the platina con- tinued ftill of the dark colour which it had contracted at firft, and no more golden grains could be perceived. On .examining the remainder of the packet of platina, the gold, which the firft ignition had exhibited, was eafily ac- counted for: the particles of gold, naturally intermingled •ng the platina, were covered with quickiilver, which had doubtlefs been added with a view to extract them; and the quickiilver, evaporating in the fire, had left the gold of its proper afpedt. It is poflible that others may have been irnpofed upon by the like appearances, and thought they had produced gold from the fubftance of the platina itfelf, when they had only collected the golden grains, which ought to be looked upon as entirely adventitious. 3' The
[457]
3. The platina, difcoloured by the two foregoing ex- periments, was put into a crucible, which was covered, and kept for half an hour in a pretty ftrong fire, furficient for the melting of caft iron. The platina loft the ill co- lour which it had contracted in a weaker heat, and became brighter and whiter than it had been at firft. The grains ftuck together, fo as to come out of the crucible in one lump; but they readily fell afunder again on a flight blow, and did not appear to have at all melted, or altered their fhape.
4. Some of this brightened platina, kept in a moderate red heat for an hour, contracted a dark colour as before; and being afterwards urged haftily with a ftrong fire, it became again bright, almoft like filver. I tried the mal- leability of feveral of the grains, both when difcoloured and when brightened by fire, and found that In both ftates, as in the crude mineral, fome bore to be confider- ably extended, while others cracked or broke from a blow or two of the hammer.
5. I proceeded to try the effect of greater degrees of heat, having fitted up for this purpofe a blaft-furnace or forge with two pair of large bellows. An ounce of pla- tina, in a black-lead crucible, was urged in this furnace with a fea-coal fire, for more than an hour. The heat was fo vehement, that the crucible in great part vitrefied ; and the flip of Windfor brick which it was covered with, though. defended by a thin coating of Sturbridge clay, as alfo the internal parts of the furnace oppolite to the bellowfes, melted and run down. The grains of platina remained unmelted, being only fuperficially united into a lump of the figure of the bottom of the crucible: their colour was a good deal brighter and more lilvery than at firft; and they feemed to cohere more firmly than thole which had undergone the weaker heat in No. 3.
6. The.
[ 4S» 3
6. The foregoing experiment was many times repeated, in different kinds of crucibles, both German and Englifh; with fires of charcoal, of common fea-coal, and of fea-coal coaked or charred. In the moft intenfe fires I was able to excite, fuch as neither the beil of the crucibles, nor the furnace, could long fupport, the platina did not appear to melt, or foften, or alter its figure. I fometimes indeed obtained a few globular drops, of the fize of fmall mot, of fmooth furfaces, which broke eafily on the anvil, and looked internally grey: thefe drops had evidently been melted, but it is probable that they were not pure platina, and that the fulion was owing to an admixture of the ferru- gineous part of the mineral, or of the golden grains : for when the purer picked grains of platina were employed, there was never any appearance of melted particles; and thofe parcels of the mineral which had once yielded fome melted drops, could never be made to afford more, though urged with fires at lean: as vehement as the firft time. The cohefion of the grains of platina feemed to begin in a moderately ftrong red heat, and to become firmer and firmer as the fire was made more violent, though I never found them cohere fo much as to refift a fmall blow of a hammer. The colour, after ftrong fire, was almofl always bright and white, except on the furface of the mafs, which was often changed to a dark brownifh, with fometimes a faint yellowiih tinge : in one experiment, the metal, when violently heated, having been quenched in cold water, the grains which compofed the internal part of the lump ac- quired a violet or purple colour.
7. I picked out fome of the larger and brighter particles of platina, to the weight of about fifty grains, and fpread them on the bottom of a fmooth crucible: the veffel be- ing covered, and kept in a vehement fire, as in the above experiments, for about an hour, the platina cohered but
nightly,
[ 459 ] flightly, and being laid again in the balance, it rather out- weighed its former counterpoife which had been left in the fcale. From this experiment, which was two or three times repeated with the fame event, I concluded, in the firft paper, published in the Philofophical Tranfadtions, that platina does not lofe of its weight in the fire : Mr. Marggraf and Mr. Macquer have fince found, that it not only does not lofe, but really gains weight, and that when the fire is long continued, the gain is very confi- derable.
8. Mr. Marggraf put two ounces of crude platina in a fcorifying difh under a muffle, and kept up a ftrong fire for two hours, ftirring the platina at times with an iron rod. He obferved that no fumes arofe ; that when grown cold, the metal looked like fhavings of lead run together, but blacker and without metallic iuftre ; and that its weight was not diminished but increafed, for it weighed two ounces and ten grains, or one part in ninety-fix more than it did at firft.
9. He repeated the experiment with one ounce of pla- tina, in a covered crucible, placed on a proper fupport, in a melting furnace,which, by means of a long pipe for con- veying in the air under the afh pit, and a long narrow chimney on the top, gives the ftrongeft fire of all the fur- naces in his elaboratory. The fire being kept up in its greateft vehemence between three and four hours, the pla- tina was found flicking together but not melted, and weighed five or nearer fix grains, that is near one part in eighty, more than at firft. He takes notice that the grains were pretty eafily feparated by a blow of a hammer ; that thofe in the internal part of the lump were whiter than at firft, but that they were ftill in their original form ; and , that fome of them bore to be flattened on the anvil. ■
O o o io. Mr.
[ 46o ] 10. Mr.Macquer put an ounce of platiaa into a German crucible, and expofed it to a flrong fire for fifty hours, in a furnace whofe heat, when continued for fuch a time, was capable of melting die mixtures which Mr. Pott fays, in his litbogeogncjia, yielded him glades the moll hard and the leafl fufible. On examining the platina after this trial, he found that it had not melted, and that the grains only ftuck together fo as to form one mafs, which had exactly the figure of the bottom of the crucible, and which had fhrunk from the vcfiel fo as to come freely out ; that all the furface of the mafs was tarnifhed and blackened, and changed to a flate colour, with a diminution of the metal- lic brilliancy; that the internal part of the crucible, where the platina had touched it, was tinged as if filings of iron had been calcined in it ; and that on weighing the platina after the operation, it was found increafed fourteen grains,, which amount (the French ounce confirming of five hun- dred and feventy-fix grains) to about one part in forty-one. The fame platina, fubmitted to another operation fimilar to the foregoing, received a further increafe of two grains, the augmentation in all being fixteen grains, or one part in thirty-fix. There could be no fufpicion, he fays, of any coals or aib.es falling in, becaufe the crucible was in a part of the furnace where fuch matters could have no accefs, and becaufe it was alio clofely covered, though not luted. As the increafe was inconfiderable in the fecond operation, he judges there would have been little or none on a third repetition. We may add, that fince after fifty hours flrong fire, a further continuance of heat occasioned ftill a very fenfible augmentation of weight, the difference between the refult of this experiment and Mr.Marggrafs, in regard to the quantity of the augmentation, may be eafily account- ed for, from the different lengths of time that the fires were continued.
U. It
[46x ] ii. It is well known to the chemifts, that the metals called imperfedt, or thofe which calcine in the fire, gain weight in their calcination ; a phenomenon not a little aftonifhing, and of which they have not been able to aflign any probable caufe, unlefs it be the abforption of air. As platina appears plainly, from many of its properties, not to be one of the imperfect metals, Mr. Macquer very juftly fufpecls, that the increafe of weight in the above experiments was owing to the calcination of fome hetero- geneous fubftances mixed with the platina. The ferrugi- neous lining which it left in the crucible, and the objura- tion of the colour, feemed to confirm this conjecture, and he further took notice, that after the fecond calcination there were fome grains of a friable matter like fcales of iron, and that the magnetic fand was no longer black and brilliant, but of the fame flate grey colour with the pla- tina. It may here be obferved, that if there was no miftake in Mr. Macquers weights, the quantity of this heteroge- neous calcinable matter muft be very confiderable. Of all the experiments I can recoiled: of the calcination of bo- dies, there is no one in which the increafe was fo great as that which Mr. Scheffer allows to iron, viz. one third of its weight, as we iliall fee hereafter in the fixth fedtion of this hittory : admitting even this augmentation to the cal- cinable matter in platina, the quantity of this matter, to produce an augmentation of fixteen grains on the ounce, mult be forty-eight grains, or one eleventh part of the platina.
12. The obfervations in the foregoing paragraph account for the difference between my experiments No. 7, and thofe of M. Marggraf and Macquer in No, 8, 9, and 10 ; mine having been made with the purer grains, and theirs with the entire mineral containing its common mixture of calcinable parts. For further latisfaclion in this point, .1 .
O o o 2 took
[ 462 ] took 360 grains of the larger and brighter particles picked out from platina, and the fame quantity of the blackifh duft feparated from it by a fieve : the two parcels, in two fmooth fcorifying difhcs, were kept under a muffle, in a very ftrong heat, for five hours ; and that both of them might undergo an heat as equal as poffible, the places of the two dirties were interchanged about the middle of the procefs. When cold, the picked platina, weighed with great exactnefs, was found to have gained two grains, or one part in a hundred and eighty ; while the duft was in- creafed near nine grains, or one part in forty. It was ob- ferved that the picked platina had become darker coloured than it was at firft, but the duft paler j and that the picked platina cohered very flightly, but the duft was agglutinated into a firm cake not ealily to be broken between the fin- gers. It mult be obferved that what is here called duft contains a confiderable proportion of true platina, divided into particles as fine as thole of the impure matter ; and confequently that the quantity of impure matter in the picked platina cannot be judged of from the proportional augmentations which the two parcels received in the fire. But we fhall here drop an enquiry, which does not feem. important enough to deferve the trouble of any further profecution, efpecially as we fhall hereafter find means of attacking thefe calcinable parts more effectually than by fimple heat.
13. The experiments I had made (No. 5 and 6 of this article) feem to prove, that platina cannot be brought in- to fufion in the common crucibles, by any heat that the vefTels themfelves can fupport. Mr. Scheffer concludes alfo from his trials, that to melt it in a crucible is impofli- ble, fince it refifts even a ftronger fire, than thatwhich vi- trifies the beft crucibles made of Waldenburg earth and qu&rtz,which we may fuppofe, frora this manner of fpeak-
ing
[463 ]
ing of them, to be of a very good kind. Neverthelefs, as the melting of platina, if it could be effected, would be a moft important acquifition in regard to its chemical hifto- ry and mechanic ufes, Mr. Macquer made lbme further trials with this view. He expofed platina to the fire of a glafshoufe furnace for five days and five nights, but with- out perceiving any other alterations than thole already men- tioned : and indeed the glafshoufe fire could not be ex- pected to fubdue this refractory metal, which had already refuted fires much greater than the glafshoufe furnace can produce, and greater than its materials or vefTels can fup- port.
14. For the laft effort, Mr. Macquer had recourfe to a forge, increafing the activity of the fire by an expedient iimilar to that which we have formerly mentioned in page 26 of the prefent work. The blaft from the bellows was divided into two pipes, which entered the furnace at two oppofite fides; and two other great bellowfes were fo dif- pofed, that their blafts entered oppofitely at the other two fides. Four ounces of platina, in a Heflian crucible, being placed in the middle of the furnace, the fire was excited by the bellowfes to fuch a degree, that in lefs than an hour and a quarter, all the internal part of the furnace melted and run towards the bottom, forming in the lower part jnaffes of glafs, which, flopping up the orifices of the blafl-pipes, made it neceffary to difcontinue the ex- periment: the crucible, which was all vitrefied, being taken out fome time after, appeared ilill of fo dazzling a whitenefs that the eye could not fupport its luflre: yet notwithstanding this extreme fire which the platina had fuffered, it was no more melted than in the foregoing ex- periments; except that in the vitrifications, which fur- rounded the crucible, there were found fome grains, of a »lver whitenefs, perfectly round, which appeared to have
had.
[464]
had a very good fufion, but which, from a fmall blow of & hammer upon a fteel anvil, fell into powder. Mr. Macquer appears therefore, in this utmoft effort, to have produced no other effects than thole which I had obtained; and his trials concur with the others in proving, that the beft of the common furnaces, and melting veffels, will themfelves melt fooner than the platina included in them.
VI. Platina expofed to the fire in contaSl with the burning fuel. As the power of fire, upon metallic as well as earthy bodies, is remarkably promoted by the immediate contadt of the burning fuel, and the impulfe of air upon the fub- jecl, platina was expofed to its action in thofe circumftances. Mr. Scheffer feems to regret that he had not fome pounds of the metal for a trial of this kind, but the procefs may be managed in fuch a manner, that a very little quantity can be made to furfice.
A crucible, having a bed of charcoal in it, was laid on its fide among the fuel, in a good blafl-furnace, with its mouth towards the nofe of the bellows; and on the char- coal were fpread four ounces of platina. The fire was ve- hemently urged for above an hour; during which an in- tenfe white flame patted through the crucible, and Lflued at an aperture made for that purpofe in the end. Great part of the crucible was vitrefied; but the grains of platina only fuperficially cohered and became brighter, without feeming to have at all foftened or altered their fliape.
The experiment was feveral times repeated and varied : common fait, whofe fumes promote the vitrification of the crucibles themfelves, was thrown on the fuel before the mouth of the velfel, and its fumes ftrongly impelled upon the platina: the lumps of platina which had undergone the preceding operations, were dropt, before the nofe of the bellows, into vielently-excited charcoal and fea-coal
fires,
[4*5 ] fires, lb ftrong as almoft inftantly to melt off a piece of the end of the forged iron rod with which the fuel was at times ftirred down. The platina flill came out unmelted, and unaltered in its form; except that there were fome- times a few globular drops like thofe mentioned in the preceding article.
VII. Platina expofed to a burning glafs.
After all thefe fruitlefs attempts for the melting of platina, no other refource remains, for determining its fufibility or non-fufibility, than the action of large burning glaffes or concaves; a trial which I have often regretted that I could not in this country find means of expoiing it to. What has earneftly been wifhed for by all thofe, whom profit, curiofity, or fcience, have interested in thefe kinds of purfuits, Mr. Macquer and Mr. Baume have endeavoured to fupply.
They ufed a concave of plate glafs, well filvered, twenty two inches in diameter, and of twenty-eight inches focus. Before they proceeded to try its effects on platina, they expofed to its action feveral other bodies, that fome judge- ment might be formed of its force.
Black iiint, powdered to prevent its crackling and flying about, and fecured in a large piece of charcoal, bubbled up, and run into a tranfparent glafs in lefs than half a minute. Heffian crucibles, and glafshoufe pots, vitrefied completely in three or four feconds. Forged iron fmoked, melted, boiled, and changed into a vitrefcent fcoria, as foon as it was expofed to the focus. The gypfum of Montmartre, when the flat fides of the plates or leaves, of which it is compofed, were prefented to the glafs, did not fhew the leaft difpofition to melt; but on prefenting a tranfverfe fection of it, or the edges of the plates, it melted in an inftant, with a hiffing noife, into a brownifh-yeUow
matter..
[ 466 ] matter. Calcareous ftones did not completely melt ; but there was detached from them a circle, more compact than the reft of the mafs, and of the fize of the focus j the fe- paration of which feemed to be occafioned by the fhrink- ing of the matter which had begun to enter into fufion. The white calx of antimony, commonly called diaphoretic antimony, melted better than the calcareous ftones, and changed into an opake, pretty gloffy fubftance, like white enamel.
They obferve that the whitenefs of the calcareous ftones and the antimonial calx are of great difadvantage to their fufion, by reflecting great part of the funs rays, fo that the fubje^l cannot undergo the full activity of the heat thrown upon it by the burning-glafs : that the cafe is the fame with metallic bodies,which melt fo much the more difficultly in the focus, as they are the more white and po- liftied : that this difference is fo remarkable, that in the focus of the concave whofe effects we have been fpeaking of, fo fulible a metal as filver, when its furface was po- lifhed, did not melt at all : and that the whitenefs of pla- tina would doubtlefs in like manner have greatly weaken- ed the action of the concave on it. Meff. Macquer and Baume therefore took the platina which they had before kept five days in a glafshoufe furnace, and which, while it had concreted into a lump large enough to be held in the focus, had at the fame time become tarnifhed and browned on the furface, fo as to be in a ftate the moft favourable for the experiment. Their account of the experiment it- felf is as follows.
" When the platina begun to feel the activity of the focus, it looked of a dazzling whitenefs : from time to time there iffued from it fiery fparks, and there arofe a fume, very fenfible, and even pretty confiderable : in fine it entered into a true and good fufion, but it was not till
the
[467 ]
the end of a minute and a half that this fufion took place. We melted it in this manner in five or fix parts : none of the melted parts however run to the ground, all of them remaining fixed to the piece of platina, probably becaufe they fet and hardened aflbon as they were no longer in the center of the focus. Thefe melted parts were diftinguifh- ed from the reft, by a filver brilliancy, and a rounded furface, fhining and polifhed. We ftruck the largeft of thefe melted maffes upon a fteel anvil, to examine its duc- tility : it flattened eafily, and was reduced into a very thin plate, without breaking or cracking in the leaft ; info- much, that it appeared to us infinitely more malleable than the grains of platina are in their natural ftate, and that we believed it might be extended into as thin plates as gold and filver. This platina grew hard and rigid un- der the ftrokes of the hammer, as gold, filver, and other metals do : this rigidity was eafily deftroyed by the me- thod praclifed for gold and lilver, that is, by heating it to a white heat and letting it cool." Mr. Baume, in his ma- nuel de chymie, printed in 176^, takes notice of another property of the platina thus melted; that it is found to be of a fpecific weight approaching (femblabk) to that of gold : on this, however, we can lav but little ftrefs, as he had faid before, in fpeaking of the crude grains of pla- tina, that their fpecific weight is equal (cgale) to that of gold.
The above experiment, though not a little curious and interefting, is by no means entirely fatisfactory ; and it were to be wifhed that fome further trials were made, with burning-glalTes of greater force, for afcertaining with more precifion the real fufion of the platina, and for ob- taining fome quantity of the melted metal, that its duc- tility, gravity, hardnefs, and other properties, may be more fatisfactorily examined. Thus much feems clear
P p p from
[ 468 ] from the experiment, that platina is a great deal more difficult of fuiion than Hint, and flint a great deal more fo than gypfum ; and rs do means have been found of pufh- ing common fire to fuch a height, as to produce either in Hint or gypfum the leaA appearance of fufion, without the concurrence of the faline or earthy parts of the fuel, which ferve as a flux for thofe bodies, though not for platina ; there appears no room to hope, as the author feems to do towards the end of his memoir, that we fhall ever be able to melt platina in great furnaces. It follows alfo, that the melted drops, which both Mr. Macquer and I obtained in our furnaces, could not be pure platina : for though it is not to bethought that our fires were of equal ■ iil:ty with that to which the platina was here expofed, drops had i'u/ivred a more perfect fuiion, than thoii parts appear to have done that were melted in the focus, of the burning-glafs : the drops likewi lc had nothing of the malleability, which platina melted by the burning- gkls is laid to poffefs in fo remarkable a degree, but on the contrary fell in powder under the hammer. If the fufion in one cafe was brought about by the mixture of fome foreign metallic matter with the platina, we cannot be certain but in the other alfo the fame caufe may have concurred in a lefs degree; and confequently it is pofllble that pure platina may require for its fuiion a heat flill more vehement.
From die experiments related in this fedtion I think it may be concluded, that platina is a filver-coloured metal, of confiderable dudlility, not fufible by the ftrongeft fires, that can be excited in the furnaces, or fuflained by the veflels, of the chemifl: or the workman; that it approaches to gold in one of the reputedly moft difcriminating cha- racters of that metal, fpecific weight ; and that it agrees v/ith gold and filver in being fixt and uncalcinable by fire.
SECT.
1 469 ]
SECT. II.
Of the aftion of Acids on Platina.
I. Platina with the Vitriolic acid.
SEVERAL parcels of the purer grains of platina were digefted for fome hours in a gentle heat, with the concentrated fpirit called oil of vitriol, and with the fame fpirit diluted with different proportions of water. No folution happened, nor any alteration either in the liquors or the metal.
2. Three ounces of ftrong oil of vitriol were boiled with one ounce of platina, in a tall narrow-necked glafs, for fome hours. The liquor remained nearly of the fame quantity as at firft, and no change could be perceived either in it or in the platina.
3. The glafs being cut off a little above the furface of the liquid, the fire was gradually increafed, till the liquor, which now begun to evaporate freely, had, in five or fix hours, wholly exhaled, and left the platina dry and red hot. The metal, when grown cold, being wafhed with water and afterwards dried, its weight was found to be the fame as at firft, and the furface of the grains ihewed no mark of corrolion. The only alteration obferved was, that many of the grains had become dull coloured and brownifhj an effect which, as we have already feen, limple heat produces, and which therefore muft not be imputed to the action of the body fuperadded, when a heat fufficient to produce it is employed at the fame time.
It appears therefore, that platina refills the pure vitriolic acid, which, by one or other of the above methods of application, diflblves or corrodes every other known me- tallic body except gold.
P p p 2 II. Platina
[ 47° ]
II. Platina with tie Marine acid.
i. Weak and ltrong fpirits of fait being digefted fe- parate-ly with one third their weight of platina, in a gentle heat, for feveral hours, the liquors remained un- coloured, and the platina unaltered. The heat was after- wards increafed, and the liquors kept ltrongly boiling till they had totally exhaled, without making any fenlible change in the platina.
2. When common fait is flrongly heated, in mixture with certain vitriolic fubitances, its acid, forced out by the vitriolic acid, and refolved into fumes by the heat, corrodes fome metallic bodies, on which, in its liquid ftate, it has no action. Two parts of decrepitated or dried fea fait were therefore mixed with three parts of green vitriol calcined to rednefs; three ounces of the mixture prefied fmooth into a cementing pot ; one ounce of platina fpread evenly upon the furface, and fome more of the mixture over it; the veflel clofely covered and luted, and kept in a moderate red heat for twelve hours. On examining it when grown cold, the faline mixture was found to have melted, and formed a fmooth uniform mafs. The platina, which had funk to the bottom, being fe- parated from the mixture by warning, appeared to have differed no change, though its weightwas a little diminifhed.
-5. The experiment was repeated with a left fufible mixture, called the regal cement, compofed of one part of common fait, one of colcothar, or vitriol ftrongly calcined, and four of powdered red bricks. An ounce of platina, furrounded as above with fix ounces of this compofition, and cemented in a clofe veflel, with a red heat, for twenty hours, fuffered no material change, though there was, as before, fome deficiency in the weight. Many of the grains were difcoloured; whereas, in the foregoing ex- periment
[ 47i ] periment they were all nearly as bright and white as at rirft, on account, perhaps, of the mixture having melted, fo as to warn and cleanfe their furfaces.
4. Of the other metallic bodies, gold is the only one which refills the marine acid in the above way of appli- cation. As the platina in thefe experiments had no mark of diilblution, it was prefumed that this metal likewife had relifted it; and that the deficiency in weight was owing to fome of the fmaller grains having been waflied off along with the ponderous metallic matter of the vi- triol. The experiment was therefore varied, by fubfti- tuting, to the foregoing mixtures, mercury-fublimate, a combination of the concentrated marine acid with quick- iilver:. when this compound is mixed with any one of the common metals, gold excepted, and the mixture expofed to a proper heat, the quickfilver feparates and exhales, while the acid unites with the metal. An ounce of pla- tina was fpread upon three ounces of powdered fublimate in a glafs veflel, which being fet in a moderate fand heat, the fublimate totally arofe, leaving the platina of its ori- ginal weight, and uncorroded, though difcoloured a little.
5. As die action of fublimate on bodies depends not only on the acid being capable of corroding them, but on its having a ftronger affinity to them than it has to the mercury, that is, a difpofition to unite with them in pre- ference to the mercury; it is poiiible that there may be bodies, really corroiihle by the acid, but which, having lefs affinity to it than mercury has, will of confequence refilt the action of fublimate. The regal cement was therefore again had recourfe to, but that none of the grains of platina might be in danger of being loft, twice their weight of gold was melted with them, and the- mixture carefully hammered into a thin plate. A piece of the plate, weighing fifty grains, was furrounded with
regal.
t 472 ] regal cement, the crucible covered and luted, and kept for twenty hours in a red heat. On examining the metal, it was found to retain the whitenefs and brittlenefs, which gold conftantly receives from fo large a proportion of pla- tina, and to have loft in weight about half a grain, or one hundredth part. This lofs proceeded perhaps from alloy in the gold employed, which was above ftandard, but not perfectly fine, or perhaps from the diflblution of fome of the heterogeneous parts of the platina, but by no means from the platina itfelf; for the fame plate, cemented again with frefh mixture for the fame length of time, fuffered no further diminution. If the marine acid was capable of corroding the platina, the corrofion would have continued in the fecond procefs, and inftead of a hundredth part, near a third part would have been eaten out. This ex- periment therefore determines with certainty the refiftence of platina to the marine fumes; and that the regal cement, fo called from its being fuppofed to purify gold from all heterogeneous metallic bodies, is incapable of feparating platina from it.
6. There are circumftancesin which gold itfelf is diffolved by the pure marine acid ; as when it has been melted with tin, and the mixture beaten into powderand calcined ; or when it has been reduced into the form of a calx by preci- pitation from other menftrua. Platina calcined with tin, 2nd fome of the precipitates of platina of which an account will be given in the next feclion, were digefted in fpirit of fait, with a moderate heat, for feveral hours : the reddifh yellow colour which the menftruum acquired, (hewed that a part of the platina was diifolved, though it feemed to .difiblve fomewhat more difficultly, and more fparingly, than gold treated in the fame manner.
III. Platinfl
[ 473 ]
III. Platina 'with the Nitrous acid.
1. Spirit of nitre diluted with water, proof aquafortis, •jnd the ftrong fmoking nitrous fpirit, were digefled fepa- rately with one third their weight of platina, in a gentle heat, for feveral hours. During the digeflion, fome fmalj. bubbles were obferved, as if a diffolution was beginning,
t the liquors acquired no colour, and the fire being in- creafed, fo as to keep them boiling till they had to- tally evaporated, the platina remained unchanged, except only that many of the grains had become difcoloured.
2. Platina was treated like wife with nitrous cements, by procenes fimilar to thoie in which it had been expofed to the marine fumes. An ounce of pure nitre, and an ounce and a half of green vitriol calcined to rcdnefs,were ground together, part of the mixture prelfed fmooth into a cru- cible, over which was fpread an ounce of platina, and the reft of the mixture above it. The crucible was covered and luted, and the fire gradually raifed, fo as to make the veffel of a full red heat, in which flate it was continued for feven or eight hours. Red nitrous fumes iffued copi- oufly through fome fmall cracks which they had forced in the luting. The crucible being grown cold, the mixture was found not melted or baked together, but loofe and powdery. The platina was of the fame weight and appear- ance as at nrft, except that many of the grains had be- come, as in the foregoing experiments, dull coloured or brownifh.
IV. Additional experiments with the foregoing acids, &c. on
platina.
Mr. Marggraf has given fome experiments on this head, which having been conducted in a fomewhat diffe- rent manner from mine, he took notice of fome pheno- mena
[ 474 ] mena which did not occur to me. They were all per- formed in fmall glafs retorts, with receivers adapted to them ; and the fire gradually raifed, fo as to make the re- torts red hot. In this manner he treated platina with eight times its weight of each of the three foregoing acids; with twice its weight of mercury-fublimate ; with twice its weight of fal ammoniac ; and with thrice its weight of the mixture called fal alembrot, compofed of one part of mercury-fublimate and two of fal ammoniac. The quantity of platina in each experiment was fixty grains.
With the nitrous and marine acids, he had a white cryftalline fublimate in the neck of the retort, which, viewed through a magnifying glafs, looked like cryftalline arfenic, but whofe quantity was too fmall to be fubmitted to any further examination. When the marine acid was ufed, there was alfo another fublimate of a reddilh colour ; and in all cafes, the remaining platina was changed in part to a reddilh brown. Mercury-fublimate arofe uncoloured, and left the platina of a dark greyifh colour, here and there reddilh. The fal alembrot arofe alfo perfectly white, but was followed by a little yellowiih matter : the remaining platina was of a bright whitenefs, almoft like filver. With fal ammoniac there was a fine yellow fublimate (errone- oufly called blue in the Berlin memoirs) like that which rifes from a mixture of this fait with iron ; the remaining platina was rather whiter than at firft, and after fome time grew a little moid in the air.
Mr. Marggraf exprefsly mentions his ufing in thefe ex- periments the crude unpicked mineral; whereas in mine only the larger white grains were employed, from which all the heterogeneous parts and ill coloured grains, that could be diftinguifhed by a good magnifier, had been care- fully picked out. It is pretty certain, that the fublimates did not proceed from the platina itfelf, but from its ad- mixtures,
[475 J mixtures, the white one poffibly from the mercurial glo- bules united with the acids, and the yellow from the fer- rugineous parts. The author himfelf concludes from the experiments, that the acids have no action on the true pla- tina, but attack in fome meafure its ferrugineous matter ; and that the marine acid feems to have this effedt in a greater degree than the other two.
V. Platina with aqua regia.
1. Aqua regia, the proper menftruum of gold, being poured upon platina, begun to adt upon it {lightly in the cold, and by the affiftance of heat flowly and difficultly diffolved it ; acquiring at firft a yellow colour, which deepened by degrees, as the menftruum became more fatu- rated, into a dark, almoft opake, brownifh red.
2. The experiment was feveral times repeated, with different forts of aquae regiae, made by diflblving fea fait and fal ammoniac, feparately, in four times their weight of aquafortis, and by abftradling the nitrous fpirit in a retort from the fame proportion of each of the falts. All thefe menftrua diffolved the platina ; and it did not appear to me that one diffolved it more readily, or in greater quan- tity than another. Mr. Macquer tried alfo feveral aquae regiae, compofed of different proportions of the nitrous and marine acids, and found that a mixture of equal parts of the two fpirits was one of thole which anfwered the be ft.
3. In order to determine the quantity of menftruum ne- ceffary for the diffolution, I prepared an aqua regia by di- luting ten ounces and a half of ftrong fmoking fpirit of nitre with eight ounces of water, and abftracting the mixture from fix ounces of common fait. Five ounces of this aqua regia, which may be reckoned to contain three ounces of very ftrong acid fpirit, were poured upon one ounce of
Q^q q platina,
[ 476 ] platina, in a retort, to which was adapted a recipient. A moderate heat being applied, the menStruum acted pretty brifkly, and red fumes arofe in abundance. When about two thirds of the liquor had come over, the action was fcarcely or not at all ieniible, though the hre was consider- ably raifed. The dillilled liquor, which appeared of a light reddifh colour, being poured back again into the re- tort, the diflblutiofl begun afreth : the vapour, which, now came over, was much paler than the firlt. The co- hobation being repeated four times, the dillilled liquor proved paler and paler every time : at length both the li . les and action eealed, though the fire was augmented,, and a considerable part of the platina remained undiffolved. The folution was therefore poured off, fome more of the menStruum added, the diitillation and cohobation renewed, and thefe proceiies repeated, till all the platina appeared to be taken up, except a little blackilh matter which feemed to be its impurities. The lafl portions of the men - ftruum feeming not to be Sufficiently faturated, fome more platina was added ; and after the acid had eealed to act, the remaining platina was dried and weighed, to fee how much of it had been diffolved. It was found, that by this way of application, one ounce of platina was diffolved by eight ounces and a quarter of the menStruum ; which quantity of the menflruum, as appears from the manner of its preparation, conSifted of about four ounces and a half of Strong acid Spirit, diluted with three ounces and three quarters of water ; whereas, when the cigeStion was performed in open veffels, and the fumes Suffered to efcape, about fourteen ounces of the above menStruum, contain- ing nearly eight ounces of Strong acid fpirit, were neceffa- ry for dillblving one ounce of platina. Platina appears to require a much greater quantity than gold, and to diffolve far more difficultly.
4. Marggraf
[ 477 1
4. Marggraf ufed an aqua regia compofed of one part of fal ammoniac and fixteen parts of aquafortis ; and found that twenty-four ounces of this menftruum were neceffary for one ounce of platina. It may be fufpected that the quantity of fal ammoniac was not here fuffickntfor enabling all the aquafortis to act on the platina, fo that the metal was difTolved only by a part of the menftruum, the reft being fuperfluous aquafortis. The author obferves, that the folution depofited in the cold fmall reddifh cryftals : yet he diftilled off one half of it in a retort, and does not take notice of any cryftallization happening in the concen- trated refiduuin ; from whence it feems to follow, that one half of the liquor was ineffential to the difiblution.
5. Macquer made an experiment of the fame kind,which agrees nearer with mine : of his aqua regia, compofed of equal parts of the nitrous and marine acids, fixteen ounces diffolved, by digeftion, one ounce of platina ; and in my trials, about fourteen ounces were found to fuffice. Acid fpirits differ fo much in their ftrength, and the diffolution is fo much influenced by the vapours being more or lefs confined during the procefs, as appears from the expe- riments above related, that an exact agreement in this point is not to be expected.
6. In all the diffolutions of platina, a portion of black- iih matter remained at the bottom, whether the platina had or had not been feparated from its black duft. In fome experiments, where the purer grains of the metal •.-.ere ufed, the quantity of this indifibluble matter amount- ed to about fix grains on the ounce, or an eightieth part : where the mineral was taken entire, without any fepara- tion of its heterogeneous mixtures, the refiduum was in one trial above a fortieth, and in another about a thir- tieth part. The proportion could not be determined with much exactnefs, the indiffoluble fubftance defending
Q^q q 2 from
[478 ] from the acfion of the acid fome minute particles of the metal itfelf.
7. Great part of this refiduum, as Marggraf obferves, is attracted by the magnet ; its ferrugineous principle being probably bedded in fandy matter, lb that the acid could not reach it. As the fineft grains of the metal leave al- ways more cr lefs of an indiffoluble fubftance, it fol- lows that platina is made fomewhat purer by the diifolu- tion.
All the experiments related in this fedtion concur in eftablifhing a ltrong agreement between platina and gold. There are fome other metals which diffolve indeed, and with much more facility, in aquaregia; but to refill: either the pure vitriolic acid, or the marine acid, or the nitrous acid, in the circumilances wherein gold and pla- tina refill them, are properties peculiar to thefe two.
SECT. III. Experiments on Solution of Platina.
I. Colour of the Solution, and trials of it for Jlaining.
SOLUTIONS of platina in aqua regia, when fatu- rated with the metal, are of a dark, almoft. opake, brown-red colour ; when only llightly impregnated, yel- low like thofe of gold. A few drops of the faturated li- quor tinge a large quantity of water of a fine golden hue. I know of no other metallic body whofe folutions in acids are fo rich or diffufive in colour, or tinge fuch large quan- tities of watery fluids.
Notwithflanding this diffufivenefs of colour of the liquor itfelf, and its refemblance when diluted to folutions of gold, it is little difpofed to communicate any colour to other bodies, and in this refpedl it differs remarkably from
gold.
[ 479 J gold. It corrodes the fkin, making it harfli and rough, but I have not obferved that it gives any ftain, not even the yellow one which the menftruum by itfelf communi- cates to the fkin. Ivory, feathers, filk, wood, and linen, were dipt in the diluted liquor and expofcd to the fun, and the dipping and drying repeated three or four times : they all became brown, from the colouring matter of the folu- tion having dried upon the furface j but water warned it off readily, and left them colourlefs as at firft, except that the filk retained a llight brownifhnefs after the waihing. The folution dropt upon warm marble, immediately cor- roded it, but without giving any colour. Dropt into infu- fions of cochineal, it did not heighten but deltroy the red or purplifh colour, and changed them to a brownifh or blackifh : fome of the mixtures applied on paper with a pencil, appeared nearly of the fame hue with Indian ink in its paler ihades.
II. Cryfiallization ofPlatina. Solutions of platina cryftallize much more eafilythan thofe of gold. As a coniiderable heat is neceffary for making the aqua regia faturate itfelf with the metal, the fatiated folution generally depofites, by the time it is grown cold, a brownifh red fediment, which is no other than a number of minute cryflals. A quantity of the folution having been fet by in an open glafs, in warm weather, the fuperfluous moifture gradually exhaling left moderately large cryftals, of a dark almofl opake red colour, of irre- gular figures, varioufly joined together, mofl of them in form of leaves, like flowers of benzoin but thicker. Their tafte was naufeous and fomewhat fharp, but not near fo corrofive as might be expected from the great quantity of nitrous and marine acids combined with the metal. Wafhed with proof fpirit, they became fomewhat paler, but ftill
remained.
[ 48o] remained of a high colour, refembling that of the deeper chives of fafFron. In a moderate heat, they feemed to melt, though only imperfectly, and emitted white fumes fmelling of fpirit of fait ; at length they fell into a duflcy afli coloured calx, ftaining the tobacco-pipe, in which they were expofed to the fire, of a pale dull reddifh colour.
III. Volatilization of Platina.
This metal, of itfelf as fixed in the fire as gold, appears to be equally volatilized by the hafiry abftraclion of an aqua regia made with fal ammoniac. Marggraf put into a glafs retort fix ounces of a folution of platina made in a mix- ture of fixteen parts of aquafortis and one part of fal am- moniac : having fet the retort in fand, and fitted to it a receiver, he drew off the liquid by a gradual fire, which at lafl was increafed, fo as to make the retort red hot and ready to melt. There remained at the bottom a reddifli- brown powder, which being further calcined under a muf- fle, became more of a brilliant blackifh hue. In the neck of the retort was found a brown-red fublimate, which, on expofure for fome days to the air, run into a red liquor refembling folution of platina. He poured fome of this liquor on a poliihed copper plate, and found that the pla- tina, after fome time, precipitated upon the copper, as it does from its common folutions, covering the copper with a ihining blackiih powder.
IV. Solution of platina, with vitriolic acid. To a folution of platina, diluted with water, I added fome ftrong fpirit, called oil, of vitriol: no precipitation or change of colour enfued, though a large quantity of the acid was at different times dropt in, and the mixture fuf- fered to ftand for feveral days. But on adding the fame ftrong vitriolic fpirit to an undiluted folution of platina,
the
[48i ] the liquor immediately became turbid, and a duiky co- loured matter foon precipitated. The precipitate was not rediffolved on the affulion of water; nor was the preci- pitation prevented by adding water immediately after the acid had been dropt in.
V. Solution of platina, with volatile aloali.
The fpirits of fal ammoniac, prepared both by quick- lime and by fixt alcaline falts, added to lolutions of platina diluted with water, precipitated a dark red fparkling pow- der: but in whatever quantity the fpirits were added, the precipitation was not total, a considerable part of the pla- tina remaining dilfolved fo as to communicate a high yellow colour to the liquor. The red precipitate, dried, and expofed to the fire in an iron ladle, became blackifh, without difcovering any thing of the fulminating power which precipitates of gold, prepared in the fame manner, have in a remarkable degree. On walking fome of the precipitate upon a filter, with repeated additions of water, greater! part of it diffolved, only a fmall quantity o»f blackifh matter remaining on the paper, and the liquor pairing through of a deep bright golden colour: a very large quantity of water was tinged of this colour by a fmall one of the powder.
VI. Solution of platina, ivitb vegetable fxt alcali. Salt of tartar, fait of wormwood, fixt nitre, and the lixivium faponarium of the London pharmacopoeia, had the fame erfecl: on folution of platina as the volatile fpirits in the foregoing article, except that the precipitates were of a much duller reddiih colour, and lefs brilliant. The precipitation was equally imperfect ; the liquor ftill con- tinuing of a deep yellow colour, and greateft. part of the precipitate being rediffolved on the addition of water.
la
[ 482 ] In the foregoing experiments, the precipitates of pla- tina by volatile alcalies were of a dark kind of red colour and confiderably lparkling, while thofe by the fixt were of a paler dull reddifh with little brilliancy. In the ac- counts which others have given of thefe precipitations, this difference, in itfelf of fmall importance, is not taken notice of. Scheffer calls the precipitates by both alcalies fimply red; and Marggraf calls them both orange yellow, a term applicable enough to the precipitates which I had obtained with the fixt alcalies, but not to thole with the volatile. It mould feem as if there had been fome real differences in the appearances of our refpe&ive produces, and I imagined that fuch differences might have arifen from differences in the folutions of platina made ufe of: fome late trials appeared to countenance this fufpicion, for while common folutions of platina yielded precipitates of the red kind, a folution of the cryftals of platina made in water gave only yellow ones.
Macquer accounts for this difference of colour in another manner. He fays the precipitate proves red only when the quantity of alcali is no more than juft fufficient to fatiate the acid ; and that the more of the alcaline liquor we add beyond this point, the precipitate proves lefs and lefs red ; agreeably to which his coadjutor Baume lays afterwards more determinately, in his ma)iiiel de chymie, that with a due quantity of fixt alcali the precipitate is orange yellow, and with an over quantity pale yellow. Mr. Macquer, fudging from hence, that the rednefs was owing to a large quantity of acid retained by the platina, digefted fome of the red precipitate in a folution of fixt alcaline fait : the alcaline liquor, abibrbing the acid, deftroyed the red co- lour of the powder, and made it white. It has long been known, that precipitates carry down with them a portion both of the diffolvent and of the body they were precipi- tated
[433 ] tated by : the author obferves that this appears mor? fenfible in our precipitate of platina, at leaft with regard to the diflblvent, than in raoft others ; and that this ob- fervation difcovers the caufe of fundry Angular phenomena, which I had remarked in the precipitation of platina, and of which 1 had not given the theory, as of the red preci- pitate being foluble in water, and of part of the platina remaining fufpended whatever quantity of alcali we add in the cold : a detail and explication of thefe phenomena, with others of the fame nature, he referves for another memoir. Some experiments I have made do not very well agree with this theory, but I forbear any further obferva- tions till the authors memoir appears.
VII. Solution of platina, ivitb mineral jixt alcali. As the two foregoing kinds of alcaline fait precipitate platina only in part, there is a third which has not even that effecl. The mineral alcali or bafis of fea fait, the method of preparing which will be defcribed in the fol- lowing part of this hiftory, produces no precipitation at all. This remarkable experiment, which we owe to Mr. Marggraf, will be further confidered hereafter.
VIII. Solution of platina, vitd P ruffian alcali. Mr. Marggraf obferves, that when iblution of pla- tina is mixed and faturated with a lixivium of fixt alcali that has been calcined with blood, it yields a fine blue precipitate, which in certain circumftances proves as beau- tiful as the belt Pruffian blue, though there falls alio at the fame time a little orange coloured matter. On re- peating this experiment, the liquors when firfl mixed ap- peared of a pretty deep blue, but when the precipitate had fettled, greateft part of it looked yellow, on account, probably, of the platina I made ufe of containing lefs fer-
R r r rugineous
[ 4*4 }
rugineous matter, or the alcaline lixivium being lefs fatu- rated with the fubftance which tinges diflblved iron blue than thofe which Mr. Marggraf employed.
To obtain a fatumted (blution of this tinging fubftance, which cannot be expected to be done by calcining alcaline faks with blood or other like matters, I digefted fome common Prufiian blue both in folution of fixt alcaline fait, and in volatile fpii it offal -ammoniac prepared with quick- en, e. Both menftrua ftx n became yellow; and the irony bafis of the Prufiian blue, thus freed from its colouring matter, remained in a nifty form. To both tinctures I dded fome more Prufiian blue, till they ceafed to hi •y action on it. The fixt alcali, along with the tinging fubftance, appeared to have taken up fome of the iron; for it ftruck a blue colour with good aquafortis, with the acid of fulphur, and with diltilled vinegar, in which there were, no grounds to fufpect any iron to be previoufly con- tained. The volatile tincture appeared free from iron, for in the fame acid fpirits it produced no change, though it inftantly turned them blue when a little iron was firft dif- folved in them.
This faturated folution of the tinging fubftance was added by degrees to folution of platina. The liquor turned at firft to a deep blue, but on further additions, to a greenifh yellow. The precipitate was of two kinds, yellowifh at the bottom, and blue on the top. The whole being {haken together and fet by till next day, a white matter appeared at the bottom, above this a yellow, and on the top a more copious brownifh grey. The liquor was of a deep gold colour.
IX. Solution of platina, with compound falts. Solutions of alum, of fal mirabile, of vitriolated tar- tar, of the fufible fait of urine, made feparately in water,
and
[485]
and folution of chalk in aqua fortis, were found by Marg- graf to produce no precipitation or apparent change in di- luted folution of platina.
Sal ammoniac, one of the ingredients to which the men- ftruum owed its power of diflblving the platina at firft, precipitated great part of it in form of a reddifh or yellow- ifli powder, nearly fimilar to that thrown down by alcalies. It is obfervable, that though neither fal ammoniac nor al- calies, feparately, occafioned a complete precipitation, the liquor ftill remaining of a high colour ; yet when one was added to the folution remaining after the action of the other, a new precipitate fell, which left the liquor colourlefs.
X. Solutkn of platina, 'with vinous fpirits.
As gold is revived from its folutions by vinous fpirits, and made to rife in yellow films to the furface; I mixed a folution of platina with a large proportion of highly rectified fpirit of wine, and expofed the mixture for many days to the fun, in a wide-mouthed glafs ilightly covered with paper to keep out dun:: there was no appearance of any yellow {kin, nor was any other alteration perceived, than that the platina had begun to crystallize from the evaporation of the fluid.
Sulpecling that though the liquor lhould really contain gold, yet the platina might ftrongly retain the gold and prevent its being leparated by the fpirit, I mixed three or four drops of folution of gold with two hundred drops of folution of platina, and after making them well together, added fome rectified fpirit of wine : the whole being ex- pofed as above to the fun, a golden film w:.s in a few days obferved upon die furface.
R r r ?. XI. Solution
[ 486 ]
XI. Solution of platina, with ejfential oils.
A colourless effential oil of rofemary was poured into about half its quantity of folution of platina, the mixture well fhaken, and fufFered to reft: the oil quickly arofe to the furface without receiving any colour, and the acid underneath remained coloured as at firft.
A competition of platina and gold, which had been melted together, being difiolved in aqua regia, and the folution treated in the fame manner, the gold was imbibed by the oil, and the platina remained diflblved by the acid : the oil, loaded with the gold, appeared of a fine yellow colour, and on ftanding for a few hours threw off great! part of the gold to the fides of the glafs, in bright yellow films, which appeared to have no mixture of platina. Some other diftilled oils were made trial of, with the fame event.
XII. Solution of platina, with at her.
The vitriolic aether or aethereal fpirit of wine, the pre- paration of which has been defcribed at the end of the eighth fedtion of the hiftory of gold, was poured into a folution of platina, and into a folution of a compofition of platina and gold. The two vials being immediately ftopt, to prevent the exhalation of the volatile fluid, and lightly fhaken; the aether received no colour from the folution of platina, but became inftantly yellow from that of the platina and gold.
XIII. Solution of platina, with tin.
Asa minute proportion of gold contained in acid fo- lutions is difcoverable by their ftriking a purple colour with tin, fome bright plates of pure tin were put into a folution of platina diluted with water. The plates in a
little
[487]
little time looked of a dark olive colour, and foon after were covered over with a reddifh brown matter. The liquor became at firft darker coloured, and afterwards, by degrees, as the precipitate fell, nearly colourlefs, without exhibiting the leaft appearance of a purplifli or reddifh hue.
Some platina was digefted in a quantity of aqua regia fufficient to diiTolve only about half of it, and the remain- der was diffolved in a frefh portion of the menftruum. The two folutions, treated as above, yielded fomewhat different phenomena, but no tendency to a purplifh calf could be perceived in either. The latter foluticn, which looked yellow from its not being fully faturatcd with the platina, was, when diluted with water, almoft colourlefs: neverthelefs, on the addition of tin, it became yellow a^.un, then of a dull red, and at laft of a dark brownifh red, confiderably deeper than the other more faturated fo- lution: on ftanding for fome time it grew perfectly clear, depofiting a paler yellowim precipitate.
To determine whether platina was capable of pre- venting a fmall proportion oi gold from difcovering itfelf in this way of trial, one drop of a folution of gold was let- fall into fome ounces of a diluted folution of platina. On adding fome plates of tin, the liquor became quickly purple.
The foregoing experiments were made with a folution of the picked grains of platina. I fubmitted alio to the four laft trials, with tin, aether, effential oils, and vinous fpirits, a folution made by digefting in aqua regia the en- tire mineral, with its mixture of yellow particles, as it comes to us; which folution, in all thefe trials, gave ex- actly the fame appearances, as the other folution did after it had been nrft mixed with a very little quantity of fo- lution of gold, ftriking a purple colour with tin, com- municating a yellow tinge to aether and effential oils, and yielding a yellow film with rectified fpirit of wine.
XIV. Precipitate
[ 488 ]
XIV. Precipitate of piatina expofed to a burning concave. Mr. M accruer and Mr. Baume\ after examining the action of a burning concave on crude piatina, as already mentioned in page 466, expofed the red precipitate of pia- tina made by alcalies, to the focus of the fame burning mirrour. " It immediately begun to boil, and diminished considerably in volume : there arofe at the fame time a very abundant and very thick fume, fmelling ftrongly of aqua regia, and which appeared fo luminous and fo white in the neighbourhood of the focus, that we could not decide whether it was not a true flame: the precipitate at the fame time loll its red colour, to refume that which is natural to piatina, and it now had the appearance of metalline lace. Being continued in the focus, the white fume fmelling of aqua regia was fucceeded by another fume or flame left copious, whofe colour inclined to violet. A little time after, there was formed, in the hotteft parr of the focus, a button of fmooth brilliant matter perfectly incited, and then the vapours ccafed. Examining this button after it was grown cold, we found it to be a vitrefcent opake mat- ter, of a hyacinth colour on the furface, internally blackifh and pretty compact. Vvre dare not affirm that this was a true vitrification of the piatina in virtue of the feline mat- ters which lined to it in the precipitate : the experi- ment ought to be rep ith pure piatina, and with a burning glafs 1 Lye ftronger than that we ufed." In- d as the piatina refamed its metallic afpecl:, it fhould feem to have been difengaged from the falts, before the time that the vitrification begun; and perhaps a button of blackifh vitreous matter, formed in the middle of the focus, may be eafily enough accounted for from the ferrugineous calx, which the precipitate cannot be fuppofed to have been free from : fee the experiments of the relation of pia- tina
[4«9 ] tina to vitreous bodies at the end of the following fetftion. The matter on which the powdery precipitate was expofed to the focus of the burning concave, might alfo have con- tributed to the vitrification ; what this matter was, the au- thor does not mention.
The experiments of this fection point out fome ftriking differences between platina and gold; net only in the power of producing, when diflblved, a purple colour with tin, and communicating a like flain to different kinds of colourlefs fubjects, a power for which gold is remarkable, and which platina wants ; but likewife in properties more importantly character if lie, as they afford means of dif- tinguifhing and parting the two metals when blended together. They have (hewn platina feparated in part from its folutions, by a fubftance which does not at all feparate gold, to wit, fal ammoniac; and gold feparated completely by fubftances which do not at all feparate pla- tina, viz. the mineral alcali, vinous fpirits, effential oils and sether. It appears likewife from thefe experiments, that befides the black dull: which remains behind in the diffolution of platina, the part diflblved is not pure pla- tina; for the blue colour produced by the Pruffian alcali amounts to a proof that the folution contains iron.
SECT. IV.
Platina expofed to jlrong /res, with faline, inflammable, Julphureous, •vitreous and earthy bodies.
HAVING feen the effects of the purer acids on pla- tina, and the general properties of its folutions, we fhall proceed to apply to it what are commonly called fluxes and dry menflrua, that is, fubftances which either barely promote the fufion of metals without corroding them, or which corrode and unite with them, when pro- perly
[ 49° 1 perly heated, nearly on the fame principle as humid men- flrua diflblve them.
I. Platina with Borax.
Half an ounce of platina was dropt into an ounce of melted borax, and urged with an intenfe fire for an hour. The platina appeared to have fuffered no alteration, but the borax was changed to a dark blackifh colour, probably from its having diflblved and vitrified fome of the ferrugineous duft. The whole was returned to the fire, which was kept up ftrong for a confiderable time longer, till the borax had funk through the crucible : it left the grains of platina of a bright white colour, (lightly cohering but unaltered in
form.
II. Platina with Alcali.
I treated platina in the fame manner both with the common fixt alcaline falts well purified, and with the cauftic alcali prepared by evaporating foapboilers ley, but could not perceive that either of them had any other effect than contributing to brighten its colour. Mr. Marggraf mixed a dram of platina with half an ounce of fait of tartar, and gave them a vehement fire, in a clofe luted crucible, for two hours. When cold, he found a hard mixt, of a yellowiih green colour, in which the platina was difperfed. The whole being feparated, as much as pofli- ble, from pieces of the crucible, by fcraping and wafhing, the water above the matter was next day found like gelly : the platina was whiter than ufual, almoft of the whitenefs of filvcr, but of its wonted figure. The gelatinous con- fidence which the water acquired in this and fome of the following experiments, is probably not owing to the pla- tina, but to fome of the earth of the crucible diflblved by the falinc matter,
III. Platina
[ 49i ]
III. Platina with Nitre.
Nitre, which reduces all the known metallic bodies, except gold, filver and quickiilver, into a calx, was mixed with equal its weight of platina, the mixture thrown into a red hot crucible, and the fire kept up for about a quarter of an hour. No deflagration happened; and the platina, freed from the fait by repeated wafhing with water, appeared to have fuffered no other change than having its colour darkened, an efredt which the fimple heat would have produced in it. The nitre was neverthelefs in great part alcalized.
Four ounces of platina and eight ounces of the pureft nitre were put into a crucible, the crucible covered with a larger one inverted over it, and kept in flrong cementation, in a wind furnace, for three days and three nights without intermiffion. The matter being now boiled in water to feparate the fait, the platina looked rufty coloured, and had loft almoft half its weight: the faline liquor, on being filtered, left a brownifh powder fomewhat more than equivalent to this diminution, and being afterwards eva- porated to drynefs, yielded a fmall quantity of a greenifh cauftic alcali. The fame platina was cemented thrice more with the fame quantities of frefh nitre, and the fire continued for three days and three nights every time. In the two firft repetitions, a (mailer quantity of a paler powder feparated, and the remaining metal in good mea- fure loft the rufty hue which it had contracted before. After the laft cementation, the little quantity of metal which remained had much the fame appearance as the pla- tina at firft: on wafhing it, there was fcarcely any further feparation of powdery matter, but the nitre was ftill alca- lized. The platina was then mixed with fal ammoniac, and the fait fublimed in a Florence rlafk: the fait arofe
S f f uncoloured,
[ 492 ]
uncoloured, and left the metal white and bright. The powders feparated in the cementation were treated in the fame manner, and the Sublimation repeated thirty times with frefh quantities of the fait: in the firft Sublimations, ferrugineous y« .roil-, but at lalt the fait re-
ceived no tinge, and the powder remained of a greyifh colour.
Mr. Mar aoiiiit of an experiment of the
fame kind, ia which he takes notice of fome phenomena, which either did not occur, or were not attended to, in mine. He threw into a red hot crucible four ounces of nitre, and one ounce, or four hundred and eighty grains, of platina : no detonation happened, but a considerable fume arofe. The fire being continued, with care to prevent the falling in of any piece of coal, the matter, after fome time, begun to fwell up, and a portion of it being tak;n out looked greeniih : it afterwards turned to a deep olive green, and grew considerably tough and thick : after fome hours of ftrong fire, it proved as thick as pap. The thick mat- ter was taken out while hot with an iron fpatula : it was of a deep olive-green colour. As much as pofiible of what adhered to the crucible was collected, and fet to digeft with the other in diftilled water : next day the whole was as thick as gelly. Being then diluted with more water, fiirred about, and Suffered to fettle, the liquid was poured off, and this repeated till all the lighter parts were waihed over : this fight matter, feparated from the faline liquor by filtration, well waihed on the filter with hot water, and dried, weighed two hundred and twenty-five grains : it was of a dark grey colour, and by ftrong calcination under a muffle became black like pitch. The more ponderous part was ground in a glafs mortar, by which fome more became fine enough to be wafhed over : this was of a clear brown colour, and amounted to thirty grains. The pla- tina
I 493 ] tina weighed three hundred and ten grains, and conic- quenfly had loft above a third : it ftill reiembled crude pla- tina, and retained its luftre; the brownifh rufly coat, with which mine appeared covered after the firft cementations, having probably been here rubbed off in the grinding. The nitre was totally deconapofed, and had acquired all the characters of alcalicity. The crucible and its fupport were tinged almcft throughout of an amethyft col6ur, as uiually happens in the calcination of manganele with nitre.
The platina v/as treated in the fame manner with three ounces of frefh nitre. The crucible and its fupport were ftill tinged of a fine amethyft colour, the nitre was to- tally alcalized, and all other circumftances happened as in the firft operation, except that the lighter parts, firft waftied oft", weighed only fixty grains: by calcination they became, as before, of a pitchy blacknefs: the remaining powdery matter was of a clear grey, and weighed forty five grains: the platina, ftill bright, weighed two hundred and fifteen grains, or lefs than half of its firft weight.
The operation was repeated with three ounces more of nitre. The crucible and its fupport were now lefs ftrongly tinged. The firft waftiings gave two grains of a light p-wder, in appearance much resembling the Eckertfberg blue earth; and by rubbing the reft of the platina in fcer, there were obtained forty grains of a light powder of a grey-brown colour. The platina loft in this operation but five grains; and fo inconfiderable a diminution giving ■little hopes of any further effect from a repetition of the procefs, the experiment was here dropt.
It had been affirmed, that platina is a compound of gold and fome other matter, fo intimately combined together, as not to be feparable without other methods of procedure than are commonly practifed or known. An adept, in the pretended art of this higher metallurgic analyfis,
Sff2 boafted
[ 494 ] boafted of having deftroyed the heterogene matter, _fo as to leave the gold pure, by long continued and repeated cementations with nitre. To remove all fcruples on this head, I permitted him to make the experiment of which I have above given a /hort account, and of which I have ventured to infert only fuch particulars as came under my own obfervation. The experiment, with which that of Mr. Marggraf, (o far as it goes, furficiently correfponds, was decilive. It fhewed much the greateft part of the platina changed to a powder, and the remaining platina as remote from the nature of gold as it. was at firft.. I tried it both by acids, and by cupellation with lead, a procefs of which an account will be given hereafter, and found it to preferve its own difcriminating characters, without any marks of gold, though it appeared to be purer than pla- tina in its common ftate. I tried alio, by the fame me- thods, the powders which had been feparated in the ce- mentations, after the reiterated fublimations offal ammo- niac from them; and found thefe likewife to be no other than platina, not reduced to a calx, but barely divided.
It may be prefumed that the action of the nitre was not directly upon the platina itfelf, but on the irony matter, adhering to the furfaces of the grains, or more intimately blended in their fubftance; which irony particles being changed to a calx, the platina intermixed became divided along with them into a powdery ftate. This fuppofitior. accounts fatisfaclorily for the principal phenomena of the procefs ; as the feparation of the powder being plentiful in the firft cementation, and more and more fparing in the following ones; the firft powder being of a deep colour, and the others paler, as if the iron prevailed in the firft, and the platina in the others; the powders yielding yellow, ferrugineous flowers with fal ammoniac, while the platina. that remained entire gave no colour to the fait.
In
[ 495 J In regard to the pretentions to the obtaining of gold by this procefs, it is not perhaps unreafonable to fuppofe, that the remarkable reparation of powdery matter in the cementation, and the appearance of fome golden grains which had been naturally intermingled among thofe of the platina, led men of warm imaginations to anticipate the further effect of the procefs, and to make the affertion which the above experiments overturn.
IV. Platina with common fait.
An ounce of common fait, dried, was kept in fulion with a dram of platina, in a cloie crucible, for an hour and a half. The fait appeared yellowifh, and on breaking the mafs, there were found in the middle of it fome red crvftalline grains. The platina was all at the bottom of the crucible, and preferved its figure, having fuffered no change, except being made very white. The experiment was repeated with what is called regenerated common fait, and the phenomena were exactly the fame. Both thefe experiments are from Marggraf.
V. Platina with vitriolic falls. . Marggraf mixed a dram of platina with fomer
pure Glaubers fal tnirabile, and kept the mixture in a flrong fire for two hours : the fait funk through the cru- cible, and left the platina of a dark grey colour: on warning the platina with water, and rubbing it in a glafs' mortar, there feparated a little light matter of a bright blackifh colour, and what remained was platina un- changed. A dram of platina, and an ounce of vitriolated tartar, were treated in the fame manner: the fait melted and became reddifh; the platina fuffered no change, ex- cept, that it looked fomewhat more grey.
VI. Platina
[ 49^ ]
VI. Platina with the cfential falts of urine.
From putrefied urine inipiflated to the confiftence of a fyrup, is obtained by crystallization a lingular faline con- crete, called the fulible or eflential fait of urine, or micro- cofmic fait, containing the acid ofphofphorus united with a volatile alcali. This fait, expofed to the fire, parts with its alcali, and aflumes a glaffy appearance; in which ftate, all the common metals, gold not excepted, are faid to be corroded by it in fufion. A hundred and eighty grains of this fait were mixed with thirty grains of platina, and urged in a crucible with a ftrong fire for two hours: the olatina was found unchanged at the bottom, covered with the fait, which likewife appeared little altered. Sixty grains of this fait, the fame quantity of calcined borax, and thirty grains of platina, were treated in the fame manner: there was a vitreous fcoria, fomewhat opake, of a yellowifh green colour: the falts and lighter parts being feparated by wafhing, the dried platina appeared of its natural form, but whiter than at firft.
After the cryftallization of the foregoing fait from urine,
there cryflallizes another, not containing the phofphorine
acid, and whofe compofition is as yet unknown. Three
drams of this fait and half a dram of platina being urged
with a ftrong fire in a clofe crucible, the fait run ail
through the crucible, and die platina, after rubbing in a
mortar and wafhing with water, appeared in its original
form, being only fomewhat whiter than before. A dram
of the fait, a dram of calcined borax, and a dram of 1 -
tina, treated in the fame manner, gave a green-yellowifl.,
dark-chryfolite-coloured vitreous mafs, under which lay
the platina unaltered, except that as in the former C2ie it
was whiter than at nrft. All thefe experiments are from
Marggraf. .
fa& VII. Platina
[ 497 J
VII. Platina ivith pbofphorme acid. When the phofphorus of urine is fet on fire under a glafs bell, nearly in the fame manner in which fulphur has been ufually burnt for obtaining its acid, it yields fpongy flowers, in appearance much like thole of zinc ; and both die flowers, and what matter remains on the glafs dim which the phofporus was placed upon, imbibe moif- ture from the air and run into a thick acid liquor, which -fed to the fire leaves a dry matter that melts into a glafiyform. Mr. Marggraf mixed fixty grains of platina widi twice as much of this acid liquor, and put them into a retort, whole juncture with the receiver was only doled with paper- The watery moiflure being drawn off by a gradual fire, the retort was fet, while hot, upon earning coals, till it begun to melt; after which, being taken from the fire, a flafh like lightening filled both the retort and receiver, and a violent exploficn followed. The author very ingenioufly, and with great probability,, attributes this effect to a regenerated phofphorus, to which the iron mixed with the platina had contributed the inflammable principle ; the action of which phofphorus could then only take place, when the abatement of the heat fuffered air to pals in through the ill-clofed juncture. . The pieces of glafs being collected, the bottom of the retort appeared covered with a white faline matter, which being fcraped off, the platina was found under it unchanged. It is evi- dent that the platina itfelf neither was, nor was fuppofed by the author to be, anywife concerned in producing the fulguration ; though Vogel makes this fulguration one of the ?iew properties of platina difcovered by Marggraf. .
VIII. Platinc
[ 493 ]
VIII. Platina with black flux, &t.
The black flux commonly employed by the chemifts for the fufion of metallic minerals and calces, compofed of one part of nitre and two of tartar mixed together and burnt in a covered veflel to an alcaline coal, was kept in fufion above an hour, in a clofe crucible, with one fourth its weight of platina. Compolitions of wood-foot, char- coal powder, common fait, and wood alhes, directed by M. de Reaumur for changing forged iron into fteel, were mixed with platina, and cemented feveral hours, in clofe crucibles, both with moderate degrees of heat, and with fires frrongly excited. In all thefe trials I could not ob- ferve that the metal fuffered any change, except that its colour was darkened.
IX. Platina ivitb fulphur.
An ounce of platina was fpread upon twice its weight of fulphur, with which fome powdered charcoal had been previoufly mixed, to prevent its becoming fluid in the fire, fo as to fufrer the platina to fubfide. The crucible, having another crucible, with a hole in the bottom, inverted into its mouth, was kept in a cementing furnace for fome hours : being then taken out, it was found that the fulphur had entirely exhaled ; and that the platina, feparated by wafh- ing from the charcoal powder, had the fame weight and appearance as at firft, except that its colour was changed to a blackifh : by rubbing it in a glafs mortar with a little alcaline fait and water, the blacknefs was deftroyed, and its original brightnefs reftored. I varied the experiment, by ftrongly heating the platina in a crucible by itfelf, and repeatedly throwing upon it pieces of fulphur : it ftill re- mained unaltered, the fulphur Iteming to have no more actionem this metal than on gold.
X. Platina
[ 499 I
X. Platina with fulphurated akali. A s fixed alcaline falts enable fulphur to diflblve gold in fufion, I expofed platina to the fire with a mixture of equal parts of fulphur and fixt alcali, called hepar fulphuris, or liver of fulphur. After a confiderable heat had been continued for fome time, and the matter now and then ftirred with a clean tobacco pipe, the crucible was taken out, and the mixture digefted in water. Among the matter that remained undiffolved, only a few particles of platina could be diftinguiihed; and the examination having been carried no further when my papers were given in to the royal fociety in 1754, it was judged that the platina had been diffolved by the fulphurated alcali, as moft of the other metals are. The experiment however feeming on a revifal not fufficiently fatisfadtory, I vvao going to repeat it with more attention, when Mr. Marggrafs memoir came to my hands, in which I find it repeated alfo by him.
Mr. Marggraf firft mixed two ounces of pure fait of tartar, one ounce of fulphur, and half an ounce of platina; and fet the crucible, with another inverted and luted on it, in a forge. After the fire had been vehemently ex- cited for three hours, the Heffian crucible and its fupport, with part of the bricks of the forge, were found melted together, and on fome fragments was feen platina, in form of little filver leaves, but not well cohering. The excefs of heat having rendered this operation fruitlefs, it was ne- ceffary to make another trial.
Half an ounce of platina, half an ounce of flowers of fulphur, and an ounce of pure fait of tartar, in a crucible carefully luted as before, were urged in a ftrong fire for two hours. On opening the crucible, the matter appeared to have melted, looked yellowilh on the outfide, and when
T 1 1 broken,
[ 5°° ] broken, fhewed -here and there fome reddifh cryftals: it was leafy, like the mineral called by the Germans eifenrahm. Some hot water was poured on it, and more water added fo long as the liquor received any tinge. The filtered lixivium was of a yellow-green colour like the common folution of hepar fulphuris. On wafhing off" the lighter parts of the undifiblved matter, the remainder looked ex- actly like the eifenrahm, being in form of broad flakes, and foft to the touch: it was alio lighter than platina, and had not the leaft refemblance to it.
He mixed forty grains of this matter with an ounce of nitre, and threw the mixture by degrees into a red hot crucible : fcarcely any detonation happened. The fire being kept up for an hour, with care to prevent the falling in of any pieces of coal, there was obtained a grey mafs inclining to greenifh, which being fet to digeft in diftilled water, the fluid became prefently like gelly. By diluting and wafhing the matter, he recovered, without alteration, the platina which he had believed to be deftroyed.
This experiment appearing ftill indecifive, I made fome further trials. I mixed four ounces of flowers of fulphur with the fame quantity of pure fixt alcaline fait, and threw the mixture by little and little into a red hot crucible, co- vering the crucible after each injection. The mixture being in perfect fufion, an ounce of platina, which had been previoully expofed to a ftrong fire by itfelf till the grains were united into a lump, was dropt in, and a mo- derate heat continued for three or four hours. The lump of platina was quickly divided, though the metal did not remain fufpended in the fulphureous mixture, but fubfided, at leaft in great part, to the bottom, from whence it was every now and then ftirred and taken up with the bole of a tobacco pipe. The crucible at length cracked and was greatly corroded. The matter being boiled in about a
quart
C 501 ]
quart of difHlled water, the filtered liquor was of a dark reddifh colour : the remainder, boiled in frefh quantities of water, gave an olive-green tincture. The boiling being repeated, and the matter rubbed in a mortar, till it gave no more tinge to the water, the part which remained at laft undifiblved was a dark-coloured powder, which had nothing of the appearance of platina, but was found to be no other than platina divided. This platina was treated in the fame manner with frefh hepar, three or four times : the crucibles always failed and were much corroded, and the platina was reduced into a powder fo fubtile, that it could not be feparated by warning ■from the parts of the crucibles which were pounded with it.
I tried alio a hepar made in Stahls manner, by melting vitriolated tartar with powdered charcoal. This mixture melted very eafily, without any addition of the alcaline fait or common falt,which are generally thought necelTary for promoting the fufion : for though vitriolated tartar is very hard of fulion by itfelf, yet here, its vitriolic acid uniting with the inflammable part of the charcoal into fulphur, the matter becomes a compound of fulphur and alcali, and melts as ealily as the hepar made directly from thofe ingredients. Platina, treated with this hepar, fuffered the fame change as from the other. The crucibles were equally corroded : the watery folutions of the mafs were partly reddifh, and partly of an olive-green colour: the grains of platina, previoufly agglutinated into a lump by flrong fire, were difunited, and in great part divided into a powdery form.
It appears therefore that platina is divided by hepar ful- phuris in fufion, nearly in the fame manner as by long ce- mentation with nitre : it remains to examine whether any part of it was truly diflblved, fo as to be taken up by the water along with the fulphureous alcaline mixture. I fil-
T t t 2 tered
[ 502 ]
tered the liquors twice through double papers, and then added by degrees fpirit of fait to neutralize the alcali : at firft a browniih precipitate fell, and afterwards a white one like the common precipitated fulphur. A little of the brown precipitate was heated in a fmall fcorifying difh, and fome nitre added to burn off the fulphur more effectu- ally: there remained on the difh feveral bright particles like platina, fprinkled all over its furface. The reft of the precipitate being burnt in like manner, I added fome pure lead, to collect the difperled particles of the platina, and afterwards worked oft" the lead in a cupel : it left a rough brittle bead, exaclly like thofe obtained in cupelling crude platina with lead, of which an account will be given here- after in the feventh feclion. It feems to follow from thefe experiments, that hepar fulphuris really diffolves platina, though very difficultly, and very fparingly.
XI. Platina with earthy bodies.
Certain earthy bodies arc found to promote the fufion, not only of fome metallic minerals, but in fome inflances, of the purer metals alio : thus forged iron, which could not be made to melt in a crucible without addition, was brought into fufion by furrounding it with gypfum or plafter-of-paris, a facf which we owe the difcovery of to Mr. de Reaumur. To fee if platina would be anywife affected by fublf ances of this kind, I intermixed an ounce of it with gypfum, and urged it with a ftrong fire in a blaif furnace for two hours : the Hefiian crucible was corroded in many parts to the thinnefs of paper, and here and there quite through, the matter of the crucible and the gypfum having in fome meafure vitrified together; but the platina remained unmelted, and unaltered.
Quicklime, and calcined flint, were likewife made trial of in the fame manner, but no change was produced by them in the platina.
[ 5°3 ]
XII. Platina with vitreous bodies. i. Half an ounce of a precipitate thrown down from folution of platina by tin, was ground in an iron mortar, with eight times its weight of common flint glafs, and the mixture put into a crucible, which was covered and luted, and placed in a wind furnace. The fire was gra- dually raifed, and kept up extremely flrong for about ten hours : the crucible being then taken out and broken, the matter proved of a dark blackim colour, untranfparent, friable, interfperfed with a bright whitifh fubftance appa- rently metallic. It is probable that this metallic matter was the platina, and that the glafs owed its opacity and dark colour, not to this metal, but to tin in the precipitate, or fome particles of iron worn off from the mortar, or other accidental caufes.
2. I ground in a glafs mortar a quarter of an ounce of a precipitate of platina made by alcaline fait, with twelve times its weight of powdered flint glafs, and committed the mixture to the fame fire as the foregoing. The refult was a compact cloudy glafs, pretty tranfparent in thin pieces, covered in part with a thin whitifh coat. Towards the upper part, and all round the fides, were obferved fe- veral particles of metal, which appeared to the eye like bright platina, and proved hard to the point of a knife. In this, as in the foregoing experiment, the glafs feems to have received nothing from the platina, the change being no other than what flint glafs is found to undergo from a flight introduction of inflammable matter.
3. Mr. Marggraf gives an account of three experiments of the mixture of platina with vitreous bodies. Five drams of pure fait of tartar, twelve drams of clean fand calcined and wafhed, one dram of calcined borax, two of nitre, and two of crude platina, were mixed together, and,
kept
[ 5°4 ] kept in a vehement fire, in a cloie crucible, for feveral hours. The rcfult was a vitreous, fomewhat opal-like mais, inclining to a fea green : the platina, no otherwife altered than in being made whiter, was difperfed partly on the furface of the glafs, and partly about the fides, and furrounded with a diftincf vitreous matter of a deep hy- acinth colour.
4. He tried alfo the powder feparated from platina by cementation with nitre, as defcribed in page 492. Six grains of this powder were mixed with a hundred and eighty grains of white fand, and ninety grains of fait of tartar. The mixture, melted with a vehement fire in a clofe crucible, proved a porous, greyilh, untranfparent glafs.
c. He prepared a precipitate from platina and tin toge- ther, and tried to vitrefy this mixt. A polifhed plate of tin being digeftcd in folution of platina, part of the platina precipitated upon the tin in form of a blackifh red pow- der, and the tin, in fome days, was quite corroded : the liquor, of a dark coffee colour inclining to black, being put into a filter, run through blackifh. This compound folution of platina and tin was precipitated with fait of tartar : the liquor now palled the filter colourlefs, and the matter which remained on the paper, being well wafhed with hot water and dried, was a black fubffance, almoft refembling, in its fracture, broken pitch or fine pit-coal. Forty grains of this fubftance, fixty of calcined borax, a hundred and twenty of purified nitre, two hundred and forty of pure fait of tartar, and four hundred and eighty of powdered flint, were well mixed together, and melted in a very ftrong fire. They yielded a greyifh glafs, in which no metallic grains could be found : a thin piece of the glafs, laid upon the nail and held up to the fun, inclined to an amethyft colour.
It
[ 5°5 1
It does not appear from thefe experiments, that any part of the platina was truly vitrified. We may rather conclude, that the disappearance of the platina in the two laft experiments was owing to its being diffufed through the mafs in a ftate of powder too fubtile to be diftinguifh- cd : the colour of the glafs cannot be afcribed to the pla- tina, fince No. 3. afforded colours more coniiderable, though the grains of platina remained unaltered.
6. In my experiments, No. 1 and 2, particularly in the latter, the platina, though it had been attenuated by folu- tion and precipitation before its mixture with the vitrefci- ble ingredients, feparated from the glafs in fufion, and was collected into very feniible particles, fome of which were of confiderable magnitude. In an experiment of Mr. Mac- quers, this effect was ftill more ftrongly marked. The red precipitate of platina made by alcalies was mixed on a por- phyry ftone with a flux compofed of one dram of calcined borax, one dram of creme of tartar, and two drams of a white glafs, which he had himfelf prepared from fix parts of white fand and eight parts of borax : the proportion of the precipitate of platina to this flux he does not fpecify. The mixture was urged with the fire of a forge, animated by feveral bellowfes, for thirty-five minutes, and the matter being then in good quiet fufion, it was Suffered to cool. The upper part of the mafs was a blackifh glafs : at the bottom of the crucible was found a well collected lump of platina, pretty brilliant and fmooth on the furface, weigh- ing ninety-fix grains. This lump had all the appearance of a metal that had received a very good fufion : neverthe- lefs, on trying to extend it under the hammer, it broke in two pieces, and fhewed an oval chamber or cavity in the middle : the fracture refembled that of large-grained brit- tle iron : in hardnefs it was nearly equal to forged iron, for it Scratched deeply gold, filver, copper, and even iron
itfelf,
[ 5°6] itfelf. The texture, brittlenefs, and cavity in the lump Shewing that the platina, though it had approached con- siderably towards fufion, had not melted perfectly, the au- thor propofes to repeat the operation with a degree of fire frill ftronger.
It mufl be obferved on this experiment, that in the pre- cipitate made ufe of, the platina cannot be fuppofed to have been pure from other metals. Solutions of platina plainly contain iron, as appears from their ftriking a blue colour with the Pruffian alcali : either fixt or volatile alcalies precipitate this iron along with the platina ; and as part of the platina remains diffolved, the precipitate may con- tain a greater proportion of iron than the grains of platina itfelf did. Though the iron is in a ftate of calx, diffoluble by the glafs, and not mifcible with metallic bodies in their perfect ftate ; yet a flight introduction of inflammable matter is Sufficient to revive it, calces of iron feeming to be eafier of revival than thofe of any other metal. The black colour of the glafs was owing doubtlefs to the iron; nor would it be furprifing, if preparations of platina mould on further trials be found to tinge glafs of all the colours that iron can communicate. If the platina really melted, the f uiion may be afcribed to a mixture of the fame metal : but moft probably the appearance of fufion was no other, than a conglutination of the impalpable atoms into which the platina had been divided, fimilar to what we find to happen when the crude mineral is urged with a ftrong fire.
From the experiments related in this fection it appears, that platina is not only of itfelf refractory in the fire; but obftinately refills the additions, and managements, by which every other known metallic body is corroded, dif- folved, or changed to a vitreous ftate. If, as the chemifts teach, metals are the more perfect, in proportion as they are the more permanent and the lefs fufceptible of changes, platina is of all known metals the moft perfect.
T
[ 5°7 ]
SECT. V.
Of the mixture of platina with metals.
H E advantages which this new metal receives from its permanence and untarnifhing whitenels, and from its refiftence to liquids by which moft of the other metals are corroded or diffolved ■, are in great meafure rendered ineffectual, by its wanting the fufibility, which might enable the workman to form it into veffels or uten- fils. We have little foundation to expedr. any ufes of this kind, from fo refradtory a body, unlefs in combination with other metals ; fome of which may, perhaps, either have their own qualities improved by the admixture of certain proportions of it, or ferve as intermediums for uniting the parts of the platina, without much injuring it in regard to thofe properties in which its excellence con- lifts. Thefe hopes contributed to animate me in the pro- fecution of a laborious fet of experiments, which however, independently of fuch confederations, could not fail of affording interefting phenomena. I regret that through- out this lection, I have little more than my own trials to relate : Marggraf and Macquer have not entered into this enquiry, and Scheffer could proceed in it but a little way for want of platina to work upon. From the united la- bours of fuch hands, difcoveries of more importance may refult.
As platina is to be diffolved by the melted metals, we (hall apply to it the feveral metallic bodies nearly in the or- der of the facility with which they become themfelves fluid in the fire ; beginning with the lingular one which we find naturally in a ftate of fulion.
U u u I. Platina
C 508 ]
I. Plat inn with Quickfilver.
One ounce of platina and fix ounces of pure quickfil- ver were rubbed together, with a little common fait and water and a few drops of fpirit of fait, in an iron mortar. After the grinding had been continued about fix hours, the grains of platina appeared coated with the quicklilver, Co as to cohere together into a kind of imperfect amalgam. The fluid quicksilver being poured off, I evaporated part of it in an iron ladle : it left a considerable quantity of a dark-coloured ro\wicr, intermingled with mining parti- cles. A part of th quickfilver was pafled through a linen cloth, and a part was {trained through thin leather : loth thefe left alio on evaporation a like powder, the quantity of which was pretty considerable from the portion which had been Strained through the linen, but very fmall from that which had palled through the leather.
Mr. Scheffer alfo tried the amalgamation of platina with mercury, and reports that it did not fucceed, though the grinding was continued, with the addition of a little aqua regia, at leaSt twice as long, as is requifite for the amalgamation of iron filings with mercury when folution of green vit-iol is added. It appears from the above ex- periment, that great part of the platina, even after long grinding, remains Hill in entire grains, not diSTolved or combined with the mercury into fuch a mafs as is called an amalgam : but the adhefion of the mercury to the fur- face Shews an affinity between the two, or a difpofition to unite ; and the powder left upon evaporating the flrained quickfilver is a proof that fome part of the platina was truly diflblved. I repeated the experiment feveral times, and always found that a part of the platina was diflblved by the mercury, and that the undiflblved grains were coated
with it.
II. Platina
[ 5°9 J
II. Platina with Bifmath.
A mixture of black flux and common fait being brought into fufion in a crucible, equal parts of platina and bifmuth were dropt in, and urged with a quick fire ftrongly excited by bellows. The two metals appeared to have melted together in a few minutes ; and the crucible being then taken out of the fire and cooled, the metallic lump at the bottom, freed from the flux, was found to weigh nearly as much as the ingredients did at firfl, the lols being only about one part in a hundred and twenty. On breaking it, no grain of platina could be feen, this me- tal feeming to be all diflblved and blended with the bif- muth.
The experiment was repeated in a wind furnace, but in this gradual heat the two metals did not well unite; nor was the union here perfect till the biimuth was increafed to about thrice the weight of the platina. By larger quan- tities of biimuth, the platina was very eafily diflblved in the wind furnace as well as in the blafl: furnace ; but in all cafes a part of it fubfided if the mixture was fuffered to cool ilowly.
I melted platina with different proportions of bifmuth, as far as twenty-four parts of the latter to one of the for- mer. All the compoiitions proved, like the bifmuth it- felf, extremely brittle : one was not remarkably more or lefs fo than another. To the file, they were fcarcely harder than pure bifmuth. On breaking them, the furface of the fracture appeared for the moil part compofed of ftrias and narrow plates ranged tranfverfely : with the larger propor- tions of biimuth, the Aria? and plates were coarfe and irre- gular ; With the fmaller proportions, finer; and when the two metals were in equal quantity, they could fcarce be diftinguifhed at all. When the mafles were newly broken,
U u u 2 they
1 510 ]
th^y looked bright and fparkling; except the compofitions with a large proportion of platina, which were of a dull greyifh colour, without any brightnefs. In the air they all tarnifhed in a remarkable manner, changing to a yel- lowiih, a purpliih or bluifh, and at length to a purpliih black : every one of them has fuffered thefe changes, though fome more flowly than others.
III. Platina with Tin.
1 . Equal parts of platina and pure tin were dropt into a mixture of black flux and common fait in ftrong fufion, and urged with a quick fire in a good blaft furnace. After a few minutes, the two metals appeared perfectly melted; and being inftantly poured out, they run freely along a narrow mould, forming a fmooth ingot, nearly of the fame weight with the platina and tin employed. The com- pound proved extremely brittle, breaking eafily from a fall. When broken, it appeared of a clofe and fmooth, though uneven, furface, and of a dull dark colour. By the file, or a knife, it was readily fcraped into a blackiih duft.
2. One part of platina and two of tin, covered with black flux, borax, and common fait, were melted in a wind furnace: the platina appeared perfectly taken up by the tin, foon after the fire had been raifed to a light white heat. The ingot was found deficient in weight about one nine- tieth part. It greatly refembled the foregoing, being only a little lefs brittle, and of a fomewhat lighter colour.
3. One ounce of platina and four of tin, covered with black flux and common fait, and urged with a quick fire, melted together, with fcarcely any lofs. This compound ftretched a little under gentle ftrokes of a flat hammer, but was by no means tough. It broke in pieces from a rude blow, and was readily fcraped into duft by a knife. The broken furface was rough, and of a granulated texture.
4. One
[5" J 4- One ounce of platina and eight of tin, dropt into a fluid mixture of black flux and common fait, united, with- out lofs, into a pretty tough compound; which bore to be considerably flattened under the hammer without break- ing, cut fmooth with a thin chifel, and Shaved with a knife. Broken, it appeared of a fparkling, dark coloured, coarfe grained texture.
5. One part of platina and twelve of tin, treated in the fame manner, formed a mixture tolerably ducfilej but Still of a dull dark hue, and a rough coarfe grain, though lefs fo than the preceding.
6. A mixture of one part of platina and twenty-four of tin, Stretched under the hammer almoSt as eafily as tin it- felf, but broke much fooner on bending. The colour was whiter, and the grain finer and evener, than thofe of the preceding compofitions, though in both reSpects it fell confiderably Short of pure tin.
7. Several of thefe compofitions, covered with black flux which had been previoufly melted by itfelf, till it ceafed to boil up, were expofed, in crucibles clofely luted, to a Strong fire in a wind furnace, which was Steadily kept up for eight hours. When taken out, they were all found to have Suffered fome diminution of weight, amounting to about one fortieth part of the tin. In their appearance and quality, there did not feem to be any alteration, except that the grain was a little finer, and the texture rather more uniform.
The foregoing mixtures feem to include a Sufficient la- titude, in the proportions of the two metals, for discover- ing their general effects on one another. We may infer from them, that within this latitude, platina diminishes the malleability of tin, renders its texture coarfer, and de- bafes its colour, more or lefs according as the platina is in greater or lefs proportion; and that, when the platina
amounts
[ J« ]
amounts to about one third of the tin or upwards, the malleability, which both metals feparately poffefs, is de- ftroyed by their combination with one another. The dif- ference in the colours of thefe compofitions was not fo confpicuous on the touchftone, as when the fractures of the ingots were -examined; although, on clofe inflection, the marks on the ftone alfo appeared all of a darker colour than thofe of pure tin, and the more fo as the proportion of platina prevailed in the mixt. Kept in a clofe room, in pill-boxes, they all tarnifhed in the fracture to a yel- lowifh hue; but pieces which were ground and polilhed have in ten years fuffered little change, except only the mixture of equal parts of platina and tin, which is grown conliderably dull and yellow.
It is obfervable, that though tin is a metal very readily deftructible by fire, yet in raoft of the foregoing fufions there was fcarcely any lofs of weight. This may be at- tributed in part to the admixture of the platina preventing the fcorification of the tin ; and in part to the flux made ufe of, and the celerity and fhort continuance of the heat. No. 2 and 7, where the heat was flowly raifed and long continued, were the onlv ones in which the lofs was at all con lider able.
IV. Platina -with Lead.
1 . Equal parts of platina and lead were injected into a mixture of black flux and common fait previouily melted together, and the lire railed haftily by bellows. A much ftronger heat was rcquilite than for the fulion of platina with an equal quantity of tin, and the lofs was conliderably greater, amounting to about one fixty-fourth part of the metallic mixture. The metal yielded difficultly to the file, broke from a moderate blow, and appeared, on the fracture, of a clofe texture, an uneven furface, and rough jagged edges. The colour was very dark, with a faint purplilh call:.
[ 5*3 ]
2. One part of platina and two of lead, covered with black flux and borax, and expofed to a gradual lire in a wind-furnace, did not come into due fulion till the lire had been raifed to a ftrong white heat. From the long con- tinuance of the fire in this experiment, the lofs was great, amounting to nearly one twenty-fourth part of the mix- ture. The ingot proved hard and brittle like the pre- ceding, but the texture filiated, and the ftria? difpoled tranfverfely.
3. One ounce of platina and three of lead, treated in the lume manner, required Hill a very ftrong fire for their per- fect fuiion, and loft about one twenty-iixth. The metal broke lefs ealily than either of the preceding, and in fome meafure ftretched under the hammer. Its colour was fomewhat darker, and inclined more to purplilh.
4. One part of platina and four of lead being covered with black flux and common fait, and committed to a wind furnace, the platina ftill did not appear perfectly taken up till the fire had been railed to a coniiderably ftrong white heat : the lofs was about one fortieth. The fame proportions of the two metals, dropt into a fluid mix- ture of the flux and fait previoufly brought to the above degree of heat, quickly melted, and loft only one part in a hundred and lixty. The ingot was much tougher than the preceding, filed well, and cut tolerably lmooth with a knife. On breaking, the upper part appeared compofed of bright plates ; the lower, of dark purplilh grains.
5. One part of platina and eight of lead united eafily in a quick fire, and loft little or nothing. The metal work- ed, and looked, like very bad lead. On breaking it, the texture appeared partly compofed of tranfverfe fibres, and partly of grains ; its colour was dull and purplilh.
6. One part of platina and twelve of lead united with- out lofs into a compound very little different from the
preceding.
1 514]
preceding. Its texture was finer, and compofed chiefly of tranfverfe fibres, with very few grains.
7: A mixture of one part of platina and twenty-four of lead proved not very much harder than lead of a middling quality. The colour was ftill fomewhat purplifh, and the texture fibrous ; but the fibres were remarkably finer than when the platina was in greater proportion.
8. The four firft of the foregoing compofitions, when newly polilhed, appeared of a dark iron colour, which quickly tarnifhed to a brownifh yellow, a deep purplifh, and at length to a blackilh. The three laft, cut with a chifel, looked of a leaden hue, which tarnifhed but little; though the fractures, and outer furfaces, of all the feven have changed nearly to a like purplifh black.
9. Upon returning thefe compounds to the fire a fecond time, it was conflantly obfei ved, after they had come into perfect fufion, that if the heat was flackened a little, great pait of the platina fubfided : that neverthelefs, the lead de- canted off even in a heat below ignition, letained fo much of the platina, as rendered it of a fine fibrous texture and purplifh colour.
10. The feveral mixtures, covered with black flux, and kept in ftrong fufion, in crucibles clofely luted, for eight hours, fuffered a diminution in weight, amounting, in moft of them, to about one thirtieth part of the lead. On breaking them, thofe with a large proportion of platina appeared of a leafy, and thofe with a fmaller of a fine fibrous texture, which feemed in general to be characterises of the perfect union of the platina and lead. They all looked whiter and brighter than at fir ft, but tarnifhed fooner in the air. One mixture in particular, of four ounces of pla- tina and twelve of lead, bioke into large, white, fhining, talk-like flakes; which, on expofure to the air, changed in a very little time, in lefs than an hour, to a reddifh,
a purple,
t sis ]
a purple, and a deep blue, and at leHgth turned flowly to a dark purplifh black colour.
The relations of platina to tin and lead appear therefore to be very different. Though a fmall proportion of it is taken up and kept fufpended by lead in a very gentle heat; a large proportion is not near fo readily diffolved as by tin, and, when united by a ftrong heat, fublides in great part upon the abatement of the heat. A little quantity fliffens and hardens lead more than it does tin, but a large one does not near fo much diminifh its malleability: a mixture of equal parts of platina and lead, though it has nothing -of the ductility which each of the metals has feparately, is much lefs brittle than the mixture of equal parts of pla- tina and tin. But the moft remarkable phenomena in the mixtures with lead are the leafy or fibrous texture, and a purplifh or bluifh colour or a difpofition to acquire thefe colours fpeedily in the air, and the black to which they at length change. Bifmuth, as we have already feen, exhi- bits with platina nearly the fame appearances, though in a fomewhat lower degree; and as none of the other me- tallic bodies I have tried was found to affect, or be affected by, platina in this manner, thefe experiments may be added to thofe of Mr. Geoffroy, in one of the late volumes of the memoirs of the French academy, for eftablifhing an analogy between bifmuth and lead.
V. Platina with Arfenic.
White arfenic is a volatile metallic calx, reducible to its metallic form by expofing it to a moderate fire with inflammable additions. A mixture of white arfenic and fixed alcaline fait, of each one ounce, with two ounces of powdered charcoal, was preffed fmooth into a crucible, and one ounce of platina fpread above it : the crucible was clofely covered and luted, and kept for twelve hours in a
X x x moderate
C 516 1
moderate cementing heat, which towards the end was in- creafed to a conhderable degree. On feparating the pla- tina from the mixture by wafhing, many of its grains ap- peared divided, and its weight was fomewhat increafed. Being afterwards expofed haftily to a very intenfe fire, it did not melt, but emitted arfenical fumes, and after thefe had ceafed, the platina was found to weigh juff one ounce as at firlt.
This experiment feeming to fhew that -platina and- arfenic huive fome dilpolition to unite, I was preparing to profecute it, to fee if more arfenic could be combined with the platina fo as to bring it into fufion, when Mr. Scheffers papers came into my hands, in which I find a remarkable experiment on this point. Mr. Marggraf likewife has fince tried platina with arfenic, in a manner not greatly different from that above mentioned.
Mr. Marggraf mixed one dram of platina with two drams of white arfenic, and expofed the mixture to the fire in a glafs retort: the arfenic rofe uncoloured, and left the platina white and undiminiihed in weight. The pro- cefs was repeated with the fame quantity of frefh arfenic, and the fire augmented to as great a degree as the coated retort could bear: the arfenic ftill rofe white, but the grains of platina were now become black, though they llill continued malleable, and weighed as much as at firft.. A dram of platina, two drams of arfenic, and one dram of fulphur, being well mixed together and treated in the fame manner; the arfenic and fqlphur fubliming together form- ed, as they ulually do when united in thefe proportion^ a red compound ; the platina becoming blackilh, and weighing two grains, or one thirtieth part, more than at firft. It feems therefore that in this way of managing the procef?, the arfenic has lefs effecl: on the platina than, tn my experiment above mentioned,
Mr,.
[ 5i7] Mr. Scheffer proceeded in a different manner. The platina was firft ftrongly heated in a crucible by itfelf, and a little arfenic being then thrown upon it, they immedi- ately melted together. He obferves, that platina melts with arfenic as eafily as copper and iron do when they are blended with arfenic; that there is no occafion for any flux; that one part of white arfenic is furhcient for four and twenty parts of the platina; and that the platina thus melted with arfenic is quite brittle, and breaks grey like arfenicated filver.
On repeating this experiment it appeared, that though the judicious author is by no means chargeable with any miftake, yet the little quantity of platina, he had to allow for the trial, made it impoflible for him to difcover the limitations, with which this ftrong action of arfenic on platina ought to be underftood. When only a few grains of platina are ufed, there is all the appearance of true fufion, but on taking larger quantities we frequently find the fufion to be only fuperficial and imperfect. An ounce of platina was ftrongly heated in a crucible, and pieces of white arfenic repeatedly thrown upon it, the arfenic amounting in all to near as much as the platina: fome of the grains melted into round drops: the greater part co- hered into a mafs, differing from thofe, into which platina itfelf is formed by fire, in the furface being fmooth and uniform, and the grains in the internal part more firmly coherent. I treated another ounce of platina in the fame manner, and with the lame event: the mafs was of a fmooth furface, as if it had been perfectly melted, but its internal part was compofed of grains of platina in their ufual form. I put both maffes into a crucible, with freih arfenic mixed with powdered charcoal, and urged them with a ftrong fire for half an hour: they run into one lump, of the figure of the bottom of the crucible, exter-
X x x 2 nally
[ 5*8 ] nalry lmooth, and of a bright white colour like quicfc- filver, very brittle, internally greyifh, of a fpongy texture,, with fome few of the grains of platina left entire in the middle: the crucible was lined with a black glafs, pro- bably a vitrification of theferrugincous part of the platina; and federal fhining metalline globules adhered to the vi- treous matter. The lump was again dropt into a ftrongly heated crucible, with more arfenic and charcoal powder, and the fire excited by the bellows for another half hour: it melted as before, into a cavernulous mafs, in which no grains of platina could now be fcen. It was again treated in the lame manner with frefli arfenic, and tried to be poured out; but though the fire was made very intenfc, die metal would not run from the crucible. Being then urged in a quick fire without addition, it concreted into a lump of the fame appearance as before: but a piece of this lump, dropt again into a crucible intenfely heated, did not feem to foften or fuffer any alteration of its figure. The reft of the lump was inclofed between two fmall pieces of charcoal, a cavity being made in each piece for receiving it: the charcoal was coated over with luting, and, when thoroughly dried, dropt in among the fuel be- fore the nofe of the bellows: the metal did not alter its figure, nor was its weight diminilhcd: I took half an ounce of the metal, and arfenicated it again in the fame manner as at firft, adding at different times more and more arfenic : it run into a lump as before, but I could not, by any increafe of the fire, or by any addition of arfenic, make it thin enough to flow from the crucible. I took half an ounce more of platina, and having com- bined with it as much arfenic as I could by repeated in- jections, I reduced the mafs into grofs powder, mixed it with black flux and fome frefh arfenic, and urged it with a quick fire in a covered crucible : the metal run into a.
fpongy
[ .519 ] fpongy lump, which retained particles of the flux here and there in its cavities, a mark that it had not flowed thin;
It appears upon the whole, that platina does melt with arfenic, but lefs perfectly than with other metals; and that it would be very difficult, if not impoffible, to bring it, on this foundation, to fufficient fufion for being poured into a mould. All the arfenicated pieces are very brittle, internally of a greyifo colour, and a loofe granulated tex- ture. It is obfervable that though arfenic foon changes in the air to a blackim hue, and when mixed with other metals difpofes moil of them to change in like manner, the arfenicated platina, after lying in a dry room for (even or eight years, continues nearly of the fame appearance as at fir ft.
VI. Platina •with Zinc.
For uniting zinc with platina, I firft tried the method in which zinc is commonly united with copper, and by which the zinc is at the fame time purified from fuch other metallic bodies as are frequently blended with it> viz. expofing the platina to the fumes, extricated by fire and inflammable additions, from calamine, one of the purer ores of zinc. But that thefe fumes might act the more effect ually on the platina, a little variation was made in the common manner of diipofing the materials.
Four ounces of calamine in fine powder were mixed with two ounces of powdered charcoal. Having often obferved that calamine, with this proportion of charcoal, acquires a kind of fluidity in the fire, fo that the platina would be apt to fink through it to the bottom; I made the powder into a mafs with a little thin tempered clay, and preffed it into the bottom of a crucible: above this mafs, the crucible was lined all round with luting to a confiderable thicknefs, fo as to leave only a final 1 paffage in tfie middle for the fumes of the zinc to iflue out; in
which;
[ 520 ]
which paflage, when the luting was thoroughly dried, an ounce of platina was placed. The crucible was covered and fet in a wind furnace, and a pretty ftrong fire kept up for fix hours. Being then taken out, fome flowers of zinc were found adhering to the cover, greateft part of the pla- tina was melted into fmall bright globules, and fuch grains, as retained their figure, appeared frofted over with minute globular protuberances, as if they had juft begun to melt. Its weight was increafed above a third part, lb that it had imbibed about as much of the zinc as copper does in the common procefs of making brafs.
Finding the fumes of zinc to act fo powerfully on pla- tina, I next tried zinc in its common metallic form. Up- on an ounce of platina, covered with borax, and heated in a blaft furnace to a ftrong white heat, I threw an equal quantity of zinc. A violent deflagration arofe, and the platina feemed to be almoft inftantly difiolved. The me- tal, being immediately poured out, run freely into the mould, and was found to have loft near half an ounce in weight ; fo that the quantity of zinc, which had furficed to keep the platina in good fufion, was very little more than one half of the platina.
I made feveral further trials of the fame kind, with dif- ferent proportions of the two metals, both in a quick fire in a blaft furnace, and in one more gradually raifed in a wind furnace : the zinc always proved a ftrong menftrnuin for the platina, though much of the zinc was difilpated by the heat requifite for rendering the mixture furficiently fluid. One ounce of platina and four ounces of zinc be- ing meked together in the blaft furnace, as in the above ex- periment, the lofs was an ounce and a half, fo that there remained with the platina about two ounces of the zinc. This compound was dropt upon another ounce of pla- tina, ftrongly heated as before with borax : the metal,
poured
C 52i 3
poured out, run clean from the crucible, and weighed jurl two ounces and a half, fo that the platina was here kept in fufion by one fourth its quantity of zinc. This mixture was put into the fame crucible, with the fame borax : it /fill deflagrated, melted, and on being poured into an iron ingot-mould, which had been newly fmoked over the flame of a torch but not heated, the fluid metal was thrown about with violence in fmall drops : this probably hap- pened, not from any particular qualities of the metal, but from fome moiflure in the mould.
Compofitions of platina with different proportions of zinc differed little in appearance from zinc itfelf ; except that where the quantity of platina was large, they were of a clofer texture and duller hue, with rather more of a blu=- iih cail. Kept for ten years in a dry room, they do not leem to have tarniihed or changed their colour. They were much harder to the file than zinc itfelf, and fell in pieces under the hammer ; without at all ifretching, as pure zinc does in a considerable degree. One twentieth of platina defrxoyed the malleability of zinc, and one fourth of zinc deftroyed the malleability of platina : within this compafs, we have no degree of ductility to expect from any mixture of the two.
VII. Platina issith Regnlus of Antimony. Equal parts of platina and regulus of antimony were dropt into a fluid mixture of black flux and common fait, and the fire ftrongly excited by the bellows. They melted perfectly together, and run freely into the mould. The compound looked of a much duller colour than the regu- lus at firft, and, when broken, fhewed a clofe and uniform, though uneven furface : it proved considerably harder to. the file, but not remarkably more or lefs brittle under, the hammer.
One.
[ 522 ]
One part of platina and twenty of regulus of antimony being treated in the fame manner, the compound looked brighter, and of a leafy texture, little different from that of the pure regulus.
The two metals were melted together in feveral of the intermediate proportions, but no other differences were obferved than thofe abovementioned ; the mixtures with a large proportion of platina being of a dull colour and clofe texture, and thofe with a fmall one bright and leafy. All of them continue untarnifhed.
Though the platina and regulus feemed to unite very well together, yet in flow cooling, part of the platina was apt to fubfide. Six ounces of platina and twenty-four of regulus of antimony having been melted together with a quick fire, and poured into a mould, the compound ap- peared uniform throughout. Being melted again, kept in fteady fulion for five or fix hours, and fuffered to cool gra- dually in the furnace ; the upper part of the mafs was bright, and of a large leafy texture, much like the regulus at firfr. ; the bottom was much duller and of a clofer tex- ture, and contained apparently a much larger proportion of the platina.
VIII. Platina with Silver. i. Twenty grains of platina, and the fame quantity of pure filver which I had revived from luna cornea, were covered with borax, and urged with a vehement fire in a blaft furnace. They melted difficultly together, and did not prove fluid enough to run freely along the mould. The metal weighed thirty-nine grains, and on the fides of the cru- cible were feen feveral fmall particles, amounting, as nearly as could be judged, to about a grain more, fo that there appeared to be no lofs of weight. The compound was hard to the file, and broke from a rude blow, though by gentle
ftrokes
[ 523 1 ftrokes it bore to be confiderably flattened. Internally U appeared of a much duller and darker colour than filver, and of a much coarfer grained texture.
2. One part of platina and two of filver, covered with nitre and common fait, did not flow thin till the fire was raifed to a ftrong white heat, and, when poured out, left many fmall particles adhering about die fides of the cru- cible. The metal proved lefs brittle than the foregoing, and not fo hard to the file: its texture was finer grained, and the colour whiter.
3. One part of platina and three of filver required alfo a very ftrong fire for their perfect fufion, and many par- ticles of the metal were thrown up almoft to the top of the crucible, as if the action of the filver upon the platina had been accompanied with a kind of ebullition or explofion. The compound was hard and brittle, though lefs fo than the preceding : by repeated nealing, it bore to be ham- mered, or flatted between fteel rolls, into thin plates.
4. One part of platina and feven of filver melted toge- ther pretty eafily, but a part of the metal was thrown up about the crucible as before. The compound hammered tolerably well, proved much harder than filver, and not fo white, nor of fo fine a grain.
5. In the foregoing experiments, the quantity of platina was from ten to twenty grains. I tried fixty grains of platina with four times, eight times, twelve times, twenty times, and thirty times as much fine filver. One of thefe mixtures was treated without any flux, another was co- vered with borax, another dropt into borax previoufly brought into fufion, another into melted black flux, and the laft into melted common fait: the fire was ftrongly excited by the bellows, and all the mixtures fuffered to cool in the crucibles. With thefe larger quantities of the two metals, the phenomenon above taken notice of was
Y y y more
[ 524 ] more remarkable: Numerous metallic globules appeared: all over the iniides of the crucibles, and many on the co- vers alfo: the differences in regard to the fluxes, and in the proportions of the two metals, ieemed to make no material difference in this rcipecl. Some of the mixtures were melted over again, in frefh crucibles, feveral times : the metal lparkled up in the lame mariner every time. On pouring them into moulds, unlefs the heat was very in- tenk, a considerable part remained' behind, the filver feeming to quit the platina on an abatement of the heat. When the heat was lb ffrong that the whole run fluid into the mould, great part of the platina feparated and fell to the bottom in cooling, unlets when the mould was very broad, fo that the compound begun to rix almoil immedi- ately, without allowing time for the platina to fubfide.
. 6. I likewife melted lilver with different proportions of a precipitate of platina obtained by adding mercury to a folution of platina in aqua regia. Here alio the event was the fame: the metal lputtered up in extremely minute grains, which feemed as it were to penetrate the crucible. 7. There appears upon the whole a ftrong repugnance between platina and lilver. Mr. Scheffer takes notice alio of the difficulty of uniting thefe two, though the fparkling up of the metal, which was not conliderable in my ex- periments when the quantities were (mail, does not leem to have been perceived at all in his. He obferves that platina melts more difficultly with filver than with lead or copper; that three parts of filver are neceffary for making one part of platina melt by a blow pipe; and that the mixture re- tains the whitenefs which both metals poffefs, but proves hard and brittle. In all my mixtures with large propor- tions of platina, the colour was greatly inferior to that of filver: belides being very dull, they had fomewhat of a yellowifh caftj and this yellowifhnefs continued fenfible
even
[ s*s ]
even when the filver amounted to twenty times the weight of the platina -, but one part of platina with thirty of filver made a mixture as white as the filver itfelf. None of them feem to have tarniflied or changed their colour in keeping.
IX. Platina with Gold.
The near and remarkable relation betwixt platina and gold in many properties hitherto iuppofed to belong to gold alone, their as manifeft difagreement in others, and the reports of gold having been debafed by the admixture of confiderable quantities of platina, induced me to exa- mine more particularly the effects of thefe two metals in combination with different proportions of one another. The proportions were adjufted according to the carat weights, as explained in the feventh fection of the hiilory of gold, the finenefs of gold being ufually exprefled in carats and their fubdivifions. The abfolute weight of what in thefe experiments is called a carat, was four grains.
i . Twelve carats of fine gold, and the fame quantity of the purer grains of platina, were urged in a blaft furnace for near an hour, with a fire lb ftrong, that the flip of Windfor brick with which the crucible was covered, though it had been dipt in thin tempered Sturbridge clay, begun to melt. Upon breaking the veflel, the metal was found in one fmooth lump or bead, which being nealed by the flame of a lamp, and boiled in alum water (the li- quor commonly ufed by the workmen for cleanfing or brightening maffes of gold and filver) appeared, both in the mafs and upon the touchftone, of a pale bell-metal colour, without any refemblance to gold. It bore feveral ftrokes, and flretchcd confiderably under the hammer, be- fore it began to crack about the edges. On viewing the fracture with a magnifying glafs, the gold and platina ap-
Y y y 2 peered *
[ 5*6 3 peared unequally mixed, and feveral fmall particles or' the
latter were ieen difiinct : nor was the mixture entirely uni- form after it had been again and again returned to the fire, and fuftered many hours of ftrong fufion.
2. Eighteen carats of gold and fix of platina were melted together as the foregoing, in an intenfe fire conti- nued about an hour. The bead, nealed and boded, was lefs pale coloured than the former, but had nothing of the colour of gold. It forged tolerably well, like coarfe gold.. To the naked eye it appeared uniform ; but a good mag- nifying glafs difcovercd, in this as well as in the other, fome inequality of mixture, notwithilanding the fufion was two or three times repeated, with as great a degree of heat as we could eafily excite by the bellows.
3. Twenty carats of gold and four of platina were kept in firong fufion above an hour and a half. Thefe united into an equal mafs, in which no granule of platina or difii- milarity of parts could be diftinguiihed.. The colour was ftill fo dull and pale, that the compound could fcarcely be judged by the eye to contain any gold. It hammered well into a pretty thin plate, but we could not draw it into wire of any considerable finenefs.
4. Twenty-two carats of gold were melted in the fame manner with two carats of platina, the fame proportion that ftandard gold contains of alloy. The mixture was uniform, and had a good deal of a golden colour, but with a particular dull dark hue, by which the eye could at once diftinguifh it, not only from fine gold, but from all the common forts of alloyed gold. It worked well, was forged into a thin plate without cracking, and drawn into mode- rately fine wire.
5. Twenty-two carats and a half of gold and one and a half of- platina, or fifteen parts of the former to one of the latter, melted into an uniform mafs, which after the ufual
nealing
[ 527 J nealing and boiling, proved fomewhat tougher than the preceding, and of a better colour.
6. Twenty-three carats of gold were melted widi one of platina, which is nearly half the proportion that ftan- dard gold contains of alloy. The compound worked ex- tremely well ; but was diftinguifhable from fine or ftandard gold by fome degree of the ill colour of the two forego- ing, which it retained after repeated forgings, fufions and boiling?.
7. Twenty-three carats and one fourth of gold, and three fourths of a carat of platina, or thirty-one parts of the former to one of the latter, formed an equal mixture, very malleable, ductile like the three foregoing while hot as well as cold, but not altogether free from their parti- cular ill colour.
8. A mixture of twenty-three carats and a half of gold with half a carat, or one forty-feventh its weight of pla- tina, was very foft and flexible, of a good colour, without any thing of the difagreeable caft by which all the forego- ing were readily diftinguifhable by the eye from any kind of alloyed gold I have feen..
9. A mixture of twenty-three carats and three fourths of gold with one fourth of a carat, or one ninety-fifth its weight of platina, could not be diftinguifhed, by the eye or the hammer, from the fine gold itfelf.
10. In all the above proceffes, even where the quantity of platina was very fmall, the fufion was performed with a vehement fire, tiiat the platina might be the more per- fectly diffoWed, and equally diffufed through the gold. This appeared to be a very neceiTary precaution. Having once melted gold with one fourth its weight of platina, the button appeared not much paler than ftandard gold with filver alloy, but on a fecond fufion it loft its yellow- Jiefs, and looked not much unlike hell metal. The gold
colour
[ 528 ] colour appeared to have been only fuperficial, from an im- perfect mixture j moft of the platina having been concealed in the internal part of the mafs, and covered as it were with a golden coat.
1 1 . In fome circumftances I have feen the gold, after it had been thoroughly mixed with the platina, fpued out again in part to the furface. The foregoing bell-metal coloured mixture, after repeated fufions with and without additions, and in different degrees of heat, became once yellow on the furface. On cupelling mixtures of platina and gold with lead, I have oftener than once feen the re- maining button covered with a golden fkin, and all the internal part grey.
12. In melting the platina and gold together, a little borax was always ufed as a flux ; with an addition of nitre, which fomewhat heightens the colour of gold, or at leaft prevents the borax from making it pale. Pieces of fome of the mixtures were remelted, with borax alone, with ni- tre alone, with common fait, with fixt alcaline fait, and with powdered charcoal : thofe with borax feemed to be the paleff, and thofe with charcoal powder the befl co- loured, though the differences were very inconliderable.
13. As a fmall portion of copper fomewhat heightens the colour of pale gold, I melted platina with eight times its weight of ftandard gold made with copper alloy ; that is, three parts of platina with twenty-two parts of fine gold and two of copper. The fufion was performed, as in the preceding experiments, with a ffrong fire, in a clofe cruci- ble, but without any flux, and continued about an hour. The metal appeared covered with a black fcurf, and had loft about a two-hundredth part of its weight. It was much duller coloured, much harder under the hammer, and cracked iboner about the edges, than mixtures of fine gold with confiderably larger quantities of platina. By
repeated
[ 529 ] repeated fufion and frequent nealing,it became a little fofter and tougher, fo as to be drawn into pretty fine wire; but the colour was flill exceeding dull, more refembling that of very bad copper than of gold.
It appears from thefe experiments, that platina dimi- nifhes the malleability of gold much lefs than it does that of the other malleable metals; and infinitely lefs than lead, tin, iron, and the brittle metals do that of gold: that in conliderable proportions it debafes the colour of gold far more than the ufual alloy, communicating a peculiar and remarkable ill colour; and that it both hardens, and de- bates the colour of ftandard gold, with copper alloy, much more than fine gold: that in fmall proportions, as one forty-feventh and downwards, it does not fenlibly injure either the colour or malleability of gold; and confequently, that large proportions of platina mixed with gold are dif- coverable at fight, but that fmall proportions, if perfectly united with the gold, will not betray themfelves either to the eye or in the workmans hands.
X. Platina with Copper.
1. Eqjjal parts of platina and copper, expofed, with- out addition, to a ifrong fire haitily excited by bellows, foon became fluid, but not thin; and loft about one fixty- fourth. The metal proved extremely hard to the file, broke difficultly on the anvil, flew afunder upon endea- vouring to cut it with a chifel, and appeared internally of a coarfe grained texture and white colour.
2. One ounce of platina and two of copper, urged with a quick fire in a blaft furnace, without addition, flowed fufficiently thin, and fcarcely fuffered any lofs. The me- tal was ftill very hard, and ftretched but little under the hammer. It looked darker coloured than the foregoing, with a flight reddifh caft.
3. One.
C 53° 1
3. One ounce of platina and four of copper, treated in the fame manner, united without lofs into a pretty tough compound, which bore to be confiderably flattened, cut with a chifel, and bent almoft double before it cracked. Internally it looked of a fine texture, and of a very pale copper colour.
4. A mixture of one ounce of platina and five of copper fbretched fomewhat more eafily under the hammer than the preceding, and appeared of a redder colour.
5. Upon increafing the copper to eight times the quan- tity of the platina, the compound proved fufficiently tough, broke difficultly, and hammered well. It was much harder than copper, and of a paler colour.
6. A mixture of one part of platina and twelve of cop- per was fomewhat more eafily extended under the hammer than the preceding, and proved fofter to the file. It (tuck a little in the teeth of the file, which the compofitions with a greater proportion of platina did not.
7. A mixture of one part of platina and twenty-five of copper was ftill fomewhat paler coloured than pure copper, and confiderably harder and ftifFer, though very malleable. On increafing the copper a little further, the mixture con- tinued fomewhat harder than the copper by itfelf, and ap- peared of a fine role colour.
8. In the foregoing fufions, though in general no flux was made ufe of, there was fcarcely any lofs of weight, ex- cept in No. 1, where the large proportion of platina re- quired the fire to be railed to a violent degree. This feems owing in great meafure to the platina preventing the fcori- fication of the copper: for on melting pure copper, a great number of times, both with and without fluxes, there was conftantlv lb me lofs.
0. The mixtures with large proportions of platina are difficultly extended under the hammer when cold, and
when
[ 53* ] vrhen red hot they fly in pieces : they bear a good polhh, and do not feem at all tarniflied in keeping ten years ; of the- mixture of equal quantities in particular, the polifhed part continues very brilliant. No. 7. has tarnilhed a little, but feemingly not lb much as pure copper.
Platina appears therefore from thefe experiments to harden copper, to dilute its colour, and diminifh its dif- pontion to tarnifh; in fmall proportions, to improve its hardnefs, without much injuring either the colour or mal- leability; and in larger proportions, to injure the mallea- bility lefs than it does that of any of the other ductile me- tals, except gold and perhaps filver. Platina and copper feem to form valuable compofitions, of which I doubt not but the workman may avail himfelf.
In a letter from Spain to a perfon in London, a tranf- lation of which has been communicated to me, there is an account of an experiment on platina and copper, which, though imperfectly related, may deferve to be mentioned here. The author firft tried platina with an equal weight of filver, and found them to melt together .... he after- wards melted it with copper, which united perfectly well; but whether it was the platina itfelf, or the mixture with filver, that was melted with the copper, is not clear from the words, though it feems to have been the former. The mixture with copper, " on trying to hammer it, flew about like glafs; but having melted it over again with a ftronger fire for fome time, and thrown in a little faltpetre, mer- cury-fublimate, and other corrofives, it became malleable, and was then made into rings, which were worn for a good while without foiling the fingers, and preferved always the fame colour and luftre as thofe called in Spanifh tombagos, which confift of two parts of copper and one
of gold.
Z z 7 A mixture
[ 532 ] A mixture of equal parts of platina and copper (No. i . of the above experiments) was tried by Mr. Scheffer, who reports, that they melted as eafily as copper by itfelf; and that the mixture proved tolerably malleable, as mixtures of gold witli a like quantity of copper: in both thele points, the little quantity he could allow for the experiment may be fuppofed to have occalioned fome deception. He adds, that when this compound is urged by a ftrong blaft impelled upon the furface, as in the purification of copper before the bellows, it throws out fparkles like iron in welding, and that thefe fparkles are found in form of round grains, which partake of both the metals; a phenomenon which gold does not exhihit with copper. After this operation, he found the mixture lefs malleable than before, like copper over-refined.
XI. Platina with Copper and Zinc.
i. Equal parts of platina and brafs, covered with borax and expofed to a quick fire in a blaft furnace, melted perfectly together, and fuffered very little lofs. The mixt was of a greyifh white colour, filed hard like bell-metal, broke from a blow of the hammer without ftretching or receiving any impreiTion, and flew afunder upon endeavour- ing to cut it with a chifel. Internally, it appeared of an uniform fine grain, a clofe texture, and a darker colour than on the outfide. It bore a very fine poliili, which in ten years does not appear to have at all tarnifhed.
2. One part of platina and two of brafs, melted together in a flow fire, loft about one thirty-fixth. The ingot was of a duller colour than the foregoing, with a faint yellow- ifh caff.. It filed fofter, and broke lefs readily from the chifel, but cracked and fell in pieces under the hammer.. It received a good polifh, and continues untarnifhed.
3. One
[ 533 J
3. One part of platina and four of brafs, covered as be- fore with borax, and expofed to a quick fire, melted toge- ther without lofs. This compound proved yellower than the preceding, and fofter to the file; it bore to be cut fome depth with a chifel before it broke, and received fome impreflion from the hammer, ftretching a little, but foon cracking in various directions.
4. Upon increasing the brafs to fix times the weight of the platina, the compound appeared yellower, though ftill very pale. It proved fofter to the file; and ftretched more under the hammer, and received a deeper imprefiion from the chifel, before it broke.
5. A mixture of one part of platina and twelve of brafs was confiderably paler, and much harder, than brafs. It broke under the chifel ; and cracked, before it had ex- tended much, under the hammer. Both this and the two preceding compofitions bore a tolerably good polifh, and have not tarniflied fo much as brafs by itfelf; though in both refpects they fall fliort of No. 1 and 2.
XII. Platina with Copper and Tin.
1. Fifty parts of platina, feventeen of copper, and fix of tin, covered with borax, became fluid in a ltrong fire, and fuffered very little lofs. The ingot proved confider- ably hard, fo as fcarce to be touched by the file; and very brittle, breaking from a moderate blow, of a rough fur- face, and dull bell-metal colour. It bore a good poliih, and continues untarnifhed.
2. Platina and copper of each one ounce, and four ounces of tin, melted perfectly together, with little or no lofs. This compound filed freely and ealily, and bore to be cut with a knife, but broke readily on the anvil ,- tl>e fracture was of an irregular furface, and a dull whitifh co- lour. Poliihed, it looked like polifhed iron: the fracture
Z z z 2 foon
[ 534 3 foon tarnilhed to a yellowifh hue; the poliflied part grew dull but retained its colour.
3. A mixture of platina and copper of each one part, and eight of tin, proved lofter than the foregoing, and flattened a little under the hammer. Broken, it fhewed a yery irregular iurface, compofed of a great number of bright white plates. It did not polifh well. The fracture foon tarnifhed; the poliLhed part retained Its colour.
XIII. Platina ivitb Iron.
x. Half an ounce of platina and an ounce of iron wire were placed on a bed of gypfum in a Heftian crucible, and covered and furrounded with more gypfum: after being urged in a blart furnace with two pair of bellows for about an hour, the crucible was ia great part vitrefied, and a large hole made in its fide, by which mo ft of the metal had* run out. The experiment was four or five times repeated, but a perfedt union of the platina and iron could not be obtained, the crucible being corroded and vitrefied by the gypfum before the iron flowed thin enough to diflblve the platina. It was obfervable that the iron, thus melted, proved very malleable; though fome have thought that forged iron, brought into fulion, is of the fame nature with common caft iron.
2. Caft iron and platina, of each three ounces, expofed without addition to a ftrong fire, united into a thick fluid, which, on adding an ounce more of the iron, flowed pretty thin. The black lead crucible having become too. foft from the great heat, to admit of being lifted with the tongs, the metal was fuffered to cool in it. On breaking it, the metal was found in one lump, not convex, but of a very concave furface : its weight was about one fixteenth lefs than that of the platina and iron employed. It proved •sxcefJively hard, fo as not to be touched by the file, and
yet
[ 535 ] yet fo tough, as not to be broken by repeated blows of a
fledge hammer, from which it received fome imprefTion.
Heated red, it broke eafily, and looked internally of an
uniform texture, not compofed of bright plates as the iron
was at firft, but of very dark coloured grains which had no
metallic luftre.
3. One ounce of platina being thrown upon four ounces of caft iron beginning to melt, and the fire kept up flrong, the whole came quickly into fuiion. The compound, like the foregoing, was extremely hard, and feemed to flretch a little under the fledge hammer, without breaking. The texture was grained, as before, but the colour ibmewhat lefs dark.
4. One part of platina and twelve of iron melted with- out difficulty,, and with little or no lofs. This mixture alfo was much harder than the iron at firft, and received fome imprefTion from the hammer. Like the others, it could not be broken while cold without extreme violence, but proved very brittle when heated red.
5. All the foregoing compofitions received a good po- Lifh. The firft, in keeping ten years, has fuffered no fen- fible change ; the fecond has fome fmall ipecks of tarnifh, and the third is tarnifhed fomewhat more, but not fo much as a piece of the iron itfelf..
6. About an ounce of a compofition of one part of pla- tina and four of iron was furrounded, in a crucible, with Reaumurs fteel-making mixture, compofed of eight parts of wood foot, four parts of wood afhes, four of charcoal powder, and three of common fait: the crucible was covered and clofely luted, and kept in a ftrong red heat for twelve hours. The metal gained an increafe of about one thirty- ninth of its weight, yielded to the file more eafily than at firft, feemed to receive no additional hardnefs on being ig- aiud and quenched in water, and. did not appear to have
acquired'.
[ 536] acquired any of the qualities by which fteel is diftinguifh- ed from iron.
7. A piece broken off from the fame ingot, weighing about three quarters of an ounce, was treated in the fame manner with the powder for foftening caft iron, viz. bone afh with a fmall mixture of charcoal powder. The me- tal was found increafed in weight about one thirty-fourth : it was lefs hard to the file than at firft, but harder than the part which had been cemented with the fteel-making mix- ture.
It may be proper to obferve, that caft iron is by no means a pure or limple metallic body, like thofe whofe re- lations to platina have been examined in the foregoing ar- ticles. It feems generally to contain mineral fulphur, to which perhaps its brittlenefs is chiefly owing, and which is feparated in the procefs by which the iron is made mal- leable. As platina appears incapable of contracting any union with pure fulphur, I have fufpected, that while the platina and caft iron unite together, fome of the fulphure- ous matter is thrown out and confumed, and that the de- gree of toughnefs, obferved in the compounds, may proceed in part from this caufe; but experiments have not yet been carried to a fufficient length to enable us to enter fatis- factorily into dilquiiitions of this kind.
If however the caft iron fliould be as effectually purified by the platina, as it is even at the finery in the iron works, yet the toughnefs of the mixtures would ftill be pretty re- markable, confidering how much platina, when its propor- tion is large, is dilpofed to diminifh this quality in all the other metals. Perhaps platina, for certain pnrpofes, may prove a \ aluable addition to this moft ufeful metal; a metal to which the workmen cannot communicate the hardnefs which is often required, without communicating at the fame time brittlenefs and intractability.
XIV. Platina
[ 537 1
XIV. Platina with metallic glajjcs.
Mr. Marggraf, after having fatisfied himfelf that platina perfectly refills the common unmetallic fluxes of the vitreous and faline kind, as related in the fourth fection of this hiftory, proceeded to try if the more active glafs of lead would ferve as a flux for it.
A glafs of lead, prepared from four parts of the fined minium and one part of pure flint, was reduced into pow- der and pafled through a fine fieve, to feparate any metallic grains that might remain in it. Eight ounces of the pow- der were mixed with one ounce and a half, or 720 grains, of platina, and the mixture urged with a ftrong fire, in a clofe luted crucible, for two hours : a white or grey- ifh brittle regulus was obtained, covered with a yellowifli koria. The regulus was remelted with more of the fame glafs of lead, and kept again two hours in fufion : it look- ed as before, had a like yellow fcoria, and weighed 606 grains, or about a fixth"part lefs than the platina employ- ed. Kept in fufion two hours in a clofe crucible, it loft fix grains, or about a hundredth part. It was then beaten in pieces in an iron mortar, and mixed with an ounce of common green glafs in fine powder : the mixture being kept melted for three hours in a covered crucible, the fcoria proved turbid, inclining to greeniih, and in fome parts to bluifh ; the metal had loft thirty grains, or about a twentieth ; it filed well, looked very white in the marks of the file, had fome toughnefs, and did not very eafily break under the hammer. It was again expofed to a ftrong tire for two hours in a clofe crucible, with half an ounce of calcined borax : the borax run through the crucible,, but the metal did not perfectly melt, only baking into a mafs, of an unequal rough furface, porous, eafy to break, Hi the fracture of a grey and white colour intermixed, in
weight
[533] weight 540 grains, fo that it had loft above a twentieth part more. It was further treated with half an ounce of calcined borax, the fame quantity of powdered white flints, and an ounce of fait of tartar: the mixture being urged for two hours in a clofe crucible, with a vehement fire, the fcoria? were of a topaz inclining fomewhat to a chryfolith colour ; the metal of a fine white colour, fpongy, rough on the furface, in weight 450 grains, fo that it had loft in this fufion one fixth part, and weighed now three eighths lefs than the platina at firft.
It may be prefumed, that the metal obtained in this ex- periment was no other than a mixture of part of the pla- tina with fome lead revived from the glafs. Though the author took care, by covering and luting the crucible, to guard againft the falling in of any inflammable matter that might revive the lead, yet fuch a matter might have hap- pened to be introduced in the pounding and lifting of the glafs; and independently of any accident of this kind, there was, perhaps, in the platina itfelf, a power fufficient for producing the effect. Common platina, fuch as Marggraf employed, plainly contains iron ; and on barely ftirring glafs of lead in fufion with an iron rod, part of the lead is revived. I mixed fome of the purer grains of platina both with glafs of lead and with glafs of antimony, and expofed both mixtures to a fire as ftrong as I could excite: the pla- tina fhewed no difpofition to melt, the grains remaining of their ufual appearance. Vogel feems therefore to have ill underftood Marggrafs experiments, when he con- cludes fsom them that platina yields a white regulus with glafs of lead.
Mr. Marggraf gives alfo another experiment of the fufion of platina, with an arfenicated glafs of lead. A glafs was prepared by melting together eight ounces of minium, two of flints, and one of white arfenic. Six
ounces
[ 539 ] ounces of this glafs in fine powder were mixed with ©nt ounce of platina, and the mixture melted in a clofe cru- cible for two hours. A brilliant regulus was obtained, greyifh on the fracture, but when filed pretty white, weighing twenty-eight grains, or about one feventeenth, more than the platina : the fcoria was of a dark, brown colour.
Here the increafe of weight is a full proof that the fufion of the platina was owing to its having imbibed either lead or arfenic from the glafs : in the brilliancy of the furface, and the grey colour of the internal part, Mr. Marggrafs metal agreed with our mafles of arfenicated platina already defcribed ; and probably the ufing of arfe- nic in a irate of vitrification with iubftances which ferve to detain it in the fire, may be the molt, effectual means of combining this volatile metallic body with platina.
XV. General obfer-cations on the mixtures of platina with
other metals. i. It appears from the foregoing experiments, that pla- tina, unfuftble by itlelf in the ftrongefl fires of our fur- naces, and proof againft the moft active unmetallic fluxes, melts with, or is dillblved by, every one of the common metallic bodies : That the different metals dilfolve it with different degrees of force, and this not in proportion to the degree of their own fufibility : That there are remarkable differences in its relation to different metals, in regard to the change which it produces in the quality of the metal; that it hardens, and diminifhes the malleability of, all the malleable metals, but feems to communicate fome degree of toughnefs to one which of itfelf haj none, viz. caft iron; that it diminifhes the malleability of tin more, and of gold lefs, than of the other metals; that in certain quantities, it debafes the colour of all the metals, communicating to
A a a a fomr
[ 54° ] fome its own whitenefs, as to copper, and producing with others a new colour, as with bifmuth, lead, and gold ; that it preferves copper and iron from tarnifhing or rufting in the air, but occalions lead and bifmuth to tarniih in a remarkable manner.
2. Though platina, when its quantity is not very large, becomes fluid with mo ft of the metals in a moderate fire, a ftrong one Hems to be always requilite for its perfect and total folution. Compolitions of copper, of iilver, and of lead, with one third their weight of platina, which had flowed thin enough to run freely into the mould, and ap- peared to the eye perfectly mixed; on being digeftcd in aquafortis till the menftruum ceafed to act, left feveral fmall grains of platina in their original form. Upon viewing theie with a microfcope, fome appeared to have fuffered no alteration: others fhewed a multitude of fmall, bright, globular protuberances, as if they had juft begiin to melt.
3. Mixtures of copper, filver, and lead with fmaller proportions of platina, which had been kept in ftrong fufion for fome hours that the platina might be wholly in- corporated, were digeftcd and boiled in Irefti portions of aquafortis, till the platina was left by itfelf in fine powder, free from any thing that aquafortis could extract. Thefe powders were expofed to very vehement fires, without ad- dition, with the addition of borax, with alcaline falts, and with flint glafs: they proved as unfufible as the platina at firft, neither melting, nor communicating any colour to the falts or glafs. It appears therefore that the platina is only fimply difiblved by the metals in fufion, and does hot by their means become truly fufible itfelf.
4. As platina unites with feveral metals into compounds of new qualities, fuch as the ingredients neither poflefs feparately, nor can be conceived, on any known mecha- nical
[54i ] nical principles, to produce by their fimple junction ; and as fuch new properties feeni to be in no metallic mixture more conipicuous than in thofe which platina affords ; k follows, that the diflblution of platina by metals is by no means a fuperficial mixture, but as perfect and intimate a coalition as we have grounds to believe that of any one metal to be with any other.
SECT. VI.
Of the fpecific gravities of mixtures of platina with different
metals.
A MONG the experiments communicated to the £f\. Royal Society by Mr. Wood, there is a remarkable one of the fpecific gravity of a mixture of equal parts of platina and gold. The gravity of the heaviefl platina he examined was to that of water as 15 to i ; and the gravity of gold, as we have feen in the hiflory of that metal, is about 19 -,-•'. If 15,0 parts of platina lofe one on being immerfed in water, and 19,3 parts of gold lofe 1 j then, if the two metals be mixed in equal quantities, 34,3 parts of the compound ihould lofe z; whence, dividing 34,3 by 2, we have 17,150 for the gravity of the compound. Such ought the gravity to be, if the two metals were joined fu- perncially, and each preferved its own proper volume ; but when melted together, the fpecific weight of the mafs is laid to have been considerably greater, amounting to no lefs than 19. If this be the cafe, 19 parts of the melted mafs muft occupy no more fpace than ij-Ai did before the fufion ; fo that near a fourth part of one metal is re- ceived into the pores of the other, without increasing the bulk of the mafs. It may be fufpedled, that the fubftance which Mr. Wood weighed by itfelf under the name of platina was the lighter caft metal mentioned at the begin-
A a a a 2 ing
[ 542 ] ing of this hiftory, and that what he melted with <»old was the true platina ; in which cafe, the gravity of pla- tina being fuppofed 17, the increafe of gravity on mixture comes out about a twentieth part, fo that about a tenth part of the platina has its bulk loft in the mafs.
To fatisfy myfelf in this point, I weighed hydroftatically the mixture, already mentioned, of equal parts of platina and gold. The fpecific gravity of the gold was 19,285 : the platina was the larger grains, whofe gravity, as we have feen in the firft fedtion, was at leaft 17. The compound weighed in air 13605, and loft in water 750, whence its gravity was 18,140: the gravity by calculation comes out 18,071 ; fo that though the platinas gravity had been no more than 17, the increafe of gravity from the mixture was not very conliderable. As a little lofs had happened in the fulion of this mixture, and as the fpecific weight of the platina employed was not certainly known, I made two frefh ones, with pieces cut off from the fame mafs of gold, and fome of the largeft grains of platina, whofe gravity was nearly 18. One of thefe mixtures, weighing 5129, loft in water 276 ; and the other.weighing 6415, loft 34 5; whence the fpecific gravity of the former turns out 18,5- . and that of the latter 18,594, which come as near to one another as can well be expected in experiments of this kind : the gravity by calculation is 18,622 ; fo that both mixtures were a little fpecifically lighter, or expanded in- to a larger volume, than if the metals had been weighed feparately, or joined by fimple appoiition of parts. As thefe experiments were made with a good deal of care, it may be prefumed that in thofe, where there feemed to be a great increafe of gravity, or contraction of volume, either fome error happened in the weighing, or the platina had not been all taken up by the gold in the fulion.
I weighed
[ 543 J
I weighed alfo hydroftatically the other mixtures of pin- tina and gold, and fundry mixtures of it with different •proportions of the other metals. Such malfes as could bear the hammer, were gently hammered a little, with care not to make them crack ; for the pure metals them- lelves, after fufion, are feldom found to come up to their true fpecific weight, till brought to greater folidity under •the hammer. The furface was filed fmooth, where any cavities or irregularities were likely to retain air; and mod: of them were kept immerfed in water for an hour or more, fhat the air might be more effectually extricated, and the water more clofely applied to them. The effect of this precaution was manifeft in fome trials purpofely made : when the metal, hanging in water from the end of the beam, had been cleared from all vifible air bubbles, and exactly counterpoifed ; on Handing for an hour or two, it fenfibly, and fometimes very coniiderably preponderated. The water was in fome of the trials melted lhow, and in others difiilled water, which were both found to be of the fame fpecific weight. The temperature of the air was from the 50th to the 60th degree of Fahrenheits thermo- meter.
The balance, made ufe of in thefe experiments, was of great feniibility, but not exactly equibrachial : and here it may be proper to obierve, that though the writers on ba- lances require, and are very folicitous about procuring, a perfect equality of the arms ; yet as this equality is ex- ceeding difficult, if not impoffible, to be obtained, fo nei- ther does it appear anywife neceffary to the accuracy of the inftrument. If ten equal fmall weights, put into one fcale, are counterpoifed by a weight in the other ; and if the ten weights be then removed, and a bit of filver or brafs plate put in their room ; it is evident, that when this plate fhall be made equiponderant to the counterpoife, it
will
[ 544 ] •will be exactly equal in weight to the ten, how unequal foever the arms of the balance may be ; and confequently, that any unequal-armed balance may, on this principle, have a fet of weights adjufted to it, which being ufed al- ways in one fcale, the inftrument mall be of the fame ac- curacy as if the arms were raoft exactly equal. The beft way of procuring equal fmall weights is, by cutting off equal lengths of the fineft filver wire : the filver thread, kept equally ftretched by a heavy body at the end, may be coiled clofe round a thicker piece of brafs wire, and all the coils cut through at once by a fharp inftrument applied lengthwife. Silver wire is drawn to fuch finenefs, and of fo uniform thicknefs, that weights, thus made by menfu- ration, are of greater nicety than it is poiTible for any ba- lance to weigh. A piece of the wire,whofe length is very fenfible and much further divifible, fhall not have weight enough to give any fenfible motion to the tendereft balance. Thefe fmall pieces, or fuch as will but juft move the ba- lance when empty, and which confequently will not move it at all when loaded, I have found to be a very ufeful ap- pendage to the adjufted feries of weights. Though a balance appears exactly in equilibrio, yet one fide may really preponderate, by anv quantity of force, lefs than that which is fuflicient to overcome the friction on the center: as lefs additional force will ierve to deprefs this fide than the other, one of the fmall weights, tried firft in one and then in the other fcale, will enable us to judge whether the equipoife is exact, or on which fide the preponderation lies.
The refults of thefe experiments were published in the Philofophical Tranfactions, together with the gravities of the feveral mixtures deduced from calculation -, from which it appeared that the experimental gravities were al- moft always lefs than the computed. But an error in
thofe
C 545 ] thofe calculations has made the computed gravities in ge- neral too great: for though. the ingredients in each mixture- were proportioned to one another by weight, the calcu- lations were inadvertently made as if they had been taken by volume. The difcovery of this miftake I owe to Mr. SchefFer, who gives a paper on this fubject in the Swedifh transactions for the year 1757.
The computed gravities being rectified, there appears to obtain, in ieveral of the mixtures, fome degree of what the above experiments fhew not to obtain in thofe with an equal quantity of gold ; the compounds being of greater gravity, or more contracted in volume, than the two me- tals confidered feparately.
This excels of the experimental gravities above the computed is attributed by Mr. SchefFer to the gravity of the platina being greater than that which I had affigned to it. He imagines, that particles of air, adhering in the cavities of the rugged grains, had occaiioned them, when weighed in water, to occupy a larger fpace than that of their own proper bulk ; and that, when the platina was dtelted into a mafs with other metals, it then difcovered its true gravity. On this foundation he endeavours to de- duce, from the fpecific weights of the mixtures, that of the platina itfelf; one of the mod: important points, as he obferves, in its philofophic hiftory, that I had left undifco- vered. Though 1 failed, on account of die above inad- vertence, of attaining to its true weight, my experiments, he thinks, lead to it; and from thofe experiments he con- cludes, that it is certainly more ponderous than pure gold.
This point feems to require fome further examination : for fuch a conclusion is not to be received without the ftrongeft proofs ; and if the principle of induction is not perfectly juft, it may give rife to fallacies of worfe confe- rence than an error in the gravity of platina,
I have
[ 546 ]
I have therefore computed the gravities anew, together with the gravity which each mixture gives for the platina. The firft column, in each of the following tables, contains the proportions of the two metals in the feveral mixtures, the lofs fuftained in fufion, where there was any, being deducted: as platina itfelf fuffers no diminution in the fire, it is from the quantity of deftructible metal mixed with it that this deduction is made. The fecond column contains the fpecific gravities of the mixtures as found by experi- ment, and the third their gravities by calculation fuppofing the platinas gravity to be ij: in the fourth is fhewn the difference between the experimental and computed gravi- ties, with the marks -J- or — according as the former is greater or lefs than the latter. The laft column gives the gravity of platina deduced on Mr. Scheffers principle from each of the mixtures.
|
Specific Gravity |
Platinas |
|||
|
By Exper iBy Calcul. |
Difference |
Gravity |
||
|
GOLD |
19,285 |
refulting |
||
|
Platina i, Gold 2 - - |
18,378 |
18,458 |
,080 ~ |
16,797 |
|
Platina 1, Gold 3 - - |
18,613 |
18,658 |
>°35- |
16,852 |
|
Platina 1, Gold 5 - - |
l8,8l2 |
18,862 |
,050 — |
l6>759 |
|
Platina 1, Gold j 1 - - |
18,835 |
19,071 |
,236- |
14,988 |
|
Platina 1, Gold 1 5 - - |
18,918 |
19,124 |
,206 — |
H>723 |
|
Platina 1, Gold 23 - - |
19,089 |
i9»i77 |
,l88 — |
15,481 |
|
Platina 1, Gold 31 - - |
19,128 |
19,204 |
,076 — |
I5»273 |
|
Platina 1, Gold 47 - - |
19,262 |
19,231 |
>°3X+ |
18,711 |
|
(Platina 1, Gold 95 - - |
i9>273 |
19*258 |
,015+ |
18,214 |
As the experiments with gold had not come to Mr. Scheffers hands when he wrote his paper, he was in hopes, that when fuch experiments fhould be made, they would give platinas gravity with certainty ; gold being
free
[ 547 ] free from fome of the caufes of error which attend the
ether metals. It appears however from the foregoing ac- count, that of twelve mixtures of platina and gold, there was not one fo heavy as the gold itfelf, whereas on Mr. Scheffers principle they ought all to have been heavier. It is plain therefore that either platina is not fo heavy as gold, or that the principle of induction does not obtain in the mixtures of gold and platina.
From the two laft mixtures, the gravity of platina comes out between 18 and 19 ; but on thefe no dependence can be had, the difference between the experimental and com- puted gravities being fo inconfiderable, that it may be at- tributed to the unavoidable imperfections of the inftru- ments ufed for the weighing; for an error of lefs than a thirty-thoufandth part of the weight makes a difference of ,012 in the fpecific gravity of the mixture, and of 1,000 in that of the platina deduced from it. The cafe is the fame in the mixtures with other metals where the platina is in fmall proportion,
The other compoiitions give the platinas gravity lefs than \y, and as the platina is found by itfelf to be 17 or more, it feems to follow, that there mult neceffarily be a diminution of gravity produced by the union of the two metals with one another. A phenomenon obferved in the fufion appeared to confirm this. Moft metallic bodies, made fluid by fire, fhrink and aflume a concave furface in their return to folidity : pure gold fhrinks perhaps rather more than any of the others. But mixtures of gold and platina, where the platina was in coniiderable proportion, were obferved to fhrink little; fome of them even ex- panded and became convex. Of this expanfion or dila- tation of volume, a decreafe of fpecific gravity is the ne- ceffary confequence.
B b b b As
[ 548 ] As the pureft grains of crude platina have fome ad- mixture of heterogeneous matter, it is poftible that this matter may prevent the intimate union of the platina and gold, and thus occafion the two metals, when blended to- gether, to occupy a larger volume than naturally belongs to them. 1 therefore melted gold with platina that had palled through fome of the operations hereafter defcribed, and which may be prefumed to have been thereby purified from moft of its heterogeneous parts.
One of the neateft beads of platina cupelled with lead, (article vi, No. 5,. of the following feclion) was melted with equal its weight of gold in a ftrong fire, and continued in fufion about an hour: the mafs proved fpongy and very light. I remelted it feveral times, with the moft vehement iires I could excite ; and in order to feparate as much as poflible of the lead,, to which its fponginefs feemed owing, I beat it in pieces, and boiled it in aquafortis, and repeat- edly injected corrofive fublimate upon it during the fufion. The mafs neverthelefs ftill turned out cavernulous, and brittle, and fpecifically lighter than either the gold or the bead of platina were by themfelves..
Platina diflblved in aqua regia was precipitated by mer- cury, and the precipitate boiled in aquafortis and well, waihed with hot water. Twenty-fix grains of this pre- paration were melted with four times as much gold: the platina feeming to be imperfectly mixed, the fufion was three or four times repeated, and the quantity of gold in- creafed to about eight times that of the platina.. This mixture proved as ponderous as the gold itfelf, or rather more fo: it weighed in air 16802, and in water 15934a whence its gravity was 19.357. It was examined by fome other gentlemen as well as myfelf, who all agreed in its being remarkably heavy: Dr. Pemberton, with a very exaft balance, found the weight in air 229.735 grains, and
Ufa
f 549 ] in water 2 17.885, from which numbers the fpecific gravity comes out 19.387.
|
SpecificGravity |
Platmas |
|||
|
By Exper. |
By Calcul |
Difference |
Gravity |
|
|
LEAD |
11,386 |
refuiting |
||
|
Platina 1, Lead 0,97 |
14,029 |
*3>679 |
>35°+ |
18,105 |
|
Platina i, Lead 1,92 |
12,925 |
12,838 |
,087+ |
J7»459 |
|
Platina 1, Lead 3,97 |
12,404 |
12,196 |
,308+ |
19,242 |
|
Platina 1, Lead 8 - |
n>947 |
11,819 |
, 128-f |
19,732 |
|
Platina 1, Lead 12 - |
n,774 |
11,682 |
,092-f |
J9>923 |
|
platina 1, Lead 24 - |
u>S7S |
">S38 |
>°37+ |
19,238 |
From this table it appears that the gravity of lead is af- fected by crude platina in a different manner from that of gold; the mixtures with gold being fuch, as if the crude grains were of lefs gravity than 1 7, but thofe with lead as if they were of greater; fo that in one or the other cafe, or in both, an alteration of volume muft neceffarily be pro- duced by the action of the two metals on one another.
|
SILVER Platina 1, Silver 1 - - Platina 1, Silver 2 - - Platina 1, Silver 3 - - Platina 1, Silver 7 - - |
Specifi ByExper. 10,980 l3>535 12,452 11,790 10,867 |
cGravity By Calcul. *3>342 1 2,449 12,046 11,488 |
Difference >I93 + ,003-f ,256-- ,621 — |
Here we fee the effects of the ebullition and difperfion of the lilver taken notice of in the hiltory of the fulion of platina with this metal. The laft mixture is lighter even than lilver itfelf, a proof that the metal is rarefied or made cavernulous by the action of the platina : the greater gra- vity of the two nrfl mixtures was probably owing to a part of the lilver having been thrown off in the fufion, and the
B b b b 2 platina
[ 5S°] platina not perfectly difiblved by the reft. I took, all pof- fible precautions for preparing a let of mixtures of thefe two metals on purpofe for this examination, but they al- ways iputtered up lb much about the crucible, that no de- pendence could be had upon the proportions of the two in the remaining mafs..
|
Specific Gravity |
Flatinas |
|||
|
By Exper. |
By Cilcul. |
Difference |
Gravity |
|
|
COPPE R |
8,8-30 |
refulting |
||
|
Platina i, Copper 0,969 |
I 1,400 |
I 1,869 |
,469 - |
|
|
Platina 1, Copper 2 - - |
10,410 |
10,514 |
,104 — |
|
|
Platina 1, Copper 4 — |
9,908 |
9>768 |
,140+ |
19,364 |
|
Platina 1, Copper 5 - - |
9>693 |
9>598 |
>°95+ |
18,970 |
|
Platina 1, Copper 8 - - |
9,30c |
9>328 |
,028-^ |
|
|
Platina 1, Copper 12 - - |
9,251 |
9,168 |
>oS3+ |
21,607 |
|
Platina 1, Copper2_5 — |
8,970 |
8,996 |
,0.26 — |
Mr. ScheiFer remarks that copper of itfelf can never be caft dole; that when melted with a weak heat, it proves fo incompact as not to bear the hammer; and that when melted in a ftrong heat, with the addition of inflammable matter, .in order to render it malleable, it proves cavernulous on the outfide. The irregularity in the above fet of ex- periments feems to fliew that fomething of the fame kind happens in the mixtures of copper and platina; fince four mixtures out of the feven were lighter than they ought to have been, and this not from any uniform action of the two metals on one another, but apparently from accidental porolity. I melted fome of the mixtures a fecond time, and found their gravities confiderably altered : that of 11,400 was increafed to 11,693; anc* that of 9,251 was diminifhed to 8,985. Little therefore can be concluded from thefe mixtures, in regard either to the gravity of the platina, or its effect in varying the gravity of copper.
IRON
[St* ]
|
. .. I |
Specific Gravity |
Platinas |
||
|
By Exp. |
ByCalc. |
Difference |
Gravity |
|
|
I RO N |
7,IOO |
refuhir.g |
||
|
Platina i, Iron 1,295 |
9>9*7 |
9'S11 |
,406 + |
20,403 |
|
Platina 1, Iron 3,333 |
8,700 |
8,202 |
>498 + |
34>963 |
|
Platina 1, Iron 5,150 |
8,202 |
7,842 |
,360 + |
40,951 |
|
Platina 1, Iron 10 - - |
7,862 |
7>496 |
,366 + |
|
|
Platina 1, Iron 12 - - |
7,800 |
7>432 |
,368+ |
The compofitions with filver have afforded a proof of the diminution of gravity from mixture, or of the mafs being dilated, from the action of the ingredients upon one another, into a larger volume than they occupied feparatcly. The above compofitions with iron feem to be as itriking inftances of a contrary effect : the gravity of the two hit cf them is fuch, as no fubftance, however ponderous, could poffibly produce by the fimple apportion of its own parts to thofe of the iron; for it appears in the calculation, that the platina and iron together occupy lefs volume than even the iron by itfelf.
Mr. Scheffer very ingenioufly accounts for this remark- able phenomenon from a fingular property of iron. When metals are deprived by calcination of their phlogifton or in- flammable principle, their abfolute weight is increafed: iron, by complete calcination, receives an augmentation of one third of its weight. Caff iron has this particularity, that it can bear a considerable diffipation of its phlogiflon, without calcining, or without lofing its metallic form; and in proportion to this diffipation its abfolute weight is in- creafed. Now, as the above ponderous mixtures were melted without any inflammable addition, he thinks a part of the phlogifton of the iron mufl neccllarily have been, burnt out in the fufion, and the metal of confequence ac- quired an additional weight; but that, as no increafe was
abfervedi
[ 552 ] obferved on weighing it, a part of the iron, equal to the acquired weight, muft have been fcorified and loft, and confequently the volume of the metal diminhhed; fo that there remained with the platina as great a weight of iron as at firft under a lefs volume.
To fatisfy myfelf whether the increafe of fpecific gra- vity, or diminution of volume, was owing wholly to this caufe, I made another mixture. But as caft iron is a very impure metal, I took a piece of a bar of the beft forged iron, and cemented it with a mixture of wood foot and powdered charcoal, till it had imbibed fo much of the in- flammable matter as to become fteel ; repeating the ce- mentation, with a frefh mixture, till the fteel melted. The metal in this ftate was very brittle, fo as without much difficulty to be reduced into powder. A portion of this powder was mixed with charcoal powder, and melted again: 7000 grains of the fteel powder, and 1000 grains of platina, were likewife mixed with charcoal powder and melted in a cloie crucible. The fpecific gravity of the forged iron was 7,795 ; which by the introduction of phlogifton in the firft cementation was diminished to 7,618. By the repeated cementation and fufion, the gra- vity was diminifhed to very little more than 7. Of the powdered fteel melted with the charcoal powder, the gra- vity was 7,032, very nearly the fame as before this laft fufion. Of the powdered fteel and platina melted with charcoal powder, the gravity was 7,760, which ftill ex- ceeds the computed gravity, though not in i'o great a degree as that of the mixtures with as large proportions of caft iron. The melted mixture weighed 30 grains lefs than two ingredients before the fufion, on account, perhap- , of fome fmall grains of the metal remaining difperfed among the charcoal powder. Though this lofs be fup- poicd to have been of the fteel only, yet, as there will re- main
[ 552 ] main 697 parts of fteci with 100 of plating, and a? 7,76 parts of the mixt lofe 1 in water, the gravity of the pla- tina comes out on calculation no lefs than 27,813.
It appears therefore that iron is very variable in its fpe- cific weight, in the different circumftanccs of being melted or forged, and impregnated more or lefs with phlogifton; but that probably fome other caufe alfo concurs in varying the gravity of mixtures of it with platina. This caufe may perhaps be found in a remarkable property of iron, which the experiments related in the former part of this work (page 261) feem to have eflablimed. Melted iron, in the inflant of its becoming folid, is dilated into a larger volume, and one of the marks of this dilatation is the con- vexity of its furface in circumftances wherein that of other metals is depreffed. Platina feems to deftroy this power in iron. In the firfl mixture I made of caft iron and pla- tina, the furface was as much hollowed as that of any metallic mafs I remember to have feen, nor was this phe- nomenon omitted in the account of the experiments printed in the Tranfactions. If then fluid iron expands in fixing, and the admixture of platina occafions it to contract, or to expand lefs, we need not wonder at the increafe of gravity in the hydroftatical experiments.
TIN
Platina 1, Tin 0,984 Platina 1, Tin 1,966 Platina 1, Tin 4 - - Platina i, Tin 8 - - Platina 1, Tin 12 - - [Platina 1, Tin 24 —
Specific Gravity By Expcr. I B) Calcul 7,180
10,827 8,972
7>794
7>7°S 7,613.
7>47l1
10,129 8,920
8,117
7*672, 7,513 ,100+
7,3491,122+
Difference
,698 + ,0- +
>323- °33+
Platinas Gravity rei'ulting
21,649 17,619
18,613. 26,745
27,368
The
[ 554 ] The firft of thefe mixtures with tin is that from which Mr. SchefFer endeavours to obtain the true gravity of pla- tina, and from this it comes out 21,649. He obferves that tin is not variable, as iron is, in its gravity, or quan- tity of phlogifton, lb long as it preferves its metallic form; and hence concludes, that when platina and tin are melted together, the excefs of the fpecific weight of the mixture above that of the tin, muft give the true fpecific weight of the platina. As the experiment on equal parts of tin and platina, makes the gravity of platina, on this principle, above 21, he feems to think that all the mixtures, whofe gravity was found fuch as to make platinas gravity lefs than this, mull have been porous, and are therefore to be difregarded in the prefent enquiry. He remarks, from the whole, that though the fpecific weights of fluids may be determined accurately enough by hydroftatical experi- ments, we cannot be fo certain about that of folids, on account of cavities, incompadtnefs, and air bubbles ad- hering ; that the experiments on the foregoing mixtures afford a proof of this, mixtures of platina with one and the fame metal being fometimes heavier, and fometimes lighter, than they ought to be by calculation ; and that the fame thing happens alio in the pure unmixed metals, according as they are caft in a weaker or ftronger heat.
The gravities of metals are doubtlefs influenced not a little by circumftances of this kind; and it muft be added, that in the mixtures with platina, there is another caufe of variation, which has not yet been attended to. When platina is melted with other metals in any confiderable proportion, a part of the platina, unlefs the mixture is cooled haftily, is apt to feparate before the fluid fets, fo that unlefs the whole mafs be weighed in the hydroftatic balance, which was not the cafe in fome of the foregoing experiments, we cannot be fure but the part weighed may
have
t 555 3
have more or lefs than its due proportion of platina. In the mixtures with fome metals, as lead, this unequal diftri- bution, or feparation of the platina, is very vifible ; and it may be prefumed to happen in a greater or lefs degree in the mixtures with all the metals, though it cannot always be diftinguifhed by the eye. Compofitions of platina with zinc, tin, and copper, by all which the platina feems to be uniformly enough difiblved, were poured into narrow cylindrical moulds : the cylinders being broken in two, the lower half of each was found to be of confiderably greater gravity than the upper.
Thus much however the experiments demonstrate, that in fome instances, in the mixtures with filver at leaft, there is a true diminution of gravity, from the action of the ingredients upon one another : and if they do not de- monflrate, they render it extremely probable, that in fome cafes, particularly in the mixtures with iron, there is a true increafe of gravity. If an increafe or diminution happen in the mixtures with one metal, we cannot be certain but they may happen alfo in thofe with another; and confe- quently the fpecific gravity of platina cannot be inferred with certainty, or even with probability, from that of any mixture of it with any metal.
Of a variation of gravity produced by mixture, there are fome remarkable inflances in the other metals alfo. Copper, whofe fpecific gravity was 8,830, was melted with half its weight of tin whofe gravity was 7, 1 80 : there was a little lofs in the fuiion, which we need not here re- gard, for the mixture was fpecifically heavier than the hea- vieft of the metals by itielf, its gravity being 8,898: both the mixture, and a piece of the copper, were examined by fome other gentlemen, who all reported the mixture to be the heavieft, although, as is ufual in trials of this kind, there were fome differences in the numbers : if, from the
C c c c gravity
[ 55* } gravity of this mixture, we were to compute that of the tin employed in it, we lhould make it above a fourth part greater than it really is.
Mr. Hooke made an experiment of the fame kind, be- fore the Royal Society, on a mixture of tin and filver. The gravity of the tin was about 7, and that of the filver 10,666 : of equal parts or the two metals melted together, the gra- vity was 10,812. By applying Mr. SchefFers principle to this mixture, if filver was a metal of unknown gravity, we ihould conclude, that its gravity mull be upwards of 23. Several other experiments of the gravities of metallic mixtures are given in Dr. Birch's hiftory of the Royal Society; but the reader mull obferve, that the computed gravities are no where to be relied on, Mr. Hooke having fallen into the fame miflake, in regard to the calculations, as I had done in the tables publilhed in the Philofophical. Tranfadlions.
Dr. Brandt, in. the Swediih acts for 1 744, where we like- wife find an inadvertence of the fame kind in the method of calculation, gives three experiments on. mixtures of lead and tin; in two of which there is fuch an increafe of gra- vity, as would make the fpeciiic weight of lead above 13,, and in the third a more remarkable one : 531 grains of fine tin left in water 757,. fo that 100 parts loft 14,218 : 531 grains of a mixture of 87 parts fine tin and 3 parts lead, loft in water jz\y fo that 100 parts of this mixture loft 1 3,653 : the quantity of tin in it ought to have loft more,, or to have occupied a greater fpace in the water, than the whole mixt did; fo that the lead and tin, by their mixture, were contracted into lefs volume than thatof thetinbyitfelf.
It appears therefore that the gravity of a metal can never be with any certainty deduced from that of its mixture with another metal, as a dilatation or contraction of the v.olume may refult from their action on one another. It- follows
[5S7l follows alfo, that when two metals of known gravities are melted together, their proportions cannot be found from the gravity of the compound, without a previous hydrofta- tic examination of known mixtures of them in different proportions; that confequently the celebrated proportion of Archimedes is of more limited ufe than it has generally been fuppofed; and that the table which Mr. Scheffer has been at the pains to calculate, in the Swedifh afts for 1755, for determining the quantities of lead and tin in any given mixtures of the two, by a ftatical examination of them, without comparifon with flandard mixtures, is little to be depended on.
As the variations of gravity arifing from the mixture of metals have been afcribed to caufes which do not obtain in fluids; it may be proper to obferve, that the fame thing often happens in fluids themfelves; and here the effect is perhaps flill more confpicuous and more ftrongly marked. One meafure of water, and one meafure of rectified fpirit of wine, mixed together, fall very feniibly fhort of two meafures ; a proof that their volume is diminifhed, or their weight, under an equal volume, increafed by the mixture. Mr. Hooke found, that twenty-one meafures of water, and three meafures of oil of vitriol, mixed toge- ther, made only twenty-three meafure?, fo that one twenty- it iiirth part of the bulk was loft.
SECT. VII.
Of the effect of fire and air on mixtures of plat in a "With
certain met ah.
I. Calcination of Tin ivitb platina.
AS gold and tin, melted together, and kept in a heat fufficient for calcining the tin, are faid byDr. Brandt, in the Swedifh tran factions, to affecl: one another in a
C c c c 2 pretty
1 553 ]
pretty remarkable manner ; the gold to become difL-luble m the pure marine acid, which gold by itfelf relifts ; and the tin to become eafily vitrefcible, though otherwife it san fcarce be vitrefied at all ^ I treated platina and tin in. the fame manner.
Two parts of the picked grains of platina and three, parts of tin were melted together, the mixture reduced in~ to powder in a clean iron mortar, and a hundred and fixty, grains of the powder fet in a cupel, under a muffle, in fuch a heat as is employed for the qupellation of filver,. The cupel being taken out, the matter appeared of a dark purpliih colour, and part of it ftuck together into a lump,. It was then put into an unglazed porcelain faucer, let again under the muffle, and ilirred every now and then for two hours : here and there fome grains appeared glow- ing, like bits of burning coal ; a phenomenon which tin ufually exhibits in its calcination. The powder,, when cold, looked of a mixed greyilh-reddiih colour, the red prevailing : it weighed thirteen grains more than at firft, fo that it had gained an increafe of. about one twelfth, ex- clufive of part of it which had ftuck both to the cupel and, to the roughifh furface of the unglazed faucer;
A part of the calx was urged in a covered crucible, with, a ftrong fire in a blaft furnace,, above an hour. It did not. in the leaft melt, and baked together but very (lightly : its colour was darkened almoft to a black. Both the red and, the black calces, digefted in fpirit of fait, gave pretty deep yellow tinctures, like diluted folutions of platina in aqua* regia; whereas neither, the grains of platina, nor the tia calcined by itfelf, give any colour to the acid.
II. Separation, of Mercury from platina. Some quickfilver, which by long trituration with pla- tiina had diffolved a part of the metal, was put into an iroik
ladle,.
r 559 ]
Iudle, and expofed to a moderate lire. The mercury eva- porated freely, and left the platina behind, in form of a dark coloured powder intermingled with fume fmall bright dining particles. It may be prefumed that the platina, by this diffolution in quickfilver, is purified from great part of its iron, a metal which quickfilver has little difpofition to unite with.
III. Separation of Arfenic from platina.
Pieces of platina, which had been melted with arfenic,. were urged with a very ftrong fire in an open crucible. Arfenical fumes, diftinguifhable by their garlic fmell, arofe in abundance for fome time :. at length the fumes entirely ceafed, and the platina remained in a fpongy mafs. On this mafs I injected a. frefh quantity of arfenic, fo as to bring it into fufion, and having then haftily excited the fire till the fumes ceafed, found the matter again fpongy, and nearly of the fame weight as after the firft operation. This was repeated three or four times, with the fame event. It did not appear that the arfenic carried off with it any part of the platina, as it does of all the other metals, gold itfelf not excepted : but a portion of the arfenic feemed to be retained by the platina even in ftrong fires. Though' the mafs was pretty compact when.fo far fatiated with the arfenic as to be in fome meafure fufible, it always became fpongy when fo much of the arfenic had been diliipated as to leave the platina unfufible. All thefe maffes were fpe— cifically lighter than the platina at firft, the gravity of the heavietr. of them being only about i6,8qo.
IV. Separation of Regulus of antimony from platina.
A mixture of platina and regulus of antimony wasv, melted in a ftrong fire, in a fhallow wide crucible, and the. aafe of- a- bellows directed obliquely upon the furface of the.
fluid..
[ 5*o ] fluid. The matter continued to flow, and to fume copl- ouily, for fome hours : at length it became confiftent in an intenie white heat, and fcarcely emitted any more fumes though ftrongly blown on. The mafs, when grown cold, broke eaiilv, appeared very porous, blebby, of a dull grey colour, and weighed coniiderably more than the quantity of platina employed. Its fpecific gravity was only about
I5-
This experiment was feveral times repeated, and the event was always the fame ; the platina not only refilling, as gold does, the volatilizing power of the antimonial regu- lus, but likewife defending a part of it from the action of the fire and air, and refilling to melt after a certain quantity had been diffipated.
I likewife treated platina with crude antimony. Four ounces of antimony and two ounces of platina, kept for fome time in a fire pretty ftrongly excited by bellows, ap- peared melted only in part : four ounces more of antimo- ny being added, and the fire renewed, a reguline matter was found partly at the bottom and fides of the crucible, and partly intermingled among black fpongy fcoriae : the whole was returned to the fire with black flux and com- mon fait : it now melted fufficiently thin, and the regulus perfectly feparated. This regulus did not differ in ap- pearance from mixtures of regulus of antimony and platina melted together, and exhibited the fame phenomena alio on trying to blow off the antimonial part.
.Mr. Scheffer likewife tried platina with antimony, and the refult of his experiments was the fame as of mine. He obferves that as platina refifts fulphur equally with gold, it cannot be fcorified by the fulphureous part of an- timony, and therefore remains, as gold does, in the regu- lus ; but that the regulus cannot be blown entirely off from it, as it is from gold, on account of the platina not, continuing fluid.
r 561 3
V. Separation of Zinc from platina. A mixture of platina and zinc, expofed haftily to a fbrong fire, deflagrated and appeared in violent agitation. This continued but a little time : the matter quickly be- came folid, and could no longer be made to flow, or the zinc, of which a considerable quantity ftill remained in it, to flame. The mafs was very brittle, dull coloured, lpongy, and, like the two foregoing, Specifically lighter than the crude platina.
VI. Cupellation of platina with Lead.
1. A mixture of platina and lead was cupelled under a muffle in an affay furnace. For ibme time the procels went on well ; the lead fmoking moderately, and changing into Scoria?, which were thrown off to the fides and ab- forbed by the cupel. In proportion as the lead worked off, the matter required a ftronger fire to keep it fluid ; and at length, collecting itfelf into a dull flat lump, it could no longer be made to flow in the greateft degree of heat which the furnace was capable of giving. The lump broke eafily under the hammer, appeared of a dull grey colour both internally and externally, and of a porous texture. It weighed near one fifth part more than the quantity of pla- tina employed.
2. This experiment was many times repeated and va- ried. I endeavoured to fcorify the lead in affay crucibles, by intenfe fires in a blaft furnace; to work it off on bone— afh preffed into the bottoms of crucibles; and to blow it off on tefts before the nofe of a bellows. The event was.. £till the fame; the platina not only refilling the power of lead, which in thefe operations deftroys or fcorifies every^ other known metallic body except gold and filver, but likewife retaining, and preventing the fcorification of, & jart of the lead itielf>
1 562 ]
3. In the hiltory of the fufion of platina with lead it has been obferved, that lead depofites, in a gentle heat, great part of the platina which had been united with it in a jftrong one. As the part which remains fufpended in the lead, might be fufpcdted to differ from that which fubfides, a quantity of lead was decanted from frefh parcels of pla- tina in a heat below ignition, and both the decanted metal and the refiduums fubmitted to cupellation feparately. The event was the fame in all; the metal becoming con- fident after the lead had been worked off to a certain point, and refufing further fcorification.
4. Mixtures of platina and lead, which had been cupelled in an allay furnace as long as they could be kept fluid, ■were expofed to ftronger fires in a blaft furnace, by them- felves, with powdered charcoal, with black flux, with bo- rax, with nitre, and with common fait. None of them perfectly melted, or fuffered any confiderable alteration ; they only became fomewhat more porous, probably from the exudation of fome of the lead and p. partial liquefaction •or foftening of the mafs. The immediate contact of burn- ing fuel, agitated by bellows, made lome of thefe mixtures flow after they had refufed to melt in crucibles acted on by intenfe fires : the beads by this means became fome- what neater and more compact, but very little of the lead was fcparated.
i;. The cupelled beads were in general brittle, breaking •eafily under the hammer, without ^retching in any con- fiderable degree. They were of a grey colour both on the upper furface and in the fracture, but pretty bright and white on the lower furface, and when ground or filed: they had nothing of the purplifli hue, which the mixtures of platina and lead (page 5 1 5) had in fo remarkable a de- gree; nor does their colour appear any wife altered after 'keeping for ten years in the fame circumftances in which
thofe
[5*3 1 thofe mixtures were kept. On weighing them hydrofta- tically, the more fpongy ones were found nearly as ponde- rous as the crude platina. Among the more compact, the gravity of one turned out 19,083, that of another 19,1 ;6, and of a third 19,240. It is probable that thefe remark- able gravities proceeded partly from the platina having been purified in the procefs from its lighter heterogeneous admixtures, and partly from an increafe of gravity occa- fioned by the coalition of the platina with the lead. The lafl: of thefe mixtures, whofe gravity was 19,240, is that which was melted with equal its weight of gold, as men- tioned in page 548.
6. A mixture of one part of platina and three of gold was cupelled with lead in an a flay furnace. The matter worked well for a considerable time : at length it collected itfelf into a bright hemifpherical lump, which by degrees became flatter, dull coloured, and rough. The button, on being weighed, was found to retain about a twelfth part of lead.
7. The experiment being repeated with a mixture of one part of platina and fix of gold, fome part of the lead appeared ftill to be retained. The bead proved rounder and brighter than the foregoing, and of a good golden co- lour on the outfide; but it broke eafily under the hammer, and appeared internally greyiiTt : fome of the fragments hung together by the outward golden coat.
8. Mixtures of platina and filver, fubmitted to the com- mon procefs of cupellation, retained likewife a little of the lead. Thefe, in becoming confident, formed not he- mifpherical beads, but flat mafles, very rough and brittle, and of a dull grey colour both externally and internally.
9. The cupellation of platina with lead was one of the experiments made by Mr. Wood, and communicated to the Royal Society in the year 1750; but the platina being
Dddd ' thai
[ 564]
then very imperfectly known, lbme deception happened in this point. Mr. Wood relates, that platina having been melted in an allay furnace on a tell with lead, and there- with expofed to a great fire for three hours, till all the lead was wrought orf, the platina was afterwards found remain- ing at the bottom of the teft, without having fuffered any alteration or diminution by this operation. Dr.Brownrigg, furprized at this refiltence of platina to lead, repeated the experiment. He melted twenty-fix grains of platina upon a cupel, with fixteen times its weight of pure lead, which he had himfelf revived from litharge : the lead being fcorified, there remained in the cupel a button of platina weighing twenty-one grains, fo that the platina loft in this operation near a fifth part of its weight. From this ex- periment he conjectured, and not without probability con- lidering the little that was then known of the properties of this new metal, that a part of the platina was fcorified by the lead; that the whole might have been fcorified by repetitions of the procefs; and that confequently gold and filver may be purified from platina, by cupellation with larger quantities of lead than are commonly employed. What the author has modeftly propofed only as a con- jecture, to be confirmed or refuted by further trials, has by fome been taken for a certainty : in a letter prefented to the Royal Society foon after, the procefs is fpoken of as a method difcovered by Dr. Brownrigg, for feparating pla- tina from gold and filver. It is plain that this experiment muft have been made, and the author has lately informed me that it was, with the caft metal mentioned at the be- ginning of this hiftory, which was then fuppofed to be true platina, and does lofe of its weight in the common procefs of cupellation.
1 o. Mr. SchefFer tried the cupellation of the grains of platina with lead, and the event was exactly the fame as in
my
[ 5^5 3 my experiments. The bead was dark coloured and rugged at top, white underneath, and retained a portion of the lead amounting to two or three parts in a hundred. He obferves that the lead cannot, by common fire, be worked off clean from this metal, as it is from gold and filver, on account of the platina not continuing fluid after the lead has been feparated to a certain point; and judges that a fufficient heat for the complete feparation of the two metals is not to be obtained by any other means than by large burning glafTes.
1 1. I have already obferved, page 494, that platina di- vided by cementation with nitre, and afterwards purified by repeated fublimations of fal ammoniac, appeared no- wife different in cupellation from the common grains. Mr. Marggraf made trial of platina attenuated by folution and precipitation. The orange coloured precipitate thrown down by flxt alcali from folution of platina in aqua regia, being well warned with hot water and ignited under a muffle, became brownifh : nine parts of this matter were melted with an ounce of pure granulated lead, and the mixture expofed to the fire in a fcorifying difli till a con- fiderable part of the lead was fcorified : the remainder worked in a cupel, left a rough bead, of a whitifh grey colour, very brittle, perfectly like that obtained in the cu- pellation of crude platina : its weight was one grain. The experiment was repeated with a precipitate made with vo- latile alcali, and the event was the lame. He tried alfo the powder which remained on diftilling a folution of pla- tina to drynefs : this powder, calcined under a muffle, ac- quired a mining blackifh colour, in which itate thirty grains of it were mixed with twenty times as much granulated lead, and the mixture worked as above, firft on a fcorify- ing difh and afterwards in a cupel : the fcoriae were of a black-brown colour : the cupelled bead was brittle and of
D d d d 2 a' grey-
[ 5^6 ] a grey-white colour like the others, and weighed forty- two grains, or two fifths more than the platina employed. This was treated in the fame manner with the fame quan- tity of frefh lead : the fcoria? were of the fame colour, and the bead ftill weighed juft forty-two grains.
12. The fame author gives an account of another ope- ration, in which platina and filver were combined together, the mixture melted with lead, the lead fccrified, the filver ieparated by aquafortis, and the remaining platina again cupelled. He took, thirty grains of crude platina, and thrice as much of the combination of filver with marine acid, called luna cornea. The mixture being expofed to as great a heat as a glafs retort would bear, no liquid pafled into the receiver, but a little white matter fublimed into the lower part of the neck of the retort, as commonly hap- pens when luna cornea is expofed to fuch a heat by itfelf. The mixt run clean together into a dark yellow hyacinth coloured mafs, and appeared well united : the glafs was flamed of a dark yellow. The mixt was pounded, along with pieces of the glafs, which could not eafily be fepa- rated, in a clean iron mortar, the powder mixed with two ounces and a half of granulated lead, and melted in a cru- cible with a ftrong fire : the fcoria was greenifh. The metal, worked on a cupel, drove well, as in the common filver affay, till towards the end of the procefs, at which time it came afunder, grew flat and rough, and looked like filver fprung on the cupel by being too haftily cooled, but without the leaft metallic brightnefs on the furface : it was very brittle under the hammer, but bore to be filed, and the mark of the file looked white : it weighed a hun- dred and ten grains. It was cupelled with an ounce more of lead, and the product was the fame as before, with the lofs of feven grains in weight. Thi3 laft bead was beaten in pieces, mixed with fix drams of pure nitre, and melted
with
( 56/ 1 with a flrong fire: the metal was of a filver whitenefs,
and weighed feventy grains : the fcoria was cauftic, liver coloured, and when liquefied in the air looked greenifh. The regulus was melted again, with half an ounce of the pureft nitre and a dram of borax : the fcoria proved clou- dy, inclining to yellow underneath and to greenifh above : the regulus was of a fine white, and weighed flill feventy grains ;. it had fomething particular in its appearance on the furface and about the fides, refembling the radiated cobalt > it ftretched pretty well under the hammer, and bore to be flatted into a thin plate, but was harder than fine filver. A part of this plate was digefled in purified aquafortis : the menftruum became firfl of a high grafs- green colour; afterwards, in a boiling heat, the plate grew black, and the folution browniih. The filver being at length diffolved, there remained at the bottom a black ponderous matter like calx of gold. This was thoroughly warned with hot diflilled water, then dried, and made red hot, but it received no gold colour. It was mixed with granulated lead, and the mixture worked firfl on a fcorify- ing difh, and then on a cupel : there remained a convex bead, without metallic luftrc, which fprung under the hammer, and refembled the other beads obtained by cu- pelling platina with lead.
13. It appears upon the whole, that Marggrafs trials for working off lead clean from platina, fucceeded no better than SchefFers and mine, fo much of the lead being always retained as to make the metal very brittle, whereas platina by itfelf, whether in its crude flate of grains or when melted by a burning-glafs, is of coiifiderable malle- ability. Macquer and Baume made another effort: they were " defirous of feeing, whether a heat of a good deal longer continuance would not produce that, which one coup defeu, perhaps jnore flrong but of fhorter duration-,
had
[ 568 ] had been unable to produce. They put upon a cupel of a proper lize one ounce of platina and two ounces of lead, and having placed the cupel in a furnace like that of Mr. Pott for the vitrification of earthy bodies, they raifed the fire by degrees, and kept it up without intermiflion for fifty hours, in Inch manner, that during the laft twenty- four hours it continued in its full violence. The cupel being then taken out, they found that the platina, inftead of being in a round brilliant button as gold and filver are after cupellation, was extended and flattened on the cupel: its upper furface was tarnifhed, dark coloured and wrinkled, from whence it was judged at firft that the operation had fucceeded no better than thofe we have been fpeaking of: the platina parted eaiily from the cupel, which was become very hard, of a yellowilh white colour, femitranfparent, and ftruck fire freely with fteel. But upon exactly weigh- ing the platina, they found, that inftead of receiving an augmentation of weight from fome of the lead remaining undeftroyed, it had loft one fixteenth of its weight: its lower furface was white and filvery : finally it was not eager, but bore very well to be extended under the ham- mer: they diilolved a part of this cupelled platina in aqua regia, and this diflblution did not mew any veftige of lead." As Mr. Macquer appears to have employed in this ex- periment the platina iuch as he received it, containing a large admixture of ferrugineous and other foreign matters undoubtedly deftrudtible in the procefs j it is obvious that it might have retained a very confiderable proportion of the lead, notwithftanding the diminution in weight. Nor can aqua regia be looked upon as an infallible tell of its having been pure from lead; for this menftruum, in certain cir- cumftances, will diflblve lead as well as platina. But whatever might be in this, the event of the experiment, in regard to the malleability of the cupelled mafs, appeared
too
[;s**J
too interesting to be pafled over, in this hiilory, without being verified by further trials.
1 4. Having at hand a wind furnace, formed of a mixture of Sturbridge clay and powdered glafs-houfe pots, fecured by iron hoops on the outiide, about two feet high from the grate to the top of the dome, fourteen inches wide in the middle and ten inches at the grate, with a chimney of nearly half the diameter of the grate and fourteen feet high ; I firfl made trial of this furnace, and found its effect to be fuch, that there was no occafion to have recourfe to any other. I fitted into it a muffle, in the manner de- fcribed by the ingenious author in a memoir on the vitre- fication of clay with chalk, formed of the fame compoli- tion with the furnace, two inches high, three inches wide, of fuch length as to reach acrofs the furnace, fupported at the height of five inches above the grate by a brick of fire-Handing clay, which was cut Hoping downwards lb as to cover as little as poflible of the grate.
15. A large cupel having been kept red hot in the muf- fle about an hour; two ounces of lead were put in, and one ounce of the picked grains of platina dropt into the melted lead. The fire being railed with coaked pitcoal to its greateft vehemence, the whole internal part of the muffle appeared of a dazzling brightnefs, and the cupel could not be diftinguiihed, till cold air was fuffered to pals- through by keeping the door open for fome time, which was done frequently, to promote the fcorification or dilli- pation of the lead. In this flate the heat was continued, untill, in five or fix hours, penetrated by the vitrefcent cinder of the coal, the muffle begun to fail : all its back part, and fome of the internal part of the furnace, melted,, forming partly irregular vitreous lumps, and partly rim- ing down through the grate in large drops of black hard glafs. The cupel was hard, yellowiih white, and femi-
tranfparent,
[ S7° ] transparent, like Macquers. The platlna was In a flat cake, coated with the femivitrified matter of the cupel and glaffy drops from the muffle, fo that nothing could be judged from the weight : it broke pretty eafdy under the hammer, and did not feem to differ from that of former cupellations.
1 6. What was here wanting in the continuance, I en- deavoured to fupply by a repetition of the fire. The pla- tina, pounded and warned, was placed under a frefh muf- fle, on a fcorifying difh ; and the heat kept up in its full violence, chiefly with wood and charcoal, for fourteen hours. Created: part of the platina ftuck fo firmly to the dim, in virtue of part of the lead which had exuded and vitrefied, as not to be got off without pounding the veffel. Where the platina on the difh was {truck with a hammer, or rubbed with a fteel burnifher, it frxetched, and acquired a continuous furface like filver or tin leaf. After the powder had been palled through a fine fieve, and wafh- cd, on beating it again fome broad flat grains appeared, which ftretched eafily under the hammer, and on being Jqueezed with a pair of plyers, bent almoft double : one of thefe bore to be opened, and bent again in feveral di- rections, without cracking. This powder, whofe particles appeared fo ductile and flexible, I tried to reunite into a mafs, by urging it with a vehement fire, in a covered cru- cible, for four hours : it cohered into a button, of the form of the crucible, not at all flicking to the veffel, and free from difcoloui ment : the button broke from a blow or two of a hammer, but not very eafily, filed tolerably fmooth, and burnifhed like fine filver.
17. I cupelled four parcels of platina, with thrice their quantity of lead, till they would no longer continue fluid in a good affay furnace ; and repeated the cupellation on frefh cupels, with the fame quantity of lead, a fecond and
a third
[ S7* ] a third time. The firfl: cupels were tinged of a deep rufty colour, probably from the irony matter in the pia- tina; the others only yellowifh, as from lead alone. The. plates of metal, after the firfl: cupellation, were dull co- loured and fluck to the cupels; after the others, they were brighter and did not flick. The four plates, weighing 3031 grains, kept for twelve hours on a fcorifying difh, in as ftrong a fire as could be excited in the affay furnace, be- came whiter, and lofl 2 1 8 grains : the white difh was co- vered all over with a yellow glazing. The plates, which had fuffered no appearance of fufion, and which flill proved brittle though much lefs fo.than before, were broken into fmaller pieces, and fet on four cupels, under a muffle, in the wind-furnace above defcribed: during eight hours ve- hement fire, the two cupels in the fore part of the muffle, which was lefs hot than the back part, were obferved, as often as the door was kept open for a little while, to fmoke confiderably ; but all the air that could pafs into the muffle, did not fo far diminifh the dazzling heat, as that any fumes could be diftinguifhed in the back part. The arch and further end of the muffle were found all over glazed by the fumes; the cupels friable and unftained; the metal of a filver whitenefs, and diminifhed 105 grains. The pieces in the front cupels were flill brittle; thofe in the further ones bore to be flattened confiderably under the hammer, and feemed nearly as foft as alloyed filver.
18. I made many other cupellations of the fame kind; of which, as no other remarkable phenomena occurred than have been already mentioned, it would be unneceflary to give a particular detail. They agree in eflablilhing an important facl, that though in the common pro- cefs of cupellation, even when performed with ftronger fires than the cupelling furnace can give, and continued ibme hours beyond the time in which the fixing of the
E e e e metal
C 57* ]
metal feems to fliew that the fire has produced its full effect, platina has been always found to retain fo much of the lead as to break under the hammer; yet by continuing thefe vehement fires for twenty hours or more, fo much of this retained lead is fcpa rated,, as to leave the platina mal- leable. Much of the lead was forced out after the metal had become folid, as appears in the experiment, No. 17, in which the quantity expelled from the cupelled plates* without their having anyvvife foftened or altered their figure, amounted to above a tenth part of their weight. The thinner the metalline plates,, the fooner and more effectually were they freed from the lead, and rendered malleable: in one cupellation, a part of the metal having run into the form of a fine wire, this wire, after fix hours ilrong heat, proved fo flexible, as to bear bending back- wards and forwards feveral times without breaking, while a thick piece of the fame mals,. after eighteen hours longer continuance of the fire, was Hill brittle : when a ffnall quantity of platina, worked in a cupel of a proportionate lize, had, from the lhape of the velTel, formed a pretty thick mals, which was the cafe in moft of the hrft cupel- lations (N.o.. 1 to 8 of this article) a vehement fire, of much longer continuance than that of Macquers experi- ment, was infutlicient for rendering the mafs malleable;, but when beaten into powder and fpread thin, a- fire not ex- tremely vehement, continued ten or twelve hours, made the particles of the powder fo ductile,, that they ftretched under the peftle into fine plates like fragments of filver leaf: the powder thus flatted, was remarkably foft or unctuous to the touch, like talk ; and being rubbed on paper, ftuck to it, fo as not to be eafily b-rufhed off, making it look like what is called filver paper. It was therefore a happy circumftance in Macquers experiment, and indeed srfTential to its fuccefs, that he ufed a conliderable quantity
t 573 1 of platina fo as to form a thin plate on the bottom of a large cupel. Much of the lead exudes at firfr. in a vitreous form, and glazes or ftains the difh or cupel on which the plate is expofed to the fire; but towards the end it feems to be forced out only in fume, no vifible mark remaining on the veffel : in one experiment, the metal loft about a twenty-fifth part of its weight, after it had ceafed to give any tinge to the cupel.
^ig. It may be proper to obferve, that in moft of the cupellations of platina with lead, eipecially where the quantity of the mixt was confiderable, and a pretty ilrohg fire made ufe of, the cupelled plates appeared of a regular and Angular figure on the furface, fuch as no other metal or metallic mixture I know of affumes in fixing. In the middle was a broad flat-bottomed depreffion, with a lip or margin round it, like a common table plate ; and the lip was frofted as it were with regular tranfverfe rows of pro- minent dots. The fmooth parts were in general foft or flippery to the touch.
VII. Cupellation of platina with Bifmutb. Mixtures of platina with bifmuth were iubmitted to the common proceffes of cupellation under a muffle, {bo- nification in aflay crucibles, and tefling before the nofe of a bellows. The general event was nearly the fame as when platina and lead were treated in the fame manner: the mixtures, which at firff, flowed eafily, became lefs and left fufible in proportion as the bifmuth was driven off, and at length could not be kept fluid in an intenfe fire, though they appeared, on weighing, to retain a confiderable quan- tity of the bifmuth. Nor could bifmuth, any more thai; lead, be worked off clean, by the common procefs of cu- pellation, from mixtures of platina with fix times iti weight of either gold or filver.
E e e e 2 When
[ 574]
When one parcel of platina was cupelled with three or four frelh quantities of bifmuth, the frrft cupels were al- ways tinged, of a blackilh rutty hue, the next paler, and the third for the moil part only of the orange yellow colour which bifmuth itfelf communicates, and which is conli- derably deeper than the {lain imparted by lead.
In many of the cupellations, the furface of the metal was found covered with a leafy fubftance like deep co- loured litharge; and fometimes, under the cupelled plate, there was a large quantity of rough fpongy greenilh matter, adhering in many parts ftrongly to the platina, running into cavities in its bottom, and in fome places lying as it were between plates or flakes of the metal. It appeared that, bifmuth, in cupellation with platina, does not diffufe or. fpread itfelf lb thin, or fink fo deep into the cupel, as lead does; but loads the parts which it touches, in luch a man- ner, as to be prevented from extending further, and to be collected there in its femivitrified ftate; fometimes lying in, large quantity on the cupel, though a considerable part of the cupel at bottom was not tinged with it. This does not feem to happen when bifmuth is worked off alone, and therefore probably proceeds from this metal being a lefs powerful menflruum than lead for the ferrugineous and. other foreign matters blended with platina. Many of the cupellations however went on well, without any appear- ances of this kind, and yielded brittle plates, fometimes dull coloured and fometimes bright,, according as lefs or more of the bifmuth was worked off, of uneven furfaces, with large protuberances irregularly and fometimes ele- gantly difpofed. Some of the minutes of thefe experi- ments having been loft, I cannot recollect whether it was with large, or with fmall proportions of bifmuth, that the procefs fucceeded befh.
From
[ S75 ] From the effccT: of a long continuance of ftrong fire on
mixtures of platina and lead in the preceding article, I was
induced to fubmit to the fame treatment mixtures of it
with bifmuth, a metal which promifed to be eafier fcpa-
rated than lead, as being itfelf much more eafily diffipated
by fire.
Some of the cupelled plates of the foregoing operations were kept for fix hours, on four cupels under a muffle, in as ftrong a heat as could be railed in a good affay furnace. Of one of the plates a part had melted, and fpread in fine filver-like leaves over the edge of the cupel : of all of them the thin edges bore to be hammered pretty well, and bent confiderably before they cracked : the cupels were ftained of a pale orange yellow. The plates being' then urged for fix hours longer on frefh cupels, in the wind furnace be- fore mentioned, they all proved of a bright filver colour, and hammered well in the thinner parts, but ftill conti- nued brittle in the thick ones: the' cupels were very flightly tinged.
From the experiments related in this feclion it appears, that platina perfectly refifts the deftructive power of lead and bifmuth, which, with the concurrent action of fire and air, reduce all the other known metallic bodies, except gold and filver, into a calx or fcoria : that it refifts anti- mony, by which filver as well as the bale metals are fcori- fied, and which has always been efteemed the fevereft teit ©f gold : that it is not fenfibly volatilized by arfenic, which in ftrong hafty fires carries off a portion of gold itfelf: that in degrees of heat confiderably ftronger and of longer con- tinuance than have hitherto been employed for thefe kinds of operations, the platina preferves a part of thofe de- ftructible metallic bodies themfelves, retaining fo much of them as to be rendered brittle ; but that by further continuance of vehement fire, thofe bodies, at leaft lead--
and
IS?*] rand 'bifmuth, may be wholly or almoft wholly diffipated, lb as to leave the platina in a mafs, as malleable as the fineft grains were feparately, and perhaps more fo, in vir* tue of their being purified in the operation from their irony or other foreign matters, as gold and filver are by the like means from all the bafe metals.
How far this diffipation of lead or bifmuth may be practicable in the large way, or on mafTes of confiderable thicknefs, cannot be abfolutely determined from the ex- periments hitherto made, for as yet, with me at leaft, the procefs has fucceeded only on thin pieces of the metal. Mr. Macquer feems to make no doubt that platina may on this foundation be rendered manageable by the work- man in large, fo as " to furnifh us with burning concaves* fpecula for telefcopes, an infinity of veffels and utenfils for chemical and culinary ufes, and almoft all the works of the lockfmith." He obferves that platina would for thefe purpofes be an excellent material, " as its vivid and brilli- ant polifh is never tarniflied by any kind of ruft, and as it not only refifts the action of air, water, fire, acids, and the veracious metals, as well as the pureft gold does, but joins to thefe admirable qualities a property itill more precious, which is wanting to gold, viz. the force and hardnefs of iron." In my experiments, the cupelled plates, both with lead and bifmuth, were considerably harder than fine gold or filver, but fofter than iron : the hardnefs here attributed ,to them appears to be inferred from the experiment on precipitate of platina related in page 505.
SECT.
I
[ 577 ]
SECT. VIII,
Of the Affinities of platina.
N this feclion it is propofed to give an account of luch. experiments as have been made relative to the com- parative affinities of platina and other metals, to one ano- ther, and to faline ditfblvents ; the feparation of platina from one metal by the intervention of another, or of one metal from another by the intervention of platina ; the ie- paration of platina by other metals, or of ether metals by platina, from their folutions in acids „ For the greater diiUnctnefs, the remits of the feveral experiments are ex- prefled in the refpeclive titles, after the fame manner as in the common tables of affinity.. The body placed upper-- moil is always to be understood to have a greater affinity to that which is immediately under, it, than to the third or lowermoft ; infomuch that if the firft and third be com- bined together, the middle one, properly applied as men- tioned in the experiment, will break their union, and. throwing out the third body, join itfelf to the firft, though the feparation is not always complete. Where no fuch: affinity, or feparation, appears in the experiment, the- feveral bodies are placed in a continued line,
I.. Mercury : Platina : Lead.
One part of platina and about four of lead were melted, perfectly together, and after the heat had fomewhat abated,, the fluid was poured gently, in a fmall ftream,. into three times it? quantity of quickfilver heated fo as to fume. On ftirring them with an iron rod, a blackifh powder was im- mediately thrown to the furface, which appeared to be:
chiefly.
[ 573 ] chiefly platina. On grinding them together in an iron mortar, a frelh powder gradually feparated, which, being occafionally warned off, in appearance greatly refembled the foregoing, but was found, on proper trials, to partici- pate more largely of mercury and lead than of platina. The amalgam was of a very dull colour, and on being ex- poled to the fire in an iron ladle, fwelled and leaped about, though the heat was fcarcely fufficient to make any of the quickfilver evaporate. I therefore had the grinding con- tinued, in a kind of mill, compofed of a thin iron plate, cut into the form of a crofs, and made to turn in an iron mortar : the plate was bent up nearly to the fhape of the bottom of the mortar, and between two of the ends was fixed a piece of wood, the other two flanding loofe, and accommodating themfelves to the mortar in virtue of their elasticity : the piece of wood received the end of an up- right fpindle, which being fecured by crofs pieces toTtecp it in the middle of the mortar, and a fmall weight, fome- times greater and fometimes lefs, placed on the top, a wheel and pulley procured a rapid motion with little la- bour. After conftant agitation in this machine, with wa- ter occaiionally renewed, for feven or eight days, the amalgam looked bright and uniform, and fuffered the quickiilver to exhale freely. The mercury being all eva- porated, there remained a dark grey powder, which proved upon examination to be platina with a very little lead. For a part of the powder being digefted in aquafortis, a fmall portion of it diffolved, and the folution appeared to be no other than a folution of lead : the undiffolved part, now of a dark purplim colour, was moffly taken up by aqua regia, to which it communicated, not indeed the common hue of folutions of platina, but a kind of dull olive colour : plates of tin, however, quickly difcovered that the matter diffolved was platina, by occafioning a pre- cipitate
[ 579 ] cipitate of the fame appearance with that which tin thr down from common folutions of platina. The reft of the powder was cupelled with lead : it left a rough, flatted, bright mafs, which would no longer melt, and which ex- actly refembled thofe obtained in cupelling crude platina with lead.
Mercury is fuppofed to have a greater affinity to lead than to any other metallic body, gold and filver excepted. In this experiment it (hewed a greater affinity to platina than to lead, fince it retained much of the platina, after the lead, which was at fint in much larger proportion, had been almoft: all thrown out.
II. Mercury: Gold: Platina. A mixture of one part of platina and two of gold, which proved very white and brittle, was well nealed, and cautioully flatted into thin plates, which were thrown red hot into boiling quickfilver. On grinding and waffling with water, a powder feparated, copioully at firft, and by degrees more fparingly. After the procefs had been con- tinued about twenty-four hours, there was no further fe- paration, except of a little blackifh matter into which a part of the mercury itfelf is always changed in thefe kinds of operations. The amalgam, which looked bright, was put into a crucible, and the quickfilver being evaporated by moderate heat, there remained a fpongy mafs, of a high colour, which being melted and caff: into an ingot, proved very loft and malleable, and not diftinguilhable by the eye from the pure gold made ufe of. How far this procefs is applicable to the feparation of platina from gold in the way of bufinefs, will be conffdered in the following feclion. It is fufBcient here to have effabliihed the greater affinity
F f f f of
[ 58° ] of mercury, to gold than platina, and to platina than lead.
III. Platina:
Lead:
Iron. One ounce of a mixture of iron and platina, and two ounces of lead, were covered with black flux, and urged with a pretty ftrong fire, but which did not prove fufficient for the fufion of the platina and iron : the lead being poured off into a cylindrical mould, the lower part of the cylinder looked of a duller hue than the lead was at nrft, and proved fpecifically heavier, in die proportion of 1 1,598 to 11,386. The lumps of iron and platina were mixed with the lead a fecond time, and expoled to a ftrong fire till the whole came into perfect fufion: on cooling the crucible too haftily in water, the fluid matter exploded and threw off the cover, and the lead was found reduced into fmall filaments, filling the crucible, which before was not one fourth part full. The iron regulus at the bottom was in a round, lmooth, very hard lump, and feemed to retain a confiderable portion of the platina. The lead, melted into a mafs with a little refin, appeared, from its fpecific gravity, and more manifeftly on cupellation, to have im- bibed more of the platina than the iron retained.
Caft iron being dropt into a melted mixture of platina and lead covered with black flux, and the fire kept up ftrong till the iron melted, moft of the platina appeared to be retained by the lead, and very little if any of it to be taken up by the iron. It was judged at firfl that this did not happen from the platina having lefs affinity to iron than to lead, but from its not having come fufficiently in. contact with the iron; for we have elfewhere feen that £reat part of the platina finks to the bottom even of the- lead, and the iron floats on the furface of the lead.
A mixture
[ 5§< ]
A mixture of platina and iron was melted with thrice its weight of lead upon a cupel, and a ftrong fire kept up till greateft part of the lead was worked off. The remain- ing mafs was rugged and cavernulous: in its cavities, and at the bottom, was a very confiderable quantity of a dark blackifh powder with fomewhat of a purplifh caft, which was attracted, though not vigoroufly, by a magnetic bar.
This experiment feems decifive of the greater affinity of platina to lead than to iron; as it fhews iron, which had been previoufly well combined with platina, thrown out again in its metallic form by lead. It may therefore be prefumed, that the abforption of part of the platina from iron by lead in the firfl. experiment, proceeded from this fuperior affinity of the platina to the lead, and not, as was at firfl fufpected, from its having an equal affinity to them both.
IV. Aqua regla:
Zinc:
Platina. Platina, digested in afaturated fblution of zinc made in aqua regia, did not appear in the leait corroded; but zinc, put into a faturated fblution of platina, foon begun to diflblve, and to precipitate the platina. The precipitate was of a brownifh black colour: the liquor, after the zinc ceafed to be adled on, continued yellow, a mark that the precipitation by zinc was not total, any more than by the unmetallic precipitants in leclion iii. Marggraf found, that when folution of zinc in aquafortis was mixed with fblution of platina, an orange-red or brick-coloured preci- pitate fell, the liquor continuing yellow as in the othcr cafe.
Ffff 2 V. Aqua
[ 5^ ]
Y. Aqua regia:
Iron:
Platina. A saturated folution of iron in aqua regia did not feniibly aft on platina: a faturated folution of platina rea- dily corroded iron, the platina precipitating. A good- quantity of yellow ochery powder fettled at the bottom, and the undilfolved part of the iron appeared incruftated with a dark coloured matter: it- could not be judged from the colour whether the precipitation was complete, the fo- lutions of platina and of iron having a great refemblance in colour.
VI. Platina:
Aqua regia and folution of iron-vitriol: Gold.
Solution of iron in the vitriolic acid, or a folution of the common green vitriol of iron made in water, which totally precipitate gold from aqua regia, made no change in folution of platina. A mixture of platina and gold, which had been melted together and kept in fufion for; dome hours, being diifolved in aqua regia, and the vitriolic folution added, the gold was precipitated and the platina remained diffolved. Solutions of iron in the nitrous and- marine acids did not precipitate either platina or gold.
VII. Aqua regia:. Copper: Platina. Platina-, put into a folution of copper in. aqua regia, was not lenfibly acted on: plates of copper, put into folii-- fyon of platina, begun quickly to diffolve, and to precipi- tate t)ie platina. The precipitate was of a dark greyi/h
colour,.
[ 5*3 ] colour, and was found on trial to have a confiderable quan- tity of the copper blended wi:h it : the liquor was of a more dufky green than folutions of pure copper, probably from its retaining fome of the platina. Solutions of cop- per in the vegetable, nitrous, marine and vitriolic acids, mixed feparately with folution of platina, produced no pre- cipitation or turbidnefs : Marggraf indeed found, that with the folution in the nitrous acid, a reddifh orange coloured powder was depofited after long ftanding, but in this pre- cipitation the copper folution probably had no fhare, for the folution of platina by itfelf, as he obferves, yields, in time, a like precipitate.
VIII. Aqua regiar Tin: Platina.
We have feen in the third fection that plates of pure tin precipitate platina, and that they do not produce with it the red or purple colour which they do with folutions of gold, but a dark brownifh or olive : it muit. here be added, for eftabliming the affinity more fully, that when platina is digefted in a folution oi tin made in aqua regia, no pre- cipitation of the tin, or corrolion of the platina enfues. The precipitation by tin is not total, any more than by the metals hitherto mentioned, but it may perhaps be quelUon- ed whether the matter which remains diifolved, and which gives colour to the liquor, be true platina, or the ferrugi- neous fubftance that was blended with it, fince in a former experiment, page 487, after the more foluble parts of the mineral, had been extracted by aqua regia, the remainder, diffolved in freili aqua regia, appeared to be completely precipitated by tin, the liquor proving perfectly colourlcfs. Solution of tin, mixed with common folution of platina, ■ fcemed to have nearly the fame effect, as tin in fubftance :
a dark:
C 584]
a dark reddifh orange coloured powder precipitated, a part of the platina or its iron remaining diflblved, fo as to give a high colour to the mcnitruum.
IX. Aqua regia: Mercury; Platina.
Mercury, which is faid to precipitate from aqua regia no one of the common metallic bodies except gold, being put into a diluted folution of platina, feemed to be in a little time corroded, and did not run freely : foon after, it appeared covered with a greyifh powdery matter, which at firft was apprehended to be a precipitate of the platina, but was found afterwards to be only a part of the mercury corroded : upon applying a moderate heat, the whole of the quicklilver, the quantity of which was very confider- able, was diflblved, without any precipitation of the pla- tina. This folution of the two metals, being evaporated a little fo as to difpofe it to moot, yielded cryflals not at all like thofe of platina, but inYorm of needles, externally of a yellowifh hue: the cryflals, flightly warned with proof ipirit, became colourlefs : expofed to the fire, they emitted copious white fumes, with a hifling or crackling noife, and left a very fmall quantity of a reddiih powder, giving a dull red ftain to the tobacco pipe which ferved for the veflel : the cryflals laid on marble, and heated almoft, if not quite, to a red heat, fcarcely ga.e it any tinge or injured its po- lilh. It appears from this experiment, that aqua regia, faturated with platina, is capable of diflblving a confider- able quantity of mercury, and that in cryflallization great part of the mercury fhoots before the platina.
To another quantity of folution of platina I added more quickfllver than it was capable of taking up. The platina now gradually fell down among the undiflblved mercury,
in
r 585 j
in form of a dark brownifh powder, leaving the liquor very little coloured. Platina therefore agrees with gold in having lefs affinity to aqua regia than mercury has, though it differs in its affinity to the mercury, gold in this preci- pitation uniting with the mercury into an amalgam, while platina remains in a diftinct powder. This observation accounts for a phenomenon obferved by Marggraf in the following experiment.
Half an ounce of quickfilver, and an ounce of folution of platina being fhaken together, the mercury run fluggiih, and foon after a quantity of yellowim white powder fettled at the bottom. The folution being fet to digefl, it ap- peared next day fomewhat greenifh. The digeftion was continued a day longer, and the mixture diluted with water; the clear liquor being decanted off, the matter at the bottom was thoroughly edulcorated, and the yellowim white powder warned off from the mercury and dried. The uncorroded mercury was not of the nature of an amalgam, but run pretty freely : being diif illcd in a retort, it left a metalline grain behind, fo fmall, that its appear- ance could not well be diftinguifhed without a micro- fcope, which fhewed it yellow. The white powder, fet to fublime in another little retort, yielded a fublimate of a reddifh yellow colour in the lower part, and whiter above: there remained a little grey matter, which being p relied looked like an amalgam. It is remarkable that the mercury had here borne a very ftrong fire, by which the whole belly of the retort had been melted, though without any hole being made in it.
It is probable that the little yellow grain, left upon
diftilling the uncorroded mercury, was a particle of gold
which the platina had contained ; and that, agreeablv to.
the foregoing remark, platina and gold, diflblved together
..\ua regia, may be parted on this principle, the gold
being
[ 586 ] being imbibed by the mercury, while the platina is preci- pitated in powder, which may be feparated from the amal- gam by waihing.
Solution of mercury in aquafortis rendered folution of platina inftantly turbid, and precipitated a greyifh brown powder. Solution of mercury-fublimate in water, poured into folution of platina, precipitated a red matter, with numerous bright fparkling particles, the liquor continuing yellow : the precipitate bore warning with water, without lofing its red colour.
X. Aqua regia: Nickel : Platina.
Marggraf relates, that a piece of pure regulus of co- balt, or cobald-Jprifi', from the fmalt works at Schneeberg in Saxony, after being repeatedly melted with glafs till all its blue-colouring matter was extracted, was readily at- tacked by folution of platina : the regulus loft its bright- nefs, and became black, a yellowifh powder precipitated, and the liquor looked greenifh.
The fubftancc by which the platina was here precipi- tated, and which communicated a green colour to the li- quor, I apprehend to have been the metallic body called nickel, difcovered and defcribed by Mr. Cronftedt, in the Swedifh tranfactions for the years 175 1 and 1754, one of \\ hofe characters is to dilTolve green in aqua regia,whereas the regulus of cobalt, ftridtly fo called, gives a reddifh fo- lution. Mr. Cronftedt obferves, that cobalt generally con- tains, befides its proper regulus, or the metal which gives a blue glafs, a quantity both of nickel and of bifmuth : that the fpeife, or metal which feparates to the bottom of the melting-pot in making the blue glafs, generally con- fifts of all the three metals ; the cobalt-regulus and bif- .muth, which of themfelves are averfe to any union with
one
£ 5S7 ]
one another, being rendered mifcible by the intervention of nickel : that when this mixture is again melted with glafs, the cobalt-regulus vitrefies firft ,- the nickel, more difficultly calcinable or vitrefcible, pret'erving its metallic form to the laft. It may be prefumed therefore that the operations, which Marggrafs metal pafTed through, fepa- rated the true cobalt-regulus, and left only the nickel.
XI. Platina, Gold, and Aqua regia. Into a faturated folution of platina made in aqua regia, Mr. Marggraf put a plate of fine gold, and digefted the whole in a moderate warmth for fome days : the gold was not in the lead acted upon, and there was no precipitation of the platina, except that a little dark orange coloured cryftalline powder fettled to the bottom, which the folu- tion of platina would have depofited by itfelf. The purer grains of platina were treated in the fame manner with a faturated folution of gold, and with the fame event, the acid mewing no difpolition to quit either of thefe metals in order to attack the other, fo that its affinity feems to be equal to both. I melted the two metals together, and di- gefted the compound in aqua regia : the menftruum dif- iblved them both, but the gold moft readily; for the firft portion of the liquor having been infufficient to diflblve the whole of the mafs, and the reft being digefted in frefh aqua regia, the firft folution was found to have the greateft proportion of gold, and the other of platina. When the quantity of gold was fuch, as to give any thing of a gold colour to the mixture, the acid foon made the plates white, by eating out the gold firft. I likewife mixed together folutions of the two metals, and did not obferve any tur- bidnefs or precipitation to enfue, though Mr. Marggraf found, in his repetition of this experiment, a reddifh orange coloured precipitate : in this refpeCt variations may happen
G g g g from
[ 588 ] from the nature of the aqua; regis: made ufe of, as from an over-proportion of fal ammoniac in the aqua regia in which the gold is diflblved, for fal ammoniac, as we have formerly feen, is of itfelf fufficient to precipitate a part of the platina. Though I could not perceive any reparation on mixing the two folutions, yet on diluting the mixture with water, and buffering it to Hand for fome days, a bright gold coloured pellicle was thrown up to the furface : that this, however, was owing to the action of the platina, I will not affirm ; for I have ken a like feparation from di- luted folutions of gold alone. Another mixture of folu- tions of gold and platina was evaporated a little, fo as to difpofe it to ihoot : it yielded firft fine red cryftals, which feemed to contain chiefly gold, with very little platina;, and afterwards deep faffron coloured cryftals, in whicL the platina apparently prevailed.
XII. Plat ma, Silvery, and Acids. Platina, digefted in a folution of filver made in aqua- fortis, was not at all acted upon, as indeed might have been expected, the platina not being foluble in the acid itfelf by this treatment. A plate of filver, digefted in fo- lution of platina, was ftrongly attacked : a white calx fet- tled upon the filver, and incrufted it all over, and the plate was fo corroded as to become friable between the fingers, the liquor ftill continuing of a gold yellow colour. This experiment is from Marggraf : itfeems to mew, that iilver abforbs the marine acid from folution of platina, and that the platina remains diflblved in die nitrous acid, for if any of the platina had precipitated we mayprefume that the calx would not have been white. He found, however, that when the filver was previoufly diflblved, either in the nitrous or vitriolic acids, it then occafioned a precipitation of the pla- tina, for on mixing thefe folutions with folution of platina9 a yellow precipitate fell. .
[589]
XIII. Platina, Lead, and Acids. Thin plates of lead put into folution of platina are foon corroded, and white cryflals form at the bottom with a blackifh matter intermixed, the liquor continuing yellow : the cryflals difTolve in water, leaving the blackifh powder, which appears to be platina. Marggraf, from whom this experiment is taken, tried alfo folutions of lead, made both in aquafortis and in diftilled wine vinegar, and relates that on mixing thefe folutions with folution of platina, no pre- cipitation enfued; a phenomenon not a little remarkable, as folutions of lead, made in either of the above menftrua, are in general precipitated by aqua regia or liquors contain- ing the marine acid. If there was no error or deception in thefe experiments, it might be concluded from them, that the marine acid has a greater affinity to platina than it has to lead; but with me the event was otherwife. A folution of lead in aquafortis, and a folution in diftilled water of cryftallized faccharum faturni which I had pre- pared myfelf, being dropt into feparate portions of folution of platina, the firft drops produced no apparent change, but on continuing to add more of the lead folutions, both mixtures grew turbid and milky, and depoiited quickly very copious white precipitate?, the liquors continuing yellow like diluted folutions of platina. I repeated the experiment three or four times with different folutions of platina, and the appearances were always the fame.
XIV. Platina, Regulus of antimony, and Aqua regia.
Marggraf found, that a piece of pure regulus of an- timony, digefted in folution of platina, was attacked by the acid. A good deal of white powder fettled at the bottom, which was doubtlefs for the moft part fome of the regulus corroded: the reft of the regulus was reduced into fmall
G g g g 2 liant
[ 59° ] brilliant parts, and appeared to be intermixed with preci- pitated platina: the liquor continued yellow.
XV. Platina, B if ninth, and Acids.
The author above-mentioned relates, that on digefting bifmuth in folution of platina, the effect was nearly the fame as with regulus of antimony, the bifmuth appearing corroded, a white powder fettling at the bottom, and the liquor continuing yellow: and that folution of bifmuth in aquafortis being mixed with folution of platina, no preci- pitation happened.
The experiments in the five laft articles of this fection- are too obfcure and ambiguous for points of fuch impor- tance as the affinities of bodies to be eftablifhed on them; but thofe of the firit ten feem to be fufficiently clear and deciiive. It may be obferved, that in fome of thefe affini- ties platina agrees with gold, as in being precipitated from aqua regia by zinc, iron, copper, tin, and quickfilver ; but that in others, it differs fo effentially from gold, that when the two metals are intimately combined together by long fufion, they may ftill be parted from one another in virtue of this contrariety in their affinities to particular bodies,, platina being rejected by quickfilver while gold is retained, and gold being rejected by aqua regis, when vitriolic fo- Unions of iron are added, while platina is retained.
SECT. IX.
Of dijihiguijliing and purifying Gold mixed ivith Platina.
WE have now finifhed a laborious examination of the properties of this new metal and its relations to other bodies. One of the moft important advantages, that were expected to refult from this enquiry, confidered in a commercial view, was the prefesving of the finenefs and
value
[59* ] value of gold ; or preventing it from being fraudulently debafed, by the admixture of a body, endowed with fo many of what have been univerfilly reckoned the modi peculiar and inimitable characters of the precious metal. This advantage has been obtained in the moll: ample man- ner that could be wifhed for; the experiments having pointed out different means, by which fmall proportions of platina mixed with gold, or fmall proportions of gold mixed with platina, miy be eafily diftinguiihed, and by which the two metals, however blended together, may be eafily parted from one another, either in the way of affay, or of bufinefs in large. The principal of thefe means it will here be proper to collect together from the different parts of the hiflory, and to confider them more particularly in regard to their ufe and applicatioa in practice.
I. Amalgamation with ^uichfilver.
In an experiment related in the laft fection, page syyr a mixture of platina and gold being united with mercury, and the amalgam ground with water for a conliderable time, the platina was gradually thrown out, and the gold retained by the quickfilver-
This procefs, fimple and convenient in the execution, is accompanied with fome uncertainties in regard to its effect, which render it of lefs general ufe than it may at firft promife to be. Repetitions of the experiment have {hewn, that though the feparation fucceeds in fome cafes, it does not perfectly in all : that if there is any particle of the platina not fully diffolved by the gold, which will generally happen unlefs the quantity of gold is three or four times greater than that of the platina and the mixture . is melted with an intenfe fire, this part will be retained in the amalgam, not diffolved by the mercury, not commi- nuted by the peffle, and too ponderous to be wafhed off in<
its
[ 592 ] its grofs form. Various mixtures of platina and gold were treated in the manner above defcribed; and the gold, recovered from the amalgams, was fubmitted to further examinations. Where the proportion of platini was large at firft, the microfcope almoft always difcovered fome grains of it remaining with the fpongy mafs of gold after the evaporation of the mercury; and even when the gold had been melted, and made fluid enough to be poured into a mould, I have fometimes feen diffrinct grains of platina on the fracture of the ingot. Where the proportion of platina had been final 1, the recovered gold was frequently, but not conftantly, found to be pure.
It appears therefore, that though mercury has a greater affinity to gold than to platina, and though platina, on this principle, is capable of being feparated from gold; yet the procefs is too vague and precarious to be applicable in the way of allay, as we can have no mark of the precife time for difcontinuing it, and as we can never be certain, with- out making another allay, whether the whole of the platina is feparated or not. As a preparatory operation, where the quantities of platina and gold to be parted are large, it is neverthelefs of good ufe ; as greateft part of the platina may by this means be waffled over with little trouble, and the gold brought into a lefs compafs, fo as to be commo- diouily fubmitted to a further purification by the means hereafter pointed out.
This procefs may be confidered as anlwering the fame purpofc, in regard to mixtures of gold and platina, as that of iiamping and wafhing does in metallic ores,which could not be reduced to pure metal in the furnace to advantage, without the previous feparation of great part of their earthy or ftony matter by water. To enfure fuccefs, the mixt, if brittle enough to be pulverable, ffiould be reduced into .very fine powder, in ftamping mills, or. in an iron mortar:
the
[ 593 1 the pulverifation may be facilitated by means of heat, both the grains of platina itfelf, and mixtures of them with other metals, being confiderably more brittle when hot than when cold. Or what is ftill better and eaiier, the mixt may be melted with a fuitable quantity of lead, and this compound fubmitted to the trituration with mer- cury and water. If there is any truth in the report, that certain gold mines are neglected, on account of their in- tractablenefs from platina contained in them, this lafl pro- cefs may turn out a very important and advantageous one.
II. Precipitation by vegetable Jixt Alcalies: As gold is totally precipitated by fixt alcaline falts, but platina only in part, and as a minute portion of platina. remaining diflblved tinges a furprizingly large quantity of the fluid of a yellow colour ; it was preiumed that a lmall admixture of platina with gold might by this means be readily diicovered. A few drops of a iblution of platina were therefore mixed with above a hundred times the quan-r tity of a folution of gold, and a pure fixt alcaline fait gra- dually added fo long as it occaiioned any effervefcence or precipitation. The remaining liquor was flill fo yellow, that it was judged the platina would have diicovered itfelf, though its proportion had been lefs than one thoufandth part of that of the gold. It may be obferved, that though it is cuftomary to dilute metallic folutions pretty largely with water in order to their precipitation, yet here, as we want only to fee whether any colour remains in the liquor after the precipitate has fettled, the lefs dilute the liquor is, the lefs quantity of colouring matter we lhall be able to diftinguiih.
It has been objected to the above -experiment, that though the platina is difcoverable when thus mingled lu-
perficially
[ 594 ] perficially with the gold, it may neverthelefs, when com- bined more intimately by fulion, elude this method of trial. Mixtures of gold with fmall proportions of platina were therefore kept in fulion for feveral hours, with a very llrong fire, and afterwards diffolved in aqua regia. The folutions were diluted considerably with water, and a fo- liation of pure fixt alcaline fait gradually added, fo long as any efFervefcence or turbidnefs enfued. The liquors proved paler than when the two metals had been diffolved fepa- rately, but retained colour enough to betray the platina. As the degree of colour was not here fo great, as might have been expected from the quantity of platina which there was reaftn tD believe they contained, I tried to dif- cover the plaina in them by fome character more con- fpicuous. I put into the filtered liquors fome plates of pure tin: the tin prefently contracted an olive hue, and threw down a large quantity of brownifh precipitate, as it does from the common folutions of platina: it was obferva- ble, that the tin plates were often fenfibly acted upon even while the liquor was overcharged with alcali.
It has been further fuggefted, that fince a part of platina ie precipitated as well as gold by fixt alcaline falts, if only this part be mixed with gold, it will elude this trial, and be thrown down by alcalies again, along with the gold, from the folution of the compound. To determine this point, I melted with gold a precipitate of platina made by fixt alcali, and kept them in ftrong fulion for an hour and a half: they feemed to unite more eafily than gold does with the crude platina, and formed a fmooth neat bead, which hammered pretty well into a thin plate before it cracked, and appeared internally uniform and equal. This compound being diffolved in aqua regia, the folution diluted with a little water, and a folution of fixt alcaline fait added by degrees till the acid was more than faturated,
the
[ 595 J the liquor became, not indeed colourlef-, but fo pale, that it could hardly be judged to contain anyplatina : ncver- thelefs, on putting into it fome tin plates, they quickly fhewed, as in the foregoing experiment, that it held a very confiderable quantity of platina. It appears there- fore that in all thefe circumftances the platina remains partially diffolved in the neutralized liquor ; and that on this foundation, fmall proportions of it, mixed with gold, may be difcovered, either by the colour of the liquor after precipitation with alcali, or, in a more fenfible manner, by further precipitation with tin. In all the above expe- riments the folutions were diluted with water, not as be- ing a circumftance advifable where gold is to be thus exa- mined, but that the ufefulnefs of this way of trial might be eftablifhed with greater certainty.
Volatile alcaline falts or fpirits have the fame effects as the fixt alcalies on folutions of platina, but their effects on folutions of gold are in fome circumftances different. After the acid has been fatiated, and all the gold precipitated, an addition of the volatile alcali beyond this point redif- folves fome part of the gold, fo that the liquor becomes yellow again though there be no platina in it. For this trial therefore, only the pure fixt alcalies are to be ufed, which, in whatever quantity they are added, have not been found to rediffolve any of the gold.
III. Precipitation by mineral fixt Alcali. The vegetable fixt alcalies ferve only for dilfinguifhing whether gold is mixed with platina or not : they are insuf- ficient for the purification of the precious metal, as they always precipitate a part of the platina along with the sold. With the mineral alcali, or the alcaline bafts of fea fait, the cafe is otherwife. Though this alcali, as appears from Marggrafs experiments, precipitates, equally witlj
Hhhh the
[ 596] the vegetable, all the common metallic bodies, gold, fil- •ver, copper, iron, tin, lead, zinc, bifmuth, regulus of an- timony, cobalt, <$cc. yet in lblution of platina it produces no precipitation or turbidnefs ; fo that when this alcali is mixed with a folution of gold containing platina, the gold alone is precipitated, and all the pl.uina remains dilfolved. The manner of obtaining this alcali from the acid with which it is united in lea fait, as it would in this place toe much interrupt the hiitory, is referred to the appendix.
The mineral alcali is in many places, particularly in the eaftern countries, found native, either in a pretty pure ftate., or blended chiefly with earthy fubftances, from which it is eaiily feparated by folution in water. I have been fa- voured by Dr. Ileberden with a quantity of this native fait lent to him from Tenerirf, and find that it anfwers the prefent intention as effectually as the alcali extracted from fea fait. The folution of platina effervefced witli it, but in whatever proportions the folution of the alcali and of the platina were mixed together, I could not obferve the leaft precipitation or cloudinefs.
A fait of the fame nature, though generally perhaps mingled with fome foreign faline matters, is obtained from the afhes of certain plants, called Kali, which growing chiefly in fait marfhes or on the fea more, the marine fait is fuppofed to be imbibed by them, and to be decom- pounded, or to have its acid feparated, partly by the power of vegetation in the plant itfelf, and partly by the burning* The beft fort of thefe allies is faid to be prepared at Ali- cant in Spain, from an annual procumbent kali with fliort leaves like thofe of houfeleek. The allies, which are one of the common kinds of potalh in France, and there called Jbude ovfoda, are brought to us, under the name of Spanifo afoes or bariglia, in hard fpongy mattes, partly whitifh or grey, and partly blackifh. From thefe mafles the faline
part
[ 597 1 part is extracted pure by powdering and digefting them in water. Though it might be fufpected that this fait, in virtue of its containing not only the mineral but a portion of vegetable alcali, would precipitate part of the platina as well as gold, I could not find that folution of platina fuf- fered the leaft alteration from it, any more than from the native or marine alcalies.
How far thefe falts may fuffice, for the perf:6l feparation of platina and gold that have been intimately combined with one another by fufion, I have not yet had direct experience. But it may be proper to obferve, that though both the native alcali and bariglia are fuppofed ge- nerally to contain fome fea fait in its whole fubftance,-. which for fome purpofes renders them unfit, yet this fait does not appear to be here of any difadvantage, for pure fea fait occafioned no precipitation or turbidnefs in a folu- tion of platina, any more than in folution of gold. The platina employed in thefe experiments was fuch as had been cupelled with lead and urged afterwards with repeated ftrong fires, page 570.
IV. Precipitation by fal ammoniac. The alcaline falts in the two foregoing articles precipitate the gold, and leave the platina wholly or partially diflblved in the liquor. Sal ammoniac has a contrary effect, preci- pitating great part of the platina, and leaving all the gold diflblved ; and on this principle platina may be dilcovered in gold as readily and as effectually as on the other. The metal being dilTolved in aqua regia, add a little folution of fal ammoniac made in water: if the gold contained any platina, the liquor will inilantly grow turbid, and a fine yellow or reddifh precipitate will quickly fall to the bot- tom : if the gold was pure, no precipitation or change of trantparency will enfue.
H h h h 2 V. Sr
[ 598 ]
V. Separation by inflammable liquors. Inflammable fpirits, which revive gold from its fo- lution in form of yellow films, have no fuch action on folution of platina. This experiment affords a fure crite- rion for diftinguiihing whether gold has been debafed by platina, or whether platina holds any gold, and likewife an infallible method of recovering the gold perfectly pure. If the compound be difiblved in aqua regia, the folution mingled with twice its quantity or more of rectified fpirit of wine, and the mixture fuffered to ftand for fome days in a glafs flightly covered, the gold rifes to the furface, leaving the platina diffolved. The golden pellicles may be collected, by pouring the whole into a filter j lift large enough to contain it : the difiblved platina will pais through, leaving the gold upon the paper, which is to- be wafhed with frefh portions of hot water till the liquor runs through perfectly colourlefs. The paper is then to be fqueezed together, and burnt in a crucible previously rub- ed en the inlide with chalk, which prevents the fmall par- ticles of the gold from lodging in the cavities : when the matter has fullv funk down, fome nitre is to be added, and the fire increafed to bring the gold into fufion. This procefs is accompanied with one inconvenience, the flow- nefs of the feparation of the gold from the folution : this may be fomewhat expedited, by employing a fpirit that has been diftilled off from fuch vegetables as give over an elTential oil.
The fame intention is anfwered very fpeedily, by pure eflential oils. The metal to be examined being difiblved in aqua regia, add to the folution about half its quantity of any colourlefs eflential oil : fhake them well together, and then luffer them to reft : the oil rifes immediately to the furface, carrying the gold with it, and leaving the pla- tina
[ 599 ] tina diflblved in the acid underneath. The oil, loaded with the gold, appears of a fine yellow colour, and on {land- ing for a few hours, throws off great part of its metal in bright films to the fides of the glafs. The oil may be taken off from the acid before this feparation happens, well fhaken with water to wafh off fuch parts of the platina as may adhere to it, and then fet on fire in a crucible : when thoroughly burnt out, the refiduum is to be melted with nitre as in the preceding experiment. After the fepa- ration of the oil employed at firfi, it may be proper, for the greater fecurity, to add a little more ; which, if any part of the gold fliould have been left in the acid, will ef- fectually take it up.
The gold is taken up fiill more readily, and perhaps more perfectly, by the fubtile fluid called aether or ajthereal fpirit of wine, the preparation of which has been already defcribed in the hiftory of gold. Though this fluid is too expenfive to be employed for the purification of gold in the way of bufinefs, it may be of ufe in the allaying of gold fulpected to be debafed with platina. Indeed the purifi- cations by the common vinous fpirits and efiential oils are not to be recommended to the refiner, who can better avail himfelf of the method pointed out in the following article.
VI. Precipitation by green vitriol. The moll effectual and advantageous method of purify- ing gold from the metallic bodies commonly found mixed with it, appears to be, by diflblving it in aqua regia, and precipitating with a large proportion of a filtered folution of green vitriol. Happily the fame procefs purifies it from platina; the vitriolic folution precipitating the gold and leaving the platina diflblved. See the hiflory of gold, page 160 of this volume. On many repetitions of this expe- riment, with mixtures of different proportions of the two
metal Si
[ 600 ] metals, I could never find that any part of the platina was precipitated with the gold, or that any part of the gold continued diffolved with the platina.
Mr. Scheffer was the firft who difcovered this property of platina, of not being precipitated by green vitriol; and the important confequence of it did not efcape him. He feems to think however, that the precipitation of the gold by vitriol, and warning the precipitate thoroughly with water, are not fufficient for completely purifying the gold from the platina, and directs an additional operation, the amalgamation of the wafhed precipitate with mercury; a procefs which did not appear to me to be at all needful.
SECT. X.
Experiments on the yellow particles ?nixcd with platina.
THE yellow particles, intermixed with platina as it comes to us, were not only by me, but by every per- fon I know of who had examined this mineral, taken to be gold; except only Mr. Marggraf, who fays they looked like the fineft gold, but no where hints that they were gold, and even relates fome experiments which feem to prove that they were not what they appeared to be.
" On fome of thefe yellow grains, in a parting-glafs, he poured aqua regis, and fet them in digeftion together. But though the aqua regis was made to boil, the grains were very little acted on, the liquor hardly receiving a yellow tinge, and folution of tin precipitating nothing from it."
" Having picked out the yellow grains from fome pla- tina that had been treated with arfenic, fal alembrot, ccc. he mixed them, their quantity being but fmall, with half a dram of lead, and cupelled them with the lead: the pro- cefs being finifhed, th; remaining button was greyifh- black, flatted, and cracked about the edges, like thofe ob- tained
[6oi ] tained in the cupellation of crude platina, and weighed
about half a grain. This little bead was put upon a frelh cupel, with one grain of gold that had been parted with filver, and twenty grains of granulated lead: after cupel- lation, he had a fair gold button, yet ftill fomewhat flat, curled, and with a kind of net- work on the furface, in co- lour like gold but paler, weighing exactly two grains, hard indeed, but bearing pretty well to be reduced into plates. To this he added four grains of the fined laminated filver, and twenty grains of granulated lead; and on repeating the cupellation obtained a button not yet quite round, and i*hing five grains. He flatted it, for it was confidcrably malleable; made it red hot; and tried to part it with puri- fied aquafortis : but the aquafortis, though made to boi1, would not futficiently act on it. He therefore poured off the aquafortis, and found the plate very little corroded. After warning it feveral times with diftilled water, and heating it red hot, it weighed four grains, and was found to be brittle, and juft perceptibly yellowifh. He added to it fix grains more of fine filver, with twenty grains of gra- nulated lead, and cupelled again : the button weighed thirteen grains, and confequently had gained an increafe of three grains. It was very malleable, and being flatted, made red hot, and digefled in purified aquafortis, the aqua- fortis attacked it briskly, leaving fome black plates, which being wafTied, and ignited under a muffle, appeared of a fine gold colour, and weighed one grain."
In this laft experiment it is probable, that the fmallnefs of the quantity occafioned fome deception. If we conclude from it, that the yellow particles were not gold, becauie the gold that was melted with them was recovered without increafe ; we muff, conclude for the fame reafon, either that they were not platina, or that the platina was deft royed in the cupellation or diffolved by the pure aquafortis. The
expe-
[ 602 ]
experiment with aqua regis feems liable to the fame diffi- culty ; for if the yellow grains were not gold becaufc they did not difTolve in aqua regis, for the fame reafon they either were not platina, or platina did not difTolve in aqua regis.
I have already mentioned the facts which induced me formerly to believe that the yellow particles mixed with platina are really gold : fee page 487. I have fince re- peated thofe experiments with the fame event, and made another which may perhaps be thought more decifive.
Twelve ounces, or 5760 grains, of platina rich in yellow particles, were placed in three fcorifying dimes under a muffle, and kept of a ffrong red heat for two or three hours, in order to diiTipate any mercurial or other foreign matter by which fome of the yellow grains might be en- veloped. All the yellow particles, that could be diftin- guifhed by a good magnifying glafs, being then picked out, which employed two perfons for feven or eight hours, their weight amounted to 47 grains : fome of them were all over yellow ; others were in part yellow, and in part like the grains of platina.
Thefe picked particles were cupelled with fomewhat more than thrice their weight, viz. 150 grains, of lead, which in lix different affayshad yielded a filverbead amount- ing to between a 9525th and a 9527th part of its weight. The cupelled mafs was of the fhape of a kidney-bean, grey, rough, brittle, with a cavity in the internal part corre- fponding to the fhape of the outfide. The mafs, broken in pieces, was laid on a frelh cupel, and urged with a verv ftrong fire for five or fix hours. It was lefs brittle than before, filed fmooth, and appeared of a pale yellowifh co- lour.
The metal being then digefted and boiled with aqua re- gia in a Florence flaik, greateft part of it diffolved, a fmall
quantity
E 6o3 ]
quantity of whitifh powder, probably filver, remaining at the bottom of the veffel. The gold-coloured folution be- ing poured into a folution of green vitriol, a precipitate like that of gold foon fell. After {landing till next day, that the precipitate might fully fettle, greateil part of the liquor was decanted off, and the remainder, with the pre-> cipitate, poured upon a filter : when the liquor had run through, the powder was warned on the filter with frefh portions of water. When dry, the filter with the preci- pitate were put into an affay crucible, and kept red hot till no more flame or fmoke appeared. Some nitre was then thrown in by little and little: at firff. a flight fulguration arofe; at length the whole appeared in quiet fufion, and being poured out into a mould, I obtained a lump of high coloured, malleable, pure gold, weighing between eighteen and nineteen grains.
o
SECT. XL
Of the mineral hijlory of Platina.
F the mineral hiftory of this metal very little is as yet known with any certainty. Though new to Europe, the hiftory even of its difcovery is as obfeure as that of the metals of moft ancient ufe : it may be pre- fumed, that the little advantage which promifed to refult from it, on account of its want of fulibility, occafioned it at firfr, to be neglected j and that the fraudulent purpofes, to which it was afterwards found to be applicable, occa- fioned the knowledge of it to be concealed.
It is fuppofed by fome, that platina is the produce of the Eaft Indies as well as of the Weil, and that its analogy with gold has been known alfo for a confiderable time in the former as well as in the latter. The foundation of this fufpicionis, that the late profeffor s'Oravelunde had in his
I i i i pofTelTion
[ 6o4] pofTeffion a ponderous metallic body, reckoned heavier than gold, fuppofed to be a mixture of gold and platina, faid to have been brought by the Dutch Eaft-India mips from China, and to have been there fold at a high price. Dr. Brownrigg however informs me, that having lately made enquiries about this fubftance in Holland, he learnt from profeftbr Allamand, that it is indeed a mixture of platina and gold, but that there was a miftake in regard to the place it had come from,which was not the eaft but the weft Indies.
That the platina brought into England is the produce of the Spanifh weft Indies, appears unquestionable ; but in what particular places or in what form it is met with, is far from being clear. Some fpeak of its being found in great abundance, like fand, in certain rivers in the pro- vince of Quito. A perfon who had been upon the fpot in- formed me, that it came from the mountains near Quito, or between Quito and the South fea ; that great part of the land at the bottom of thofe mountains is covered with it, the floods, which come after heavy rains, warning the mi- neral down with them.. Another perfon, concerned alfo in the importation of it, affirmed that it was found in Peru, in a gold mine, which had been formerly deftroyed by an inundation and lately drained ; whether originally con- tained in the mine, or brought by the flood, was not known.
It has been reported, and without contradiction, ever fince the platina became known here, that in order to pre- vent the frauds which might be practifed with a fubftance of fuch qualities, the king of Spain had ordered the mines that afford it to be ftopt up ; an account which, if literally underftood, feems to imply that platina is not plentiful, on the furface of the earth. Whatever may be in this, whether the prohibition was made againft the working of
mines
[ 6o5] mines of platlna, or the exportation of platina lying at day, or both ; we may obferve, that the fetting at large even of the little quantity that has hitherto been made publick, far from being productive of any ill confequences, has been the means of effectually preventing thofe abufes, which platina could not fail of giving occafion to, while confined to a particular part of the world, and while the exiftence of fuch a fubftance was in general unknown. In the papers laid before the royal fociety foon after the pla- tina came here, there is an account of gold having been taken in payment from fome Spaniards, which being mixed with platina was fo brittle that it could not be difpofed of, and which could not be refined in London, fo that it was quite ufelefs. I have been informed that the Dutch re- finers at Dort have long complained of their meeting with gold adulterated with a fubftance which they could not fe- parate, which they called diabolus metallorum, and which they now judge to have been no other than platina; and that our jewellers, &c. for many years part, have avoided making ufe of the Spanifh gold for any curious works, on account of its having frequently a mixture of a fubftance which renders it intractable, and which is often vifible to the eye in fmall diftinct grains like thofe of platina, as if the gold had been melted by a heat not fufficient for the perfect diflblution of the platina, which when diflblved would have given an ill colour to the mafs (fee page 526). The more the platina became known, the lefs danger there was of any frauds of this kind, and we have now nothing to fear from it j the experiments already made having dis- covered eafy means of diftinguiihingwith certainty gold de- bafed by platina, and of completely parting the two metals, however they may be blended together by accident or de- fign. The refining of gold from platina is now no more difficult than the refining of it from any other metal,
I i i i 2 It
[ 606 ] • It is the general opinion that platina is found in the fama form in which it is brought to us. The obfervations on the appearance of the grains, and the matters mixed with them, mentioned at the beginning of this effay, induced me, on the rirft examination of it, to think that it had been ground in mills with quick/ilver. Marggraf, whofe pla- tina came from London, and probably from the fame parcel with that in which I had obferved the drops of quickfilver, feems to have entertained a fufpicion of the lame kind ; for he doubts whether the platina is a native mineral, or a metallic recrement from which the Spaniards have extracted the perfect metal it contained. I have fince been informed that the quicklilver we obferved among it, which doubtleis influenced Marggraf as well as me, did- not come with it from the weft Indies, but was added here by the proprietor with a view to get out the golden particles.
Some accounts however feem to countenance the above conjecture, that platina is found in large maffes, and. is re- duced into fmooth grains by ftamping and grinding. A. lubllance in final! grains, like the platina as brought to us,, one mould think could hardly be called a.ftone, as platina. is by don Antonio d'Ulloa.
D'Ulloa is the rirft writer I have met with who mentions' platina by name. In his voyage to South America, in 1735 and the following years, fpeaking of the gold and iilver mines of Quito, he relates, that in the territory of Choco there are mines, in which the gold is fo enveloped in other mineral fubftances, bitumens, and ftones, that they are obliged to ufe quickfilver for its feparation; that fome- times they find mineral fubftances which, from their being mixed with platina, they chufe to neglect ; that this pla- tina is a ftone (piedra) of luch refiftence, that it is not eafily broken by a blow upon an anvil; that it is not fub- dued by calcination ; and that it is very difficult to extract, the metal it contains even with much labour and expence.
[6o7] ' Some have fufpedted that the piedras del Inga or Ihcd, defcribed by the fame author as being untranfparerit and of a leaden colour, and which were made into mirrours by the ancient Indians, confiit of platina mixed with a ltony matter. This mineral cannot be the fame with that to which he gives the name of platina in the foregoing para- graph, for he exprefsly mentions that the piedra del Ingo is foft, and liable to be broken by a flight blow. The Inca ftone is now pretty common, and as the French tranflator of the papers on platina (fee page 447) obferves, appears to be no other than a ferrugineous mineral of the pyrites or rather mundick kind.
Alonfo Barba mentions a fubflance, under the name of chwnpi, which feems to have more refcmblance to the pla- tina of d'Ulloa. He defcribes the chumpi as a hard ftone, of the emery kind, participating of iron, of a grey colour ihining a little, very hard to work becaufe it refills the fire much, found in Potoli, Chocaya, and other places, along with blackifh and reddiih ores that yield gold. If platina is really found in large mafles, either generally or only now and then, one might reafonably expedt thole mafTes to be iuch as are here defcribed.
Of the fame kind perhaps alio is the mineral mentioned by feveral authors under the name of Spanilh emery, Smiris Hifpanica, which fhould feem, from the accounts given of it, to be no other than platina or its matrix. T\\q fmhis is faid to be found in the gold mines, and its exportation prohibited; to contain films or veins of native gold; to be in great requeft among the alchemifts; to have been fome- times ufed for the adulteration of gold ; to Hand, equally with the noble metal, cupellation, quartation, antimony, and the regal cement; and to be feparable from it bv amalgamation with mercury, which throws out the fmiris and retains the gold; properties flrongly characlerittic of
plating
[ 6o3 ] platina, and which do not belong to any known fubftance befides. This debasement of gold per extraSlum fmiridis Hifpanici is mentioned by Becher in his minera arenaria, and feveral times hinted at in his phyfica fubterranea. Both Becher and Stahl indeed call the SubStance, which the gold receives from the emery, an earth, whereas platina is undoubtedly a metal; but this does not at all invalidate our fuppofition, for they give the name of earth alfo to the SubStance which copper receives from calamine in being made into brafs, which is now known to be metallic.
From thefe obfervations I have been led to SuSpect that the European emerys likewife might poffibly participate of platina. If this was certain, it would account Satis- factorily for the ufe which fome of the alchemifts are faid to have made of emerys and other ferrugineous ores ; and we fhould no longer doubt, or wonder, that by treating gold with thefe kinds of minerals, they obtained a perma- nent augmentation ; that this augmentation, though it refilled lead, antimony, aquafortis, and the regal cement, was Separable, as Becher owns it was, by quickfilver ; and that, when it exceeded certain limits, it rendered the gold pale and brittle.
If emery contains platina, I imagined it might be dis- coverable by boiling the powdered mineral in melted lead, and afterwards working off the lead upon a teft or cupel. The experiment was made with eight ounces of the fineff. powder of common emery, and the fame quantity of lead, which were covered with black flux to prevent the Scori- fkation of the lead, and urged with a ftrong fire for two or three hours. The lead became hard, rigid, of a dark colour, and a granulated texture, as if it had really im- bibed fome platina from the emery ; but in cupellation it worked almoit entirely off", leaving only a bead about the Size of a Small pins head, which was probably no other than filver contained in the lead.
[6o9 ]
I repeated the experiment, with Tome variation, think- ing to obtain a more perfect refolution of the emery by vitrefying it with the lead. Two ounces of line emery, and fix ounces of minium, were well mixed together, and urged with a ftrong fire, in a clofe crucible, for an hour : they melted into an uniform dark brownifh glafs. The glafs was powdered, mixed with four ounces of fixt alca- line fait and fome powdered charcoal, and put into a frem crucible, with fome common fait on the furface : The fire was pretty ftrongly excited, but the fulion was not fo perfect as could be wifhed, and only about two ounces of lead were found revived- This lead had fuffered nearly the fame change as that in the foregoing experiment, and like it, gave no appearance of platina on being cupelled.
It feems to follow from thefe experiments, that the emery employed in them contained no platina ; but as it is not to- be fuppofed that all emerys are of one compofition, other forts may deferve to be fubmitted to the fame trials. As' gold is contained in fome parcels of common minerals, and by no means in all the individuals of any one fpecies ; pla- tina may pofiibly in like manner be found in fome Euro- pean ores, though there is not the leail footltep of it in other parcels of the fame kind of ore.
SECT. XII.
General Obfervations.
TH E foregoing hiflory has brought us acquainted, with a mineral fubftance, whole metallic afpecl, great weight,, malleability, and perfect mifcibility with all the common metallic bodies, are fufficient characters of its being a true metal : — which abides fixt and uncalcined in the firongeft fires, is nowife fcorified by nitre, or by lead or bifmuth, nor diflblved by vitreous bodies, and which
i:
[ 6io ] k therefore a perfe<£t metal, of the fame clafs with gold and filver, and perhaps more perfect, or lefs alterable, than they : — which, with the colour of filver, poffefles the fpecific weight, and feveral other of the reputedly moft difcriminative properties of gold j refitting, equally with gold, many agents, which difcolour, corrode, diffolve, or fcorify filver and the bafe metals, as air and fulphureous exhalations, the nitrous, marine, and vitriolic acids both in their liquid ftate and when refolved by fire into fume, and fulphur and antimony in fufion : — with thefe valua- ble properties of gold, it adds fome to gold itfelf, making it both lefs foft and lefs fufible, which no other alloy does : hence a due proportion of it bids fair to remove thofe inconveniencies, which the enamellers complain of, when they work upon plates either of fine gold or of alloyed gold.
2. Though platina undoubtedly belongs to the fame genus of bodies with gold and filver, of which genus no more than thefe three fpecies have hitherto been difcover- ed ; and though it agrees with gold in many properties which have been univerfally fuppofed difr.inc~t.ive of the fpecies : yet there are other important characters in which it remarkably differs from gold. Its white colour; its want of fufibility ; the fingular alterations which it pro- duces in fome of the other metals, and in gold itfelf; its being difficultly and fparingly acted on by hepar fulphuris, by which gold is plentifully diffolved ; its folution in aqua regis giving no ftain to the fubftances which by folutions of gold are tinged red or purple ; its being in part precipi- tated from its folution by fal ammoniac, which does not in the l«aft precipitate gold ; its being precipitated only partially by vegetable fixt alcalies and by volatile alcalies, ajid not in the leaft by the mineral alcali or by folution of green vitriol, by all which gold is precipitated entirely;
its
[6n ] its precipitates by alcalies having nothing of the fulmi- nating power, which precipitates of gold poffeis in a more eminent degree than any other known kind of matter; its folutions in aqua regia fufFering no decompofition from ef- fential oils or aether, by both which gold is imbibed from the acid, nor from inflammable fpirits by which gold is re- vived and thrown out in its proper form; its being reject- ed, on trituration, from its folution in quicklilver, while gold is retained and continues diffolved; its being feparable from gold, in virtue of thefe diverfities of affinity, without augmentation or diminution of either metal, as eafily and as perfectly as any one metal is feparable from any other; are characters abundantly more than fufficient for eftablilh- ing a fpecific difference between platina and gold.
3. The author of the letter from Venice, mentioned in page 447, enters into fome alchemical fpeculations on this fubjecT:, which the nature of the prefent hiftory requires that I iliould give fome account of. He imagines, that as platina is a fpecies of the fame genus with gold, its dif- ferences from gold are only accidental, proceding either from fome heterogeneous body radically united with it, or from the want of a glutinous tinging fulphur. To which of thefe caufes its imperfection is owing, he does not de- termine: its being lefs ponderous than gold, the black points difcovered on its grains by a microftope, and a part of it being precipitated from aqua regia by alcalies while a part continues diffolved, are brought as arguments for the former; its want of fuiibility, its folution wanting the power of flaming animal fubitances and of producing a purple with tin, and its not being feparated from its fo- lution by inflammable liquors which have an affinity with fulphurs, for the latter. In the one cafe by purging the platina of its heterogeneous matter, and in the other by introducing the tinging fulphur, he thinks it will become gold. This lafl, it feems, is eafy enough to be done,
K k k k bodies
[ 612 ]
bodies having a natural difpofition and appetite for re- ceiving the principle which is wanting for their perfection : but in the fLrft cafe there are no hopes of fucceeding, for to root out an impure matter, with which a metal is radi- cally combined in its firft formation, he admits not to be in the power of any other agent than the philofophers ftone itfelf. Of thefe notions it is fufficient to oblerve, that they are drawn from a fuppofition which cannot be ad- mitted till fome fads fhall be produced, to make it proba- ble, viz. that all the bafe metals are no other than gold vitiated by fome impure fubftance.
4. Vogel adopts an opinion, that platina is not a true metal or iemhr.etal of a peculiar kind, but a mixed mine- ral, the drofs of the amalgamation-works in which gold is feparated by quickfilver from a mixed ore. This opinion he attributes to Marggraf; and in a periodical pamphlet publifhed at London, it is faid that Marggraf fuppofes pla- tina to be not only the effect of reiterated amalgamation, but to be a part of the mercury itfelf fixed by fome matter in the ore or metal it was amalgamated with. All I can find in Marggraf relative to this point is the following pafiage. " We cannot fay with certainty whether platina is an actual ore, or whether it is a ftream-mineral which has been torn off and carried away by water from entire veins, or whether thirdly it may not poffibly be a mere metallic recrement, from which the Spaniards, as being the owners of the works, have already perhaps extracted the perfect metal." I do not apprehend that the latter part of this fentence will admit of the improbable inter- pretation that has been given of it. The author feems to- me to have meant no more, than that the platina poflibly has not come to us in its native form, but has been ground with quickfilver to extract the gold intermixed with it; a fufpicion which I had myfelf expreffed alfo in the firft paper in theTranfactions, and which the mercurial globules found among the platina could not fail to produce.
[6i3]
APPENDIX.
Page 4, 5. Portable Furnaces.
A CONVENIENT grate for thefe furnaces may be formed of four or five iron rings, placed within one another, each furnifhed with three pins at equal diftances projecting from its circumference : the pins of the inner ring drop down into correfponding notches made in the next, and the pins of this fecond ring into notches in the third : the notches are made in the middle of the fpaces between every two pins. Thus we have a grate compofed of moveable parts, and which we can enlarge or diminifh at pleafure, fo as to fit into furnaces of different widths, by adding or removing a ring on the outfide: the fpaces for admitting air to pafs up through the fuel, and the afties to drop down, are likewife more equally diftributed than in thofe of the common con- ftruction. I have had fome iron grates of this kind caft, and find them to anfwer well: the rings are about five eighths of an inch deep, a quarter of an inch thick, and their diftances from one another fomewhat more than half an inch: the inner ring is an inch and a half in diameter, and three pins projecting inwards prevent any coals from falling through this fpace.
Inftead of binding the black lead pots with wire, thev may be furrounded with iron or copper hoops. The ad- vantage of a hoop round the mouth has been already men- tioned: another may be fitted between the doors, and as the wideft part of the furnace is that included between the
K k k k 2 hoops,
[6i4] hoops, two flips of iron or copper, pafling from one hoop to the other on oppofite fides, and fcrewed to each, will keep them immoveable in their places: thefe flips ferve likewife to carry handles for lifting the furnace by.
It may be proper to obferve that even the unfound or cracked pots, unfit for the purpofes of crucibles, and which are fold for a much lower price than the found ones, if properly fecured with hoops or wires, will make ufcful and durable furnaces. In all cafes it will be expe- dient to wafh over the infides with Sturbridge clay diluted with water ; and where the furnace is defigned for conti- nued ftrong fire, or for ftanding the action of corrofive bodies, as in fuiion trans carbones (page 21) it may be lined to fome thicknefs, with a lute compofed of the fame clay, beaten up with about twice its meafure of coarfe. fand, or rather of glafs-houfe pots in coarfe powder,
Page 66. GlaJJ'es gilt on the edges. Since the publication of the firft part of this work, and probably on the principles there pointed out, thefe glaffes have been prepared in England, with as durable gilding as thofe brought from Bohemia and Thuringia. Of the compofition of the varnifh I can fay no more with certainty,, than that oil of turpentine is an ingredient in it. My. worthy friend Mr. Ziegler, in an elegant German tranfla- tion with which he has honoured this work, defcribes- a varnifh, with the method of ufing it, which appeared from his experiments to be the beft. Fine tranfparent amber, reduced to powder, is boiled in a brafs veflel having a valve in its cover (fee page 368) with as much drying oil as will juft cover it : generally in five or fix hours the amber is perfectly diffolved. Dilute the folution with four or five times its quantity of oil of turpentine, and let it ftand fome
days
[6i5] days that all the impurities may fettle to the bottom. That the varnifh may dry the eafier and acquire the more firm- nefs, it is to be ground with a little white lead, or rather with a mixture of white lead and minium. It is to be applied very thin on the glafs, and the gold leaf rather blown upon the part, fo as it may flick raft, than preffed down with cotton. The glaifes may be laid in a warm place, free from dufl, till the varnifh is fully hardened; after which the gold may be burnifhed, a piece of fmooth paper being laid between the tooth or fled burnifher and the gold. He obferves that this gilding is durable and of a fine luflre ; and that as the toughefl varnifhes naturally deferve the preference, the amber varnifh above deicribed, promifes, in virtue of that quality, to be the befl.
Page 64. Gilding on the covers of books. The bookbinders dilute the whites of eggs with water, and moiflen the part to be gilt three or four times with this liquor: when fo far dried as that the gold may not flick without preffing, the part is flightly oiled over before^ the gold is laid on. Mr. Ziegler, after taking notice of thefe particulars in a note on the above palfage, adds, that for gilding on taffeta or other fluffs, fome fine powdered maflich, or white of eggs dried and powdered, is dufled thinly on the fluff; and the gold leaf, firil cut to a proper fize, is laid on a hot fomewhat oiled flamp, and prefied down; with care that none of the powder touch the flamp, which would occafion the gold to Hick to it. Our book- binders, for gilding on rough leather, follow a practice of the fame kind, ufing common refin inftead of the maflich or dried whites of eggs : the refin melting only in thofe parts where the hot flamp is applied and the gold fixed on it, the other parts of the leather remain rough as at firfl,
and
[ 6x6 ] and on this account only they prefer the dry refin to the liquid glutinous fubftances.
Page 45, 67. Me/ting of Gold. Black lead crucibles are laid by foreign writers to be accompanied with an inconvenience of rendering the gold brittle and fomewhat pale, efpecially when a new crucible is ufed for the firit time. I had often melted gold in thefe crucibles myfelf, and had been told by different workmen that they generally employed this kind, without obferving any ill effect from them. On further enquiry among the gold-beaters, whofe daily labour is one of the fevereft tri- als of the toughnefs of the gold which they melt, I cannot find that they have any fufpicion of its being injured by black lead crucibles, though they now make ufe of the Heflian or Englifh more frequently than the black lead, on. account chiefly of their greater cheapnefs : one of thefe workmen informed me that " he had once found gold, which was melted in a black lead crucible, to be brittle, but imagined the brittlenefs to have proceeded only from want of lufficient heat, for on melting the gold a fecond time, in the fame crucible, it had the proper tough- nefs." The degree of heat is a very material article in the melting of gold : if the gold is but juft brought into fuiion it proves always brittle, a pretty confiderable increafe of the fire beyond this point being requifite for giving it full malleability, or for procuring a perfect folution, and an uniform mixture and cohefion of its parts : and when this neceffary fluidity has been obtained, the pouring of the metal into a cold mould will render it as brittle as if the heat had been infufficient at firfl. It is probable that the cafe is the fame in all the other metals, though in no one, perhaps, fo eminently as in gold: and we may hence account
for
[6i7] /or the brittlenefs, which gold, after fufion, is frequently found to have contracted, and which has commonly been afcribed to other caufes. I have already taken notice, (page 69) that the general opinion among the chemical writers, of gold being made brittle by a piece of charcoal falling on it in fufion, appeared to be a miftake, and I have fince found the fame observation made by Mr. SchefYer, in an excellent paper on the parting of metals printed in the Swedifh tranfactions : he fays that in the royal mint of Stockholm, the gold is always covered with charcoal in melting, and yet retains the full malleability which it had before.
Page 82. Fufibility of mixtures of gold and copper. It has been affirmed, that though mixtures of gold and copper melt eafier than gold itfelf, infomuch as to ferve as a folder for it ; yet this does not proceed from any in- creafe of fufibility occafioned by the mixture of the two- metals with one another,, but merely from copper beino- more fufible than gold, lb that the more copper the mix- ture contains, the eafier it ought to melt. Admitting however that copper does melt eafier than gold, which by no means appears to be the cafe, an increafe of fufibi- lity would ftill follow; for a mixture of gold and copper is a folder for fine copper as well as for fine gold.
Page 89. Calcination, &c. of tin ivitb gold. The experiment in which a mixture of gold and tin, calcined to an alh grey colour, is faid to have melted with eafe into a yellow glafs with a regulus at the bottom, I have now tried, but without obferving any appearance of vitrification — 1200 afTay weights of fine gold, and 1800
of
[ 6i8 ] of pure tin, being melted together, the mixt was of a white colour without the leaft yellownefs, rough on the furface, tolerably bright underneath, eafy to break, of a broad leafy texture like the bell: regulus of antimony, in weight 2981. Beaten into powder and calcined under a muffle, with a moderate heat, for five hours, it appeared of a light grey colour, and Weighed 3283; fo that it had gained in the calcination above a tenth part of the weight of the mixt, or between a fifth and a fixth part of that of the tin. The calx was put into an affay crucible, which was inclofedin a larger, and urged with a ftrong fire for two hours: the cover of the outer crucible, made of Sturbridge clay, re- mained found j that of the inner one, made of clay and chalk, melted entirely, and lined the infide of the crucible with an opake dark-coloured glafs. The powder, at the bottom, continued unmelted, and to the naked eye looked nearly of the fame appearance as at firft, but on viewing it with a magnifying glafs, feveral diftindt particles of gold were obferved in it. A little of the powder was mixed with about ten times its quantity of powdered flint glafs, and expofed to a ftrong fire in a wind furnace for feveral hours: the glafs proved nearly tranfparent, uncoloured, a little cloudy, with grains of gold at the bottom.
Page 114. Gold with fal microcofmicus . Mr. Pott, in a curious differtation on this remarkable fait of urine, gives fome experiments which feem to fhew that this fait has little or no action on gold. Ground with gold leaf, and then melted with a blow pipe on a coal, it forms a pearl-like mafs, which in the air becomes a tranf- parent flime or gelly : this melts again into the fame ap- pearance as before, and on continuing the fufion, the gold feparates and riles to the furface in form of a maffive leaf,
the
[ 6i9 ]
the fait remaining whitifh. When the fait was melted in a crucible with equal its weight, or a third of its weight, of gold or aurum fulminans, it received no purple or rofe colour, and did not appear to take up any part of the gold. Ground with gold leaf, and expofed to the focus of a burn- ing-glafs about a foot in diameter, it fmoked, frothed, and flowed long, but at length the gold rofe up to the furface, leaving the fait clear. One part of the purple calx of gold, precipitated from aqua regia by tin, being mixed with ten parts of the fait, and melted in a crucible with a ftrong fire; mod of the fait rofe over the crucible, leaving a brownifh glafs, and the gold revived into grains. One part of a calx of gold made with quickfilver, and ten of the fait, melted in a ftrong fire, gave likewife a yellow- brown glafs, and the gold was found revived. One part of gold, two parts of fal ammoniac, and eight parts of the microcofmic fait, being melted together, the fait run all through the crucible, the gold remaining in grains.
Solution of gold in aqua regis is precipi taped by folution of the microcofmic fait, provided the aqua regis is fully faturated with the gold, and the microcofmic folution added in fumcient quantity. If the mixture of the two folutions be poured upon compolitions for glafs, the whole boiled down to drynefs, then well ground together, and brought into fufion, the gold either wholly disappears, or hardly a footftep of it is to be feen. A compolition of three parts of powdered flint, two parts of faltpetre, and one part of calcined borax, being treated in this manner with the mixt folutions; a blue fandiver, dmeft like tur- cois, was found on the top of the melted matter, and a pure blue glafs underneath. With regard to the blue co- lour, which may feem pretty extraordinary, the author obferves that fome variation may happen from the degree of fire, and that the common ruby glafs, when in giving
L 1 1 1 it
[ 620 ]
it colour by the flame of wood it is kept too long in iirong heat, changes to an amethyft blue.
Page 114. Gold plates for enamelling. That a certain mixture of alloy is neceffary in the gold plates defigned for being enamelled on, I have related on the authority of a writer in the French Encyclopediei who fays the gold muft be of the finenefs of twenty-two- carats at moft, that if finer it will not have ftrcngth enough, and that if coarfer it will melt. I am told bv an experienced artift that this is a miftake; that ducat gold is generally ufed, whofe finenefs appears to be from 23 * to 23! carats; and that the fineflgold is for this ufe the beft, unlefs where fome parts of the gold are left bare and after- wards poliihed, as is often done in watch cafes, fnufF boxes, 8tc. for which purpofes a mixture of alloy is neceffary; that filver is preferred as an alloy to copper, the latter dil- pofing the plates to tarnifh and turn green ; and that the plates are ftrengihened, by covering them on the back fides with enamel. Thus much is certain, that the finer the gold, the more foft and flexible ; and the coarfer, the more fufible it proves.
Page 121, 123. cTouchftone — not of the marble kind.
Mr. Pott al fo has taken notice, that the touchftone is not of the marble kind; and that black marbles, how well foever they may anfwer for receiving a coloured ftroke from metals, are unfit for the ufe of touchftones, on ac- count of their being difiblved by aquafortis. He makes the touchftone a clayey flate, partaking of iron; and finds, that in a ftrong fire, like many other ferrugineous clayey minerals, it melts perfectly into a blackifh brown flag, and
that.
[621 ]
that a fmall quantity of it, mixed with vitreous com- pofitions, gives them a notable green tinge. He oblerves that the imperfe&ion of flints, &c. for the ufe of touch-, ftones, confifts in their hardnefs, which occafions them to give too great brightnefs to the metalline ftroke, fo that its colour, and confequently the proportion of alloy, cannot be ^xa&ly judged of.
Page 1 72, &c. Coffins s precipitate — Ruby glafs. Cassius does not appear, from his treatife de auro, to have been the difcoverer either of the precipitation by tin, or of the tinging of glafs by the precipitate. He defcribes the preparation of the precipitate, and flightly mentions its ufe in this intention ; but gives no account of the manner of employing it, nor any practical hint in regard to the operation, except that in fpeaking of the fmallnefs of the parts of gold, he fays that one dram of gold duly prepared will tinge ten pounds of glafs.
A procefs for making the ruby glafs has been communi- cated to me by an artift, who fays he was affured it came from Kunkel, and that he had found it a good one for enamelled colour, but had never tried it for glafs. The gold is directed to be diffolved in a mixture of one part of fpirit of fait and three of aquafortis; and the tin, in a mixture of one part of the former of thefe acids with two of the latter. The folution of gold being properly diluted with water, the due proportion of which is to be found by trials made on a fmall quantity as mentioned in page 176, the folution of tin is added, and the mixture fuffered to (land till the purple matter has fettled to the bottom. The colourlefs liquor is then poured off; and the purple fedi- ■ment, while moift and not very thick, is thoroughly mixed with powdered flint or fand: this mixture is well ground
L 1 1 1 2 with
[ 622 ]
with powdered nitre, tartar, borax, and arfenic, and the compound melted with a fuitable fire. The proportions of the ingredients are, 2560 parts of fand, 384 of nitre> 240 of tartar, 240 of borax, 28 of arfenic, 5 of tin, and 5 of gold.
I have not yet had an opportunity of trying this procef?, but am convinced, that the mixing of the precipitate with the land, &c. in. a moid flate is a very material circum- ftance, if not the principal one upon which the fuccels of the operation depends. Perhaps the mofr. certain way v/ould be, not to wait for any precipitation at all, but to moiften the powders with the purple liquor, grinding them well together, in a moderate warmth, till dry, and if ne- ceflary, repeating the humectation . Mr. Potts experi- ments with the microcofmic fait, mentioned in page 619, confirm the utility of this method of mixture.
Page 185. Quantity of gold collected in rivers. In a paper drawn up by Mr.Guettard, from the obfer- vations of Mr. Pailhes, and publiflied in the volume of the French memoirs for the year 1761, che gold found in rivers is reckoned an object of more importance than it has been ufually reprefented. It is faid that the mint of To- loufe received commonly every year two hundred marcs, or one hundred pounds weight, of gold collected from the Ariege, Garonne, and Salat j and that iince the year 1750, twelve pounds have been carried into the bureau of Fa- rmers, though this bureau comprehends at moil an extent of only two leagues round, and though the whole of the gold is not fent thither, ilrangers and hawkers buying it up every day.
Page
[ 6*3 ]
Page 1 86. Source of the gold found it rivers. . It has been generally thought that the particles of gold, found among the fands of rivers, have been torn off by tl*e violence of the ftream, in paffing over fome rich beds cr veins. The oblervations of Mr. Pailhes, in the memoir before mentioned, feem to prove, that the gold is not con- fined to any particular fpot, but diffeminated, though very Sparingly, through all the adjacent earths ; and that the particles found in the rivers proceed from part of the banks warned down by floods and rains, the lighter earth being carried away by the current, while the gold particles, with the ponderous black fands and flints, fettle to the bottom. The author relates, that thofe who employ themfelves in collecting the gold, fometimes anticipate the effect of the floods, by privately cutting down or undermining the banks, that the gold particles may be feparated, which occafions frequent law-fuits between them and the proprietors of the grounds : That in the town of Pamiers, fituated on one of the celebrated auriferous rivers, Ariege, on digging for wells or foundations of buildings, the earth thrown up is always found to contain particles of gold : That he has dis- covered abundance of auriferous tracts in other parts of the territory of Foix, infomuch that he imagines it would even be more difficult to procure water for the warning than to find the gold : and that belldes the gold met with in detached particles, the flints that accompany them con- tain alfo gold, which might be feparated to advantage by ftamping and warning. A quantity of thefe flints was fent by Mr. Pailhes to the academy, but in the aflays made of them they appeared to be merely ferrugineous, yielding aear half their weight of iron, without any mark, of gold.
Page-
[ 624]
Page 221. Gold coloured metal. The celebrated Mr. Pott, in a German letter to Von Jufti printed in 1760, affirms that tombac, or a gold coloured metal, may be made from a mixture of copper and tin ; and in a further difcuffion of Von Juflis objecti- ons, printed in 1762, he gives the particular compoiition of this metal. " Take one half-ounce of tin afhes, and four half-ounces of copper : melt them well together, in a clofc luted crucible, with a ftrong fire. Or take one half- ounce of the pureft tin cut in pieces, and fixteen half- ounces of pure copper beaten into thin plates : lay the tin between the copper plates, lute the crucible clofe, and melt with a ftrong fire."
Page 224. Gold coloured varnifh.
The compoiition of a gold coloured varnifh, ufed by the Englifh artifts for brafs and filver, was communicated to fome of the French academicians, in 1720 by Mr.Scar^- let, and in 1738 by Mr. Graham, and has lately been pub- lished in the volume of the French memoirs for 1761- Though I do not apprehend that this varnifh is anywife fuperiour to that defcribed in the page above referred to, I fhall here infert it for the fatisfaclioh of the reader. " Take two ounces of gum lac, two ounces of yellow am- ber, forty grains of dragons blood in tears, half a dram of faffron, and forty ounces of good fpirit of wine : infufe and digeft in the ufual manner, and then {train through a Jinen cloth. The piece to be varnifhed muft be heated before the liquid is applied : it receives from the varnifh a gold colour, little different from that of the mercurial gilding, and may be cleaned, when fullied, with warm wa- iter." It may be prefumed that the amber is of no great
ufe
[625 J ufe in the competition, this concrete being very Sparingly diffolved by the fpirit.
As the fpirit for varnifhes ought to be freed as much as poSfible from its phlegm or watery part, and as this is mod conveniently effected by means of fixt alcaline falts, I ac- cordingly directed the fpirit to be Shaken with fo much alcali as mould be fufficient to imbibe the phlegm. In Meyers very ingenious German treatife on quicklime, pub- lished in 1764, it is obferved, that fpirit rectified with thefe falts is unfit for varnifhes, particularly for fuch as are to be applied on gilt works, the fpirit taking up a part or the fait, which darkens the colour, and prevents the fpeedy drying of the varnifh. This obfervation feems to relate to fpirits that have been dephlegmated, not fimply by Suffer- ing them to Stand Some hours on the alcali in the cold, and occaiionally making the veffel, but by the application of a considerable heat ; in which laSb cafe the fpirit is known •to take up a Small portion of the alcaline fait, but that it diffolves any in the firSl is not fo clear. If, however, the fpirit Should in either way have received an alcaline im- pregnation, it may eaSily be purified by means of a little •alum well dried and powdered, the alcali being abfprbcd by the acid of the alum, -and forming therewith a com- pound not combinable with vinous fpirits. Some perfons, when a perfectly pure fpirit is required, firfl dephlegmate •it with alcaline falts, and afterwards purify it from fuch part of the alcali as remains in it, by a freSh distillation from a quantity of alum : perhaps fimple infuiion and agi- tation with the alum would be as effectual, at leafl for the purpofe of making varnifhes, as the more troublefome pro- cefs of distillation.
Gold
[ 626 ]
Gold coloured Glafs, with metallic fubjlances. Precipitates of filver, baked on glafs, ftain it yellow, and likewife give a yellow colour on being mixed and melted with forty or fifty times their weight of vitreous compositions : the precipitate from aquafortis by fixt al- cali feems to anfwer beft. I have likewife obtained yel- low glaffes with certain preparations of iron, particularly with Pruffian blue. But neither with filver nor with iron does the colour fucceed conftantly, or approach to the high yellow of gold.
The neareft imitations I have obtained of the colour of gold in glafs, were produced with antimony and lead. A quantity of crude antimony in fine powder was calcined by a little at a time in a flat iron pan, with care to prevent as much as poffible its running into lumps, by ufing a very gradual fire, and keeping it conftantly ftirring, till at length, when brought to a full red heat, it neither foftened nor emitted any fumes. The afh coloured calx, weighing little more than half of the crude antimony, was put into a crucible, and urged with a ftrong fire in a blaft furnace: it melted into a glafs, dark coloured and opake in pieces of any tonfiderable thicknefs, and of a tranfparent yellow when drawn out thin.
Some of this glafs, reduced to powder, was mixed and melted with four times, three times, and twice its weight of powdered flint glafs : the glafs refulting from the firft mixture was of a tranfparent pale yellow, from the fecond deeper, and from the third of a pretty deep yellow, with- out any mixture of greenim or brown. Equal parts of the glafs of antimony, of flint calcined and powdered, and of minium, formed a glafs of a high yellow, and with two parts of glafs of antimony, two of minium, and three of powdered flint, the colour approached ftill more to that of
gold.
[ 6V ] gold. All thefe compofitions were bright and transpa- rent, without any fcum on the furface or regulus at the bottom. The laft exhibited a multitude of fmall fparkles interfperfed through its whole fubftance, which gave it a beautiful appearance in the mafs, though in the lapidaries hands they were found to be imperfections, ariiing from air bubbles. It is pretty remarkable, that in feveral repe- titions of this experiment, in a glafshoufe furnace as well as in my own elaboratory, the product was always full of thefe brilliant fpecks.
Glafs of lead and glafs of antimony make likewife a gold coloured glazing for porcelain and earthen ware. The nneft gold glazing is faid to be made with an addition of filver. A glafs of lead is prepared by melting minium or litharge with a third or a fourth part of its weight of pow- dered flint. This yellow glafs, reduced into fine po\v»der, is either fprinkled on the porcelain made red hot, or mixed with beer or other glutinous liquids to a due confiftence, and applied with a pencil : the ware is then placed in die furnace, under a muffle, till the glafs begins to melt, which is known by its gliftening ; after which, while warm from the fire, it is moiftened with a diluted folution of filver, and baked again. Or the powdered glafs of lead is moift- ened with the filver folution, then melted, and the glazing of the ware finifhed in one procefs, by applying on it this compound glafs. After the baking, the glazed veffels, while ftill red hot, are held over the fmoke of burning ftraw, &c.
Gold coloured glajfes without metallic fubjlances.
There are fundry earthy bodies, as chalk and gypfum,
which make a yellow colour in glafs, efpecially when the
vitrification is procured with borax or alcaline falts.
Thefe glafles however have generally more or lefs of a
M m m m green
[ 628 J
green tinge, and never, fo far as I have obferved, a gold yellow.
Neri directs, for a gold yellow colour, one part of red tartar, and the fame quantity of manganefe, to be mixed with a hundred parts of fritt or the compofition for glafs.
Kunkel, in his experimental remarks on Neri, fays that this procefs gave him more trouble than any other in the book ; that the proportions are quite faulty; that the quan- tity of manganefe is too much for the tartar, and the tar- tar too little for the fritt ; that one part, or one and a quar- ter of manganefe, is fufficient for a hundred of fritt, but that fix parts of tartar are hardly enough, efpecially unlefs the tartar is of a dark red colour almoft blackifh ; and that: he found it expedient to add to the tartar about a fourth of its weight of powdered charcoal. He takes notice that the compofition fwells up greatly in melting; that if the glafs be much ftirred with the iron, as is cuftomary for other kinds of glafs, it will rife up fo as to run over the pot, though at firft no more than half full ; and that there- fore Itr muft be left unftirred, and worked as it ftands in fu- fion. Speaking afterwards of a yellow enamel tinged with the fame materials, he adds, that the colour muft be carefully warehed, too long a continuance of fire deftroy-
mg it.
My ingenious friend Mr. Samuel More, in repeating ^nd varying this procefs with a view to render the colour more perfect, found that the manganefe is entirely inefi'en- tial to the gold colour, and that the tartar is no otherwife of ufe, than in virtue of the coaly matter to which it is in part reduced by the fire. Different kinds of coals, as that of tartar, common charcoal, foot, dried blood, Sec. on be- ing melted with colourlefs fritts or glaffes, gave always, pale yellow, dark yellow, reddifh, browniih, or blackifli colours, according- as the inflammable matter was in fmaller
or.
[ 629 1 er larger proportion ; the phlogiflon, or inflammable part of the coal, feeming to be the direct tinging fubflance. When the phlogiilic matter was thus diffufed through glafs, he did not find it to be affected by continued flrong fire, any more than charcoal is when excluded from the air : though fome pots of the coloured glafs flood for a fortnight in the glafshoufe furnace, they flill retained their colour; nor did the mofl intenfe fire of a lamp alter it in the leaf!:. How fixt the colour when once united with the glafs is, we may judge from the indeflructibility, by very flrong fire in open veifels, even of the fuperficial browns and blacks which charcoal and foot communicate to glafs in its converfion into porcelain : fee page 242.
Mr. Pott, in his neue wkhtige pbyjicalifch-chymifche ma~ terien, &c. printed in 1762, cbferves alio that common coals give a yellow colour to glafs ; and that even by ce- mentation, provided the heat is not furficient to change it towards porcelain, they flain it of the fame colour. He fays that different coaly matters differ in their tinging power ; that caput mortuum of foot, and lamp black, an- fwer better than common charcoal, and that he has known fome perfons employ the coal of indigo : That the fpark- ling coal, which remains in the retort after the rectifica- tion of the thick empyreumatic animal oils, is one of the moft active of thefe kinds of preparations, being as it were the heavier part of the inflammable fubflance of the oil, and very rich in colour : That this preparation, powdered and then again burnt a little in a clofe vefTel, is excellent for tinging glafs, and gives yellow, brown, reddifh or black- ifh colours according to its quantity: That the fritt mufl not be very hard of fufion, for if it is, the flrong fire will deflroy the colouring fubflance before the glafs melts j and that he has found the following compofitions to be nearly the beft, viz, fand 2 parts, alcali 3 parts ; or fand 2, al-
M m m m 2 cali
[630 J
cab 3, calcined borax 1 ; or fand 2, alcali 2, calcined bo- rax 1 : That though faltpetre is hardly, or very fpa- ringly, ufed for yellow glafles, as it too much volatilizes the colouring fubftance ; yet here for the molt part a cer- tain proportion of it, which proportion will be eafily found by trial, is very neceflury, for without it the concentrated colouring matter is apt to make the glafs too dark, and even of an opake pitchy blacknefs.
That there is any. material diverfity in the effects of dif- ferent coals, may be juftly queftioned ; for Mr. More ob- tained, with common charcoal,, the fame colours, as with the coals which are here fuppofed to be of greater excel- lence and adtivity : it is probable, that the only difference- confifts in- their containing different quantities of the in- flammable matter, fo that a little more fhall be required of one kind than of another, for producing the fame de- gree of colour in the glafs. Nor does the foftnefs or full- bility of the fritt appear to be anywife neceffary, for my> friend informs me that he has tinged, with coal, glafles. which were fo hard of fufion, that the glafsmen could not work them, whereas the above compolitions are all rather too fcft to be ferviceable in the large way.
Page 2-3,3. Convcrfion of green glafs Into porcelain. Some have complained, that on repeating thefe experi- ments, the change did not fucceed; and further enquiry; has fliewn, that fome forts of green glafs are unfit for this, operation. Green glafs has been chiefly made, of vegetable afhes and fand, brought into fufion together by a ftrong fire: with this kind, which is the green glafs common about London, the experiments fucceed in the manner de- fcribed. In fome parts of this kingdom, inftead of vege-. taUe afhes, the vitrification of the fand has been procured ,
princi-
principally by means of another ingredient, the flags of the iron furnaces; and glafs of this compofition is found not to be convertible into porcelain. The failure of this kind of glafs ferves to confirm the general refults drawn from the former experiments ; that earthy and metallic glafles made without faline matter, are not fufceptible of this change ; and that the change depends on the faline fub- ftance contained in the vegetable afhes.
Page 314. Machines for blowing air by a fall of water.
I have received an account, from a worthy corre- fpondent in Swifferland, of a machine which he has con- (tructed for a fmelting furnace according to the foregoing directions : he fays, it has fo much the advantage of all other kinds of bellows, that it deferves to be introduced univerfally wherever the fituation of the place will permit. The only inconvenience he finals in it is, that the cullender and gratings are liable to be ftopt up by leaves, Sec. With regard to the cullender, the obftruclion msy be obviated by enlarging the holes. The gratings ought to be of r: large furface : the wire grating in the ciftern on the top may be a cylinder nearly as large as the ciftern will re- ceive, for if it is no more than fufficient to cover the mouth of the pipe, it will doubtlefs be foon choaked up : when fo much of the cylinder becomes ftopt, that the water has no longer a free paflage through, it may be lifted up and cleaned,, another being placed in the room of" it, without the. trouble of turning off the water, or inter- rupting the going of the machine. The gratings here can be liable to no other inconveniences, than thofe which are common in other water machines, mills, aqua;du<£ts, Sec.
Some further improvements have occurred in the con- ftrudtion of thefe machines, by which they may be made
effectual
effectual in cafes where the quantity or fall of water would ctherwife be inefficient.
Of conJlrutli?ig blowing machines with falls of water of
great height.
Where the height of the fall is great, the quantity of water is ufually fmall; and in all the ways of application that have hitherto been contrived, the height will by no means make amends for the deficiency in quantity.
In the common conftrudtion of thefe machines, where the upper pipe or funnel is no more than three, four, or five feet high ; though the fall mould be fuch as to admit of the lower pipe being thirty or forty feet or more, it does not appear that any material advantage could refult from fuch a height. For, as the air is admitted into the water only at the top of this long pipe, it cannot, I think, be fup- pofed, that the quantity admitted will be the greater for the length of the patfage under the place of its admifTion. Water indeed has been found by Mariotte to run fafter, through an upright long pipe, than through a fhort one : a quantity of water which was forty-five feconds in running through a pipe three feet long, was difcharged in thirty- feven feconds, or near a fixth part lefs time, through a pipe of the fame bore and a double length; lb that as more 4vater paries fucceifively through the long pipe than through a fhort one, in equal times, more air alio mufr. be carried down by it. But in the cafe which we are here confidering, no benefit can be expected on this principle; for as the fupply of water is fuppofed to b.e limited, the bore of the pipe mufl necefTarily be made lefs, in pro- portion to the increafe which its length may produce in the velocity. If the lower pipe is of fuch height, that the watery column it contains may fufficiently refift the force of the comprefi'ed air in the air-veflel, it fliould feem that
any
.[633 ] any further addition to its height could be of no manner of ufe.
We have feen, in the foregoing part of this effay, that it would be more advifable, in fuch cafes, to fliorten the lower pipe, and to lengthen the upper one: by this means the water, acquiring greater velocity at the place of its difcharge from the upper pipe into the lower, is enabled to divide or fpread more, and thus to receive more air into its interftices. T.he advantage, thus obtained, does not how- ever increafe in fo great a proportion as the height does. From an experiment related in page 310 it appears, that by increafing the height four-fold, the effect was not in- creafed three-fold; and this even in fmall heights, where the effect is much more influenced by a variation of the height than in great ones.
The obfervations already mentioned point out means of availing ourfelves more advantageouily of high falls; fo as to produce always with certainty, from a fall of a double or treble height, a double or treble effect if the quantity of water be the fame; or an equal effect, with one half or one third the quantity of water.
Experiments have convinced me, that a fall of fourteen feet is more than fufficient for compreffing the air to fuch a degree, as to be able to fuftain the gage at the height of four feet; or to raife, on an opening of a fquare inch, a weight of about a pound and three-quarters averdupois, or above two pounds troy; a compreffure, which is appre- hended to be as great as there will in general be occafion for. Where we have plenty of water, with fuch a fall, we can drive in air, with this force, in any quantity: for if one machine, with a certain portion of the ilream, pro- duces a continued blaft of this flrength through a pipe of a certain bore, as an inch or three-quarters of an inch; it is evident,- that the quantity of air may be doubled, trebled, ccc
At
[ 634 ] at pleafure, without diminishing the compreffure or force of the blaft, by adding another and another machine, till all the ftream is employed. It is plain, in like manner, that the fame advantage may be received from high falls, by placing one machine over another ; that after the wa- ter has performed its office in falling through one machine, it is ftill capable of exerting the fame aftion in another and another machine, fo long as equal fpaces remain for it to fall through ; fo that the total effeft mud be the fame, as if, a quantity of water, fufficient for working all the ma- chines, came at firft in one ftream.
A fall thus divided into two machines is reprefented in the middle of the annexed plate. In the lower machine, v/hofe air-veffel is funk to a confiderable depth in a pit made in the ground, the water is forced up in the pit, on the outiide of the veffel, four feet higher than the furface of the water within the veffel, or of the ftone on which the water daffies, called by the workmen the dam-board, (fee page 287). The air-veffel of the upper machine having an additional part at one fide, which performs the fame office as the pit, the water is in like manner forced up to the fame height in this outer part ; which outer veffel ferving as a refervoir for the machine under it, the water begins to aft in this lower machine four feet higher up than the daffi-board of the firft. Whatever number of machines the fall will admit of, the cafe is the fame in them all : though in each of them the water falls eighteen feet, yet as it is preffed up again four feet for the fucceed- ing machine, one machine takes up but fourteen feet of the real fall.
- The outer veffel, and its communication with the air- veffel, may be conveniently formed by an upright partition in the air-veffel itfelf, not reaching quite to the bottom. The outer divilion may be open at top, and needs not be
fo
fo high as the clofe air-veffel ; it is Sufficient if it reach? ; a little more than four feet above the level of the dash- board, the water, which it is defigned to receive, not rifing higher than this. In other xefpects, the ftructure of thefe machines agrees entirely with that of the lingle ones already defcribed. It muft be obferved only that the cullenders of the lower machines fhould be, as nearly as pofhble, of the fame dimenfions with thofe of the upper ones. For if they are of fmaller bores, they will not admit of all the water which paffes through the upper ones, fo that part of it muft run to wafte : if they are. larger, the water will pafs off too faft, without producing its due effect. The regulators, defcribed in page 312, are here particularly ufeful, affording ready means of increaiing or diminilhing the apertures occafionally while the machines are at work.
Of blowing machines with low falls of water. The dimenfions hitherto given are fuch as appear the moft advantageous. Much lower falls, however, than thofe which the foregoing machines are calculated for, as ten, eight, or perhaps feven feet, may be made to afford a ftrong blaft. To produce fuch a compreffure of the air in the air-veffel, as to raife the gage four feet, a fall of about fix feet is neceffary for the lower pipe. If the upper pipe is only about a foot and a half or two feet, the water, when divided by means of the cullender, will carry down a certain quantity of air? and though the quantity, from an equal ftream of water, will not be fo great as when the fall is higher, yet, as there are in many parts of the king- dom, large bodies of water running with fuch a defcent, the deficiency may be compenfatcd, as already taken notice, by enlarging or multiplying the machines.
N n n n For
[ 636 J
For many pur poles ftill lefs falls will fufRce. The fmiths bellows, as we have formerly feen, raifes the gage only about fourteen inches; and fuch a compreffure, it is prefumed, may be gafned from a fall of five feet or lefs. Small falls may be applied alio to another purpofe, of no little importance, the ventilation of mines and coal-pits, or the driving in of frelh air, in the room of that, which the mineral vapours have rendered unwholefome or pernicious.
In all thefe machines it mull be obferved, that the* height of the column of water, falling through the pipe, determines, not the actual force of the Mail, but the greateft force which can be given it in that machine; that the height of the gage is always the mealure of the actual force; that this force depends on the width of the pipe through which the air is difcharged from the air-velTel, and may be diminilhed, or increafed in any degree up to the greatelt that the column of water can refill, by widen- ing or narrowing the aperture of the pipe; that different machines will give blaifs of equal force through pipes of greater or lefs width, according to the greater or lefs quan- tity of air which the water carries down with it; and that therefore the iize of the blaft-pipe mull be adjuited by trial for each particular machine.
The dillance of the daih-board under the pipe may likewife admit of lbme variation, and require to be re- gulated according to the fize of the pipe. In fome of the common machines, this diilance is three or four feet or more; but lb large a fpace is apparently a difadvantage; for fo much of it, as is more than fufficient for the free pafling off of the water, is entirely ufelefs, being, in effect, fo much taken off from the height of the fall. The diftance of fix inches, laid down in the foregoing machines, is defigned for a circular pipe of twelve inches diameter; in which cafe, the area, by which the water is difcharged
all
[637] all round, is jufl double to the area of the pipe, and confe^- quently more than large enough for letting the water oir without impediment.
Explanation of the Plate.
The two blowing machines, reprefented on the plate, are both drawn to one fcale, that the eye may judge more readily of their comparative heights and dimenfions. The fupports of the refervoirs, &cc. are not exprefTed, that the ellential parts may be the more diftinct.
The machine on the left hand is that of Dauphiny, de- fcribed in page 274, with a fall of about thirty feet. The other is a natural fall of twenty-eight feet, formed into two artificial ones of eighteen feet each; fee page 3x0 and 634. This double machine, though fomewhat lower than the other, may be prefumed to have twice its effect, in virtue of the divifion; beiides the advantage of the more free ad- mirlion of air, and the fpreading of the dream through.a pipe of a much larger bore, by which it is enabled to carry down in its interflices a much greater quantity of air. The dotted lines, in the upper refervoir, reprefent a cy- lindrical grating of iron wire, to keep back weeds, &:c. Thedivifion of the air-velfel,and theccurfeof the water from the upper machine to the lower, are jpparentfrom the figure.
On the right hand is a perfpective view of the cullender, fcrewed to the upper pipe, drawn to a larger fcale, to {hew the difpoiition of the holes. The holes may be made wider than formerly propoled, as an inch each iide, to pre- vent any danger of their being choaked up.
I'.-ge 321. Black Diamond. I have been favoured with a fight of this Hone, and am aiTured it is a true diamond. At a diflance, it looks uni-
N n n n 2 formly
[ 633 ] h rinly black j but on clofer examination, it appears in iome parts tranfparent, and in others charged with foul- nefs, on which the black hue depends.
Page 359. Ind'un ink from lamp-black and glue. Since the experiments on the compolition of Indian ink were made, I have met with an account in Du Haldes hiftory of China, which feems to confirm them. He gives three receipts for the preparation of this commodity, two taken from Chinefe books, and the third com- municated by a native to one of the milfionaries. The colouring material in all of them is lamp-black, to which is added, in one, a quantity of horfe-chethut burnt till the fmoke ceafes : he does not determine whether the wood or the fruit of the horfe-chelhut tree is meant, but adds, from the Chinefe author, that if ufed in over-proportion, it inclines the black colour to a violet. The congluti- nating matter, in one of the prescriptions, is a thin lize of neats leather; in another, a folution of gum tragacanth; and in the other, a mixture of fize with a decoction of certain vegetables to us unknown. The firft, viz. lamp- black and lize, which is that from verbal communication, is the very compofition which our experiments pointed- out. As to gum tragacanth, it certainly is not the con- glutinating ingredient in any of the famples of Indian ink that have come under my examination, the vegetable gum not putrefying with water as the Indian ink does: if gum was really made ufe of, gum tragacanth ihould feem the moli unfit for the purpofe, on account of its difficult and imperfect folution in water. The vegetable decoctions or infuiions I cannot apprehend to be of any manner of ufe where fize is employed, unlefs it fhould be to give a fcent to the compofition, in which intention, muik and other
perfumes
[639 ] perfumes are faid to be frequently mixed with it. The author obferves that the Chinefe have inks of different goodnefs and price; that the moft effeniial difference pro- ceeds from the quality of the lamp-black ; and that the beft lamp-black is the foot of oil,, which is burnt in lamps, in apartments fitted up for this purpofe. The Chinefe, according to his account, imagine the differences in the foot of different oils, ccc. to be much greater than the experiments related in page 342 give room to believe they arc
Page 369. Black varnifh for metals. The workmen frequently employ for this purpofe, as I am informed, a mixture of lamp-black with the feum- mings, &c. of different oil paints: the mixture is applied with a pencil, and the piece afterwards baked in an oven, with a heat fomewhat greater than is ufed for the papier mache. Naples yellow, a fuperfluous ingredient in the black varnifh, is the bafis of the dark brown which we fee on fome iron fnuff-boxes, this pigment changing to a brown in baking with the varnifh.
Page 596. Separation of the alcali cffeafalt.
1 . Purification offeafalt.
Pure marine fait is a combination of the mineral alca- line fait with marine acid : but all the common forts of this fait have a mixture of one or more faline matters of a different compofition, their bails, inftead of an alcaline fait, being an earth; which earth is generally the fame with that called magnefia, though fometimes, perhaps, it may be of the calcareous kind.
1. Thefe
[ 64o ] i . Thefe falts with an earthy balls are difcovered, by dilfolving the marine fait in water, and dropping in a fo- lution of any alcaline fait. The earth, of whatever kind it be, precipitates ; the acid, which held it diffolved, quitting it, to unite with the fuperadded alcali; fo that by continuing to drop in more of the alcaline folution, till it ceafes to occafion any precipitation or cloudinels, we pro- duce in the liquor, inftead of the fait with an earthy bafis, a true neutral fait with an alcaline bafis.
2. In fome forts of marine fait, the acid united with the earth is the vitriolic. This may be known, by dropping, into a folution of the fait, a folution of chalk, or other calcareous earth, made in the nitrous, marine, or vegetable acids. The vitriolic acid quits the earth which it was be- fore combined with, and joins itfelf to the calcareous earth, forming therewith a felenitic concrete, not dilToluble or exceeding fparingly, and which therefore fettles to the bottom in a powdery form; fo that by continuing to drop in a due quantity of the calcareous folution, all the vitriolic acid may be feparated with the calcareous earth, while the macrnelia, now combined with the acid in which the cal- careous earth was before diffolved, remains in the liquor along with the marine fait.
3. There is another method in which we can feparate the vitriolic acid, and this without communicating any fcreign impregnation lo the liquor. Add to the folution of the marine fait, fome ftrong lime-water : the vitriolic
i unites and precipitates with the lime; and the mag- Cfaos deprived of its acid folvent, precipitates alio. Though ibis limple procefs effectually purines the fait lr m the combination of vitriolic acid and magnelia (corn- lily called bitter fait, or bittern) it does not anfwer fo .1, for merely distinguishing that acid, as the foregoing method; lime-water producing a turbidnefs and precipi- tation in many liquors which contain no vitriolic acid.
[64i ]
4. In many kinds of marine f tit, the heterogeneous earth is united with the true marine acid : we may always judge that this is the cafe, when the method of trial No. 1 difcovers that the fait contains an earth, and when the cal- careous folution No. 2, by producing no cloudinefs, Shews that the acid is not the vitriolic. The combination of either magneSia or calcareous earth, with the marine acid, or with the nitrous acid if fuch an acid fhould ever exift in marine fait, I know of no other means of feparating, than decompounding it by alcalies as in No. 1, or careful crystallization.
The combination of earth with marine acid I have found to be by much the mofl frequent and mod: consider- able admixture in the common marine flits ufed among us at table. This compound liquefies ealily in the air, a well known imperfection in the common forts of marine fait; and on this difpofition to liquefy depends its being in great meafure Separable by cryftaUization. The bay falts, cryftallized by the Slow evaporation produced by the funs heat, have much lefs of this deliquiable fait, and hence are much lefs Subject to grow moiSt in the air, than thofe prepared by the hafty boiling down of the brine ; though they generally have a pretty large admixture of the bitter fait, which cryStallizes as perfectly, though not io foon, as the marine fait itfelf.
On this bitter fait probably depends a property of the common marine falts, which has given occaiion to fome mistakes in regard to their composition. When common Salt has been melted in the fire, it afterwards deliquiates very fpeedily in the air, though before it was of fuch a kind as to be little difpoSed to grow moiSt. This does not Seem to proceed from the fait being rendered alcaline, or lofing any of its acid, but from fuch a tranlpoSition of its acids as we find to happen when artificial mixtures of the
lame
[ 642 ] fame ingredients are treated in the fame manner : the vi- triolic acid of the bitter fait, loofened from its earth by the heat, unites with fo much as it can faturate of the alcali of the marine fait ; and the marine acid, difengaged by the other from this part of the alcali, unites with the magne- iia which the vitriolic acid has forfaken, forming there- with, inftead of the cryftallizable bitter fait, the very deli- quiable compound above mentioned. It has been found indeed, that common fait gives out a portion of marine acid, when folutions of it are haftily boiled down, or when the dry fait is expofed to ftrong fire : but the compound of earth and marine acid parts with fome of its acid in the lame circumftances, and Mr. Baume has fhewn, in his manuelde cbymie, that marine fait, purified from that com- pound, does not.
The purification of marine fait from its earth, by the addition of alcaline falts, No. i, how ufeful foever it may be to the faltboiler, muft never be had recourfe to in the prefent intention, unlefs we have an alcali exactly the fame with the marine alcali itfelf ; for by whatever means we can disjoin the marine alcali from its acid, we fhall disjoin alone with it this extraneous alcali. Nor indeed is fuch a purification any wife wanted here ; for in feparating the acid from the alcali, we feparate it from the earth alio, and the alcali is afterwards purified from this earth, along ■with the other earthy matter which it has contracted in the operation, by folution in water. For the two firft rroceffes of the following article, it is furficient if the fait i? well purified from vitriolic acid j and for the third, even that purification is unneceffary.
2 . Preparation of cubic nitre. The acid of common fait can neither be expelled from its alcali by fire, nor transferred from it, fo far as is known,
to
[ 643 1 to any other body. But though we cannot transfer the marine acid from the alcali; we can transfer the alcali, from the marine acid, to the nitrous acid ; and from this laft acid we can feparate the alcali pure. The combi- nation of this alcali with the nitrous acid is called, from the figure which it aflumes in crystallization, cubic nitre.
I. Cubic nitre may be prepared, by putting into a glafs retort lbme common fait, pure from vitriolic acid, tho- roughly dried over the fire, and reduced to powder ; fetting the retoft, on as much fand as will keep it fteady, in an iron pot placed in a proper furnace ; pouring in thrice the weight of the fait, of ftrong fmoking fpirit of nitre, with care to avoid the fumes ; immediately luting on a large receiver, with fome water in it to promote the condenfa- tion of the fumes ; and proceeding to diftillation, with a very gradual fire, increafed at lafl fo as to make the bot- tom of the retort red hot. The marine acid, with part of the nitrous, comes over into the receiver : the marine aU cali, combined with the reft of the nitrous acid, remains in the retort. The mafs of fait is to be diffolved and waihed out of the retort with diflilled water or pure rain water* the folution filtered, evaporated with a moderate heat till a pellicle begins to appear on the furface, and then fet in the cold : the fait moots into cubical or rather rhomboidal cryftals, generally cluttered together.
Mr. Marggraf, in a dhTertation on the beft method of feparating the alcaline fubftance of common fait, found that two parts of fmoking fpirit of nitre, of fuch ftrength as initantly to fire pure oil of cloves, were fufficient for one part of purified common fait ; but of the weaker ni- trous fpirit, called aquafortis, he recommends eight times the weight of the fait. He fays the cryftals obtained with the fmoking fpirit (for he does not feem to have actually
O o o o tried
[ 644] tried the weaker one) were pure cubic nitre, which defla- grated on a burning coal without crackling, and had not the leaft mixture of common fait. Some have reported, that though a pretty llrong fpirit of nitre was ufed in more ihan double the weight of the fait, the refiduum after the diftillation conlifted chiefly of marine fait unchanged, with only a fmall proportion of cubic nitre intermixed. On what caufe the failure depended, the few experiments I have made on thib head do not enable me to judge: per- haps it may be necelfary that the nitrous fpirit mould be very ftrong, for a concentrated acid may produce decom- pofitions, as well as diffolutions, which the fame acid, di- luted, is incapable of effecting.
II. Cubic nitre may be obtained alfo in the procefs of making filver into luna cornea, which is the moll effectual way of purifying iilver. Solution of common fait in water being dropt by degrees into a folutlon of filver made in aquafortis, fo long as any cloudinefs enfues, the marine acid precipitates with the filver, as the vitriolic did with chalk in No. 2 of the foresoin? article, and the re- maining liquor is a folution of cubic nitre, blended with the copper which the filver contained. How far this copper may be injurious in the intention for which cubic nitre is here wanted, has not been fully examined.
III. The ffrong affinity of the vitriolic acid to calca- reous earth affords a method of obtaining cubic nitre, more eligible than either of the foregoing. Spirit of fait is commonly prepared by diftillation with the vitriolic acid; and in this cafe,, what remains in the retort is a combination of that acid with the alcali of the .marine fait, This compound is common in the fhops, under the name of Glaubers fait or Jal mirabik. If a faturated folution of fal mirabile be made in water, and a folution of chalk in aquafortis added by degrees fo long as it occafions any
doudinefsj
[645 J cloudinefs; the vitriolic acid and the chalk precipitate to- gether, and the nitrous acid and mineral alcali remain in the liquor, which accordingly, on cry flail ization, yields a true cubic nitre. The folutions ought to be well fatu- rated, that the milkinefs, which grows fainter and fainter as we continue to add more of the calcareous folution, may be the better diftinguifhed; and after the cloudinefs feems to have entirely ceafed, a little more of this laft fo- lution may be dropt in, for a fmall excels in its quantity will be of no inconvenience, but a fmall deficiency, by leaving part of the fil mirabile undecompounded, will oc- calion the mineral alcali, for which this procefs is only preparatory, to be impure, as will appear in the following operation.
3 . Separation of the mineral alcali from cubic nitre. The marine alcali being by the above methods com- bined with the nitrous acid, the acid is to be feparated from it by deflagration with inflammable fubftances. Mix the cubic nitre with one fifth or one iixth of its weight of powdered charcoal, grinding them thoroughly together : the coal of animal fubftances is preferable to that of vege- tables, as the latter will leave, after burning, fome fmall portion of an alcaline fait, of a different nature from that which is here required. Throw the mixture, by a very little at a time, into a large crucible made juft red hot, covering the crucible, as fpeedily and as clofe as may be, after each injection, to prevent the matter from being dif- fipated by the ftrong deflagration which enfues. When the mixture has been all thrown in, and the detonation has ceafed, the fire may be augmented, and a pretty ftrong red heat kept up for half an hour or more, the crucible during this time being left uncovered. The nitrous acid being thus burnt out, there remains in the crucible a blu-
O o o o 2 ifh-greenifh,
1 646 1
iih-greenifh alcaline mafs, which is to be purified by folu- tion in diddled water. It diflblves more difficultly than the vegetable alcalies, and on duly evaporating the folution ihoots into fine white cryflals, which do not liquefy in the air. This laft property of the marine alcali tends to con- firm the obfervation already mentioned, that the deliquia- tion of marine fait after fufion does not proceed from a part of the alcali having been divefted of its acid.
If the marine fait, ufed for the preparation- of cubic ni- tre by the firft and fecond proceffes, contained any fait with an earthy bafis, or if the folution of chalk in the third way of preparation was employed in too great quantity, the crystallization of the cubic nitre will in great meafure fe- parate thofe deliquiable compounds ; and indeed, without' cryftallization, as the nitrous acid is diffipated or defiroyed in the fire, it will leave with the alcali only the earth, which will be feparated, as well as the allies of the coal, by the diflblution in water. If the cubic nitre contained any marine or vitriolic fait, the marine fait will continue after the deflagration unchanged, and the vitriolic fait will produce with the inflammable matter a fulphureous com- pound,
I'NDE X,
INDEX.
A.
ACID, marine,, contained in moll ^ll fpring waters — — — — 138 Means of dillinguilhing in liquidsi 38 Purification of liquids from it 1 3 3, 64 1 Gold difLlved in it — — — — 94 Piatina difTolved in it — — 472,558 Forms with earths a deliquiable fak 641
Acid, nitrous, Gold difTolved in it — 91 Forms with earths a deliquiable fait 646
Acirl,vitriolie, diftinguifhed in liquids 640
Purification of liquids from it — 640
Forms with earths a cryffallizable
fait 641
Aiol'ipile, defcribed — — 20
In fome circumltances animates fire 21 In others extinguishes fire 21
AZtherealffirit, Preparation of 143
Imbibes gold from aqua regii — 101
Does not imbibe piatina 4S6
Its ufe in afTaying gold 142
Agate, Staining of it black — — 457 Dillinguifhing of natural from artificial
ftains 439
Drawing of regular figures on it — 439
Producing vegetations on it like mocho
itone 440
.^•.influences folution and precipitation 93 Its power in changing or deltroying
colours 378-
Probably abforbed by metals in calcina- tion — 461
Air, different means of fupplying to
fmelting furnaces — — 267
See Bellows and Blowing machines
Experiments of its pa fling down through
pipes with filling water — — 291
General principles of its pacing down
— 296, 298
Gage for (hewing its degree of com- preffiire in blowing machines — 285 Quantity required by large fmelting-
furnaces 282,286
Quantity required for faiths forge 286 Different force of the blaft for different
fuels 236
Vapour from the atolipile whether ca- pable of fupplying its office for fur- naces — — 21
Means of renewing in mines, &c. 273 Its comparative gravity to water — 43 Apparent weights of bodies how influ- enced by variations of its gravity 43
Alc.ili, mineral, native — 596
Method of extracting from fea fait 639- Obtained from certain vegetables — 596
Cryltallized 646
Precipitates moll metals, like vegetable
alcalies 596
Does not precipitate piatina — 483, 596
Alchemy, hiflory of 1 99
lis principles 200
Its pretenfions inconfiflent with its
principles 201, 232
Deceptions in regard to the traiafmutabi-
Jity of bafe metals into goldg3,96,203 Frauds herein 202
Alloy of gold 1 114
How far ufeful 1 14, 1 15, 620
Its proportion in different coins — 1 16
Amalgamation of gold 7 e
Of piatina 508
Of iron 508
Piatina and gold feparated by 591
Mill for grind'ng amalgams — 57S, 194
Atticr)
I N D
Ambtr, method ofdiflblving in cxpreffed
oils 367
Mere effectual method of di(Tr,lving in
oils ^6S,6o6
Advantages ot" this folution — 369,61 5 Varnilhes 366, 614.
Amcardi'im, Staining of linen with its juice 328
Annitt;, Its preparation 224
Its ule in gold coloured varnilhes 225
Aatirrvny, Refining of gold by it — 156 Does not make gold completely pjrei 58
Platina treated with it j6o
Platir.a with its regulus 521, 559
Afuafbrlis, Preparation of 13-
Purification 138
Adjustment of its Strength for the part- ing affay 1 39,140
Recovered after parting i>2
Recovered from fjlution of gold in aqua regia 99
Aquz regia, different ways of preparing 96
Differences in the volatilizing power of
different aquae regia; 100
Archel, gives durable blackifh /lain to mar- ble 436
Ar-enU, with gold 87
with platina 5 ' S» S 59
Method of uniting wi.h difficultly fuii- bie bodies ; 39
Jijfi-yn.g of gold 127
Cupellati in 1:9
Pcrting with aquafortis 135
EuriheraQay., lo di covering platina 142
Quantity ailjwcd for an afT.y 127
' .crs Report of the rr.e c:s — 141
Ajlringnt'. matter of wgM iu'e5, la general properties 34;
Phenomena on mixrt.r^ with green vi- triol • — 346
Differences in the pher.o;r.eria produced by different aftringents — 3S2, +i3
A*ruirif,i!nii>ir.i, Preparation of 104, 105 Jtaexplolivc power compared wiilrtfiat
of gunpowder 106
Conjecture on the ciufe cf its flrong a'tion on the body which i'upporo it . 10.7
E X.
The gold recovered — 107, lo3
Aurum mujivum, Preparation of — — 222
B.
Balances, obfervations on — 543
Equility of the arms not neceffary for
their exa&nefs 543
Adjultment of the weights — — 544
Bariglia, of the fame nature with mineral alcali 596
Bai reliefs of gold, made on works of gold or filver — 80
Bttbs, chemical, their ufe — 3 \
S:rd bath g
■\Varer bath 9
Imperfections of fand and water — 3 t
Mercurial bath -i\
Compound metalline bath 32
Belhxcs, for large furnaces 268
for frtall furnaces — — — — 18 Wooden bellows 268
their inconveniences — 269
Water bellows — 269
See Biawrrsg -machines Quantity of air afforded by bellows of
iron furnaces — 282
by bellows of the iron finery 282 Quantity of air retained in them when
clofed 2S3
Made to deliver neariy all their air 283,
2S4-
Made to give an equV Waft 18
Made to ri;ove with lei's urce 20
Bi/mutb, me!:ed with platina 509
irked on a cupel — ry .
Agreement with lead — 515
B (ire. Preparation of 14.0
Black ieJifi receive heat from the fun more
freely than others — — 318
not from common fire — — — 31^
B :,;.knefs producedhy ab brption of 1 i^h 1 3 1 7
By lirong reflexion of light 318
By the concentration oi other colours
By the combination of other colours 420, 441, 442, 354
By the mutual aclion of bodies on mixture — — — — — — 344
By
I N D
By fire 334
By air and fun 329, 350, 438
Black, diluted, becomes grey 319
Means of judging of the fpecies oi the colour — 319, 320
Black natural fubfl antes 320
Minerals 320, 323
Vegetable juices 322, 329
Aniiial juice 333
B.htk Csmpofitions 356
Enamel 442
Glafs 441
Ink3 — 371
Paints — 356
Sealing wax — 370
Varnilhes — 366, 639
Fjr marking fheep 359
For preferving wood 362
For backing leather 365
£t:A Dye 401
on Cotton 433
on K.rs 4^8
on Linen — 433, 429
on Si k — — 422
on Thread 433, 429
on Woollen 40*
Black Stains 434
on Agate 438
on Bone « 435
on Hair 43 5
on Ivory 43 5
en Marble — 436
on Wood 434
BJttlftry hearths for running down iron ore z66
BkmBg WlhlHIi by a fall of water with- out compieffible bellows — 269
at Tivoli — 1 — 270
in Dauphiny 271,274
on the Tiburiine mountain — 272
at Lead hills in Scotland 373
in the territory of Foix 276
in Languedoc 277
Of the quantity of water they require279 Quantity of air afforded by them — 280 Mrans ot determining the force of the
air 28$
Of the itructureof the air vefTe! — 286 Method of drying the ait — — — 290
E X.
Experiments for improving the ma- chines 279
for afcertainhig their principles of
action 201
for regulating the flructu-e of the
funnel and pipe -293
for increafing the effect by dividing
the itream 303
Defcription of a machine deduced fram
the t.\-p:rii]ieots 310
Midlines lor fa. Is of great height — '.32 for low falls 635
Bh-.v-pipe for concentrating flame 19
Jt> ufes 20
Blue gt*f*t with Z'ffre 343, 141
wi.ll gold 619
Blue liquors, from oak duftand vitrij! 3S2
From logwood and verdegris 407
From logwood and blue vitriol — 403 From an eifcnlial oil and volatile fpiric
Bone ftained purple 97
itained black 43;
Brafs m^Ired with phtina — 532
Made harder and lefs difpofed to tar- Djfe J32j S3j
Breath human, its fcrce — — — 286
Brocades, method of fpinning gilt wire for them — 61
Slips of gilt paper ufed infiead of gilt thread 62
Inconveniences cfthe paper — • — 62
The gold extended by palling the : between rolls 2 i
Method of cleaning when fullied — 39
Burning-glifs, flint melted by it — 461;
Gyplum melted by it 465 .
Platina melted by it 467
Effects faid to be produced "by it on
gold 70
Examination thereof — 71
Whitenefs of bodies impedes its action
on them — 319, 466
Does not melt poliQied filver — 466
C.
Ca/i/nine, an ore of zinc 519
Means of extracting the zinc pure C19
Method, j
I N D
Mc-hoJ of combining its zinc with
difficultly fufible metals 519
with plitina 520
Calcination, increafe of weight produced
by it 46s
in iron 55*
in mineral platina — 459
in a mixture of platina and tin 558
in a mixture of gold and tin — 618
Conjecture on thecaufe of the incteafe
461
Cj',:: weights explained — 1 1 5
Cajli from medals, a matter for taking354
Cement which quickly hardens in water 64
Cementation, purification of gold by 90, 1 54 Jmpcrfeclionof the prccels — 155, 47- lis ufe I$S
Chalk, black, defcription and ufes — 325 found in England 324
Charcoal, as a pigment 33 c
Its differences from foot black 357, 376
Crayons — — — — 3 ? 5
Experiments of the difference* of diffe- rent coa!s 336
Vegetable, how dillinguiihed from that
of bones 3 33, 377
Stains glaO yellow by batting — 629
Tiroes clafs in fufion yellow, reddifh,
A*. ■ 6:8
Cfol, its contraction in drying 258
Its purity iudged from its contraction _ , .-'— ° 259
Coalpits, means of renewing air in them
Cobalt, a ready method of diftinguifhing
irom othe'r minerals — 343
■Stones tinged bv f-lutian of its metallic
part __. . 437
Vitrification of its metallic f.irt — 586
pectineal, ir.fufion turned blacluin by fo-
lution of pUtina —-—.479
Corni, gold, ofc bfa'.by in them it 114 Finenefs of thofe ..of different cuuntiies
116, 117
Allowance of finenefs in the Ej glim ige ■■ — — — ' — lot
E X.
Copper mtked with geld 82, 8 j
Promote; the fufion of gold — — 8a Melted with platina in different pro- portions — — .529
ILrdened and made lefs difpofed to
tarnifh — — 530, 531
Always incompafl after melting, 550 Produces a gold colour with tin — 624 Stones ftaincd by folutions of it — 440
Diffulved by vitriol of iron 390
Precipitation of gold by it — — 109
Glafs coloured with it, on being baked
with white earth, gave a red tinge to
the earth — 247
Cotton, Rained black 433
purple 97, 434
red 434
Cram, air imbibed by water paffing througii
it 29 1
Q entity of the water influenced by the fucking-pipe being of en or clofe 292
CraT'"<, Charcoal — — — 335
Differences of coals for crayons ^— 337 Excellence of that of willow — 237
BUck lead 328
Different forts, how made — 328 Bell way of obtaining good ones 329
Crueiblei, method of preparing fo as To Hand fulphureous mixtures — 156
Black-lead, tieir advantages 67
whether hurtful to gold — — - 616 made into portable furnaces 2
Cupels, method of preparing 129
Cupellation, furnace lor 12
Pri: ciple of the operation 89
Method of' conducting the procefs 131
Proportion of lead 13-2
I'T.uks for regulating the fire 132, 133 Gold not made quite pure by cupella-
tion — 114
Efflorefcence of Giver in the cupe! — 260
Ctitlle tone, its ufe for taking 'nr.prefii ;ns,
&c. 333
Cuttle fjb ink 334
D-
Diamond, its foil — — — ?zt
one laid to be black — — 321, 637
Dijlillation,
I N D
Dijlillaliex, apparatus for ' — — 9
of aquafortis in iron pots 13S
of aquafortis from metallic fo'utions, in
(lone ware veflels 1 $3
of acid fpirits in glafs retort* — — 643 of aether — — 143
DjedCktbs, method of comparing famples
Dying, apparatus for experiments in — 10
Dying black, on woollen 401
on filk — — 422
on hats 428
on linen and thread 429
Differences in woollen, filk and linen, in regard to their receiving this dye
426, 427, 429
E (Ten tial differences of the dying liquor
fr:m writing ink 401, 407
Neceffitv of a blue ground, or of log- wood, for woollen 404, 407
Means of difcovering whether the cloth
has been blued 4C4
Whether maddering ufeful 405
Improvement of the dye by verdegris
411, 429
Effedl of the verdegris on the other in- gredients — 407
Whether cloth weakened by this dye 402 The dye made to take uniformly — 409
Subftitutes to galls 416
Black dye from combination of colours 420
E.
Eirtbs, fjparately unfufible, brought into fuficn when mixed — — 245, 618
EleSlricity, colours impreffed by it on glafs, from metallic bodies — — — 175
Emery, Spanilh, probably platina 607
Common, its effjfls on lead 608
En/me!, bafis for — 442
Tinged black by concentration of other
colours — 441
by cDmbination of colours — 442 Black imitated on dial plates of watches,
&c. 370
Hiltory of tinging it purple or red 170 Preparations of gold for colourisg it 176
E X.
Procefs of colouring it with gold 621
Enamelling furnace \z
Gold plates for enamelling izz
Ether, fee jE:b;r
Expansion, does not always proceed uni- formly by equil augmentations of
heat 256, 260
Some fluids expand in congealing 2^7
water, in freezing 257
gypfum diluted with water, in felting
— - — i-j
call iron in becoming folid 262
Extenjibility of metals, comparative, how influenced by their gravities 61
F.
Feathers ftained pnrp'e — 97
Fire common, not reflecled from elliptic fur- naces into a focus 26
Not animated by the elaflic fleam of
water 2f
Concentrated by dreams of air 26
A furnace for this concentration of fire 27 Its differences from folar heat in pro- ducing colours — — 3 jo
in the burning of black bodies 3 1 S
Fijhes, method of gilding on them — 64
Flame concentrated into a kind of focus 19, 26
Flatting-mill for gold wire — — — —57
Hint melted by a burning-glafs — — 465
Frankfort black — — — 376
a vegetable coal — 377
Freezing expands watery fluids — ■ — 258 Contraflsoilyor.es — — 258
Futh coaici fea coal, its advant3ges — 246 its inconveniences 569
Furnaces, defcription of a fet of portable
ones for experiments 1,613
For open fire — — 7
For a fand pot — — 8
Ira (till — ~~9
For enamelling — — . — 12
For cupellation — — • — 12
P ppp F.i
I N D
For milling fm.)kc and fumes p.ifs down s
through the grate 79
For ilrongfire — 16
For fmehingorcs, &c. among the fuehi
Wind furnace 11, 24, 569
Dull fjrnace — — 18
Aftion of wind and biall furnaces com-
Aifay furnace — — 12
Reverberatory — 13
Lamp furnace 28
Fujtiility of metals increafed by mixture 32, 82, 617
G.
Ca'li, their pr duilicn on oak trees — 344.
Difference in ftrength of the bluifh and whilifh forts 344
1'hcnomena on mixture with green vi- triol — — — — 346
Subliitutes fur in dying — 345, 416
Gilding on paper, wood, &c. 62
on books — 64, 61 ;
on glafs 65, 100, 614
on live fifhes 64
on copper, with amalgam of g>,ld — 77
means of collecting the mercury, and
preferring the workmen from its
fumes r — — — 79
of heightening the colour of the gild- ing <■ 78
on iron, wi-.h the amalgam 81
on iron, with folution of gold — 108 on copper, earthen-ware, &c. by regulus
of antimony and gold — 88
Cold gilding 98
Gilding imitated by a varnifh — 225
Means of dilcovering this counterfeit
gilding -. r 225
Gilding wax — — 78
Glaf:, its general compofition — — 245 From fimple earths — 245, 246, 618
From metallic bodies 245, 247
From earths with falts 246, 248
Effects of cementation on the different
forts of glafs — 231, 246
earthy and metallic ones not altered
— . — 246, 630
green glafs, from earths with fait, changed
into porcelain — — — •— — 230.
E X.
fucceffion of the changes — — jig
of the caufe of the change — z$ 1
effefts of different cementing mateiiala
— — 241
of the qualities of the porcelain — 236
Glafs, a very fufible one 442
Tinged blue, by gold 619
E'.ack, by manganefe — — 4^2
Black] by iron 44.1
Erown, by iron 44.1
Green, by iron 621
Gold coloured, by antimony and lead 626 With gold coloured fpanglcs, by yellow
talk — — 223
Yellow, by earths 627
Yellow, by iron 626
Yellow, reddifh, &c. by phlogifton 6:3
Purple, by niangantfe — 173
Purple or ruby-coluured, by gold 171 hillory of the ruby-glafs — — 1 70
prepatations of gold for it i;6
expeiiments on a glafs tinged by
gold 1-9
method of preparing the rub) glafs 621
Glnfi ftained ) eHow without fufion 6261629 Method of gilding on — 65, 100,614
GUzing, gold coloured 627
Glue, common and fifh-glue, diiference in the mifcibility of folutions of them with certain liquids — 387
Goltl, its mineral hillory 181
Where chiefly found 182
Sometimes in large maiTes 1 82
]n the fands of European rivers — 1 84 Diffeminated through various earths —
185, 623
Thence wafhed out by rivers 623
Marks of the richeft (pots in rivers 1 86
Found in Britain — 187
Intimately mixed in moft fands — 188
Gold, Separation from fands, &c. by warn- ing 190
Separation from earths, &c. by quick- silver — - — — 193
Extraction from fands by fufion — 195 Extraction from the ores of other me- tals 199
Extraction from large proportions of /li- ver — — 161
of copper — — —.— — — 1 66
Ex,
I N D
Extra&ion from large proportions of icon — — 168
Means of readily difcovering it when diflblved in acids 102,1 10
Diftinguifljed from other yellow me- tals 123, 184
Very minute quantities in minerals, in fome circumftances extracted to ad- vantage 1 64.
Not feparable from copper, as filver is, by eliquation with lead 167
Separation from gilt works 168
Gold, ufe of mixing alloy with it — 1 1 4
Metals fit for its alloy 1 1 5
Englifh ilandard gold 116
Crown gold 117
Way of accounting its finenefs in diffe- rent countries 1 1 j
Finenefs of different coins — 116, 117 Latitude of finenefs in the Englifh coin- age — — 116
Its proportional value to filver in diffe- rent nations 1 18
Gold, of judging of its finenefs from its weight — 125
from its colour on the touch ftonei 19
Affayingof gold — - 127
Refining of gold on the teft 144
by aquafortis — — — 147
by cementation — — — 1 54
by antimony — 156
by vitriolic acid — — — — 9;
by vitriol — 159
Purification by nitre 68
by fublimate 70, 94
Different means of purifying it from pla-
tina 590
Separated from bale metals by calcination
and amalgamation — — 89
Means of avoiding impofition in the
Chinefegold trade — >— • — 124, 126 Purification from emery 147
Gold, Recovery of its luflre when fullied38
Its colour heightened — 83, 68
Made pale 68,78, 112
Obfervations on meking.it 67
Marks of its finenefs and toughnefs in
fufion — — 69
Of preferving or reftoring its toughnefs
67
Whether black-lead crucibles injuiious
, 616
E X.
Solders for it — ■ 82,617
Its great duelility, and the arts depend- ing thereon 44
Expanfion by heat 42
Degree of extenfion in wire and leaf^9
Experiment; of it! gnvity 41
Variations of its gravity from heat and
cold — — — — 42
Difference of its comparative weight to
brafs in heavy and light air — 43 Said to be vitrefcible by large burning
glades 70
Examinatirn of this account — — 71 Changed purple — 74, 10c, 107, 10S
Amalgamation with mercury 75
Mixture with different metals 82
Melted with different proportions of
platina 32;
Not always diffufed equally through
other metals 86
Separated from filver by fufion — 86 Its habitus to different acids, Sec. — 90
DifTolved in the nitrous acid go
Diflblved in the marine acid 94
Corroded by phofphorine acid — 114
Solution in aqua regis 96
Cryftallized gg
Volatilized 100
Recovered after volatilization — 101 Separation from acids by inflammable
liquors — — *_ 101
Precipitation by alcalies 104
Diflblved in volatile alcali 104
Precipitation by metallic bodies — 108 Diflblved by hepar fulphuris — 1 1 1 Recovered from the hepar — 1 1 1 , 1 1 z
Gold, enquiry into its producibility and
dellrudiibility by art — igg
Debalements and augmentations of ic
6oj, 60S
Gold, white, fee Platina
Gold beaten siin, how prepared 46
Repaired when grown unfit for nfe 47
Gold powder 63, 76 99, 109
Gold-Jtze, for gilding 63
for varnifh 365
Gold coloured metal 213, 624
Pigments — — 2zz
Claffes 626
Varnifh or lacquer 223, 614
P p p p z Gran
I N D
Critnu'.uij!.' ofgvlJand ("Over — 14S, 161
Cr.uti for portable furnace, of .1 new can-
ftruclion 613
gravity, fpaifit, of met.!!?, difficulty of
determining with exadnds — 554.
cautions in the weighing — 42. 543
variations produced by hc.it 42
Method of computing the gravities of mixi's from thole of then ingredients 1 25. 541
Mithkes in this computation 545, 556
Method of computing the gravity of ingredients from that of mixts 454
Variation of gravity from mixture, in
metals 547, 555
in liquids 557
Two metals contracted by mixture into lefs volume than one of them occu- pied feparately 551, 556
Tables of the gravities of mixtures of piatina with different metals l\b,Jeq.
Grey, a mixture of white and black 319. Produced frombl-e, red and yellow356
Method of dying 413
Produced in painting with bbek pig- ments diluted — 319
Gur.pctcder, its force compared with that ofaurum fulminans — — — 106
L \f.'r/:rj, melted by a burning glafs — 46; Promotes the fufion of iron 502, 534 Vi^ed with water, expands in becoming
folid — — 259
Its ufes in cenfequence of this property
H.
Hair fhined black 1 — 435-
Hats, method of dying black 428
Ileal, means of communicating an equal
one to different fubjeds — 31, 32
Produced by the mixture of two cold
liquors ■ 1 157
Horn Ilained black 435
Ire, lighter than water _
257
E X.
Becomes (till lighter from a continuance
of cold — — — — 258
Arrangement cf its parts 258
lmpicjftons from medals, &c. matters for
taking 333, 354,442
Matter lor making feals from imprefiions in wax 76
Ink, lni'u.n, its analyfis 358
Imitation of it 359
How faid to be made in China — 638
Ink, priming, its preparation 37 1
Ink for the rolling-prefi, its prcparatioii376- Differences from printing ink 376
Inks, fympatbttic, made vifible by fire 3 1 8 by palling a liquor over the writing 353 by expofure to fumes 353
Ink, black writing, with vitriol and aflrin-
gents 361
Phenomena of the produdien of the co- lour 346
Properties of the colouring matter 347
Analogy with Pruilian blue 34a
Experiments for determining its bell
preparation . 362
Compofition of ink deduced from the
experiments 39 z
Cauies of its fading or changing colour
380,384
Its colour difcharged . 349
Improved by keeping 386
Injured by fome metalline veflels — 390
Made to write black at once 392
Duration of writings prolonged by pre- paration of the paper 393
Ink from more durable materials — 396 Ink of the ancient6 — — 399,334-
Iron, brought into fufion by gypfum 502,
S34.
Malleable after fufion 534
Melted with gold 82, 85
Melted withplatina 1 534,, 552
Improved by piatina — 536
Soldered by gold 82
Amalgamation of it with mercury 508 Experiments of its expanfion in fixing
261
lncreafe of weight in calcination — 552
Variations in its gravity from its having
more or lefs phlogifton — — 552
Colours
I N D
Colours it gives toglafs 441, 620, 626 Subitances which tinge diffolved iron
blick 1 34;
General properties of the black
matter 347, 348
Colour doea not fucceed unlefs the
iron is dilTolved in acids 349
Subftance which tinges dilTolved iron
Means of obtaining a faturated loliuion of this fubflance 484
Method of gilding iron with amalgam
of gold 81
with lolution of gold 108
3-uan bianco 443
Ivory ftained black 43;
purple 97
K.
Kiltov) 324,315
L.
Lac 223
Solution of it in fpirit for vamilhes224, — 624
Laces, gold, method of cleaning when ful-
!ied 39
Separation of the gold from them by fufion — — — 165
Lamp, improved 28
Lamp furnace 28
Lamp black, method of preparing — 341
A finer fort 342,639
From pitcoal 342
Differences from other kinds of foot 3 41
Difference from charcoal as a pigment
3S7,376
Lend, melted with different proportions of
platina — 512
Changed blue, purple, &c. — 5 1 6, 5 1 7
Agreement with bifmulh 515
Cupelled with platina <;6i
Meked with emery 608
Lead, black, where found 32 J
Its ufes s, 326, 328
Difference in goodnefs 326
Its analyfis — — 326
E X.
Iti Raining quality deftroved by fire 327 Grcitc.'l part 'if it volatile in the Ji re 326
How made into pencils 328
Genuine pencils dillinguiQitd f'om counterfeit 328
Leaf gold, the procefs of making it — 4 J
Its fizes 50
Calculation of its thinnefs 59
Green gold leaf 50
Party leaf e, 1
Methods of fixing it on other bodies 62
Linen embelliflicd with flowers of it
226
Leather, ftained black 365
purple 97
Light, blacknefs produced by its abforptkm
317
by its Itrong reflection 318
Lightening, wood preferved from, by a coating of tar and lamp black — 364
Linen, embeliifhed with gold leaf — 226 Stained black by vegetable juices 329,
330, 33,
Dyed black 429,433
Marked of durable colour by floeb — 332
Stained of buff colours 43 3
red 434
purple 97. 434
Logwood, its ufe in the black dye — 406 in writing ink 382
Produces blue and black with verdegris 4°7
Colour of its decoction improved by keeping — 411
M.
Madder, produces a b'ack dye on blue
cloth — 420
Produces a black dye on yellow iron
ftain 433
Produces a purple on dilute iron (lain
— —433
A boiling heat injurious for the red and
purple, ufeful for the black 421, 434
Matiganefe, colours it imparts to glafs 174,
44.1
Incteafes the iuubility of1 glafs — 442
in
I N D
In calcination with nitre, tinges the cru- cible and its fupport amethyft co- loured 493
Marble, ftained black 436
violet or purple 93
Medals, a matter for taking calls from 354
Mtnftraumfintflrepitu 97
Mercury, amalgamated with gold 75
Means of preferving the workmen from
its fumes in gilding 78
Amalgamated with platina ;o8
Has greater affinity to platina than to
lead 577
Has greater affinity to gold than to pla- tina 579
Precipitation of gold by it no
Extraction of gold and filverby it from
earths '93
from bafe metals 89,167
Mines, of renewing the air in them 273, 636
Mocko f.cne imitated 44°
Mirtars, glaf>, hardened into a fubftance like porcelain 24°
Af&^fcdefcribed 130
Supplied by a crucible laid on its fide 1 2
N.
Kiln; its ufe in purifying gold 69
tfitre, cubic, preparation of — 642
Separation of marine alcali f.-om it 645
O.
Oik faw-duji produces a blue with vitriol
' 382.333
SubRituted to galls in dying 419
to oak bark for tanning 382
Oili, made mifcib'.e with water 397
Oi/i, ejfential, feparategold from acids 102
Of thfme, changed blue by volatile
fpirit 436
0/7.', exprejjed, made to dryfoon for paint- ing 357, 371
Differences in their drying quality 371
E X.
Conjecture on the caufe of their diffe- rent difpofition to dry 374,
Differences in their diffolving amber369
Deprived of their property of growing thick in the air 375
Aherations which they undergo from
„ fi/e— 37-2. 374
Reduced to a thick varnifh — — 371
Ores, method of mcking among the fuel 21
P.
Paint, black, with oil — 356
with water— — 357
Paper, method of preparing fo as to pro- long the duration of writings — 393
Papier macbe, how made — — — 366 varnifli for it — — — 367
Parchment, prepared fo as to prolong the duration of writings — 395
Parting of gold from filver 135
Bell prcj ortion of the filver to the gold
in t!ie way of affay 128
in the refining bufinefs — 1 48
Method of conducing the procefs by
the afliyer 135
by the refiner 148
of fecuring the glaffes from breaking
149
of pteventing any gold from being
difTolved 93
Does not make gold compleat! •• r>ure 96, " _ ,4J
Pencils, bUik lead 32g
Different forts, how made 328
True dillinguiflied from counterfeit 328
Phofpbo'us, with gold 1 1 3
with platina 497
Separation of its acid 497
Pitdra del inca, what — 607
Pitcoal, as a pigment 324
Plajler of par is, fee Gypftrn
Platina, a metal of the fame genu- with
gold and filver 610
Stands all the tefts by which gold is aflayed — — — — — ■— — 445
Gold
I N D
Gold adulterated with it — 445, 605
Its mineral hiflory 603
Hiftory of its introduction into Europe
4 + 2
Suppofed to be contained in Come Euro- pean minerals 608
hence the ufe of fuch minerals for the augmentation of gold 608
Hillory of experiments mad; on it by different perfons 445
Flatitiii, Defcription of — 449
Subihnces mixed with it 450
Quantity ofg.Id feparated from it 596
Malleability 4 j j
Gravity, nearly equal to that of gold4j2
Effects of fire on it 4 56
Gains weight in the fire 459
the increafe rung to its admixtures46t Not fufible in fiies that vitrefy all com- mon veffels and furnaces 458, 462,
— 463, 468
Melted by burning glafs 466
ReGfts the fhnple acids, equally with
gold 4-9/7-
Diffolved, like gold, in certain circum- ftmces, by marine acid — — 471 DiiTolved, like gold, by aqua regia 475 Quantity of menftruum neceflary 476 Colour of the folution, and trials for
ftaining 478
Volatilization of the phtina 480
Cr) ftallization 479
Precipitation by different alcalies 4 S 1 Not precipitated bv mineral alcali 483 Precipitated in part by fa! ammoniac 485 Not feparated from its folution by vi- nous fpirits — — 4S j
Not imbibed by effential oils or aether . 4s5
Produces no purple with tin ,87
Precipitates expored to burning glaf .
GMs melted with the precipitates in ftrong fires — 5C3
Its particles agglutinated by fire into a metalline lump 4,7, 505, =70
Expofed to ftrong fires with various fluxes 489
Divided, and an irony matter feparated, by long cementation with nitre 491
Diflolvcd, fparingly, by fulphurated al- cali 502
Unites intimately with all the common metals — — — — — 541
EX.
Strong fire neceflary for its perfect fi-
lution by metals 54a
Produces with them compjt:r.dsof nev
qualities 539, -40
Examination of the fpecific gravities of
the corr.p' unds e±\
The gravity of platina not deducihle from
that of any mixture of it v. ith other
Amalgamation of it with mercur .
Fuffn with arfenic 515
Bifmuth
Brafs 53 z
Copp:r 527
Gold 525
Iron — 534,
Lead 515
Re cuius of antimony — 5.21
Tin 510
SiK'er — — 522
Zinc 519
Calcination w i:h tin — — 5:7
Expofed to flrong fire with reguks of
antimony -$g
with arfenic — — — 559
with zinc 561
Cupellati n with lead ;6s
\w;h bifmuth — 573
Retains in cupellation fome lead and
bifmuth, fo as to prove brittle 56 l^
freed from them and made malleable
by continued ftrong fire — ;63
limitations herein 572
Experiments of its greater affinity to q;.e
metal than to another 577
of its greater or lefs affinity to acids than Other metals have 581.
Pl.ulni, mear.s of diftinguifhing gold mix- ed with it — — — — 593, 59y o' purif)irg gold from it 591 ,595^99. ot extracting gold from large pro- por.ions of it $92-
Its excellence for fj ecula ard other ro- ftruments 576
Hints for forming it into malleable mafTes — 571
Some metals improved and made more ferviceable by a final! admixture of it 531, 536, 610
Poifonajb, black juice from it 330
Kaifcd in England — — 331
Porcelain, .
I N D
Porcelain, gilding on 65, 88
(i ild coloured glazing for — — 627
Glafs changed into porcelain — 230
experiments of the fucccflion of the
changes — — — 23 J
of the qualities and ulcs of the por- celain — — 236
of the effccls of different cementing
materials 241
of the cementing of different forts of
glafs — 245, 630
ol the caule of the change 251
Printing-ink, procefs of prepaiing it 371,
374
Difcharged 372
Purple dye, on linen and cotton — 434
Powders of gold 75, 105, 107, 108,110
I76
Q,
Stuart at ion, fee parting — — 148
^uickfilver, fee mercury
R.
ReJ dye on linen and cotton — — 434
Refining of gold, by cementation 90, 154
Bv aquafortis — go, 93, 147
By vitriolic acid — 9;
By sq-.ta regia — 96
By telling 144
with bifmuth, &c. 14
By aniimony — — 156
Regal cement 470
Reverberator] furnace — — 13
Rilling prefi ink — 376
Rdh f<5f flitting gold «ire 58
Improved method of making them 58, — — 263
Ruby glafs, fee glafs S.
Sal ammtniat, rough and fublimed, differ in their power of volatilizing gold 100
Sal microcofmicus, how prepared from urine 496
E X.
Melted with platina — — ■ — — 496
Gold precipitated by it 619
Vitrefied with gold — — 619
Salt, common, methods of purifying — 639 Caufe of its liquefying in the air —641
Prevented from deliquiating 642
Separation of its alcaline bads — 639
Vitrification of earthen veffels promoted
by its fumes — — 464
Sands, black, as a colouring material — 325
Containing gold 1 88
Method of extracting gold from — 195
Sealing-wax, black 370
Seals, matter for making them from im- preflions in wax — 76
Steep, compofition for marking 379
Shell gold 63
SUA, Raw, method of cleanfing 422
Lofs of weight in cleanfing — 423,425 Imperfections of the common method
423
obviated — — 424
Method of dying black — — — 42 $ Increafe of weight from the black dye
—-423
Difference from woollen in receiving a black dye 426, 427
Silver, tarnifhed to a golden hue — 40,221
Mixtures of it with gold 83
As alloy for gold, lofes its value — 119
Parted from gold 1 59, 147
Recovered from its folution 150
Sulphurated 161
Efflorefcence in the cupel 260
Saturated folution not precipitated by
copper till more acid is added — 151 Polifhed, does not melt by a burning
glafs 466
Melted with platina $22
Particular repugnance to platina — 524 Glafs tinged yellow by precipitates of ir.
! 626
Subltances fhined black by its folution
3 to, 436
Solar light neceffary for producing the
blacknefs 3 ij'o
The colour deflroyed by fire 439
deftroyed by aquafortis, recovered by
fun ______ 439
Sizt,
I N D
&zt, goUffot gikiing— * 62
for varnifhes — — — — 369
Smokt and fumes in furnaces, made to pafs
down through the grate —■ 79
Dome for carrying them off — — 33
Sot/a, fait, whence obtained — 596
The fame with thebafis of lea fait 596
Solar light, differences from common fire in producing colours from metallic
folutions — — 1 50, 98
in the combultion of black bodies 318
Solder, for gold 82,617
for iron — 8z
Soot, its difference from charcoal as a pig- ment 357, 376
Difference of different foots 339
depend more on the manner of burn- ing than on the fubjeft 342, 343 Tinftures in different menllrua — 339 Brown pigment prepared from — 340
Specula, metals propofed for 86, 576
Spirit of wine, its ufe in cleaning gold- laces, Sec. 39
Separates gold from acids — 103
Gold feparated by it from platina 485
Method of dephlegmating for vamifhes,
&c. 1 224, 625
Steel irillrumenti, way of tempering equal- ly a number of fmall ones all at once 32
Stones, flained purple 97
llained black — 436
Method of drawing defigns with the
colouring liquids — 439
Artificial colours diftinguifhed from na- tural 439
Stove, for warming a room with little fuel
Sublimate corrofive, its aftion on metals
47»
Its ufe in purifying gold 70, 94
Sulphurated alcali, its effecTs on gold 1 1 1 ,
on platina — — — — 499
T. Talc, gold coloured, contains no gold 190
E %
lis ufe for making g'oJJ coloured fp.m' gles in glafi f2i
Tanning, oak fiw duff, fubftituEed to oak bark , JS,
Tar, corfipofit'on of it for preferring word,
&C _ 36*
Tejls, method of making 14;'
From mineral Hones 14.4
tejling of gold and filver, procefs of <— 146
Ibermometert, made with different fluids
not corr>p-rib'e from twofixt points257
A mercurial and ipirit one made to cor-
refpond 256
Not influenced by difference in thegiaf» 250
Thread, common method of dying bla.L
' 1 429
Propofal for a more durable black 433
Tiki, painted black in imitation of black glazed ones 362
Tin, Its eftefts on gold 85
Gold coated with it by boiling in tin
veffel with alcali 39
Precipitation of gold by its folution 1 1 r,
— — — — — (^5
Produces a gold colour with copper 624 Melted with platina in different pro- portions — 510
Calcined, &c. with gold 89, 61 7
with platina — 557
Touch need lei, manner of preparing them »i9
Touch Jlone, for metals 1 z\
Directions for its ufe «i2;>
Its requifite qualities 121, 12J
Of what genus ol flones 620
Toxiccdendra, juices from them which (lain, linen of durable black 33c<33«
Trees, conjecture on the caufe of fome re- filling, and others being burlt, by ftrong froih 258
V.
Vcirnijhei, Amber 367, 614
Black, for japanning, &c. 366
hard, for p.ipier mache 366
Q^q q q Farn0es,
I N D
ttmijbts, Black, for metals — 369, 639 of China and Jipjn — — — 329 a limilar cne trom plants raifed here
330
Brown for metals — — 639
Gold coloured — — — — 2:3,624 Printers, ink for — 372
Vegetations of metals in the cupel — 260 from metallic foluiion.% on llones 440
Feriegrit, improves the black dye 407,
411,429
Debafes black ink 339
Produces blue and black with logwood
— — 407
Precipitation of gold by it 109
Verditer, its preparation hji
Vtjfili, g'afs, made harder and more ca- pable of bearing fire, by baking — 240
Vitriol, green, how prepared in England _ _ . j4-
Whether Englifh or foreign preferable
in dying — — 408
Marks of its goodnefs 1 4C9
Phenomena on mixing it with alfrin-
gents 346
Calcination of — > — 135
Its ufe in purifying gold 109
Vitriolic acta*, how diftinguifhed in liquids
— — 640
Purificatkn of liquids from it — 640
Via urfi, asanallringent in dying 41 6, 418 Differences from other allringents 418
w.
Wadl — 325
H'atir, its bulk iocreafed in freezing 257,
2.5S
Mark of fmall degrees ofalcalefcer.ee cr
putrefaction in it — 346
Its vapour does not animate fire — 21
Weight of a cubic inch of it 43
Its expence thiough different apertures
at different depths 279
Runs faller through an upright long than
fhort pipe . 632
Carries air down with it in falling
though pipes — 291,7?;,
Account of blowing machines on this
principle 270
IJxpcriirients for their improvement z.79
FIN
E X.
Wax, gilding — — — — — . — -3 Black fealing — 370
Weights, fmall ones how obtained txacl 544
Willow, its excellence for charcoal crayons whence — — — 337.
Wire, gold or gilt 51
Pioctls of gilding and drawing the wiic
53
Method of flatting it 57
Method of fpinning the flatted wire on
thread 71
Obfervations on the finenefs of the gold
ufed for it 51
Proportion cf the gold to the filver 5.} Calculation of the thinnef; of the golden
coat 59
The filver extracted, fo as to leave a tube
of gold 60
Wire gilt only on one fide 61
Colour of the gilding altered, by heat 56 Method of neajing the wire -»» 55, 5*) wirefrom piatina and gold 326
Wire-draztting plates, of it«l — — 53 Of Lyons metal, their qualities — 54 Iron the bafisof the Lyons metal — 54
Wood, ftained black —+3+
Stained purple — — — — 97
Compofition for preferving 363
In fome cafes damaged by painting 365
Wool, method of cleanfing from its natural
greaie 415
of dying black 415
Woollen dub, procefs of dying black 41c
ofdyinggrey — 413
flained purple 97
Writingbhck with colourlefs liquors 39; Sympathetic, fee ink c.ilt letters — — — — 64
Z.
Zajfre, eolours it gives to glafs — 44 f Mark for difiinguifhing the minerals fit for making it — . — 343
Zinc, way of extracting pure from its ore,
and combining with metals 519
Melted with platina -1 52-5
with gold 3"5> S7
/. s.
v
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