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UNIVERSITY OF
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SANTA CRUZ

SANTA CRUZ

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Mrs. Robert Towne

SANTA CRUZ

THE PRINCIPLES OF
CHEMISTRY

BY

D. MENDEL£EFF

PART FOUR

NEW YORK
P. F. COLLIER & SON

28

/

PEINOIPLES OP CHEMISTBY

(PART FOUR)

CHAPTER XXI

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE

SULPHUR, selenium, and tellurium belong to the uneven series of the
Sixth group. In the even series of this group there are known chro-
mium, molybdenum, tungsten, and uranium ; these give acid oxides
of the type R03, like SO3. Their acid properties are less sharply
defined than those of sulphur, selenium, and tellurium, as is the case
with all elements of the even series as compared with those of the
uneven series in the same group. But still the oxides Cr03, Mo03,
WOo, and even U03, have clearly defined acid properties, and form
salts of the composition MO,nR03 with bases MO. In the case of the
heavy elements, and especially of uranium, the type of oxide, U03,
is less acid and more basic, because in the even series of oxides the
element with the highest atomic weight always acquires a more and
•more pronounced basic character. Hence UO3 shows the properties of
A base, and gives salts UO2X2. The basic properties of chromium,
molybdenum, tungsten, and uranium are most clearly expressed in the
lower oxides, which they all form. Thus chromic oxide, Cr2O3, is as.
distinct a base as alumina, A12O3. »

Of all these elements chromium is the most widely distributed,
and the most frequently used. It gives chromic anhydride, Cr03, and
chromic oxide, Cr203 — two compounds whose relative amounts of
oxygen stand in the ratio 2:1. Chromium is, although somewhat
rarely, met with in nature as a compound of one or the other type.
The red chromium ore of the Urals, lead chromate or crocoisite
PbCrO4, was the source in which chromium was discovered, by
Vauquelin, who gave it this name (from the Greek word signifying
colour) owing to the brilliant colours of its compounds ; the chromatea
(salts of chromic anhydride) are red and yellow, and the chromic salts
(from Cr2O3) green and violet. The red lead chromate is, however, a
rare chromium ore found only in the Urals and in a few other localities.
Chromic oxide, Cr2O3, is more frequently met with. In small quantities
it forms the colouring matter of many minerals and rocks — for example,

276

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 277

of some serpentines. The commonest ore, and the chief source of the
chromium compounds, is the chrome iron ore or chromite, which occurs
in the Urals l and Asia Minor, California, Australia, and other
localities. This is magnetic iron ore, FeO,Fe2O3, in which the ferric
oxide is replaced by chromic oxide, its composition being FeO,Cr2O3.
Chrome iron ore crystallises in octahedra of sp. gr. 4*4 , it has a feeble
metallic lustre, is of a greyish -black colour, and gives a brown powder.
It is very feebly acted on by acids, but when fused with potassium
acid sulphate it gives a soluble mass, which contains a chromic salt,
besides potassium sulphate and ferrous sulphate. Iri practice the
treatment of chrome iron ore is mainly carried on for the preparation
of chromates, and not of chromic salts, and therefore we will trace the
history of the element by beginning with chromic acid, and especially
with the working up of the chrome iron ore into potassium dichromate,
K2Cr2O7, as the most common salt of this acid. It must be remarked
that chromic anhydride, CrO3, is only obtained in an anhydrous state,
and is distinguished for its capacity for easily giving anhydro-salts
with the alkalis, containing one, two, and even three equivalents of the
anhydride to one equivalent of base. Thus among the potassium salts
there is known the normal or yellow chromate, K2CrO4, which corre-
sponds to, and is perfectly isomorphous with, potassium sulphate, easily
forms isomorphous mixtures with it, and is not therefore suitable for a
process in which it is necessary to separate the salt from a mixture
containing sulphates. As in the presence of a certain excess of acid,
thedichromate, K2Cr207 = 2K2CrO4 + 2HX — 2KX — H2O, is easily
formed from K2CrO4, the object of the manufacturer is 'to produce
such a dichromate, the more so as it contains a larger proportion of the
elements of chromic acid than the normal salt. Finely-ground chrome
iron ore, when heated with an alkali, absorbs" oxygen almost as easily
(Chapter III., Note 7) as a mixture of the oxides of manganese, with
an alkali. This absorption is due to the presence of chromic oxide,
which is oxidised into the anhydride, and then combines with the
alkali Cr2O3 -f- O3 = 2Cr03. As the oxidation and formation of the
chromate proceeds, the mass turns yellow. The iron is also oxidised,
but does not give ferric acid:, because the capacity of the chromium for
oxidation is incomparably greater than that of the iron.

A mixture of lime (sometimes with potash) and chrome iron ore
is heated in a reverberatory furnace, with free access of air and at a

The working of the Ural chrome iron ore into chromium compounds ha8 been
firmly established in Russia, thanks to the endeavours of P. K. Ushakoff, who con-
structed large works for this purpose on the river Kama, near Elabougi, where as much
as 2,000 tons of ore are treated yearly, owing to which the importation of chromium pre-
parations into Russia has ceased,

278 PRINCIPLES OF CHEMISTRY

red heat for several hours, until the mass becomes yellow ; it then
contains normal calcium- chromate, CaCrO4, which is insoluble in
water in the presence of an excess of lime.1 bis The resultant mass is
ground up, and treated with water and sulphuric acid. The excess of
lime forms gypsum, and the soluble calcium dichromate, CaCr2O7,
together with a certain amount of iron, pass into solution. The
solution is poured off, and chalk added to it ; this precipitates the
ferric oxide (the ferrous oxide is converted into ferric oxide in the
furnace) and forms a fresh quantity of gypsum, while the chromic acid
remains in solution — that is, it does not form the sparingly-soluble
normal salt (1 part soluble in 240 parts of water). The solution then
contains a fairly pure calcium dichromate, which by double decom-
position gives other chromates ; for example, with a solution of potassium
sulphate it gives a precipitate of calcium sulphate and a solution of
potassium dichromate, which crystallises when evaporated.2

Potassium dichromate, K2Cr207, easily crystallises from acid solu-
tions in red, well- formed prismatic crystals, which fuse at a red heat
and evolve oxygen at a very high temperature, leaving chromic oxide
and the normal salt, which undergoes no further change : 2K2O207
= 2K2Cr04 + Cr2O3 + 03. At the ordinary temperature 100 parts
of water dissolve 10 parts of this salt, and the solubility increases as
the temperature rises. It is most important to note that the
dichromate' does not contain water, it is K2CrO4 + CrO3 , the acid
salt corresponding to potassium acid sulphate, KHSO4, does not exist.
It does not even evolve heat when dissolving in water, but on the con*
trary produces cold, i.e. it does not form a very stable compound with
water. The solution and the salt itself are poisonous, and act as
powerful oxidising agents, which is the character of chromic acid in
general. When heated with sulphur or organic substances, with
sulphurous anhydride, hydrogen sulphide, &c., this salt is deoxidised,
yielding chromic compounds.2 bis Potassium dichromate 3 is used in the
arts and in chemistry as a source for the preparation of all other

1 bl" But the calcium chromate is soluble in water in the preseiice of an excess of
chromic acid, as may be seen from the fact that a solution of chromic acid dissolves
lime.

2 There are many variations in the details of the manufacturing processes, and these
must be looked for in works on technical chemistry. But we may add that the chromate
may also be obtained by slightly roasting briquettes of a mixture of chrome iron and
lime, and then leaving the resultant mass to the action of moist air (oxygen is absorbed,
and the mass turns yellow).

2 bis The oxidising action of potassium dichromate on organic substances at the
ordinary temperature is especially marked under the action of light. Thus it acts on
gelatin, as Poutven discovered ; this is applied to photography in the processes of photo-

For Note 3 see p. 279.

CHROMIUM, MOLYBDENUM, TUNGSTEN) URANIUM, ETC. 279

chromium compounds. It is converted into yellow pigments by means
of double decomposition with salts of lead, barium, and zinc. When
solutions of the salts of these metals are mixed with potassium
dichroraate (in dyeing generally mixed with soda, in order to obtain
normal salts), they are precipitated as insoluble normal salts ; for
example, 2BaCl2 + K2Cr207 + H2O == 2BaCrO4 + 2KC1 + 2HC1. It
follows from this that these salts are insoluble in dilute acids, but
the precipitation is not complete (as it would be with the normal salt).
The barium and zinc salts are of a lemon yellow colour ; the lead salt
has a still more intense colour passing into orange. Yellow cotton
prints are dyed with this pigment. The silver salt, Ag2CrO4, is of a
bright red colour.

When potassium dichromate is mixed with potassium hydroxide

gravure, photo-lithography, pigment printing, &c. Under the action of light this gelatin
is oxidised, and the chromic anhydride deoxidised into chromic oxide, which unites with
the gelatin and forms a compound insoluble in warm water, whilst where the light has not
acted, the gelatin remains soluble, its properties being unaffected by the presence of
chromic acid or potassium dichromate.

5 Ammonium and sodium dichromates are now also prepared on a large scale. The
sodium salts may be prepared in exactly the same manner as those of potassium. The
normal salt combines with ten equivalents of water, like Glauber's salt, with which it is
isomorphous. Its solution above 80° deposits the anhydrous salt. Sodium dichromate
crystals contain Na2Cr2O7,2H2O. The ammonium salts of chromic acid are obtained
by saturating the anhydride itself with ammonia. The dichromate is obtained by
saturating one part of the anhydride with ammonia, and then adding a second part of
anhydride and evaporating under the receiver of an air-pump. On ignition, the normal
and acid salts leave chromic oxide. Potassium ammonium chromate, NH4KCrO4, is
obtained in yellow needles from a solution of potassium dichromate in aqueous ammonia ;
it not only loses ammonia and becomes converted into potassium dichromate when
ignited, but also by degrees at the ordinary temperature. This shows the feeble energy
of chromic acid, and its tendency to form stable dichromates. Magnesium chromate is
soluble in water, as also is the strontium salt. The calcium salt is also somewhat soluble,
but the barium salt is almost insoluble. The isomorphism with sulphuric acid is shown
in the chromates by the fact that the magnesium and ammonium salts form double salts
containing six equivalents of water, which are perfectly isomorphous with the corre-
ponding sulphates. The magnesium salt crystallises in large crystals containing seven
equivalents of water. The beryllium, cerium, and cobalt salts are insoluble in water.
Chromic acid dissolves manganous carbonate, but on evaporation the solution deposits
manganese dioxide, formed at the expense of the oxygen of the chromic acid. Chromio
acid also oxidises ferrous oxide, and ferric oxide is soluble in chromic acid.

One of the chromates most used by the dyer is the insoluble yellow lead chromate,
PbCrO4 (Chapter XVIII., Note 46), which is precipitated on mixing solutions of
PbX2 with soluble chromates. It easily forms a basic salt, having the composition
PbO,PbCrO4) as a crystalline powder, obtained by fusing the normal salt with nitre and
then rapidly washing in water. The same substance is obtained, although impure and
in small quantity, by treating lead chromate with neutral potassium chromate, especially
on boiling the mixture ; and this gives the possibility of attaining, by means of these
materials, various tints of lead chromate, from yellow to red, passing through different
orange shades. The decomposition which takes place (incompletely) in this case ia
as follows: 2PbCrQ4 + K3CrO^= PbCrO4,PbO + K2Cr2O7— that is, potassium dichromate
is formed in solution.

280 PRINCIPLES OF CHEMISTRY

or carbonate (carbonic anhydride being disengaged in the latter case) it
forms the normal salt, K2Cr04, known as yellow chromate of potassium.
Its specific gravity is 2*7, being almost the same as that of the dichro-
mate. It absorbs heat in dissolving ; one part of the salt dissolves in
1*75 part of water at the ordinary temperature, forming a yellow
solution. When mixed even with such feeble acids as acetic, and more
especially with the ordinary acids, it give's the dichromate, and Graham
obtained a trichromate, K2Cr3Ol0 = K2Cr04,2Cr03, by mixing a
solution of the latter salt with an excess of nitric acid.

Chromic anhydride is obtained by preparing a saturated solution of
potassium dichromate at the ordinary temperature, and pouring it in a
thin stream into an equal volume of pure sulphuric acid.4 On mixing,
the temperature naturally rises , when slowly cooled, the solution
deposits chromic anhydride in needle-shaped crystals of a red colour
sometimes several centimetres long. The crystals are freed from the
mother liquor by placing them on a porous tile.4 bis It is very important
at this point to call attention to the fact that a hydrate of chromic
anhydride is never obtained in the decomposition of chromic compounds,

* The Sulphuric acid should not contain any lower oxides of nitrogen, because they
reduce chromic anhydride into chromic oxide. If a solution of a chromate be heated
with an excess of acid— for instance, sulphuric or hydrochloric acid — oxygen or chlorine
is evolved, and a solution of a chromic salt is formed. Hence, under these circum-
stances, chromic acid cannot be obtained from its salts. One of the first methods
employed consisted in converting its salts into volatile chromium hexafluoride, CrF6.
This compound, obtained by Unverdorben, may be prepared by mixing lead chromate
with fluor spar in a dry state, and treating the mixture with fuming sulphuric acid in a
platinunt vessel : PbCrO4 + 8CaF2 + 4H2SO4 = PbSO4 + 3CaSO4 + 4H2O + CrF6. Fuming
sulphuric acid is taken, and in considerable excess, because the chromium fluoride which
is formed is very easily decomposed by water. It is volatile, and forms a very caustic,
poisonous vapour, which condenses when cooled in a dry platinum vessel into a red,
exceedingly volatile liquid, which fumes powerfully in air. The vapours of this
substance when introduced into water are decomposed into hydrofluoric acid and
chromic anhydride : CrF6 + 8H2O = CrO3 -f 6HF. If very little water be taken the hydro-
fluoric acid volatilises, and chromic anhydride separates directly in crystals. The
chloranhydride of chromic acid, CrO2Cl2 (Note 5), is also decomposed in the same
manner. A solution of chromic acid and a precipitate of barium sulphate are formed by
Creating the insoluble barium chromate with an equivalent quantity of sulphuric acid.
If carefully evaporated, the solution yields crystals of chromic anhydride. Fritzsche
gave a very convenient method of preparing chromic anhydride, based on the relation
of chromic to sulphuric acid. At the ordinary temperature the strong acid dissolves
both chromic anhydride "and potassium chromate, but if a certain amount of water is
added to the solution the chromic anhydride separates, and if the amount of water be
increased the precipitated chromic anhydride is again dissolved. The chromic anhy-
dride is almost all separated from the solution when it' contains two equivalents of
water to one equivalent of sulphuric acid. "Many methods for the preparation of chromic
anhydride are based on this fact.

4 bis They cannot be filtered through paper or washed, because the chromic
anhydride is reduced by the filter-paper, and is _ dissolved during the process of
washing.

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 281

but always the anhydride, Cr03. The corresponding hydrate, Cr04H2,
or any other hydrate, is not even known. Nevertheless, it must be
admitted that chromic acid is bibasic, because it forms salts isomorphous
or perfectly analogous with the salts formed by sulphuric acid, which is
the best example of a bibasic acid. A clear proof of the bibasicity o£
Cr03 is seen in the fact that the anhydride and salts give (when heated
with sodium chloride and sulphuric acid) a volatile chloranhydride,
Cr02Cl2, containing two atoms of chlorine as a bibasic acid should.5

5 Berzelius observed, and Rose carefully investigated, this remarkable reaction,
which occurs between chromic acid and sodium chloride in the presence of sulphuric
acid. If 10 parts of common salt be mixed with 12 parts of potassium dichromate, fused,
cooled, and broken up into lumps, and placed in a retort with 20 parts of fuming sul-
phuric acid, it gives rise to a violent reaction, accompanied by the formation of brown
fumes of chromic chloranhydride, or chromyl chloride, CrO2Cl2, according to the re-
action : Cr03 + 2NaCl -H H2S04 = Na2S04 + H2O -h CrO2Cl2. The addition of an excess of
sulphuric acid is necessary in order to retain the water. The same substance is always
formed when a metallic chloride is heated with chromic acid, or any of its salts, in the
presence of sulphuric acid. The formation of this volatile substance is easily observed
from the brown colour which is proper to its vapour. On condensing the vapour in a
dry receiver a liquid is obtained having a sp. gr. of T9, boiling at 118°, and giving a
vapour whose density, compared with hydrogen, is 78, which corresponds with the above
formula. £hromyl chloride is decomposed by heat into chromic oxide, oxygen, and
chlorine: 2CrOoCl2 = Cr3O3 + 2Cl2 + 0; so that it is able to act simultaneously as a
powerful oxidising and chlorinating agent, which is taken advantage of in the investiga-
tion of many, and especially of organic, substances. When reated with water, this
substance first falls to the bottom, and is then decomposed into hydrochloric and chromic
acids, like all chloranhydrides : CrO2Cl2 + H2O = CrO5 + 2HC1. When brought into con-
tact with inflammable substances it sets fire to them ; it acts thus, for instance, on
phosphorus, sulphur, oil of turpentine, ammonia, hydrogen, and other substances. It
attracts moisture from the atmosphere with gredt energy, and must therefore be kept in
closed vessels. It dissolves iodine and chlorine, and even forms a solid compound with
the latter, which depends upon the faculty of chromium to form its higher oxide,
Cr207. The close analogy in the physical properties of the chloranhydrides, CrO2Cl2 and
SO2C12, is very remarkable, although sulphurous anhydride is a gas, and the corresponding
oxide, CrO2, is a non- volatile solid. It may be imagined, therefore, that chromium di-
oxide (which will be mentioned in the following note) presents a polymerised modification
of the substance having, the composition CrOj ; in fact, this is obvious from the method
of its formation.

If three parts of potassium dichromate be mixed with four parts of strong hydrochloric"
acid and a small quantity of water, and gently wanned, it all passes into solution,
and no chlorine is evolved ; on cooling, the liquid deposits red prismatic crystals, known
as Peligot's salt, very stable in air. Thfs has the composition KCl,CrO3, and is formed
according to" the equation K2Cr2O7-f 2HCl = 2KCl,CrO5 + H2O. It is evident that this
is the first chloranhydride of chromic acid, HCrO3Cl, in which the hydrogen is re-
placed by potassium. It is decomposed by water, and on evaporation the solution yields
potassium dichromate and hydrochloric acid. This is a fresh instance of the reversible
reactions so frequently encountered. With sulphuric acid Peligot's salt forms chromyl
chloride. The latter circumstance, and tHe fact that Geuther produced Peligot's salt
from potassium chromate and chromyl chloride, give reason for thinking that it is a
compound of these two substances . 2KCl,CrO3=K2CrO4 + Cr02Cl2. It is also sometimes
regarded as potassium dichromate in which one atom of oxygen is replaced by chlorine —
that is, K2Cr206Ci2, corresponding with K2Cr2O7. When heated it parts with all its
chlorine, and on further heating gives chromic oxide.

282 PRINCIPLES : OF CHEMISTRY

Chromic anhydride is a red crystalline substance, which is converted
into a black mass by heat ; it fuses at 190°, and disengages oxygen
above 250°, leaving a residue of chromium dioxide, CrO2,6 and, on still
further heating, chromic oxide, Cr203. Chromic anhydride is exceed-
ingly soluble in water, and even attracts moisture from the air, but, as
was mentioned above, it does not form any definite compound with
water. The specific gravity of its crystals is 2 '7, and when fused it has
a specific gravity 2*6. The solution presents perfectly defined acid
properties. It liberates carbonic anhydride from carbonates ; gives
insoluble precipitates of the chromates with salts of barium, lead, silver,
and mercury.

The action of hydrogen peroxide on a solution of chromic acid or of
potassium dichromate gives a blue solution, which very quickly becomes
colourless with the disengagement of oxygen. Barreswill showed that
this is due to the formation of a perchromic anhydride, Cr207, corre-
sponding with sulphur peroxide. This peroxide is remarkable from the
fact that it very easily dissolves in ether and is much more stable in
this solution, so that, by shaking up hydrogen peroxide mixed with a
small quantity of chromic acid, with ether, it is possible to transfer all
the blue substance formed to the ether.6 bis

With oxygen acids, chromic acid evolves oxygen ; for example, with

<* This intermediate degree of oxidation, CrO2, may also be obtained by mixing solu-
tions of chromic salts with solutions of chromates. The brown precipitate formed
contains a compound, Cr2O3)CrO3, consisting of equivalent amounts of chromic oxide
and anhydride. The' brown precipitate of chromium dioxide contains water. The same
substance is formed by the imperfect deoxidation of chromic anhydride by various redu-
cing agents. Chromic oxide, when heated, absorbs oxygen, and appears to give the same
substance. Chromic nitrate, when ignited, also gives this substance. When this sub-
stance is heated it first disengages water and then oxygen, chromic oxide being left. It
corresponds with manganese dioxide, Cr20.3,CrO3 = 3CrO2. Kriiger treated chromium
dioxide with a mixture of sodium chloride and sulphuric acid, and found that chlorine
gas was evolved, but that chroniyl chloride was not formed. Under the action of light,
a solution of chromic acid also deposits the brown dioxide. At the ordinary temperature
chromic anhydride leaves a brown stain upon the skin and tissues, which probably pro-
ceeds from a decomposition of the same kind. Chromic anhydride is soluble in alcohol
containing water, and this solution is decomposed "-in a similar manner by light.
Chromium dioxide forms K2CrO4 when treated with H2O2 in the presence of KHO.

6 bis NOW that persulphuric acid H2S2O8 is well known it might be supposed that
perchromic anhydride, Cr2O7, would correspond to perchromic acid, H2Cr2Oa, but as yet
it is not certain whether corresponding salts are formed. Pe'chard (1891) on adding an
excess of H2O2 and baryta water to a dilute solution of CrO2 (8 grm. per litre), observed
the formation of a yellow precipitate, but oxygen was disengaged at the same time and
the precipitate (which easily exploded when dried) was found to contain, besides an
admixture of BaO2, a compound BaCrO5, and this = BaO3 + CrO3, and does not correspond
to perchromic acid. The fact of its decomposing with an explosion, and the mode of ite:
preparation, proves, however, that this is a similar derivative of peroxide of hydrogen to.
persulphuric: acid (Chapter XX.)

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC, 283

Sulphuric acid the following reaction takes place 2CrO3 + 3H2S04
=Cr2(SO4)3 + 03 -f 3H2O. It will be readily understood from this that
a mixture of chromic acid or of its salts with sulphuric acid forms an
excellent oxidising agent, which is frequently employed in chemical
laboratories and even for technical purposes as a means of oxidation.
Thus hydrogen sulphide and sulphurous anhydride are converted into
sulphuric acid by this means. Chromic acid is able to act as a powerful
oxidising agent because it passes into chromic oxide, and in so doing
disengages half of the oxygen contained in it : 2CrO3 = Cr2O3-f O3.
Thus chromic anhydride itself is a powerful oxidising agent, and is
therefore employed instead of nitric acid in galvanic batteries (as a
depolariser), the hydrogen evolved at the carbon being then oxidised,
and the chromic acid converted into a non-volatile product of deoxida-
tion, instead of yielding, as nitric acid does, volatile lower oxides of
offensive odour. Organic substances are more or less perfectly oxidised
by means of chromic anhydride, although this generally requires the aid
of heat, and does not proceed in the presence of alkalis, but generally
in the presence of acids. In acting on a solution of potassium iodide,
chromic acid, like many oxidising agents, liberates iodine ; the reaction
proceeds in proportion to the amount of Cr03 present, and may serve
for determining the amount of CrO3, since the amount of iodine liberated
can be accurately determined by the iodometric method (Chapter XX.,
Note 42). If chromic anhydride be ignited in a stream of ammonia, it
gives chromic oxide, water, and nitrogen. In all cases when chromic
acid acts as an oxidising agent in the presence of acids and under the
action of heat, the product of its deoxidation is a chromic salt, CrX3,
which is characterised by the green colour of its solution, so that the
red or yellow solution of a salt of chromic acid is then transformed into
a green solution of a chromic salt, derived from chromic oxide, Cr203,
which is closely analogous to A12O3, Fe2O3, and other bases of the com-
position E2O3. This analogy is seen in the insolubility of the anhydrous
oxide, in the gelatinous form of the colloidal hydrate, in the formation
of alums,7 of a > volatile chloride of chromium, &c.7 bis

7 As a mixture of potassium dichromate and sulphuric acid is usually employed
for oxidation, the resultant solution generally" contains a double sulphate of potas-
sium and chromium— that is, chrome alum, isomorphous with ordinary alum —
K2Cr2O7 + 4H2SO4 + 20H2O = 63 + K2Cr2(SO4)4,24H20 or 2(KCr(SO4)2,12H20). It is pre-
pared by dissolving potassium dichromate in dilute sulphuric acid ; alcohol is then added
and the solution slightly heated, or sulphurous anhydride is passed through it. On the
addition of alcohol to a cold mixture of potassium dichromate and sulphuric acid, the
gradual disengagement of pleasant-smelling volatile products of the oxidation of alcohol,
and especially of aldehyde, C2H4O, is remarked. If the temperature of decomposition

For Note 7 bis see p. 285.

$84 PRINCIPLES OF CHEMISTRY

Chromic oxide, Cr203, rarely found, and iri small quantities, in chrome'
ochre, is formed by the oxidation of chromium and its lower oxides, by*'

does not exceed 85°, a violet solution of chrome alum is obtained, but if the tempera-
ture be higher, a solution of the same alum is obtained of a green colour. As chrome
alum requires for solution 7 parts of water at the ordinary temperature, it follows that il
a somewhat strong solution of potassium dichromate be taken (4 parts of water and 1J
of sulphuric acid to 1 part of dichromate), it will give so concentrated a solution
of chrome alum that on cooling, the salt will separate without further evaporation. If
the liquid, prepared as above or in any instance of the deoxidation of chromic acid,
be heated (the oxidation naturally proceeds- more rapidly) somewhat strongly, for in-
stance, to the boiling-point of water, or if the violet solution already formed be raised to
the same temperature, it acquires a bright green colour, and on evaporation the
same mixture, which at lower temperatures so easily gives cubical crystals of chrome
alum, does not give any crystals whatever If the green solution be kept, however, for
several weeks at the ordinary temperature, it deposits violet crystals of chrome alum.
The green solution, when evaporated, gives a non-crystalline mass, and the violet
crystals lose water at 100° and turn green. It must be remarked that the transition of
the green modification into the violet is accompanied by a decrease in volume (Lecoq de
Boisbaudran, Favre). If the green mass formed at the higher temperature be evaporated
to dryness and heated at 30° in a current of air, it does not retain more then 6 equi-
valents of water. Hence Lbwel, and also Schrbtter, concluded that the green and violet
modifications of the alum depend on different degrees of combination with water, which
may be likened to the different compounds of sodium sulphate with water and to the
different hydrates of ferric oxide.

However, the question in this case Is not so simple, as we shall afterwards see.
Jtfot chrome alum alone, but all the chromic salts, give two, if not three, varieties. At
least, there is no doubt about the existence of two— a green and a violet modification.
The green chromic salts -are obtained by heating solutions of the violet salts, the violet
solutions are produced on keeping solutions of the green salts for a long time. The con-
version of the violet salts into green by the action of heat is itself an indication of the
possibility of explaining the different modifications by their containing different propor-
tions (or states) of water, and, moreover, by the green salts having a less amount of
water than the violet. However, -there are other explanations. Chromic oxide is a base
like alumina, and is therefore able to give both acid and basic salts. It is supposed that
the difference between the green and violet salts is due to, this fact. This opinion of
Kriiger is based on the fact that alcohol separates out a salt from the green solution
which contains less sulphuric acid than the normal violet salt. On the other hand,
Lbwel showed that all the acid cannot be separated from the green chromic salts by
suitable reagents, as easily as it can be from the same solution of the violet salts ; thus
barium salts do not precipitate all the sulphuric acid from solutions of the green salts.
According to other researches the cause of the varieties of the chromic salts lies in a
difference in the bases they contain— that is, it is connected with a modification of the
properties of the oxide of chromium itself. This only refers to the hydroxides, but as
hydroxides themselves are only special forms of salts, the differences observed as yet
in this direction between the hydroxides only confirm the generality of the difference
observed in the chromic compounds (see Note 7 bis).

The salts of chromic oxide, like those of alumina, are. easily decomposed, give basic
and double salts, and have an acid reaction, as chromic oxide is a feeble base. Potas-
sium and sodium hydroxides give a precipitate of the hydroxide with chromic salts,
CrX3. The violet and green salts give a hydroxide soluble in an excess of the
reagent ; but the hydroxide is held in solution by very feeble affinities, so that it is
partially separated by heat and dilution with water, and completely so on boiling.
In an alkaline solution, chromic hydroxide is easily converted into chromic acid
by the -action of lead dioxide, chlorine, and other oxidising agents. If the chromic
oxide occurs together with such oxides as magnesia, or zinc oxide, then on precipitation

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 285

the reduction of chromates (for example, of ammonium or mercurio
chromate) and by the decomposition (splitting up) of the saline corn-
it separates out from its solution in combination with these oxides, forming, for example,
ZnO,Cr2O5. Viard obtained compounds of Cr2O3 with the oxides of Mg, Zn, Cd, &c.)
On precipitating the violet solution of chrome alum with ammonia, a precipitate contain-
ing Cr2O3,6H2O is obtained, whilst the precipitate from the boiling solution with causth
potash was a hydrate containing four equivalents of water. When fused with borax chromic
Baits give a green glass. The same coloration is communicated to ordinary glass by the
presence of traces of chromic oxide. A chrome glass containing a large amount of
chromic oxide may be ground up and used as a green pigment. Among the hydrates
of oxide of chromium Guignet's green forms one of the widely-used green pigments which
have been substituted for the poisonous arsenical copper pigments, such as Schweinfurt
green, which formerly was much used. Guignet's green has an extremely bright green
colour, and is distinguished for its gre,it stability, not only under the action of light but
also towards reagents ; thus it is not altered by alkaline solutions, and even nitric acid
does not act on it. This pigment remains unchanged up to a temperature of 250° ; it
contains Cr2O3,2HO2, and generally a small amount of alkali. It is prepared by fusing 3
parts of boric acid with 1 part of potassium dichromate ; oxygen is disengaged, and a
green glass, containing a mixture of the boratesof chromium and potassium, is obtained.
When cool this glass is ground up and treated with water, which extracts the borio
acid and alkali and leaves the above-named chromic hydroxide behind. This hydroxide
only parts with its water at a red heat, leaving the anhydrous oxide.

The chromic hydroxides lose their water by ignition, and in so doing become spon-
taneously incandescent, like the ordinary ferric hydroxide (Chapter XXII.). It is not
known, however, whether all the modifications of chromic oxide show this phenomenon.
The anhydrous chromic oxide, Cr<,O5, is exceedingly difficultly soluble in acids, if it
has passed through the above recalescence. But if it has parted with its water, or the
greater part of it, and not yet undergone this self-induced incandescence (has not lost a
portion of its energy), then it is soluble in acids. It is not reduced by hydrogen. It is
easily obtained in various crystalline forms by many methods. The chromates of mer-
cury and ammonium give a very convenient method for its preparation, because when
ignited they leave chromic oxide behind. In the first instance oxygen and mercury are
disengaged, and in the second case nitrogen and water : 2Hg2Cr04 = Cr2O3 + 05 + 4Hg or
(NH4)2Cr2O7 = Cr205 + 4H20 + N2. The second reaction is very energetic, and the mass
of salt burns spontaneously if the temperature be sufficiently high. A mixture of potas-
sium sulphate and chromic oxide is formed by heating -potassium dichrbmate with an
equal weight of sulphur : K2Cr2O7 + S = K2SO4 + Cr2O5. The sulphate is easily extracted
by water, and there remains a bright green residue of the oxide, whose colour is more
brilliant the lower the temperature of the decomposition. The oxide thus obtained is
used-as a green pigment for china and enamel. The anhydrous chromic oxide obtained
from chromyl chloride, Cr02Cl2, has a specific gravity of 5'21, and forms almost black
crystals, which give a green powder. They are hard enough to scratch glass, and have a
metallic lustre. The crystalline form of chromic oxide is identical with that of the oxide
of iron and alumina, with which it is isomorphous.

7 bis The most important of the compounds corresponding with chromic oxide is chromic
chloride, Cr2Cl6, which is known in an anhydrous and in a hydrated form. It resembles
ferric and aluminic chlorides in many respects. There is a great difference between
the anhydrous and the hydrated chlorides ; the former is insoluble in water, the latter
easily dissolves, and on evaporation its solution forms a hygroscopic mass which is very
unstable and easily evolves hydrochloric acid when heated with water. The anhydrous
form is of a violet colour, and Wb'hler gives the following method for its preparation : an
intimate mixture is prepared of the anhydrous chromic oxide with carbon and organic
matter, and charged into a wide infusible glass or porcelain tube which is heated in a
combustion furnace ; one extremity of the tube communicates with an apparatus generat-
ing chlorine which is passed through several bottles containing sulphuric acid in order

286 PRINCIPLES OF CHEMISTRY

pounds of the oxide itself, CrX3 or Cr2X6, like alumina, which it
resembles in forming a feeble base easily giving double and basic salts,
which are either green or violet.

to dry it perfectly before it reaches the tube. On heating the portion of the tube in
which the mixture is placed and passing chlorine through, a slightly volatile sublimate of
chromic chloride, CrCl3 or Cr2Cl6, is formed. This substance forms violet tabular
crystals, which may be distilled in dry chlorine without change, but which, however, re-
quire a red heat for their volatilisation. "These crystals are greasy to the touch and in-
soluble in water, but if they be powdered and boiled in water for a long time they pass
into a green solution. Strong sulphuric acid does not act on the anhydrous salt, or
only acts with exceeding slowness, like water. Even aqua regia and other acids do not
act on the crystals, and alkalis only show a very feeble action. The specific gravity of
the crystals is 2'99. When fused with sodium carbonate and nitre they give sodium
chloride and potassium chromate, and when ignited in air they form green chromic oxide
and evolve chlorine. On jgnition in a stream of ammonia, chromic chloride forms
sal-ammoniac and chromium nitride, CrN (analogous to the nitridesBN,A!N). Mosberg and
Peligot showed that when chromic chloride is ignited in hydrogen, it parts with one-third
of its chlorine, forming chromous chloride, CrCl2 — that is, there is formed from a com-
pound corresponding with chromic oxide, Cr2O3, a compound answering to the suboxide,
chromous oxide, CrO — just as hydrogen converts ferric chloride into ferrous chloride with
the aid of heat. Chromous chloride, CrCl2, forms colourless crystals easily soluble in
water, which in dissolving evolve a considerable amount of heat, and form a blue liquid,
capable of absorbing oxygen from the air with great facility, being converted thereby
into a chromic compound.

The blue solution of chromous chloride may also be obtained by the action of metallic
zinc on the green solution of the hydrated chromic chloride ; the zinc in this case takes
up chlorine just as the hydrogen did. It must be employed in a large excess. Chromic
oxide is also formed in the action of zinc on chromic chloride, and if the solution remain
for a long time in contact with the zinc the whole of the chromium, is converted into
chromic oxychloride. Other chromic salts are also reduced by zinc into chromous salts,
CrX2, just as the ferric salts FeX3 are concerted into ferrous salts FeX2 by it. The
chromous salts are exceedingly unstable and easily oxidise and pass into chromic salts;
hence the reducing power of these salts is very great. From cupric salts they -separate
cuprous salts, from stannous salts they precipitate metallic tin, they reduce mercuric
salts into mercurous and ferric into ferrous salts. Moreover, they absorb oxygen from
the air directly. With potassium chromate they give a brown precipitate of chromium
dioxide or of chromic oxide, according to the relative amounts of the substances taken :
Cr03 + CrO = 2CrO2 or CrO3 + 3CrO = 2Cr2Or. Aqueous ammonia gives a blue precipi-
tate, and in the presence of ammoniacal salts a blue liquid is obtained which turns red
in the air from oxidation. This is accompanied by the formation of compounds analo-
gous to those given by cobalt (Chapter XXII.) A solution of chromous chloride with a
hot saturated solution of sodium acetate, C2H3NaO2, gives, on coojing, transparent red
crystals of chromous acetate, C4H<jCrQ4,H2O. This salt is also a powerful reducing
agent, but may be kept for a long time in a vessel full of carbonic anhydride,

The insoluble anhydrous chromic chloride CrCl3 very easity passes into solution in
the presence of a trace (0'004) of chromous chloride CrCl2. This remarkable phe-
nomenon was observed by Peligot and explained by Lb'wel in the following manners
chromous chloride, as a lower stage of oxidation, is capable of absorbing both oxygen
and chlorine, combining with various substances. It is able to decompose many
chlorides by taking up chlorine from them; thus it precipitates mercurous chloride from
a solution of mercuric chloride, and in so doing passes into chromic chloride : 2CrCl2
+ 2HgCl2 = Cr2Cl6 + 2HgCl. Let us suppose that the same phenomenon takes p\&ce
when the anhydrous chromic chloride is mixed with a solution of chromous chloride,,
The latter will then take up a portion, of the chlorine of the former, and pass into %

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 287

The reduction of chromic oxide — for instance, in a solution by zinc
and sulphuric acid — leads to the formation of chromous oxide, CrO, and

soluble hydrate of chromic chloride (hydrochloride of oxide of chromium), and the
original anhydrous chromic chloride will pass into chromous chloride. The chromous
chloride re-formed in this manner will then act on a fresh quantity of the chromio
chloride, and in this manner transfer it entirely into solution as hydrate. This view is
confirmed by the fact that other chlorides, capable of absorbing chlorine like chromous
chloride, also induce the -solution of the insoluble chromic chloride — for example, ferrous
chloride, FeCl2, and cuprotfs chloride. The presence of zinc also aids the solution of
chromic chloride, owing to its converting a portion of it into chromous chloride^ The
solution of chromic chloride in water obtained by these methods is perfectly identical
with that which is formed by dissolving chromic hydroxide in hydrochloric acid. On
evaporating the green solution obtained in this manner, it gives a green mass, con-
taining water... On further heating it leaves a soluble chromic oxychloride, and when
ignited it first forms an insoluble oxychloride and then chromic oxide ; but no anhy-
drous chromic chloride, Cr2Cl6, is formed by heating the aqueous solution of chromio
chloride, which forms an importa.nt fact in support of the view that the green solu-
tion of chromic chloride is nothing else but hydrochloride of oxide of chromium. At
100° the composition of the green hydrate is Cr2Cl6)9H2O, and on evaporation at the
ordinary temperature over H2S04 crystals are obtained with 12 equivalents of water ;
the red mass obtained at 120° contains Cr2O3,4Cr2Cl6,24H2O. The greater portion of
it is soluble in water, like the mass which is formed at 150° The latter contains
Cr2O3,2Cr2Cl6,9H2O = 8(Cr2OCl4,3H2O)— that is, it presents the same composition as
chromic chloride in which one atom of oxygen replaces two of chlorine. And if the
hydrate of chromic chloride be regarded as Cr2O3,6HCl, the substance which is ob-
tained should be regarded as Cr303,4HCl combined with water, H20. The addition
of alkalis — for example, baryta— to a solution of chromic chloride immediately produces
a precipitate, which, however, re-dissolves on shaking, owing to the formation of one of
the oxychlorides just mentioned, which may be regarded as basic salts. Thus we may
represent the product of the change produced on chromic chloride under the influence
of water and heat by the following formulae . first Cr2O3,6HCl or Cr-jCl^SHoO is formed,
then Cr2O3,4HCl,H2O or Cr2OCl4,3H2O, and lastly Cr2O3,2HCl,2H2O or Cr2O2Cl2,3H20.
In all three cases there are 2 equivalents of chromium to at least 3 equivalents of
water. These compounds may be regarded as being intermediate between chromio
hydroxide and chloride ; chromic chloride is Cr-jCl^, the first oxychloride Cr2(OH)2Clj,
the second Cr2(OH)4Cl2, and the hydrate Cr2(OH)6— that is, the chlorine is replaced by
hydroxyl.

It is very important to remark two circumstances in respect to this . (1) That the
whole of the chlorine in the above compounds is not precipitated from their solutions
by silver nitrate ; thus the normal salt of the composition Cr2Cl6,9H2O only gives up
two-thirds of its chlorine , therefore Peligot supposes that the normal salt contains the
oxychloride combined with hydrochloric acid: Cr2Cl6 + 2H2O = Cr2O2Cl2,4HCl, and that
the chlorine held as hydrochloric acid reacts with the silver, whilst that held in the
oxychloride does not enter into reaction, just as we observe a very feebly-developed
faculty for reaction in the anhydrous chromic chloride; and (2) if the green aqueous
solution of CrCl3 be left to stand for some time, it ultimately turns violet ; in this form
the whole of the chlorine is precipitated by AgN03, whilst boiling re-converts it into the
green variety. Lbwel obtained the violet solution of hydrochloride of chromic oxide by
decomposing the violet chromic sulphate with barium chloride. Silver nitrate precipi-
tates all the chlorine from this violet modification ; but if the violet solution be boiled
and so converted into the green modification, silver nitrate then only precipitates a portion
of the chlorine.

Recoura (1890-1893) obtained a crystallohydrate of violet chromium sulphate,
Cr2(S04)3, with 18 or 15 H20 By boiling a solution of this crystallohydrate, he

288 PRINCIPLES OF CHEMISTRY

its salts, CrX2, of a blue colour (see Notes 7 and 7 b!s). The further

converted it into the green salt, which, when treated with alkalis, gave a precipitate
of Cr2O3,2H2O, soluble in 2H2SO4 (and not 3), and only forming the basic salt,
Cra(OH)2(SO4)2. He therefore concludes that the green salts are basic salts. The
cryoscopic determinations made by A. Speransky (1892) and Marchetti (1892) give a
greater 'depression' for the violet than the green salts, that is, indicate a greater
molecular weight for the green salts. But as Etard, by heating the violet sulphate to
100°, converted it into a green salt of the same composition, but with a smaller amount
of H2O, it follows that the formation of a basic salt alone is insufficient to explain the
difference between the green and violet varieties, and this is also shown by the fact that
Bad, precipitates the whole of the sulphuric acid of the violet salt, and only a portion
of that of the green salt. A. Speransky also showed that the molecular electro-
conductivity of the green solutions is less than that of the violet. It is also known that
the passage of the former into the latter is accompanied by an increase of volume, and,
according to Recoura, by an evolution of heat also.

Piccini's researches (1894) throw an important light upon the peculiarities of the
green chromium trichloride (or chromic chloride) ; he showed (1) that AgF (in contra-
distinction to the other salts of silver) precipitates all the chlorine from an aqueous
solution of the green variety ; (2) that solutions of green CrCl3,6H2O in ethyl alcohol
and acetone precipitate' all their chlorine when mixed with a similar solution of AgNO5 ;
(3) that the rise of the boiling-point of the ethyl alcohol and acetone green solutions of
CrClsjGHoO (Chapter VII., Note 27 bis) shows that i in this case (as in the aqueous solu-
tions of MgSO4 and HgCl2) is nearly equal to 1, that is, that they are like solutions of
non-conductors ; (4) that a solution of green CrCl3 in methyl alcohol at first precipitates
about J of its chlorine (an aqueous solution about §) when treated with AgNO3, but
after a time the whole of the chlorine is precipitated ; and (5) that an aqueous solution
of the green variety gradually passes into the violet, while a methyl alcoholic solution
preserves its green colour, both of itself and also after the whole of the chlorine has
been precipitated by AgNO5. If, .however, in an aqueous or methyl alcholic solution
only a portion of the chlorine be precipitated, the solution gradually turns violet.
In my opinion the general meaning of all these observations requires further elucidation
and explanation, which should be in harmony with the theory of solutions. Recoura,
moreover, obtained compounds of the green salt, Cr2(SO4)3, with 1, 2, and 3 molecules of
'H2SO4, K2SO4, and even a compound Cr2(SO4)3H2CrO4. By neutralising the sulphuric
acid of the compounds of Cr2(SO4)3 and H2SO4 with caustic soda, Recoura obtained an
evolution of 83 thousand calories per each 2NaHO, while free H2SO4 only gives 80'8
thousand calories. Recoura is of opinion that special chromo sulphuric acids, for
instance (CrSO4)H23O4 = $Cr2(SO4)3H2SO4, are formed. With a still larger excess of
sulphuric acid, Recoura 6btained salts containing a still greater number of sulphuric
acid radicles, but even this method does not explain the difference between the green and
violet salts.

These facts must naturally be taken into consideration in order to arrive at
any complete decision as to the cause of the different modifications of the chromic salts.
We may observe that the green modification of chromic chloride does not give double
salts with the metallic chlorides, whilst the violet variety forms compounds Cr2Cl6,2RCl
(where R = an alkali metal), which are obtained by heating the chromates with an excess
of hydrochloric acid and evaporating the solution until it acquires a violet colour. As
the result of all the existing researches On the green and violet chromic salts, it appears
to me most probable that their difference is determined by the feeble basic character of
chromic oxide, by its faculty of giving basic salts, and by the colloidal properties of its
hydroxide (these three properties are mutually connected), and moreover, it seems to me
that the relation between the green and violet salts of chromic oxide best answers to the
relation of the purpureo to the luteo cobaltic salts (Chapter XXIL, Note 85). This
subject cannot yet be considered as exhausted (see Note 7).

We may here observe that with tin the chromic salts, CrX3, give at low temperatures

CHROMIUM. MOLYBDENUM, TUNGSTEN, URANIUM, ETC 289

reduction8 of oxide of chromium and its corresponding compounds
gives metallic chromium. Deville obtained it /-(probably containing
carbon) by reducing chromic oxide with carbon,- at a temperature near
the melting point of platinum, about} 1750°, but the metal itself does
not fuse at this temperature. Chromium has a steel-grey colour and is
very hard (sp. gr. 5-9), takes a good polish, and dissolves in hydro-
chloric acid, but cold dilute sulphuric and nitric acids have no action,
upon it. Bunsen obtained metallic chromium by decomposing a solution
of chromic chloride, Cr.2Cl6, by a galvanic current, as scales of a grey
colour (sp, gr. 7'3). Wb'hler obtained crystalline chromium by igniting
a mixture of the anhydrous chromic chloride Cr2Clc (see Note 7 bis)
with finely-divided zinc, and sodium and potassium chlorides, at the
boiling-point of zinc. When the resultant mass has cooled the zinc may

CrX2 and SnXj, whilst at high temperatures, on the contrary, CrX2 reduces the metal
from its salts SnX2. The -reaction, therefore, belongs to the class of reversible reac-
tions (Beketoff).

Poulenc 'obtained' anhydrous CrP3 (sp. gr. 8'78) and CrF2 (sp. gr. 4-11) by the
action of gaseous HF upon CrCl2. A solution of fluoride of chromium is employed as &
mordant in dyeing. Recoura (1890) obtained green and violet varieties of Cr2Br6,6H2O.
The green variety can only be kept in the presence of en excess of HBr In the solution ,
if alone its solution easily passes into the violet variety with evolution of heat.

8 The reduction of metallic chromium proceeds with comparative ease in aqueous
solutions. Thus the action of sodium amalgams upon a strong solution of Cr2Cl6 gives
(first CrCl2) an amalgam of chromium from which the mercury may be easily driven off
by heating (in hydrogen to avoid oxidation), and there remains a spongy mass of easily
oxidizablo chromium. Plaset (1891), by passing an electric current through a solution of
chrome alum mixed with a small amount of H2SO4 and K2SO4, obtained hard scales of
chromium of a bluish-white colour possessing great hardness and stability (under the
action of water, air, and acids). Glatzel (1890) reduced a mixture of 2KC1 + Cr2ClB by
heating it to redness with shavings of magnesium. The metallic chromium thus
obtained has the appearance of a fine light-grey powder which is seen to be crystalline
tinder the microscope ; its sp. gr. at 16 J is 6*7284. It fuses (with anhydrous borax) only at
the highest temperatures, and after fusion presents a silver- white fracture. The strongest
magnet has no action upon it.

Moissan (1893) obtained chromium by reducing the oxide Cr.jO3 with carbon in the
electrical furnace (Chapter VIII., Note 17) in 9-10 minutes with a current of 850 amperes and
60 volts. The mixture of oxide and carbon gives a bright ingot weighing 100-110 grams.
A current of 100 amperes and^ 50 volts completes the experiment upon a smaller quantity
of material in 15 minutes ; a current of 30 amperes and 50 volts gave an ingot of 10 grama
in 80-40 minutes. The resultant carbon alloy is more or less rich in chromium
(from 87-37-91-7 p.c.). To obtain the metal free from carbon, the alloy is broken into
large lumps, mixed with oxide of chromium, put into a crucible and covered with a
layer of oxide. This mixture is then heated in the electric furnace and the pure metal
is obtained. This reduction can also be carried on with chrome iron ore FeOCr2O5
which occurs in nature. In this case a homogeneous alloy of iron and chromium
is obtained If this alloy be thrown in lumps into molten nitre, it forms insoluble
sesquioxide of iron and a soluble alkaline chromate. This alloy oj iron and
chromium dissolved in molten steel (chrome steel) renders it hard and tough, so that
Such eteel has many valuable applications. The alloy, containing about 3 p.c. Cr and
about 1-3 p.c. carbon, is«even harder than the ordinary kinds of tempered steel and has a
fine granular fracture. The usual mode of preparing the ferrochromes for adding to
iteel is by fusing powdered chrome iron ore under fluxes in a graphite crucible.

290

PRINCIPLES OF CHEMISTRY

be dissolved in dilute nitric acid, and grey crystalline chromium (sp. gr.
6-81) is left behind. Fre"my also prepared crystalline chromium by the
action of the valour of sodium on anhydrous chromic chloride in a
stream of hydrogen, using the apparatus shown in the accompanying
drawing, and placing the sodium and the chromic chloride in separate
porcelain boats. The tube containing these boats is only heated when
it is quite full of dry hydrogen. The crystals of metallic chromium
obtained in the tube are grey cubes having a considerable hardness and
withstanding the action of powerful acids, and even of aqua regia.
The chromium obtained by Wohler by the action of a galvanic current
is, on the contrary, acted on under these conditions. The reason of
this difference must be looked for in the presence of impurities, and in
the crystalline structure. But- in any case, among the properties of

FlG 92.— Apparatus for the preparation of metallic chromium by igniting chromic chloride
and sodium iu a stream of hydrogen.

metallic chromium, the following may be considered established : it is
white in colour, with a specific gravity of about 6-7, is extremely hard
in a crystalline form, is not oxidised by air at the ordinary temperature,
and with carbon it forms alloys like cast iron and steel.

The two analogues of chromium, molybdenum and tungsten (or wol-
fram), are of still rarer occurrence in nature, and form acid oxides, R03,
which are stillless energetic than Cr03. Tungsten occurs in the some-
what rare minerals, scheelite, CaWO4, and wolfram ; the latter being an
isomorphous mixture of the normal tungstates of iron and manganese,
(MnFe)W04. Molybdenum is most frequently met with as molybdenite,
MoS2, which presents a certain resemblance to graphite in its physical
properties and softness. It also occurs, but much more rarely, as a
yellow lead ore, PbMo04. In both these forms molybdenum occurs m
the primary rocks, in granites, gneiss, &c., and in iron and copper ores

CHROMIUM, MOLYBDENUM TUNGSTEN, URANIUM, ETC. 291

in Saxony, Sweden, and Finland. Tungsten ores are sometimes met
with in considerable masses in the primary rocks of Bohemia and
Saxony, and also in England, America, and the Urals. The pre-
liminary treatment of the ore is very simple ; for example, the sulphide,
MoS2, is roasted, and thus converted into sulphurous anhydride and
molybdic anhydride, MoO3, which is then dissolved in alkalis, generally
in ammonia. The ammonium molybdate is then treated with acids,
when the sparingly soluble molybdic acid is precipitated. Wolfram is
treated in a different manner. Most frequently the finely -ground ore is
repeatedly boiled with hydrochloric and nitric acids, and the resultant
solutions (of salts of manganese and iron) poured off, until the dark
brown mass of ore disappears, whilst the Dungstic acid remains, mixed
with silica, as an insoluble residue ; it is treated also with ammonia,
and is thus converted into soluble ammonium tungstate, which passes
into solution and yields tungstic acid when treated with acids. This
hydrate is then ignited, and leaves tungstic anhydride. The general
character of molybdic and tungstic anhydrides is analogous to that of
chromic anhydride ; they are anhydrides of a feebly acid character,
which easily give polyacid salts and colloid solutions.8 bis

8.bli The atomic composition of the tungsten and molybdenum compounds is taken as
being identical with that of the compounds of sulphur and chromium, because (1) both
these metals give two oxides in which the amounts of oxygen per given amount of metal
stand in the ratio 2:3; (2) the higher oxide is of the latter kind, and, like chromio
and sulphuric anhydrides, it has an acid character , (3) certain of the molybdates are iso-
moiphous with the sulphates ; (4) the specific heat of tungsten is 0'0334, cpnsequently
the product of the atomic weight and specific heat is 6'15, like that of the other elements
—it is the same with molybdenum, 96'0 x 0 0722 = 6'9 ; (5) tungsten forms with chlorine
not only compounds WC16, WC15, and WOC14) but also WO2C12, a volatile substance the
analogue of chromyl chloride, Cr02Cl2, and sulphuryl chloride, SO2C12. Molybdenum
gives the chlorine compounds, MoCl2, MoCl3(?), MoCl4 (fuses at 194°, boils at 268° ;
according to Debray it contains MoCl5), MoOCl4, Mo02Cl2, and MoO2(OH)Cl. The
•existence of tungsten hexachloride, WC16, is an excellent proof of the fact that the type
SX0 appears in the analogues of sulphur- as in S03 ; (6) the vapour density accurately
determined for the chlorine compounds MoCl4, WC16, WC15, WOC14 (Roscoe) leaves no
doubt as to the molecular composition of the compounds of tungsten and molybdenum,
for the observed and calculated results entirely agree.

Tungsten is sometimes called scheele in honour of Scheele, who discovered it in 1781
and molybdenum in 1778. Tungsten is also known as wolfram ; the former name was the
name given to it by Scheele, because he extracted it from the mineral then known as
tungsten and now called scheelite, CaWO4. The researches of Roscoe, Blomstrand and
others have subsequently thrown considerable light on the whole history of the compounds
of molybdenum and tungsten.

The ammonium salts of tungsten and molybdic acids when ignited leave the anhy-
drides, which resemble each other in many respects. Tungsten anhydride, WO3, is a
yellowish substance, which only fuses at a strong heat, and has a sp. gr. of 6'8. It is
insoluble both in water and acid, but solutions of the alkalis, and even of the alkali car-
bonates, dissolve it, especially when heated, forming alkaline salts. Molybdic anhydridet
MoOj, is obtained by igniting the acid (hydrate) or the ammonium salt, and forms a
white mass which fuses at a red heat, and solidifies to a yellow crystalline mass of sp. gr.

292 PRINCIPLES OF CHEMISTRY

Hydrogen (which does not directly form compounds with Cr, Mo,

4'4; whilst on further heating in open vessels or in a stream of air this anhydride
sublimes in pearly scales— this enables it to be obtained in a tolerably pure state. Water
dissolves it in small quantities — namely, 1 part requires 600 parts of water for its solution.
The hydrates of molybdic anhydride are soluble also in acids (a hydrate, H.^MoC^, is
obtained from the nitric acid solution of ihe ammonium salt), which forms one of their
distinctions from the tungstic acids. But after ignition molybdic anhydride is insoluble
in acids, like tungstic anhydride ; alkalis dissolve this anhydride, easily forming molybdates.
Potassium bitartrafce dissolves the anhydride with the aid of heat. None of the acida
yet considered by us form so many different salts with one and the name base (alkali) as
molybdic and tungstic acids. The composition of these salts, and their properties also,
vary considerably. The most important discovery in this respect was made by Margue-
rite and Laurent, who showed that the salts which contain a large proportion of tungstic
acid are1 easily soluble in water, and ascribed this property to the fact that tungstic acid
may be obtained in several states. The common tungstates, obtained with an excess of
alkali, have an alkaline reaction, and on the addition of sulphuric or hydrochloric acid
first deposit an acid salt and then a hydrate of tungstic acid, which is insoluble both in
water and acids ; but if instead of sulphuric or hydrochloric acids, we add acetic or phos-
phoric acid, or if the tungstate be saturated with a fresh quantity of tungstic acid, which,
may be done by boiling the solution of the alkali salt with the precipitated tungstic acid,
a solution is obtained which, on the addition of sulphuric or a similar acid, does not-
give a precipitate of tungstic acid at the ordinary or at higher temperatures. Thesolutioa
then contains peculiar salts of tungstic acid, and if there be an excess of acid it also-
contains tungstic acid itself ; Laurent, Riche, and others called it tnetatungstic acid, and
it is still known by this name. Those salts which with acids immediately give the in,
soluble tungstic acid have the composition R2W04, RHWO4, whilst those which give
the soluble metatungstic acid contain a far greater proportion of the acid elements.-
Scheibler obtained the (soluble) metatungstic acid itself by treating the soluble barium
(meta) tetratungstate, BaO,4WO3, with sulphuric acid. Subsequent research showed the-
existence of a similar phenomenon for molybdic acid. There is no doubt that this is a
case of colloidal modifications.

Many chemist's have worked on the various salts formed by molybdic and tungstic
acids. The tungstates have been investigated by Marguerite, Laurent, Marignac,.x
Riche, Scheibler, Ahthon, and others. The molybdates were partially studied by the
same chemists, but chiefly by Struve and Svanberg, Delafoutaine, and others. It appears-
that for a given amount of base the salts contain one to eight equivalents of molybdic or
tungstic anhydride ; i.e. if the base have the composition RO, then the highest proportion
of base will be contained by the salts of the composition ROWO3 or ROMoO3 — that is, by
those salts which correspond with the normal acids H2WO4 and H2MoO4, of the same nature-
as sulphuric acid ; but there also exist salts of the composition RO,2WO3, KO,3WO3
RO,8WOs. The water contained in the composition of many of the acid salts is often
not taken into account in the above. The properties of the salts holding different pro-
portions of acids vary considerably, but one salt may be converted into another by the
addition of .acid or base with great facility, and the greater the proportion of the elements
of the acid in a salt, the more stable, within a certain limit, is its solution and the salt-
itself.

The most common ammonium molybdate has the composition (NH4HO)6,H2O,7Mopj
(or, according to Marignac and others, NH4HMoO4), and is prepared by evaporating an
ammoniacal solution of molybdic acid. It is used in the laboratory for precipitating
phosphoric acid, and is purified for this purpose by mixing its solution with a small
quantity of magnesium nitrate, in order to precipitate any phosphoric acid present, filter-
ing, and then adding nitric acid and evaporating to dryness. A pure ammonium molyb-
date free from phosphoric acid may then be extracted from the residue.

Phosphoric acid forms insoluble compounds with the oxides of uranium and iron,
tin, bismuth, &c., having feeble basic and even acid properties. This perhaps depends.

CHROMIUM. MOLYBDENUM, TUNGSTEN, URANIUM, ETC, 293
and W) reduces molybdic, and tungstic anhydride at a red heat ; and

on the fact that the atoms cf hydrogen in phosphoric acid are of a very different
character, as we saw above. Those atoms of hydrogen whicli are replaced with difficulty
by ammonium, sodium, &c., are probably easily replaced by feebly energetic acid
groups— that is, the formation of particular complex substam.-es may be expected to
take place nt the expense of these atoms of the hydrogen of phosphoric acid and of
certai'n feeble metallic acids ; and these substances will still be acids, because the
hydrogen of the phosphoric acids and metallic acids, which is easily replaced by metals,
is not removed by Iheir mutual combination, but remains in the resultant compound.
Such a conclusion is verified in the phosphomolybdic acids obtained (1888) by Debray
If a solution of ammonium molybdate be acidified, and a small amount of a solution (it
may be acid) containing orthophosphoric acid or its salts be added to it (so that there are
at least 40 parts of molybdic acid present to 1 part of phosphoric acid), then after a period
of twenty-four hours the whole of the phosphoric acid is separated as a yellow precipitate,
containing, however, not more than 8 to 4 p.c. of phosphoric anhydride, about 8 p.c. of
ammonia, about 90 p.c. of molybdic anhydride, and about 4 p.c. of water. The formation
Of this precipitate is so distinct and so Complete that this method is employed for the dis-
covery and separation of the smallest quantities of phosphoric acid. Phosphoric acid was
found to be present in the majority of rocks by this means. The precipitate is soluble
in ammonia and its salts, in alkalis and phosphates, but is perfectly insoluble in nitric,
sulphuric, and hydrochloric acids in the presence of ammonium molybdate. The compo-
sition of the precipitate appears to vary under the conditions of its precipitation, but its
nature became clear when the acid corresponding with it was obtained. If the above-
described yellow precipitate be- boiled in aqua regia, the ammonia is destroyed, and
an acid is obtained in solution, which, when evaporated in the air, crystallises out in yellow
oblique prisms of approximately the composition P2O5,20MoO3,26H2O Such an unusual
proportion of component parts is explained by the above-mentioned considerations. We
saw above that molybdic acid easily gives salts R.2OwMoO5mH2O, which we may imagine
to correspond to a hydrate MoO2(HO).j«MoO5mH2O. And suppose that such a hydrate
reacts on orthophosphoric' acid, forming water and compounds of the composition
MoO2(HPO4)7iMoO3?/iH20 or MoO5(H2PO4)j7iMoO5>rcHoO ; this is actually the composi-
tion of phosphomolybdic acid. Probably it contains a portion of the hydrogen replaceable
by metals of both the acids H5PO4 and of H2Mo0.i. The crystalline acid above is
probably H-,MoPO7,9Mo03,12H.jO. This acid is really tribasic, because its aqueous
Solution precipitates salts of potassium, ammonium, rubidium (but not lithium and
sodium) from acid solutions, and gives a yellow precipitate of the composition
R3MoPO7,9MoO3,8H2O, where R = NH4. Besides these, salts of another composition
xnay be obtained, as would be expected from the preceding. These salts are only stable
in acid solutions (which is naturally due to their containing an excess of acid oxides),
whilst under the action of alkalis they give colourless phosphomolybdates of the compo-
sition R3MoPOr),MoO2,8H20. The corresponding salts of potassium, silver, ammonium,
are easily soluble in water and crystalline.

Phosphomolybdic acid is an example of the complex inorganic acids first obtained
by Marignac and afterwards generalised and studied in detail by Gibbs. We shall
afterwards meet with several examples of such acids, and we will now turn attention to
the fact that they are usually formed by wtfak polybasic acids (boric, silicic, molybdic,
&c.), and in certain respects resemble the cobaltic and such similar complex compounds,
with which we shall become acquainted in the following chapter. As an example we will
here mention certain complex compounds containing molybdic and tungstic acids, aathey
will illustrate the possibility of a considerable complexity in the composition of salts.
The action of ammonium molybdate upon a dilute solution of purpureo-cobaltic salts (see
Chapter XXII.) acidulated with acetic acid gives a salt which after drying at 100° has the
composition Co.jO3lONH37MoO33H2O. After ignition this salt leaves a residue having the
composition 2CoO?MoOs. An analogous compound is also obtained for tungstic acid, having
'ibe composition Co.jO510NH510WO39H,,0. In this case after ignition there remains a salt

294 PRINCIPLES OF CHEMISTRY

this forms the means of obtcaining metallic molybdenum and tungsten,

of the composition CoO5WO5 (Carnbt, 1889). Professor Kournakoff, by treating a solution of
potassium and sodium molybdates, containing a certain amount of suboxide of cobalt, with
bromine obtained salts having the composition : 8K2OCo.2O312MoO320HaO (light green)
and SKjOCo^OjlOMo^lOBLjO (dark green). Pe'chard (1893) obtained saltsof the four complex
phosphotungstic acids by evaporating equivalent mixtures of solutions of phosphoric acid
and metatungstic acid (see further on): phosphotrimetatungstic acid P3O5l2WO348H2Or
phosphotetrameiatungstic acid P205l6WO569H2O, phosphopentametatungstic acid
P2O520WO3H2O, and phosphphexametatungstic acid PaO524WO559H2O. Kehrmann
and Frankel described still more complex salts, such as: 8Ag2O4BaOP20522WO3H,2O,
6BaO'2K2OP2O322WO348H2O. Analogous double salts with 22WO5 were also obtained
with KSr, KHg, BaHe, and NH4Pb. Kehrmann (1892) considers the possibility of
obtaining an unlimited number of such salts to be a general characteristic of such com-
pounds. Mahom and Friedheim (1892) obtained compounds of similar complexity for
raolybdic and arsenic acids.

For tungstic acid there are known : (1) Normal salts— for example, KaWO4 ; (2) the
so-called acid salts have a composition like 8K2O,7WO3)6H2O or K6H8(WO4)7,2H2O ; (3>
the tritungstates like Na3O,3WO3)3H5p = NaJH4(WO4)3,H2O. All these three classes of
salts are soluble in water, but are precipitated by barium chloride, and with acids in solu-
tion give an insoluble hydfate of tungstic acid ; whilst those salts which are enumerated
below do not give >a precipitate either with acids or with the salts of the heavy metals, be-
cause they form soluble salts even with barium and lead. They are generally called meta-
tungstates. They all contain water and a larger proportion of acid elements than the
preceding salts ; (4) the tetratungstates, like Na,iO,4WO3,10H2O and BaO,4WO3,9H2O for
example ; (5) the octatungstates — for example, Na-jO.SW Oj^H^O. Since the metatung-
states lose so much water at 100° that they leave salts whose composition corresponds
with an acid, 8H2O,4WO3— that is, H6W4O15— whilst in the meta salts only 2 hydrogens
ore replaced by metals, it is assumed, although without much ground, that these salts-
contain a particular soluble metatungstic acid of the composition H6W4Oi5.

NAs an example we will give a short description of the sodium salts. The normal
Bait, Na2WO4, is obtained by heating a strong solution of sodium carbonate with tungstic-
ftcid to a temperature of 80° ; if the solution be filtered hot, it crystallises in rhombic
tabular crystals, having the composition Na2WO4,2H2O, which remain unchanged in the
air and are easily soluble in water. When this salt is fused with a fresh quantity of
tuugstic acid, it gives a ditungstate, which is soluble in water and separates from ita
solution in crystals containing water. The same salt is obtained by carefully adding
hydrochloric acid to the solution of the normal salt so long as a precipitate does not
appear, and the liquid still has an alkaline reaction. This salt, was first supposed to
have the composition Na2W2O7,4H2O, but it has since been found to contain (at 100°)
NaeW7O24,16H2O — that is, it corresponds with the similar salt of molybdic acid.

(If this salt be heated to a red heat in a-stream of hydrogen, it loses a portion of its
oxygen, acquires a metallic lustre, and" turns a golden yellow colour, and, after being-
treated with water, alkali, and acid, leaves golden yellow leaflets and cubes which are
very like gold. This very remarkable substance,. discovered by W6hler, has, according1
to Malaguti's analysis, the composition Na2!W5O9 ; that, isj it, as it were, contains a-
double tungstate of tungsten oxide, WO2, and of sodium, Na.2WO4;WO2WO3r ' The
decomposition of the fused sodium salt is best effected by finely-divided tin. This sub-
stance has a sp. gr. 6'6; it conducts electricity, like metals, and like them has a metallic
lustre. When brought into contact with zinc and sulphuric acid it disengages hydrogen,
and it becomes covered with a coating of copper in a solution of copper sulphate in the
presence of zinc — that is, notwithstanding its complex composition it presents to a
certain extent the appearance and reactions of the metals. It is not acted on by aqua-
rcgia or alkaline solutions, but it is oxidised when ignited in air.)

The ditungstate mentioned above, deprived of water (having undergone a modifica-
tion similar to that of metaphosphoric acid), after being treated with water, leaves anr

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC, 295
Both metals are infusible, and both under the action of heat form

anhydrous, sparingly soluble tetratungstate, Na2WO4,3WO5) which, when heated at 120°
in a closed tube with water, passes into an easily soluble metatungstate. It may there-
fore be said that the metatungstates are hydrated compounds. On boiling a solution of
the above-mentioned salts of sodium with the yellow hydrate of tungstic acid they give
a solution of metatungstate, which is the hydrated tetratungstate. Its crystals contain
NaoW4O13,10H2O. After the hydra£e of tungstic acid (obtained from the ordinary tung-
states by precipitation with an acid) has stood a long time in contact with a solution (hot
or cold) of sodium tungstate, it gives a solution which is not precipitated by hydrochloric
acid ; this must be filtered and evaporated over sulphuric acid in a desiccator (it is de-
composed by boiling). It first forms a very dense solution (aluminium floats m it) of
sp. gr. S'O, and octahedral crystals of sodium metat'ungstate, Na2W4O13,10H2O, sp. gr.
3'85, then separate. It effloresces and loses water, and at 100° only two out of the
ten equivalents of water remain, but the properties of the salt remain unaltered. If the
salt be deprived of water by further heating, it becomes insoluble. At the ordinary
temperature one part of water dissolves ten parts of the metatungstate. The other
metatungstates are easily obtained from this salt. Thus a strong and hot solution,
mixed with a like solution of barium chloride, gives on cooling crystals of barium meta-
tungstate, BaW4O13,9H2O. These crystals are dissolved without change in water con-
taining hydrochloric acid, and also in hot water, but they are partially decomposed by
cold water, with the formation of a solution of metatungstic acid and of the normal
barium salt BaWO4.

In order to explain the difference in the properties of the salts of tungstic acid, we
may add that a mixture of a solution of tungstic acid with a.solution of silicic acid does
not coagulate when heated, although the silicic acid alone would do so ; this is due to
the formation of a silicotungstic acid, discovered by Marignac, which presents a fresh
example of a complex acid. A solution of a tungstate dissolves gelatinous silica, just as
it does gelatinous tungstic acid, and when evaporated deposits a crystalline salt of
silicotungstic acid. This solution is not precipitated either by acids (a clear analogy to
the metatungstates) or by sulphuretted hydrogen, and corresponds with a series of salts.
These salts contain one equivalent .of silica and 8 equivalents of hydrogen or metals, in
the same form as in salts, to 12 or 10 equivalents of tungstic anhydride ; for example,
the crystalline potassium salt has the composition K8Wi2SiO42,14H.jO = 4K2O,12WO3,
SiOjjlilLjO. Acid salts are also known in which half of the metal is replaced by
hydrogen. The complexity of the composition of such complex acids (for example, of
the phosphomolybdic acid) involuntarily leads to .the idea of polymerisation, which we
were obliged to recognise for silica, lead oxide, and other compounds. This polymerisa-
tion, it seems to me, may be understood thus : a hydrate A (for example, tungstic acid)
is capable of combining with a hydrate B (for example, silica or phosphoric acid, with
or without the disengagement of water), and by reason of this faculty it is capable of
polymerisation — that is, A combines with A — combines with itself — just as aldehyde,
GjH4O, or the cyanogen compounds are able to combine with hydrogen, oxygen, &c.,
and are liable to polymerisation. On this view the molecule of tungstic acid is probably
much more complex than we represent it , this agrees with the easy volatility of such
compounds a* the chloranhydrides, CrOaCl2, MoO2Clj, the analogues of the volatile
eulphuryl chlcride, SO C12, and with the non-volatility, or difficult volatility, of chromic .
and molybdic annydrides, the analogues of the volatile sulphuric anhydride. Such a
view also finds a certain confirmation in the researches made by Graham on the colloidal
state of tungstic acid, because colloidal properties only appertain to compounds of a very
complex composition. The observations made by Graham on the colloidal state of
tungstic and molybdic acids introduced much new matter into the history of these sub'-
stances. When sodium tungstate, mixed in a dilute solution with an equivalent quantity
of dilute hydrochloric acid, is placed in a dialyser, hydrochloric acid and sodium chloride
pass through the membrane, and a solution of tungstic acid remains in the dialyser.
Out of 100 parts of tucgstic acid about 80 parts remain in the dialyser. The solution
*B

296 PRINCIPLES OF CHEMISTRY

compounds with carbon and iron (the addition of tungsten to steel
renders the latter ductile and hard).9 Molybdenum forms a grey powder,,
which scarcely aggregates under a most powerful heat, and has a specific
gravity of 8 '5 It is not acted on by the air at the ordinary tempera-
ture, but when ignited it is first converted into a brown, and then into a
blue oxide, and lastly into molybdic anhydride. Acids do not act on it
— that is, it does not liberate hydrogen from them, not even from
hydrochloric acid— but strong sulphuric acid disengages sulphurous
anhydride, forming a brown mass, containing a lower oxide of molyb-
denum. Alkalis in solution do not act on molybdenum, but when fused

has a bitter, astringent taste, and does not yield gelatinous tungstio acid (hydrogel)
either when heated or on the addition of acida or salts. It may also be evaporated to
dryness; it then forms a vitreous mass of the hydrosol of tungstic acid, which adheres
strongly to the walls of the vessel in which it has been evaporated, and is perfectly
Soluble in water. It does not even lose its solubility after having been heated to 200°,
and only becomes insoluble when heated to a red heat, when it loses about 2^ p.c. of
water. The dry acid, dissolved in a small quantity of water, forms a gluey mass, just
like gum arabic, which is one of the representatives of the hydrosols of colloidal
substances. The solution, containing 5 p.c of the anhydride, has a sp. gr. of T047 ; with
20 p.c., of 1'217; with 50 p.c., of T80; and with 80 p.c., of 8'24. The presence of a
polymerised trioxide in the form of hydrate, H2OW5O0 or HSO4WO5, must then be
recognised in the solution : this is confirmed by Sabaneeffs cryoscopic determinations
(1889). A similar stable solution of molybdic acid is obtained by the dialysis of a
mixture of a strong solution of sodium molybdate with hydrochloric acid (the precipitate
which is formed is re-dissolved). If MoCl4 be precipitated by ammonia and washed with
water, a point is reached at which perfect solution takes place, and the molybdic acid"
forms a colloid solution which is precipitated by the addition of ammonia (Muthmann).
The addition of alkali to the solutions of the hydrosols of tungstic and molybdic acids
immediately results in the re-formation of the ordinary tungstates and molybdates.
There appears to be no doubt but that the same transformation is accomplished in the
passage of the ordinary tungstates into the metatungstates as takes place in the passage
of tungstic acid itself from an insoluble into a soluble state ; but this may be even
actually proved to be the case, because Scheibler obtained a solution of tungstic acid,
before Graham, by decomposing barium metatungstate (BaO4WO3,9H2O) with sulphunc
acid. By treating this salt with sulphuric acid in the amount required for the precipi-
tation of the baryta, Scheibler obtained a solution of metatungstic acid which, when
containing 43-75 p.c. of acid, had a sp. gr. of T634, and with 27*61 p.c. a sp. gr. of 1*327 —
that is, specific gravities corresponding with those found by Graham.

Pechard found that as much heat is evolved by neutralising raetatungstic acid as with
sulphuric acid.

Questions connected with the metamorphoses or modifications of tungstic and
molybdic acids, and the polymerisation and colloidal state of substances, as well as the
formation of complex acids, belong to that class of problems the solution of which
will do much towards attaining a true comprehension of the mechanism of a number of
chemical reactions. I think, moreover, that questions of this kind stand in intimate con-
nection with the theory of the formation of solutions and alloys and other so-called inde-
finite compounds.

9 Moissan (1893) studied the compounds of Mo and W formed with carbon in the
electrical furnace (they are extremely hard) from a mixture of the anhydrides and carbon.
Poleck and Griitzner obtained definite compounds FeW2 and FeW2C5 for tungsten.
Metallic W and MO displace Ag from its solutions but not Pb. There is reason for believing
"that the sp. gr. of pure molybdenum is higher than that (8'5) generally ascribed to it.

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 297

with it hydrogen is given off, which shows, as does its whole character,
the acid properties of the metal. The properties of tungsten are almost
identical ; it is infusible, has an iron-grey colour, is exceedingly hard, so
that it even scratches glass. Its specific gravity is 19'1 (according to
Roscoe), so that, like uranium, platinum, &c., it is one of the heaviest
metals.9 bis Just as sulphur and chromium have their corresponding
persulphuric and perchromic acids, H2S2O8 and H2CrO8, having the
properties of peroxides, and corresponding to peroxide of hydrogen, so
also molybdenum and tungsten are known to give permolybdic and per-
tungstic acids, H2Md2O8 and H2W,>08, which have the properties of true
peroxides, i.e. easily disengage iodine from KI and chlorine from HC1,
easily part with their oxygen, and are formed by the action of peroxide
of hydrogen, into which they are readily reconverted (hence they may
(be regarded as compounds of H202 with 2Mo05 and 2WO3), <kc. Their
formation (Boerwald 1884, Kemmerer 1891) is at once seen in the
coloration (not destroyed by boiling), which is obtained on mixing a
solution of the salts with peroxide of hydrogen, and on treating, for in-
stance, molybdic acid with a solution of peroxide of hydrogen (Pochard
1892). The acid then forms an orange-coloured solution, which after
evaporation in vacuo leaves Mo2H2084H2O as a crystalline powder,
and loses 4H2O at 100°, beyond which it decomposes with the evolu-
tion of oxygen. 9trl

Uranium, U = 240, has the highest atomic weight of all the
analogues of chromium, and indeed of all the elements yet known. Its

9 bis We may conclude our description of tungsten and molybdenum by stating that
their sulphur compounds have an acid character, like carbon bisulphide or stannic sul-
phide. If sulphuretted hydrogen be passed through, a solution of a molybdate it doea
not give a precipitate unless sulphuric acid be present, when a dark brown precipitate of
molybdenum trisulphide, MoS3, is formed. When this sulphide is ignited without access
of air it gives the bisulphide MoS.^ ; the latter is not able to combine with potassium sulphide
like the trisulphide MoS3, which forms a salt, K^MoS^ corresponding with K2Mo04.
This is soluble in water, and separates out from its solution in red crystals, which have a
metallic lustre and reflect a green light. It is easily obtained by heating the native
bisulphide, MoS2, with potash,,sulphur, and a small amount of charcoal, which serves for
deoxidising the oxygen compounds. Tungsten gives similar compounds, R^WS^ where
E = NH4, K, Na. They are decomposed by acids, with the separation of tungsten trisul-
phide, WS-5, and molybdenum trisulphide, MoS3. Eideal (1892) obtained WSN5 by heating
WO3 in NH3. This compound exhibited the general properties of metallic nitrides.

9n-j When peroxide of hydrogen acts upon a solution of potassium molybdate well-
formed yellow crystals belonging to the triclinic system separate out in the cold. When
these crystals are heated in vacuo they first lose water and then decompose, leaving a
residue composed of the salt originally taken. They are soluble in water but insoluble
in alcohol. Their composition Is reprasented by the formula K4Mo2Os2H2O. An am-
monium salt is obtained by evaporating peroxide of hydrogen with ammonium molybdate.
The following salts haice also been obtained by the action of peroxide of hydrogen upon the
corresponding molybdates : Na^MojOeeHjO— in yellow prismatic crystals ; MgMo2O810H2O
— stellar needles; BaMo..O82H2O — in microscopic yellow octahedra. A corresponding
sodium pertungstate has been obtained by Pe'chard by boiling sodium tungstate with a

298 PRINCIPLES OF 'CHEMISTRY

highest salt-forming oxide, UO3, shows very feeble acid properties,
Although it gives sparingly-soluble yellow compounds with alkalis,
which fully correspond with the dichromates — for example, Na.2U2Ov
==Na20,2U03,10 — yet it more frequently and easily reacts with acids,-HX,

Solution of peroxide of hydrogen for several minutes. The solution rapidly turns yellow,
and no longer gives a precipitate of tungstic anhydride when treated with nitric acid.
When evaporated in vacuo the solution leaves a thick syrupy liquid from which ray-like
Crystals separate out ; these crystals are more soluble in, water than the salt originally
taken. When heated they also lose water and oxygen. Their composition answers to
the formula M2W2O82H,,O, where M = Na, NH4) &c. The permolybdates and per-
tungstates have similar properties. When treated with oxygen acids they give peroxide
of hydrogen, and disengage chlorine, and iodiae from hydrochloric acid and potassium
iodide.

Piccini (1891) showed that peroxide of hydrogen not only combines with the oxygen
compounds of Mo and W, but also with their fluo-compounds, among which ammonium
fluo-molybdate MoO2F2 2NH4 and others have long been known. (A few new salts of
similar composition have been obtained by F. Moureu in 1893.) The action of peroxide
of hydrogen upon these compounds gi-ves salts containing a larger amount of oxygen ; for
instance, a solution of MoO2F,/2KFH20 with peroxide Of hydrogen gives a yellow solu-
tion which after cooling separates out yellow crystalline flakes of Mo03F22KFHoO, resem-
bling the salt originally taken in their external appearance. By employing a similar method
Piccini also obtained : MoO3Fo2RbFH20— yellow monoclinic crystals ; MoO3F82CsFHjO,
—yellow flakes, and the corresponding tungstic compounds. All these salts re-act like
peroxide of hydrogen.

In speaking of these compounds I for my part think it may be well to call attention
to the fact that, in the first place, the composition of Piccini's oxy-fluo compounds does
not correspond to that of permolybdic and pertungstic acid. If the latter be expressed
by formulae with one equivalent of an element, they will be HMoO4 and HWO^ and the
oxy-fluo form corresponding to them should have the composition Mo03F and WO5F
while it contains M03F2 and WO3F2, i.e. answers as it were to a higher degree of oxida-
tion, MoH2O5 and W HO5. But if permolybdic acid be regarded as 2MoO3 + H2 O2, i.e.
as containing the elements of peroxide of hydrogen, then Piccini's compound will also be
found to contain the original salts + H2O ; for example, from MoOjF22KFH3O there is
obtained a compound MoO2F22KFH.jO2, i.e. instead of H2O they contain H2O.;. In the
second place the capacity of the salts of molybdenum and tungsten to retain a further
amount of oxygen or H4O2 probably bears some relation to their property of giving com-
plex acids and of polymerising which has been considered in Note 8 bis. There is,
however, a great chemical interest in the accumulation of data respecting these high
peroxide compounds corresponding to molybdic and tungstic acids. With regard to the
peroxide form of uranium, see Chapter XX., Note 66.

10 Uranium trioxide, or uranic oxide, shows its feeble basic and acid properties in a
great number of its reactions. (1) Solutions of uranic salts give yellow precipitates with
alkalis, but these precipitates do not contain the hydrate of tho oxide, but compounds of
it with bases ; for example, 2UO2(NO3)3 + 6KHO = 4KNO3 + 3H2O + K2U.,O7. There are
other urano-alkali compounds of the same constitution ; for example, (NH4).iU.4O7
(known commercially as uranic oxide), MgU207, BaU2O7. They are the analogues of the
dichromates. Sodium uranate is the most generally used under the name of uranium
yellow, Na.jU2O7. It is used for imparting the characteristic yellow -green tint to glass
and porcelain. Neither heat nor water nor acids are able to extract the alkali from
sodium uranate, Na2U^O7, and therefore it is a true insoluble salt, of a yellow colour, and
clearly indicates the acid character (although feeble) of uranic oxide. (2) The carbonates
of the alkaline earths (for instance, barium carbonate) precipitate uranic oxide from its
salts, as they do all the salts of feeble bases; for example, R2O3. (3), The alkaline car-
bonates, when added to solutions of uranic salts, give a precipitate, which is soluble in

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC.

forming fluorescent yellowish-green salts of the composition U02X2,
and in this respect uranic trioxide, U03, differs from chromic anhydride,
CrO3, although the latter is able to give the oxychloride, Cr02Cl2. In
molybdenum and tungsten, however, we see a clear transition from
chromium to uranium. Thus, for example, chromyl chloride, Cr02Cl2,
is a brown liquid which volatilises without change, and is completely
decomposed by water ; molybdenum oxychloride, MoO2Cl2, is a crys-
talline substance of a yellow colour, which is volatile and soluble in
water (Blomstrand), like many salts. Tungsten oxychloride, WO2C12,
stands still nearer to uranyl chloride in its properties ; it forms yellow
scales on which water and alkalis act, as they do on many salts (zinc
chloride, ferric chloride, aluminium chloride, stannic chloride, &o.), and
perfectly corresponds with the difficultly-volatile salt, UO2C12 (obtained
by Peligot by the action of chlorine on igniteil uranium dioxide, UO2),
which is also yellow and gives a yellow solution with water, like all the

an excess of the reagent, and particularly so if the acid carbonates be taken. This is
due to the fact that (4) the uranyl salts easily form double salts with the salts of the
alkali metals, including the salts of ammonium. Uranium, in the form of these double
salts, often gives salts of well-defined crystalline form, although the simple salts are little
prone to appear in crystals. Such, for example, are the salts obtained by dissolving potas-
sium uranate, K2U2O7, in acids, with the addition of potassium salts of the same acids.
Thus, with hydrochloric acid and potassium chloride a well-formed crystalline salt,
K2(UOS)C14,2H2O, belonging to the monoclinic system, is produced. This salt decom-
poses in dissolving in pure water. Among these double salts we may mention the
double carbonate with the alkalis, R4(UO2)(COS)3 (equal to 2R2CO3 + UO2CO3) ; the
acetates, R(UO2)(CiH3Ov)3— for instance, the sodium salt, Na(UO2)(OjH3O2)3, and the
potassium salt, K(UO2)(C2H3O2)5,H2O; the sulphates, R.2(UO2)(SO4)3,2H2O, &c. In the
preceding formula R = K, Na, NH4, or R2 = Mg, Ba, &"c. This property of giving
comparatively stable double salts indicates feebly developed basic properties, because
double salts are mainly formed by salts of distinctly basic metals (these form, as it were,
the basic element of a double salt) and salts of feebly energetic bases (these form the acid
element of a double salt), just as the former also give acid salts ; the acid of the acid
salts is replaced in the double salts by the salt of the feebly energetic base, which, like
water, belongs to the class of intermediate bases. For this reason barium does not
give double salts with alkalis as magnesium does, and this is why double salts are
more easily formed by potassium than by lithium in the series of the alkali metals.
(5) The most remarkable property, proving the feeble energy of uranic oxide as a base, is
Been in the fact that when their composition is compared with that of other salts those of
oranic oxide, always appear as basic salts. It is well known that a normal salt, RaX6;
corresponds with the oxide R2O3, where X= Cl, NO3, &c.,orX-, = S04, CO3, &c. ; but there
also exist basic salts of the same type where X = HO or X^ = O. We saw salta of all
kinds among the salts of aluminium, chromium, and others. With uranic oxide no salts
are known of the types UX6 (UC16, U(SO4)3, alums, &c., are not known), nor even salts,
U(HO)iX4 or UOX4, but it always forms salts of the type U(HO)4X2 or UO2X2.
Judging from the fact that nearly all the salts of uranic oxide retain water in crystallising
from their solutions, and that this water is difficult to separate from them, it may be
thought to be water of hydration. This is seen in part from the fact that the composition
of many of the salts of uranic oxide may then be expressed without the presence of water
Of crystallisation; for instance, U(HO)4K2C14 (and the salt of NH4, U(HO)4K2(S04) ,
U(HO)4(C8H3O2)2.. Sodium uranyl acetate however does not contain water.

300 PRINCIPLES OP CHEMISTRY

salts UO2X2. The property of uranic oxide, U03, of forming salts
UO2X2 is shown in the fact that the hydrated oxide of uranium,
UO2(HO).2, which is obtained from the nitrate, carbonate, and other
salts by the loss of the elements of the acid, is easily soluble in acids,
as well as in the fact that the lower grades of oxidation of uranium are
able, when treated with nitric acid, to form an easily crystallisable
uranyl nitrate, UO2(N03)2,6H20 ; this is the most commonly occurring
uranium salt.11

Uranium, which gives an oxide, UO3, and the corresponding salt
tT02X2 and dioxide U02, to which the salts UX4 correspond, is rarely
met with in nature. Uranite or the double orthophosphate of uranio

11 Uranyl nitrate, or uranium nitrate, UO^NO^jBH^O, crystallises from its solu-
tions in transparent yellowish- green prisms (from an acid solution), or in tabular crystals
(from a neutral solution), which effloresce in the air and are easily soluble in water,
alcohol, and ether, have a sp. gr. of 2'8, and, fuse when heated, losing nitric acid and water
in the process. If the salt itself (Berzelius) or its alcoholic solution (Malaguti) be
heated up to the temperature at which oxides of nitrogen are evolved, there then remains
a mass which, after being evaporated with water, leaves uranyl hydroxide, UOa(HO)4
(sp. gr. 5'93), whilst if the salt be ignited there remains the dioxide, UO^, as a brick-red
powder, which on further heating loses Oxygen and forms the dark olive uranoso- uranio
oxide, U3O8. The solution of the nitrate obtained from the ore is purified in the following
manner : sulphurous anhydride is first passed through it in order to reduce the arsenic
acid present into arsenious acid ; the solution is then heated to 60°, and sulphuretted
hydrogen passed through it ; this precipitates the lead, arsenic, and tin, and certain
other metals, as sulphides, insoluble in water and dilute nitric acid. This liquid is then
filtered and evaporated with nitric acid to crystallisation, and the crystals are dissolved
in ether. Or else the solution is first treated with chlorine in order to convert the ferrous
chloride (produced by the action of the hydrogen sulphide) into ferric chloride, the
oxides are then precipitated by ammonia, and the resultant precipitate, containing the
oxides Fe2O3,UO3, and compounds of the latter with potash, lime, ammonia, and other
bases present in the solution (the latter being due to the property of uranic oxide of
combining with bases), is washed and dissolved in a strong, slightly-heated solution of
ammonium carbonate,,which dissolves the uranic oxide but not the ferric oxide. The
solution is filtered, and on cooling deposits a well-crystallising uranyl ammonium car-
bonate, U0.4(NH4)4(CO3)3, in brilliant monoclinic crystals which on exposure to air slowly
give off water, carbonic anhydride, and ammonia; the same decomposition is readily
effected at 300°, the residue then consisting of uranic oxide. This salt is not very soluble
in water, but is readily so in ammonium carbonate; it is obvious that it may readily be
converte'd into all the other salts of oxides of uranium. Uranium salts are also purified
in the form of acetate, which is very sparingly soluble, and is therefore directly precipi-
tated from a strong solution of the nitrate by mixing it with acetic acid.

We may also mention the uranyl phosphate, HUPO6, which must be regarded as an
orthophosphate in which two hydrogens are replaced by the radicle uranyl, UO.^, i.e. as
H(UO2)PO4. This salt is formed as a hydrated gelatinous yellow precipitate, on mixing
a solution of uranyl nitrate with disodium phosphate. The precipitation occurs in the
presence of acetic acid, but not in the-presence of hydrocMoric acid. If moreover an
excess of an ammonium salt be present, the ammonia enters into the composition of
the bright yellow gelatinous precipitate formed, in the proportion UO.2NH4PO4. This
precipitate is not soluble in water and acetic acid, and its solution in inorganic acida
when boiled entirely expels all the phosphoric acid. This fact is taken advantage of for
removing phosphoric acids from solutions— for instance, from those containing salts of
calcium and magnesium.

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 301

oxide, R(U02)H2P2O8,7H20, where R=Cu or Ca, uranium- vitriol
U(S04)2,H2O, samarakite, and seschynite, are very rarely found, and
then only in small quantities. Of more frequent and abundant
occurrence is the non-crystalline, earthy brown uranium ore known as
pitchblende (sp. gr. 7*2), which is mainly composed of the intermediate
oxide, TJ3Og= U02,2UO3. This ore is found at Joachimsthal in Bohemia
and in Cornwall. It usually contains a number of different impuri-
ties, chiefly sulphides and arsenides of lead and iron, as well as lime
and silica compounds. In order to expel the arsenic and sulphur it is
roasted, ground, washed with dilute hydrochloric acid, which does not
dissolve the uranoso-uranic oxide, U3O8, and the residue is dissolved
in nitric acid, which transforms the uranium oxide into the nitrate,
U02(N03)2.

It must be observed that the oxide of uranium, first distinguished
by Klaproth (1789), was for a long time regarded as able to give
metallic uranium under the action of charcoal and other reducing agents
(with the aid of heat). But the substance thus obtained was only the
uranium dioxide, UO2. The compound nature of this dioxide,12 or the
presence of oxygen in it, was demonstrated by Peligot (1841), by igniting
it with charcoal in a stream of chlorine. He thus obtained a volatile
uranium tetrachloride* UC14,13 which, when heated with sodium, gave

13 Uranium dioxide, or uranyl, UO , which 'is contained in the salts UO<jX2, has
the appearance and many of the properties of a metal. Uranic oxide may be regarded as
•uranyl oxide, (U02)O, its salts as salts of this uranyl ; its hydroxide, (UOo)H2O2, is consti-
tuted like CaILjO2. The green oxide of uranium, uranoso-uranic oxide (easily formed from
uranic salts by the loss of oxygen), U5Og = UOo,2UO3, when ignited with charcoal or
hydrogen (dry) gives a brilliant crystalline substance of sp. gr. about ll'O (Urlaub), whose
appearance resembles that of metals, and decomposes steam at a red heat with the
evolution of hydrogen ; it does not, however, decompose hydrochloric or sulphurio
acid, but is oxidised by nitric acid. The same substance (i.e. uranium dioxide UO^) is
also obtained by igniting the compound (UO.yjILjC^ in a stream of hydrogen, according"
to the equation UO..K.Cl4 + Hi = UO2 + 2HCl + 2KCl. It was at first regarded as the
metal, In 1841 Peligot found that it contained oxygen, because carbonic oxide and
anhydride were evolved when it was ignited with charcoal in a stream of chlorine, and
from 272 parts of the substance which was considered to be metal he obtained 882 parts
of a volatile product containing 142 parts of chlorine. From this it was concluded that
the substance taken contained an equivalent amount of oxygen. As 142 parts of chlorind
correspond with 32 parts of oxygen, it followed that 272-82 = 240 parts of malal were
combined in the substance with 82 parts of oxygan, and also in the chlorine compound
obtained with 142 parts of chlorine. These calculations have been made for the now
accepted atomic weight of uranium (U = 240, see Note 14). Peligot took another atomic.
weight, but this does not alter the principle of the argument.

15 Uranium tetrachloride, uranous chloride, UC14, corresponds with uranous oxide
as a base. It was obtained by Peligot by igniting uranic oxide mixed with charcoal in a-
stream of dry chlorine: UO3 + 8C + 2Clj-UCl4-h3CO. This green volatile compound
(Note 12) crystallises in reg_ular octahedra, is very hygroscopic, easily soluble in water,
with the development of a considerable amount of heat, and no longer separates out
from its solution in an anhydrous state, but disengages hydrochloric acid when

302 PRINCIPLES OF CHEMISTRY

metallic uranium as a grey metal, having a specific gravity of 187, and
liberating hydrogen from acids, with the formation of green urarious
•salts, UX4, which act as powerful reducing agents.14

rated. The solution of uranous chloride in water is green. It is also formed by the
action of zinc and copper (forming cuprous chloride) on a solution of uranyl chloride,
UO.^Cli, especially in the presence of hydrochloric acid and sal-ammoniac. Solutions of
uranyl salts are converted into uranous salts by the action of various reducing agents,
and among others by organic substances or by the action of light, whilst the salts UX4
are converted into uranyl salts, UO2X^, by exposure to air or by oxidising agents. Solu-
tions of the green uranyl salts act as powerful reducing agents, and give a brown precii
pitate of the uranous hydroxide, UH4O4, with potash and other alkalis. This hydroxide
is easily soluble in acids but not in alkalis On ignition it does not form the oxide UO2l
because it decomposes water, but when the higher oxides of uranium are ignited in a
stream of hydrogen or with charcoal they yield uranous oxide. Both it and the chloride
UC14 dissolve in strong sulphuric acid, forming a green salt, U(SO4)o,2H2O. The same
salt, together with uranyl sulphate, UO^SO.^, is formed when the green oxide, U5O8, is
dissolved in hot sulphuric acid. The salts obtained in the latter instance may be
separated by adding alcohol to the solution, which is left exposed to the light ; the alcohol
reduces the uranyl salt to uranous salt, an excess of acid being required. An excess of
water decomposes this salt, forming a basic salt, which is also easily produced under
.other circumstances, and contains UO(SO4),2H2O (which corresponds to the uranic salt).

14 The atomic weight of uranium was formerly taken as half the present one, U = 120,
And the oxides U2O3, suboxide UO, and green oxide U3O4, were of the same types as the
•oxides of iron. With a certain resemblance to the elements of the iron group, uranium
presents many points of distinction which do not permit its being grouped with them.
Thus uranium forms a very stable oxide, 17.^05(11 = 120), but does not give the corre-
sponding chloride U8C16 (Roscoe, however, in 1874 obtained UC15, like MoCl3 and WC15),
and under those circumstances (the ignition of oxide of uranium mixed with charcoal, in
a~stream of chlorine), when the formation of this.compound might be expected, it gives
XU = 120) the chloride UClj, which is characterised by its volatility; this is not a pro-
perty, to such an extent, of any of the bichlorides, RC18, of the iron group.

The alteration or doubling of the atomic weight of uranium — i.e. the recognition of
U = 240 — was made for the first time in the first (Russian) edition of this work (1871), and
in my memoir of the same year in Liebig'a Annalen, on the ground that with an atomic
weight 1'20, uranium could not be placed in the periodic system. I think it will not be super-
fluous to add the following remarks on this subject : (1) In the other groups (K — Rb — Cs,
Ca— Sr— Ba, Cl— Br— I) the acid character of the.oxides decreasesand their basic charac-
ter increases with the rise of atomic weight, and therefore we should expect to find the
same in the group Cr — Mo — W — U, and if CrO3, Mo03, W03 be the anhydrides of acids
then we indeed find a decrease in their acid character, and therefore uranium trioxide,
JJO3, should be a very feeble anhydride, but its basic properties should also be very
feeble. Uranic oxide does indeed show these properties, as was pointed out. above (Note
10). (2) Chromium and its analogues, besides the oxides RO3, also form lower grades of
•oxidation RO2, R%Qz> and the same. is seen in uranium; it forms U03, UO2, U^O.-, and
their compounds. (3) Molybdenum and tungsten, in being reduced from RO->( easily and
frequently give an intermediate oxide of a blue colour, and uranium shows the same
property ; giving the so-called green oxide which, according to present views, must
be regarded as U3O6 = UO22y03, analogous to Mo3O8. (4) The higher chlorides, RCld,
possible for the elements of this group, are either unstable (WC1J or do not exist at all
{Cr) ; but there is one single lower volatile compound, which is decomposed, by water,
and liable to further reduction into a non-volatile chlorine product and the metal. The
same is observed in uranium, which forms an easily volatile chloride, UC14, decomposed
by water. (5) The high sp. gr. of uranium (18'6) is explained by its analogy to tungsten
<i(sp. gr, 19'1). (6) For uranium, as fox chromium and tungsten, yellow tints pre-

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 303

As the salts of uranic oxide are reduced in the absence of organic
matter by the action of light, and as they impart a characteristic
coloration to glass,15 they find a certain application in photography and
glass work.

If we compare together the highly acid elements, sulphur, selenium,
and tellurium, of the uneven series, with chromium, molybdenum,
tungsten, and uranium of the even series, we find that the resemblance
of the properties of the higher form RO3 does not extend to the lower
forms, and even entirely disappears in the elements, for there is
not the smallest resemblance between sulphur and chromium and their
analogues in a free state. In other words, this means that the small
periods, like Na, Mg, Al, Si, P, S, Cl, containing seven elements, do
not contain any near analogues of chromium, molybdenum, &c., and
therefore their true position among the other elements must be looked
for only in those large periods which contain two small periods, and
whose type is seen in the period containing : K, Ca, Sc, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br. These large periods contain
Ca and Zn, giving RO, Sc, and Ga of the third group, Ti and Ge
giving R02, V and As forming R2O5, Cr and Se of the sixth group,
Mn and Br of the seventh group, and the remaining elements, Fe,
fclo, Ni^ form connective members of the intermediate eighth group, to
the description of the representatives of which we shall turn in the
following chapters. We will now proceed to describe manganese,
Mn = 55, as an element of the seventh group of the even series, directly
following after Cr — 52, which corresponds with Br=80 to the same
degree that Cr does with Se = 79. For chromium, selenium, and
bromine very close analogues are known, but for manganese as yet
none have been obtained — that is, it is the only representative of the
even series in the seventh group. In placing manganese with the

dominate in the form R05, whilst the lower forms are green and blue. (7) Zimmermann
(1881) determined the vapour densities of uranous bromide, UBr4, and chloride, UC14
(19'4 and 13'2), and' they were found to correspond to the formulae giver* above— that is,
they confirmed the higher atomic weight U = 240. Roscoe, a great authority on the
metals of this group, was the first to accept the proposed atomic weight of uranium,
U = 240, which since Zimmerraann's work has been generally recognised.

15 Uranium glass, obtained by the addition of the yellow salt K2U207 to glass, has a
green yellow fluorescence, and is sometimes employed for ornaments ;• it absorbs the
violet rays, like the other salts of uranic oxide — that is, it possesses an absorption spec-
trum in which the violet rays are absent. The index of refraction of the absorbed rays
is altered, and they are given out again as greenish-yellow rays ; hence, compounds of
uranic acid, when placed in the violet portion of the spectrum, emit a greenish-yellow
light, and this forms one of the best examples (another is found in a solution of quinine
sulphate) of the phenomenon of fluorescence. The rays of light which pass through
urauic compounds do not contain the rays which excite the phenomena of fluorescence
and of chemical transformation, as the researches of Stokes prove.

804 PRINCIPLES OF CHEMISTRY

halogens in one group, the periodic system of the elements only requires
that it should bear an analogy to the halogens in the higher type of
oxidation— i.e. in the salts and acids — whilst it requires that as great
a difference should be expected in the lower types and elements as there
exists between chromium or molybdenum and sulphur or selenium.
And this is actually the case. The elements of the seventh group form
a higher salt-forming oxide, R2O7, and its corresponding hydrate,
HR04, and salts— for example, KC1O4. Manganese in the form of
potassium permanganate, KMnO4, actually presents a great analogy in
many respects to potassium perchlorate, KC1O4. The analogy of the
crystalline form of both salts was shown by Mitscherlich. The salts of
permanganic acid are also nearly all soluble in water, like those of
perchloric acid, and if the silver salt of the latter, AgClO4, be sparingly
soluble in water, so also is silver permanganate, AgMn04. The specific
volume of potassium perchlorate is equal to 55, because its specific
gravity =2-54 ; the specific volume of potassium permanganate is equal
to 58, because its specific gravity =2-71. So that the volumes of
equivalent quantities are in this instance approximately the same
whilst the atomic volumes of chlorine (35-5/1-3 = 27) and manganese
(55/7-5) are in the ratio 4 : 1. In a free state the higher acids HC104
and HMn04 are both soluble in water and volatile, both are powerful
oxidisers — in a word, their analogy is still closer than that of chromic
and sulphuric acids, and those points of distinction which they present
also appear among the nearest analogues — for example, in sulphuric and
telluric acids, in hydrochloric and hydriodic acids, &c. Besides Mn2O7
manganese gives a lower grade of oxidation, Mn03, analogous to
sulphuric and chromic trioxides, and with it corresponds potassium
manganate, K2MnO4, isomorphous with potassium sulphate.16 In the
still lower grades of oxidation, Mn2O3 and MnO, there is hardly any
similarity to chlorine, whilst every point of resemblance disappears
when we come to the elements themselves — i.e. to manganese and
chlorine — for manganese is a metal, like iron, which combines directly
with chlorine to form a satine compound, MnCl2, analogous to magne-
sium chloride.17

Manganese belongs to the number of metals widely distributed in

16 The comparison of potassium permanganate with potassium perchlorate, or of
potassium manganate with potassium sulphate, shows directly that many of the physical
and chemical propertieo of substances do not depend on the nature of the elements,,
but on the atomic types in which they appear, on the kind of movements, or on the posi-
tions in which the atoms forming the molecule occur.

17 If, however, we compare the spectra (Vol. L p. 665) of chlorine, bromine, and
iodine with that of manganese, a certain resemblance or analogy is to be found connect-
ing manganese both to iron and to chlorine, bromine, and iodine.

CHROMIUM MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 305

nature, especially in those localities where iron occurs, whose ores
frequently contain compounds of manganous oxide, MnO, which presents
a resemblance to ferrous oxide, FeO, and to magnesia. In many minerals
magnesia and the oxides allied to it are replaced by manganous oxide;
calcspars and magnesites — i.e. R"C03 in general — are frequently met
with containing mauganous carbonate, which also occurs in a separate
state, although but rarely. The soil also and the ash of plants generally
contain a small quantity of manganese. In the analysis of minerals
it is generally found that manganese occurs together with magnesia,
because, like it, manganous oxide remains in solution in the presence of
ammoniacal salts, not being precipitated by reagents. The property of
this manganous oxido, MnO, of passing into the higher grades of oxida-
tion under the influence of heat, alkalis, and air, gives an easy means
not only of discovering the presence of manganese in admixture -with
magnesia, but also of separating these two analogous bases. Magnesia is
not able to give higher grades of oxidation, whilst manganese gives them
with great facility. Thus, for instance, an alkaline solution of sodium
hypochlorite produces a precipitate of manganese dioxide in a solution of
a manganous salt : MnCl2 + NaClO + 2NaHO = Mn02 + H20 + 3NaCl •
whilst magnesia is not changed under these circumstances, and remains
in the form of MgCl2. If the magnesia be precipitated owing to the
^presence of. alkali, it may be dissolved in acetic acid, in which manganese
tHoxicle is insoluble. The presence of small quantities of manganese
may also be recognised by th6 green coloration which alkalis acquire
when heated with manganese compounds in the air. This green colora-
tion depends on the property of manganese of giving a green alkaline
manganate : MnCl.2-f 4KHO + O2=K2Mn04 +2KC1 +-2H2O. Thus
the faculty of oxidising in the presence of alkalis forms an essential
character of manganese. The higher grades of oxidation containing
Mn207 and Mn03 are quite unknown in nature, and even Mn02 is not
so widely spread in nature as the ores composed of manganous com-
pounds which are met with nearly everywhere. The most important
ore of manganese is its dioxide, or so-called peroxide, Mn02, which is
known in mineralogy as pyrolusite. Manganese also occurs as an
oxide corresponding with magnetic iron ore, MnO,Mn2O3=Mn304,
forming the mineral known as hausmannite. The oxide .Mn2Q3 also
occurs in nature as the anhydrous mineral braunite, and in a hydrated
form, Mn2O3,H20, called manganite. Both of these often occur as an
admixture in pyrolusite. Besides which, manganese is met with in
nature as a rose-coloured mineral, rhodonite, or silicate, MnSi03. Very
fine and rich deposits of manganese ores have been found in. the
Caucasus, the Urals, and along the Dnieper. Those at the Sharapansky

806 PRINCIPLES OF CHEMISTRY

district of the Government of Kutais and at Nicopol on the Dnieper
are particularly rich. A large quantity of the ore (as much as 100,000
tons yearly) is exported from these localities.

Thus manganese gives oxides of the following forms MnO,
manganous oxide, and manganous salts, MnX2, corresponding with the
base, which resembles magnesia and ferrous oxide in many respects ;
Mn203, a very feeble base, giving salts, MnX3, analogous to the
aluminium and ferric salts, easily reduced to MnX2 ; MnO2, dioxide,
generally called peroxide, an almost indifferent Oxide, or feebly acid ; 18
MnO3, manganic anhydride, which forms salts resembling potassium
sulphate ;I8dis Mn2O7, permanganic anhydride, giving salts analogous
to the perchlorates.

All the oxides of manganese when heated with acids give salts, MnX2,
corresponding with the lower grade of oxidation, manganous oxide,
MnO. Manganic oxide, Mn.2O3, is a feebly energetic base ; it is true
that it dissolves in hydrochloric acid and gives a dark solution con-
taining the salt MnCl,, but the latter when heated evolves chlorine
and gives a salt corresponding with manganous oxide MnCl2 — i.e. at
first: Mn2O3 + 6HCl==3H2O + Mn2Cl6, and then the- Mn2Cl6 decom-
poses into 2MnCl2-rCl2. None of the remaining higher grades of
oxidation have a basic character, but act as oxidising agents in the
presence of aiids, disengaging oxygen and passing into salts of the lower
grade of oxidation of 'manganese, MnO. Owing to this circumstance,
the manganous salts are often obtained ; they are, for instance, left in
the residue when the dioxide is used for the preparation of oxygen and
chlorine.19

18 The name 'peroxide' should only be retained for those highest oxides (and MnO2
stands between MnO and MnOs) which either by a direct method of double decomposition
are able to give hydrogen peroxide or contain a larger proportion of oxygen than the
base or the acid, just as hydrogen peroxide contains more oxygen than water. Their
type will be H2O2, and they are exemplified by barium peroxide, BaO2, and sulphur
peroxide, S2O7> &c. Such a dioxide as MnO2 is, in all probability, a salt— that is, a
manganous manganate, MnO3MnO, and also, as a basic salt of a feeble base, capable of
combining with alkalis and acids. Hence the name of manganese peroxide should
be abandoned, and replaced by manganese dioxide. PbO2 is better termed lead dioxide
than peroxide. Bisulphide of manganese, MnS2, corresponding to iron pyrites, FeS.j,
sometimes occurs in nature in fine octahedra (and cube combinations), for instance, in
Sicily ; it is called Hauerite.

18 bis On comparing the manganates with the permanganates— for example, K2MnO.j
with KMnO4 — we find that they differ in composition by the abstraction of on<* equivalent
of the metal. Such a relation in composition produced by oxidation is of frequent
occurrence— for instance, K4Fe(CN)6 in oxidising gives K3Fe(CN)6 ; H2S04 in oxidising
gives persulphuric acid, HSO4, or H2S7O8 ; H^O forms HO or H2O2, &c.

w In the preparation of oxygen from the dioxide by means of H2S04, MnSO4 is
formed ; in the preparation of chlorine from HC1 and MnOa, MnClj is obtained. These
two manganous salts may be taken as examples of compounds MnX-z. Manganoua
sulphate generally contains various impurities, and also a large amount of iron salt

CHROMIUM, MOLYBDENUM, TUNGSTEN, UBANIUM, ETC. 307

As the salts of manganous oxide MnX2 closely resemble (and are
isomorphous with) the salts of magnesia MgX2 in many respects (with

<from the native MnO.J, from which it cannot be freed by crystallisation. Their
removal may, however, ba effected by mixing a portion of the liquid with a solution of
sodium carbonate ; a precipitate of manganous carbonate is then formed. This pre-
cipitate is collected and washed, and then added to the remaining mass of the impure
solution of manganous sulphate; on heating the solution with this precipitate, the
whole of the iron is precipitated as oxide. This is due to the fact that in the solution of
the manganese dioxide in sulphuric acid the whole of the iron is converted- into the
ferric state (because the dioxide acts as an oxidising agent), which, as an exceedingly
feeble base precipitated by calcium carbonate and other kindred salts, is also precipitated
by manganous carbonate. After being treated in this manner, the solution of manganous
sulphate is further purified by crystallisation. If it be a bright red colour, it is due to
the presence of higher grades of oxidation of manganese ; they may be destroyed by
boiling the solution, when the oxygen from the oxides of manganese is evolved and a
very faintly coloured solution of manganous sulphate is obtained. This salt is remarkable
for the facility with which it gives various combinations with water. By evaporating
the almost colourless solution of manganous sulphate at very low temperatures, and by
cooling the saturated solution at about 0°, crystals are obtained containing 7 atoms of
water of crystallisation, MnSO.^TH.jO, which ara isomorphous with cobaltous and ferrous
sulphates. These crystals, even at 10°, lose 5 p.c. of water, and completely effloresce at
15-", losing about 20 p.c. of water. By evaporating a solution of the salt at the ordinary
temperature, but not above 20°, crystals are obtained containing 5 mol. H2O, which
are isomorphous with copper sulphate ; whilst if the crystallisation be carried on between
20 J and 30°, large transparent prismatic crystals are formed containing 4 mol. H2O (see
Nickel). A boiling solution also deposits these crystals together with crystals containing
8 mol. H2O, whilst the first salt, when fused and boiled with alcohol, gives crystals
containing 2 mol. H.^O. Graham obtained a monohydrated salt by drying the salt at
about 200 ^ The last atom of water is eliminated with difficulty, as is the case with all
salts like MgSO^iH^O. The crystals containing a considerable amount of water are
rose-coloured, and the anhydrous crystals are colourless. The solubility of MnSO4,4H2Q
(Chapter I., Note 24) per 100 parts of water is : at 10°, 127 parts; at 870'5, 149 parts ; at
75°, 145 parts ; and at 101°, 92 parts. Whence it is seen that at the boiling-point this salt
is less soluble than at lower temperatures, and therefore a solution saturated at the
ordinary temperature becomes turbid when boiled. Manganous sulphate, being analogous
to magnesium sulphate, is decomposed, like the latter, when ignited, but it does not then
leave manganous oxide, but the intermediate oxide, Mn3O4. It gives double salts with
the alkali sulphates. With aluminium sulphate it forms fine radiated crystals, whose
composition resembles that of- the alums— namely, MiiA^SC.^.,, 24HjO. This salt is
easily' soluble in water, and occurs in nature.

Manganous chloride, MC12, crystallises with 4 mol. HoO, like the ferrous salt, and
not with 6 mol. H.2O like many kindred salts — for example, those of cobalt, calcium, and
magnesium ; 100 parts of water dissolve 88 parts of the anhydrous salt at 10° and 55.
parts at 62°. Alcohol also dissolves manganous chloride, and the alcoholic solution
burns with a red flame. This salt, like magnesium chloride, readily forms double salts.
A solution of borax gives a dirty rose-coloured precipitate having the composition
MnH^BOj^H.O, which is used as a drier in paint-making. Potassium cyanide pro-
duces a yellowish-grey precipitate, MnC.^No, with manganous salts, soluble in »n excess of
the reagent, a double salt, K4MnC(<Ne, corresponding with potassium ferrocyanide,
being formed. On evaporation of this solution, a portion of the manganese is oxidised
And precipitated, whilst a salt corresponding to Gmelin's red salt, K^MnCBN6 (see
Chapter .XXII.), remains in solution. Sulphuretted hydrogen does not precipitate
salts of manganese, not even the acetate, but ammonium sulphide gives a flesh-coloured
precipitate, MnS ; at 320° this sulphide of manganese passes into a green variety (Antony).
Oxalic acid in strong solutions of manganous salts gives a white precipitate of the

308 PRINCIPLES OF CHEMISTRY

the exception of the fact that MnX2 are rose coloured and are easily
oxidised in the presence of alkalis), we will- not dwell upon them, but

oxalate, MnC2O4. This precipitate is insoluble in water, and is used for the preparation
of manganous oxide' itself because it decomposes like oxalic acid when ignited (in a tube
without access of air), with the formation of carbonic anhydride, carbonic oxide, and
manganous oxide. Manganous oxide thus" obtained is a green powder, which however
oxidises with such facility that it burns in air when brought into contact with an
incandescent substance, and passes into the red intermediate oxide Mn304. In solutions
of manganous salts, alkalis produce a precipitate of the hydroxide MnH2O2, which
rapidly absorbs oxygen in the presence of air and gives the brown intermediate oxide,
or, more correctly speaking, its hydrate.

Manganous oxide, besides being obtained by the above-described method from man-
ganous oxalate, may also be obtained by igniting the higher oxides jn a stream of
hydrogen, and also from manganese carbonate. The manganous oxide ignited in the
presence of hydrogen acquires a great density, and is no longer so easily oxidised. It
may also be obtained in a crystalline form, if during the ignition of the carbonate or
higher oxide a trace of dry hydrochloric acid gas be passed into the current of hydrogen.
It Is thus obtained in the form of transparent emerald green crystals of the regular
system, and in this state is easily soluble in acids.

Manganous oxide in oxidising gives the red oxide of manganese, Mn3O4. This is the
most stable of all the oxides of manganese ; it is not only stable at the ordinary but also
at a high temperature — that is, it does not absorb or disengage oxygen spontaneously.
When ignited, all the higher oxides of manganese pass into it by losing oxygen, arid
manganous oxide by absorbing oxygen. This oxide does pot give any distinct salts,
but it dissolves in sulphuric acid, forming a dark red solution, which contains both
manganous and manganic (of the oxide, Mn2O3) sulphates. The latter with potassium
sulphate gives a manganese alum, in which the alumina is replaced by its isomorphoua
oxide of manganese. But this alum, like the solution of the intermediate oxide in sul-
phuric acid, evolves oxygen and leaves a manganous 'salt when slightly heated.

Manganese dioxide is still less basic than the oxide, and disengages oxygen or a
halogen in the presence of acids, forming manganous salts, like the oxide. However, if it
be suspended in ether, and hydrochloric acid gas passed into the mixture, which is kept
cool, the ether acquires a green colour, owing to the formation of tetra- chloride of
manganese, MnCl4, corresponding with the dioxide which passes into solution. It is
however very unstable, being exceedingly easily decomposed with the evolution of
chlorine. The corresponding fluoride, MnF4, obtained by Nickles is much more stable.
At all events, manganese dioxide does not exhibit any well-defined basic character, but
has rather an acid character, which is particularly shown in the compounds MnF4 and
MnCl4 just mentioned, and in the property of manganese dioxide of combining with
alkalis. If the higher grades of oxidation of manganese be deoxidised in the presence of
alkalis, they frequently give the dioxide combined with the alkali— for example, in the
presence of potash a compound is formed which contains K20,5MnO2, which shows the
weak acid character of this oxide. When ignited in the presence of sodium compounds
manganese dioxide frequently forms Na2O,SMnOa and Na2O,12Mn02, and lime when
heated with MnO2 gives from CaO,3MnO2 to (CaO.)2,MnO2 (Rousseau) according to the
temperature. Besides which, perhaps, MnO.> is a saline compound, containing
MnOMnO3 or (MnO)5Mn2O7, and there are reactions which support such a view (Spring,
Richards, Traube, and others) ; for instance it is known that manganous chloride and
potassium permanganate give the dioxide in the presence of alkalis.

Manganese dioxide may be obtained from manganous salts by the action of oxidis-
ing agents. If manganous hydroxide or carbonate be shaken up in water through
which chlorine is passed, the hypochlorite of the metal is not formed, as is the case
with certain other oxides, but manganese dioxide is precipitated • 2MnO2H2 + Cla
= MnCl2 + MnO2,H3O + H2O. Owing to this fact, hypochlorites in the presence of alkalis
and acetic acid when added to a solution of manganous salts give hydrated manganese

CHROMIUM, MOLYBDENUM, TUNGSTEN URANIUM, ETC. 309

limit ourselves to illustrating the chemical character of manganese by
describing the metal and its corresponding acids. The fact alone that
the oxides of manganese are not reduced to the metal when ignited in
hydrogen (whilst the oxides of iron give metallic iron under these
circumstances), but only to manganous oxide, MnO, shows that
manganese has a considerable affinity for oxygen — that is, it is difficult
to reduce. This may be effected, however, by means of charcoal or
sodium at a very high temperature. A mixture of one of the oxides of
manganese with charcoal or organic matter gives fused metallic man-
ganese under the powerful heat developed by coke with an artificial
draught. The metal was obtained for the first time in this manner by
Gahn, after Pott, and more especially Scheele, had in the last century
shown the difference between the compounds of iron and manganese
(they were previously regarded as being the same). Manganese is pre-
pared by mixing one of its oxides in a finely-divided state with oil and
soot ; the resultant mass is then first ignited in order to decompose
the organic matter, and afterwards strongly heated in a charcoal crucible.
The manganese thus obtained, however, contains, as a rule, a consider-
able amount of silicon and other impurities. Its specific gravity varies
between 7-2 and 8'0. It has a light grey colour, a feebly metallic
lustre, and although it is very hard it can be scratched by a file. It
rapidly oxidises in air, being converted into a black oxide ; water acts
on it with the evolution of hydrogen — this decomposition proceeds very
rapidly with boiling water, and if the metal contain carbon.20

dioxide, as was mentioned above. Manganous nitrate also leaves manganese dioxide
when heated to 200° It is also obtained from manganous and manganic salts of the
alkalis, when they are decomposed in the presence of a small amount of acid ; the prac-
tical method of converting the salts MnX2 into the higher grades of oxidation is given in
Chapter II., Note 6.

20 Other chemists have obtained manganese by different methods, and attributed
different properties to it. This difference probably depends on the presence of carbon
in different proportions. Deville obtained manganese by subjecting the pure dioxide,
mixed with pure charcoal (from burnt sugar), to a strong heat in a lime crucible until the
resultant metal fused. The metal obtained had a rose tint, like bismuth, and like it
was very brittle, although exceedingly hard. It decomposed water at 'the ordinary
temperature. Brunner obtained manganese having a specific gravity of about 7'2, which
decomposed water very feebly at the ordinary temperature, did not oxidise in air, and
was capable of taking a bright polish, like steel ; it had the grey colour of cast iron, was
very brittle, and hard enough to scratch steel and glass, like a diamond. Brunner's
method waa as follows , He decomposed the manganese fluoride (obtained as a soluble
compound by the action of hydrofluoric acid on manganese carbonate) with sodium, by
mixing these substances together in a crucible and covering the mixture with a layer of
ealt and fluor spar , after which the crucible was first gradually heated until the reaction
began, and then strongly heated in order to fuse the metal separated. Glatzel (1889)
obtained 25 grms. of manganese, having a grey colour and sp. gr. 7'39, by heating a
mixture of 100 grms. of MuClj with 200 grms. KC1 and 15 grms. Mg to a bright white
heat. Moissan and others, by heating the oxides of manganese with carbon in the electric

310 PRINCIPLES OF CHEMISTRY

It has been shown above that if manganese dioxide, or any
lower oxide of manganese, be heated with an alkali in the presence of
air, the mixture absorbs oxygen,21 and forms an alkaline manganate of a
green colour: 2KHO + Mn02 4- O = K2Mn04 + H20. Steam is disen-
gaged during the ignition of the mixture, and if this does not take
place there is no absorption of oxygen. The oxidation proceeds much
more rapidly if, before igniting in air, potassium chlorate or nitre be
added to the mixture, and this is the method of preparing potassium
manganate, K2MnO4. The resultant mass dissolved in a small quantity
of water gives a dark green solution, which, when evaporated under the
receiver of an air pump over sulphuric acid, deposits green crystals of
exactly the same form as potassium sulphate — namely, six-sided prisms
and pyramids. The composition of the product is not changed by being
redissolved, if perfectly pure water free from air and carbonic acid be
taken. But in the presence of even very feeble acids the solution of
this salt changes its colour and becomes red, and deposits manganese
dioxide. The same decomposition takes place when the salt is heated
•with water, but when diluted with a large quantity of unboiled water
manganese dioxide does not separate, although the solution turns red,
This change of colour depends on the fact that potassium manganate,
K2MnO4, whose solution is green, is transformed into potassium per-
manganate, KMnO4, whose solution is of a red colour. The reaction
proceeding under the influence of acids and a large quantity of water

furnace, obtained carbides of manganese — for example, Mn3C — and remarked that the metal
volatilised in the heat of the voltaic arc. Metallic manganese is, however, not prepared on'
a large scale, but only its alloys with carbon (they readily and rapidly oxidise) and ferro-
manganese or a coarsely crystalline alloy of iron, manganese and carbon, which is
smelted in blast-furnaces like pig-iron (see Chapter XXII.) This ferro-manganese is
employed in the manufacture of steel by Bessemer's and other processes (see Chapter
XXII.) and for the manufacture of manganese bronze. However, in America, Green and
Wahl (1895) obtained almost pure metallic manganese on a large scale. They first treat
the ore of MnO.3 with 30 p.c. sulphuric acid (which extracts all the oxides of iron
present in the ore), and then Iveat it in a reducing flame to convert it into MnO, which
they mix with a powder of Al, lime and CaFo (as a flux), and heat the mixture in a
crucible lined with magnesia ; a reaction immediately takes place at a certain temperature,
and a metal of specific gravity 7'3 is obtained, which only contains a small trace of iron.

Manganese gives two compounds with nitrogen, Mn5N2 and Mn^No. They were
obtained by Prelinger (1894) from the amalgam of manganese Mn?Hg5 (obtained on a
mercury anode by the action of an electric current upon a solution of MnCh) ; the
mercury may be removed from this amalgam by heating it in an atmosphere of hydrogen,
and then metallic manganese is obtained as a grey porous mass of specific gravity 7'42.
If this amalgam be heated in dry nitrogen it gives Mn5N2 (grey powder, sp. gr. 6'58), but
if heated in an atmosphere of NH3 it gives (as also does Mn5N2) Mn5N2, (a dark mass
with a metallic lustre, sp. gr. G-21), which, when heated in nitrogen is converted into
Mn5N.;, and if heated in hydrogen evolves NH5 and disengages hydrogen from a solution
of NH4C1. At all events, manganese is a metal which decomposes water more easily
than iron, nickel, and cobalt.

« Volume I. p. 157. Note 7.

CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 811

is expressed in the following manner : 3K2Mn04 -f 2H20=2KMn04
-f MnO2 + 4KHO. If there is a large proportion of acid and the de-
composition is aided by heat, the manganese dioxide and potassium
permanganate are also decomposed, with formation of raanganous salt.
Exactly the same decomposition as takes place under the action of acids
is also accomplished by magnesium sulphate, which reacts in many cases
like an acid. When water holding atmospheric oxygen in solution acts
on a solution of potassium manganate, the oxygen combines directly
with the manganate and forms potassium permanganate, without
precipitating manganese dioxide, 2K2MnO4 + O + H2O = 2KMnO4
+ 2KHO. Thus a solution of potassium manganate undergoes a very
characteristic change in colour and passes from green to red ; hence this
salt received the name of chameleon mineral.2'2

Potassium permanganate, KMnO4, crystallises in well-formed, long
red prisms with a bright green metallic lustre. In the arts the potash
is frequently replaced by soda, and by other alkaline bases, but no salt
of permanganic acid crystallises so well as the potassium salt, and
therefore this salt is exclusively used in chemical laboratories. One
part of the crystalline salt dissolves in 15 parts of water at the ordinary
temperature, The solution is of a very deep red colour, which is so
intense that it is still clearly observable after being highly diluted with
wa'ter. In a solid state it is decomposed by heat, with evolution of-

23 It was known -to the alchemists by this name, but the true explanation of the>
change in colour is due to the researches of Cbevillot, Edwards, Mitscherlich, and
Forchhammer. The change in colour of potassium manganate is due to its insta-
bility and to its splitting up into two other manganese compounds, a higher and &
lower 8MnO3 = Mn . O7 + MnO.^. Manganese trioxide is really decomposed in this manner
by the action of water (see later) : 8MnO3 + H2O = 2MnHO4 + MnO2 (Franke, Thorpe,
and Humbly). The instability of the salt is proved by the fact of its being deoxidised by
organic matter, with the formation of manganese dioxide .and alkali, so that, for instance,
a solution of this salt cannot be filtered through paper. The presence of an excess of
alkali increases the stability of the salt ; when heated it breaks up in the presence of
water, wif,h the evolution of oxygen.

The method of preparing potassium permanganate will be understood from the above.
There are many recipes for preparing this substance, as it is now used in considerable
quantities both for technical and laboratory purposes. But in all cases the essence of
the methods is one and the same : a mixture of alkali with any oxide of manganese
(even manganous hydroxide, which may be obtained from manganous chloride) is first
heated in the presence of air or of an oxidising substance (for the sake of rapidity, with
potassium chlorate) , the resultant mass is then treated with water and heated, when
manganese dioxide is precipitated and potassium permanganate remains in solution.
This solution may be boiled, as the liquid will contain free alkali ; but the solution
cannot be evaporated to dryness, because a strong solution, as well as the solid salt, ia
decomposed by heat.

By adding a dilute solution of manganous sulphate to a boiling mixture of lead
dioxide and dilute nitric acid, the whole of the manganese may be converted into per«
manganic acid (Crtun)

312 PRINCIPLES OF CHEMISTRY

oxygen, a residue consisting of the lower oxides of manganese ancT
potassium oxide being left.22bi3 A mixture of permanganate of potas-
sium, phosphorous and sulphur takes fire when struck or rubbed, a
mixture of the permanganate with carbon only takes h're when heated,
not when struck. The instability of the salt is also seen in the fact
that its solution is decomposed by peroxide of hydrogen, which at the
same time it decomposes. A number of substances reduce potassium
permanganate to manganese dioxide (in which case the red solution
becomes colourless).23 Many organic substances (although far from
all, even when boiled in a solution of permanganate) act in this manner,
being oxidised at the expense of a portion of its oxygen. Thus, a
solution of sugar decomposes a cold solution of potassium permanganate.
In the presence of an excess of alkali, with a small quantity of sugar,
the reduction leads to the formation of potassium manganate, because
2KMn04 + 2KHO=0 + 2K2Mn04 + H2O. With a considerable amount
of sugar and a more prolonged action, the solution turns brown and
precipitates manganese dioxide or even oxide. In the oxidation of
many organic bodies by an alkaline solution of KMnO4 generally three-
eighths of the oxygen in the salt are utilised for oxidation : 2KMn04
=K2O -j-2Mn02 + O3. A portion of the alkali liberated is retained by
the manganese dioxide, and the other portion generally combines with
the substance oxidised, because the latter most frequently gives an acid
with an excess of alkali. A solution of potassium iodide acts in a
similar manner, being converted into potassium iodate at the expense of
the three atoms of oxygen disengaged by two molecules of potassium
permanganate.

In the presence of acids, potassium permanganate acts as an oxidising
agent with still greater energy than in the presence of alkalis. At any
rate, a greater proportion of oxygen is then available for oxidation,
namely, not f , as in the presence of alkalis, but f , because in the first
instance manganese dioxide is formed, and in the second case mangan-
ous oxide, or rather the salt, MnX2, corresponding with it. Thus, for

22 bis The solution of this salt with an. excess of impure commercial alkali generally
acquires a green tint.

23 A solution of potassium permanganate gives a beautiful absorption spectrum
(Chapter XIII.) If the light in passing through this solution loses a portion pf its rays
in it (if one may so account for it), this is partially explained by the increased oxidising
power which the solution then acquires. We may here also remark that a dilute solution
of permanganate of potassium forms a colourless solution with nickel salts, because
the green colour of the solution of nickel salts is complementary to the red. Such a
decolorised solution, containing a large proportion of nickel and a small proportion of
manganese, decomposes after a time, throws down a precipitate, and re-acquires the
green colour proper to the nickel salts. The addition of a solution of a cobalt salt (rose-
red) to the nickel salt also destroys the colour of both salts.

CHKOMIUM, MOLYBDENUM, TUNGSTEN, UKANIUM, ETC. 313

instance, in the presence of an excess of sulphuric acid, the decom-
position is accomplished in the following manner : 2KMn04 + 3H2SO4
==K2S04 + 2MnSO4 + 3H2O + 5O. This decomposition, however, does
not proceed directly on mixing a solution of the salt with sulphuric
acid, and crystals of the salt even dissolve in oil of vitriol without the
evolution of oxygen, and this solution only decomposes by degrees after
a certain time. This is due to the fact that sulphuric acid liberates
free permanganic acid from the permanganate,24 which acid is stable
in solution. But if, in the presence of acids and a permanganate, there

24 If sulphuric acid is allowed to act on potassium permanganate without any special
precautions, a large amount of oxygen is evolved (it may even explode and inflame), and
a violet spray of the decomposing permanganic acid is given off. But if the pure salt
(i.e. free from chlorine) be dissolved in pure well-cooled sulphuric acid, without
any rise in temperature, a green-coloured liquid settles at the bottom of the vessel.
This liquid does not contain any sulphuric acid, and consists of permanganic anhydride,
MnaO7 (Aschoff, Terreil). It is impossible to prepare. any considerable quantity of the
anhydride by this method, as it decomposes with an explosion as it collects, evolving
oxygen and leaving red oxide of manganese. Permanganic anhydride, Mn.^O;, in
dissolving in sulphuric acid, gives a green solution, which (according to Franke, 1887) con-
tains a compound Mn2S010 = (MnO3)iSO4— that is, sulphuric acid in which both hydro-
gens are replaced by the group MnO3, which is combined with OK in permanganate of
potassium. This mixture with a small quantity of water gives Mn.^,O7, according to the
equation: (MnO3)2SO4-t- H^O =» IL^O.^ + Mn207, and when heated to 80° it gives man-
ganese- trioxide, (MnO5J2SO4 + H.jO = 2MnOa + H2SO4 + O. Pure manganese trioxide is
obtained if the solution of (Mn03)2SO4 be poured in drops on to sodium carbonate. Then,
together with carbonic anhydride, a spray of manganese trioxide passes over, which
may be collected in a well-cooled receiver, and this shows that the reaction proceeds
according to the equation . (MnO3)2SO4 + Na^COj - Na.;S04 + 2MnO3 + C02 + O (Thorpe).
The trioxide is decomposed by water, forming manganese dioxide and a solution of
permanganic acid: 8MnO3 + H;O = MnOi + 2HMn04. The same acid is obtained by
dissolving permanganic anhydride in water.

Barium permanganate when treated with sulphuric acid gives the same acid. This
barium salt may be prepared by the action of barium chloride on the difficultly soluble
silver permanganate, AgMnO4, which is precipitated on mixing a strong solution of the
potassium salt with silver nitrate. The solution of permanganic acid forms a bright red
liquid which reflects a dark violet tint. A dilute solution has exactly the same colour
as that of the potassium salt. It deposits manganese dioxide when exposed to the action
of light, and also when heated above 60°, and this proceeds the more rapidly the more
dilute the solution. It shows its oxidising properties in many cases, as already
mentioned. Even hydrogen gas is absorbed by a solution of permanganic acid ; and
charcoal and sulphur are also oxidised by it, as they are by potassium permanganate.
This may be taken advantage of in analysing gunpowder, because when it is treated
with a solution of potassium permanganate, all the sulphur is converted into sulphuric
acid and all the charcoal into carbonic anhydride. Finely-divided platinum immediately
decomposes permanganic acid. With potassium iodide it liberates iodine (which may
afterwards be oxidised into iodic acid) (Mitscherlich, Fromherz, Aschoff, and others).
Ammonia does not form a corresponding salt with free permanganic acid, because it is
oxidised with evolution of nitrogen. The oxidising action of permanganic acid in a
strong solution may be accompanied by flame and the formation of violet fumes of
permanganic acid ; thus a strong solution of it takes fire when brought into contact with
paper, alcohol, alkaline sulphides, fats, &c.

We may add that, according to Franke, 1 part of potassium permanganate with 18

814 PRINCIPLES OF CHEM1STBY

is a substance capable of absorbing oxygen— for instance, capable o£
passing into a higher grade of oxidation — then the reduction of the
permanganic acid into manganous oxides sometimes proceeds directly
at the ordinary temperature. This reduction is very clearly seen,
because the solutions of potassium permanganate are red whilst the
manganous salts are almost colourless. Thus, for instance, nitrous acid
and its salts are converted into nitric acid and decolorise the acid solution,
of the permanganate. Sulphurous anhydride and its salts immediately
decolorise potassium permanganate, forming sulphuric acid. Ferrous
salts, and in general salts of lower grades of oxidation capable of being
oxidised in solution, act in exactly the same manner. Sulphuretted
hydrogen is also oxidised to sulphuric acid ; even mercury is oxidised
at the expense of permanganic acid, and decolorises its solution, being
converted into mercuric oxide. Moreover, the end point of these reactions
may easily be seen, and therefore, having first determined the amount
of active oxygen in one volume of a solution of potassium permanganate,
and knowing how many volumes are required to effect a given oxidation,
it is easy to determine the amount of an oxidisable substance in a
solution from the amount of permanganate expended (Marguerite's
method).

The oxidising action of KMnO4, like all other chemical reactions,
is not accomplished instantaneously, but only gradually. And, as the
course of the reaction is here easily followed by determining the amount
of salt unchanged in a sample taken at a given moment,25 the oxidising
reaction of potassium permanganate, in an acid liquid, was employed by
Harcourt and Esson (1865) as one of the first cases for the investigation
of the laws of the rate of chemical change^ as a subject of great import-
ance in chemical mechanics. In their experiments they took oxalic acid,-

parts of sulphuric acid at 100° gives brown crystals of the salt Mn.2(SO4)3,H2SO4,4H_,0,
which gives a precipitate of hydrated -manganese dioxide, H2MnO5 = MnOjH^O, when
treated with water.

Spring, by precipitating potassium permanganate with sodium sulphite and washing
the precipitate by decantation, obtained a soluble colloidal manganese oxide, whose
composition was the mean between Mn3O5 and MnO2 — namely, MnaO3,4(Mn02HQO).

25 For rapid and accurate determinations of this kind, advantage is taken of those
methods of chemical analysis which are known as ' titrations ' (volumetric analysis), and
consist in measuring the volume of solutions of known strength required for the complete
conversion of a given substance. Details respecting the theory and practice of titration,
in which potassium permanganate is very frequently employed, must be looked for in
works on analytical chemistry.

20 The measurements of velocity and acceleration serve for determining the measure
of forces in mechanics, but in that case the velocities are magnitudes of length or paths
passed over in a unit of .time. The velocity of chemical change embodies a conception of
quite another kind. In the first place, the velocities of reactions are magnitudes of the
masses which have entered into chemical transformations ; in the second place, these
velocities can onlv be relative Quantities. Hence the conceotion of ' velocity ' has auite a

CHKOMIUM, MOLYBDENUM, TUNGSTEN, UKANIUM, ETC. 315

C2H204, which in oxidising gives carbonic anhydride, whilst, with
an excess of sulphuric acid, the potassiym permanganate is converted
into manganous sulphate, MnS04, so that the ultimate oxidation
will be expressed by the equation: 5C2H204-f2MnK04 + 3H2S04
= 10C02 + K2S04 + 2MnS04 + 6H2O. The influence of the relative
amount of sulphuric acid is seen from the annexed table, which gives
the measure of reaction p per 100 parts of potassium permanganate,
taken four minutes after mixing, using n molecules of sulphuric acid,
H2S04, per 2KMnO4 + 5C2H204

n = 2 4 6 8 12 16 22

p =22 36 51 63 77 86 92.

showing that in a given time (4 minutes) the oxidation is the more
perfect the greater the amount of sulphuric acid taken for given amounts
of KMnO4 and C2H2O4. It is obvious also that the temperature and
relative amount of every one of the acting and resulting substances
should show its influence on the relative velocity of reaction ; thus; for
instance, direct experiment showed the influence of the admixture
of manganous sulphate. When a large proportion of oxalic acid (108
molecules) was taken to a large mass of water and to 2 molecules of
permanganate 14 molecules of manganous sulphate were added, the
quantity x of the potassium permanganate acted on (in percentages
of the potassium permanganate taken) in t minutes (at 16°)- was as
follows :

*=2 5 8 11 14 44 47 53 61 68
JB= 5-2 12-1 18-7 25-1 31-3 68-4 717 75-8 79-8 83-0

These figures show that the rate of reaction — that is, the quantity of
permanganate changed in one minute — decreases proportionally to the
decrease in the amount of unchanged potassium permanganate. At the

different meaning in chemistry from what it has in mechanics. Their only common factor
is time. If dt be the increment of time and dx the quantity of a substance changed in
this space of time, then the fraction (or quotient) dx.'dt will express the rate of the
reaction. The natural conclusion, come to both byHarcourt andEsson, and previously to
them (1850) by Wilhelmj (who investigated the rate of conversion, or inversion, of sugar
in its passage into glucose), consists in establishing that this velocity is proportional to
the quantity of substances still unchanged— i.e. that dx'dt = C(A.-x), where C is a
constant coefficient of proportionality, and where A is the quantity of a substance taken
for reaction at the moment when t = 0 and a? = 0 — that is, at the beginning of the
experiment, from which the time t and quantity x of substance changed is counted.
On integrating the preceding equation we obtain \og(A./A-x)--=kt, where A; is a new
constant, if we take ordinary (and not natural) logarithms. Hence, knowing A, x, and t,
for each reaction, we find k, and it proves to be a constant quantity. Thus from the
figures cited in the text for the reaction 2KMnO4 + 108C2H2O4+14MnSO4, it may be
calculated that /t = 0'0114; for example, < = 44, a; = 68'4 (A = 100), whence A;i = 0'5004 and
# = 0-0114, (see also Chapter XIV., Note 3, and Chapter XXVII., Note 25 bis).

316 PRINCIPLES OF CHEMISTRY

commencement, about 2-6. per cent, of the salt taken was decomposed in
the course of one minute, whilst after an hour the rate was about
0'5 per cent. The same phenomena are observed in every case which
has been investigated, and this branch of theoretical or physical
chemistry, now studied by many,27 promises to explain the course of
chemical transformations from a fresh point of view, which is closely
allied to the doctrine of affinity, because the rate of reaction, without
doubt, is connected with the magnitude of the affinities acting between
the reacting substances.

2l The researches' made by Hood, Van't Hoff, Oetwald, Warder, Menschutkin, Kono-
valoff, and others have a particular significance in this direction. Owing to the com-
parative novelty of this subject, and the absence of applicable as well as indubitable
deductions, I consider it impossible to enter into this provinc'e of theoretical chemistry,
although I am quite confident that its development should lead to very important results,
especially in respect to chemical equilibria, for Van't Hoff has already shown that
the limit of reaction in reversible reactions is determined by the attainment of equal
velocities for tho opposite reactions.

317

CHAPTER XXII

IRON, COBALT, AND NICKEL

JUDGING from the' atomic weights, and the forms of the higher oxides
of the elements already considered, it is easy to form an idea of
the seven groups of the periodic system. Such are, for instance, the
typical series Li, Be, B, C, N, O, F, or the third series, Na, Mg, Al, Si,
P, S, Cl. The seven usual types of oxides from R2O to R2O7 correspond
with them (Chapter XV.) The position of the eighth group is quite
separate, and is determined by the fact that, as we have already seen,
in each group of metals having a greater atomic weight than potassium
a distinction ought to be ..made between the elements of the even and
uneven series* The series of even elements, commencing with a
strikingly alkaline element (potassium, rubidium, caesium), together with
the uneven series following it, and concluding with a haloid (chlorine,
bromine, iodine), forms a large period, the properties of whose members
repeat themselves in other similar periods. The elements of the eighth
group are situated between the elements of the even series and the ele-
ments of the uneven series following them. And for this reason elements
of the eighth group are found in the middle of each large period. The
properties of the elements belonging to it, in many respects independent
and striking, are shown with typical clearness in the case of iron, the
well-known representative of this group.

Iron is one of those elements which are not only widely diffused in
the crust of the earth, but also throughout the entire universe. Its-
oxides and their various compounds are found in the most diverse
portions of the earth's crust ; but here iron is always found combined
with some other element. Iron is not found on the earth's surface in
a free state, because it easily oxidises under the action of air. It is
occasionally found in the native state in meteorites, or aerolites, which
fall upon the earth.

Meteoric iron is formed outside the earth.1 Meteorites are fragments
"which are carried round the sun in orbits, and fall upon the earth

The composition of meteoric iron is variable. It generally contains nickel, phos-
phorus, carbon, &c* The echreibersite of meteoric stones contains Fe4Ni2P.

818 PRINCIPLES OF CHEMISTRY

when coming into proximity with it during their motion in space. The
meteoric dust, on passing through the upper parts of the atmosphere,
and becoming incandescent from friction with the gases, produces that
phenomenon which is familiar under the name of falling stars.2 Such is

2 Comets and the rings of Saturn ought now to b6 considered as consisting of an
accumulation of such meteoric cosmic particles. Perhaps the part played by these
minute bodies scattered throughout space is much more important in the formation
of the largest celestial bodies than has hitherto bean imagined. The investigation of
this branch of astronomy, due to Schiaparelli, has a bearing on the whole of natural
Science.

The question arises as to why the iron in meteorites is in a free state, whilst on earth
it is in a state of combination. Does not this tend to show that the condition of our
globe is very different from that of the rest? My answer to this question has been
already given in Volume I. p. 877, Note 57. It is my opinion that inside the earth there
is a mass similar in composition to meteorites— that is, containing rdcky matter and
metallic iron, partly carburetted. In conclusion, I consider it will not be out of place
to add the following explanations. According to the theory of the distribution of pres-
sures (sae my treatise, On Barometrical Levelling, 1876, pages 48 et scq.) in an atmo.
sphere of mixed gases, it follows that two gases, whose densities are d and dlt and whose
grelative quantities or partial pressures at a certain distance from the centre of gravity
Are h and h^ will, when at a greater distance from the centre of attraction, present a
-different ratio of their masses x : a^— that is, of their partial pressures— which may be
•found by the equation fZ^log h — log x) — d(\og h\ — log Xi). If, for instance, d : di = 2 : 1,
and h = hi (that is to say, the masses are equal at the lower height) = 1000, then when
# = 10 the magnitude of Xi will not be 10 (i.e. the mass of a gas at a higher level whose
density =1 will not be equal to the mass of a gas whose density =2, as was the case at
a lower level), but much greater — namely, xl = 100 — that is, the lighter gas will pre»
dominate over a heavier one at a higher level. Therefore, when the whole mass of the
earth was in a state of vapour, the substances having a greater vapour density accumu-
lated about the centre and those with a lesser vapour density at the surface. And as
•the vapour densities depend on the atomic and molecular weights, those substances which
"have small atomic and molecular weights ought to have accumulated at the surface, and
those with high atomic and molecular weights, which are the least volatile and the easiest
to condense, at the centre. Thus it becomes apparent why such light elements as
liydrogen, carbon, nitrogen, oxygen, sodium, magnesium, aluminium, silicon, phosphorus,
•sulphur, chlorine^ potassium, calcium, and their compounds predominate at the surface
.and largely form the earth's crust. There is also now much iron in the sun, as spectrum
analysis shows, and therefore it must have entered into the composition of the earth
And other planets, but would have accumulated at the centre, because the density of
its vapour is certainly large and it easily condenses. There was also oxygen near the
centre of the earth, but not sufficient to combine with the iron. The former, as a much
lighter element, principally accumulated at the surface, where we at the present time
find all oxidised compounds and even a remnant of free oxygen. This gives the
possibility not only of explaining in accordance with cosmogonic theories the pre-
dominance of oxygen compounds on the surface of the earth, with the occurrence of
unoxidised iron in the interior of the earth and in meteorites, but also of understanding
•why the density of the whole earth (over 5) is far greater than that of the rocks (1 to 3)
composing its crust. And if all the preceding arguments and theories (for instance
the supposition that the sun, earth, and all the planets were formed of an elementary
homogeneous mass, formerly composed of vapours and gases) be -true, it must be ad-
mitted that the interior of the earth and other planets contains metallic (unoxidised) iron,
which, however, is only found on t'ae surface as aerolites. And then assuming that
.aerolites are the fragments of plrnets which have crumbled to pieces so to say
.during cooling (this has been held to be the case by astronomers, judging from the paths

IRON, COBALT AND NICKEL 319

the doctrine concerning meteorites, and therefore the fact of their
containing rocky (siliceous) matter and metallic iron shows that outside
the earth the elements and their aggpegation are in some degree the
same as upon the earth itself.

The most widely diffused terrestrial compound of iron is iron
bisulphide, FeS2, or iron pyrites. It occurs in formations of both
aqueous and igneous origin, and sometimes in enormous masses. It is
a substance having a greyish-yellow colour, with a metallic lustre, and a
specific gravity of 5*0 ; it crystallises in the regular) system.2 bis

The oxides are the principal ores used for producing metallic iron.
The majority of the ores contain ferric oxide, Fe.2O3, either in a
free state or combined with water, or else in combination with ferrous
oxide, JTeO The species and varieties of iron ores are numerous and
diverse. Ferric oxide in a separate form appears sometimes as crystals
of the rhombohedric system, having a metallic lustre and greyish steel
colour ; they areJbrittle, and form a red powder, specific gravity about
5-25. Ferric oxide in type of oxidation and properties resembles
alumina ; it is, however, although with difficulty, soluble in acids even
when anhydrous. The crystalline oxide bears the name of specular
iron ore, but ferric oxide most often occurs in a non-crystalline form,
in masses having a red fracture, and is then known as red hcematite.
In this form, however, it is rather a rare ore, and is principally found
in veins. The hydrates of ferric oxide, ferric hydroxides,3 are most.

of aerolites), it is readily understood why they should be composed of metallic
iron, and this would explain its occurrence in the depths of the earth, which we
assumed as the basis of our theory of the formation of naphtha (Chapter VIII., Notes
67-60).

*bls Immense deposits of iron pyrites are known in various parts of Russia. On the
river Msta, near Borovitsi, thousands of tons are yearly collected from the detritus of
the neighbouring rocks. In the Governments of Tou!a, Riazan, and in the Donets district
continuous layers of pyrites occur among the coal seam". Very thick beds of pyrites
are also known in many parts of the Caucasus. But the deposits of the Urals are par»
ticularly vast, and have been worked for a long time. Amongst these I will only indicate
the deposits on the Soymensky estate near the Kishteimsky works; the Kaletinsky
deposits near the Virhny-Isetsky works (containing 1-2 p.c. Cu) ; on the banks of the
river Koushaivi near Koushvi (3-5 p.c. Cu), and the deposits near the Bogoslovsky
works (3-5 p.c. Cu). Iron pyrites (especially that containing copper which is extracted
ifter roasting) is now chiefly employed for roasting, as a source of SO^ for the manufac*
ture of chamber sulphuric acid (Vol.1, p. 291), but the remaining oxide of iron is per-
fectly suitable for smelting into pig iron, although it gives a sulphurous pig iron (the
sulphur may be easily removed by subsequent treatment, especially with the aid of
ferro-manganese in Bessemer's process). The great technical importance of iron pyrites
leads to its sometimes being imported from great distances; for instance, into England
from Spain. Besides which, when heated in closed retorts FeS^ gives sulphur, and if
allowed to oxidise in damp air, green vitriol, FeSO4.

3 The hydrated ferric oxide is found in nature in a dual form. It is somewhat rarely
met with in the form of a crystalline mineral called gttthite, whose specific gravity is i'4

*c

820 PRINCIPLES OF CHEMISTRY

often found in aqueous or stratified formations, and are knovrn as
brown hcematites ; they generally have a brown colour, form a yellowish-
brown powder, and have no metallic lustre but an earthy appearance.
They easily dissolve in acids and diffuse through other formations, espe-
cially clays (for instance, ochre) ; they sometimes occur in reniform and
similar masses,, evidently of aqueous origin. Such are, for instance,
the so-called bog or lake aiidvpeat ores found at the bottom of marshes
and lakes, and also under and in peat beds. This ore is formed from
water' containing ferrous carbonate in solution, which, after absorbing1
oxygen, deposits ferric hydroxide. In rivers and springs, iron is found
in solution as ferrous carbonate through the agency of carbonic
acid : hence the existence of chalybeate springs containing FeC03.
This ferrous carbonate, or siderite, is either found as a non -crystalline
product of evidently aqueous origin, or as a crystalline spar called
spathic iron ore. The reniform deposits of the former are most re-
markable ; "they are called spherosiderites, and sometimes form whole
strata in the Jurassic and carboniferous formations. Magnetic
iron ore, Fe3O4 = FeO,Fe2O3, in virtue of its purity and practical
uses, is a very important ore ; it is a compound of the ferrous and
ferric oxides, is naturally magnetic, has a specific gravity of 5-1,
.crystallises in well-formed crystals of the regular system, is with diffi-
culty soluble in acids, and sometimes forms enormous masses, as, for
instance, Mount Blagodat in the Ural.. However, in most cases— for
instance, at Korsak-Mogila (to the north of Berdiansk and Nogaiska,
near the Sea of Azov), or at Krivoi Rog (to the west of Ekaterinoslav)-^.
the magnetic iron ore is mixed with other iron 'ores. In the Urals, the
Caucasus (without mentioning Siberia), and in the districts adjoining the
basin of the Don, Russia possesses the richest iron ores in the world.
To the south of Moscow, in the Governments of Toula and Nijni-
novgorod, in the Olonetz district, and in the Government of Orloffsky
(near Zinovieff in the district of Kromsky), and in many other places,
there are likewise abundant supplies of iron ores amongst the deposited
aqueous formations ; the siderite of Orloffsky, for instance, is dis-
tinguished by its great purity.4

and composition Fe^H204, or FeHO.j— that is, one of oxide of iron to one of water,
Fe^Os,!!./); frequently found as brown ironstone, forming a dense mass of fibrous,
reniform deposits containing SFejO^SH^O— that is, having a composition Fe4H6O9. In
bog ore and other similar ores we most often find a mixture of this hydrated ferric oxide
with clay and other impurities. The specific gravity of such formations is rarely as high
as 4-0.

4 The ores of iron, similarly to all substances extracted from veins and deposits, are
worked according to mining practice by means of vertical, horizontal, or inclined
shafts which reach and penetrate the veins and strata containing the ore deposits.
The mass of ore excavated is raised to the surface, then sorted either by hand or else in

1BON. COBALT, AND NICKEL 821

Iron is also found in the form of various other compounds — for
instance, in certain silicates, and also in some phosphates ; but these
forms are comparatively rare in nature in a pure state, and have not
the industrial importance of those natural compounds of iron pre-
viously mentioned. In small quantities iron enters into the composi-
tion of every kind of soil and all rocky formations. As ferrous oxide,
FeO, is isomorphous with magnesia, and ferric oxide, Fe203, with
alumina, isomorphous substitution is possible here, and hence minerals
are not unfrequently found in which the quantity of iron varies con-
siderably ; such, for instance, are pyroxene, amphibole, certain varieties
of mica, &c. Although much iron oxide is deleterious to the growth of
vegetation, still plants do not flourish without iron ; it enters as an
indispensable component into the composition of all higher organisms ;
in the ash of plants we always tind more or less of its compounds. It
also occurs in blood, and forms one of the colouring matters in it ;
100 parts of the blood of the highest organisms contain about 0*05 of iron.

The reduction of the ores of iron into metallic iron is in prin-
teiple very simple, because when the oxides of iron are strongly heated
with charcoal, hydrogen, carbonic oxide, and other reducing agents,*
they easily give metallic iron. But the matter is rendered more

special sorting apparatus (generally acting with water to wash the ore), and is subjected
to roasting and other treatment. In every case the ore contains foreign matter. In the
extraction of iron, which is one of the cheapest metals, the dressing of an ore is in most
cases unprofitable, and only ores rich in metal are worked— namely, those containing at
least 20 p.c. It is often profitable to transport very rich and pure ores (with as much as
70 p.c. of iron) from long distances. The details concerning the working and extraction
of metals will be found in special treatises on metallurgy and mining.

4 The reduction of iron oxides by hydrogen belongs to the order of reversible re-
actions (Chapter II.), and is therefore determined by a limit which is here expressed
by the attainment of the same pressure as in the case where hydrogen acts on iron
oxides, and as in the case where (at the same temperature) water is decomposed by
metaHRc iron. The calculations referring to this matter were made by Henri Sainte-Claire
Deville (1870). Spongy iron was placed in a tube having a temperature t, one end of
which was connected with a vessel containing water at 0° (vapour tension = 4*6 mm.)
and the other end with a mercury pump and pressure gauge which determined the
limiting tension attained by the dry hydrogen p (subtracting the tension of the water
vapour from the tension observed). A tube was then taken containing an excess of iron
oxide. It was filled with hydrogen, and the tension pl observed of the residual hydrogen
when the water was condensed at 0°.

t = 200° 440° 860° 1040°

p = 95' 9 25'8 12'8 9'2 mm.

p±= — — 12' 8 9 '4 mm.

The equality of the pressure (tension) of the hydrogen in the two cases is evident. The
hydrogen here behaves like the vapour of iron or of its oxide.

By taking ferric oxide, Fe2O3, Moissan observed that at 850° it passed into
magnetic oxide, Fe5O4, at 500° into ferrous oxide, PeO, and at 600° into metallic iron.
Wright and Luff (1878), whilst investigating the reduction of oxides, found that (a) the
temperature of reaction depends on the condition of the oxide taken — for instance

322 PRINCIPLES OF CHEMISTRY

difficult by the fact that the iron does not melt at the heat developed
by the combustion of the charcoal, and therefore it does not separate
from those mechanically mixed impurities which are found in the iron
ore. This is obviated by the following very remarkable property of
iron : at a high temperature it is capable of combining with a small
quantity (from 2 to 5 p.c ) of carbon, and then forms cast iron, which
easily melts in the heat developed by the combustion of charcoal in air.
For this reason metallic iron is not obtained directly from the ore, but
is only formed after the further treatment of the cast iron } the first
product extracted from the ore being cast iron. The fused mass dis-
poses itself in the furnace below the slag — that is, the impurities of the
ore fused by the heat of the furnace. If these impurities did not fuse
they would block up the furnace in which the ore was being smelted,
and the continuous smelting of the cast iron would not be possible ; 6
it would be necessary periodically to cool the furnace and heat it up
again, which means a wasteful expenditure of fuel, and hence in the
production of cast iron, the object in view is to obtain all the earthy
impurities of the ore in the shape of a fused mass or slag. Only
in rare cases does the ore itself form a mass which fuses at the
temperature employed, and these cases are objectionable if much iron
oxide is carried away in the slag. The impurities of the ores most
often consist of certain mixtures — for instance, a mixture of clay and
sand, or a mixture of limestone and clay, or quartz, &c. These

precipitated ferric oxide is reduced by hydrogen at 85°, that obtained by oxidising the
metal or from its nitrate at 175° ; (b) when other conditions are the same the reduction
by carbonic oxide commences earlier than that by hydrogen, and the reduction by
hydrogen still earlier than that by charcoal ; (c) the reduction is effected with greater
facility when a greater quantity of heat is evolved during the reaction. Ferric oxide
obtained by heating ferrous sulphate to a red heat begins to be reduced by carbonic
oxide at 202°, by hydrogen at 260°, by charcoal at 480°, whilst for magnetic oxide, Fe3C>4,
the temperatures are 200°, 290°, and 450° respectively.

c The primitive methods of iron manufacture were conducted by intermittent pro-
cesses in hearths resembling smiths' fires. As evidenced by the uninterrupted action
of the steam boiler, or the process of lime burning, and the continuous preparation and
condensation of sulphuric acid or the uninterrupted smelting of iron, every industrial
process becomes increasingly profitable and complete under the condition of the con-
tinuous action, as far as possible, of all agencies concerned in the production. This
continuous method of production is the first condition for the profitable production
on the large scale of nearly all industrial products. This method lessens the cost of
labour, simplifies the supervision of the work, renders the product uniform, and fre-
quently introduces a very great economy in the expenditure of fuel and at the same time
presents the simplicity and perfection of an equilibrated system. Hence every manu-
facturing operation should be a continuous one, and the manufacture of pig iron and
sulphuric acid, which have long since become so, may be taken as examples in many
respects. A study of these two manufactures should form the commencement of an
acquaintance with all the contemporary methods of manufacturing both from a tech*
nical and economical point of view.

IRON, COBALT, AND NICKEL 323

impurities do not separate of themselves, or do not fuse. The difficulty
of the industry lies in forming an easily-fusible slag, into which the
whole of the foreign matter of the- ore would pass and flow down to the
bottom of the furnace above the heavier cast iron. This is effected by
mixing certain fluxes with the ore and charcoal, A flux is a substance
which, when mixed with the foreign matter of the ore, forms a fusible
vitreous mass or slag. The flux used for silica is limestone with clay ;
for limestone a definite quantity of silica is used, the best procedure
having been arrived at by experiment and by long practice in iron
smelting and other metallurgical processes*7

Thus the following materials have to be introduced into the furnace
where the smelting of the iron ore is carried on : (1) the iron ore,
composed of oxide of iron and foreign matter ; (2) the flux required to
form a fusible slag with the foreign matter ; (3) the carbon which is
necessary (a) for reducing, (6) for combining with the reduced iron
to form cast iron, (c) principally for the purpose of combustion and
the heat generated thereby, necessary not only for reducing the iron
and transforming it into cast iron, but also for melting the slag, as well
as the cast iron — and (4) the air necessary for the combustion of the
charcoal. The air is introduced after a preparatory heating in order to
economise fuel and to obtain the highest temperature. The air is
forced in under pressure by means of a special blast arrangement.
This permits of an exact regulation of the heat and rate of smelting.
All these component parts necessary for the smelting of iron must be
contained in a vertical, that is; shaft furnace, which at the base must
have a receptacle for the accumulation of the slag and cast iron formed,
in order that the operation may proceed without interruption. The
walls of such a furnace ought to be built of fireproof materials if it be

7 The composition of slag suitable for iron smelting most often approaches the
following : 50 to 60 p.c. SiO2, 5 to 20 A18O3, the rest of the mass consisting of MgO,
CaO, MnO, FeO. Thus the most fusible slag (according to the observations of
Bodeman) contains the alloy Al2O5,4CaO,7SiO2. On altering the quantity of magnesia
and lime, and especially of the alkalis (which increases the fusibility) and of1 silica
(which decreases it), the temperature of fusion changes with the relation between the total
quantity of oxygen and that in the silica. Slags of the composition) RO,SiO2 are easily
fusible, have a vitreous appearance, and are very common. Basic slags approach the
composition 2RO,Si02. Hence, knowing the composition and quantity of the foreign
matter in the ore, it is at once easy to find the quantity and quality of the flux which
must be added to form a suitable slag. The smelting of iron is rendered more complex
by the fact that the silica, SiO2, which enters into the slag and fluxes is capable of form-
ing a slag with the iron oxides. In order that the least quantity of iron may pass into
the slag, it is necessary for it to be reduced before the temperature is attained at which
the slags are formed (about 1000°), which is effected by reducing the iron, not with char-
coal itself, but with carbonic oxide. From this it will be understood how the progress of
the whole treatment may be judged by the properties of the slags. Details of this
complicated and well-studied subject will be found in works on metallurgy.

824 PRINCIPLES OF CHEMISTRY

designed to serve for the continuous production of cast iron by charging
the ore, fuel, and flux into the mouth of the furnace, forcing a blast of
air into the lower part, and running out the molten iron and slag from
below. The whole operation is conducted in furnaces known as blast
furnaces. The annexed illustration, fig. 93 (which is taken by kind
permission from Thorpe's Dictionary of Applied Chemistry), represents
the vertical section of such a furnace. These furnaces are generally
of large dimensions — varying from 50 to 90 feet in height. They are
sometimes built against rising ground in order to afford easy access to
the top where the ore, flux, and charcoal or coke are charged.8

8 The section of a blast furnace is represented by two truncated cones joined at their
bases, the upper cone being longer than the lower one ; the lower cone is terminated by
the hearth, or almost cylindrical cavity in whichj the cast iron and slag collect, one
side being provided with apertures for drawing off the iron and slag. The air is blown
into the blast furnace through special pipes, situated, over the hearth, as shown in the
section. The air previously passes through a series ;of cast-iron pipes, heated by the
combustion of the carbonic oxide obtained from Jthe upper parts of the furnace, where
it is formed as in a ' gas-producer.' The blast furnace acts continuously until it is worn
out ; the iron is tapped off twicer day, and the furnace is allowed to cool a little from
time to time so as not to be spoilt by the increasing heat, and to enable it to withstand
long usage.

Blast furnaces worked", with charcoal fuel are not so high, and in general give a
smaller yield than those using coke, because the latter are worked with heavier charges
than those in which charcoal is employed. Coke furnaces yield 20,000 tons and over of pig
iron a year. In the United States there are blast furnaces 30 metres high, and upwards
of 600 cubic metres capacity, yielding as much as 130,000 tons of pig iron, requiring a blast
of about 750 cubic metres of air per minute, heated to' 600°, and consuming about 0*85
part of coke per 1 part of pig iron produced. At the present time the world produces as
much as 30 million tons of pig iron a year, about T% of which is converted into wrought
iron and steel. The chief producers are the United States (about 10 million tons a year)
and England (about 9 million tons a year) ; Russia yields about 1£ million tons a year.
The world's production has doubled during the last 20 years,, and in this respect the
United States have outrun all other countries. The reason of this increase of production
must be looked for in the increased demand for iron and steel for railway purposes, for
structures (especially ship-building), and in the fact that : (a)' the cost of pig iron has
fallen, thanks to the erection of large furnaces and a fuller study of the processes taking
place in them, and (6) that every kind of iron ore (even sulphurous and phosphoritic) can
now be converted into a homogeneous steel.

In order to more thoroughly grasp the chemical process which takes place in blast
furnaces, it is necessary to follow the course of the material charged in at the top and of
'the air passing through the furnace. From 50 to 200 parts of carbon are expended on 100
parts of iron. The ore, flux, and coke are charged into the top of the furnace, in
layers, as the cast iron is formed in the lower parts and flowing down to the bottom
causes the whole contents of the furnace to subside, thus forming an empty space at
the top, which is again filled up with the afore-mentioned mixture. During its down*
ward course this mixture is subjected to increasing heat. This rise of temperature
first drives off the moisture of the ore mixture, and then leads to the formation of
the products of the dry distillation of coal or charcoal. Little by little the subsiding
mass attains a temperature at which the heated carbon reacts with the carbonic anhydride
passing upwards through the furnace and transforms it into carbonic oxide. This is
the reason why carbonic anhydride is not evolved from the furnace, but only carbonic
oxide. As regards the ore itself, on being heated to about 600° to 800° it is reduced at
the expense of the carbonic oxide ascending the furnace, and formed by the contact of

IRON, COBALT, AND NICKEL

325

The cast iron formed in blast furnaces is not always of the same
quality. When slowly cooled it is soft, has a grey colour, and is not

the carbonic anhydride with the incandescent charcoal, so that the reduction in the blast
furnace is without doubt brought about by the formation and decomposition of carbonic
oxide and not by carbon itself— thus, Fe2O3 + 3CO = Fe2 + 8C02. The reduced iron, on
further subsidence and contact with carbon, forms cast iron, which flows to the bottom
of the furnace. In these lower layers, where the temperature is highest (about 1,800°),

15-0 1

Flo. 93.— Vertical section of a modern Cleveland blast furnace capable of producing 300 to 1,000 tons
of pig iron weekly. The outer casing is of riveted iron plates, the furnace being lined with re-
fractory fire-brick. It is closed at the top by a • cap and cone ' arrangement, by means of which
the charge can be fed into the furnace at suitable intervals by lowering the moveable cone.

the foreign matter of the ore finally forms slag, which also is fusible, with the aid of
fluxes. The air blown in from below, through the so-called tuyeres, encounters carbon
in the lower layers of the furnace, and burns it, converting it into carbonic anhydride.
It is evident that this develops the highest temperature in these lower layers of the
furnace, because here the combustion of the carbon is effected by heated and compressed
air. This is very essential, for it is by virtue of this high temperature that the
process of forming the slag and of forming and fusing the cast iron are effected

826 PRINCIPLES OF CHEMISTRY

completely soluble in acids. When treated with acids a residue of
graphite remains ; it is known as grey or soft cast iron. This is the
general form of the ordinary cast iron used for casting various objects,
because in this state it is not so brittle as in the shape of white cast
iron, which does not leave particles of graphite when dissolved, but
yields its carbon in the form of hydrocarbons. This white cast iron
is characterised by its whitish-grey colour, dull lustre, the crystalline
structure of its fracture (more homogeneous than that of grey iron), and
such hardness that a file will hardly cut it. When white cast iron is
produced (from manganese ore) at high temperatures (and with an ex-
cess of lime), and containing little sulphur and silica but a considerable
amount of carbon (as much as 5 p.c.), it acquires a coarse crystalline
structure which increases in proportion to the amount of manganese,
and it is then known under the name of ' spiegeleisen ' (and 'ferro-

9

simultaneously in these lower portions of the furnace. The carbonic acid formed in
these parts rises higher, encounters incandescent carbon, and forms with it carbonic
oxide. This heated carbonic oxide acts as a reducing agent on the iron ore, and is re-
converted by it into carbonic anhydride ; this gas meets with more carbon, and again
forms carbonic oxide, which again acts as a reducing agent. The final transformation
of the carbonic anhydride into carbonic oxide is effected in those parts of the furnace
where the reduction of the oxides of iron does not take place, but where the temperature^
is still high enough to reduce the carbonic anhydride. The ascending mixture of
carbonic oxide and nitrogen, COj, &c., is then withdrawn through special lateral
apertures formed in the upper cold parts of the furnace walls, and is conducted through
pipes to those stoves which are used for heating the air, and also nometimes into other
furnaces used for the further processes of iron manufacture. The fuel of blast furnaces
consists of wood charcoal (this is the most expensive material, but the pig iron pro-
duced is the purest, because charcoal does not contain any sulphur, while coke does),
anthracite (for instance, in Pennsylvania, and in Russia at Pastouhoff's works in the
Don district), coke, coal, and even wood and peat. It must be borne in mind that the
utilisation of naphtha and naphtha refuse would probably give very profitable results
in metallurgical processes.

The process just described is accompanied by a series of other processes. Thus, for
instance, in the blast furnace a considerable quantity of cyanogen compounds are formed.
This takes place because the nitrogen of the air blast comes into contact with incan-
descent carbon and various alkaline matters contained in the foreign matter of the ores.
A considerable quantity of potassium cyanide is formed when wood charcoal is employed
for iron smelting, as its ash is rich in potash.

9 The specific gravity of white cast iron is about 7'5. Grey cast iron has a much lower
specific gravity, namely, 7'0. Grey cast iron generally contains less manganese and
more silica than white ; but both contain from 2 to 8 p.c. of carbon. The difference
between the varieties of cast iron depends on the condition of the carbon which
enters into the composition of the iron. In white cast iron the carbon is in combination
with the iron — in all probability, as the compound CFe4 (Abel and Osmond and others
extracted this compound, which is sometimes called ' carbide,' from tempered steel,
which stands to unannealed steel as white cast iron does to grey), but perhaps in the state
of an indefinite chemical compound resembling a solution. In any case the compound of
the iron and carbon in white cast iron is chemically very unstable, because when slowly
cooled it decomposes, with separation of graphite, just as a solution when slowly cooled

IRON, COBALT, AND NICKEL 827

Cast iron is a material which is either suitable for direct application
for casting in moulds or else for working up into wrought iron and
eteel. The latter principally differ from cast iron in their containing
less carbon — thus, steel contains from 1 p.c. to 0*5 p.c. of carbon and
far less silicon and manganese than cast iron ; wrought iron does
not generally contain more than 0'25 p.c. of carbon and not more than
0-25 p.c. of the other impurities. Thus the essence of the working up
of cast iron into steel and wrought iron consists in the removal of the
greater part of the carbon and other elements, S, P, Mu, Si, &c. This
is effected by means of oxidation, because the oxygen of the atmosphere,
oxidising the iron at a high temperature, forms solid oxides with it ;
and the latter, coming into contact with the carbon contained in the
cast iron, are deoxidised, forming wrought iron and carbonic oxide,
which is evolved from the mass in a gaseous form. It is evident that
the oxidation must be carried on with a molten mass in a state of
agitation, so that the oxygen of the air may be brought into contact
with the whole mass of carbon contained in the cast iron, or else the
operation is effected by means of the addition of oxygen compounds
of iron (oxides, ores, as in Martin's process). Cast iron melts much
more easily .than wrought iron and steel, and, therefore, as the carbon
separates, the mass in the furnace (in puddling) or hearth (in the
bloomery process) becomes more and more solid ; .moreover the degree of
hardness forms, to a certain extent, a measure of the amount of carbon
separated, and the operation may terminate either in the formation of
steel or wrought iron.10 In any case, the iron used for industrial pur-
yields a portion of the substance dissolved. The separation of carbon in the form of
graphite on the conversion of white cast iron into grey is never complete, however slowly
the separation be carried on; part of the carbon remains in combination with the
iron in the same state in which it exists in white cast iron. Hence when grey cast iron is
treated with acids, the whole of the carbon does not remain in the form of graphite, but a
part of it is separated as hydrocarbons, which proves the existence of chemically-combined
carbon in grey cast iron. It is sufficient to re-melt grey cast iron and to cool it quickly to
transform it into white cast iron. It is not carbon alone that influences the properties of
cast iron ; when it contains a considerable amount of sulphur, cast iron remains white
even after having been slowly cooled. The same is observed in cast iron very rich in
manganese (5 to 7 p.c.), and in this latter case the fracture is very distinctly crystalline
and brilliant. When cast iron contains a large amount of manganese, the quantity of
carbon may also be increased. Crystalline varieties of cast iron rich in manganese are in
practice called ferro-manganese (p. 810), and are prepared for the Bessemer process.
Grey cast iron not having an uniform structure is much more liable to various changes
than dense and thoroughly uniform white cast iron, and the latter oxidises much more
slowly in air than the former. White cast iron is not only used for conversion into wrought
iron and steel, but also in those cases where great hardness is required, although it be ac-
companied by a certain brittleness , for instance, for making rollers, plough- shares, &c.

10 This direct process of separating the carbon from cast iron is termed puddling. It
is conducted in reverberatory furnaces. The cast iron is placed on the bed of the
furnace and melted ; through a special aperture, the puddler stirs up the oxidising raasa

928 PRINCIPLES OF CHEMISTRY

poses contains impurities. Chemically pure iron may he obtained by
precipitating iron from a solution (a mixture of ferrous sulphate with

of cast iron, pressing the oxides into the molten iron. This resembles kneading dough,
and the process introduced in England became known as puddling. It ia evident that
the puddled mass, or bloom, is a heterogeneous substance obtained by mixing, and
hence one part of the mass will still be rich in carbon, another will be poor, some parts
will contain oxide not reduced, &c. The further treatment of the puddled mass consists
in hammering and drawing it out into flat pieces, which on being hammered become
more homogeneous, and when several pieces are welded together and again hammered
out a still more homogeneous mass is obtained. The quality of the steel and iron thug
formed depends principally on their uniformity. The want of uniformity depends on
the oxides remaining inside the mass, and on the variable distribution of the carbon
throughout the mass. In order to obtain a more homogeneous metal for manufac-
turing articles out of steel, it is drawn into thin rods, which are tied together m
bundles and then again hammered out. As an example of what may be attained in this
direction, imitation Damascus steel may be cited ; it consists of twisted and plaited
wire, which is then hammered into a dense mass. (Real damascened wootz steel
inay be made by melting a mixture of the best iron with graphite (fa) and iron rust;
the article is then corroded with acid, and the carbon remains in the form of a pattern.)
Steel and wrought iron are manufactured from cast iron by puddling. They are, how-
ever, obtained not only by this method but also by the bloomery process, which is carried
out in a fire similar to a blacksmith's forge, fed with charcoal and provided with a blast j
a pig of cast iron is gradually pushed into the fire, and portions of it melt and fall to the
bottom of the hearth, coming into contact with an air blast, and are thus oxidised. Tho
bloom thus formed is then squeezed and hammered. It is evident that this process is
only available when the charcoal used in the fire does not contain any foreign matter
which might injure t"he quality of the iron or steel — for instance, sulphur or phosphorus
—and therefore only wood charcoal may be used with impunity, from which it follows
that this process can only be carried on where the manufacture of iron can be conducted
With this fuel. Coal and coke contain the above-mentioned impurities, and would
therefore produce iron of a brittle nature, and thus it would be necessary to have
recourse to puddling, where the fuel is burnt on a special hearth, separate from the
cast iron, whereby the impurities of the fuel do not come into contact with it. The
manufacture of steel from cast iron may also be conducted m fires ; but, in addition to
this, it is also now prepared by many other methods. One of the long-known processes
is called cementation, by which steel is prepared from wrought iron but not from cast
iron. For this process strips of iron are heated red-hot for a considerable time whilst
immersed, in powdered charcoal ; during this operation the iron at the surface combines
with the charcoal, which however does not penetrate ; after this the iron strips are
re- forged, drawn out again, and cemented anew, repeating this process until a steel of the
desired quality is formed — that is, containing the requisite proportion of carbon. The
JBessemer process occupies the front rank among the newer methods (since 1856) ; it
is so called from the name of its inventor. This process consists in running melted
cast iron into converters (holding about 6 tons of cast iron)— that is, egg-shaped
receivers, fig. 94, capable of revolving on trunnions (in order to charge in the
cast iron and discharge the steel), and forcing a stream of air through small apertures
at a considerable pressure. Combustion of the iron and carbon at an elevated tempera-
ture then takes place, resulting from the bubbles of oxygen thus penetrating the mass
of the cast iron. The carbon, however, burns to a greater extent than the iron, and
therefore a mass is obtained which is much poorer in carbon than cast iron. As the
combustion proceeds very rapidly in the mass of metal, the temperature rises to such an
extent that even the wrought iron which may be formed remains in a molten condition,
whilst the steel, being more fusible than the wrought iron, remains very liquid. In
half an hour the mass is ready. The purest possible cast iron is used in the Bessemer

IKON, COBALT, AND NICKEL 329

magnesium sulphate or ammonium cnloride) by the prolonged action of
a feeble galvanic current ; th« iron may be then obtained as a dense

process, because sulphur and phosphorus do not burn out like carbon, silicon, and
manganese.

The presence of manganese enables the sulphur to be removed with the slag, and the
presence of lime or magnesia, which are introduced into the lining of the converter,

loo 50 ;tt o t J sMeter

yiO. 94.— Bessemer converter, constructed of iron plate and lined with ganister. The air is carried
by the tubes, L, O, D to the bottom, M, from which it passes by a. number of holes into the con-
verter. The converter is rotated on the trunnion d by means of the rack and pinion H, when it
is required either to receive molten cast iron from the melting furnaces or to pour out the steel.

facilitates the removal of the phosphorus. This basic Bessemer process, or Thomas
Oilchrist process, introduced about 1880, enables ores containing a considerable amount
if phosphorus, which bad hitherto only been used, for cast iron, to be used for making
vrought iron and steel. Naturally the greatest uniformity will be obtained .by re-melting
the metal. Steel is re- melted in small wind furnaces, in masses not exceeding 80 kilos ;
a liquid metal is formed, which may be cast in moulds, A-tnixture of wrought and cast
iron is often used for making cast steel (the addition pf a small amount of metallic Al
improves the homogeneity of the castings, by facilitating the passage of the impurities
into slag). Large steel castings are made by simultaneous fusion in several furnaces and
•crucibles ; in this way, castings up to 80 tons or more, such as large ordnance, may be
made. This molten, and therefore homogeneous, steel is called cast steel. Of late years
the Martin's process for the manufacture of steel has come largely into use ; it was
invented in France about 1860, and with the use of regenerative furnaces it enables large
quantities of cast steel to be made at a time. It is based on the melting of cast iron with
iron oxides and iron itself — for instance, pure ores, scrap, &c. There the carbon of the
cast iron and the oxygen of the' oxide form carbonic oxide, and the carbon therefore
burns out, and thus cast steel is obtained from cast iron, providing, naturally, 'that there
is a requisite proportion and corresponding degree of heat. The advantage of this

830 PRINCIPLES OF CHEMISTRY

mass. This method, proposed by Bottcher and applied by Klein, gives,
as R. Lenz showed, iron containing occluded hydrogen, which is dis-

process is that not only do the carbon, silicon, and manganese, but also a great part of
the sulphur and phosphorus of the cast iron burn out at the expense of the oxygen of the
iron oxides. During the last decade the manufacture of steel and its application for
rails, armour plate, guns, boilers, &c., has developed to an enormous extent, thanks to
the invention of cheap processes for the manufacture of large masses of homogeneous
cast steel. Wrought iron may also be melted, but the heat of a blast furnace is insufficient
for this. It easily melts in the oxyhydrogen flame. It may be obtained in a molten
state directly from cast iron, if the latter be melted with nitre and sufficiently stirred up.
Considerable oxidation then takes place inside the mass of cast iron, and the temperature
rises to such an extent that the wrought iron formed remains liquid. A method is also
known for obtaining wrought iron directly from rich iron ores by the action of carbonic
oxide : the wrought iron is then formed as a spongy mass (which forms an excellent
filter for purifying water), and may be worked up into wrought iron or steel either by
forging or by dissolving in molten cast iron.

Everybody is more or less familiar with the difference in the properties of steel and,
wrought iron. Iron is remarkable for its softness, pliability, and small elasticity, whilst
steel may be characterised by its capability of attaining elasticity and hardness if it be
cooled suddenly after having been heated to a definite temperature, or, as it is termed,
tempered. But if tempered' steel be re-heated and slowly cooled, it becomes as soft as
wrought iron, and can then be cut with the file and forged, and in general can be made
to assume any shape, like wrought iron. In this soft condition it is called annealed steel.
The transition from tempered to annealed steel thus takes place in a similar way to the
transition from white to grey cast iron. Steel, when homogeneous, has considerable
lustre, and such a fine granular structure that it takes a very high polish. Its fracture
clearly shows the granular nature of its structure. The possibility of tempering steel
enables it to be used for making all kinds of cutting instruments, because annealed steel
can be forged, turned, drawn (under rollers, for instance, for making rails, bars, &c.), filed,
&c., and it may then be tempered, ground and polished. The method and temperature
of tempering and annealing steel determine its hardness and other qualities. Steel is
generally tempered to the required degree of hardness in the following manner : It is
first strongly heated (for instance, up to 600°), and then plunged into water— that is,
hardened by rapid cooling (it then becomes as brittle as glass). It is then heated until
the surface assumes a definite colour, and finally cooled either quickly or slowly.
When steel is heated up to 220°, its surface acquires a yellow colour (surgical instru-
ments) ; it first of all becomes straw-coloured (razors, &c.), and then gold-coloured ; then
at a temperature of 250° it becomes brown (scissors), then red, then light blue at 285°
(springs), then indigo at 300° (files), and finally sea-green at about 340°. These colours
are only the tints of thin films, like the hues of soap bubbles, and appear on the steel
because a thin layer of oxides is formed over its surface. Steel rusts more slowly than
wrought iron, and is more soluble in acids than cast iron, but less so than wrought iron.
Its specific gravity is about 7'6 to 7'9.

As regards the formation of steel, it was a long time before the process of cementation
was thoroughly understood, because in this case infusible charcoal permeates unfused
wrought iron. Caron showed that this permeation depends on the fact that the charcoal
used in the process contains alkalis, which, in the presence of the nitrogen of the
air,. form metallic cyanides; these being volatile and fusible, permeate the iron, and,
giving up their carbon to it, serve as the material for the formation of steel. Thia
explanation is confirmed by the fact that charcoal without alkalis or without nitrogen
will not cement iron. The charcoal used for cementation acts badly when used over
again, as it has lost alkali. The very volatile ammonium cyanide easily conduces to the
formation of steel. Although steel is also formed by the action of cyanogen compounds,
nevertheless it does not contain more nitrogen than cast or wrought iron (O'Ol p.c.), and

IRON, COBALT, AND NICKEL 381

engaged on heating. This galvanic deposition of iron is used for
making galvanoplastic cliches, which are distinguished for their great

these latter contain it because their ores contain titanium, which combines directly with
nitrogen. Hence the part played" by nitrogen in steel is but an insignificant one. Ifc
may be useful here to add some information taken from Caron's treatise concerning the
influence of foreign matter on the quality of steel. The principal properties of steel are*
those of tempering and annealing. The compounds of iron with silicon and boron have
not these properties. They are more stable than the carbon compound, and this latter1
is capable of changing its properties; because the carbon in it either enters into
.combination or else is disengaged, which determines the condition of hardness or softness
of steel, as in white and grey cast iron. When slowly cooled, steel splits up into &
mixture of soft and carburetted iron ; but, nevertheless, the carbon does not separate
from the iron. If such steel be again heated, it forms a uniform compound, and hardens,
when rapidly cooled. If the same steel as before be taken and heated tPlong time, then,
after being slowly cooled, it becomes much more soluble in acid, and leaves a residue of
pure carbon. This shows that the combination between the carbon and iron in steel
becomes destroyed when subjected to heat, and the steel becomes iron mixed with
carbon. Such burnt steel cannot be tempered, but may be corrected by continued
forging in a heated condition, which has the effect of redistributing the carbon equally
throughout the whole mass. After the forging, if the iron is pure and the carbon has
not been burnt out, steel is again formed, which may be tempered. If steel be re-
peatedly or strongly heated, it becomes burnt through and cannot be tempered or
annealed ; the carbon separates from the iron, and this is effected more easily if the
steel contains other impurities which are capable of forming stable combinations with
iron, such as silicon, sulphur, or phosphorus. If there be much silicon, it occupies the
place of the carbon, and then continued forging will not induce the carbon once
separated to re-enter into combination. Such steel is easily burnt through and cannot
be corrected; when burnt through, it is hard and cannot be annealed— this is tougb,
steel, an inferior kind. Iron which contains sulphur and phosphorus cements badly,
combines but little with carbon, and steel of this kind is brittle, both hot and cold.
Iron in combination with the above-mentioned substances cannot be annealed by alow
cooling, showing that these compounds are more stable than those of carbon and ironfc
and therefore they prevent the formation of the latter. Such metals as tin and zino
combine with iron, but not with carbon, and form a brittle mass which cannot be
annealed and is deleterious to steel. Manganese and tungsten, on the contrary, ajfe,
capable of combining with charcoal ; they do not hinder the formation of steel, but even
remove the injurious effects, of other admixtures (by transforming these admixed sub-
stances into new compounds and slags), and are therefore ranked with the substances
which act beneficially on steel ; but, nevertheless, the best steel, which is capable of
renewing most often its primitive qualities after burning or hot forging, is the purest.
The addition of Ni, Cr, W, and certain, other metals to steel renders it very suitable for,
certain special purposes, and is therefore frequently made use of.

It is worthy of attention that steel, besides temper, possesses many variable
properties, a review of which may be made in the classification of the sorts of steel
(1878, Cockerell). (1) Very mild steel contains from 0'05 to 0'20 p.c. of carbon, breaks
with a weight of 40 to 50 kilosjper square millimetre, and has an extension of 20 to
80 p.c.; it may be welded, like wrought iron, but cannot be tempered ; is used in sheets
for boilers, armour plate and bridges, nails, rivets, &c., as a substitute for wrought iron ;
(2) mild steel, from 0'20 to 0'35 p.c. of carbon, resistance to tension 50 to 60 kitos,
extension 15 to 20 p.c., not easily welded, and tempers badly, used for axles, rails, and
railway tyres, for cannons and guns, and for parts of machines destined to resist bending
and torsion ; (3 hard steel, carbon 0'85 to 0'50 p.c., breaking weight 60 to 70 kilos per
square millimetre, extension 10 to 15 p.c., cannot be welded, takes a temper ; used for
rails, all kinds of springe, swords, parts of machinery in motion subjected to friction,

332 PRINCIPLES OF CHEMISTRY

hardness. Electro- deposited iron is brittle, but if heated (after the
separation of the hydrogen) it becomes soft. If pure ferric hydroxide,
which is easily prepared by the precipitation of solutions of ferric
salts by means of ammonia, be heated in a stream of hydrogen, it
forms, first of all, a dull black powder which ignites spontaneously in
air (pyrophoric iron), and then a grey powder of pure iron. The
powdery substance first obtained is an iron suboxide ; when thrown
into the air it ignites, forming the oxide Fe3O4. If the heating in
hydrogen be continued, more water and pure iron, which does not
ignite spontaneously, will be obtained. If a small quantity of iron be
I fused in the oxyhydrogen flame (with an excess of oxygen) in a piece
of lime and mixed with powdered glass, pure molten iron will be
formed, because in the oxyhydrogen flame iron melts and burns, but
the substances mixed with the iron oxidise first. The oxidised im-
purities here either disappear (carbonic anhydride) in a gaseous form,
or turn into slag (silica, manganese, oxide, and others) — that is, fuse
with the glass. Pure iron has a silvery white colour and a specific
gravity of 7 '84 ; it melts at a temperature higher than the melting-
points of silver, gold, nickel, and steel, i.e. about 1400°-! 500° and

spindles of looms, hammers, spades, hoes, &c. ; (4) very hard steel, carbon 0'5 to 0'65
p.c., tensile breaking weight 70 to 80 kilos, extension 5 to 10 p.c., does not weld, but
tempers easily ; used for small springs, saws, files, knives and similar instruments.

The properties of ordinary wrought iron are well knowa. The best iron is the most
tenacious — that is to say, that which does not break up when struck with the hammer
or bent, and yet at the same time is sufficiently hard. There is, however, a distinction
between hard and soft iron. Generally the softest iron is the most tenacious, and can
best be welded, drawn into wire, sheets, &c. Hard, especially tough, iron is often
characterised by its breaking when bent, and is therefore very difficult to work, and
objects made from it are less serviceable in many respects. Soft iron is most adapted
for making wire and sheet iron and such small objects as nails. Soft iron is characterised
by its attaining a fibrous fracture after forging, whilst tough iron preserves its granular
structure after this operation. Certain sorts of iron, although fairly soft at the ordinary
temperature, become brittle when heated and are difficult to weld. These sorts are
less suitable for being worked up into small objects. The variety of the properties of
iron depends on the impurities which it contains. In general, the iron used in the arts
still contains carbon and always a certain quantity of silicon, manganese, sulphur,
phosphorus, &c. A variety in the proportion of these component parts changes the
quality of the iron. In addition to this the change which soft wrought iron, having a
fibrous structure, undergoes when subjected to repeated blows and vibrations is con-
siderable ; it then becomes granular and brittle. This to a certain degree explains the
want of stability of some iron objects — such as truck axles, which must be renewed after
a certain term of service, otherwise they become brittle. It is evident that there are
innumerable intermediate transitions from wrought iron to steel and cast iron.

At the present day the greater part of the cast iron manufactured is converted into
steel, generally cast steel (Bessemer's and Martin's). I may add the Urals, Donetz
district, and other parts of Russia offer the greatest advantages for the development of
an iron industry, because these localities not only contain vast supplies of excellent iron
ore, but also coal, which is necessary for smelting it.

IKON, COBALT, AND NICKEL 888

below the melting point of platinum (17500).11 But pure iron becomes
soft at a temperature considerably below that at which it melts, and
may then be easily forged, welded, and rolled or drawn into sheets and
wireiibis Pure iron may be rolled into an exceedingly thin sheet,
weighing less than a sheet of ordinary paper of the same size. This
ductility is the most important property of iron in all its forms, and is
most marked with sheet iron, and least so with cast iron, whose
ductility, compared with wrought iron, is small, but it is still very
considerable when compared with other substances — such, for instance,
as rocks.12

The chemical properties of iron have been already repeatedly
mentioned in preceding chapters. Iron rusts in air at the ordinary
temperature — that is to say, it becomes covered with a layer of iron
oxides. Here, without doubt, the moisture of the air plays a part,
because in dry air iron does not oxidise at aH, and also because, more

11 According to information supplied by A. T. Skinder's experiments at the Oboukoff
Steel Works, 140 volumes of liquid molten steel give 128 volumes of solid metal. By
means of a galvanic current of great intensity and dense charcoal as one electrode, and
iron as the other, Bernadoss. welded iron and fused holes through sheet iron. Soft
wrought iron, like steel and soft malleable cast iron, may be melted in Siemens'
regenerative furnaces, and in furnaces heated with naphtha.

11 bu <3.ore (1869), Tait, Barret, Tchernoff, Osmond, and others observed that at a
temperature approaching 600° — that is, between dark and bright red .heat — all kinds of
wrought iron undergo a peculiar change called recalescence, i.e. a spontaneous rise of
temperature. If iron be considerably heated and allowed to cool, it may be observed
that at this temperature the cooling stops— that is, latent heat is disengaged, corre-
sponding with a change in condition. The specific heat, electrical conductivity, magnetic,
and other properties then also change. In tempering, the temperature of recalescence
must not be reached, and so also in annealing, <fcc. It is evident that a change of the •
internal condition is here encountered, exactly similar to the transition from a solid to a
liquid, although there is no evident physical change. It is probable that attentive study
would lead to the discovery of a similar change in other substances.

13 The particles of steel are linked together or connected more closely than those of
the other metals ; this is shown by the fact that it only breaks with a tensile strain of
60-80 kilos per sq. mm., whilst wrought iron only withstands about 80 kilos, cast iron
10, copper 85, silver 28, platinum 80, wood 8. The elasticity of iron, steel, and other
metals ig expressed by the so-called coefficient of elasticity. Let a rod be taken whose
length is L ; if a weight, P, be hung from the extremity of it, it will lengthen to L
The less it lengthens under other equal conditions, the more elastic the material, if ife
resumes its original length when the weight is removed. It has been shown by experiment
that the increase in length I, due to elasticity, is directly proportional to the length L
and the weight P, and inversely proportional to the section, but changes with the
material. The coefficient of elasticity expresses that weight (in kilos per sq. mm.)
under which a rod having a square section taken as 1 (we take 1 sq. mm.) acquires
double the length by tension. Naturally in practice materials -do not withstand such a,
lengthening, under a certain weight they attain a limit of elasticity, i.e. they stretch
permanently (undergo deformation). Neglecting fractions (as the elasticity of metal*
varies not only with the temperature, but also with forging, purity, &c.), the coefficient
of elasticity of steel and iron is 20,000, copper and brass 10,000, silver 7,000, glass 6,000,
lead 2,000, and wood 1,200.

334 PRINCIPLES OF CHEMISTRY

particularly, ammonia is always found in iron rust ; the ammonia must
arise from the action of the hydrogen of the water, at the moment of its.
separation, on the nitrogen of the air. Highly -polished steel does not
rust nearly so readily, but if moistened with water, it easily becomes
coated with rust. As rust depends on the access of moisture, iron may
be preserved from rust by coating it with substances which prevent
the moisture having access to it. Thus arises the practice of covering
iron objects with paraffin,13 varnish, oil, paints, or enamelling it
with a glassy-looking flux possessing the same coefficient of expansion as
iron, or with a dense scoria (formed by the heat of superheated steam),
or with a compact coating of various metals. Wrought iron (both as
sheet iron and in other forms), cast iron, and steel are often coated with
tin, copper, lead, nickel, and similar metals, which prevent contact with
the air. These metals preserve iron very effectually from rust if they
form a completely compact surface, but in those places where the iron
becomes exposed, either accidentally or from wear, rust appears much
more quickly than on a uniform iron surface, because, towards these
metals (and also towards the rust), the iron will then behave as an
electro-positive pole in a galvanic couple, and hence will attract
•oxygen. A coating of zinc does not produce this inconvenience, because
iron is electro-negative with reference to zinc, in consequence of which
galvanised iron does not easily rust, and even an iron boiler containing
some lumps of zinc rusts less than one without zinc.14 Iron oxidises
at a high temperature, forming iron scale, Fe3O4, composed of ferrous
and ferric oxides, and, as has been seen, decomposes water and acids
with the evolution of hydrogen. It is also capable of decomposing
salts and oxides of other metals, which property is applied in the arts
for the extraction of copper, silver, lead, tin, &c. For this reason
iron is soluble in the 'solutions of many salts — for instance, in cupric
sulphate, with precipitation of copper and formation of ferrous sul-
phate.15 When iron acts on acids it always/orms compounds FeX2 —

18 Paraffin is one of the best preservatives for iron against oxidation in the air. I
found this "by experiments about 1860, and immediately published the fact. This method
is now very generally applied.

»* See Chapter XVIII., Note 84 bis. Based on the rapid oxidation of iron and its
increase in volume in the presence of water and salts of ammonium, a packing is used
for water mains and steam pipes which is tightly hammered into the socket joints.
This packing consists of a mixture of iron filings and a small quantity of sal-ammoniac
(and sulphur) moistened with water ; after a certain lapse of time, especially after the
pipes have been used, this mass swells to such an extent that it hermetically seals the
joints of the pipes.

16 Here, however, a ferric salt may also be formed (when all the iron has dissolved
and the cupric salt is still in excess), because the cupric salts are reduced by ferrous
salts. Cast iron is also dissolved.

IRON, COBALT, AND NICKEL 835

that is, corresponding to the suboxide FeO — and answering to magnesium
compounds — and hence two atoms of hydrogen are replaced by one
atom of iron. Strongly oxidising acids like nitric acid may transform
the ferrous salt which is forming into the higher degree of oxidation or
ferric salt (corresponding with the sesquioxide, Fe2O3), but this is a
secondary reaction. Iron, although easily soluble in dilute nitric acid,
loses this property when plunged into strong fuming nitric acid ; after
this operation it even loses the property of solubility in other acids
until the external coating formed by the action of the strong nitric
acid is mechanically removed. This condition of iron is termed the
passive state. The passive condition of iron depends on the formation,
on its surface, of a coating of oxide due to the iron being acted on by
the lower oxides of nitrogen contained in the fuming nitric acid.16
Strong nitric acid which does not contain these lower oxides, does not
render iron passive, but it is only necessary to add some alcohol or
other reducing agent which forms these lower oxides* in the nitric acid,
and the iron will assume the passive state.

Iron readily combines with non-metals — for instance, with chlorine,
iodine, bromine, sulphur, and even with phosphorus and carbon ; but
on the other hand the property of combining with metals is but little
developed in it — that is to say, it does not easily form alloys. Mercury,
which acts on most metals, does not act directly on iron, and the iron
amalgam, or solution of iron in mercury, which is used for electrical
machines, is only obtained in a particular way — namely, with the
co-operation of a sodium amalgam, in which the iron dissolves and by
means of which it is reduced from solutions of its salts.

When iron acts on acids it forms ferrous salts of the type FeX2,
and in the presence of air and oxidising agents they change by degrees
into ferric salts of the type FeX3. This faculty of passing from the
ferrous to the ferric state is still further developed in ferrous hydroxide.
If sodium hydroxide be added to a solution of ferrous sulphate or
green vitriol, FeSO4,17a white precipitate of ferrous hydroxide, FeH2O2,

16 Powdery reduced iron is passive with regaxd to nitric acid of a specific gravity of
1*87, but when heated the acid acts en it. This passiveness disappears in the magnetic
field. Saint-Edme attributes the passivenesa of iron (and nickel) to the formation of
nitride of iron on the surface of the metal, because he observed that when heated in dry
hydrogen ammonia is evolved by passive iron.

Eemsen observed that if a strip of iron be. immersed in acid and placed in the mag-
netic field, it is principally dissolved at its middle part— that is, the acid acts more feebly
at the poles. According to Etard (1891) strong nitric acid dissolves iron in making it
passive, although the action is a very slow one.

17 Iron vitriol or green vitriol, sulphate of iron or ferrous sulphate, generally crys-
tallises from solutions, like magnesium sulphate, with seven molecules of water,
FeS04,7H2O. This salt is not only formed by the action of iron on sulphuric acid, but

836 PRINCIPLES OF CHEMISTRY

is obtained ; but on exposure to the air, even under water, it turns
green, becomes grey, and finally turns brown, which is due to the
oxidation that it undergoes. Ferrous hydroxide is very sparingly
soluble in water ; the solution has, however, a distinct alkaline reaction,
which is due to its being a fairly energetic basic oxide. In any case,
ferrous oxide is far more energetic than ferric oxide, so that if ammonia
be added to a solution containing a mixture of a ferrous and ferric
salt, at first ferric hydroxide only will be precipitated. If barium
carbonate, BaC03, be shaken up in the cold with ferrous salts, it
does not precipitate them — that is, does not change them into ferrous
carbonate ; but it completely separates all the iron from the ferric
salts in the cold, According to the equation Fe2Cl6 -}- 3BaCO3 + 3H2O
== Fe203,3H20 + 3BaCl2 + 3CO2. If ferrous hydroxide be boiled with,
a solution of potash, the water is decomposed, hydrogen is evolved, and
the ferrous hydroxide is oxidised. The ferrous salts are in all. respects
similar to the salts of magnesium and zinc ; they are isomorphous
with them, but differ from them in that the ferrous hydroxide is not
soluble either in aqueous potash or ammonia. In the presence of an
excess of ammonium salts, however, a certain proportion of the iron

also by the action of moisture and air on iron pyrites, especially when previously roasted
(FeS2 + O3 = FeS + SO2), and in this condition it easily absorbs the oxygen of damp air
(FeS + O4 = FeS04). Green vitriol is obtained in many processes as a bye-product.
Ferrous sulphate, like all the ferrous salts, has a pale greenish, colour hardly perceptible
in solution. If it be desired to preserve it without change — that is, so as not to contain
ferric compounds — it is necessary to keep it hermetically sealed. This is best done by.
expelling "the air by means of sulphurous anhydride or ether , sulphurous anhydride,
SO2, removes oxygen from ferric compounds, which might be formed, and is itself
changed into sulphuric acid, and hence the oxidation of the ferrous compound does not
take place in its presence. Unless these precautions are -taken, green vitriol turns
brown, partly changing into the ferric salt. "When turned brown, it is not completely
Soluble in water, because during its oxidation a certain amount of free insoluble ferric
oxide is formed: 6FeSO4 + O3 = 2Fe2(SO4)3 + Fe2O3. In order to cleanse such mixed
green vitriol from the oxide, it is necessary to add some sulphuric acid and iron and boil
the mixture; the ferric salt is then transformed into the ferrous state: Fe8(S04)3 + Fe
= 8FeS64.

Green vitriol is used for the manufacture of Nordhausen sulphuric acid (Chapter
XX.), for preparing ferric .oxfde, in many dye works (for preparing the indigo vats and
reducing blue indigo to white), and in many other processes, it is also a very good
disinfectant, ''and is the cheapest salt from which other compounds of iron may be
obtained.

The other ferrous salts (excepting the yellow prussiate, which will be mentioned later
are but little used, and it is therefore unnecessary to dwell upon them. We will only
mention ferrous chloride, which, in the crystalline state, has the composition
FeCl2,4H2O. It is easily prepared . for instance, by the action of hydrochloric acid on
iron, and in the anhydrous state by the action of hydrochloric acid gas on metallic iron
at a red heat. The anhydrous ferrous chloride then volatilises in the form of colourless
cubic crystals. Ferrous oxalate (or the double potassium salt) acts as a powerful
reducing agent, and is frequently employed in photography (as a developer).

ERON, COBALT, AND NICKEL 337

is not precipitated by alkalis and alkali carbonates, which fact points
to the formation of double ammonium salts.18 The ferrous salts have
a dull greenish colour, and .form solutions also of a pale green colour,
whilst the ferric salts have a brown or reddish-brown colour. Thu
ferrous salts, being capable of Oxidation, form very active reducing
agents— for instance, under their action gold chloride, AuCl3, deposits
metallic gold, nitric acid is transformed into lower oxides, and the
nighest oxides of manganese also pass into the lower forms of oxidation.
All these reactions take place with especial ease in the presence of an
excess of acid. This depends on the fact that the ferrous oxide, FeO
(or salt), acting as a reducing agent, turns into ferric oxide, Fe2C>3 (or
salt), and in the ferric state it requires more acid for the formation
of a normal salt than in the ferrous condition. Thus in the normal
ferrous sulphate, FeSO4, there is one equivalent of iron to one
equivalent of sulphur (in the sulphuric radicle), but in the neutral
ferric salt, Fe2(S04)3, there is one equivalent of iron to one and 'a
half of sulphur in the form of the elements of sulphuric acid.19

The most simple oxidising agent for transforming ferrous into ferric
salts is chlorine in the presence of water— for instance, 2FeCl2 4- Cla

18 Ferrous sulphate, like magnesium sulphate, easily forms double salts — for instance,
(NH4)aSO4,FeSO4,6H2O. This salt does not oxidise in air so readily as green vitriol, and
is therefore used for standardising K^MnO4

19 The transformation of ferrous oxide into ferric oxide is not completely effected in
air, as then only a part of the suboxide is converted into ferric oxide. Under these
circumstances the so-called magnetic oxide of iron is generally produced, which contains
atomic quantities of the suboxide and oxide— namely, FeO,Fe2O5 = Fe3O4. This sub-
stance, as already mentioned, is found in nature and in iron scale. It is also formed
when most ferrous and ferric salts are heated in air; thus, for instance, when ferrous
carbonate, FeCO5 (native or the precipitate given by soda in a solution of FX2), is
heated ifc loses the elements of carbonic anhydride, and magnetic oxide remains. This
oxide of iron is attracted by the magnet, and is on this .account called magnetic oxide,
although it does not always show magnetic properties. If magnetic oxide be dissolved in
any acid — for instance, hydrochloric — which does not act as an oxidising agent, a ferrous
salt is first formed and ferric oxide remains, which is also capable of passing into
solution. The best way of preparing the hydrate of the magnetic oxide is by decomposing
a mixture of ferrous and ferric salts with ammonia ; it is, however, indispensable to pour
this mixture into the ammonia, and not vice versd, as in that case the ferrous oxide would
at first be precipitated alone, and then the ferric oxide. The compound thus formed has a
bright green colour, and when dried forms a black powder. Other combinations of
ferrous with ferric oxide are known, as are also compounds of ferric oxide with other
bases. Thus, for instance, compounds are known containing 4 molecules of ferrous oxide
to 1 of ferric oxide, and also 6 of ferrous to 1 of ferric oxide. These are also magnetic,
and are formed by heating iron in air. The magnesium compound MgO,Fe2O3 is
prepared by passing gaseous hydrochloric acid over a heated mixture of magnesia and
ferric oxide. Crystalline magnesium oxide is then formed, and black, shiny, octahedral
crystals of the above-mentioned composition. This compound is analogous- to the
aluminates— for instance, to spinel. Bernheim (1888) and Rousseau (1891) obtained
many similar compounds of ferric oxide, and their composition apparently corresponds
to the hydrates (Note 22) known for the oxide.

838 PRINCIPLES OF CHEMISTRY

= Ye2Cl6, or, generally speaking, 2FeO -f C12 + H2O = Fe2O3 + 2HC1
When such a transformation is required it is best to add potassium
chlorate and hydrochloric acid to the ferrous solution ; chlorine is
formed by their mutual reaction and acts as an oxidising agent.
Nitric acid produces a similar effect, although more slowly Ferrous
salts may be completely and rapidly oxidised into ferric salts by means
of chromic acid or permanganic acid, HMnO4, in the presence of acids —
for example, 10FeSO4 + 2KMn04 + 8H2SO4 = 5Fe2(S04)3 + 2MnS04
4- K2S04 + 8H20. This reaction is easily observed by the change of
colour, and its termination is easily seen, because potassium perman-
ganate forms solutions of a bright red colour, and when added to a
solution of a ferrous salt the above reaction immediately takes place in
the presence of acid, and the solution then becomes colourless, because all
'the substances formed are only faintly coloured in solution. Directly all
the ferrous compound has passed into the ferric state, any excess of
permanganate which is added communicates a red colour to the liquid
(see Chapter XXI.)

Thus when ferrous salts are acted on by oxidising agents, they pass
into the ferric form, and under the action of reducing agents the
reverse reaction occurs. Sulphuretted hydrogen may, for instance, be
used for this complete transformation, for under its influence ferric salts
are reduced with separation of sulphur — for example, Fe2Cl6 + H2S
= 2FeCl2 + 2HC1 + S. Sodium thior,ulphate acts in a similar way :
Fe2Cl6 + Na2S203 + H2O = 2FeCl2 -f Na2SO4 + 2HC1 + S. Me-
tallic iron or zinc,20 in the presence of acids, or sodium amalgam, <fec.,
acts like hydrogen, and has also a similar reducing action, and this
furnishes the best method for reducing ferric salts to ferrous salts —
for instance, Fe2Cl6 + Zn = 2FeCl2 + ZnCl2. Thus the transitionfrorn
fsrrous salts to ferric softs and vice versd is always possible?1

30 Copper and cuprous salts also reduce ferric oxide to ferrous oxide, and are them*
selves turned into cupric salts. The essence of the reactions is expressed by the following
equations . Fe2O3 + CajO+ 2FeO + 2CuO ; Fe8O3 + Cu = 2FeO + CuO. This fact is made
use of in analysing copper compounds, the quantity of copper being ascertained by the
amount of ferrous salt obtained. An excess of ferric salt is required to complete the
reaction. Here we have an example of reverse reaction ; the ferrous oxide or its salt in
the presence of alkali transforms the cupric oxide into cuprous oxide and metallic copper,
as observed by Lovel, Knopp, and others.

J1 We will here mention the reactions by means of which it may be ascertained
whether the ferrous compound has been entirely converted into a ferric compound or
vice versd. There are two substances which are best employed for this purpose:
potassium ferricyanide, FeE^C^Ne, and potassium thiocyanate* KCNS. The first salt
gives with ferrous salts a blue precipitate of an insoluble salt, having a composition
Fe5C12N13 , but with ferric salts it does not form any precipitate, and only gives a brown
colour, and therefore when transforming a ferrous salt into a ferric salt, the completion
of the transformation may be detected by taking a drop of the liquid on paper or on ft
porcelain plate and adding a drop of the ferricyanide solution. If a blue precipitate be

IRON, COBALT, AND NICKEL 339

Ferric oxide, or sesquioxide of iron, Fe2O3, is found in nature,
and is artificially prepared in the form of a red -powder by many
methods. Thus after heating green vitriol a red oxide of iron remains,
called colcothar, which is used as an oil paint, principally for painting
wood. The same substance in the form of a very tine powder (rouge)
is used for polishing glass, steel, and other objects. If a mixture of
ferrous sulphate with an excess of common salt be strongly heated,
crystalline ferric oxide will be formed, having a dark violet colour, and
resembling some natural varieties of this substance. When iron pyrites
is heated for preparing sulphurous anhydride, ferric oxide also remains
behind ; it is used as a pigment. On the addition of alkalis to a
solution of ferric salts, a brown precipitate of ferric hydroxide is formed,
which when heated (even when boiled in water, that is, at about 100°,
according to Tomassi) easily parts with the water, and leaves red
anhydrous ferric oxide. Pure ferric oxide does not show any mag-
netic properties, but when heated to a white heat it loses oxygen and is
converted into the magnetic oxide. Anhydrous ferric oxide which has
been heated to a high temperature is with difficulty soluble in acids
(but it is soluble when heated in strong acids, and also when fused with
potassium hydrogen sulphate), whilst ferric hydroxide, at all events
that which is precipitated from salts by means of alkalis, is very readily
soluble in acids. The precipitated ferric hydroxide has the composition
2Fe20;i3H2O, or Fo4H609. If this ordinary hydroxide be rendered
anhydrous (at 100°), at a certain moment it becomes incandescent
• — that is, loses a certain quantity of heat. This self -incandescence
depends on internal displacement produced by the transition of the
easily- soluble (in acids) variety into the difficultly-soluble variety,
and does not depend on the loss of water, since the anhydrous oxide
.undergoes the same change. In addition to this there exists a ferric
hydroxide, or hydrated oxide of iron, which, like the strongly-heated
anhydrous iron oxide, is difficultly soluble in acids. This hydroxide on
losing water, or after the loss of water, does not undergo such self-
incandescence, because no such state of internal displacement occurs
(loss of energy or heat) with it as that which is peculiar to the ordinary
Oxide of iron. The ferric hydroxide which is difficultly soluble in acids
has the composition Fe203,H2O. This hydroxide is obtained by a pro-,

formed, then part of the ferrous salt still remains ; if there is none, the transformation is
complete. The thiocyanate does not give any marked coloration with ferrous salts ; but
,with ferric salts in the most diluted state it forms a bright rod soluble compound, and
therefore when transforming a ferric salt into a ferrous salt we must proceed as before,
testing a drop of the solution with thiocyanate, when the absence of a red colour will
prove the total transformation of the ferric salt into the ferrous state, and if a red colour
Is apparent it shows that, the transformation is not yet complete.

840 PEINCIPLES OF CHEMISTRY"

longed ebullition of water in which ferric hydroxide prepared by the
oxidation of ferrous oxide is suspended, and also sometimes by similar
treatment of the ordinary hydroxide after it has been for a long time,
in contact with water. The transition of one hydroxide to another is
apparent by a change of colour ; the easily-soluble hydroxide is redder,
and the sparingly-soluble hydroxide more yellow in colour.22

The normal salts of the composition Fe2X6 or FeX3 correspond
with ferric oxide — for example, the exceedingly volatile ferric chloride,
Fe2Cl6, which is easily prepared in the anhydrous state by the action
of chlorine on heated iron.23 Such also is the normal ferric nitrate,

22 The"* two ferric hydroxi'des are not only characterised by the above-mentioned
properties, but also by the fact that the first hydroxide forms immediately with potassium
ferrocyanide, K4FeCbNtJ> a blue colour depending on the formation -of Prussian blue,
whilst the second hydroxide does not give any reaction whatever with this salt. The
first hydroxide is entirely soluble in nitric, hydrochloric, and all other acids; whilst the
second sometimes (not always) forms a brick-coloured liquid, which appears turbid
and does not give the reactions peculiai^ to the ferric salts (Pean de Saint-Gilles,
Scheurer-Kestner). In addition to this, when the smallest quantity of an alkaline salt
is added to this liquid, ferric oxide is precipitated. Thus a colloidal solution is formed
(hydrosol), which is exactly similar to silica hydrosol (Chapter XVII.), according to
which example the hydrosol of ferric oxide may be obtained.

If ordinary ferric hydroxide be dissolved in acetic acid, a solution of the colour of red
wine is obtained, which has all the reactions characteristic of ferric salts. But if this
solution (formed in the cold) be heated to the boiling-point, its colour is very rapidly
intensified, a smell of acetic acid becomes apparent, and the solution then contains a
new variety of ferric oxide. If the boiling of the solution be continued, acetic acid is
evolved, and the modified ferric oxide is precipitated. If the evaporation of the acetic
acid be prevented (in a closed or sealed vessel), and the liquid be heated for some time,
the whole of the ferric hydroxide then passes into the insoluble form, and if some alkaline
salt be added (to the hydrosol formed), the whole of the ferric oxide is then precipitated,
in its insoluble form. This method may be applied for separating ferric oxide from
solutions of its salts.

All phenomena observed respecting ferric oxide (colloidal properties, various forms,
formation of double basic salts) demonstrate that this substance, like silica, alumina,
lead hydroxide, &c., is polymerised, that the composition is represented by (Fe^QjJn.

JS The ferric compound which is most used in practice (for instance, in medicine, for
cauterising, stopping bleeding, &c.— Oleum Martis) is ferric chloride, Fe^Cle, easily
obtained by dissolving the ordinary hydrated oxide of iron in hydrochloric acid. It" ia
obtained in the anhydrous state by the action of chlorine on heated iron. The experi-
ment is carried on in a porcelain tube, and a solid volatile substance is then formed in
the shape of brilliant violet scales which very readily absorb moisture from the air, and
when heated with water decompose into crystalline ferric oxide and hydrochloric acid :
Fe.2Cl6 + 3H2O = 6HCl + Fe2O.v Ferric chloride is so volatile that the density of its
vapour may be determined. At 440° it .is equal to 164'0 referred to hydrogen ; the
formula Fe2Cl6 corresponds with a density of 162'5. An aqueous solution of this salt
has a brown colour. On evaporating and cooling this solution, crystals separate con-
taining 6 or 12 molecules of H.>O. Ferric chloride is not only soluble in water, but also
in alcohol (similarly to magnesium chloride, &c.) and in ether. If the latter solutions
axe exposed to the rays of the sun they become colourless, and deposit ferrous chloride,
FeCl2, chlorine being disengaged. After a certain lapse of time, the aqueous solutions
of ferric chloride decompose with precipitation of a basic salt, thus demonstrating the
instability of ferric chloride, like the other salts of ferric oxide (Note 22). This salt is

IRON, COBALT, AND NICKEL

341

Fe2(N03)6 ; it is obtained by dissolving iron in an excess of nitric acid,

much more stable in the form of double salts, like all the ferric salts and also the
salts of many other feeble bases. Potassium or ammonium chloride forms with it very
beautiful red crystals of a double salt, having the composition Fe2Cl6,4KCl,2H2O.
When a solution of this salt is evaporated it decomposes, with separation of potassium
chloride.

B. Eoozeboom (1892) studied in detail (as for CaCl2, Chapter XIV., Note 50) the
separation of different hydrates from saturated solutions of Fe2Cl6 at various concen-
trations and temperatures ; he found that there are 4 crystallohydrates with 12, 7, 5, and
4 molecules of water. An orange yellow only slightly hygroscopic hydrate, Fe2Cl6,12H20,
is most easily and usually obtained, which melts at 37° ; its solubility at different tempera-
tures is represented by the curve BCD in the accompanying figure, where the point B

0' 50'

FIG. 95.— Diagram of the solubility of FeaCl.

100'

corresponds to the formation, at -55°, of a cryohydrate containing about Fe2Cl,j + S6H20,
the point C corresponds' to the melting-point ( + 37°) of the hydrate Fe2Cl6,12H2O, and
the curve CD to the fall in the temperature of crystallisation with an increase in the
amount of salt, or decrease in the amount of water (in the figure the temperatures are
taken along the axis of abscissae, and the amount of n in the formula nEe^Clg -4- 100H O
along the axis of ordinates). When anhydrous Fe2Cl6 is added to the above hydrate
(12H20), or some of the water is evaporated from the latter, very hygroscopic
crystals of Fe.,Cl6)5H2O (Fritsche) are formed ; they melt at 56°, their solubility is
expressed by the curve HJ, which also presents a small branch at the end J This
again gives the fall in the temperature of crystallisation with an increase in the amount
of Fe.tC\6. Besides these curves and the solubility of the anhydrous salt expressed by
the line KL (up to 100°, beyond which chlorine is liberated), Roozeboom also gives the
two curves, EFG and JK, corresponding to the crystallohydrates, Fe2Cl6,7H2O (melts at
+ S2°-5, that is lower than any of the others) and Fe.jCla^HjO (melts at 730<5), which
he discovered by a systematic research on the solutions of ferric chloride. The curve
AB represents the separation of ice from dilute solutions of the salt.

The researches of the same Dutch chemist upon the conditions of the formation of
Crystals from the double salt (NH4Cl)4Fe2ClcJ2H2O are even more perfect. This salt
was obtained in 1889 by Fritsche, and is easily formed from a strong solution of Fe4Cl<,
by adding sal-ammoniac, when it separates in crimson rhombic crystal.?, which, after
dissolving in water, only deposit again on evaporation, together with the sal-ammoniac.

Roozeboom (1892) found that when the solution contains b molecules of FeuClc, and

342

PRINCIPLES OF CHEMISTRY

b-i-

taking care as far as possible to prevent any rise of temperature.1* Tht
normal salt separates from the brown solution when it is concentrated

a molecules of NH4C1, per 100 molecules H2O, then at 15° one of the following separa-
tions takes place : (1) crystals, Fe2Cl6,12H2O, when a varies between 0 and 11, and 6
between 4'65 and 4'8, or (2) a mixture of these crystals and the double salt, when a,
-1-86, and 6 = 4-47, or (8) the double salt, Fejd^NEUCl^iHjO, when a varies
between 2 and 11'8, and 6 between 8'1 and 4'56, or (4) a mixture of sal-ammoniac with
the iron salt (it crystallises in separate cubes, Retgers, Lehmann), when a varies
between 7'7 and 10'9, and 6 is less than 8'88, or (5) sal-ammoniac, when a = ll'88. And
as in the double salt, a ' 6-4 : 1 it is evident that the double salt only separates out
when the ratio a ' b is less than 4 . 1 (i.e. when Fe2Cl6 predominates). The above is

seen more clearly in the accom-
panying figure, where a, or the
number of molecules of NH4C1
per 100H2O, is taken along the
axis of abscissae, and 6, or the
number of -molecules of FeaCl6,
along the ordinates. The curves
ABCD correspond to saturation
and present an iso- therm of 15°.
The portion AB corresponds to
the separation of chloride of iron
(the ascending nature of this
curve shows that the solubility of
Fe2Cle is increased by the pre-
sence of NH4C1, while that, of
NH4C1 decreases in the presence
of Fe2Cl6), the portion EC to the
double salt, and the portion CD
to a mixture of sal-ammoniac- and
ferric chloride, while the straight

line OF corresponds to the ratio Fe2Cl6,4NH4Cl, or a : &"4 : 1. The portion CE shows
that more double salt may be introduced into the solution without decomposition, but-
then the solution deposits a mixture of sal-ammoniac and ferric chloride (see Chapter
XXIV. Note 9 bb). If there were more such well-investigated cases of solutions, our
knowledge of double salts, solutions, the influence of water, equilibria, isomorphous
mixtures, and such-like provinces of chemical relations might be considerably advanced.
24 The normal ferric salts are decomposed by heat and even by water, forming basic
salts, which may be prepared in various ways. Generally ferric hydroxide is dissolved
in solutions of ferric nitrate ; if it contains a double quantity of iron the basic salt is
formed which contains Fe2O3 (in the form of hydroxide) + 2Fe2(NO3)6=3Fe./)(NO3)4,
a salt of the type Fe2OX4. Probably water enters into its composition. With con-
siderable quantities of ferric oxide, insoluble basic salts are obtained containing various
amounts of ferric hydroxide. Thus when a solution of the above-mentioned basic acid
is boiled, a precipitate is formed containing 4(Fe2O3)8,2(N2O5),8H2O, which probably
Contains 2Fe2Os(NO3)2 + 2Fe8O3,8H2O If a solution of basic nitrate be sealed in a
tube and then immersed in boiling water, the colour of the solution changes just in
the same way as if a solution of ferric acetate had been employed (Note 22). The
solution obtained smells strongly of nitric acid, and on adding a drop of sulphuric or
Hydrochloric acid the insoluble variety of hydrated ferric oxide is precipitated.

Normal ferric orthopjiosphaf is soluble in sulphuric, hydrochloric, and nitric acids,
but insoluble in others, such as, for instance, acetic acid. The composition of this salt
in the anhydrous state is FePO4, because in orthophosphoric acid there are three atoms
of hydrogen, and iron, in the ferric state, replaces the three atoms of hydrogen. This
.Bait is obtained from ferric acetate, which, with disodium phosphate, forms a white pre-

II IS

FIG. 96.— Diagram of the formation, at 15°, of the double
salt FeaCle4NH4C12HaO or FeCNH^Cl.E^O, (After
Roozeboom.)

IRON, COBALT, AND NICKEL 343

under a bell jar over sulphuric acid. This salt, Fe2(N03)6,9H20, then
orystallises in well-formed and perfectly colourless crystals,25 which
deliquesce in air, melt at 35°, and are soluble in and decomposed by
water, The decomposition may be seen from the fact that the solution
is brown and does not yield the whole of the salt again, but gives
partly basic salt. The normal salt (only stable in the presence o£ an
excess of HN03) is completely decomposed with great facility by heat-
ing with water, even at 130°, and this is made use of for removing iron
(and also certain other oxides of the form R203) from many other
bases (of the form RO) whose nitrates are far more stable. The ferric
salts, FeX3, in passing into ferrous salts, act as oxidising agents, as is
seen from the fact that they not only liberate S from SH2, but also
iodine from KI like many oxidising agents.25 bis

cipitate of FeP04, containing water. If a solution of ferric chloride (yellowish-red
colour) be mixed with a solution of sodium acetate in excess, the liquid assumes an
intense brown colour which demonstrates the formation of a certain quantity of ferric
acetate ; then the disodium phosphate directly forms a white gelatinous precipitate of ferrio
phosphate. By this means the whole of the iron may be precipitated, and the liquid which
was brown then becomes colourless. If this normal salt be dissolved in orthophosphorio
acid, the crystalline acid salt Fel^POJ^ is formed. If there be an excess of ferric oxide
in the solution, the precipitate will consist of the basic salt. If ferric phosphate be
dissolved in hydrochloric acid, and ammonia be added, a salt is precipitated on heating
which, after continued washing in water and heating (to remove the water), has the
composition Fe4P;jOu — that is, 2Fe.iO3)P3O5. In an aqueous condition this salt may be
considered as ferric hydroxide, Ee2(OH)6, in which (OH)3 is replaced by the equivalent
group PO4. Whenever ammonia is added to a solution containing an excess of ferric
salt and a certain amount of phosphoric acid, a precipitate is formed containing the
whole of the phosphoric acid in the mass of the ferric oxide".

Ferric oxide is characterised as a feeble base, and also by the fact of its forming double
salts— for instance, potassium iron alum, which has a composition FeJ(S04)3,K2SO4,
24H3O or FeK(SO4)2,12H3O. It is obtained in the form of almost colourless or light
rose-coloured large octahedra of the regular system by simply mixing solutions of
potassium sulphate and the ferric sulphate obtained by dissolving ferric oxide in sul-
phuric acid.

. 25 It would seem that all normal ferric salts are colourless, and that the brown colou*
which is peculiar to the solutions is really due to basic ferric salts. "A remarkable
example of the apparent change of colour of salts is represented by the ferrous and ferric
oxalates. The former in a dry state has a yellow colour, although as a rule the ferrous
salts are green, and the latter is colourless or pale green. When the normal ferric salt is
dissolved in water it is, like many salts, probably decomposed by the water into acid
and basic salts, and the latter communicates a brown colour to the solution. Iron alum
is almost colourless, is easily decomposed by water, and is the best proof of our asser-
tion. The study of the phenomena peculiar to ferric nitrate might, in my opinion, give
ft very useful addition to our knowledge of the aqueous solutions of salts in general.

23 bis The reaction FeX5 + KI = FeX3 + KX + I proceeds comparatively slowly in solu-
tions, is not complete (depends upon the mass), and is reversible. In this connection we
may cite the following data from Seubert and Rohrer's (1894) comprehensive researches.
The investigations were conducted with solutions containing T\j gram— equivalent
weights of Fea(S04)3 (i.e.' containing 20 grams of salt per litre), and a corresponding
solution of KI ; the amount of iodine liberated being determined (after the addition of
starch) by a solution (also T\j normal) of Na^S-jO^ (see Chapter XX., Note 42). The pro-

*D

$44 PRINCIPLES OF CHEMISTRY

'Iron forms one other oxide besides the ferric and ferrous oxides ;
this contains twice as much oxygen as the former, but is so very
unstable that it can neither be obtained in the free state nor as a
hydrate. Whenever sjjch conditions of double decomposition occur as
should allow of its serration in the free state, it decomposes into
Oxygen and ferric oxide. It is known in the state of salts, and is only
stable in the presence of alkalis, and forms salts with them which have
a decidedly alkaline reaction ; it is therefore a feebly acid oxide. Thus
when small pieces of iron are heated with nitre or potassium chlorate
a potassium salt of the composition K2Fe04 is formed, and therefore
the hydrate corresponding with this salt should have the composition
H2FeO4. It is called ferric acid. Its anhydride ought to contain
Fe03 or Fe2O6 — twice as much oxygen as ferric oxide. If a solution
of potassium ferrate be mixed with acid, the free hydrate ought
to be formed, but it immediately decomposes (2K2FeO4 -f SH.^SC^
= 2K2S04 + Fe2(SO4)3 + 5H2O + O3), oxygen being evolved. If a
small quantity of acid be taken, or il; a solution of potassium ferrate
be heated with solutions of other metallic salts, ferric oxide is sepa-
rated— for instance :

2CuS04 + 2K2Fe04 = 2K2S04 + 03 -f Fe203 + 2CuO.

Both these oxides are of course deposited in the form of hydrates.
This shows that not only the hydrate H2FeO4, but also the salts of the
heavy metals corresponding with this higher oxide of iron, are not
formed by reactions of double decomposition. The solution of potas-
sium ferrate naturally acts as a powerful oxidising agent ; for instance,
it transforms manganous oxide into the dioxide, sulphurous into
sulphuric acid, oxalic acid into carbonic anhydride and water, &c.26
Iron thus combines with oxygen in three proportions : RO, R203,

gross of the reaction was expressed by the amount of liberated iodine in percentages

of the theoretical amount. For instance, the following amount of iodide of potassium
was decomposed when Fe2(S04)5+2nKI was taken:

»•> 1 28 6 10 20

After 15' 11-4 26'3 40'6 78'5 01'8 96'0

p 80* 14-0 85-8 47-8 78'5 94'3 97'4

„ Ihour 19-0 42'7 66'0 84'0 95'7 97'6

„ 10 '„ 82-6 56-0 75-7 93'2 96'5 97'6

„ 48. „ 89'4 C7'7, 82'6 93'4 96'6 97*6

Similar results were obtained for FeCl3, but then the amount of iodine liberated was
eomewhat greater. Similar results .were also obtained by increasing the mass of FeX$
per KI, and by replacing it by HI (see Chapter XXI., Note 26).

|,a 26 If chlorine be passed through a strong solutidn of potassium hydroxide in which
hydrated ferric oxide is suspended, the turbid liquid acquires a dark pomegranaie-red
colour and contains potassium ferrate : 10KHO + Fe303 + 8C13 -' 2KjFeO4 + 6KC1 + SlLjO.
The chlorine must not be in excess, otherwise the salt is again decomposed, although the
mode of decomposition is unknown ; however, ferric chloride and potassium chlora ,0
are probably formed. Another way in which the above-described salt is formed is also

IRON, COBALT. AND NICKEL 345

and RO3. It might have been expected that there would be inter-
mecliate stages RO2 (corresponding to pyrites FeS2) a^nd R2O5, but for
iron these are unknown. 26bis The lower oxide has a, distinctly basic
character, the higher is feebly acid. The only one which is stable in
the free state is ferric oxide, Fe2O3 ; the suboxide, FeO, absorbs
oxygen, and ferric anhydride, FeO3, evolves it. It is also the same
for other elements ; the character of each is determined by the relative
degree of stability of the known oxides. The salts FeX2 correspond
with the suboxide, the salts FeX3 or Fe2X6 with the sesquipxide, and
FeX6 represents those of ferric acid, as its potassium salt is Fe02(OK)2,
corresponding with K2SO4, K2Mn04, K2Cr04, &c. Iron therefore
forms compounds of the types FeX2, FeX3, and FeX6, but this latter,
like the type NX5, does not appear separately, but only when X re-
presents heterogeneous elements or groups ; for instance, for nitrogen
in the form of NO2(OH), NH4C1, &c., for iron in the form of
Fe02(OK)2. But still the type FeX6 exists, and therefore FeX2 and
FeX3 are compounds which, like ammonia, NH3, are capable of further
combinations up to FeX6 ; this is also seen in the property of
ferrous and ferric salts of forming compounds with water of crystallisa-
tion, besides double and basic salts, whose stability is determined by
the quality of the elements included in the types FeX2 and FeXs.26 tri
It is therefore to be expected that there should be complex compounds

remarkable ; a galvanic current (from 6 Grove . elements) is passed through cast-iron
and platinum electrodes into -a strong solution of potassium hydroxide. The cast-
iron electrode is connected with- the positive pole, and the platinum electrode is sur-
rounded by a porous earthenware cylinder. Oxygen- would bo evolved at the cast-
iron electrode, but it is used up in oxidation, and a dark solution of potassium ferrate is
, therefore formed about it. It is remarkable that the cast iron cannot be replaced by
wrought iron.

86t>i» When Mond and his assistants obtained the remarkable volatile compound
Ni(CO)4 (described later, Chapter XXII.), it was shown subsequently by Mond and
Quincke (1891), and also by Berthelot, that iron, under certain conditions, in a stream of
carbonic oxide, also volatilises and forms a compound Jike that given by nickel. Koscoe
and Scudder then showed that when water gas is passed through and kept under
pressure (8 atmospheres) in iron vessels, a portion. of the iron volatilises from the
sides of the vessel, and that when the gas is burnt it deposits a certain amount of oxides
of iron (the same result is obtained with ordinary coal gas which contains a small amount
of CO). To obtain the volatile compound of iron with carbonic oxide, Mond prepared
•a finely divided iron by heating the oxalate in & stream of hydrogen, and after cooling it
lo 80°— 45° he passed CO over the powder. The iron then formed (although very slowly)
ft volatile compound containing Fe(CO)6 (as though it answered to a very high type,
PeXjo), which when cooled condenses into a liquid (slightly coloured, -probably owing to
Incipient decomposition), sp. gr. 1-47, which solidifies at -21°, boils at about 103°, and
has a vapour density (about 6'5 with respect to air) corresponding to the above formula ;
> decomposes at 180°. Water and dilute acids do not act upon it, but it decomposes
wider the action of light and forms a hard, non-volatile crystalline yellow compound
ITe3(CO)7 which decomposes at 80° and again forms Fe(CO)5.

80 tn When the molecular Fe8Cl6 is produced instead of FeCl3 this complication o'i.
tthe type also occurs,

346 'PRINCIPLES OF CHEMISTRY

derived from ferrous and ferric oxides. Amongst these the series,
of cyanogen compounds is particularly interesting j their formation,
and character is not only determined by the property which, irort
•possesses of forming complex types, but also by the similar faculty of
the cyanogen compounds, -which, like nitriles (Chapter IX.), have
clearly developed properties of polymerisation and in general of forming
complex compounds.27

In the cyanogen compounds of iron, two degrees might be expected :
Fe(CN)2, corresponding 'with ferrous oxide, and Fe(CN)3, correspond-
ing with ferric oxide. There are actually, however, many other known
compounds, intermediate and far more complex. They correspond
with the double salts so easily formed by metallic cyanides. The two
following double salts are particularly well known, very stable, often^
used, and easily prepared. Potassium ferrocyanide or yellow prussiate
of potash, a double salt of cyanide of potassium and ferrous, cyanide,
has the composition FeC2N2,4KCN ; its crystals contain 3 mol. of water :
K4FeC6N6,3H20. The other is potassium ferricyanide or red prussiate
of potash. It is also known as Gmelin's salt, and contains cyanide of
potassium with ferric cyanide ; its composition is Fe(CN)3,3KCN or
K3FeC6N6. Its crystals do not contain water. It is obtained from,
the first by the action of chlorine, which removes one atom of the
potassium. A whole series of other ferrocyanic -compounds correspond-
with these ordinary salts.

Before treating of the preparation and properties of these two
remarkable and very stable salts, it must be observed that with ordi-
nary reagents neither of them gives the same double decompositions
as the other ferrous and ferric salts, and they both present a series of
remarkable properties. Thus these salts have a neutral reaction, are
unchanged by air, dilute acids, or water, unlike potassium cyanide and
even some of its double salts. When solutions of these salts are treated
with caustic alkalis, they do not give a precipitate of ferrous or fecric
hydroxides, neither are they precipitated by sodium carbonate. This
led the earlier investigators to recognise special independent groupings
in them. The yellow prussiate was considered to contain the complex
radicle FeC6N6 combined with potassium, namely with K4, and K$
was attributed to the red prussiate. This was confirmed by the feic*
that whilst in both salts any other metal, even hydrogen, might be
substituted for potassium, the iron remained unchangeable, just as
nitrogen in cyanogen, ammonium, and nitrates does not enter into

27 Some light may be thrown upon the faculty of Fe of forming various compounds with
CN, by the fact that Fe not only combines with carbon but also with nitrogen. Nitride
of iron Fe2N was obtained by Fowler by heating finely powdered iron in a stream ol
NH3 at the temperature of jnelting lead.^

IRON, COBALT, AND. NICKEI* 847'

double decomposition, being in the state of- the complex radicles CJ$y
NH4, NO2. Such a representation is, however, completely superfluous
for the explanation of the peculiarities in the reactions of such com-
pounds as double salts. If a magnesium salt which can be precipitated
by potassium hydroxide does not form a precipitate in the presence of
ammonium dbloride, it is very clear that it is owing to the formation
of a soluble double salt which is not decomposed by alkalis. And
there is no necessity to account for the peculiarity of reaction of a
double salt by the formation of a new complex radicle. In the same
way also, in the presence of an excess of tartaric acid, cupric salts do
not form a precipitate with potassium hydroxide, because a double salt
is formed. These peculiarities are more easily understood in the case
of cyanogen compounds than in all others, because all cyanogen com-
pounds,, as unsaturated compounds, show a marked tendency to.
complexity. This tendency is satisfied in double salts. The appear-
ance of a peculiar character in double cyanides is the more easily
understood since in the case of potassium cyanide itself, and also in
hydrocyanic acid, a great many peculiarities have been observed
which are not encountered in those haloid compounds, potassium
chloride and hydrochloric acid, with which it was usual to compare
cyanogen compounds. These peculiarities become more comprehensible
on comparing cyanogen compounds with ammonium compounds. Thus
in the presence of ammonia the reactions of many compounds change
considerably. If in addition to this it is remembered that the
presence of many carbon (organic) compounds frequently completely
disturbs the reaction of salts, the peculiarities of certain double cyanides
will appear still less strange, because they contain carbon. The fact
that the presence of carbon or another element in the compound pro-
duces a change in the reactions, may be compared to the action of
oxygen, which, when entering into a combination, also very materially
changes the nature of reactions. Chlorine is not detected by silver
nitrate when it is in, the form of potassium chlorate, KC103, as it is
detected in potassium chloride, KC1. The iron in ferrous and ferric
compounds varies in its. reactions. In addition>to the above-mentioned
facts, consideration ought to be given to the circumstance that the
easy mutability of nitric acid undergoes modification in its alkali
salts, and in general the properties of a salt often differ much from
those of the acid. Every double salt ought to be regarded as a pecu*
liar kind of saline compound : potassium cyanide is, as it were, a basic*
and ferrous cyanide an acid, element. They may be unstable in the
separate state, but form a stable double compound when combined
together ; the act of combination disengages the energy of the elements,

848 PRINCIPLES OF CHEMISTRY

and they, so to speak, saturate each other. Of course, all this is not a
definite explanation, but then the supposition of a special complex radicte
can even less be regarded as such.

Potassium ferrocyanide, K4FeC6N6, is very easily formed by mixing
solutions of. ferrous sulphate and potassium cyanide. First, a white
precipitate of ferrous cyanide, FeC2N2, is formed, which becomes blue
on exposure to air, but is soluble in an excess of potassium cyanide,
forming the ferrocyanide. The same yellow prussiate is obtained on
heating animal nitrogenous charcoal or animal matters — such as
horn, leather cuttings, &c. — with potassium carbonate in iron
vessels,27 bls the mass formed being afterwards boiled with water with
exposure to air, potassium cyanide first appearing, which gives yellow
prussiate. The animal charcoal may be exchanged for wood charcoal,
permeated with potassium carbonate and heated in nitrogen or
ammonia ; the mass thus produced is then boiled in water with ferric
oxide.28 In this manner it is manufactured on the large scale, and is
called 'yellow prussiate' ('prussiate de potasse/ Blutlaugensalz).

It is easy to substitute other metals for the potassium in the yellow
prussiate. The hydrogen salt or hydroferrocyanic acid, H4FeC6N6, is
obtained by mixing strong solutions of yellow prussiate and hydro-
chloric acid. If ether be added and the air excluded, the acid is
obtained directly in the form of a white scarcely crystalline precipitate
which becomes blue on exposure to air (as ferrous cyanide does from the
formation of blue compounds of ferrous and ferric cyanides, and it is
on this account used in cotton printing). It is soluble in water and
alcohol, but not in ether, has marked acid properties, and decomposes
carbonates, which renders it easily possible to prepare ferrocyanides of

27 bU The sulphur of the animal refuse here forms the compound FeKS2, which
by the action of potassidm cyanide yields potassium sulphide, thiocyanate, and ferro-
cyanide.

28 Potassium ferrocyanide may also be obtained from Prussian blue by boiling with a
solution of potassium hydroxide, and from the ferricyanide by the action of alkalis and
reducing substances (because the red pruasiate is a product of oxidation produced by
the action of chlorine : a ferric salt is reduced to a ferrous salt), &c. In many works
(especially in Germany and France) yellow prussiate is prepared from the mass, con-
taining oxide of iron, and employed for purifying coal gas (Vol. L, p. 861), which
generally contains cyanogen compounds. About 2 p.c. of the nitrogen contained in coal is
converted into cyanogen, which forms Prussian blue and thiocyanates in the mass used
for purifying the gas. On evaporation the solution yields large yellow crystals containing
8 molecules of water, which is easily expelled by heating above 100°. 100 parts of water
at the ordinary temperature are capable of dissolving 25 parts of this salt ; its sp. gr. is
1-83. Wnen ignited it forms potassium cyanide and iron carbide, FeC2 (Chapter XIII.,
Note 12). Oxidising substances change it into potassium ferricyanide. With strong
sulphuric acid it gives carbonic oxide, and with dilute sulphuric acid, when heated,
prussic acid is evolved according to the equation: 2K4FeC6N6 + 8H2S04 = Ka

+ 3K.,SO4 + 6HCN; hence in the yellow prussiate K* replaces Fe.

IRON, COBALT, AND NICKEL 849

the metals of the alkalis and alkaline earths ; these are readily soluble,
have a neutral reaction, and resemble the yellow prussiate. Solutions
of these salts form precipitates with the salts of other metals, because the
ferrocyanides of the heavy metals are insoluble. Here either the whole
pf the potassium of the yellow prussiate, or only a part of it, is exchanged
lor an equivalent quantity of the heavy metal. Thus, when a cupric
salt is added to a solution of yellow prussiate, a red precipitate is obtained
which still contains half the potassium of the yellow prussiate :

K4FeC6N6 + CuS04 = K2CuFeC6N6 + K2S04.

But if the process be reversed (the salt of copper being then in excess)
the whole of the potassium will be exchanged for copper, forming a
reddish-brown precipitate, Cu2FeC6N6,9H2O. This reaction and
those similar to it are very sensitive and may be used for detecting
metals in solution, more especially as the colour of the precipitate
very often shows a marked difference when one metal is exchanged
for another. Zinc, cadmium, lead, antimony, tin, silver, cuprous and
aurous , salts form white precipitates ; cupric, uranium, titanium
and molybdenum salts reddish-brown; those of nickel, cobalt,
and chromium, green precipitates ; with ferrous salts, ferrocyanide
forms, as has been already mentioned, a white precipitate — namely,
uFe2Fe06N6, or FeC2N2 — which turns blue on exposure to air, and
with ferric salts a blue precipitate called Prussian blue. Here the
potassium is replaced by iron, the reaction being expressed thus :
2Fe2Clc + 3K4FeC6N6 = 12KCl4-Fe4Fe3C!8N18, the latter formula
expressing the composition of Prussian blue. It is therefore the
compound 4Fe(CN)3 + 3Fe(CN)2. The yellow prussiate is prepared in
chemical works on a large scale especially for the manufacture 'of this
blue pigment, which is used for dyeing cloth and other fabrics and
also as one of the ordinary blue paints. It is insoluble in water, and
the stuffs are therefore dyed by first soaking them in a solution of a
ferric salt and then in a solution of yellow prussiate. If however
an excess of yellow prussiate be present complete substitution between
potassium and iron does not occur, and soluble Prussian blue is
formed ; KFe2(CN)6= KCN,Fe(CN)2,Fe(CN)3. This blue salt is
colloidal, is soluble in pure water, but insoluble and precipitated
when other salts— for instanpe, potassium or sodium chloride — are
present even in small quantities, and is therefore first obtained as a
precipitate.29

** Skraup obtained this salt both from potassium ferrocyanide with ferric chloride
and from ferrjcyanide with ferrous chloride, which evidently shows that it contains iron

$50 PRINCIPLES OF CHEMISTRY

Potassium ferricyanide, or red prussiate of potash, K3FeC6N6, is
called 'Gmelin's salt,' because this savant obtained it by the action
of chlorine on a solution of the -yellow prussiate : K4FeC6N6 + Cl
e= K3FeC6NG 4- KC1. The reaction is due to the ferrous salt being
changed by the action of the chlorine into a ferric salt. It separates
from solutions in anhydrous, well-formed prisms of a red colour, but
the solution has an oliye colour ; LOO parts of water, at 10°, dis-
solve 37 parts of the salt, and at 100°, 78 parts.30 The red prus-
siate gives a blue precipitate with ferrous salts, called TurnbidVs blue,
very much like Prussian blue (and the soluble variety), because it also
contains ferrous cyanide and ferric cyanide, although in another propor-

in both the ferric and ferrous states. With ferrous chloride it forms Prussian blue, and
with ferric chlbfide Turnbull's blue.

Prussian blue was discovered in the beginning of the last century by a Berlin
manufacturer, Diesbach. It was then prepared, as it sometimes is also at present,
directly from potassium cyanida obtained by heating animal charcoal with potassium
carbonate. The mass thus obtained Is dissolved in water, alum is added to the
solution in order to saturate the- free alkali, and then a solution of green vitriol is added
which has previously been sufficiently exposed to tho air to contain both ferric and
ferrous salts. If the solution of potassium cyanide be mixed with a solution containing
both salts, Prussian blrte will be formed, because it is a -compound of ferrous cyanide,
FeC8N2, and ferric cyanide, Fe2C6N6. A ferric salt with potassium ferrocyanide forma
a blue colour, because ferrous cyanide is obtained from the first salt and ferric cyanide
from the second. During the preparation of this compound alkali must be avoided, aa
otherwise the precipitate would contain oxides of iron. Prussian blue has not a crystal-
line structure ; it forms a blue mass with a copper-red metallic lustre. Both acids and
alkalis act on it. The action is at first confined to the ferric salt it contains. Thus
•alkalis form ferric oxide and ferrocyanide in solution: 2Fe2C6N6,8FeC2N2+'12KHO
, =» 2(Fe<2O-,,3H.40) + 8K4FeC6N6. Various ferrocyanides may thus be prepared. Prussian
blue is soluble in an aqueous solution of oxalic acid, forming blue ink. In air, when
exposed to the action of light, it fades; but in the dark again absorbs oxygen and
becomes blue, which fact is also sometimes noticed in blue cloth. An excess of potassium
ferrocyanide renders Prussian blue soluble in water, although insoluble in various saline
solutions — that is, it converts it into the soluble variety. Strong hydrochloric acid also
Dissolves Prussian blue.

80 An excess ;f chlorine must not be employed in preparing this compound, otherwise
'-'the reaction goes further. It is easy to find out when the action of the chlorine on potassium
ierrocyanide must cease ; it is only necessary to take a sample of the liquid and add a
solution of a ferric salt to it'. If a precipitate of Prussian blue is formed, more chlorine
must be added, as there is still- some undecomposed ferrocyanide, for the ferricyanide
does not give a precipitate with ferric salts. Potassium' ferricyanide, like the ferro-
cyanide, easily exchanges its potassium for hydrogen and various metals by double
decomposition. With the salts of tin, silver, and mercury it forms yellow precipitates,
and with those of uranium, nickel, cobalt, copper, and bismuth brown precipitates. The
lead salt under the action of sulphuretted hydrogen forms lead sulphide and a hydrogen
salt or acid, H3FeCbN6, corresponding with potassium ferricyanide, which is soluble,
crystallises in red needles, and resembles hydroferrocyanic acid, H4FeC6NG. Under th*
action of reducing agents— for instance, sulphuretted hydrogen, copper— potassium ferri-
.cyanide is changed into ferrocyanide, especially in the presence of alkalis, and thus forma
a rather energetic oxidising agent— capable, for instance, of changing manganous oxide
into dioxide, bleaching tissues, &c.

IRON. COBALT. AKD NICKEL 351

tion, being formed accordirig to the equation: 3FeCl2+2K3FeC6N(}
.=6KCl+Fe3Fe2ai2Nl2, or 3FeC2N2,Fe206N6 ; in Prussian blue we
have Fe7Cy18, and here Fe5Cy,2. A farric salt ought to form, ferric
^cyanide Fe2C6N6, with red priissiate, but ferric cyanide is solublei
and therefore no precipitate is obtained, and the liquid only becomes
•brown.31

If chlorine ancl sodium are representatives of independent groups
of elements, the same may also be said of! iron. Its nearest ana-
logues show, besides a similarity in character, a likeness as regards
physical properties and a proximity in atomic weight. Iron, occupies a,
medium position amongst its nearest analogues, both with respect to
properties and faculty of forming saline oxides, and also as regards
-atomic weight. On the one hand, cobalt, 58, and 'nickel, 59, approach

S! It is important to mention a series of readily e.rystallisable salts formed by the-
action of nitric acid on potassium and other ferrocyanides and ferricyanides. Thes*
salt contain the elements of nitric oxide, and are therefore called nitrO'(nitro9o}
ferricyanides (nitroprussides). Generally a crystalline sodium salt is obtained»
Na2FeC5N6O,2H3O. In its composition this salt differs from the red sodium salt,
Na3FeC6N6, by the fact that in it one molecule of sodium, cyanide, NaCN, is replaced by
nitric oxide, NO. In order to prepare it, potassium ferrocyanida in powder is mixed
with five-sevenths of its weight of nitric acid diluted with 'an equal volume of water.
The mixture is at first left at the ordinary temperature, and then heated x>n a.
water-bath. Here ferricyanide is first of all formed (as shown by the liquid giving a.
precipitate with ferrous chloride), which then disappears (no precipitate with ferrous,
chloride), and forms a green precipitate. The liquid, when cooled, deposits crystals.
•of nitre. The liquid is then strained off and mixed with sodium carbonate, boiled*,
filtered, and evaporated ; sodium nitrate and the salt described are deposited in crystals.
It separates in prisms of a red colour. Alkalis and salts of the alkaline earths do not
give precipitates: they are soluble, but the salts of iron, zinc, copper, and silver form
precipitates where sodium is exchanged with these metals. It is remarkable that the-
sulphides of the alkali metals give with this salt an intense bright purple coloration.
This series of compounds was discovered by Gmelin and studied by Playfair and others.
(1849).

This series to a certain extent resembles the nitro-sulphide series described by
Eoussin. Here the primary compound consists of black crystals, which are obtained aa
follows :— Solutions of potassium hydrosulphide and nitrate are mixed, and the mixture
is agitated whilst ferric chloride is added, then boiled and filtered;- on cooling, black
Crystals aro deposited/having the composition Fe6S3 (NO)10,IL)O (Rosenberg), or, acfcord-
ing to Demel, FeNO^NHgS. They have a slightly metallic, lustre, and are soluble im
water, alcohol, and ether. They absorb the latter as easily as calcium chloride absorbs,
water. In the presence of alkalis these crystals remain' unchanged, but with acids they
evolve nitric oxides. There are several compounds which are capable of interchanging,,
and correspond with Roussin's salt. Here we enter into the series of the nitrogen
compounds which have been as yet but little investigated, and will most probably in
time form most instructive material for studying the nature of that element. These-
Beries of compounds are as unlike the usual saline compounds of inorganic chemistry as.
are organic hydrocarbons. There is no necessity to describe these series in detail, because-,
their connection, with othei compounds is not yet clear, and they have nob yet any
application.

852 PRINCIPLES OF CHEMISTS?

iron, 56 ; they are metals of a more basic character, they do not fonn
stable acids or higher degrees of oxidation, and are a transition to
copper, 63, and zind, 65. On the other hand, manganese, 55, and
•chromium, 52, are the nearest to iron ; they form both basic and acid
oxides, and- are a transition to the metals possessing acid properties.
In addition to having atomic weights approximately alike, chromium,
manganese, iron, cobalt, nickel, and .copper have also nearly the same
specific gravity, so. that the atomic volumes and the molecules of their
analogous compounds are also near to one another (see table at the
beginning- of -this volume). Besides this, the likeness between the
*above-mentioned elements is also seen from the following :

They form suboxides, RO, fairly energetic bases, isornorphous with
magnesia— for instance, the salt RS04,7H20, akin to MgS04,7H2O,
And FeS04,7H20, or to sulphates containing less water j with alkali
^sulphates all form double salts crystallising with 6H20 ; all are capable
of forming ammonium salts, &c. The lower oxides, in the cases of
nickel and cobalt, .are tolerably stable, are not easily oxidised (the
•nickel compound with more difficulty than cobalt, a transition to
copper); with manganese, and .especially with chromium, they are
•more easily oxidised than with iron and pass into higher oxides.
They also form oxides of the form R203, and with nickel, cobalt,
.and manganese this oxide is veify unstable, and is more easily reduced
than ferric oxide ; but, in the case of chromium, it is very stable, and
forms the ordinary kind of salts. It is_isomorphous with ferric oxide,
forms alums, is a feeble base, &c. Chromium, manganese, and iron are
oxidised by alkali and oxidising agents, forming salts like Na2S04 j
but cobalt and nickel are difficult to oxidise ; their acids are not known
with any certainty, and are, in all probability, still less stable than the
•ferrates. Cr.Mn and Fe form compounds R2C1G which are like Fe2Cl3
in many respects ; in Co this faculty is weaker and in Ni it has almost
disappeared. The cyanogen compounds, especially of manganese and
cobalt, are very near akin to the corresponding ferrocyanides. The
oxides of nickel and cobalt are more easily reduced to metal than those
of iron, but those of manganese and chromium are not reduced so
easily as iron, and the metals themselves are not easily obtained in a
pure state j they are capable of forming varieties resembling cast iron.
The metals Cr,Mn,Fe,Co, and Ni have a grey iron colour and are very
-difficult to melt, but nickel and cobalt can be melted in the reverbera-
'tory furnace and are more fusible than iron, whilst chromium is more
-difficult to melt than platinum (Deville). 'These metals decompose
water, but with greater difficulty as the atomic weight rises, forming a
transition to copper, which does not decompose water. All the com-

IRON, COBALT, AND NICKEL 35$

pounds' of thes& metals have various colours,- which are sometimes very
bright, especially, in the higher stages of oxidation.

These metals of the iron group are often met with together in
nature. Manganese nearly everywhere accompanies iron, and iron is
always an ingredient in the ores of manganese. Chromium is found
principally as chrome ironstone — that is, a peculiar kind of magnetio
oxide in which Fe203 is replaced by Cr2O3.

Nickel and cobalt are as inseparable companions as iron and
manganese, The similarity between them even extends to such
remote properties as magnetic qualities. In this series of metals we
find those which ar6 the most magnetic : iron, cobalt, and nickel.
There is even a magnetic oxide among the chromium compounds, such
being unknown in the other series. Nickel easily becomes passive in
strong nitric acid. It absorbs hydrogen in just the same way as iron.
Tn short, in the series Cr, Mn, Fe, Co, and Ni, there are many points
in common although there are many differences, as • will be seen still
more "clearly on becoming acquainted with cobalt and nickel.

In nature cobalt is principally found in combination with arsenio
and sulphur. Cobalt arsenide, or cobalt sjieiss, CoAs2, is found in
brilliant crystals of the regular system, principally in Saxony. CobdU
glance, CoAs2CoS2, resembles it very much, and 'also belongs to the
regular system ; it is found in Sweden, Norway, and the Caucasus.
Kupfernickel is a. nickel ore in combination with arsenic, but of a
different composition from cobalt arsenide, having the formula NiAs ;
it is found in Bohemia and Saxony. It has a copper-red colour and is
rarely crystalline ; it is so called because the miners of Saxony first
mistook it for an ore of copper (Kupfer), but were unable to extract
copper from it. Nickel glance, NiS2,NiAs2, corresponding with cobalt
glance, is also known. Nickel accompanies the ores of cobalt and.
cobalt those of nickel, so that both metals are found together. The
ores of cobalt are worked in the Caucasus in the Government of
Elizavetopolsk. Nickel ores containing aqueous hydrated nickel silicate
are found in the Ural (Revdansk). Large quantities of a similar ore
are exported into Europe from New Caledonia. Both ores contain
about 12 per cent. Ni. Garnierite, (RO)5(SiO2)4l^H2O, where R=Ni
and Mg, predominates in the New Caledonian ore. Large deposits of
nickel have been discovered in Canada, where the ore (as nickelous
pyrites) is free from arsenic. Cobalt is principally worked up into
cobalt compounds, but nickel is generally reduced to the metallic statet in
which it is now often used for alloys — for instance, for coinage in many
European States, and for plating other metals, because it does not
oxidise. Cobalt arsenide and cobalt glance are principally used for jthe

.854 ?KINCIPLES OF -CHEMISTRY

preparation of cobalt compounds ; they are first sorted by discarding
the rocky matter, and then roasted. During this process most of the
sulphur and arsenic disappears ; the arsenious anhydride volatilises
with the sulphurous anhydride and the metal also oxidises.32 It is a
simple matter to obtain nickel and cobalt from their oxides. In order
vto obtain the latter, solutions of their salts are treated with sodium

52 The residue from the roasting of cobalt ores is called zqfflor, and is often met with
in commerce. From this the purer compounds of cobalt may be prepared. The ores of
nickel are also first roaste'd, and the oxides dissolved in acid, nickelous salts being then
obtained.

The further treatment of cobalt and nickel ores is facilitated if the arsenic can be
almost entirely removed, which may be effected by roasting the ore a second time with a
small addition of nitre and sodium carbonate 5 the nitre combines with the arsenic,
forming an arsenious salt, which may be extracted with water. The remaining mass is

Dissolved in hydrochloric acid, mixed with a small quantity of nitric acid. Copper, iroa,

'manganese, nickel, cobalt, &c., pass into solution. By passing hydrogen sulphide
through the solution, copper, bismuth, lead, and arsenic are deposited as metallic sul-
phides ; but iron, cobalt, nickel, and manganese remain in solution. If an alkaline solu-
tion of bleaching powder be then added to the .remaining solution, the whole of the
manganese will first be deposited in the form of dioxide, then the cobalt as hydrated
cobaltio oxide, and finally the nickel also. It is, however, impossible to rely on this
method for effecting a complete separation, the more so since the higher oxides of the

» three above-mentioned metals haye all a black colour; but, after a few trials, it will be
easy to find how much bleaching powder is required to precipitate the manganese, and
the amount which will precipitate all the cobalt. The manganese may also be separated

* from cobalt by precipitation .from a mix! ure of the solutions of both metals (in the form ol
the ' ous ' salts) with ammonium sulphide, and then treating the precipitate with acetic
acid or dilute hydrochloric acid, in which manganese sulphide is easily soluble and cobalt
sulphide almost insoluble. Further particulars relating to the separation of cobalt from
nickel may be found in treatises on analytical chemistry. In practice It is usual to rely on
the rough method of separation founded oirthe fact that nickel is more easily reduced and
more difficult to oxidise than cobalt. The New Caledonian ore is smelted with CaS04
end CaC03 on coke, and a metallic regulus is obtained containing all the NI, Fe, and 8.
This is roasted with Si02, which converts all the iron into slag, whilst the Ni remains
combined with the S ; this.residue on further roasting gives NiO, which is reduced by the
carbon to metallic Ni. The Canadian ore (a pyrites containing 11 p. c. Ni) is frequently
treated in America (after a preliminary dressing). by smelting it with NagSC^ and
charcoal ; the resultant fusible Na-jS then dissolves the CuS and FeSft while the NiS la
obtained in a bottom layer (Bartlett and Thomson's process) from which Ni is obtained
in the manner described above.

For manufacturing purposes somewhat impure cobalt compounds are frequently used,
which are converted into smalt. This is glass containing a certain amount of cobalt
oxide ; the glass acquires a bright blue colour from this addition, so that when powdered
it may be used as a blue pigment; it is also unaltered at high temperatures, so
that it used to take the place now occupied by Prussian blue, ultramarine, &o. At
present smalt is almost exclusively used for colouring glass and china. To prepare
emalt, ordinary impure cobalt ore (zaffre) is fused in a crucible with quartz and potassium
carbonate. A fused mass of cobalt glass is thus formed, containing silica, cobalt oxide,

i and potassium oxide, and a metallic mass remains at the bottom ol the crucible, con-
taming almost all the other metals, x arsenic, nickel, copper, silver, &o. This 'metallic
niass is called speiss, and is used as nickel ore for the extraction of nickel. Smalt usually
contains 70 p.c. of silica, 20 p.c. of potash and soda, and about 5 to 6 p.o. of cobaltous
<mde; the remainder consisting of other metallic oxides.

IRON, COBALT, iAND NICKEL 355

carbonate and the precipitated " carbonates are heated ; the suboxides
are thus obtained, and these latter are reduced' In a stream ofi
hydrogen, or even by heating with ammonium chloride. They easily
oxidise when in the state of powder. When the chlorides of nickel
and cobalt are heated in a stream of hydrogen, the metal is deposited
In brilliant scales. Nickel is~ always much more easily and quickly
reduced than cobalt. Nickel melts more easily than cobalt, and this
even furnishes a means of testing the heating powers of a reverberatory
furnace. Cobalt fuses at a temperature only a little lower than that
•at which iron does. In general, cobalt is nearer to iron than nickel,
nickel being nearer to copper. 32.tli3 Both nickel and cobalt have mag-
netic properties like iron, but Co is less magnetic than Fe, and Ni stiU
less so. The specific gravity of nickel reduced by hydrogen is 9'1 and'
that of cobalt 8-9. Fused .cobalt has -a specific gravity of 8-5, the
density of ordinary nickel being almost the same. Nickel has ,a greyish,
silvery-white colour ; it is brilliant and very ductile, so that the finest
wire may be easily drawn from it. This wire has a resistance to
tension equal to iron wire. The beautiful colour of nickel, and the
high polish which it is capable of receiving and retaining, as it does
not oxidise, render it a useful metal for many purposes, and in
many ways it resembles silver.32 trf It is now very common to cover

33t>i» All we know respecting the relations of Co and Ni to 'Fe and Cu confirms the
fact that Co is more closely related to Pe and Ni to Cu ; and as the atomic weight of
Fe =* 56 and of Cu = 68, then according to the principles of the periodic system it would
be expected that the atomic weight of Co would be about-59H50, whilst that of Ni should
be greater than that of Co but less than that of Cu, i.e. about. 50;5-.60-5. Hpwever, aft
yet the majority of the determination)* of the atomic -weights ,bf Co and Ni give a
different result and show that a lower atomic weight is obtained for Ni than for Co.
Thus K. Winkler (1894) obtained (etnploying metals deposited electrolytically and deter-
mining the amount of iodine which combined with them) Ni = 68'72 and Co = 59'87 (if
H = 1 and I <= 126'53). In my opinion this should not be regarded as proving that the
principles of the periodic system cannot be applied in this instance, nor as a reason for
altering the position of these elements in the'system (i.e. by placing Ni after Fe, and Co
next to Cu), because in the first place .the figures given by different chemists (for instance,
Zimmerinann, Kriiss, and others) are somewhat divergent, and in the second place the
majority of the latest modes of determining the 'atomic weights of Co and Ni aim at
finding what weights of these metals react witjh known weights of other elements without
taking into account the faculty they have of absorbing hydrogen; since this faculty is
triore developed hi Ni than in Co the hydrogen (occluded in Ni) should lower the atomio
'weight of Ni more than that of Co. On the whole, the question of the atomio
weights of Co and Ni cannot yet be considered as. .decided, notwithstanding • the
•numerous researches which have been made ; still there can be no doubt that the atomio
weights of these two metals are very nearly equal, and greater than that of Fe, but les»
than that of Cu. This question is of great interest, not only for completing our know-
ledge of these metals, but also for perfecting our knowledge of the periodic system'of the
Elements.

6»tri por stance, the alkalis may be fused in nickel vessels as well as in silvefj
because they have no action upon either metal. Nickel, like silver, is not acted upon b$

856 PRINCIPLES OF CHEMISTRY

<other metals with a layer of nickel (nickel plating). This is done by a
process of electro-plating, using a solution of a nickel salt. The
colour of cobalt is dark and redder ; it is also ductile, and has a
greater tensile resistance than iron. Dilute acids act very slowly oni
nickel and cobalt; nitric acid may be considered as the best solvent 1
for them. The solutions in every case contain salts corresponding with
the ferrous salts — that is, the salts CoX2, NiX2, Correspond with the
suboxides of these metals. These salts in their types are similar to the
magnesium salts. The salts of nickel when crystallising with water
liave a green colour, and form bright green solutions, but in the anhy-
drous state they most frequently have a yellow colour. The salts of
cobalt are generally rose-coloured, and generally blue when in the
anhydrous state. Their aqueous solutions are rose-coloured. Cobaltous
'chloride is easily soluble in alcohol, and forms a solution of an intense
blue colour.83

•dilute acids. Only nitric acid dissolves both metals well. Nickel is harder, and fuses at
•a higher temperature than silver. For castings, a small quantity of magnesium (O'OOl
part by weight) is added to nickel to render it more homogeneous (just as aluminium la
added to steel). Nickel forms many valuable, alloys. Steel containing 8 p.c. Ni is par-
ticularly valuable, its limit of elasticity is higher and its hardness is greater; it is used.
for armour plate and other large pieces. The alloys of nickel, especially with copperand
zinc (melchior, see later), aluminium and silver^ although used in certain cases, are now
replaced by nickel-plated or nickel-deposited goods (deposited by electricity from a
solution of the ammonium salts).

53 The change of colour is dependent in all probability on the combination with
.water, or according to others on polymeric transformation. It enables a solution of
cobalt chloride to be used as sympathetic ink. If something be written with cobalt
chloride on white paper, it will be invisible on account of the feeble colour of the solution,
and when dry nothing can be distinguished. If, however, the paper be heated before the
fire, the rose-coloured salt will be changed into a less hydrous blue salt, and the writing
will become quite visible, but fade away when cool.

The change of colour which takes place in solutions of CoCl3 under the influence not
only of solution in water or alcohol, but also of a change of temperature, is a character-
istic Of all the halogen salts of cobalt. Crystalline iodide of cobalt, CoI36H20, gives a
• dark red solution between — 220%and +20°; above +20° the solution turns brown and
passes from olive to green, from -I- 85° to 820° the solution remains green. According to
Etard the change of colour is due to the fact that at first the solution contains the
hydrate CoI2H20, and that above 85°. it contains CoI24H2O. These hydrates can be
crystallised from the solutions; the former at ordinary temperature and the latter oil
beating the solution. The intermediate olive colour of the solutions corresponds to the
incipient decomposition of the hexahydraled salt and its passage into CoI24H2O. A
solution of the hexahydrated chloride of cobalt, CoCl26H2O, is rose-coloured between
- 22° and -f 25° ; but the colour changes starting from + 25°, and passes through all
the tints between red and blue right up to 50° ; a true blue solution is only obtained
at 65° and remains up 'to 800°. This true blue solution contains another hydrate.
CoCl22H2O.

The dependence between the solubility of the iodide and chloride of cobalt and
he temperature is expressed by two almt>st straight lines corresponding to 'the hexa-
and di-hydrates ; th« passage of the one into the other hydrate being expressed by a
«\jrve. The same character ofc phenomena is Seen also in the variation of the vapour

IKON, COBALT, AND NICKEL 857

If a solution of potassium hydroxide be added to a solution of a
cobalt salt, a blue precipitate of the basic salt will be formed. If a

tension of solutions of chloride of cobalt with the temperature. We have repeatedly
seen that aqueous solutions (for instance, Chapter XXII., NotQ 28 for Fe3Cl6) deposit
different crystallo-hydrates at different temperatures, and that the amount of water
in the hydrate decreases as the temperature t rises, so that it is not surprising that
CoCl22H2O (or according to Potilitzin CoCl2H2O) should sepauate out above 55° and
CoCl26H2O at 25° and below. Nor 'ia it exceptional that the colour of a salt varies
according as it contains different amounts of H20. But in this instance it is character*
istio fflmt the change of colour takes place in solution in the presence of an excess of
water. This apparently shows that the actual solution may contain either CoC1^6H2O or
CoCl22H2O And as we know that ar solution may contain both metaphosphoric PHOj
end orthophosphoric acid H3P04 = HP03+H2O, as well as certain other anhydrides,
the question of the s.tate of substances in solutions becomes still more complicated.

Nickel sulphate crystallises from neutral solutions at a temperature of from 15° to 20°
in rhombic crystals containing 7H.jO. Its form approaches very closely to that of the
Salts of zinc and magnesium. The planes of a vertical prism for magnesium salts are
inclined at an angle of 90° 80', for zinc salts at an angle of 91° 7', and for nickel salts at
an, angle of 91° 10'. Such is also the form of the zinc and magnesium selenates and
chromates. Cobalt sulphate containing 7 molecules of water is deposited in crystals
of the monoclinic system, like the corresponding salts of iron and manganese. The angle
of a vertical prism for the iron salt = 82° 20', for cobalt = 82° 22', and the inclination of
the horizontal pinacoid to the vertical prism for the iron salt = 99° 2', and for the cobalt
salt 99° 86' All the isomorphous mixtures of the salts of magnesium, iron, cobalt,
nickel and manganese have the same form if they contain 7 mol. H2O and iron or cobalt
predominate, whilst' if there is a preponderance of magnesium, zinc, or nickel, the
crystals have a rhombic form like magnesium sulphate. Hence these sulphates are
'dimorphous, but for some the one form is more stable and for others the other. Brooke,
"Moss, Mitscherlich, Rammelsberg, and Marignac have explained these relations. Brooke
and Mitscherlich also supposed that NiSO4,7H2O is not only capable of assuming these
forms, but also that of the tetragonal system, because it is deposite'd in this form from
acid, and especially from slightly-heated solutions (80° to 40°). But Marignac demon-
strated that the tetragonal crystals do not contain 7, but 6, molecules of water, NiSO4,6H2O.
He also observed that a solution evaporated at 60° to 70° deposits monoclinic crystals,
but of a different form from ferrous sulphate, FeSO4,7H2O— namely, the angle of the
prism is 71° 62', that of the pinacoid 95° 6'. This salt appears to be the same with 6
molecules of water as the tetragonal. Marignac also obtained magnesium and zmo
salts with 6 molecules of water by evaporating their solutions at a higher tem-
perature, and these salts were found to be isomorphous with the monoclinic "nickel salt.
In addition to this it must be observed that the rhombic crystals of nickel sulphate with
7H30 become turbid under the influence of heat and light, lose water, and change into
the tetragonal salt. The monoclinic crystals in time also become turbid, and change
their structure, so that the tetragonal form df this salt is the most stable. Let us also
add that nickel sulphate in all its shapes forms very beautiful emerald green crystals,
which, when heated to 280°, assume a dirty greenish-yellow hue and then contain one
molecule of water.

Klobb (1891) and Langlot and Lenoir obtained anhydrous CoS04 and NiS04by
Igniting the hydrated salt with (NH4)2SO^ until the ammonium (Bait had completely
volatilised and decomposed.

We may add that when equivalent aqueous solutions of NiX2 (green) and CoX2 (red)
ere mixed together they give an almost colourless (grey) solution, in which the green and
tred' colour of the component parts disappears owing to the combination of thecomple*
mentary colours.

A double salt NiKF3 fs obtained by heating NiCJ2 withKFHPin a platinum'cruciblej
ECoFs is formed in a similar manner. The nickel gait occurs in fine green plates, easily

658 PRINCIPLES OF CHEMISTRY

solution of a cobalt salt be heated almost" to the boiling-point, and the
solution be then mixed with a boiling solution of an alkali hydroxide,
a pink precipitate of cobaltous hydroxide^ CoH2O2,"wiil be formed. If
air be not completely excluded during the precipitation by boiling, the
precipitate will also contain brown cobaltic hydroxide formed by the •
further oxidation of the cobaltous oxide.34 Under similar circumstances
nickel salts form a green precipitate of nickelous hydroxide> the forma-
tion of which is not hindered by the presence of ammonium salts, butt
in that case only requires more alkali to completely separate the
nickel. The nickelous oxide obtained by heating the hydroxide, or
from the carbonate or nitrate, is a grey powder, easily soluble in acids
'and easily reduced, but the same substance may be obtained in the
crystalline form as an ordinary product from the ores ; it crystallises
in regular octahedra, with a metallic lustre, and is of a grey colour.
In this state the nickelous oxide almost resists the action of acids.34 bls
soluble in water but scarcely soluble in ethyl and methyl alcohol. They decompose into
..green oxide of nickel and potassium fluoride when heated in a current of air. The
analogous salt of cobalt crystallises in crimson flakes.

If instead of potassium fluoride, CoCl2 or NiCla be fused with ammonium fluoride,
they also form double salts with the latter. This gives the possibility of obtaining
anhydrous fluorides NiP2 and CoF^. Crystalline fluoride of nickel, obtained by
heating the amorphous powder formed by decomposing the double ammonium salt in
a stream of hydrofluoric acid, occurs in beautiful green prisms, sp. gr. 4'63, which are
insoluble in water, alcohol, and ether; sulphuric, hydrochloric, and nitric acids also have
no action upon them, even when heated ; ^NiF3 is decomposed by steam, with the forma-
tion of black oxide, which retains the crystalline structure of the salt. Fluoride of
cobalt, obtained as a r6se-coloured powder by decomposing the double ammonium salt
with the aid of heat in a stream of hydrofluoric acid, fuses into a ruby-coloured mass
which bears distinct signs of a crystalline structure; sp. gr. 4'48. The molten saH
only Volatilises at about 1400°, which forms a clear distinction between CoF3 and the
volatile NiFjj/ Hydrochloric, sulphuric, and nitric acids act upon CoFg even in the oold,
although slowly, while when heated the reaction proceeds rapidly (Poulenc, 1892).

w Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the following
manner. A solution of 10 grams of CoCl26H2O in 60 c.c. of water is heated in a flask
with 250 grams of caustic potash and a stream of coal gas is passed through the solution.
When heated the hydrate of the suboxide of cobalt which separates out, dissolves in the
caustic potash and forms a dark blue solution. This solution is allowed to stand for 24
hours in an atmosphere of coal gas (in order to prevent oxidation). The crystalline mass
which separates out has a composition Co(OH)2, and to the naked eye appears as a violet
powder, which is seen to be crystalline under the microscope. The specific gravity of
this hydrate is 8'597 at 15°. It does not undergo change in the air ; warm acetic acid
dissolves it, but it is insoluble in warm and cold solutions of ammonia and sal-
ammoniac.

54 bis The following reaction may be added to those of the cobaltous and nickelous
salts : potassium cyanide forms a precipitate with • cobalt salts which is soluble ify an
excess of the reagent and forms a green solution. On heating this and adding a certain
quantity of acid, a double. cobaZi cyanide is formed which corresponds with potassium
lerricyanide'. Its formation. is Accompanied with the evolution of hydrogen, and is
founded upon the property which cobalt has of oxidising in an alkaline solution, the de.«
velopment of which has been^observed in such a considerable measure in the cobaltamino
euJts. The process which goes on here may be expressed by the following equation |

IRON, COBALT, AND NICKEL

It is interesting to note the relation of the cobaltous and nickelous
"hydroxides to ammonia ; aqueous ammonia dissolves the precipitate of
cobaltous and nickelous hydroxide. The blue ammoniacal solution of
nickel resembles the same solution of cupric oxide, but has a somewhat
reddish tint. It is characterised by the fact that it dissolves silk in
the same way as the ammoniacal cuptic oxide dissolves cellulose. Am-
monia likewise dissolves the precipitate of cobaltous hydroxide, forming
a brownish liquid, which becomes darker in air and finally assumes a
bright red hue, absorbing oxygen. The admixture of ammonium chloride
prevents the precipitation of cobalt salts by ammonia j when the am-
monia is added, a brown solution is obtained from which, as in the
case of the preceding solution, potassium hydroxide does not separate
the cobaltous oxide. Peculiar compounds are produced in this solution j
they are comparatively stable, containing ammonia ancl an excess of
oxygen ; they bear the name cobaltoamine and cobaltiamine salts. They
have been principally investigated by Oenth, Fremy, Jorgenson and
others. Genth found that when a cobalt salt, mixed with an excess of
ammonium chloride, is treated with ammonia and exposed to the air,
after a certain lapse of time, on adding hydrochloric acid and boiling,
a red powder is precipitated and the remaining solution contains an
orange salt. The study of these compounds led to the discovery of a
whole series of similar salts, some of which correspond with particular
higher degrees of oxidation of cobalt, which are described later.35

CoC2N2 + 4KCN first forms CoK4C6N6, which salt with water, H2O, forms potassium
hydroxide, KHO, hydrogen, H, and the salt, K3CoC6N6. Here naturally the presence of
the acid is indispensable in consequence of its being required to combine with the alkali.
Prom aqueous solutions this salt crystallises in transparent, hexagonal prisms of a yellow
colour, easily soluble in water. The reactions of double decomposition, and even the
formation of the corresponding acid, are here completely the same as in the case of the
ferricyanide. If a nickelous salt be treated in precisely the same manner as that just
described for a salt of cobalt, decomposition will occur.

33 The cobalt salts may be divided into at least the following classes, which repeat
themselves for Cr, Ir, Rh (we shall not stop to consider the latter, particularly as they
closely resemble the cobalt salts) :—

(a) Ammonium cobalt salts, which are simply direct compounds of ths cobaltous
Ipalts €0X2 with ammonia, similar to various other compounds of the salts of silver,
copper, and even calcium and magnesium, with ammonia. They are easily crystallised
from an ammoniacal solution, and have a pink colour. Thus, for instance, when
cobaltous chloride in solution is mixed with sufficient ammonia to redissolve the
precipitate first formed, octahedral crystals are deposited which have a composition
CoCl2,H3O,6NHg. These salts are nothing else but combinations with ammonia of
crystallisation— if it may be so termed— likening them in this way to combinations with
water of crystallisation. This similarity is evident both from their composition and from
their capability of giving off ammonia at various temperatures. The most important
point to observe is that all these salts contain 6 molecules of ammonia to 1 atom of cobalt,
and this ammonia is held in fairly stable connection. Water decomposes these salts. (Nickel
behaves similarly without forming other compounds corresponding to the true cobaltic.)

(6) The solutions of the above-mentioned salts are rendered turbid by the action of

860 PRINCIPLES OF CHEMISTRY

Nickel does not possess this property of absorbing the oxygen of the air
when in an aramoniacal solution. In order to understand this distinc-

the art ; they absorb oxygen and become covered with a crust of oxycobaltamitie salts.
The latter are sparingly soluble in aqueous ammonia, have a brown colour, and are
characterised by the fact that with warm water they evolve oxygen, forming salts of the
following category : The nitrate may be taken as an example of this kind of salt ; its
composition is CoN207,6NH3,H20. It differs from cobaltous nitrate, Co(NO3)2, in con-
taining an extra atom of oxygen — that is, it corresponds with cobalt dioxide, Co02, in
the same way that the first salts correspond with cobaltous oxide ; they contain 5, and
not 6, molecules of ammonia, as if NH3 had been replaced by O, but we shall afterwards
meet compounds containing either 5NH3 or 6NH3 to each atom of cobalt.

(c) The luteocobaltic salts are thus called because they have a yellow (luteus)
colour. They are obtained from the salts of the first kind by submitting them in dilute
solution to the action of the air ; in this case salts of the second kind are not formed,
be'cause they are decomposed by an excess of water, with the evolution of oxygen and
the formation of luteocobaltic salts. By the action of ammonia the salts of the fifth
kind (roseocobaltic) are also converted into luteocobaltic salts. These last-named salts
generally crystallise readily, and have a yellow colour ; they are comparatively much
more stable than the preceding ones, and even for a certain time resist the action ol
boiling water. Boiling aqueous potash liberates ammonia and precipitates hydrated
cobaltic oxide, Co2O3,8H2O, from them. This shows that the luteocobaltic salts corre-
spond with cobaltio oxide, Co2C>5, and those of the second kind with the dioxide.
When a solution of luteocobaltic sulphate, Co2(S04)3,12NH5,4H2O, is treated with
baryta, barium sulphate is precipitated, and the solution contains luteocobaltio
hydroxide, Co(OH)3,6NH5, which is soluble in water, is powerfully alkaline, absorbs
the oxygen of the air, and when heated is decomposed with the evolution of am-
monia. This compound therefore corresponds to a solution of cobaltic hydroxide in
ammonia. The luteocobaltic salts contain 2 atoms of cobalt and 12 molecules of
ammonia — that is, 6NH3 to each atom of cobalt, like the salts of the first kind. The
CoXj salts have a metallic taste, whilst those of luteocobalt and others have a purely
saline taste, like the salts of the alkali metals. In the luteo-salts all the X's react (are
ionised, as some chemists say) as in ordinary salts — for instance, all the C12 is pre-
cipitated by a solution of AgNO3 ; all the (S04)5 gives a precipitate with BaX2, &c.
The double salt formed with PtCl4 is composed in the same manner as the potassium
salt, K2PtCl4 = 2KCl + PtCl4, that is, contains (CoCl3,6NH3)?,8PtCl4, or the amount of
chlorine in the PtCl4 is double that in the alkaline salt. In the rosepentamine (e), and
rosetetramine (/), salts, also all the X's react or are ionised, but in the (g) and (h) salts
only a portion of the X's react, and they are equal to the (e) and (/) salts minus water ;
this means that although the water dissolves them it is not combined with them, as
PH03 differs from PH303; phenomena of this class correspond exactly to what has
been already (Chapter XXI., Note 7) mentioned respecting the green and violet salts of
oxide of chromium.

(d) The fuscocobaltic salts. An ammoniacal solution of cobalt salts acquires a brown
colou in the air, due to the formation of these salts. They are also produced by the
decomposition of salts of the second kind ; they crystallise badly, and are separated from
their solutions by addition of alcohol or an excess of ammonia. When boiled they give
Up the ammonia and cobaltic oxide which they contain. Hydrochloric and nitric acids
give a yellow precipitate with these salts, which turns red when boiled, forming salts of
the next category. The following is an example of the composition of two of the fusco-*
cobaltic salts, Co2O(S04)2,8NH3,4H2O and Co2OCl4,8NH3,3H2O. It is evident that the
fuscocobaltic salts are aramoniacal compounds of basic cobaltic salts. The normal co-
baltic sulphate ought to have the composition Co2(SO4)3 - Co2O3,8SO5 ; the simplest
basic salts will be Co2O(SO4)2=Co203,2SO3, and Co2O2(S04) = Co2O3,SO3. The fusco-
cobaltio salts correspond with the first type of basic salts. They are changed (in con-
centrated solutions) into oxycobaltamine salts by absorption of one atom of oxygen,

IRON, COBALT, AND NICKEL 361

tion, and in general the relation of nickel, it is important to observe
that cobalt more easily forms a higher degree of oxidation — namely,

Co202(S04)2. The whole process of oxidation will be as follows : first of all €03X4, a
cobaltous salt, is in the solution (X a univalent haloid, 2 molecules of the salt being
taken), then Co2OX4, the basic cobaltic salt (4th series), then Co2O2X4, the salt of the
dioxide (2nd series); The series of basic salts with an acid, 2HX, forms water and a
normal salt, Co2Xe (in -8, 5, 6 series). These salts are combined with various amounts ol
water and ammonia. tJnder many conditions the salts .of fuscocobalt are easily trans-
formed into salts oi the next series. The salts of the series that has just been described
contain 4 molecules of ammonia to 1 atom of cobalt,

(e) 'The roseocobaltic (or rosepentamine), CoXaBLjOjSNHj, salts, like the luteo-
cobaltic, correspond with the normal cobaltic salts, but contain less ammonia, and an
extra molecule of water. Thus the. sulphate is obtained from cobaltous sulphate
dissolved in ammonia and left exposed to the air until transformed into a brown solution
of the fuscocobaltic salt ; when this is treated with sulphuric acid a crystalline powder
of the roseocobaltic salt, Co2(S04)3,10NH3,5H2O, separates. The formation of this salt
is easily understood : cobaltous sulphate in the presence of ammonia absorbs oxygen, and
the solution of the fuscocobaltic salt will therefore contain, like cobaltous sulphate, one
part of sulphuric acid to every part of cobalt, so that the whole process of formation may
be expressed by the equation: 10NH3 + 2CoSO4 + H2SO4 + 4H20 + O = Co2(S04)3)iONH3,
6H2O. This salt forms tetragonal crystals of a red colour, slightly soluble in cold, but
readily soluble in warm water. Wh6n the sulphate is treated with baryta, roseocobaltio
hydroxide is formed in the solution, which absorbs the carbonic anhydride of the air.
It is obtained from the next series by the action of alkalis.

(/) The rosetetramine cobaltic salts CoCl2,2H20,4NH5 were obtained by Jorgenson,
and belong to the type of the luteo-salts, only with the substitution of 2NH3 for H2O.
Like the luteo- and roseo-salts they give double salts with PtCl4, similar to the alkaline
double salts, for instance (Co2H2O,4NH3)2(SO4)2Cl2PtCl4. They are darker in colour
than the preceding, but also crystallise well. 'They are formed by dissolving CoC03 in
sulphuric acid (of a given strength), and after NH3 and carbonate of ammonium have
been added, air is passed through the solution (for oxidation) until the latter turns red.
It is then evaporated with lumps of carbonate of ammonium, filtered from the precipi-
tate and crystallised. A salt of the composition Co2(CO3)2(SO4), (2H2O,4NH3)3 is thus
obtained, from which the other salts may be easily prepared.

(<7) The purpureocobaltic salts, CoX3,5NH3, are also products of the direct oxidation
of ammoniacal solutions of cobalt salts. They are easily obtained by heating the roseo-
cobaltic and luteo-salts with strong acids. They are to all effects the same as the
roseocobaltic salts, only anhydrous. Thus, for instance, the purpureocobaltic chloride,
Co2Cl6,10NH3, or CoCl3,5NH3, is obtained by boiling the oxycobaltamine salts with
ammonia. There is the same distinction between these salts and the preceding ones as
between the various compounds of cobaltous chloride with water. In the purpureo-
cobaltic only X2 out of the X3 react (are ionised) To the rosetetramine salts (/) there
correspond the purpureotetramine salts, CoX3H20,4NH3. The corresponding chromium
purpureopentamine salt, CrCl3,5NH3 is obtained with particular ease (Christensen, 1898).
Dry anhydrous chromium chloride is treated with anhydrous liquid ammonia in a
freezing mixture composed of liquid CO2 and chlorine, and after some time the mixture
is taken out of the freezing mixture, so that the excess of 'NH3 boils away ; the violet
crystals then immediately acquire the red colour of the salt, CrCl3,5NH3, which is formed.
The product is washed with water (to extract the luteo-salt, CrCl3,6NH3), which does not
dissolve the salt, and it is then recrystallised from a hot solution of hydrochloric acid.

(h) The prazeocobaltic salts, CoX3,4NH3, are green, and form, with respect to the
fosetetramine salts (/), the products of ultimate dehydration (for example, like meta-
phosphoric acid with respect to orthophosphoric acid, but in dissolving in water they give
neither rosetetramine nor tetramine salts. (In my opinion one should expect salts with
a still smaller amount of NH3, of the blue colour proper to the low hydrated compounds

862 PRINCIPLES OF CHEMISTRY"

tesquioxide of cobalt, cobaltic oxide, Co2O3— than nickel, especially in
the presence of hypochlorous acid. If a solution of a cobalt salt be'

of cobalt ; the green colour of the prazeo-salts already forms a step towards the blue.)
Jorgenson obtained salts for ethylene-diamine, N2H4C2H4 which replaces 2NH3. After
being kepi a long time" 'in aqueous solution they give rosetetramine salts, just as met%«
phosphoric acid gives orthophosphoric acid, while the rosetetramine salts are converted
into prazeo-salts by Ag2O and NaHO. Here only one X is ionised out of the X3. There
are also basic salts of the same type; but the best known is the chromium salt called the
fhodozochromic salt, Cr2(OH)3Cl3,6NH3,2H2O, which is formed by the prolonged action
of water upon the corresponding roseo-salt.-

The cobaltamine compounds differ essentially but little from the ammoniacal com*
pounds of other metals. The only difference is that here the cobaltic oxide is obtained
from the cobaltous oxide in the presence of ammonia. In any case it ia a simpler question
than that of the double cyanides. Those forces in virtue of which each a considerable
number of ammonia molecules are united with a molecule of a cobalt compound, apper-
tain naturally to the series of those slightly investigated forces which exist even in the
highest degrees of combination of the majority of elements. They are the same forced
which lead to the formation of compounds containing water of crystallisation, double
salts, isomorphous mixtures and complex acids '( Chapter XXI., Note 8 bis). The
simplest conception, according to my opinion, of cobalt compounds (much more so than
by assuming special complex radicles, with Schiff, Weltzien, Claus, and others), may be
formed by comparing them with other ammoniacal products. Ammonia, like water, com*
bines in various proportions with a multitude of molecules. Silver chloride and calcium
chloride, just like cobalt chloride, absorb ammonia, forming compounds which are some-
times slightly stable, and easily dissociated, sometimes more stable, in exactly the same
way as water combines with certain substances, forming fairly stable compounds called
hydroxides or hydrates, or less stable compounds which are called compounds with water
of crystallisation. Naturally the difference in the properties in both oases depends on
4he properties of those elements which enter into the composition of the given substance,
and on those kinds of affinity towards which chemists have not as yet turned their
attention. If boron fluoride, silicon fluoride, &c., combine with hydrofluoric acid, if
platinic chloride, and even cadmium chloride, combine with hydrochloric acid, these
compounds may be regarded as double salts, because acids are salts of hydrogen. But
evidently water and ammonia have the same saline faculty, more especially as they, like
haloid acids, contain hydrogen, and are both capable of further combination— for instance,
ammonia with hydrochloric acid. Hence it is simpler to compare complex ammoniacal
with double salts, hydrates, and similar compounds, but the ammonio-metallic salts
present a most complete qualitative and quantitative resemblance to the hydrated salts
of metals. The composition of the latter is MXjiwH^O, where M •= metal, X = the
haloid, simple or complex, and n and m the quantities of the haloid and so-called water
of crystallisation respectively. The composition of the ammoniacal salts of metals is
MXnmNHj. The water of crystallisation is held by the salt with more or less stability, and
some salts even do not retain it at all ; some part with water easily when exposed to the air,
others when heated, and then with difficulty. In the case of some metals all the salts com-
bine with water, whilst with others only a, few, and the water so combined may then be
easily disengaged. All this applies equally well to the ammoniacal, salts, and therefore the
combination of ammonia may be termed the ammonia of crystallisation. Just as the
water which is combined with a salt is held by it with different degrees of force, so it is with
ammonia. In combining with 2NH3, PtCl2 evolves 81,000 cals. ; while CaCl9 only evolves
14,000 cals. ; and the former compound parts with its NH3 (together with HC1 in this
case) with more difficulty, only above 200°, while the latter disengages ammonia at 180°.
ZnCl2,2NH3 in forming ZnCl2, 4NH3 evolves only 11,000 cals., and splits up again into
its components at 80°. The amount of combined ammonia is as variable as the amount
of water of crystallisation— for instance, Snl48NE3,CrCl28NH5,CrCl56NH3,CrCl35NH3,
PtCMNH*, &c. are Tmown. Very often NHX is replaceable by OH-, and conversely. A,

IRON, COBALT, AND NICKEL 36$

mixed with barium carbonate and an excess of hypochlorous acid be
added, or chlorine gas be passed through it, then at the ordinary

colourless, anhydrous cupric salt—for instance, cupric sulphate — when combined with
water forms blue and green salts, and violet when combined with ammonia. If steam be
passed through anhydrous copper sulphate the salt absorbs water and becomes heated ; if
ammonia be substituted for the water the heating becomes much more intense, and the
ealt breaks up into a fine violet powder. With water CuS04,5H2O is formed, and with
ammonia CuS04,5NH3, the number of water and; ammonia molecules retained by the
Bait being the same in each case, and as a proof of this, and that it is not an isolated .
coincidence, the remarkable fact must be borne in mind that water and ammonia con-
secutively, molecule for molecule, are capable of supplanting each other, and forming the
compounds CuSO4,5K20, CuS04,4H20,NH3; CuS04)8H20,2NH3 ; CuS04,2H30,3NH5 ;
CuS04,H20,4NH3, and CuSO4,5NH3. The last of these compounds was obtained by
Henry Rose, and my experiments have shown that more ammonia than this cannot be
retained. By adding to a strong solution of cupric sulphate sufficient ammonia to
dissolve the whole of the oxide precipitated, and then adding alcohol, Berzelius obtained
the compound CuS04,H3O,4NH3, &c. The law of substitution also assists in rendering
these phenomena clearer, because a compound of ammonia with water forms ammonium
hydroxide, NH4HO, and therefore these molecules combining with one another may also
interchange, as being of equal value. In general, those salts form stable ammoniacal
compounds which are capable of forming stable compounds with water of crystallisation ; ,
and as ammonia is capable of combining with acids, and as some of the salts formed by
slightly energetic bases in their properties more closely resemble acids (that is, salts of
hydrogen) than those salts containing more energetic bases, we might expect to find
more stable and more easily-formed ammonio-metallio salts with metals and their
oxides having weaker basic properties than with those which form energetic bases. Thi#
explains why the salts of potassium, barium, &c., do not form ammonio-metallic salts,
whilst the salts of silver, copper, zinc, &c., easily form them, and the salts RX5 still
more easily and with greater stability. This consideration also accounts for the great
stability of the ammoniacal compounds of cupric oxide compared with those of silver
oxide, since the former is displaced by the latter. It also enables us to see clearly the
distinction which exists in the stability of the cobaltamine salts containing salts corre-
ponding with cobaltous oxide, and those corresponding with higher oxides of cobalt,
for the latter are weaker bases than cobaltous oxides. The nature of the forces
and quality of the phenomena occurring during the formation of the most stable sub'
stances, and of such compounds as crystallisable compounds, are one and the same it
although perhaps exhibited in a different degree. This, in my opinion, may be best
confirmed by examining the compounds of -carbon, because for this element the nature
of the forces acting during the formation of its compounds is well known. Let us take
as an example two unstable compounds of carbon. Acetic acid, C2H4O2 (specific gravity
1-06), with water forms the hydrate, C2H402,H20, denser (1'07) than either of the com-
ponents, but unstable and easily decomposed, generally simply referred to as a
solution. Such also is the crystalline compound of oxalic acid, C2H2O4, with water,
C2H2O4)2H20. Their formation might be predicted as starting from the hydrocarbon
C2H6, in which, as in any other, the hydrogen- may be exchanged for chlorine, the
water residue (hydroxyl), &o. The first substitution product with hydroxyl, C2H5(HO),
is stable ; it can be distilled without alteration, resists a temperature higher than 100°,
and then does not give off water. This is ordinary alcohol. The second, C2H4(HO)2,
can also be distilled without change, but can be decomposed into water and C2H4O
(ethylene oxide or aldehyde) ; It boils at about 197°, whilst the first hydrate boils at 78°,
ft difference of about 100° The compound C2H3(HO)S will be the third product of such
substitution ; it ought to boil at about 300°, but does not resist this temperature— it de-
composes into H20 and CiH402, where only one hydroxyl group remains, and the other
atom of oxygen is left in the same condition as in ethyler.e oxide, C2H4O. There is a proof
of this. Glycol, C2H4(HO)2, boils at 197°, and forms water and ethylene oxide, which

864 PEINCIPLES OF CHEMISTRY

temperature on shaking, the whole of the cobalt will be separated
in the form of black cobaltic oxide : 2CoS04 + C1HO + 2BaC03

boils at 13° (aldehyde, its isomeride, boils at 21°) ; therefore the product disengaged by
the splitting up of the hydrate boils at 184° lower than the hydrate C2H4(HO)2. Thus
the hydrate C2H3(HO)3, which ought to boil at about 800°, splits up in exactly the same
way into water and the product C2HjO2, which boils at 117°— that is, nearly 183° lower
than the hydrate, C2H3(HO)3. But this hydrate splits up before distillation. The
above-mentioned hydrate of acetic acid .is such a decomposable hydrate— that is to
say, what is called a solution. Still less stability may be expected from the following
hydrates. C2H2(HO)4 also splits up into water and a hydrate (it contains two hydroxyl
groups) called glycollic acid, C2H2O(HO)2 = C2H4O3. The next product of substitution
will be 02H(HO)5; it splits up into water, H2O, and glyoxylio acid, C2H4O4 (three
hydroxyl groups). The last hydrate which ought to be obtained from C2H6, and ought
to contain C2(HO)6, is the crystalline compound of oxalic acid, C2H2O4 (two hydroxyl
groups), and water, 2H20, which has been already mentioned. The hydrate C2(HO)a
«= C2H2O4,2H2O, ought, according to the foregoing reasoning, to boil at about 600°
(because the hydrate, C2H4(HO)2, boils at about 200°, and the substitution of 4 hydroxyl
groups for 4 atoms of hydrogen will raise the boiling-point 400°). It does not resist this
temperature, but at a much lower point splits up into water, 2H20, and the hydrate
C2O2(HO)2, which is also capable of yielding water. Without going into further dis-
cussion of this subject, it may be observed that the formation of the hydrates, or com-
pounds with water of crystallisation, of acetic and oxalic acids has thus received aa
accurate explanation, illustrating the point we desired to prove in affirming that com-
pounds with water of crystallisation are held together by the same forces as those which
act in the formation of other complex substances, and that the easy displaceability
of the water of crystallisation is only a peculiarity of a local character, and not
a radical point of distinction. All the above-mentioned hydrates, C2X<3, or pro-
ducts of their destruction, are actually obtained by the oxidation of the first hydrate,
C2H5(HO), or common alcohol, by nitric acid (Sokoloft and others). Hence the forces
which induce salts to combine with nH2O or with NH3 are undoubtedly of the same
•order as the forces which govern the formation of ordinary ' atomic ' and saline com-
pounds. (A great impediment in the study of the former was caused by the conviction
which reigned in the sixties and seventies, that 'atomic' were essentially different
from 'molecular* compounds like crystallohydrates, in which it was assumed that
there was a combination of entire molecules, as though without the participation of the
atomic forces,) If the bond between chlorine and different metals is not equally strong,
eo also the bond uniting ?iH2O and «NH3 is exceeding variable; there is nothing very
surprising in this. And in the fact that the combination of different amounts of NH$
and H20 alters the capacity of the haloids X of the salts RX2 for reaction (for instance,
in the luteo-salts all the X3, while in the purpureo, only 2 out of the 3, and in the prazeo-
salts only 1 of the 8 X's reacts), we should see in the first place a phenomenon similar
to what we met with in Cr2Cle (Chapter XXI., Note 7 bis), for in both instances the essence
of the difference lies in the removal of water; a molecule RC13,6H2O or RCl^GNHj
contains the halogen in a perfectly mobile (ionised) state, while in the molecule
HC13)5H2O or RC13,5NH3 a portion of the halogen has almost lost its faculty for reacting
•with AgNO3, just as metalepsical chlorine has lost this faculty which is fully developed in
the chloranhydride. tlntil the reason of this difference be clear, we cannot expect that
ordinary points of view and generalisation can give a clear answer. However, we may
assume that here the explanation lies in the nature and kind of motion of the-atoms in tb.6
molecules, although as yet it is not clear how. Nevertheless, I think it well to call
attention again (Chapter I.) to the fact that the combination of water, and hence, also,
Of any other element, leads to most diverse consequences ; the water in the gelatinous
hydrate of alumina or in the decahydrated Glauber salt is very mobile, and easily reacts
like water in a free state ; but the same water combined with oxide of calcium, or C2H4
(for instance, in C2H60 and in C4H10O), or with P205, has become quite different, and no

IRON, COBALT, AND NICKEL 365

=Co203 4- 2BaS04 -f HC1 4- 2C02. Under these circumstances nickelous-
oxide does not immediately form black sesquioxide, but after a consider-
able space of time it also separates in the form of sesquidxide, Ni203,
but always later than cobalt. This is due to the relative difficulty of
further oxidation of the nickelous oxide. It is, however, possible to-
oxidise it ; if, for instance, the hydroxide NiH2O2 be shaken in water
and chlorine gas be passed through it, then nickel chloride will be.
formed, which is soluble in water, and insoluble nickelic oxide in the
form of a black precipitate: 3NiH202 + Cl2=NiCl2 + Ni203,3H8O,
Nickelic oxide may also be obtained by adding sodium hypochlorite
mixed with alkali to a solution of a nickel sialt. Nickelic and cobaltio
hydrates are black. Nickelic oxide evolves oxygen with all acids, and
in consequence of this it is not separated as a precipitate in the presence
of acids ; thus it evolves chlorine with hydrochloric acid, exactly like
manganese dioxide. When nickelic oxide is dissolved in aqueous
ammonia it liberates nitrogen, and an ammoniacal solution of nickelous
oxide is formed. When heated, nickelic oxide loses oxygen, forming

longer acts like water in a free state. We see the same phenomenon in many other
cases— for example, the chlorine in chlorates no longer gives a precipitate of chloride of
silver with AgN03. Thus, although the 'instance which is found in the difference
between the roseo- and purpureo-salts deserves to be fully studied on account of its sim-
plicity, still it is far from being exceptional, and we cannot expect it to be thoroughly
explained unless a mass of similar instances, which are exceedingly common among
chemical compounds, be conjointly explained. (Among the researches which add to
our knowledge respecting the complex ammoniacal compounds, I think it indispensable
to call the reader's attention to Prof. Kournakofi's dissertation ' On complex metallic-
bases,' 1893.)

Kournakoff (1894) showed that the solubility of the luteo-salt, CoCl3)6NH3, at 0°
= 4-80 (per 100 of water), at 20° = 7'7, that in passing into the roseo-salt, CoCl3H205NH3,
the solubility rises considerably, and at 0° = 16'4, and. at 20°= about 27,- whilst the
passage into the purpureo-salt, CoCl3,5NH3, is accompanied by a great fall in the
solubility, namely, at 0° = 0'28, and at 20° = about 0'5. And as crystallohydrates with a-
smaller amount of water are usually more soluble than the higher crystallohydrates (Le
Chatelier)/ whilst here we find that the solubility falls (in the purpureo-salt) wkh a loss
of water, that water which is contained in the roseo-salt cannot be compare^ with the
water of crystallisation. Kournakoff, therefore, connects the fall in solubility (in the
passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive
Capacity of the chlorine.

In conclusion, it may be observed that the elements of the eighth group — that is, the
analogues of iron and platinum— according to my opinion, will yield most fruitful results
when studied as to combinations with whole molecules, as already shown1 by the examples
of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed
In this eighth group, and are remarkable for their stability. This faculty of the elements
of the eighth group for forming the complex compounds alluded to, is in all probability
connected with the position which the eighth group occupies with regard to the others.
Following the seventh, whi'ch forms the type RX7, it might be expected to contain the
most complex type, RX8. This is met with in OsO4. The other elements of the eighth
group, however, only form the lower types RX2, RX3, RX4 .... and these accordingly
should be expected to aggregate themselves into the higher types, which is accom-
plished in the formation of the above-mentioned complex compounds.

866 PRINCIPLES OF CHEMISTRY

nickelous oxide. Cobaltic oxide, Co2O3, exhibits more stability thao
nickelic oxide, and shows feeble basic properties j thus it is dissolve^
in acetic acid without the evolution of oxygen.35 bls But ordinary acids,
especially on heating, evolve oxygen, forming a' solution of a cobaltous
salt. The presence of a cobaltic salt in a solution of a cobaltous salt
may be detected by the brown colour of the solution and the blacfc
precipitate formed by the addition of alkali, and also from the fact that
euch solutions evolve chlorine when heated with hydrochloric acid,
Cobaltic oxide may not only be prepared by the above-mentioned
methods, but also by heating cobalt nitrate, after which a steel-coloured
mass remains which retains traces of nitric acid, but when heated
further to incandescence evolves oxygen, leaving a compound of
cobaltic and cobaltous oxides, similar to magnetic ironstone. Cobalt
(but not nickel) undoubtedly forms besides Co2O3 a dioxide. Co02«
This is obtained 36 when the cobaltous oxide is oxidised by iodine or
peroxide of barium.37

55 bis Marshall (1891) obtained cobaltic sulphate, CoafSO^.lSHjjO, by the action of an
electric current upon a strong solution of CoSO4.

56 rk6 action of an alkaline hypoohlorite or hypobromite upon a boiling solution of
cobaltous salts, according to Sohroederer (18S9), produces oxides, whose composition
varies between Co^Og (Rose's compound) and Co20s, and also between Co508 and
Co12O19. If caustic potash and then bromine be added to the liquid, only Co203 is
formed. The action of alkaline hypochlorites or hypo-bromites, or of iodine, upon
cobaltio salts, gives a highly-coloured precipitate which has a different colour to the
hydrate of the oxide Co.2(OH)e. According to Carnot the precipitate produced by the
hypochlorites has a composition Co^O^, whilst that given by iodine in the presence of
an alkali contains a larger amount of oxygen. Fortuuann (1891) reinvestigated the
composition of the higher oxygen oxide obtained by iodine in the presence of alkali, and
found that the greenish precipitate (which disengages oxygen when heated to 100°)
corresponds to the formula Co02. The reaction must be expressed by the equation:
CoX2 + 12 + 4KHO = Co02 +.2KX + 2KI + 2H2O.

37 Prior to Fortmann, Rousseau (1889) endeavoured to solve the question as to
whether Co02 was able to combine, with bases. He succeeded in obtaining a barium
compound corresponding to jthia oxide. Fifteen grams of BaCl2 or BaBra are triturated
with 5-G grams of oxide of barium, and the mixture heated to redness in a closed
platinum crucible ; 1 gram of oxide of cobalt is then gradually added to the fused mass.
Bach addition of oxide is accompanied by a violent disengagement of oxygen. After a
short time, however, the mass fuses quietly, and a salt settles at the bottom of the
crucible, which, when freed from the .residue, appears as black hexagonal, very brilliant
crystals. In dissolving in water this substance evolves chlorine ; its composition corre*
sponds to the formula 2(Co02)BaO. If the original mass be neated for a long time
(40 hours), the amount of dioxide in the resultant mass decseases. The author ob-
tained a neutral salt having the composition CoOaBaO (this compound «=-Ba02CoO)
by breaking up the mass a,s it agglomerates together, and bringing the pieces into
/contact with the more heated surface of the crucible. This salt is formed between th*
Somewhat narrow limits of temperature 1,000°-1,100° ; above and below these limit*
compounds richer or poorer in Co02 are formed. The formation of Co02 by the action
of BaO2, and the easy decomposition, of Co02 with the evolution of oxygen, give reason
for thinking that it belongs *Q the class of peroxides (like Cr2O7, Ca02, &c.) ; it is not yet
fenovra whether they give peroxide of hydrogen like the true peroxides. The fact that

IRON, COBALT, AND NICKEL 867

Nickel alloys possess qualities which render them valuable for
technical purposes, the alloy of nickel with iron being particularly
remarkable. This alloy is met With in nature as meteoric iron. The,
Pallasoffsky mass of meteoric iron, preserved in the St. Petersburg
Academy, fell in Siberia in the last century ; it weighs about 15 cwt.
and contains 88 p.c. of iron and about 10 p.c. of nickel, with a
small admixture of other metals. In the arts German silver is most
extensively used ; it is an alloy containing nickel, copper, and zinc in
various proportions. It generally consists of about 50 parts of copper,
25 parts of zinc, and 25 parts of nickel. This alloy is characterised by
its white colour resembling that of silver, and, like this latter metal, it
does not rust, and therefore furnishes an excellent substitute for silver
in the majority of cases where it is used. Alloys which contain silver
in addition to nickel show the properties of silver to a still greater
extent. Alloys of nickel are used for currency, and if rich deposits of
nickel are discovered a wide field of application lies before it, not only
in a pure state (because it is a beautiful metal and does not rust) but
also for use in alloys. Steel vessels (pressed or forged out of sheet
steel) covered with nickel have such practical merits that their manu-
facture, which has not long commenced, will most probably be rapidly
developed, whilst nickel steel, which exceeds ordinary steel in its
tenacity, has already proved its excellent qualities for many purposes
(for instance, for armour plate).

Until 1890 no compound of cobalt or nickel was known of sufficient
volatility to determine the molecular weights of the compounds of these
metals ; but in 1890 Mr, L. Mond, in conducting (together with Langer
and Quincke) his researches on the action of nickel upon carbonic oxide
(Chapter IX., Note 24 bis), observed that nickel gradually volatilises in
a stream of carbonic oxide ; this only takes place at low temperatures,
and is seen by the coloration of the flame of the carbonic oxide. This
observation led to the discovery of a remarkable volatile compound of
nickel and carbonic 'oxide, having as molecular composition Ni(CO)4,38

it is obtained by means of iodine (probably through H10), and its great resemblance
to Mn02, leads rather, to the supposition that CoD3 is a very feeble saline oxide. The
form Co02 is repeated in the cobaltic compounds (Note 85), and the existence of CoO<j
should have long ago been recognised upon this basis.

58 This compound is known as nickel tetra-carbonyl. It appears to me yet premature
to judge of the structure of such an extraordinary compound as Ni(CO)4. , It has long
been known that potassium combines with CO forming Kn(CO)n (Chapter IX., Note 81),
but this substance is apparently saline and non-volatile, and has as little in common
•with Ni(CO)4 as N^H has with SbH3. However, Berthelot observed that when NiC4O4
is kept in air, it oxidises and gives a colourless compound, Ni3C3O5,10H80, having
apparently saline properties. We may add that Schutzenberger, on reducing NiClj by
heating it in a current of hydrogen, observed that a nickel compound partially volatilises
with the HC1 and gives metallic nickel when heated again. The platinum compound,
*E

868 PRINCIPLES OP CHEMISTRY

as determined by the vapour density and depression of the freezing
point. Cobalt and many other metals do not form volatile compounds
under these conditions, but ir&n gives a similar product (Note 26 bis).
Ni(CO)4 is prepared by taking finely divided NL(obtained by reducing
NiO by heating it in a stream of hydrogen, or by igniting the oxalate
NiC2O4) 39 and passing (at a temperature below 50°, for even at 60°
decomposition may take place and an explosion) a stream of CO over
it j the latter carries over the vapour of the compound, which condenses
(in a well-cooled receiver) into a perfectly colourless extremely mobile
liquid, boiling without decomposition at 43°, and crystallising in needles
at —25° (Mond and Nasini, 1891). Liquid Ni(CO)4 has a sp. gr. 1-356
at 0°, is insoluble in water, dissolves in alcohol and benzene, and burns
with a very smoky flame due to the liberation of Ni. The vapour when
passed through a tube heated to 180° and above deposits -a brilliant
coating of metal, and disengages CO. If the tube be strongly heated
the decomposition is accompanied by an explosion. If Ni(CO)4 as
vapour be passed through a solution of CuCl2, it reduces the latter to
metal ; it has the same action upon an ammoniacal solution of AgCl, strong
nitric acid oxidises Ni(CO)4, dilute solutions of acids have no action ;
if the vapour be passed through strong sulphuric acid, CO is liberated,
chlorine gives NiCl and COCla ; no simple reactions of double decom-
position are yet known for Ni(CO)4, however, so that its connection
with other carbon compounds is not clear. Probably the formation of
this compound could be applied for extracting nickel from it ores.40

PtCl2(CO)3 (Chapter XXIII., Note 11), offers the greatest analogy to Ni(CO)4. This
compound' was obtained as a volatile substance by Schutzenberger by moderately
heating (to 285°) metallic platinum in a mixture of chlorine and carbonic oxide. If we
designate CO by Y, and an atom of chlorine by X, then taking into account that,
according to the periodic system, Ni is an analogue of Pt, a certain degree of corre-
spondence is seen in the composition NiY4 and PtX8Y2. It would be interesting to
compare the reactions of the two compounds.

59 According to its empirical formula oxalate of nickel also contains nickel and
carbonic oxide.

40 The following are the thermo-chemical data (according to Thomson, and referred
to gram weights expressed by the formula, in large calories or thousand units of heat)
for the formation of corresponding compounds of Mn, Fe, Co, Ni, and Cu (-*• Aq signifies
that the reaction proceeds in an excess of water) :

R=Mn Fe Co Ni Cu

128 100 95 94 63

106 78 73 72 41

76 48 43 41 82

95 68 63 61 88

193 169 163 163 130

4-16 18 18 19 11

These examples show that for analogous reactions the amount of heat evolved in
passing from Mn to Fe, Co, Ni, and Cu varies in regular sequences as the atomic weight
increases. A similar difference is to be found in other groups and series, and proves
that thermo-chemlcal phenomena are subject to the periodic law-

S69

CHAPTER

THE PLATINUM METALS

THE six metals : ruthenium, Ru, rhodium, Rh, palladium, Pd, osmium,
Os, iridium, Ir, and platinum, Pt, are met with associated together in
nature. Platinum always predominates over the others, and hence
they are known as the platinum metals. By their chemical character
their position in the periodic system is in the eighth group, correspond-
ing with iron, cobalt, and nickel.

The natural transition from titanium and vanadium, to copper and
einc by means of the elements of the iron group is demonstrated by all
the properties of these elements, and in exactly the same manner a
transition from zirconium, niobium, and molybdenum to silver, cadmium,
and indium, through ruthenium, rhodium, and palladium, is in perfect
accordance with fact and with the magnitude of the atomic weights, as
also is the position of osmium, iridium, and platinum between tantalum
and tungsten on the one side, and gold and mercury on the other. la
all these three cases the elements of smaller atomic weight (chromium,
molybdenum, and tungsten) are able, in their higher grades of
oxidation, to give acid oxides having the properties of distinct but
feebly energetic acids (in the lower oxides they give bases), whilst the
elements of greater atomic weight (zinc, cadmium, mercury \ even in
their higher grades of oxidation, only give bases, although with feebly
developed basic properties. The platinum metals present the same
intermediate properties such as we have already seen in iron and the
elements of the eighth group.

In the platinum metals the intermediate properties qf feebly acid
and feebly basic metals are developed with great clearness, so tbat
there is not one sharply-defined acid anhydride among their oxides,
although there is a great diversity in the grades of oxidation from the
type R04 to R2(X The feebleness of the chemical forces observed in
the platinum metals is connected with the ready decomposability of
their compounds, with the small atomic volume of the metals them?

870 PRINCIPLES OF CHEMISTRY

selves, and with their large atomic -weight. The oxides of platinum,
indium, and osmium can scarcely be termed either basic or acid ; they
are capable of combinations of both kinds, each of which is feeble,
Tjhey are all intermediate oxides.

The atomic weights of platinum, iridium, and osmium are nearly
191 to 196, and of palladium, rhodium, and ruthenium, 104 to 106.
Thus, strictly speaking, we have here two series of metals, which
are, moreover, perfectly parallel to each other ; three members in
the first series, and three members in the second — namely, platinum
presents an analogy to palladium, iridium to rhodium, and osmium
to ruthenium. As a matter of fact, however, the whole group of the
platinum metals is characterised by a number of common properties,
both physical and chemical, and, moreover, there are Several points of
resemblance between the members of this group and those pf the iron
group (Chapter XXII.) The atomic volumes (Table III, column 18)
of the elements of this group are nearly equal and Very small. The iron
metals have atomic volumes of nearly 7, whilst that of the metals allied
to palladium is nearly 9, and of those adjacent to platinum (Pt, Ir, Os,)
nearly 9 '4. This comparatively small atomic volume corresponds with
the great infusibility and tenacity proper to all the iron and platinum
metals, and to their small chemical energy, which stands out very
clearly in the heavy platinum metals. All the platinum metals are
very easily reduced by ignition and by the action -of various reducing
agents, in which process oxygen, or a haloid group, is disengaged from
their compounds and the metal left behind. This is a property of the
platinum metals which determines many of their reactions, and the
circumstance of their always being found in nature in a native state.
In Russia in the Urals (discovered in 1819) and in Brazil (1735)
platinum is obtained 'from alluvial deposits, but in 1892 Professor
Inostrantseff discovered a vein deposit of platinum in serpentine near
Tagil in the Urals.1 The facility with which they are reduced is so
great that their chlorides are even decomposed by gaseous hydrogen,
especially when shaken up and heated under a certain pressure. Hence
it will be readily understood that such metals as zinc, iron, &c.., separate
them from solutions with great ease, which fact is taken advantage of
in practice and in the chemical treatment of the- platinum metals.1 bl8

1 Wells and Penfield (1888) have described a mineral sperryllite found in the Canadian
gold-bearing quartz and consisting of platinum diarsenide, PtAso. It is a noticeable fact
that this mineral clearly confirms the position of platinum in the same group as iron,
because it corresponds in crystalline form (regular octahedron) and chemical composition
with iron pyrites, FeS2.

t t>i» Some light is thrown upon the facility with which the platinum compounds
decompose by Thomsen's data, showing that in an excess of water ( + Aq) the formation

THE PLATINUM METALS 371

All the platinum metals, like those of the iron group, are grey, with
a comparatively feeble metallic lustre, and are very infusible. In this
respect they stand in the same order as the metals of the iron series ;
nickel is more fusible and whiter than cobalt and iron, so also palla-
dium is whiter and more fusible than rhodium and ruthenium, and
platinum is comparatively more fusible and whiter than iridium or
osmium. The saline compounds of these metals are red or yellow, like
those of the majority of the metals of the iron series, and like the
latter, the different forms of oxidation present different colours. More-
@yer, certain complex compounds of the platinum metals, like certain
complex compounds of the iron series, either have particular character-
istic tints or else are colourless.

The platinum metals are found in nature associated together in
alluvial deposits in a few localities, from which they are washed,
owing to their very considerable density, which enables a stream of
water to wash away the sand and clay with which they are mixed.
Platinum deposits are chiefly known in the Urals, and also in Brazil
and a few other localities. The platinum ore washed from these
alluvial deposits presents the appearance of more or less coarse grains,
and sometimes, as it were, of semi-fused nuggets.2

All the platinum metals give compounds with the halogens, and th«
highest haloid type of combination for all is RX4. For the majority
of the platinum metals this type is exceedingly unstable ; the lower
compounds corresponding to the type RX2, which are formed by th^
separation of X2, are more stable. In the type RX8 the platinum
metals form more stable salts, which offer no little resemblance to

from platinum, of such a double salt as PtCl2,2KCl, is accompanied by a comparatively
small evolution of heat (see Chapter XXI., Note 40), for instance, Pt+Cl2-f-2KCl-f Aq
only evolves about 88,000 calories (hence the reaction, Pt-f-Cl2+Aq,. will evidently
disengage still less, because PtCl2 + 2KCl evolves a certain amount of heat), whilst on
the other hand, Fe + Cl2+Aq gives 100,000 calories, and even the reaction with copper
(for the formation of the double salt) evolves 68,000 calories.

2 The largest amount of platinum is extracted in the Ura^s, about five tons annually.
A certain amount of gold is extracted from the washed platinum by means of mercury,
which does not dissolve the platinum metals but dissolves the gold accompanying the
platinum in its ores. Moreover, the ores of platinum always contain metals of the iron
series associated with them. The washed and mechanically sorted ore in the majority
of cases contains about 70 to 80 p.c. of platinum, about 5 to 8 p.c. of indium, and a some-
what smaller quantity of osmium. The other platinum metals — palladium, rhodium, and
ruthenium — occur in smaller proportions than the three above named. Sometimes grains
of almost pure osmium-iridium, containing only a small quantity of other metals, are
found in platinum, ores. This osmium-iridium may be easily separated from the other
platinum metals, owing to its being nearly insoluble in aqua regia, by which the latter
are easily dissolved. There are grains of platinum which are magnetic. The grains of
osmium-iridium are very hard and malleable, and are therefore used for certain pur-
poses, for instance, for the tips of gold pens.

87'2 PRINCIPLES 0£ CHEMISTRY

the kindred compounds of the iron series— for example, to nickeloufr
chloride, NiCl2, cobaltous chloride, CoCl2, &c. This even expresses
itself in a similarity of volume (platinous chloride, PtCl2, volume, 46 ;
nickelous chloride, NiCl2 = 50), although in the type RX2 the true iron
metals give very stable compounds, whilst the platinum metals fre-
quently react after the manner of suboxides, decomposing into the
metal and higher types, 2RX2 = R -h RX4. This probably depends on
the facility with which RX2 decomposes into R and X2, when Xa
combines with the remaining portion of RX2

As in the series iron, cobalt, nickel, nickel gives NiO and Ni203,
whilst cobalt and iron give higher and varied forms of oxidation, so
also among the platinum metals, platinum and palladium only give the
forms RX2 and RX4, whilst rhodium and iridium form another and
intermediate type, RX3, also met:. with in cobalt, corresponding with
the oxide, having the composition R2O3, besides which they form
an acid oxide, like ferric acid, which is also known in the form of
salts, but is in every respect unstable. Osmium and ruthenium, like
manganese, form still higher oxides, and in this respect exhibit the
greatest diversity. They not only give RX2, RX3, RX4, and RX6,
but also a still higher form of oxidation, RO4, which is not met with in
any other series. This form is exceedingly characteristic, owing to the
fact that the oxides, Os04 and Ru04, are volatile and have feebly acid
properties. In this respect they most resemble permanganic anhydride,
which is also somewhat volatile.3

When dissolved in aqua regia (PtCl4 is formed) and liberated from
the solution by sal-ammoniac ( (NH4)2 PtCl6 is formed) and reduped by
ignition (which may be done by Zn and other reducing agents, direct
from a solution of Pt014) platinum 3 bis forms a powdery mass, known

* In characterising the platinum metals according to their relation to the iron metals,
{t is very important to add two more very remarkable points. The platinum metals are
capable of forming a sort of unstable compound with hydrogen ; they absorb it and only
part with it when somewhat strongly heated. This faculty is especially developed io
platinum and palladium, and it is very characteristic that nickel, which exactly corresponds
with platinum and palladium in the periodic system, should exhibit the same faculty for
detaining a considerable quantity of hydrogen (Graham's and Raoult's experiments).
Another characteristic property of the platinum metals consists in their easily giving
(like cobalt which forms the cobaltio salts) stable and characteristic saline compounds
with ammonia, and like Fe and Co, double salts with the cyanides of the alkali metals,
especially in their lower forms of combination. All the above so clearly brings the
elements of the iron series in close relation to the platinum metals, that the eighth group
acquires as natural a character as can be required, with a certain originality or indivi-
duality for each element.

3 t>u Platinum was first obtained in the last century from Brazil, where it was called
silver (platinus). Watson in 1750 characterised platinum as a separate independent
metal. In 1803 Wollaston discovered alladium and rhodium in crude platinum, and at

THE PLATINUM METALS 373

as spongy platinum or platinum black. If this powder of platinum be
heated and pressed, or hammered in a cylinder, the grains aggregate or
forge together, and form a continuous, though of course not entirely
homogeneous, mass. Platinum was formerly, and is even now, worked
up in this manner- The platinum money formerly used in Russia was
made in this way. Sainte-Claire Deville, in the fifties, for the first
time melted platinum in considerable quantities by employing a special
furnace made in the form of a small reverberatory furnace, and com-
posed of two pieces of lime, on which the heat of ^the oxyhydrogen flame
has no action. Into this furnace (shown in fig. 34, Vol. 1. p. 175) — or,
more strictly speaking, into the cavity made in the pieces of lime — the
platinum is introduced, and two orifices are made in the lime ; through
one, the upper, or side orifice, is introduced an oxyhydrogen gas burner,
in which either detonating gas or a mixture of oxygen and coal-gas is
burnt, whilst the other orifice serves for the escape of the products of
combustion and certain impurities which are more volatile than the
platinum, and especially the oxidised compounds of osmium, ruthenium,
and palladium, which are comparatively easily volatilised by heat. In
this manner the platinum is converted into a continuous metallic form
by means of fusion, and this method is now used for melting consider-
able masses of platinum 4 and its alloys with iridiura.

about the same time Tennant Distinguished iridium and osmium in it. Professor Glaus,
of Kazan, in his researches on the platinum metals (about 1840) discovered ruthenium
in them, and to him are due many important discoveries with regard to these elements,
such as the indication of the remarkable analogy between the series Pd— Rh— Rn and
Pt-Ir-0*.

The treatment of platinum ore is chiefly carried on for the extraction of the platinum
itself and its alloys with iridium, because these metals offer a greater resistance to the
action of chemical reagents and high temperatures than any of the other malleable and
ductile metals, and therefore the wire so often used in the laboratory and for technical
purposes is made from them, as also are various vessels used for chemical purposes in
the laboratory and in works. Thus sulphuric acid* is distilled in platinum retorts, and
many substances are fused, ignited, and evaporated in the laboratory in platinum
crucibles and on platinum foil. Gold and many other substances are dissolved in dishes
made of iridium-platinum, because the alloys of platinum and iridium are but slightly
attacked when subjected to the action of aqua regia.

The comparatively high density (about 21'5), hardness, ductility, and infusibility (it
does not melt at a furnace heat, but only in the oxyhydrogen flame or electric furnace),,
as well as the fact of its resisting the action of water, air, and other reagents, renders an
alloy of 90 parts of platinum and 10 parts of iridium (Deville's platinum-iridium alloy) a
most valuable material for making standard weights and measures, such as the metrej
kilogram, and pound, and therefore all the newest standards of most countries are made
of this alloy.

1 This process has altered, the technical treatment of platinum to a considerable
extent. It has in particular facilitated the manufacture of alloys of platinum with
Iridium and rhodium from the pure platinum ores, since it is sufficient to fuse the
ore in order for the greater amount of the osmium to burn off, and for the mass to fuse
Into a homogeneous, malleable alloy, which can be directly made use of. There is very
little ruthenium in the ores of platinum. If during fusion lead be added, it dissolves

874 PRINCIPLES OF CHEMISTRY

To obtain pure platinum, the ore is treated with aqua regia in which
only the osmium and iridium are insoluble. The solution contains the
platinum metals in the form RC14, and in the lower forms of chlorina-
tion, RC13 and RC12, because some of these metals — for instance,
palladium and rhodium — form such unstable chlorides of the type RX4
that they partially decompose even when diluted with water, and pass
into the stable lower type of combination j in addition to which the
chlorine is very easily disengaged if it comes in contact with substances
on which it can act. In this respect platinum resists the actiop of
heat and reducing agents better than any of its companions — that is,
it passes with greater difficulty from PtCl4 to the lower compound
PtCl2. On this is based the method of preparation of more or less
pure platinum. Lime or sodium hydroxide is added to the solution in
aqua regia until neutralised, or only containing a very slight excess of
alkali. It is best to first evaporate and slightly ignite the solution, in
order to remove the excess of acid, and by heating it to partially con-
vert the higher chlorides of the palladium, &c., into the lower. The
addition of alkalis completes the reduction, because the chlorine held
in the compounds RX4 acts on the alkali like free chlorine, converting
it into a hypochlorite. Thus palladium chloride, PdCl4, for example,
is converted into palladious chloride, PdCl2, by this means, according
to the equation PdCl4 + 2NaHO=PdCl2 + NaCl+NaC10 + H20. In
a similar manner iridic chloride, IrCl4, is converted into the trichloride,
IrCl3, by this method. When this conversion takes place the platinum
still remains in the form of platinic chloride, PtCl4. It is then possible
to take advantage of a certain difference in the properties of the higher
and lower chlorides of the platinum metals. Thus lime precipitates the
lower chlorides of the members of the platinum metals occurring in
solution without acting on the platinic chloride, Pt014, and hence the
addition of a large proportion of lime immediately precipitates the
associated metals, leaving the platinum itself in solution in the form
of a soluble double salt, PtCl4,CaCl2. A far better and more perfect

the platinum (and other platinum tnetals) owing to Its being able to form a very charac-
teristic alloy containing PtPb. If an alloy of the two metals be left exposed to moiet
air, the excess of lead is converted into carbonate (white lead) in \he presence of the
water and carbonic acid of the air, whilst the above platinum alloy remains unchanged.
The white lead may be extracted by dilute acid, and the alloy PtPb remains unaltered.
The other platinum metals also give similar alloys with lead, The fusibility of these,
alloys enables the platinum metals to be separated from the gangue of the ore, and they
may afterwards be separated from the -lead by subjecting the alloy to oxidation in
furnaces furnished with a bone ash bed, because the lead is then oxidised and absorbed
by the bone ash, leaving the platinum metals untouched. This method of treatment
was proposed by H. Sainte-Claire Deville in the sixties, and is also used in the analysis ofi
those metals (see further on).

THE PLATINUM METALS 875

separation is effected by means of ammonium chloride, which gives, with
platinio chloride, an insoluble yellow precipitate, PtCl4,2NH4Cl, whilst
it forms soluble double salts with the lower chlorides RC12 and RC13,
so that ammonium chloride precipitates the platinum only from the
solution obtained by the preceding method. These methods are
employed for preparing the platinum which is used for the manufacture
of platinum articles, because, having platinum in solution as calcium
platinochloride, PtCaCl6, or as the insoluble ammonium platinochloride,
Pt(NH4)2Cl6, the platinum compound in every case, after drying or
ignition, loses all the chlorine from" the platinic chloride and leaves finely-
divided metallic platinum, which may be converted into homogeneous
metal by compression and forging, dr by fusion.9

s For the ultimate purification of platinum from palladium and iridium the metals
must be re-dissolved in aqua regia, -and the solution evaporated until the residue begins
to evolve chlorine. The residue is then re-precipitated with ammonium or potassium
chloride. The precipitate may still contain a certain amount of iridium, which passes
with greater difficulty from the tetraohloride, IxCl*, into the trichloride, IrCl5, but it will
be quite free from palladium, because the latter easily loses its chlorine and passes into
palladious chloride, PdCl2, which gives an easily-soluble salt with potassium chloride.
The precipitate, containing a small quantity of ioidium, is then heated with sodium
carbonate in a crucible, when the mass decomposes, giving metallic platinum and
iridium oxide. If potassium chloride has been employed, the residue after ignition is
washed with water and treated with aqua regia. The iridium oxide remains undissolved,
and the platinum easily passes into solution. Only cold and dilute aqua regia must be
used. The solution will then contain pure platinic chloride, which forms the starting-
point for the preparation of all platinum compounds. Pure platinum for accurate
researches (for instance, for the unit of light, according to Violle's method) may be
obtained (Mylius and Foerster, 1892) by Finkener's method, by dissolving the impure
metal in aqua regia (it should be evaporated to drive off the nitrogen compounds), and
adding NaCl so as to form a double sodium salt, which is purified by crystallising with a
email amount of caustic soda, washing the crystals with a strong solution of NaCl, and
then dissolving them in a hot 1 p.c. solution of soda, repeating the above and ultimately
igniting the double salt, previously dried at 120°, in a stream of hydrogen ; platinum
black and NaCl are then formed. The three following are very sensitive tests (to
thousandths of a per cent.) for the presence of Ir, Eu, Rh, Pd (osmium is not usually
present in platinum which has once been purified, since it easily volatilises with Cl»
and CO2, and in the first treatment of the crude platinum either passes off as Os04
or remains undissolved), Fe, Cu, Ag, and Pb : (1) the assay is alloyed with 10 parts of
pure lead, the alloy treated with dilute nitric acid (to remove the greater part of the
Pb), and dissolved in aqua regia; the residue will consist pf Ir and Ru; the Pb is
precipitated from the nitric acid solution by sulphuric acid, whilst the remaining
platinum metals are reduced from the evaporated solution by formic acid, and the
resultant precipitate fused with KHSO4 ; the Pd and Rh are thus converted into soluble
salts, and the former is then precipitated by HgC2N2. (2) Iron may be detected by the
usual reagents, if the crude platinum be dissolved in aqua regia, and the platinum
metals precipitated from the solution by formic acid. (3) If crude platinum (as foil or
sponge) be heated in a mixture of chlorine and carbonic oxide it volatilises (with a
Certain amount of Ir, Pd, Fe, &c.) as PtCl2,2CO (Note 11), whilst the whole of the Rh,
Ag, and Cu it may contain remains behind. Among other characteristic reactions for
the platinum metals, we may mention : (1) that rhodium is precipitated from the solution
obtained after fusion with KHS04 (in which Pt does not dissolve) by NH3, acetic and
formic acids ; (2) that dilute aqua regia dissolves precipitated Pt, but not Rh ; (3) thai

876 PRINCIPLES OF CHEMISTRY^

Metallic platinum in 'a fused state has a specific gravity of 21 ; ft
is grey; softer than iron but harder than copper, exceedingly ductile,
and therefore easily drawn into wire and rolled into thin sheets, and
may be hammered into crucibles and drawn into thin tubes, &c. In
the state in which it is obtained by the ignition of its compounds*
it forms a spongy mass, known as spongy platinum, or else as powder
(platinum black).6 In either case it is dull grey, and is characterised,
as we already know, by the faculty of absorbing hydrogen and other
gases. Platinum is not acted on by hydrochloric, hydriodic, nitric, and
sulphuric acids, or a mixture of hydrofluoric and nitric acids. Aqua
regia, and any liquid containing chlorine or able to evolve chlorine or
bromine, dissolves platinum. Alkalis are decomposed by platinum at
a red heat, owing to the faculty of the platinum oxide, Pt02, formed to
combine with alkaline bases, inasmuch as it has a feebly-developed acid
character (see Note 8). Sulphur, phosphorus (the phosphide, PtP2f

if the insoluble residue of the platinum metals (Ir, Ru, Os) obtained, after treating with
'aqua regia, be fused with a mixture of 1 part of KNOj and 6 parts of KgCOj (in a gold
crucible), and then treated with water, it gives a solution containing the Ru (and a
'portion o£ the Ir), but which throws it all down when -saturated with chlorine and
boiled ; (4) that if indium be fused with a mixture of EHO and KN03, it gives a soluble
'potassium salt, IrK204 (the solution is blue), which, when saturated with chlorine, gives
IrCI4, which is precipitated by NH4C1 (the precipitate is- black), forming a double salt,
leaving metallic Ir after ignition; (5) that rhodium mixed with NaCl and ignited in a
current of chlorine gives a soluble double salt (from which sal-ammoniac separates Pfc
and Ir>, which gives (according to Jb'rgensen) a difficultly soluble purpureO'Salt (Chapter
XXH, Note 85), Rh2Cl3,5NHa, when treated with NH5; in this form the Rh may be
easily purified and obtained in a metallic form by igniting in hydrogen ; and (6) that
palladium, dissolved in aqua regia and dried (NH^Cl ^hrows down any Pt), gives soluble
PdCl2, which forms an easily crystallisable yellow salt, PdCl2NH3, with ammonia ; this-
salt (Wilm) may be easily purified by crystallisation, and gives metallic Pd when
ignited. These reactions illustrate the method of separating the platinum metals from
each other.

6 We have already become acquainted with the effect of finely-divided platinum on
many gaseous substances. It is best seen in the so-called platinum black, which is a
coal-black powder left by the action of sulphuric acid, on the alloy of zinc and platinum,
or which is precipitated by metallic zinc from a dilute solution of platinum. In any
case, finely-divided platinum absorbs gases more powerfully and rapidly the more
finely divided and porous it is. Sulphurous anhydride, hydrogen, alcohol, and many
organic substances in the presence of such platinum are easily oxidised by the oxygen of
the air, although they do not combine with it directly. The absorption of oxygen is as
much as several hundred volumes per one volume of platinum, acd the oxidising power
of such absorbed oxygen is taken advantage of not only in the laboratory but even in
manufacturing processes. Asbestos or charcoal, soaked in a solution of platinic chloride
and ignited, is very useful for this purpose, because by this means it becomes coated with
pjatintim black. If 60 grams of PtCl4 be dissolved in 60 c.c. of water, and 70 c.ctjof a
strong (40 p.c.) solution of formic aldehyde added, the mixture cooled, and then a
flolu'tion of 60 grams of NaHO in 60 grams of water added, the platinum is pre-
cipitated. After washing with water the precipitate passes into solution and forms *
black liquid containing soluble colloidal platinum (Loew, 1890). If the precipitated
platinum be allowed to absorb oxygen on the filter, the temperature rises 40°, and a
very porous platinum black is obtained which vigorously facilitates oxidation.

THE PLATINUM METALS 877

Is formed), arsenic and silicon all act more or less rapidly on platinum,
under the influence of heat. Many of the metals form alloys with it.
Even charcoal combines with platinum when it is ignited with it, and
therefore carbonaceous matter cannot be subjected to prolonged and
powerful ignition in platinum vessels. Hence a platinum crucible soon
becomes dull on the surface in a.smoky flame. Platinum also forms
alloys with zinc, lead, tin, copper, gold, and silver.7 Although mercury
does not directly dissolve- platinum, still it forms a solution or amalgam
with spongy platinum in the presence of sodium amalgam ; a similar
amalgam is also formed by the action of sodium amalgam on a solution
of platinum chloride, and is used for physical experiments.

There are two kinds of platinum compounds, PtX4 and PtX2.
The former are produced by an excess of halogen in the cold, and the
latter by the aid of heat or by the splitting up of the former. The
starting-point for the platinum compounds is platinum tetrachloride,
platinic chloride, PtCl4, obtained by dissolving platinum in aqua
regia.7bis The solution crystallises in the cold, in a desiccator, in the
form of reddish-brown deliquescent crystals which contain hydrochloric
acid, PtCl4,2HCl,6H20, and behave like a true acid whose salts cor-
respond to the formula K2I>tCl6-- ammonium platinochloride, for
example.7 tri The hydrochloric acid is liberated from these crystals by
gently heating or evaporating the solution to dryness ; or, better still,
after treatment with silver nitrate a reddish-brown mass remains
behind, which dissolves in water, and forms a yellowish-red solution
which on cooling deposits crystals of the composition PtCl4,8H20.
The tendency of PtCl4 to combine with hydrochloric acid and water —
that is, to form higher crystalline compound^ — is evident in the
platinum compounds, and must be taken into account in explaining
the properties of platinum and the formation of many other of its
complex compounds. Dilute solutions of platinic chloride are yellow,
and are completely reduced by hydrogen, sulphurous anhydride, and
many reducing agents, which first convert the platiuic chloride into

* It is necessary to remark that platinum when alloyed wifh silver, or as amalgam,
is soluble in nitric acid, and in this respect it differs from gold, so that it is possible,
by alloying gold with silver, and acting on the alloy with nitric acid, to recognise
the presence of platinum in the gold, because nitric acid does not act on gold alloyed
with silver.

7 t>ta Ptd4 is also formed by the action of a mixture of HC1 vapour and air, and by
the action of gaseous chlorine upon platinum.

7 lrl Pigeon (1891) obtained fine yellow crystals of PtH3Cl6,4H20 by addingstrong sul-
phuric acid to a strong solution of PtH2Cl6,6H20 If crystals of H2PtCl6)6H20 be
melted in vacuo (60°) in the presence of anhydrous potash, a red-brown solid hydrate is
obtained containing less water and HC1, which parts with the remainder at 200°, leaving
anhydrous PtCl4. The latter does not disengage chlorine before 220°, and is perfectly
eoluble in water.

878 PEINCIPLES OF CHEMISTRY

the lower compound platinoua chloride, PtCl2. That faculty "which
reveals itself in platinum tetrachloride of combining with water of
crystallisation and hydrochloric acid is distinctly marked in its pro*
perty, with which we are already acquainted, of giving precipitates
with the salts of potassium, ammonium, rubidium, &c. In general it
readily forma double salts, R2PtCl6=:PtCl4 + 2RCl, where R is a
univalent metal such as potassium or NH4. Hence the addition of a
solution of potassium or ammoniunj chloride to a solution of platinio
chloride is followed by the formation of a yellow precipitate, which is
sparingly soluble in water and almost entirely insoluble in alcohol and
ether (platinic chloride is soluble in alcohol, potassium iridiochloride,
IrK8Cl6, i.e. a compound of IrCl8, is soluble in water but not in alcohol),
It is especially remarkable in this case, that the potassium compounds
here, as in a number of other instances, separate in an anhydrous form,
whilst tjie sodium compounds, which are soluble in water and alcohol,
form red crystals containing water. -The composition NaaPtCl6,6H2O
exactly corresponds with the above-mentioned hydrochloric compound.
The compounds with barium, BaPt016,4H2O, strontium, 8rPtCl6,8H20,
calcium, magnesium, iron, manganese, and many other metals are all
soluble in watfer.8

e Nilson (1877), who investigated the platmochlorides of various metals subsequently
to Bonsdorff, Topsoe, Cleve, Marignao, atjd others, found that univalent and bivalent
metals — such as hydrogen, potassium, ammonium . . . beryllium, calcium, barium—
give compounds of euch a composition that there is always twice as much chlorine in
the platinic chloride as in the combined metallic chloride; for example, K2Cl8tPtCl4}
BeC^PtCl^SHaO, to. Such trivalent metals as aluminium, iron (ferric), chromium, di«
dyrnium, cerium (cerou8)'form compounds of the type ECljPtCl^ in which the amounts of
chlorine are in the ratio 8 : 4. Only indium and yttrium-give salts of a different composi-
tion—namely, 2lnCl5,5PtCl4,86HaO and 4YCl5,5PtCl4)5lHaO. Such quadrivalent metato
as thorium, tin, zirconium give compounds of the type RCl^PtC^, in which the ratio of
the chlorine is 1:1. In this manner the valency of a metal may, to a certain extent, be
judged from the composition of the double salts formed with platinic chloride.

Platinic bromide, PtBr4, and iodide, Ptl*, are analogous to the tetrachloride, but the
todide is decomposed still more easily than the chloride. If sulphuric acid be added to
platinic chloride, and the solution evaporated, it forms a black porous mass like char-
coal, which deliquesces in the air, and has the composition Pt(SOi)2. But this, the
only oxygen salt of the type PtXj, is exceedingly unstable. This is due to the fact that
platinum oxide, the oxide of the type PtO2, has a feeble acid character. This is shown
in a number of instances. Thus if a strong solution of platinic chloride treated with
sodium carbonate be exposed to the action of light or evaporated to dryness and then
washed with water, a sodium platinate, PtjNaaO^HsO, remains.. The composition ol
this salt, if we regard it in the same sense as we did the salts of silicic, titanic, molybdio
and other acids, will be PtO(ONa)8,2PtO2,6H20— that is, the same type is repeated as
we saw in the crystalline compounds of platinum tetrachloride with sodium chloride, or
with hydrochloric acid—namely, the type PtX^SY, where Y is the molecule H8O,HC1, &c.
Similar compounds are also obtained with other alkalis. They will be platinates of the
alkalis in which the platinio oxide, PtQ2, plays the part of an acid oxide. Eousseaa
(1889) obtained different grades of combination BaOPt03> 8(BaO)2Pt08, &c., by igniting
a mixture of PtCl^ and caustic b&ryta. If «uch an alkaline compound of platinum be

THE PLATINUM METALS 379

Platinous chloride, PtCl2, is formed when hydrogen platinoghloride,
PtH2Cl6, is ignited at 300°, or when potassium is heated at 230° in a
fitream of chlorine. The undecomposed tetrachloride is extracted from
the residue by washing it with water, and a greenish-grey or brown
insoluble mass of the dichloride (sp. gr. 5'9) IB then obtained. It is
soluble in hydrochloric acid, giving an acid solution of the composition
PtCl8,2HCl, corresponding with the type of double salts PtR2Cl4.
Although platinous chloride decomposes below 500°, still it is formed to
a small extent at higher temperatures. Troost and Hautefeuille, an<$
Seelheim observed that when platinum was strongly ignited in a stream of
chlorine, the metal, as it were, slowly volatilised and was deposited in
crystals j a volatile chloride, probably platinous- chloride, was evidently
formed in this case, and decomposed subsequently to its formation,
depositing crystals of platinum.

The properties of platinum above- described are repeated more or less
distinctly, or sometimes with certain modifications, in the above-men-
tioned associates and analogues of -this metal. Thus although palladium
fortns PdCl4, this form passes into PdCla with extreme ease.9 Whilst

treated with acetic acid, the alkali combines with the latter, and a platinio hydroxide^
Pt(OH)4, remains as a brown mass, which loses water and oxygen when ignited, and in
BO doing decomposes with a slight explosion. When slightly ignited this hydroxide first
loses water and gives the very unstable oxide Pt02. Piatinio sulphide,. PtS2, belongs to
the same type ; it is precipitated by the action of sulphuretted hydrogen on a solution
of platinum tetrachloride. The moist precipitate is capable of attracting oxygen, and ia
then converted into the sulphate above mentioned, which is soluble in water. This
absorption of oxygen and conversion into sulphate is another illustration of the basio
nature of PtO2, so that it clearly exhibits both basio and acid properties. The latter
appear, for instance, in the fact that platiuic sulphide, PtS2, gives crystalline compounds
with the alkali sulphides.

* In comparing the characteristics of the platinum metals, it must be observed that
palladium in its form of combination PdXg gives saline compounds of considerable)
stability. Amongst them palladoua chloride is formed by the direct action of chlorine
or aqua regia (not in excess or in dilute solutions) on palladium. It forms a brown
solution, which gives a black insoluble precipitate of palladoua iodide, PdI2, with
solutions of iodides (in this respect, as in many others, palladium resembles mercury in
the mercuric compounds HgX8). With a solution of mercuric cyanide it gives a yellowish
white precipitate, pallodous cyanide, PdC8N2, which is soluble in potassium cyanide, and
gives other double salts, M2PdC4N4.

That portion of the platinum ore which dissolves in aqua regia and is precipitated
by ammonium or potassium chloride does not contain palladium. It remains in solu-
tion, because the palladic chloride, PdClj, is decomposed and the palladoua chloride
formed is not precipitated by ammonium chloride; the same holds good for all the other
lower chlorides of the platinum metals. Zinc (and iron) separates out all the unprecipi.
tated platinum metals (and also copper, &c.) from the solution. The palladium is found
to these platinum residues precipitated by einc. If this mixture of metals be treated
with aqua regia, all the palladium will pass into solution as palladous chloride, with,
some platinic chloride. By this treatment the main portion of (he irtdium, rhodium, &c.
remains almost undtseolved, the platinum is separated from the mixture of palladia
tod platinto chlorides by a solution of ammonium chloride, and the eolation of

880 PRINCIPLES OP CHEMISTRY

rhodium and indium in dissolving in aqua regia also form RhCl4 and

is precipitated by potassium iodide or mercuric cyanide. Wilm (1881) showed, that
palladium may be separated from an impure solution by saturating it with ammonia; all
the iron present is thus precipitated, and, after filtering, the addition of hydrochloric
acid to the filtrate gives a yellow precipitate of an ammonio-palladium compound^
PdCIa,2NH3, whilst nearly all the other metals remain in solution. Metallic palladium
is obtained by igniting the aromonio-compound or the cyanide, PdC2N3. It occurs
native- although rarely, and is a metal of a whiter colour than platinum, sp. gr. 11'4,
melts at about 1,500° ; it is much more volatile than platinum, partially oxidises on the
surface when heated (Wilm obtained spongy palladium by igniting PdCla,2NH3, and
observed that it gives PdO when ignited in oxygen, and that on further ignition thia
oxide forms a mixture of PdaO and Pd), and loses its absorbed oxygen on a further rise
of temperature. It does not blacken or tarnish (does not absorb sulphur) in the air at
the ordinary temperature, and is therefore better suited than silver for astronomical and
other instruments In which fln.e divisions have to be engraved on a white metaj, in order
that the fine lines should be clearly visible. The most remarkable property of palladium,
discovered by Graham, consists in its capacity for absorbing a large amount of hydrogen.
Ignited palladium absorbs as much as 940 volumes of hydrogen, or about 0'7 p.c. of its
own weight, which closely approaches to the formation of the compound Pd5H3r and-
probably indicates the formation of palladium hydride, Pd2H. This absorption
also takes place at the ordinary temperature—for example, when palladium serves as
an electrode at which hydrogen is evolved. In absorbing the hydrogen, the palladium
does not change in appearance, and retains all its metallic properties, only its volume
increases by about 10 p.c.— that is, the hydrogen pushes out and separates the atoms of
the palladium from each other, and is itself compressed to ^ of its volume. This com-
pression indicates a great force of chemical attraction, and is accompanied by the evolu-
tion of heat (Chapter II., Note 88). The absorption of 1 grm. of hydrogen by metallic
palladium (Favre) is accompanied by the evolution of 4'2 thousand calories (for Pt 20,
for Na 13, for K 10 thousand junits of 'heat), trooat showed that the dissociation
pressure of palladium hydride is inconsiderable at the ordinary temperature, but reaches
the atmospheric pressure at about 140°. This subject was subsequently investigated by
A. A. Cracow of 8*. Petersburg -(1894), who showed that at first the absorption of
hydrogen by the palladium proceeds like solution, according to the law of Dalton and
Henry, but that towards the end.it proceeds hire a dissociation phenomenon in definite
compounds; this forms another link between the phenomenon of solution and of the
formation of definite atomic compounds. Cracow's observations for a temperature 18°,
showed that the electro-conductivity and tension vary until a compound PdjH is reached,
and namely, that the tension p rises with the volume v of hydrogen absorbed, according
fb the law of Dalton and Henry— for instance) for

p = 2~l 8'2 5-5 7-7 mm,

v=14 20 84 47

The maximum tension at 18° is 9 mm. At a temperature of about 140° (in the vapour of
xylene) the maximum tension is about 760mm., and when v = 10-50 vols. the tension
(according to Cracow's experiments) stands at 90— 450 mm.— that is, increases in pro-
portion to the volume of hydrogen absorbed. But from the point of view of chemical
mechanics it is especially important to remark that Moutier clearly showed, through
palladium hydride, the similarity of the phenomena which proceed in evaporation and
dissociation, which fact Henri Sainte-Claire Deville placed as a fundamental proposition
in the theory of dissociation. It is possible upon the basis of the second law of the
theory of heat, according to the law of the variation of the tension p of evaporation with
the temperature T (counted from —273°), to calculate the latent heat of evaporation
L {see works on physics) because 424 L=*T (1/d-l/D) dp/dt, where d and D are the
weights of cubic measures of the gas (vapour) and liquid. (Thus, for instance, for
r, when <cBlOO°,T=873, d = 0'603,D= 960, dp/dt~ 0'027 m., 18,596=867, L«530,

THE PLATINUM METALS 881

IrCl4, bat they pass into RhCla and IrCl3 9 bi8 very easily when heated

whence 424 L= 227,264, and the Second portion of the equation 226,144, which is
sufficiently near, within the limits of experimental error, see Chapter I., Note 11.)
The same equation is applicable to the dissociation of Na2H and K3H — (Chapter XII.,
Note 42)— but it has only been verified in this respect for Pd2H, since Mbutier, by
calculating the amount of heat L evolved', for £ = 20, according to the variation of
the tension (dp/dt) obtained 4'1 thousand calories, which is very near the figure
obtained experimentally by Favre (see Chapter XII., Note 44). The absorbed hydrogen
is easily disengaged by ignition or decreased pressure. The resultant compound
does not decompose at the ordinary temperature, but when exposed to air the metal
sometimes glows spontaneously, owing to the hydrogen burning at the expense of
the atmospheric oxygen. The hydrogen absorbed by palladium acts towards many
solutions as a reducing agent ; in a word, everything here points to the formation of a
definite compound and at the same time of a physically-compressed gas, and forms one
of the beat examples of the bond existing between chemical and physical processes, to*
which wo have many tunes drawn attention.' It must be again remembered that the
other metals of the eighth group, even copper, are, like palladium and platinum, able to>
combine with hydrogen. The permeability of iron and platinum tubes to hydrogen is
naturally due to the formation of similar compounds, but palladium is the mosfe
permeable.

s W» Rhodium is generally separated, together with iridium, from the residues left
after the treatment of native platimun, because the palladium is entirely separated from
them, and the ruthenium is present in them in very small traces, whilst the osmium at
any rate is easily separated, as we shall soon see. The mixture of rhodium and iridium
which is left undissolved in dilute aqua regia is dissolved in chlorine water, or by the>
action of chlorine on a mixture of the metals with sodium chloride. In either case both
metals pass into solution. They may be separated by many methods. In either case
(if the action be aided by heat) the rhodium is obtained in the form of the chloride
RhClj, and the iridium as iridious chloride, IrClj. They both form double salts with
sodium chloride which are soluble in water, but the iridium salt is also partially soluble
in alcohol, whilst the rhodium salt is not. A mixture of the chlorides, when treated with
dilute aqua regia, gives iridic chloride, IrCl^, whilst the rhodium chloride, RhClj, re-
mains unaltered ; ammonium chloride then precipitates the iridium as ammonium ividio-
chloride, Ir(NH4)2Cl6, and on evaporating -the rose-coloured filtrate the rhodium gives
ft crystalline salt, RhtNH^Cle. Rhodium and its various oxides are dissolved when
fused with potassium hydrogen sulphate, and give a soluble double sulphate (whilst
iridium remains unacted on) ; this fact is very characteristic for this metal, which offers
in its properties many points of resemblance with the iron metals. When fused with
potassium hydroxide and chlorate it is oxidised like iridium, but it is not afterwards
soluble in water, iu which respect it differs from ruthenium. This is taken advantage of for
separating rhodium, ruthenium, and iridium. In any case, rhodium under ordinary
conditions always gives salts of the type EX3, and not of any other type ; and not only
halogen salts, but also oxygen salts, are known in Ibis type, which is rare among
the platinum metals. Rhodium chloride, RhCls, is known in an insoluble anhydrous
and also in a soluble form (like CrX5 or salts of chromic oxides), in which it easily gives
double salts, compounds with water of crystallisation, and forms rose-coloured solutions.
In this form rhodium easily gives double salts of the two types RhMjClg and RhM2Cl5—
for example, K3RhCl6,8H2O and K2RhCl5,H2O. Solutions of the salts (at least, the
ammonium salt) of the first kind give salts of the second kind when they are boiled. If
a strong solution of potash be added to a red solution of rhodium chloride and boiled, a
black precipitate of the hydroxide Rh(OH)3 is formed ; but if the solution of potash is
added little by little, it gives a yellow precipitate containing more water. This yellow
hydrate of rhodium oxide gives a yellow solution when it is dissolved in acids, which
only becomes rose-coloured after being boiled. It is obvious a change here takes place,
like the transmutations of the salts of chromio oxide. It is also a remarkable fact thai

882 PRINCIPLES OF CHEMISTRY

or when acted upon by substances capable of taking up chlorine (even
alkalis, which form bleaching salts). Among the platinum metals,
ruthenium and osmium have the most acid character, and although they
give RuCl4 and OsCl4 they are easily oxidised to Ru04 and Os04 by
the action of chlorine in ibhe presence of water ; the latter are "volatile
and may be distilled with the water and hydrochloric acid, from a
solution containing other platinum metals.9 trl Thus with respect to

the black hydroxide, like many other oxidised compounds of the platinoid metals, does
not dissolve in the ordinary- oxygen acids, whilst the yellow hydroxide is easily soluble
and gives yellow solutions, which deposit imperfectly crystallised salts. Metallic,
rhodium ifl easily obtained by igniting its oxygen and other compounds in hydrogen, or
by precipitation with zinc. It resembles platinum, and has a sp. gr. of 12-1. At the
ordinary temperature it decomposes formic acid into hydrogen and carbonic anhydride,
with development of heat (Deville). With the alkali sulphites, the salts of rhodium and
iridiuni of the type #X3 give sparingly-soluble precipitates of double sulphites of the
composition R(S03Na)3,HaO, by means of which these metals may be separated from
solution, and also may be separated from each other, for a mixture of these salts when
treated with strong sulphuric acid gives a soluble iridium sulphate and leaves a red in-
soluble double salt of rhodium and sodium. It may be remarked that the oxides IraO3 and
Eh2O3 are comparatively stable and are easily formed, and that they also form different
aouble salts (for instance, IrCl3,3KCl3H2O, RhCl3)2NH4Cl4H30, RhCl^SNHiClliH-jO)
and compounds like the cobaltia compounds (for instance, luteo-salts RhX5,6NH3, roseo-
salts, RhX3H2O5NH3, and purpureo-salts IrX3,5NH3. &c.) Iridious oxide, Ir2O3, ia
obtained by fusing iridious chloride and its compounds with sodium carbonate, and
treating the mass with water. The oxide is then left as a black powder, which, when
strongly heated, is decomposed into iridium and oxygen; it is easily reduced, and ia
insoluble in acids, which indicates the feeble basic character of this oxide,, in many
respects resembling such oxides as cobaltic oxide, cerio or lead dioxide, &o. It does not
dissolve when fused with potassium hydrogen sulphate. Rhodium oxide, Rh-jOs, is a
tar more energetic base. It dissolves when fused with potassium hydrogen sulphate.

From what has been said respecting the separation of platinum and rhodium it will
be understood how the compounds of iridium, which is the main associate of platinum,
are obtained. Ia describing the treatment of osmiridium we shall again have an
opportunity of learning the method of extraction of the compounds of this metal, which
has in recent times found a technical application in the form of its oxide, Ir2O3&
this is obtained from many of the compounds of iridium by ignition with water, is
easily reduced by hydrogen, and is insoluble in acids. It is used in painting on china,
for giving a black colour. Iridium itself is more difficultly fusible than platinum, and
when fused it does not decompose acids or even aqua regia ; it is extremely hard, and is
not malleable ; its sp. gr, is 22*4. In the form of powder it dissolves in aqua regia, and
is even partially oxidised when heated in air, sets fire to hydrogen, and, in a word, closely
resembles platinum. Heated in an excess of chlorine it gives iridio chloride. IrCl^ but
this loses chlorine at 50° ; it Is, however, more stable in the form of double salts, which
have a characteristic black colour— for instance, Ir(NH4)2Cl6— but they give iridious
chloride, IrCl3, when treated with sulphuric aeid.

9 tr| We have yet to become acquainted with the two remaining associates of platinum
—ruthenium and osmium—whose most important property is that they are oxidised
even when heated in air, and that they are able to give volatile oxides of the form Ru04
and Os04 ; these have a powerful odour (like iodine and nitrous anhydride). Both these
higher oxides are solids ; they volatilise with great ease at 100° ; the former is yellow
and the latter white. They are known as ruthenic and osmic anhydrides, although their
aqueous solutions (they both slowly dissolve in water) do not show an acid reaction, and
although they do not even expel carbonic anhydride from potassium carbonate, do not

THE PLATINUM MEf ALS 888

the types of combination, all the platinum metals, under certain circun>
stances, give compounds of the type RX4 — for instance, HQ2, RC14, &o,

give crystalline salts with bases, and their alkaline solutions partially deposit them
again when boiled (an excess of water decomposes the salts). The formulae Os04 and
Eu04 correspond with the vapour density of these oxides, Thus Deville found the
vapour density of osmic anhydride to be 128 (by the formula 127*6) referred to hydrogen.
Tennant and Vauquelin discovered this. con\pound, and Berzelius, Wb*hler, Fritzsche,
Struve*, Deville, Claus, Joiy,and others, helped in its investigation; nevertheless there
are still many questions concerning it which remain unsolved. It should be observed
(hat BO* is the highest known form for an oxygen compound, and BH4 is the highest
known form for a compound of hydrogen; whilst the highest forms of acid hydrates
contain SLE^O*, PH304, SH204, .ClHOf-oll with four atoms of oxygen, and therefore in
this number there is apparently the limit for the simple forms of combination of hydrogen
dad oxygen. In combination with several atoms of an element, or several elements,
there may be more than O4 or EE^ but a molecule never contains more than four atoms
of either 0 or H to one atom of another element. Thus the simplest forms of combina*
tion of hydrogen and oxygen are exhausted by the list BB^, BH$, BH2, BH, BO, B0fl,
BOsj B04. The extreme members are BH4 and B04, and are' only met with for such
elements as carbon, silicon, osmium, ruthenium, which also give BtJla w^h chlorine.
In these extreme forms, BH4 and BO^ the compounds are the least stable (com.
pare SiH^, PH5, SH2, C1H, or BuO^ MoOj, Zr08>.SrO), and easily give up part, or even
all, their oxygen or hydrogen.

The primary source from which the compounds of ruthenium and osmium are
obtained is^either osmiridium (the osmium predominates, from IrOs to IrOs4, sp.gr.
from 16 to 2*1), which occurs tn platinum ores (it is distinguished from the grains of
platinum by its crystalline structure, hardness, and insolubility in aqua regia), or else
those insoluble residues which -are obtained, as we saw above, after treating platinum
with aqua regia. Osmium predominates in these materials, which sometimes 'contain
from 80 p.o. to 40 p.c. of it, and rarely more than 4 p.o. to 5 p.c. of ruthenium. The
process for their treatment is as follows : they are -first fused with 6 parts of zinc, %nd
the zinc is then extracted with dilute hydrochloric acid* The osmiridium thus treated
is, according to Fritzsche and Struv^s method, then added to a fused mixture of
potassium hydroxide and chlorate in an iron crucible ; the mass as it begins to evolve
oxygen acts on the- metal, and the reaction afterwards proceeds spontaneously. The
dark product is treated with water,, and gives a solution of osmium and ruthenium in
the form of soluble salts, B«jOs04 and B^BuO^ whilst the ins&luble residue contains a
mixture of oxides of iridium fancl pome osmium, rhodium, and ruthenium), and grains
of metallic iridium still unactefl on. According to Fre*my's method the lumps of
osmiridium are straightway heated to whiteness in a porcelain tube in a stream of air or
oxygen, when the very volatile osmio anhydride is obtained directly, and is collected in
a well-cooled receiver, whilst the ruthenium gives a -crystalline sublimate ci the dioxide,
Bu02, which is, however, very difficultly volatile (it volatilises together with osmio
anhydride), and therefore remains in the cooler portions of the tube j this method does
not give volatile ruthenic anhydride, and thevirldium and other metals are not oxidised
or give non-volatile products, This method $s simple, and at once gives dry, pure osmio
anhydride in the receiver, and ruthenium dioxide in the sublimate. The air which
passes through the tube should be previously passed through sulphuric acid, not only in
order to dry R, but also to remove the organic and reducing dust. The vapour of osmio
anhydride must be powerfully cooled, and ultimately passed over caustic potash. A
third mode of treatment, which is most frequently employed, was proposed by Wohler,
and consists in slightly heating (in order that the sodium chloride, should not melt) an
intimate mixture of osmiridium and common salt in a stream of moist chlorine. The
metals then form compounds with chlorine and sodium chloride, whilst the osmium
forms the chloride, OsCl^ which reacts with the moisture, and gives osmic anhydride,
which is condensed. The ruthenium in this, as in the other processes, does not* directly

884 PRINCIPLES OF CHEMISTRY

But this is the highest form for only platinum and palladiuraY
The remaining platinum metals further, like iron, give acids of the type

give ruthenic anhydride, but is always extracted as the soluble ruthenium salt, K2Ru04,
obtained by fusion with potassium, hydroxide and chlorate or nitrate. When the orange-
coloured ruthenate, K2RuO4, is mixed with acids, the liberated ruthenic acid immediately
decomposes into the volatile ruthenio anhydride and the insoluble ruthenic oxide:
2K2Ru04 + 4HN05=Ru04+Ru02,2H2O + 4KN05. When once one of the above com-
pounds of ruthenium or osmium is procured it is easy to obtain all the remaining
compounds, and by reduction (by metals, hydrogen, formic acid, &c.) the metals
themselves.

Osmic anhydride, Os04, is 'very easily deoxidised by many methods. It1 blackens
organic substances, owing to reduction, and is therefore used in investigating vegetable
and animal, and especially nerve, preparations under the microscope. Although osmic
anhydride may be distilled in hydrogen, still complete reduction is accomplished when a
mixture of hydrogen and osmic anhydride is slightly ignited (just before it inflames). If
osmium be placed in the flame it is oxidised, and gives vapours of osmic anhydride, which
become reduced, and the flame gives a brilliant light. Osmio anhydride deflagrates
like nitre on red-hot charcoal ; zinc, and even mercury and silver, reduce osmic anhydride
from its aqueous solutions into the lower oxides or metal ; such reducing agents as
hydrogen sulphide, ferrous sulphate, or sulphurous anhydride, alcohol, &c., act in the
same manner with great ease.

The lower oxides of osmium, ruthenium, and of the other elements of the platinum
series are not volatile, and it is noteworthy that the other elements behave differently.
On comparing S02, S03; As203, As2O5 ; P203, P2O5; CO, C02, Ac., we observe a
converse phenomenon ; the higher oxides are less volatile than the lower. In the case
of osmium all the oxides, with the exception of the highest, are non-volatile, and it may
therefore be thought that this higher form is more simply constituted than the lower.
It is possible that osmic oxide, OsO^ stands in the same relation to the anhydride as
C2H4 to CH4 — i.e. the lower oxide is perhaps Os2O4, or is still more polymerised, which
would explain why the lower oxides, having a greater molecular weight, are less volatile
than the higher oxides, just as we saw in the case of the nitrogen oxides, N20 and NO.

Buthenium and osmium, obtained by the ignition or reduction of their compounds
in the form of powder, have a density considerably less than in the fused form, and differ
in this condition in their capacity for reaction ; they are much more difficultly fused than
platinum and iridium, although ruthenium is more fusible than osmium. Ruthenium
in powder has a specific gravity of Q'5, the fused metal of 12-2 ; osmium in powder has a
specific gravity of 20'0, and when semi-fused— or, more strictly speaking, agglomerated—
in the oxy -hydrogen flame, of 21'4, and fused 22'5. The powder of slightly-heated osmium
oxidises very easily in the air, and when ignited burns like tinder, directly forming the
odoriferous osmic anhydride (hence its name, from the Greek word signifying odour) ;
ruthenium also oxidises when heated in air, but with more difficulty, forming the oxide
Bu02. The oxides of the types RO, R203, and RO2 (and their hydrates) obtained by
reduction from the higher oxides, and also from the chlorides, are analogous to those
given by the other platinum metals, in which respect osmium and ruthenium closely
resemble them. We may also remark that ruthenium has been found rn the platinum
deposits of Borneo in the form of laurite, Ru2S3, in grey octahedra of sp. gr. 7'0.

For osmium, Moraht ' and Wischin (1893) obtained free osmic acid, H2OsO4, by
decomposing K2OsO4 with water, and precipitating with alcohol in a current of hydrogen
(because in air volatile Os04 is formed) ; with H2S, osmic acid gives Os03(HS)2 at the
ordinary temperature.

Debray and Joly showed that ruthenic anhydride, RuO4, fuses at 25°, boils at 100°,
and evolves oxygen when dissolved in potash, forming the salt KRu04 (not isomorphous
with potassium permanganate).

Joly (1891), who studied the ruthenium compounds in greater detail, showed that the
easily-formed KRu04 gives RuK04Ru03 when ignited, but it resembles KMn04 in many

THE PLATINUM METALS 885

R03or hydrates, HaRO^ROafHO^ (the type o£ sulphuric acid) ; bu$
they, like ferric and manganic acids, are chiefly known in the form of
salts of the composition K2R04 or K2R207 (like the dichromate). These
salts are obtained, like the manganates and ferrates,, by fusing the oxides,
or even the metals themselves, with nitric, or, better still, with potassium
peroxide. They are soluble iji water, are easily deoxidised and do not
yield the acid anhydrides under the action of acids, but break up, either
(like the ferrate) forming oxygen and a basic oxide (indium and rhodium
react in this manner, as they do not give higher forms of oxidation), Or
passing into a lower and higher form of oxidation — that is, reacting
like a manganate (or partly like nitrite 'or phosphite). Osmium and
ruthenium react according to the latter form, as they are capable of
giving higher forms of oxidation, Os04 and RuO4, and therefore their
reaction.^, of decomposition may be essentially represented by the equa-
tion • 20sC>3=OsOa

respects. In general, Eu has much in common with Mn, Joly (1889) also showed that
If KNO5 be added to a solution of EuClj containing HC1, .the solution, becomes hot, and
ft salfr, EuCl3N02KCl, is formed, which enters into double .decomposition and is very
Btable. Moreover, if Eu.Cl3 be treated with an excess of nitric acid, it forms a salt^
EuCl3NOEaO, after being heated (to boiling) and the addition of HCL The vapour
density of Eu04, determined by Pebray and Joly, corresponds to that formula,

10 Although palladium gives the same type.s of combination (with chlorine) as
platinum, its reduction to EX3 is incomparably easier tnan that of platinic-chloride, and
(a the case of iridium iij is also very easy. Iridic chloride, IrCl4, ^ts as an oxidising
agent, readily parts with a fourth of its- chlorine to a number of substances, readily
evolves chlorine when heated, an,d it is only at low temperatures that chlorine and aqua
regia convert iridium into iridio chloride. In' .disengaging chlorine iridium more often
and easily gives the very* stable iridious chloride, IrCl3 (perhaps this substance is
lr2Cle=IrCl2,IrCl4, insoluble in water, but soluble in potassium chloride, because it
forms the double salt K3IrCl6), than the dichloride, IrCl^. This compound, corresponding
to IrX2, is very stable, and corresponds with the basic oxide, Ir203> resembling the
oxides Fe203, Co203. To this form there correspond ammoniacal compounds similar to
those given by cobaltic oxide. Although iridium also gives an acid in the form of the
salt K2Ir207, it does not, like iron (and chromium),, form the corresponding chloride^
IrC-lc. In general, in this as in the other elements, it is impossible to predict the chlorine
compounds from those of oxygen. Just as there is no chloride SC16> but only SC12, so
also, although Ir03 exists, IrCl6 is wanting, the only chloride being IrCl4^ and this
is unstable, like SC12, and easily parts >with its chlorine. In this respect rhodium, is
very much like iridium (as platinum is like palladium). For KhCl4 decomposes with
extreme ease, whilst rhodium chloride, EhCl3> is very -stable,.like many of the salts of
the type EhX5).although like the platinum elements these salts are easily reduced to
metal by the actjon of heat and powerful reagents. There 'is as close a resemblance
between osmium and ruthenium. Osmium when submitted to the riction of dry chlorine
gives osmic chloride, OsCl^ but the' latter is converted by water (as is osmium by moist
chlorine) into osmic anhydride, although the greater portion is then decomposed .into
Os(HO)4 and 4HC1, like a chloranhydride of an acid. In general this acid character is
more developed in osmium than in platinum and iridium. Having parted with chlorine,
osmio chloride, OsCl4, gives the unstable trichloride, OsCl3, an.d the stable soluble
dichloride, OsCl2> which corresponds with platinous chloride in its properties and
reactions. 'The relation of ruthenium to the halogens is of the «^amQ nature*

386 PRINCIPLES OF CHEMISTRY

Platinum and its analogues, like iron and its analogues, are able to
form complex and comparatively stable cyanogen and ammonia com-
pounds, corresponding with the ferrocyanides and the ammoniacal com-
pounds of /cobalt, which we have already considered in the preceding
chapter.

If platinous chloride, PtCla (insoluble in water), be added by degrees
to a solution of potassium cyanide, it is completely dissolved (like
silver chloride), and on evaporating the solution deposits rhombic
prisms of potassium platinocyanidet PtK2(CN)4,3H20. This salt, like
all those corresponding with it, has a remarkable play of colours, due to
the phenomena of dichromism, and even polychromism, natural to all
the platinocyanides. Thus it is yellow and reflects a bright blue
light. It is easily soluble in water, effloresces in air, then turns red,
and at 100° orange, when it loses all its water. The loss of water
does not destroy its stability—that is, it still remains unchanged, and
its stability is further shown by the fact that it is formed whea
potassium ferrocyanide, K4Fe'(CN)6, is heated with platinum black.
This salt, first obtained by Gmelin, shows a neutral reaction with
litmus ; it is exceedingly stable under the action of air, like potassium
ferrocyanide, which it resembles in many respects. Thus the platinum
In it cannot be detected by reagents such as sulphuretted hydrogen ;
the potassium may be replaced by other metals by the action of their
salts, so that it corresponds with a whole series of compounds, R2Pt(CN)4,
and it is stable, although the potassium cyanide and platinous salts, of
which it is composed, individually easily undergo change. When
treated with oxidising agents it passes, like the ferrocyanide, into a
higher form of combination of platinum. If salts of silver be added
to its solution, it gives a heavy white precipitate of silver platino-
cyanide, PtAg2(CN)4, which when suspended in water and treated
with sulphuretted hydrogen, enters into double decomposition with the
latter and forms insoluble silver sulphide, Ag2S, and soluble hydro-
platinocyanic acid, H2Pt(CN)4. If potassium platinocyanide be mixed
with an equivalent quantity of sulphuric acid, the hydroplatino-
cyanic acid liberated may be extracted by a. mixture, of alcohol and
ether. The ethereal solution, when evaporated in a desiccator, deposits
bright red crystals of the composition PtH2(CN)4,5H80. This acid
colours litmus paper, liberates carbonic anhydride from sodium car-
bonate, and saturates alkalis, so that it presents an analogy to hydro-
ferrocyanic acid.11

" This acid character is explained by the influence of the platinum on the hydrogen,
and by the attachment of the cyanogen groups. Thus cyanuric acid, H3(CN)303, is an
energetic acid compared with cyanic acid, HCNO. And the formation of a compound

THE PLATINUM METALS 887

Ammonia, like potassium cyanide, has the faculty of easily reacting
with platinum dichloride, forming compounds similar to the platino

with five molecules of water of crystallisation, (PtH2(CN)4>5H20), confirms the opinion
that platinum is able to form compounds of still higher types than that expressed
in its saline compounds, and, moreover, the combination of hydroplatinocyanio acid
with water does not reach the limit of the compounds which appears in

A whole series of glatinocyanides of the common type PtB2(CN)4wH20 is obtained
by means of double decomposition with the potassium or hydrogen or silver salts. For
example, the salts of sodium and lithium contain, like the potassium salt, three molecules
of water» The sodium salt is soluble in water and alcohol. The ammonium salt has the
composition Pt(NH4)2(CN)4,2H20 and gives crystals which reflect blue and rose-coloured
light. This ammonium salt decomposes at 800°, with evolution of water and ammonium
cyanide, leaving a greenish platinum dicyanide, Pt(CN)3, which is insoluble in water
and acid but dissolved in potassium cyanide, hydrocyanic acid, and other cyanides. The
eatne platinous cyanide is, obtained by the action of sulphuric acid on the potassium
salts in the form of a reddish-brown amorphous precipitate. The most characteristic of the
platinocyanides are those of the alkaline earths. The magnesium salt PtMg(CN)4,7H2O
crystallises in regular prisms, whose side faces are of a metallic green colour and terminal
planes dark blue. It shows a carmine-red colour along the main axis, and dark red
along the lateral axes ; it easily loses water, (2H2O), at 40°, and then turns blue (it then
contains 5H20, which is frequently the case with the platinocyanides). Its aqueous
solution is colourless, and an' alcoholic solution deposits yellow crystals. The remainder
of the water is given off at 280°. It is obtained by saturating platinocyanio acid with
magnesia, or else by double decomposition between the barium salt and magnesium sul«
phate. The strontium salt, SrPt(CN)4,4jH2O crystallises in milk-white plates having
a violet and green iridescence. When it effloresces in a desiccator, its surfaces have
a violet and metallic green iridescence. A colourless solution of the barium salt
PtBa(CN)4,4H20 is obtained by saturating a solution of hydroplatinocyanio acid with
baryta, or by boiling the insoluble copper platinocyanide in baryta water It crystallises
in nronoclinic prisms of a yellow colour, with blue and green iridescence ; it loses half its
water at 100°, and the whole at 150°. The ethyl salt, Pt(C2H5)2(CN)4)2HovO, is also
very characteristic ; its crystals are isomotphous with those of the potassium salt, and
are obtained by passing hydrochloric acid into an alcoholic solution of hydroplatino-
cyanic acid. The, facility with which they crystallise, the regularity of their forms,
and their remarkable play of colours, renders the preparation of the platinocyanides one
of the most attractive lessons of the laboratory.

By the action of chlorine or dilute nitric acid, the platinocyanides are converted into
salts of the composition PtM3(CN)5, which corresponds with Pt(CN)3,2KCN— that is,
they express the type of a non-existent form of oxidation of platinum, PtX$ (i.e. oxide
PtsOj), just as potassium ferricyanide (FeCy3,8KCy) corresponds with ferric oxide, and
the ferrocyanide corresponds with the ferrous oxide. The potassium salt of this series
contains PtK^CN^SHgO, and forms brown regular prisms with a metallic lustre, and is
soluble in water but insoluble in alcohol. Alkalis re-convert this compound into the
ordinary platinocyanide K2Pt(CN)fc. taking up the excess of cyanogen. It is remarkable
that the salts of the type J?tMjjCy5 contain the same amount of water of crystallisation
as those of the type PtM2Cy4. Thus the salts of potassium and lithium contain three,
and the salt of magnesium seven, molecules of water, like the corresponding salts of the
type of platinous oxide. Moreover, neither platinum nor any of its associates gives any
cyanogen compound corresponding with the oxide, i.e. having the composition PtK2Cy6,
just as there are no compounds higher than those which correspond to BCy5wMCy for
cobalt or iron. This would appear to indicate the absence of any such cyanides, and
indeed, for no element are there yet known, any poly-cyanides containing more than three
equivalents ol cyanogen for one equivalent of the element. The phenomenon, is perhapa

888 PRINCIPLES OF CHEMISTRY

oyanide and cobaltia compounds, which are comparatively stable.- 'But
as ammonia does not. contain any hydrogen easily replaceable by

connected with the faculty or cyanogen of giving tricyanogen polymeridea, such as cyanurio.
acid, solid cyanogen chloride, &c. Under the action of an excess of chlorine, a solution
of PtK2(CN)4 gives (besides PtK2Cy6) a product PtK2Cy4Cl2, which evidently contains
the form PtX4, but at first the action of the chlorine (or the electrolysis of, or addition
of dilute peroxide of hydrogen to, a solution of PtK2Cy4, acidulated with hydrochloric
acid) produces an easily soluble intermediate salt which crystallises in thin copper-red
needles (Wilm, Hadow, 1889). It only contains a email amount of chlorine, and
apparently corresponds to a compound 6PtK2Cy4+ PtK2Cy4Cl2 + 24H20. Under the
action of an excess of ammonia both these chlorine products are converted either oon>
pletety or in part (according to Wilm ammonia does not act upon PtK2Cy4) into
PtCy2,2NH5, i.e. a platino-ammonia compound (see further on). It is also necessary to
pay attention to the fact that ruthenium and osmium—which, as we know, give higheir
forms of oxidation than platinum — are also able to combine with a larger proportion of
potassium cyanide (but not of cyanogen) than platinum. Thus ruthenium forms a
crystalline hydroruth&wcyanic ac^d, RuH4(CN)6, which js soluble in water and alcohol,
and corresponds with the salts M4Ru(CN)6. There are exactly similar osmic com-
pounds—for example, "K4Os(CN)6,8H2O. The latter is obtained in the form of colourless,
sparingly-soluble regular tablets on evaporating the solution obtained from a fused
mixture of potassium osrciochloride, K2OsCl6,.and potassium cyanide. These osmic and
ruthenic compounds fully correspond with potassium ferrocyanide, K4Fe(CN)6,8H20, not
Only in their composition but also jn their crystalline form and reactions, which again
demonstrates the close analogy between iron, ruthenium, and osmium, which we have
shown by giving these three elements a similar position (in the eighth group) in the
periodic system. For rhodium and indium only salts of the same type as the ferricyanides,
M3RCy6, are known, and for palladium only of the type M2PdCy4, which are analogous
to the platinum salts. In all these examples a constancy of the types of the double
cyanides is apparent. In the eighth group we have iron, cobalt, nickel, copper, and their
analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum,
gold. The double cyanides of iron, ruthenium, osmium have the type K^CNJeJ of
cobalt, rhodium, iridium, the type K5R(CN)e ; of nickel, palladium, platinum the type
K2R(CN)4 and K2R(CN)5; and for copper, silver, gold there are known KR(CN)2, so
that the presence of 4, 8, 2, and 1 atoms of potassium corresponds with the
order of the elements in the periodic system. Those types which we have seen
in the ferrocyanidea and ferricyanidea of iron repeat themselves in all the platinoid
metals, and this naturally leads to the conclusion that the formation of similar
so-called double salts is of exactly the 8ame*bature as that of the ordinary salts. If, in
expressing the union of the elements in the oxygen salts, the existence of an
aqueous residue (hydroxyl group) be admitted, in which the hydrogen is replaced by a
metal, we have then only to apply this mode of expression to the double salts and the
analogy will be obvious, if only we remember that C12, (CN)2, SO4, &c., are equivalent to
O, as we see in RO, RC12, RS04, &c. They all «=X2, and, therefore, in point of fact,
wherever X ( = C1 or OH, &c.) can be placed, there (C12H), (S04H), &c., can also stand.
And as ClaH = Cl + HC1 and S04H <= OH + SO5, &c., it follows that molecules EDC1 or SQs,
or, in general, whole molecules — for instance, NH8, B^O, salts, &c., can annex themselves
to a compound containing X. (This is an indirect consequence of the law of substitution
which explains the origin of double salts, ammonia compounds, compounds with water
of crystallisation, &c., by one general method.) Thus the double salt MgS04,K2S04,
according to this reasoning, may be considered as a substance of the same type as
MgCl2, namely, as =Mg(S04K)2, and the alums as derived from A1(OH)(S04), namely, as
AKSO^HSO^, Without stopping to pursue this digression further, we will apply
these considerations to the type of the ferrocyanides and ferricyanides and their
platinum analogues. Such a salt aa KgPtCy4 may accordingly be regarded as Pt(CyaS)^

THE PLATINUM METALS 889

metals, and as ammonia itself is able to combine -with acids, the
PtX2 plays, as it tfere, the part of an acid with reference to the*

like Pt(OH)2 ; and such a salt as PtK2Cy5 as PtCy(CyaK)2, the analogue of PtX(OH)2,
or A1X(OH)2, and other compounds of the type RXj. 'Potassium ferricyanide and the
analogous compounds of cobalt, iridium, and rhodium, belong to the same'type, with the
same difference as there is between KX(OH)3 and R(OH)S, since FeK3Cy6=Fe(Cy2K)5.
Limiting myself to these considerations, which may partially elucidate the nature
of double salts, I will now pasa 'again to the complex saline compounds known for
platinum.

(A) On mixing a solution of potassium thiocyanate with a solution of potassium
platinosochloride, K2PtCl4, they form a double thiocyanate, PtB^CNS)^ which is easily
soluble in water and alcohol, crystallises in red prisms, and gives au orange-coloured
solution, which precipitates salts of the heavy metals. The action of sulphuric acid
on the lead .salt of the same 'type gives the acid jtself, PtH2(SCN)4, which corre-
sponds with these salts. The type of these compounds is evidently the same as that of
the cyanides.

(J5) Platinous chloride, PtCl2> which is insoluble fn water, forms double $alt$ .with
the metallic chlorides. These double chlorides are soluble in water, and capable of.
crystallising. Hence when a hydrochloric acid solution of platinous chloride is mixed
with solutions of metallic salts and evaporated it forms crystalline salts of a red or
yellow colour. Thus, for example, the potassium salt, PtKjjCl^ is red, and easily
soluble in water; the sodium salt is also soluble in alcohol;' the barium salt,
PtBaCl4,8H2O, is soluble hi water, but the silver salt^ PtAg2Cl4, is insoluble in water,
and may be used for obtaining the remaining salts by means of double decomposition
with their chlorides.

(0) A remarkable example of the complex compounds of platinum was observed by
Schiitzenberger (1868). He showed that finely-divided platinum in the presence of
chlorine and carbonic oxide at 250°-800° gives phosgene and a volatile compound con-
taining platinum. The same substance is formed by the action of carbonic oxide on
platinous chloride. It decomposes with an explosion in contact with water. Carbon
tetracbloride dissolves a portion of this substance, and on evaporation gives crystals of
2PtCl2,8CO, whilst the compound PtCl2,2CO remains undissolved. When fused and
sublimed it gives yellow needles of PtCl^CO, and in the presence of an excess of
carbonic oxide PtCl2,2CO is formed. These compounds are fusible (the first at 250°, the
second at 142°, and the third at 195°). In this case (as in the double cyanides) com-
bination takes place, because both carbonic oxide and platinous chloride are unsaturated
compounds capable of further combination. The carbon tetrachloride solution absorbs
NH5 and gives PtCl2,CO,2NH3, and PtCl5,2CO,2NH5, and these substances are analo-
gous (Foerster, Zeisel, Jorgensen) to similar compounds containing complex amines (for
instance, pyridine, C5H5N), instead of NH$, and ethylene, &c., instead of CO, so that here
we have a whole series of comple? platino-compounds. The compound PtC^CO
dissolves in hydrochloric acid without change, and the solution disengages all the
carbonic oxide when KCN is added to it, which shows that those forces which bind
9 molecules of KCN to PtClj can also bind the molecule CO, or 2 molecules of CO.
When 'the hydrochloric acjd solution of PtCLjCO is mixed with a solution of sodium
acetate or acetic acid, it gives a precipitate of PtOCO, i.e, the C12 is replaced by oxygen
(probably because the acetate is decomposed by water). This oxide, PtOCO, splits up
into Pt+ C02 at 850°, PtSCO is obtained by the action of sulphuretted hydrogen upon
PtCl2CO. All this leads to the conclusion that the group PtCO is able to assimilate
X2=C12, S, O, &c. (Mylius, FperstSlr, 1891). Pullinger (1891), by igniting spongy
platinum at 250°, first in a stream of chlorine, and then in a stream of carbonic oxide,
obtained (besides volatile products) a non-volatile yellow substance which remained
unchanged in air and disengaged chlorine and phosgene gas when ignited ; its compo-
sition was PtCl$(CO)2, which apparently proves it to be a compound of PtCl2 and

890 PRINCIPLES OF CHEMISTRY

ammonia. Owing to the influence of the ammonia, the X2 in th«
resultant compound will represent the same character as it has in

2COC12, as PtCla is able to combine with oxychlorides, and forms somewhat stable
compounds.

(D) The faculty of platinous chloride for forming stable compounds with divers sub-
stances shows itself in the formation of the compound PtCl2,PCl3 by the action of phos-
phorus pentachloride at 250° on platinum powder (Pd reacts in a similar manner,
according to Fink, 1892), The product contains both phosphorus pentachloride and
platinum, whilst the presence of PtCLj is shown in the fact that the action of water
produces chlorplatino-phosphorous acid, PtCl2P(OH)3.

(E) After the. cyanides, the double salts of platinum formed by sulphurous acid are
most distinguished for their stability and characteristic properties. This is all the more
instructive, as sulphurous acid is only feebly energetic, and, moreover, in these, as m all
its compounds, it exhibits a dual reaction. The salts of sulphurous acid, R2S03) either
react as salts of a feeble bibasic acid, where the group SO3 presents itself as bivalent,
and consequently equal to X2, or else they react after the manner of salts of a monobasic
acid containing the same residue, RSOj^lis occurs in the salts of sulphuric acid. In sul-
phurous acid this residue is combined with hydrogen, H(803H), whilst in sulphuric acid
it is united with the aqueous residue (hydroxyl), OH(S03H). These two forms of action
of the sulphites appear in their reactions with the platinum salts — that is to say, salts of
both kinds are formed, and they both correspond with the type PtH^. The one
eeries of salts contain PtH2(S03)2, and their reactions are due to the bivalent residue
of sulphurous acid, which replaces X2. The others, which have the composition
PtR2(S03H)4, contain sulphoxyl. The latter salts will evidently react like, acids ; they
are formed simultaneously with the salts of the first .kind, and pass into them. These
salts are obtained either by directly dissolving platinous oxide in water containing sul-
phurous acid, or by passing sulphurous anhydride into a solution of platinous chloride
in hydrochloric acid. If a solution of platinous chloride or platinous oxide in sulphurou»
acid be saturated with sodium carbonate, it forms a white, sparingly soluble precipitate
containing PtNa2(S08Na)4,7H20. If this precipitate be dissolved in a small quantity ol
hydrochloric acid and left to evaporate at the ordinary temperature, it deposits a salt of
t*he other type, PtNa!,(S03)3,H3O, in the form of a yellow powder, which is sparingly
eoluble in wafer. The potassium salt analogous to the first salt, PtK2(S03K)4.2H2O, is
precipitated by passing sulphurous anhydride into a solution of potassium sulphite in
which platinous oxide is suspended. A similar salt is known for ammonium, and with
hydrochloric acid it gives a salt of the second kind, Pt(NH4)2(S03)2,H20. If ammonio-
chloride of platinum be added to an aqueous solution of sulphurous anhydride, it is first
deoxidised, and chlorine is evolved, forming a salt of the type PtX2 ; a double decompo-
sition then takes place with the ammonium sulphite, and a salt of the composition
Pt(NH4)3Cl8(803H) is formed (in a desiccator). The acid character of this substance is
explained by the fact that it contains the elements SO3H— sulphoxyl, with the hydrogen
not yet displaced by a metal. On saturating a solution of this acid with potassium
carbonate it gives orange-coloured crystals of a potassium salt of the composition
Pt(NH4)2Cl3(S03K). Here it is evident that an equivalent of chlorine in PtfNH^C^ is
replaced by the univalent residue of sulphurous acid. Among these salts, that of the
composition Pt(NH4)2Cla(S03H)2,H20 is very readily formed, and crystallises in well-
formed colourless crystals; it is obtained by dissolving ammonium platinosochloride,
Pt(NH4)2Cl4, in an aqueous solution of sulphurous acid. The difficulty with which sul-
phurous anhydride and platinum are separated from these salts indicates the same basio
character in these compounds as is seen in the double cyanides of platinum. In their
passage Into a complex salt, the metal platinum and the group S03 modify their
relations (compared with those of PtXa or S03X2), just as the chlorine in the salts KC1O,
KC10«, and KC104 is modified in its relations as compared with hydrochloric acid or
potassium chloride.

THE PLATINUM' METALS 891

ammotoiacal salts.; consequently, the ammonlacal compounds produced
from PtX2 will be salts in which X will be replaceable by various
other haloids, just as the metal is replaced in the cyanogen salts ; such
is the nature of the platino-ammonium compounds. PtX2 forms com-
pounds with 2NH3 and with 4NH3, and so also PtX4 gives (not
directly from PtX4 and ammonia, but from the compounds of PtX2 by
the action of chlorine, <fec.) similar compounds with 2NH3 and with
4NH3.12

(F) No less characteristic are the plat inonitrites formed by platinous oxide. They
correspond with nitrous acid, whose salts, ENO2, contain the univalent radicle, NO2,
which is capable of replacing chlorine, and therefore the salts of this kind should form a
common type PtR2(NO2)4, and such a salt of potassium has actually been obtained by
mixing a solution of potassium platinosochloride with a solution of potassium nitrite,
when the liquid becomes colourless, especially if it be heated, which indicates the change
in the chemical distribution of the elements. As the liquid decolorises it gradually
deposits sparingly soluble, colourless prisms of the potassium salt K2Pt(NO2)4, which
does not contain any water. With silver nitrate a solution of this salt gives a precipitate
of silver platinonitrite, PtAg2(tf 02)4. The silver of this salt may be replaced by other
metals by means of double decomposition with metallic chlorides. The sparingly soluble
barium salt, when treated with an equivalent quantity of sulphuric acid, gives a soluble
acid, which separates, under the receiver of an air-pump, in red crystals ; this acid has
the composition PtH2(N02)4. To the potassium salt, K2Pt(NO2)4, there correspond
(Vezes, 1892) K2Pt(NO2)4Br2 and K2Pt(NO2)4Cl2 and other compounds of the same type
K3PtX6, where X is partly replaced by Cl or Br and partly by (NO2), shpwing a
transition towards the type of the double salts like the platino-ammoniacal salts. (The
corresponding double sodium nitrite salt of cobalt is soluble in water, while he K,NH4
and many other salts are insoluble in water, as I was informed by Prof. K. Winkler
in 1894).

In all the preceding complex compounds of Pt we see a common type PtX2)2MX
(i.e. of double salts corresponding to PtO) or PtM2X4 = Pt(MX2)2, corresponding to
Pt(HO)2 with the replacement of O by its equivalent X2. Two other facts must also be
noted. In the first place these X's generally correspond to elements (like chlorine) or
groups (like CN, NO2, SO3, &c.), which are capable of further combination. In the
second place all the compounds of the type PtM2X4 are capable of combining with
chlorine or similar elements, and thus passing into compounds of the types PtX3 or PtX4.

12 The platinum salt and ammonia, when once combined together, are no longer
subject to their ordinary reactions but form compounds which are comparatively very
stable. The question at once suggests itself to all who are acquainted with these pheno-
mena, as to what is the relation of the .elements contained in these compounds. The first
explanation is that these compounds are salts of ammonium in which the hydrogen is
partially replaced by platinum. This is the view, with certain shades of difference, held
by many respecting the platino-ammonium compounds. They were regarded in this
light by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we suppose the hydro-
gen in 2NH4X to be replaced by bivalent platinum (as in the salts PtX2), we shall

obtain JJ^Pt^— that is, the compound PtX2,2NH3. The compound with 4NH3 will

then be represented by a 'further substitution of the Hydrogen in ammonia 'by ammo-
nium itself— i.e. as NH2(NH4X)2Pt or PtX2,4NH3. A modification of this view is found
in that representation of compounds of this kind which is based on atomicity. As
platinum in PtX2 is bivalent, has two affinities, and ammonia, NH3, is also bivalent,
because nitrogen is quinquivalent and is here only combined with H3, it is evident what
bonds should be represented inPtX2>2NH3and inPtX2,4NH3. In the former, Pt(NH3Cl)2,
the nitrogen of each atom of ammonia is united by three affinities with H3, by one with

*F

392 PRINCIPLES OF CHEMISTRY

If ammonia acts on a boiling solution of platinous chloride in
hydrochloric acid, it produces the green salt of Magnus (1829),

platinum, and by the fifth with chlorine. The other compound is Pt(NH3$:NH3Cl)y— that
is, the N is united by one affinity with the other N, whilst the remaining bonds are the
same as in the first salt. It is evident that this union or chain of ammonias has no
obvious limit, and the most essential fault of such a mode of representation is that it does
not indicate at all what number of ammonias are capable of being retained by platinum.
Moreover, it is hardly possible to admit the bond between nitrogen and platinum in such
stable compounds, for these kinds of affinities are, at all events, feeble, and cannot lead
to stability, but would' rather indicate explosive and easily-decomposed compounds.
Moreover, it is not clear why this platinum, which is capable of giving PtX4, does not act
with its remaining affinities when the addition of ammonia to PtXj takes place. These,
and certain other considerations which indicate the imperfection of this representation of
the structure of the platino-ammonium salts, cause many chemists to incline more to the
representations of Berzelius, Glaus, Gibbs, and others, who suppose that NH^is able to
combine with substances, to adjoin itself or pair itself with them (this kind of combination
is called ' Paarung ') without altering the fundamental capacity of a substance for further,
combinations. Thus, in PtX2,2NH3, the ammonia is the associate of PtX2, as is
expressed by the formula N^HePtX-j. Without enlarging on the exposition of the details
of this doctrine, we will only mention that it, like the first, does not render it possible to
foresee a limit to the compounds with ammonia; it isolates compounds of this kind
into a special and artificial class ; does not show the connection between .compounds of
this and of other kinds, and therefore it essentially only expresses the fact of the com-
bination with ammonia and the modification in its ordinary reactions. For these
reasons we do not hold to either of these proposed representations of the ammonio-
platinum compounds, but regard them, from the point of view cited above with reference
to double salts and water of crystallisation — that is, we embrace all these compounds
under the representation of compounds of complex types. The type of the compound
PtX2,2NH3 is far more probably the same as that of PtX2,2Z— i.e. as PtX^ or, still more
•accurately and truly, it is a compound of the same type as PtX2,2KX or PtX2,2H20, &c.
Although the platinum first entered into PtK2X4 as the type PtX2, yet its character has
changed in the same manner as the character of sulphur changes when from SO2 the com-
pound S02(OH)2 is obtained, or when KC104, the higher form, is obtained from KC1. For us
as yet there is no question as to what affinities hold X2 and what hold 2NH3, because this
is a question which arises from the supposition of the existence of different affinities in the
atoms, which there is no reason for taking as a common phenomenon. It seems to me
that it is most important as a commencement to render clear the analogy in the formation
of various complex compounds, and it is this analogy of the ammonia compounds with
those of water of crystallisation and double salts that forms the main object of the
primary generalisation. We recognise in platinum, at all events, not only the four.
affinities expressed in the compound PtCl4, but a much larger number of them, if only
the- summation of affinities is actually possible. Thus, in sulphur we recognise not two
but a much greater number of affinities; it is clear that at least six affinities can act.
So also among the analogues of platinum : osmic anhydride, Os04, Ni(CO)4) PtH2Cl6, &o.
indicate the existence of at least eight affinities ; whilst, in chlorine, judging from the
compound KC1O4 = C1O3(OK) = C1X7, we must recognise at least seven affinities,
instead of the one which is accepted. The latter mode of calculating affinities is a tribute
to that period of the development of science when only the simplest hydrogen compounds
•were considered, and when all complex compounds were entirely neglected (they were
placed under the class of molecular compounds). This is insufficient for the present
state of knowledge, because we find that, in complex compounds as in the most simple, the
same constant types or modes of equilibrium are repeated, and the character of certain
elements is greatly modified in the passage from the most simple into very complex
compounds.

THE PLATINUM METALS 893

PtCl2,2NH3, insoluble in water and hydrochloric acid. But, judging
by its reactions, this salt has twice this formula. Thus, Gros (1837),
on boiling Magnus's salt with nitric acid, observed that half the chlorine
was replaced by the residue of nitric acid and half the platinum was
disengaged : 2PtCl2(NH3)2 + 2HNO3==PtCl2(NO3)2(NH3)4 + 2PtCl2.
The Gros's salt thus obtained, PtCl2(NO3)24NH3 (if Magnus's salt

Judging from the most complex platino-ammonium compounds PtCl4,4NH3, we
should admit the possibility of the formation of compounds of the type PtX4Y4, where
Y4 = 4X2 = 4NH3,' and this shows that those forces which form such a characteristic
Series of double platinocyanides PtK2(CN)4,3H2O. probably also determine the formation
o! the higher ammonia derivatives, as is seen on comparing —

PtCl2 NH3 C12 8NH3
Pt(CN)2 KCN KCN 3H20.

Moreover, it is obviously much more natural to ascribe the faculty for combination
with nY to the whole of the acting elements— that is; to PtX2 or PtX4, and not to
platinum alone. Naturally such compounds are not produced with any Y. With
certain X's there only combine certain Y's. The best known and most frequently-;',
formed compounds of this kind are those with- water— that is, compounds with water of
crystallisation. Compounds with salts are double salts; also we know that similar
compounds are also frequently formed by means of ammonia. Salts of zinc, ZnXj,
copper, CuX2, silver, AgX, and many others give similar compounds, but these and many
other ammonio-metallic saline compounds are unstable, and readily part with their
combined ammonia, and it is only in the elements of the platinum 'group and in the
group of the analogues of iron, that we observe the faculty to form stable ammonio-
metallic compounds. It must be remembered that the metals of the platinum and iron
groups are able to form several high grades of oxidation which have an acid character,
.and consequently in the lower degrees of combination there yet remain affinities capable
of retaining other ' elements, and they probably retain ammonia, and hold it the more
stably, because all the properties of the platinum compounds are rather acid than basic
—that is, PtXn recalls rather HX or SnX» or CXn than KX, CaX2, BaX2, &c., and
ammonia naturally will rather combine with an acid than with a basic substance.
Further, a dependence, or certain connection of the forms of oxidation with the ammonia
compounds, is seen on comparing the following compounds :

PdClo,2NH3,H20 PdCl2,4NH3,H20

PtCl2,2NH3 PtCl4,4NH3

BhCl3,5NH3 BuCl2,4NH3,8H2O

IrCl3)5NH3 OsCl2,4NH3,2H2O

We know that platinum and palladium give compounds of lower types than iridium
and rhodium, whilst ruthenium and osmium give the highest forms of oxidation ; this
shows itself in this case also. We have purposely cited the same compounds with 4NH5
for^osmium and ruthenium as we have for platinum and palladium, and it is then seen
that Eu and Os are capable of retaining 2H.2O and 8H2O, besides C12 and NH3, which
.the compounds of platjnum and palladium are unable to do. The same ideas
which were developed in Note 85, Chapter XXII. respecting the cobaltia compounds are
perfectly applicable to the present case, i.e. to the platinia compounds or ammonia
compounds of the platinum metals, among which Bh and Ir give compounds which are
perfectly analogous to the cobaltia compounds.

Iridium and rhodium, which easily give compounds of the type RX3, give compounds
(Claus) of the type IrX3,6NH3, of a rose colour, and BhX3,5NH3, of a yellow colour.
.Jbrgensen, in his researches on these compounds, showed their entire analogy with the
.cobalt compounds, as was to be expected from the periodic system.

394 PRINCIPLES OF CHEMISTRY

belongs to the type PtX2, then Gros's salt belongs to the type PtX4), is
soluble in water, and the elements of nitric acid, but not the chlorine,,
contained in it are capable of easily submitting themselves to double
saline decomposition. Thus silver nitrate does not enter into double
decomposition with the chlorine of Gros's salt. Most instructive was
the circumstance that Gros, by acting on his salt with hydrochloric
acid, succeeded in substituting the residue of nitric acid in it by
chlorine, and the chlorine thus introduced, easily reacted with silver
nitrate. Thus it appeared that Gros's salt contained two varieties of
chlorine — one which reacts readily, and the other which reacts with
difficulty. The composition of Gros's first salt is PtCl2(NH3)4(N03)2 ;
it may be converted into PtCl2(NHa)4(S04), and in general into
PtCl2(NH3)4X2.i*

The salt of Magnus when boiled with a solution of ammonia gives
the salt (of Reiset's first base) PtCl2(NH3)4, and this, when treated
with bromine, forms the salt PtCl2Br2(NH3)4, which has the same
composition and reactions as Gros's salt. To Reiset's salts there
corresponds a soluble, colourless, crystalline hydroxide, Pt(OH)2(ISrH3)4,
having the properties of a powerful and very energetic alkali ; it
attracts carbonic anhydride from the atmosphere, precipitates metallic
salts like potash, saturates active acids, even sulphuric, forming
colourless (with nitric, carbonic, and hydrochloric acids), or yellow
(with sulphuric acid), salts of the type PtX2(NH3)4.!4 The com-

13 Subsequently, a whole series of such compounds was obtained with various
elements in the place of the (non-reacting) chlorine, and nevertheless they, like the
chlorine, reacted with difficulty, whilst the second portion of the X's introduced into
such salts easily underwent reaction. This -formed the most important reason for the
interest which the study of the composition and structure of the platino-ammonium
salts subsequently presented to many chemists, such as Keiset, Blomstrand, Peyrone,
Raeffski, Gerhardt, Buckton, Cleve, Thomsen, Jorgensen, Kournakoff, Verner, and
others. The salts PtX^NHs, discovered by Gerhardt, also exhibited several different
properties in the two pairs of X's. In the remaining platino-ammonium salts all the
X's appear to react alike.

The quality of the X's, retainable in the platino-ammonium salts, maybe considerably
modified, and they may frequently be wholly or partially replaced by hydroxyl. For
example, the action of ammonia on the nitrate .of Gerhardt's base, Pt(NO5)4,2NH3, in a
boiling solutiont gradually produces a yellow crystalline precipitate which is nothing
else than a basic hydrate or alkali, Pt(OH)4,2NH3. It is sparingly soluble in water, but
gives directly soluble salts PtX4,2NH3 with acids. The stability of this hydroxide is
such that potash does not expel ammonia from it, even on boiling, and it does not change
below 130°. Similar properties are shown by the hydroxide Pt(OH)2,2NH3 and the
oxide PtO,2NH3 of Eeiset's second base. But the hydroxides of the compounds con-
taining 4NH3 are particularly remarkable. The presence of ammonia renders them
soluble and energetic. The brevity of this work does not permit us, however, to
mention many interesting particulars in connection with this subject.

14 Hydroxides are known corresponding with Gros's salts, which contain one hydroxyl
group in the place of that chlorine or haloid which in Gros's salts reacts with difficulty,

THE PLATINUM METALS 895

parative stability (for instance, as compared with AgCl and NH3) of
such compounds, and the existence of many other compounds analogous

and these hydroxides do not at once show the properties of alkalis, just as the chlorine
which stands in the same place does not react distinctly ; but still, after the prolonged
action of acids, this hydroxyl group is also replaced by acids. Thus, for example, the action
of nitric acid on Pt(N03)2Cl2,4NH3 causes the non-active chlorine to react, but in the
product all the chlorine is not replaced by NO3, but only half, and the other half is replaced
by the hydroxyl group : Pt(NO3)2Cl2,4NH3 + HN03 + H2O = Pt(NO3)3(OH),4NH3 + 2HC1 ;
and this is particularly characteristic, because here the hydroxyl group has not reacted
with the acid — an evident sign1 of the non-alkaline character of this residue. I think it
may be well to call attention to the fact that the composition of the. ammonio-metalla-
salts very often exhibits a correspondence between the amount of X's and the amount
of NH3) of such a nature that we find they contain either XNH3 or the grouping
X2NH3; for example, Pt(XNH3)2 and Pt(X2NH3)2, Co(X2NH3)3, Pt(XNH3)4, &o.
Judging from this, the view of the constitution of the double cyanides of platinum
given in Note 11 finds some confirmation here, but, in my opinion, all questions
respecting the composition (and structure) of the ammoniacal, double, complex, and
crystallisation compounds stand connected with the solution of questions respecting the
formation of compounds of various degrees of stability, among which a theory of
solutions must be included, and therefore I think that the time has not yet come for a
complete generalisation of the data which exist for these compounds ; and here I again
refer the reader to Prof. Kournakoff's work cited in Chapter XXII., Note 85. However,
we may add a few individual remarks concerning the platinia compounds.

To.the common properties of the platino-ammonium salts, we must add not only their
stability (feeble acids and alkalis do not decompose them, the ammonia is not evolved
by heating, &c.), but also the fact that the ordinary reactions of platinum are concealed
in them to as great ah extent as those of iron in the ferricyanides. Thus neither alkalis
nor hydrogen sulphide will separate the platinum "from them. For example, sulphuretted
hydrogen in acting on Gros's salts gives sulphur, removes half the chlorine by' means of
its hydrogen, and forms salts of Reiset's first base. This may be understood or explained
by considering the platinum in the molecule as covered, walled up by the ammonia, or
situated in the centre of the molecule, and therefore inaccessible to reagents. On this
assumption, however, we should expect to find clearly-expressed ammoniacal properties,
and this is not the case. Thus ammonia is easily decomposed by chlorine, whilst in
acting on the platino-ammonium salts containing PtX2 and 2NH3 or 4NH3, chlorine
combines and does not destroy the ammonia ; it converts Reiset's salts into those of.
Gros and Gerhardt. Thus from PtX2,2NH3 there is formed PtX2Cl2,2NH3, and from
PtX2,4NH3 the salt of Gros's base PtX2Cl2,4NH3. This shows that the amount .of
chlorine which combines is not dependent on the amount of ammonia present, but is due
to the basic properties of platinum. Owing to this some chemists suppose the ammonia
to be inactive or passive in certain compounds. It appears to me that these relations,
these modifications, in the usual properties of ammonia and platinum are explained
directly by their mutual combination. Sulphur, in sulphurous anhydride, SO2, and
hydrogen sulphide, SH2, is naturally one and the same, but if we, only knew of it in the
form of hydrogen sulphide, then, having obtained it in the form of sulphurous anhydride,
we should consider its properties as hidden. The oxygen in magnesia, MgO, and in
•nitric peroxide, NO2, is so different that there is no resemblance. Arsenic no longer
reacts in its compounds with hydrogen as it reacts in its compounds with chlorine, and
in their compounds with nitrogen all metals modify both their reactions and their physical
properties. We are accustomed to judge the metals by their saline compounds with
haloid groups, and ammonia by its compounds with acid substances, and here, in the
platino-compounds, if we assume the platinum to be bound to the entire mass of the
ammonia — to its hydrogen and nitrogen — we shall understand that both the platinum
and ammonia modify their characters. Far more complicated is the question why a por-

896 PRINCIPLES OF CHEMISTRY

to them, endows them with a particular chemical interest. Thus
Kournakoff (1889) obtained a series of corresponding compounds contain-

tion of the chlorine (and other haloid simple and complex groups) in Gros's salts acts iu
a different manner from the other portion, and why only half of it acts in the usual way.
But this also is not an exclusive .case. The chlorine in potassium chlorate or in carbon
tetrachloride does not react with the same ease with metals as the chlorine in the salts
corresponding with hydrochloric acid. In this case it is united to oxygen and carbon,
•whilst in the platino-ammonium compounds it is united partly to platinum and partly to
the platino-ammonium group. Many chemists, moreover, suppose that a part of the
chlorine is united directly to the platinum and the other part to the nitrogen of the
ammonia, and thus explain the difference of the reactions ; but chlorine united to
platinum reacts as well with a silver salt as the chlorine of ammonium chloride, NH4C1,
or nitrosyl chloride, NOC1, although there is no doubt that in this case there is a
union between the chlorine and nitrogen. Hence it is necessary to explain the absence
of a facile reactive capacity in a portion of the chlorine by the conjoint influence
of the platinum and ammonia on it, whilst the other portion may be admitted as
being under the influence of the platinum only, and therefore as reacting as in other
salts. By admitting a certain kind of stable union in the platino-ammonium grouping,
it is possible to imagine that the chlorine does not react with its customary facility,
because access to a portion of the atoms of chlorine in this complex grouping is difficult,
and the chlorine union is not the same as we usually meet in the saline compounds of
chlorine. These are the grounds on which we, in refuting the now accepted explanations
of the reactions and formation of the platino-compounds, pronounce the following opinion
as to their structure.

In characterising the platino-ainmonmm compounds, it is necessary to bear in mind
that compounds which already contain PtX4 do not combine directly with NH3, and that
such compounds as PtX4,4NH3 only proceed from PtX2, and therefore it is natural to
conclude that those affinities and forces which cause PtX2 to combine with X2 also cause
it to combine with 2NH3. And having the compound PtX2,2NH3, and supposing that in
subsequently combining with C12 it reacts with those affinities which produce the com-
pounds of platinic chloride, PtCl4, with water, potassium chloride, potassium cyanide,
hydrochloric acid, and the like, we explain not only the fact of combination, but also
many of the reactions occurring in the transition of one kind of platino-ammonium salts
into another. Thus by this means we explain the fact that (1) PtX2)2NH3 combines
with 2NH3, forming salts of Beiset's first base ; (2) and the fact that this compound
•(represented as follows for 'distinctness), PtX2,2NH3,2NH3, when heated, or even when
boiled in solution, again passes into PtX2)2NH3 (which resembles the easy disengage-
ment of water of crystallisation, &c.) ; (3) the fact that PtX2,2NH3 is capable of absorbing,
under the action of the same forces, a molecule of chlorine, PtX2,2NH3,Cl2, which it
then retains with energy, because it is attracted^ not only by the platinum, but also by the
hydrogen of the ammonia; (4) the fact that this chlorine held in this compound (of
Gerhardt) will have a position unusual in salts, which will explain a certain (although
very feebly-marked) difficulty of reaction; (5) the fact that this does not exhaust the
faculty of platinum for further combination (we need only recall the compound
PtCl4,2HCl,16H2O), and that therefore both PtX2,2NH3,Cl.2 and PtX2,2NH3,2NH3 are still
capable of combination, whence the latter, with chlorine, gives PtX2,2NH3)2NH3,Cl2,
after the type of PtX4Y4 (and perhaps higher) ; (6) the fact that Gros's compounds
thus formed are readily re-converted into the salts of Beiset's first base when acted on
by reducing agents ; (7) the fact that in Gros's salts, PtX2)2NH5(NH3X);2, the newly-
attached chlorine or haloid will react with difficulty with salts of silver, &c., because it is
attached both to the platinum and to the ammonia, for both of which it has an attraction ;
(8) the fact that the faculty for further combination is not even yet exhausted in the
type of Gros's salts, and that we actually have a compound of Gros's chlorine salt with
platinous chloride and with platinic chloride; the salt PtS04,2NH3,2NH3,SO4 com-
bines further also with H30 ; (9) the fact that such a faculty of combination with new

THE PLATINUM METALS S97

'ing thiocarbamide, CSN2H4, in the place of ammonia, PtCl2,4CSN2H4,
and others corresponding with .Reisetfs salts. Hydroxylamine, and
other substances corresponding with ammonia, also give similar com-
pounds. The common properties and composition of such compounds
show their entire analogy to the cobaltia compounds (especially for
ruthenium and iridium) and correspond to the fact that both, the
platinum- metals, and cobalt occur in the same, eighth, group.

molecules is naturally more developed in the Idwer forms of combination than in the
higher. Hence the salts of Reiset's first base— for example, PtCl2,2NH3,2NH3— both
combine with water and give precipitates (soluble in water but not in hydrochloric acid)
of double salts with many salts of the heavy metals— for example, with lead chloride,
cupric chloride, and also with platinic and platinous chlorides* (Buckton's salts). The
latter compounds^ will have the composition PtCl2?2NH3,2NH3,PtCl2 — that is, the same
composition as the • salts of Reiset's second base, but it cannot be identical with it.
Such an interesting case does actually exist. The first salt, PtCl2,4NH3)PtCl2, is green,
insoluble in water and in hydrochloric acid, and is known as Magnus's salt, and the
second, PtCl2,2NH3, is Reiset's yellow, sparingly soluble (in water). They are polymeric,
namely, the first contains twice the number of elements held in the second, and at the
same time they easily pass into each other. If ammonia be added to a hot hydrochloric
acid solution of platinous chloride, it forms the salt PtCl2,4NH3, but in the presence of
an excess of platinous chloride it gives Magnus's salt. On boiling the latter in ammonia it
gives a colourless soluble salt of Reiset's first base, PtCl2',4NH3, and if this be boiled with
water, ammonia is disengaged, and a salt of Reiset's second tase, PtCl2)2NH3, is obtained.
A class of platino-ammonium isomerides (obtained by Millon and Thomsen) are also
known. Buckton's salts — for example, the copper salt — were obtained by them from the
salts of Reiset's first base, PtCl2,4NH3, by treatment with $ solution of cupric chloride,
«fec., and therefore, according to our method of expression, Buckton's copper salt will be
PtCl2,4NH3,CuCl2. This salt is soluble in water, but not in hydrochloric acid. In io
the ammonia must be considered as united to the platinum. But if cupric chloride be
dissolved in ammonia, and a solution of platinous chloride in ammonium chloride is
added to it, a violet precipitate is obtained, of the same composition as Buckton's
3alt, which, however, is insoluble in water, but soluble in hydrochloric acid. In this a
portion, if not all, of the ammonia must be regarded asN united to the copper, and it must
therefore be represented as CuCl2,4NH3,PtCl2. This form is identical in composition
but different in properties (is isomeric) with the preceding salt (Buckton's). The salt of
Magnus is intermediate between them, PtCl2,4NH3,PtCl.j ; it is insoluble in water. and
hydrochloric acid. These and certain other instances of isomeric compounds in the
series of the platino-ammQnium salts throw a light on the nature of the compounds in
question, just as the study of the isomerides of the carbon compounds has served and'
still serves as the chief cause of the rapid progress of organic chemistry. In conclusion,
we may add that (according to the law of substitution) we must necessarily expect all
kinds of intermediate compounds between the platino and analogous ammonia deriva-
tives on the one hand, and the complex compounds of nitrous acid on the other.
Perhaps the instance of the reaction of ammonia upon osmic anhydride, Os04, observed
by Fritsche, Fr&tny, and others, and more fully- studied by Joly (1891), belongs to this
class. The latter showed that when ammonia acts upon an alkaline solution of OsO4
the reaction proceeds according to the equation : OsO4 + KHO + NH3 = OsNK03 + 2H2O.
It might be imagined that in this case the ammonia is oxidised, probably forming the
residue of nitrous acid (NO), while the type OsO4 is deoxidised into OsO2, and a salt,
OsO(NO)(KO), of the type 03X4 is formed. This salt crystallises well in light yellow
octahedra. It corresponds to osmiamic acid, OsO(ON)(HO), whose anhydride,
[OsO(NO)]o, has the composition Os2N2O5, which equals 2Os + N2O5 to the same extent
as the above-mentioned compound PtC02 equals Pt + CO? (see Note 11).

398 PRINCIPLES OF CHEMISTRY

CHAPTER XXIV

COPPER, SILVER, AND GOLD

THAT degree of analogy and difference which exists between iron,
cobalt, and nickel repeats itself in the corresponding triad ruthenium,
rhodium, and palladium, and also in the heavy platinum metals,
osmium, iridium, and platinum. These nine metals form Group VIII.
pf the elements in the periodic system, being ther intermediate group
between the even elements of the large periods and the uneven, among
which we know zinc, cadmium, and mercury in Group II. Copper,
silver, and gold complete l this transition, because their properties
place them in proximity to nickel, palladium, and platinum on the one
hand, and to zinc, cadmium, and mercury on the other. Just as Zn,
Cd, and Hg ; Fe, Ru, and Os ; Co, Rh, and Ir ; Ni, Pd, and Pt,
resemble each other in many respects, so also do Cu, Ag, and Au.
Thus, for example, the "atomic weight of copper Cu = 63, and in all its
properties it stands between Ni = 59 and Zn = 65. But as the tran-
sition from Group VIII. to Group II., where zinc is situated, cannot be
otherwise than through Group I., so in copper there are certain pro-
perties of the elements of Group I. Thus it gives a suboxide, Cu20,
and salts, CuX, like the elements' of Group I., although at the same
time it forms an oxide, CuO, and salts CuX2, like nickel and zinc. In
the state of the oxide, CuO, and the salts, CuX2, copper is analogous to
zinc, judging from the insolubility of the carbonates, phosphates, and
similar salts, and by the isomorphism, and other characters.2 In the
cuprous salts there is undoubtedly a great resemblance to the silver

1 The perfectly unique position held by copper, silver, and gold in the periodic system
of the elements, and the degree of affinity which is found between them, is all the more
remarkable, as nature and practice have long isolated these metals from all others by
having employed them— for example, for coinage— and determined their relative
importance and value in conformity with the order (silver between copper and gold) of
their atomic weights, &c.

2 Cupric sulphate contains 5 molecules of water, CuS04,5H2O, and the isomorphous
mixtures with ZnSO4,7H2O contain either 5 or 7 equivalents, according to whether copper
or zinc predominates (Vol. II. p. G). If there be a large proportion of copper, and if the
mixture contain 5HoO, the form of the isomorphous mixture (triclinic) will be isomorphous
with cupric sulphate, CuS04,5H2O, but if a large amount of zinc (or magnesium, iron,
nickel, or cobalt) be present the form (rhombic or monoch'nic)"will be nearly the same

COPPER, SILVER, AND GOLD 899

salts — thus, for example, silver chloride, AgCl, is characterised by its
insolubility and capacity of combining with ammonia, and in this respect
cuprous chloride closely resembles it, for it is also insoluble in water,
and combines with ammonia and dissolves in it, &c. Its composition is
also BC1, the same as AgCl, NaCl, KC1, <fec., and silver in many com-
pounds resembles, and is even isomorphous with, sodium, so that this
again justifies their being brought together. Silver chloride, cuprous
chloride, and sodium chloride crystallise in the regular system.
Besides which, the specific heats of copper and silver require that they
should have the atomic weights ascribed to them. To the oxides Cu2O
and Ag20 there are corresponding sulphides Ag2S and Cu2S. They
both occur in nature in crystals of the rhombic system, and, what is
most important, copper glance contains an isomorphous mixture of
them both, and retains the form of copper glance with various pro-
portions of copper and silver, and therefore has the composition B2S
•where R = Cu, Ag.

Notwithstanding the resemblance in the atomic composition of the
cuprous compounds, CuX, and silver compounds, AgX, with the com-
.pounds of the alkali metals KX, NaX, there is a considerable degree
of difference between these two series of elements. This difference is
clearly seen in the fact that the alkali metals belong to those elements
which combine with extreme facility with oxygen, decompose water,
and form the most alkaline bases ; whilst silver and copper are
oxidised with difficulty, form less energetic oxides, and do not decom-
pose water, even at a rather high temperature. Moreover, they only
displace hydrogen from very few acids. The difference between them
is also seen in the dissimilarity of the* properties of many of the
corresponding compounds. Thus cuprous oxide, Cu2O, and silver oxide,
Ag20, are insoluble in water : the cuprous and silver carbonates,
chlorides, and sulphates are also sparingly soluble in water. The
oxides of silver and copper are also easily reduced to metal. This
difference in properties is in intimate relation with that difference in
the density of the metals which exists in' this case. The alkali metals
belong to the lightest, and copper and silver to the heaviest, and there-
fore the distance between the molecules in these metals is very dis-
similar— it is greater for the former than the latter (tables in Chapter
XV.). From the point of view of the periodic law, this difference
between copper and silver and such elements of Group I. as potassium
and rubidium, is clearly seen from the fact that copper and silver

as that of zinc sulphate, ZnSO4,7H20. Supersaturated solutions of each of these salts
crystallise in that form and with that amount of water which is contained in a crystal
of one or other of the Baits brought in contact with the solution (Chapter XIV., Note 27).

400 PRINCIPLES OF CHEMISTRY.

stand in the middle of those large periods (for example, K, Ca, Sc, Tf,
V, Or, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br) which start with
the true metals of the alkalis — that is to say, the analogy and difference
"between potassium and copper are of the same nature as that between
chromium and selenium, or vanadium and arsenic.

Copper is one of the few metals which have long been known in a
metallic form. The Greeks and Romans imported copper chiefly from
the island of Cyprus — whence its Latin name, cuprum. It was known
to the ancients before iron, and was used, especially when alloyed with
other metals, for arms and domestic utensils. That copper was known
to the ancients will be understood from the fact that it occurs, although
rarely, in a native state, and is easily extracted from its other natural
compounds. Among the latter are the oxygen compounds of copper.
When ignited with charcoal, they easily give up theii oxygen to
it, and yield metallic copper j hydrogen also easily takes up the
oxygen from copper oxide when heated. Copper occurs in a native
fctate, sometimes -in association with other ores, in many parts of the
Urals angl in Sweden, and in considerable masses in America, espe-
cially in the neighbourhood of the great American lakes ; and also in,
Chili, Japan, and China, The oxygen compounds of copper are also of
somewhat common occurrence in certain localities ; in this respect
certain deposits of the Urals are especially famous. The geological
period of the Urals (Permian) is characterised by a considerable dis-
tribution of copper ores. Copper is met with in the form of cuprous
oxide, or suboxide of copper, Cu20, and is then known as red copper
ore, because it forms red masses which not unf requently are crystallised
in the Tegular system. It is found much more rarely in thevstate of
cupric oxide, CuO, and is then called black copper ore. The mosf
common of the oxygenised compounds of copper are the basic carbonates
corresponding with the oxides. That these compounds are undoubtedly
of aqueous origin, is apparent, not only from the fact that specimens
are frequently found of a gradual transition from the metallic, sul-
phuretted, and oxidised copper into its various carbonates, but also from
the presence of water in their composition, and from the laminar,
reniform. structure which many of them present. In this respect mala-
chite is particularly well known ; it is used as a green paint and also
for ornaments, owing to the diversity of the shades of colour presented
by tho different layers of deposited malachite. The composition of
malachite corresponds with "the basic carbonate containing one molecule
of cupric carbonate to one of hydroxide : • CuCO3,CuH202.' In this
'form- the copper frequently occurs in admixture with yarious sedi-
.mentary rocks, forming large strata, which confirms the abuebus origin

COPPEK, SILVER, AND GOLD 401

of these compounds. There are many such localities in the Perm and
other Governments bounding the Urals. Blue carbonate of copper, or
azurite, is also often met with in the same localities ; it contains the
same ingredients as malachite, but in a different proportion, its com-
position being CuH2O2,2CuCO3. Both these substances may be ob
tained artificially by the action of the alkali carbonates on solutions
of cupric salts at various temperatures. These native carbonates are
often used for the extraction of copper, all the more as they very
readily give metallic copper, evolving water and carbonic anhydride
when ignited, and leaving the easily-reducible cupric oxide. Copper
is, however, still more often met with in the form of the sulphides.
The sulphides of copper generally occur in chemical combination
with the sulphides of iron.3 These copper-sulphur compounds (copper
pyrites CuFeS2, variegated copper ore Cu3FeS3, &c.) generally occur in
veins in a rock gangue.

The extraction of copper from its oxide ores does not present any
difficulty, because the copper, when ignited with charcoal and melted;,
is reduced from the impurities which accompany it. This mode of
smelting copper ores is carried on in cupola- or cylindrical furnaces,
fluxes forming a slag being added to the mixture of ore and charcoal.

8 Iron pyrites, PeS2, very often contain a small quantity of copper sulphide (see
Chapter XXII., Note 2 bis), and on burning the iron pyrites for sulphurous anhydride the
copper oxide remains in the residue, from which the copper is often extracted with profit,
For this purpose the whole of the sulphur is not burnt off from the iron pyrites, but a
portion is left behind in the ore, which is then slowly ignited (roasted) with access of air.
Cupric sulphate is then formed, and is extracted by water ; or what is better and more
frequently done, the residue from the roasting of the pyrites is roasted with common
salt, and the solution of cuprio chloride obtained by lixiviating is precipitated with iron,
A far greater amount of copper ia obtained from other sulphuretted ores. Among these
copper glance, Cu2S, is more rarely met with. It has a metallic lustre, is grey, generally
crystalline, and is obtained in admixture with organic matter ; so that there is no doubt
that its origin is due to the reducing action of the latter on solutions of cupric sulphate.
Variegated copper ore, which crystallises in octahedra, not infrequently forms art
admixture in copper glance ; it has a metallic lustre, and is reddish-brown ; it has a
superficial play of colours, due to oxidation proceeding on its surface. Its composition is
CusFeSj. But the most common and widely-distributed copper ore is copper pyrites^
which crystallises in regular octahedra ; it has a metallic lustre, a sp. gr. of 4'0, and
yellow colour. Its composition is CuFeS2. It must be remarked that the sulphurous
ores of copper are oxidised in the presence, of water containing oxygen in solution,
and form cupric sulphate, blue vitriol, which is easily soluble in water. If this water
contains calcium carbonate, gypsum and cupric carbonate are formed by double
decomposition: CuSC^ + CaCOs = CuCO5 + CaS04. Hence copper sulphide in the form
of different ores must be considered as the primary product, and the many other copper
ores as secondary products, formed by water. This is confirmed by the fact that at the
present time the water extracted from many copper mines contains cupric sulphate in
eolution. From this liquid it is easy to extract cupric oxide by* the action of organic
matter and various impurities of water. Hence metallic copper is sometimes found in
natural products of the modification of copper sulphide and is probably deposited by
the action of organic matter present in the water.

402 PRINCIPLES OF CHEMISTRY

The smelted copper still contains sulphur, iron, and other metallic
impurities, from which it is freed by fusion in reverberatory furnaces,
with access of air to the surface of the molten metal, as the iron and
sulphur are more easily oxidised than the copper. The iron then
separates as oxides, which collect in the slag.4

4 "Copper ores rich in oxygen are yeryrare"; the sulphur ores are of more common
occurrence, but the extraction of tho copper from' them is much more difficult. The
problem here n'ot only consists in the removal of the sulphur, but also in the removal of
the iron combined with the sulphur and copper. This is* attained by a -whole series of
operations, after which "there still sometimes remains the extraction of the metallic silver
which generally accompanies the copper, although in but small quantity. These
Jprocesses commence with the roasting1— i.e. calcination — of the.ore with access of air, by
which means the sulphur is converted into sulphurous anhydride. It should here bej
Remarked that iron sulphide is more easily oxidised than copper sulphide, and therefore
the greater part of the iron in the residue from roasting is no longer 'in the form of
Bulphide but of oxide of iron. The roasted ore is mixed with charcoal, and siliceous fluxes,.
and smelted in a cupola furnace. The iron then passes into the slag, because its oxide-
gives an easily-fusible mass wifh the silica, whilst the copper, KL the form of sulphide,,
fuses and collects under the slag. Tho greater .part of -the iron is removed from, the'
mass by this smelting. The resultant coarse metal is again roasted in order to
remove the greater part of the" sulphur from the copper sulphide, and to convert the
toetal into oxide, after which the mass is again smelted. These processes are repeated
several times, according to the richness of the ore. During these smeltings a portion of
the copper is already obtained in a metallic form, because copper sulphide gives
metallic copper with the oxide (CuS + 2CuO = 8Cu + SO2). We will not here describe
the furnaces used. or the details of this process, but the above remarks include the ex-
planation of those chemical processes which are accomplished in- the various tech-
nical operations which are made use of in the process (for details see works on.
metallurgy).

Besides the smelting of copper there also exist methods for its extraction from
solutions in the wet way, as it is called. Recourse is generally had to these methods for
poot copper ores. The copper is brought into solution, from which it is separated by
means of metallic iron or by other methods (by the action'of an electric current). The
sulphides are roasted in such a manner that the greater part of the copper is oxidised
into cupric sulphate, whilst at the same time the corresponding iron salts are as-far as
possible decomposed. This process is based on the fact that the copper sulphides absorb
oxygen when they are calcined .in ..the presence of air, forming cupric sulphate. The
roasted ore is treated with water, to which acid is sometimes added, and after lixivia-
tion the resultant solution containing copper is treated either with metallic iron or with
milk of lime, which precipitates cupric hydroxide from the solution. Copper oxide
ores poor in metal may be treated with dilute acids in order to obtain the copper
oxides in solution, from which the copper is then easily precipitated either by iron or
as hydroxide by lime. According to Hunt and Douglas's method, the copper in the ore
is converted by calcination into the cupric oxide, which is brought into solution by
the action of a mixture of solutions -.of ferrous sulphate and sodium chloride ;
the oxide converts the ferrous chloride into ferric oxide, forming copper chlorides,
according to the equation 3CuO + 2FeCl2 = CuCl2 + 2CuCl + Fe2O3. The cupric chloride
is soluble in water, whilst the cuprous chloride is dissolved in the solution of sodium
chloride, and therefore all the copper passes into solution, from which it is precipitated
ty iron.

The same American metallurgists give the fojlowing wet method foi? extracting the
Ag and Au occurring in many copper ores, especially in sulphurous ores : (1) The Cu2S is
first converted into oxide by roasting in a calciner ; (2) the CuO is extracted by the
dilute sulphuric acid obtained in the fourth process, the Cu then passes into solution,

COPPER, SILVER, AttD GOLJD 403

'Copper is characterised by its^red colour, which distinguishes it
from all other metals. Pure copper is soft, and may be beaten out by
a hammer at the ordin'ary temperature, and when hot may be rolled
into very thin sheets. Extremely thin leaves of copper transmit a
green light. The tenacity of copper is also considerable, and next' to
iron it is one of the most durable metals in this respect. Copper wire
of 1 sq. millimetre in section only breaks under a weight of 45 kilograms.
The specific gravity of copper is 8-8, unless it contains cavities due to the
fact that molten copper absorbs oxygen from the air, which is disen-
gaged on cooling,, and therefore gives a porous mass whose density is
much less. Rolled copper, and also that which is deposited by the electric
current, has a comparatively high density. Copper melts at a bright
red heat, about 1050°, although below the temperature at which many
kinds of cast iron, melt. At a high temperature it is converted into
vapour, which communicates a green colour to the flame. Both native
copper and that cooled from a molten state crystallise in regular
octahedra. Copper is not oxidised in dry air at the ordinary tempera-
ture, but when calcined it becomes coated with a layer of oxide, and it
does not burn even at the highest temperature. Copper, when calcined
in air, forms either the red cuprous oxide or the black cupric oxide,

while the Ag, Au and oxides of iron remain behind in the residue (from which the noble
metals may be extracted) ; (3) a portion of the copper in solution is converted into CuCLj
(and.CaSO4 precipitated) by means of the CaCl2 obtained in the fifth process ; (4) the
mixture of solutions of CuS04 and CuCl2 is converted into the insoluble CuCl (salt of the
suboxide) by the action of the SOj obtained by roasting the ore (in the first operation),
sulphuric acid is then formed in the solution, according to the equation : CuSO^H- CuClj
? + SO.j + 2H2O = 2H2SO4 + 2CuCl; (5) the precipitated CuCl is treated with lime and
•water, and gives CuCl2 in solution and CuO in the residue ; and lastly (6) the Cu2O is
reduced to metallic Cu by carbon in a furnace. According to Crboke's, method the impure
copper regulus obtained by roasting and smelting the ore. is broken up and immersed
.repeatedly in molten lead, which extracts the Ag and Au occurring in the regulus. The
regulus is then heated in a reverberatory furnace to run off the- lead, and is then smelted
for Cu.

The copper brought into the market often contains small quantities of varioijs impuri-
ties. Among these there are generally present iron, lead, silver, arsenic, and sometimes
small quantities of oxides of copper. As copper, when mixed with ,a small amount of
foreign substances, loses its tenacity to a certain degree, the manufacture of very thin
sheet copper requires the use of Chili copper, which is distinguished for its great softness,
and therefore when it is desired to have pure copper, it is best to take thin sheet copper,
like that which is used in the manufacture of cartridges. But the purest copper is electro-
lytic copper— that is, that which is deposited from a solution by the action of an
electric current.

If the copper contains silver, as is often the case, it is used in gold refineries for the
precipitation of silver from its solutions in sulphuric acid. Iron and zinc reduce copper
salts, but copper reduces mercury and silver salts. " The precipitate contains not only the
silver which was previously in solution, but also all that which was in the9 copper.. The
silver solutions in sulphuric acid are obtained in the separation of silver from gofcl by
treating their alloys with sulphuric acid, which only dissolves the silver.

404 PRINCIPLES OF CHEMISTRY

according to the temperature and quantity of air supplied. Tn an
at the ordinary temperature, copper — as everyone knows — becomes
coated with a brown layer of oxides or a green coating of basic salts,
due to the action of the damp air containing carbonic acid. If this
action continue for a prolonged time, the copper is covered with a thick
coating of basic carbonate, or the so-called verdigris (the aerugo nobilis
of ancient statues). This is due to the fact that copper, although
scarcely capable of oxidising by itself,5 in the presence of water and
acids — even very feeble acids, like carbonic acid — absorbs oxygenfrom
the air and forms salts, which is a very characteristic property of it (and
of lead).6 Copper does not decompose water t and therefore does not disen-

* Schiitzenberger showed that when the basic carbdnate of copper is decomposed by
an electric current it gives, besides the ordinary copper, an allotropic form which grows
on the negative platinum electrode, if its surface be smaller than that of the positive
copper electrode, in the form of brittle crystalline growths of sp. gr. 8'1. It differs from
ordinary copper by giving not nitric, oxide but nitrous oxide when treated with nitric
acid, and in being very easily oxidised in air, and coated with red shades of colour. It
is possible that this is copper hydride, or copper which has occluded hydrogen. Spring
(1892) observed that copper reduced from the oxide by hydrogen at the lowest possible
temperature was pulverulent, while that reduced from CuClj at a somewhat high tem-
perature appeared in bright crystals. The same difference occurs with many other
metals, and is probably partly due to the volatility of the metallic chlorides.

6 This is taken advantage of in practice ; for instance, by pouring dilute acids ovei
copper turnings on revolving tables in the preparation of copper salts, such as verdigris,
or the basic acetate 2C4H6CuO4,C uH2O2,5H2O, which is so much used as an oil paint (i.e.
with boiled oil). The capacity of copper for absorbing oxygen in the presence of acids
is so great that it is possible by this means (by taking, for example, thin copper shavings
moistened with sulphuric acid) to take up all the oxygen from a given volume of air, and
this is even employed for the analysis of air.

The combination of copper with oxygen is not only aided by acids but also by alkalis,
although cupric oxide does not appear to have an acid character. Alkalis do hot act jon
copper except in the presence of air, wheja they produce cupric oxide, which does not
appear to combine with such alkalis as caustic potash or soda. But the action of
ammonia is particularly distinct (Chapter V., Note 2). In the action of a solution of
ammonia not only is oxygen absorbed by the copper, but it also acts on the ammonia,
and a definite quantity of ammonia is always acted on simultaneously with the passage
of the copper into solution. The ammonia is then converted into nitrous acid, according
to the reaction : NH3 + O3 = NH02 + H-jO, and the nitrous acid thus formed passes into the
state of ammonium nitrite, NH4NO^. In this manner three equivalents of oxygen are
expended on the oxidation of the ammonia, and six equivalents of oxygen pass over to
the copper, forming six atoms of cupric oxide. The latter does not remain in the state
of oxide, but combines with the ammonia.

A strong solution of common salt does not act on copper, but a dilute solution oi tUe
salt corrodes copper, converting it into oxychloride— that is, in the presence of air.
This action of salt water is evident in those cases where the bottoms of ships are coated
.with sheet copper. From what has been said above it will be evident that copper vessels
should 'not be employed in the preparation of food, because this contains salts and acids
which act on copper in the presence of air, and give copper salts, which are poisonous,
and therefore the food prepared in untinned copper vessels may be poisonous. Hence
tinned vessels are employed for this purpose — that is, copper vessels coated with a thin
layer of tin, on which acid and saline solutions do not act.

COPPER, SILVER, AND GOLD 405

gage hydrogen from it either at the ordinary or at high temperatures.
Nor does copper liberate hydrogen from the oxygen acids , these act on
it in two ways : they either give up a portion of their oxygen, form-
ing lower grades of oxidation, or else only react in the presence of
air. Thus, when nitric acid acts on copper it evolves nitric oxide, the
copper being oxidised at the expense of the nitric acid. In the same
way copper converts sulphuric acid into the lower grade of oxidation —
into sulphurous anhydride, S02. In these cases the copper is oxidised
to copper oxide, which combines with the excess of acid taken, and
therefore forms a cupric salt, CuX2. Dilute nitric acid does not act
on copper at the ordinary temperature, but when heated it reacts
with great ease ; dilute sulphuric acid does not act on copper except
in presence of air.

Both the oxides of copper, Cu20 and CuO, are unacted on by-
air, and, as already mentioned, they both occur in nature.6 bis How-
ever, in the majority of cases copper is obtained in the form oi!
"cupric oxide and its salts— and the copper compounds used indus-
trially generally belong to this type. This is due to the fact that the
cuprous compounds absorb oxygen from the air and pass into cupric
compounds. The cupric compounds may serve as the source for the
preparation of cuprous oxide, because many reducing agents are
capable of deoxidising the oxide into the suboxide. Organic sub-
stances are most generally employed for this purpose, and especially
saccharine substances, which are able, in the presence of alkalis, to
undergo oxidation at the expense of the oxygen of the cupric oxide,
and to give acids which combine with the alkali : 2CuO — O = Cu2O,
In this case the deoxidation of the copper may be carried further and
metallic copper obtained, if only the reaction be aided by heat. Thus,
for example, a tine powder of metallic copper may be obtained by heat-
ing an ammoniacal solution of cupric oxide with caustic potash
and grape sugar. But if the reducing action of the saccharine
substance proceed in the presence of a sufficient quantity of alkali in

6 bu Copper, besides the cuprous oxide, Cu2O, and cupric oxide, CuO, gives two khown
higher forms of oxidation, but they have scarcely been investigated, and even their
composition is not well known. Copper dioxide (CuO2, or CuO2,H.2O, perhaps CuOH2O2)
is obtained by the action of hydrogen peroxide on cupric hydroxide, when the green
colour of the latter is changed to yellow. It is very unstable, and is decomposed even
by boiling water, with the evolution of oxygen, whilst the action of acids gives cuprio
Baits, oxygen being also disengaged. A still higher copper peroxide is formed by heating
a mixture of caustic potash, nitre, and metallic copper to a red heat, and by dissolving
cupric hydroxide in solutions of the hypochlorites of the alkali metals. A slight heating
of the soluble salt formed is enough for it to be decomposed into oxygen and copper
dioxide, which is precipitated. Judging from Fremy's researches, the composition of the
copper-potassic compound should be KaCuO4. Perhaps this ia a compound of the
peroxides of potassium, K,O2, and of copper, Cu03.

406 PBINCtfLES OF CHEMISTRY

solution, and afc not too high a (temperature, cuprous oxide is ob-
tained. To see this reaction clearly, it is not sufficient to take any
cripric salt, because the alkali necessary for the reaction might pre-
cipitate cupric oxide — it is necessary to add previously some substance
•which will prevent this precipitation. Among such substances,
tartaric acid, C4H606, is one of the best. In the presence of a suffi-
cient quantity of tartaric acid, any amount of alkali may be added to a
solution of cupric salt without producing a precipitate, because a soluble
double salt of cupric oxide and alkali is then formed. If glucose (for
instance, honey or molasses) be added to such an alkaline tartariq
solution, and the temperature be slightly raised, it" first gives a yellow
precipitate (this is cuprous hydroxide, CuHO), and then, on boiling,
a red precipitate of (anhydrous) cuprous oxide. If such a mixture
be left for a long time at the ordinary temperature, it deposits well-
formed crystals of anhydrous cuprous oxide belonging to the regular
system.7

7 Colourless solutions of cuprous salts may also be obtained by the action of sul-
phurous or phosphorous acid and similar lower grades of oxidation on the blue solutions of
the cupric salts. This is very clearly and easily effected by means of sodium thio-
solphate, N^SgOj, which is oxidised in the process. Cuprous oxide can not only be
obtained by the deoxidation of cupric oxide, but also directly from metallic copper itself,
because the latter, in oxidising at a red heat in air, first givee cuprous oxide. It is pre-
pared in this manner on a large scale by heating sheet copper rolled into spirals in
reverberatory furnaces. Care must be taken that the air is not in great excess, and that
the coating of red cuprous oxide formed does not begin to pass into the black cupric oxide.
If the oxidised spiral sheet is then unbent, the brittle cuprous oxide falls away from
the soft metal. The suboxide obtained in this manner fuses, with ease. It is necessary
to prevent the access of air during thev fusion, and if the mass contains cupric oxide it
must be mixed with charcoal, which reduces the latter. Cuprous chloride, CuCl, corre-
sponding with cuprous oxide (as sodium chloride corresponds with sodium oxide), when
calcined with sodium carbonate, gives sodium chloride and cuprous oxide, carbonio
anhydride being evolved, because it doe's not combine with the cuprous oxide under these
conditions. The reaction can b 9 expressed, by the following equation: 2CuCl + Na2CO3
= Cu20'+ SNaCl + CO2. The cupric oxide itself, when calcined with finely-divided copper
this copper powder may be obtained by many methods — for instance, by immersing zinc
in a solution of a copper salt, or by igniting cupric oxide in hydrogen), give3 the fusible
cuprous oxide: Cu + CuO = Cu3O. Both the native and artificial cuprous oxide haye-a
sp. gr. of 5'6. It is insoluble in water,' and is not acted on by (dry) air. When heated
with acids the suboxide forms a solution of a cupric salt and metallic copper— for example,
Cu20 + H2SO4 = Cu + CuSO4 + H2O. However, strong hydrochloric acid does not separate
metallic copper on dissolving cuprous oxide, which is due to the fact that the cuprous
cnloride formed is soluble in strong -hydrochloric acid. Cuprous oxide also, dissolves in,
a solution of ammonia, and in the absence of air gives a colourless solution, which turni
blue in the airr absorbing oxygen, owing to the conversion of the cuprous oxide into
cupric oxide. The blue..solution thus formed may be again reconverted into a colourless
cuprous solution by immersing a copper strip in it, because the metallic copper then
deoxidises the cupric oxide in the solution into cuprous oxide. • Cuprous oxide ,is charac-
terised by the fact that it gives red glasses when fused with glass or with salts forming
vitreous alloys. Glass tinted with cuprous oxide is used for ornaments. The access of
•air must be avoided during its preparation, because the colour then becomes green, owing

COPPEB, SILVEB, AND GOLD 407

Cupric chloride, CuCl2, when ignited, gives cuprous chloride, CuCl
—i.e. the. salt corresponding with sub.oxide of copper — and therefore
cuprous chloride is always formed when copper enters into reaction
with chlorine at a high temperature. Thus, for example, when copper
is calcined with mercuric chloride, jt forms cuprous chloride arid vapours
of mercury. The same substance is obtained on heating metallic
Copper in hydrochloric acid, hydrogen being dis.engaged ; but this reac-
tion only proceeds with finely-divided copper, as hydrochloric acid acts
very feebly on compact masses of copper, and, in the presence of air,
gives cupric chloride. The green solution of cupric chloride is decolo-
rised by metallic copper, cuprous chloride being formed ; but this
reaction is only accomplished with ease when the solution is very con-
centrated and in the presence of an excess of hydrochloric acid to
dissolve the cuprous chloride. The' addition of water to the solu-
tion precipitates the cuprous chloride, because it is less soluble ia
dilute than in strong hydrochloric acid. Many reducing agents which
'are able to take up half the oxygen from cupric oxide are able, in the
presence of hydrochloric acid, to form cuprous chloride. Stannous
salts, sulphurous anhydride, alkali sulphites, phosphorous- and hypo-
phosphorous acids, and many similar reducing agents, act in this
manner. The usual method of preparing cuprous chloride consists in
passing sulphurous anhydride into a very strong solution of cupric
chloride : 2CuCl2 + S02 + 2H20 = 2CuCl + 2HC1 + H^SO^ Cuprous
chloride forms colourless cubic crystals which are insoluble in water.
It is easily fusible, and even -volatile. Under the action of oxidising
agents, it passes into the cupric salt, and it absorbs oxygen from moist
air, forming cupric oxychloride, Cu-jC^O. Aqueous ammonia easily
(dissolves, cuprous chloride as well as cuprous oxide ; the solution also
turns blue on exposure to the air. Thus an ammoniacal solution of
cuprous chloride serves as an excellent absorbent for oxygen ; but this
solution absorbs not only oxygen, but also certain other gases — for
example, carbonic oxide and acetylene.8

to the formation of cupric oxide, which colours glass blue. This may even be taken
fwlvantage of in testing for copper under the blow-pipe by heating the copper compound
with borax in the flame of a blow-pipe ; a red glass is obtained in the reducing flame,
and a blue glass in the oxidising flame, owing to the conversion of the cuprous into cuprio
oxide.

Etard (1882), by passing sulphurous anhydride into a .solution of cupric acetate, ob«
tained a white precipitate of cuprous sulphite, Cu2SO3,H2O, whilst he obtained the same
salt, of a red colour, from the double salt of sodium and copper ; but there are not any
convincing proofs of isomerism in this case.

8 The solubility of cuprous chloride in ammonia is due to the formation of compounds
between the ammonia and the chloride. In a warm solution the compound NH3^CuCl.
is formed, and at the ordinary- temperature CuCl,NH3. This salt is soluble in hydro,
chloric acid, and then forms a corresponding double salt of cuprous chloride and ammo*

408 PRINCIPLES OF CHEMISTRY

When copper is oxidised with a considerable quantity of oxygen at
a high temperature, or at the ordinary temperature in the presence of
acids, and also when it decomposes acids, converting them into lower
grades of oxidation (for example, when submitted to the action of
nitric and sulphuric acids), it forms cupric oxide, CuO, or, in the
presence of acids, cupric salts. Copper rust, or that black mass which
forms on the surface of copper when it is calcined, consists of cupric
oxide. The coating of the oxidised copper is very easily separated
from the metallic copper, because it is brittle and very easily peels off,
when it is struck or immersed in water. Many copper salts (for

nium chloride. By the action of a certain excess of ammonia on a hydrochloric acid
solution of cuprous chloride, very well formed crystals, having the composition
CuCl,NH3,H3O, are obtained. Cuprous chloride is not only soluble in ammonia and
hydrochloric acid, but it also dissolves in solutions of certain other salts — for example,
in sodium chloride, potassium chloride, sodium thiosulphate, and certain others. All
the solutions of cuprous chloride act in many cases as very powerful deoxidising
substances; for example, it is easy, by means of these solutions, to precipitate
gold from its solutions in a metallic form, according to the equation AuCls + SCuCl
= Au + 8CuCl2.

Among the other compounds corresponding with cuprous oxide, cuprous iodide, Cul,
is worthy of remark. It is a colourless substance which is insoluble in water and
sparingly soluble in ammonia (like silver iodide), but capable of absorbing it, and in this
respect it resembles cuprous chloride. It is remarkable from the fact that it is exceed-
ingly easily formed from the corresponding cupric compound Culs- A solution of cuprio
iodide easily decomposes into iodine and cuprous iodide, even at the ordinary tempera-
ture, whilst cupric chloride only suffers a similar change on ignition. If a solution of a'
cupric salt be mixed with a solution of potassium iodide the cupric iodide formed imme-
diately decomposes into free iodine and cuprous iodide, which separates out as a precipi-
tate. In this case the cupric salt acts in an oxidising manner, like, for example, nitrous
acid, ozone, and other substances which liberate iodine from iodides, but with this differ-
ence, that it only liberates half, whilst they set free the whole of the iodine from potas-
sium iodide : 2KI + CuCl2 = 2KC1 + Cul + 1.

It must also be remarked that cuprous oxide, when treated with hydrofluoric acid,
gives an insoluble cuprous fluoride, CuF. Cuprous cyanide is also insoluble in water,
and is obtained by the addition of hydrocyanic acid to a solution of capric chloride
saturated with sulphurous anhydride. This cuprous cyanide, like silver cyanide, gives
a double soluble salt with potassium cyanide. The double cyanide of copper and
potassium is tolerably stable in the air, and enters into double decompositions with
various other salts, like those double cyanides of iron with which we are already
acquainted.

Copper hydride, CuH, also belongs to the number of the cuprous compounds. It
was obtained by Wiirtz by mixing a hot (70°) solution of cupric sulphate with a solution
of hypophosphorous acid, H3PO2. The addition of the reducing hypophosphorous acid
must be stopped when a brown precipitate makes its appearance, and when gas begins
to be evolved. 'The brown precipitate is the hydrated cuprous hydride. When gently
heated it disengages hydrogen ; it gives cuprous oxide when exposed to the air, burns
in a stream of chlorine, and liberates hydrogen with hydrochloric acid: CuH + HCl
= CuCl + H2. Zinc, silver, mercury, lead, and many other heavy metals do not form
such a compound with hydrogen, neither tinder these circumstances nor under the action
of hydrogen at the moment of the decomposition of salts by a galvanic current. The
greatest resembance is seen between cuprous hydride and the hydrogen compounds of
potassium, sodium, Pd, Ca, and Ba.

COPPER, SILVER, AND GOLD 409

instance, the nitrite and carbonate) leave oxide of copper 8bis in the
form of friable black powder, after being ignited. If the ignition be
carried further, Cu2O may be formed from the CuO.8 trl Anhydrous
cupric oxide is very easily dissolved in acids, forming cupric salts, CuX2.
They are analogous to the salts MgX2, ZnX?, ]STiX2, FeX2, in many
respects. On adding potassium or ammonium hydroxide to a solution
of a cupric salt, it forms a gelatinous blue precipitate of the hydrated
oxide of "copper, CuH202, insoluble in water. The resultant precipitate
is redissolved by an excess of ammonia, and gives a very beautiful
azure blue solution, of so intense a colour that the presence of small
traces of cupric salts may be discovered by this means.9 An excess of

8 bit The oxide of copper obtained by igniting the nitrate is frequently used for.
organic analyses. It is hygroscopic and retains nitrogen (1*5 c.c. per gram) when the
Citrate is heated in vacuo (Richards and Rogers, 1893);

8 trl Oxide o.f copper is also capable of dissociating when heated. Debray and
Joannis showed that it then disengages oxygen, whose maximum tension is constant
lor a given temperature, providing that fusion does not take place (the CuO then
dissolves in the molten Cu2O) ; that this loss of oxygen is followed by the formation of
fiuboxide, and that on cooling, the oxygen is again absorbed, forming CuO.

9 Cupric oxide and many of its salts are able to give definite, although unstable,
compounds with ammonia. This faculty already shows itself in the fact that cupric
•oxide, as well as the, salts of copper, dissolves in aqueous ammonia, and also in the fact
that salts of copper absorb ammonia gas. If ammonia be added to a solution of any
cupric salt, it first forms a precipitate of cupric hydroxide, which then dissolves in an
excess of ammonia. The solution thus formed, when evaporated or on the addition of
alcohol, frequently deposits crystals of salts containing both the elements of the salt of
copper taken and of ammonia. Several such compounds are generally formed. Thus
cupric chloride, CuCl2, according to Deherain, forms four compounds with ammonia —
namely, with one, two, four, and six molecules of ammonia. Thus, for example,
if ammonia gas be passed into a boiling saturated solution of cupric chloride, on
cooling, small octahedral crystals of a blue colour separate out, containing
CuCl2,2NH3,H2O. At 150° this substance loses half the ammonia and- all the water
contained in it, leaving the compound CuCl2,NH3. Nitrate of copper forms the com-
pound Cu(NO3)2,2NHj. This compound remains unchanged on evaporation. Dry
cupric sulphate absorbs ammonia gas, and gives-a compound containing five molecules of
ammonia to one of sulphate (Vol. I., p. 257, and Chapter XXII., Note 85). If this com-
pound is dissolved in aqueous ammonia, on evaporation it deposits a crystalline substance
containing CuS04,4NH3,H20. At 150° this substance loses the molecule of water and
one-fourth of its ammonia. On ignition all these compounds part with the remaining
ammonia in the form of an ammoniacal salt, so that the residue consists of cupric oxide.
Both the hydrated and anhydrous cupric oxide are soluble in aqueous ammonia.

The solution obtained by the action of aqueous ammonia and. air on copper turnings
(Note 6) is remarkable for its faculty of dissolving cellulose^ which is insoluble in water,
dilute acids, and alkalis. Paper soaked in such a solution acquires the property of not
rotting, of being difficultly combustible, and ^aterproof, &c. It has therefore been
applied, especially in England, to many practical purposes— for example, to the con-
struction of temporary buildings, for covering roofs, &c. The composition of the
substance held in solution is Cu(HO)2,4NH3.

If dry ammonia gas be passed over cupric oxide heated to 265°, a portion of the oxide
of copper remains unaltered-, whilst the other portion gives copper nitride, the oxygen of
the eopper oxide combining with the hydrogen and forming water. The oxide of copper
which" remains unchanged is easily removed by washing the resultant product with

410 PRINCIPLES OF CHEMISTRY

potassium or sodium hydroxide does nob dissolve cupric hydroxide.
A hot solution gives a black precipitate of the anhydrous oxide
instead of the blue precipitate, and the precipitate of the hydroxide
of copper becomes granular, and turns black when the solution
is heated. This is due to the fact that the blue hydroxide is
exceedingly unstable, and when slightly heated it loses the elements
of water and gives the black anhydrous cupric oxide : CuH202
= CuO + H2O.

Cupric oxide fuses at a strqng heat, and on cooling forms a heavy
crystalline mass, which is black, opaque, and somewhat tenacious. It
is a feebly energetic base, so that not only do the oxides of the metals
of the alkalis and alkaline earths displace it from its compounds, but
even such oxides as those of lead and silver precipitate it from solutions,
which is partially dueto these oxides beingsoluble, although butslightly
so, in water. However, cupric oxide, and especially the hydroxide,
easily combines with even the least energetic acids, and does not give
any compounds with bases ; but, on the other hand, it easily forms
basic salts, 9 bis and in this respect outstrips magnesium and recalls the

aqueous ammonia. Copper nitride is very stable, and is insoluble ; it has the composi-
tion Cu3N (i.e. the copper is monatomic here as in Cu2O), and is an amorphous green
powder, which is decomposed when strongly ignited, and gives cuprous chloride and
ammonium chloride when treated with hydrochloric acid. Like the other nitrides, copper'
nitride, Cu3N, has scarcely been investigated.. Granger (1892), by heating copper in the
vapour of phosphorus, obtained hexagonal prisms of 'Cu5P, which passed into_Cu6P
(previously obtained by Abel), when heated in nitrogen. Arsenic is easily absorbed by
copper, and its presence (like P), even in small quantities, has a great influence upon
the properties of copper — for instance, pure copper wire 1 sq. mm. in section breaks
under a load of 85 kilos, while, the presence of 0'22 p.c. of arsenic raises the breaking
load to 42 kilos.

9 bis As a comparatively feeble base, oxide of copper easily forms both basic and
double salts. As an instance we may mention the. double sails composed of th»
dichloride CuCl2,2H20 and potassium chloride. The, double salt CuK2Cl4,2H2O
crystallises from solutions in blue plates, but when heated alone or with substances
taking up water easily gives brown needles CuKCl3 and at the same time KC1, and this
reaction is reversible at 92 ' as Meyerhoffer (1889) showed (i.e. above 92° the simpler
double salt is formed and below 92° the more complex salt). With an excess of the
copper salt, KC1 gives another double salt, Cu2KCl5,4H2O, the_transition temperature of
which is 65°. The instances of equilibria which are encountered in such complex
relations (see Chapter XIV-., Note 25, .astrakhanite, and Chapter XXII., Note 23)- are
embraced by the law of phases given by Gibbs (Transactions of the Connecticut
Academy of Sciences, 1875-1878, in J. Willard Gibbs' memoir ' On the equilibrium of
heterogeneous substances : ' and in" a clearer and more accessible form in H. W,
Bakhuis Roozeboom's papers, Rec. trav. chim., Vol. VI., and in W. Meyerhoffer's memoir
Die Phasenregel und ihre Anwendungent 1893, to which sources we refer those desiring
fuller information respecting this law). Gibbs calls ' bodies ' substances (simple or com-
pound) capable of forming homogeneous complexes (for instance, solutions or inter-
combinations) of a varied composition ; &phase — a mechanically separable portion of such
bodies or of their homogeneous complexes (for instance, a vapour, liquid or precipitated
. solid), perfect equilibrium-^-mdh a state • of bodies and t>f their complexes as. Is

COPPEE, SILVER, AND GOLD 411

oxides of lead! or mercury. Hence the hydroxide of copper dissolves in.
solutions of neutral cupric salts. The cupric salts are generally blue or
green, because cupric hydroxide itself is coloured. But some of the
salts in the anhydrous state are colourless.10

characterised by a constant pressure at a constant temperature even under a change in
the amount of one of the component parts (for instance, of a salt in a saturated solution),
while an imperfect equilibrium is such a one for which such a change corresponds with
a change of pressure (for instance, an unsaturated solution). The law of phases consists
in the fact that : n "bodies only give a perfect equilibrium when n + 1 phases participate*
in that equilibrium — for example, in the equilibrium of a salt in its saturated solution
in water there are two bodies (the salt and water) and three phases, namely, the salt,
solution, and Tapour,- which can be mechanically separated from each other, and to this
equilibrium there corresponds a definite tension. At the same time, n bodies may
occur in n + Z phases, but only at one definite temperature and one pressure ; a change
of one of these may bring about another state (perfect or not— equilibrium stable or
unstable). Thus water when liquid at the ordinary temperature offers two phases
{liquid and vapour) and is in perfect equilibrium (as also is ice below 0°)1Lbut water, ice,
and vapour (three phases and only one body) can only be in equilibrium at 0°, and at the
ordinary pressure ; with a change of t there will remain either only ice and vapour or
only liquid water and vapour ; whilst with a rise of pressure not only will the vapour
pass into the liquid (there again only remain two phases) but also the temperature of
the formation of ice will fall (by about 7° per 1000 atmospheres). The same laws of
phases are applicable to the consideration of the formation of simple or double salts
from saturated solutions and to a number of other purely chemical relations. Thus, for
example, in the above-mentioned instance, when the bodies are KC1, GuCl2, and H20,
perfect equilibrium (which here has reference to the solubility) consisting of four phas_es,
corresponds to the following seven cases, considering only the phases (above 0°)
A = CuCL,2KCl,2H2O; B = CuCl2KCl ; C = CuCl2,2H20,KCl, solution and vapour:
(1) A + B + solution +jvapour; (2) A + C + solution t vapour ; (3) A + KC1 + solution,
•f vapour; (4) A + B + C + vapour (it follows that B + KC1 + solution gives A); {5}
A + C + KC1 + vapour ; (6) B + C + solution + vapour ; and (7) B + KC1 + solution + vapour.
Thus above 92° A gives B + KC1. The law of phases by bringing complex instances of
chemical reaction under simple physical schemes, facilitates their study in detail and
gives the means' of seeking the simplest chemical relations dealing with solutions, dis-
sociation, double decompositions and similar cases, and therefore deserves consideration,
but a detailed exposition of this subject must be looked for" in works on physical
chemistry.

10 The normal cupric nitrate, CuNoO6,3H20, is obtained as a deliquescent salt of a blue
colour (soluble in water and in alcohol) by dissolving copper or cupric oxide iix nitric acid.
It is so easily decomposed by the action of heat that it is impossible to drive off the water
of crystallisation from it before it begins to decompose. During the ignition of the normal
salt the cupric oxide formed enters into combination with the remaining undecomposed
normal salt, and gives a basic salt, CuN266,2CuH2O2. The same basic salt is obtained
if a certain quantity of alkali or cupric hydroxide or carbonate be added to -the solution
of the normal salt, which is even decomposed when boiled with metallic copper, and forms,
the basic salt as a green powder, which easily decomposes under the action of heat and
leaves a residue of cupric oxide. The basic salt, having the composition CuN206,3CuH2O2,
is nearly insoluble in water.

The normal carbonate of copper, CuCO3, occurs in nature, although extremely rarely.
If solutions of cupric salts be mixed with solutions of alkali carbonates, then, as in the
case of magnesium, carbonic anhydride is evolved arid basic salts are formed, which vary
in composition according to the temperature and conditions of the reaction. By mixing-
cold solutions, a voluminous blue precipitate is. formed, containing an equivalent pro-
portion of cupric hydroxide and carbonate (after standing or heating, its composition

412 PEINCIPLES OF CHEMISTRY

The commonest normal salt is blue vitriol — i.e. the normal cupric
sulphate. It generally contains five molecules of water of crystallisa-
tion, CuS04J5H2O. It forms the product of the action of strong sul-
phuric acid on copper, sulphurous anhydride being evolved. The same
salt is obtained in practice by carefully roasting sulphuretted ores of
copper, and also by the action of water holding oxygen in solution on
them : CuS -fO4 = CuSO4. This salt forms a by-product, obtained in
gold refineries, when the silver is precipitated from the sulphuric acid
solution by means of copper. It is also obtained by pouring dilute
sulphuric acid over sheet copper in the presence of air, or by heating
cupric oxide or carbonate in sulphuric acid. The crystals of this salt
belong to the triclinic system, have a specific gravity of 2-19, are of a
beautiful blue colour, and give a solution of the same colour. 100
parts of water at 0° dissolve 15, at 25° 23, and at 100° about 45 parts
of cupric sulphate, CuS04.10 bi8 At 100° this salt loses a portion of its

is the same as malachite, ep. gr. 8'5) : 2CuS04 + 2Na2COs + H20 = CuCO5,CuH203
+ 2Na2SO4 + C02. If the resultant blue precipitate be heated in the liquid, it loses water
and is transformed into a granular green mass of the composition Cu2CO4 — i.e. into a
compound of the normal salt with anhydrous cupric oxide. This salt of the oxide corre-
Bponds with or£hocarbonic acid, C(OH)4 = CH4O4, where 4H is replaced by 2Cu. On
further boiling this salt loses a portion of the carbonic acid, forming black cupric oxide,
BO unstable is the compound of copper with carbonic anhydride. Another basic salt which
occurs in nature, 2CuCO3,CuH2O2, is known as azurite, or blue carbonate of copper ; it
also loses carbonic acid when boiled with water. On mixing a solution of cupric sulphate
with sodium sesquicarbonate no precipitate is at first obtained, but after boiling a pre-
cipitate is formed having the composition of malachite. Debray obtained artificial
azurite by heating cupric nitrate with chalk.

10 bis Although sulphate of copper usually crystallises with 5H2O, that is, differently
to the sulphates of Mg, Fe, and Mn, it is nevertheless perfectly isomorphous with them,
as is seen not only in the fact that it gives isomorphous mixtures with them, containing
a similar amount of water of crystallisation, but also in the ease with which it forms,
like all bases analogous to MgO, double salts, R2Cu(SO4).3,6H2O, where R=^K, Rb, Cs,
of the monoclinic system.

Salts of this kind, like CuCl2,2KCl,2H2O,PtK2Cy4, &€., present a composition
CuX2 if tho representation of double salts given in Chapter XXIII., Note 11, be
admitted, because they, like Cu(HO)o, contain Cu(X2K)2, where X2=SO4, i.e. the
resuhie of sulphuric acid, which combines with H2, and is therefore able to replace
the H2 by X2 or O. A detailed study of the crystalline forms of these salts, made by
Tutton (1893) (see Chapter XIII., Note 1), showed : (1) that 22 investigated salts of the
composition R2M(SO4),6H2O, where R = K, Rb, Cs, and M = Mg, Zn, Cd, Mn, Fe, Co, Ni,
Cu, present a complete crystallographic resemblance ; (2) that in all respects the Rb
salts present a transition between the K and Cs salts ; (3) that the Cs salts form
crystals most easily, and the K salts the most difficultly, and that for the K salts of Cd
and Mn it was even impossible to obtain well-formed crystals ; (4) that notwithstanding
the closeness of their angles, the general appearance (habit) of the potassium compound
differs very clearly from the Cs salts, while the Rb salts present a distinct transition in
this respect ; (5) that the angle of the inclination of one of the axes to the plane of the two
Other axes showed that in the K salts (angle from 75° to 75° 38') the inclination is least,
in the Cs salts (from 72° 62' to 73° 50') greatest, and in the Rb salts (from 73° 57' to
74° 42') intermediate between the two ; the replacement of Mg. . . Cu produces but a

COPPER, ,SILVER, AND GOLD 413

water of crystallisation, which it only parts with entirely at a high
temperature (220°) and then gives a white powder of the anhydrous
sulphate ; and the latter, on further calcination, loses the elements of
sulphuric anhydride, leaving cupric oxide, like all the cupric salts. The
anhydrous (colourless) cupric sulphate is sometimes used for absorbing
water ; it turns blue in the process. It offers the advantage that it
retains both hydrochloric acid and water, but not carbonic anhydride.11
Cupric sulphate is used for steeping seed corn ; this is said to prevent
the growth of certain parasites on the plants. In the arts a consider-
able quantity of cupric sulphate is also used in the preparation of other
copper salts— for instance, of certain pigments u bis — and a particularly

very small change in this angle; (6) that the other angles and the ratio of the
axes of. the crystals exhibit a similar variation ; and (7) that thus the variation of the
form is chiefly determined by the atomic weight of the alkaline metal. As an example
we cite the magnitude of the inclination of the axes of R2M(SO4)2,6H2O.

R= K Rb Cs

M = Mg 75d 12' 74° 1' 472° 54'

Zn 75° 12' 74° 7' 72° 59'

Cd 74° 7' 72° 49'

Mn 78° 8' 72° 53'

Fe 75° 28' 74° 16' 78° 8'

Co 75° 5' 73° 59' 72° 52'

Ni 75° 0' . 73° 57' 72° 58'

Cu 75° 32' 74° 42' 73° 50'

This shows clearly (within the limits of possible error, •which may be as much as
W) the almost perfect identity of the independent crystalline forms notwithstanding the
difference of the atomic weights of the diatomic elements, M = Mg : . . Cu.

" In addition to what has been said (Chapter I., Note 65, and Chapter XXII.,
Note 85) respecting the combination of CuSO4 with water and ammonia, wo may add
that Lachinoff (1893) showed that CuSO4,5H2O loses 4|H2O at 180°, that CuSO4,5NH5
also loses 4|NH3 at 320°, and that only £H2O and iNH3 remain in combination with
the CuSO4. The last $H2O can only be driven off by heating to 200°, and the last
|NH5 by heating to 860° Ammonia displaces water from CuSO4,5H2O, but water
cannot displace the ammonia from CuSO^SNfi^. If hydrochloric, acid gas "be passed
over CuSO4,5H2O at the ordinary temperature, it first forms CuSQ4,5H2Or8&Cl, and
then CuSO4,2H20,2HCl. When air is passed over the latter compound it passes into
CuSO4H2O with a small amount of HC1 (about £HC1). At 100°. Cu.SO4,5H2O in a
stream of hydrochloric acid gas gives CuSO44H2O,2HCl, and then CuSO^ILjOHCl,
whilst after prolonged heating- CuS04 remains, which rapidly passes into CuS04,5H20
when placed under a bell jar over water. Over sulphuric acid, however, CuSO4,5H2O
only parts with 8H2O, and if CuSO4,2H2O be placed over water it again forma
CuS04,5H2O, and so on.

11 bis Commercial blue vitriol generally contains ferrous sulphate. The salt is purified
by converting the ferrous salt into a ferric salt by heating the solution with chlorine or
nitric acid. The solution is then evaporated to dryness, and the unchanged cupric sul-
phate extracted from the residue, which will 'con tain the larger portion of the ferric
oxide. The remainder will be separated if cupric hydroxide is added to the solution andi
boiled ; the cupric. oxide, CuO, then precipitates the ferric oxide, Fe2O3, just as it is itself
precipitated by silver oxide. But the solution will contain a small proportion1 of a basic
salt of copper, and therefore sulphuric acid must be added to the filtered solution, and the
salt allowed to crystallise. Acid salts are not formed, and cupric sulphate itself has an
acid reaction on litmus paper.

414 PRINCIPLES OP CHEMISTRY

large quantity is used in the galvanoplastic process, tvhich consists in
the deposition of copper from a solution of cupric sulphate by tho
action of a galvanic current, when the metallic copper is deposited
on the negative pole and takes the shape of the latter. The d£-
ficription of the processes of galvanoplastic art introduced by Jacobi
in St. Petersburg forms a part "of applied physics, and will not be
touched on here, and we will only mention that, although first intro-
duced for small articles, it is now used for such articles as type moulds
(cliches], for maps, prints, &c., and also for large statues, and for the
•deposition of iron, zinc, nickel, gold, silver, tfec. on other metals and
materials. The beginning of the application of the galvanic current to
the practical extraction of metals from solutions has also been estab-
lished, especially since the dynamo- electric machines of Gramme,
Siemens, and others have rendered it possible to cheaply convert the
mechanical motion of the steam engine into an electric current. It is
to be expected that the application of the electric current, which has
long since given such important results in chemistry, will, in the near
future, play an important part in technical processes, the example being
fihown by electric lighting.

The alloys of copper with certain metals, and especially with zinc
*nd tin, are easily formed by directly melting the metals together.
They are easily cast into moulds, forged, and worked like copper,
whilst they are much more durable in the air, and are therefore fre-
quently used in the arts. Even the ancients used exclusively alloys
of copper, and not pure copper, but its alloys with tin or different
kinds of bronze (Chapter XVIII., Note 35). The alloys of copper with
zinc are called brass or ' yellow metal.' Brass contains about 32 p.c. of
zinc ; generally, however, it does not contain more than 65 p.c. of
copper. The remainder is composed of lead and tin, which usually
occur, although in small quantities, in brass. Yellow metal contains
about 40 p.c. of zinc.12 The addition of zinc to copper changes the

12 Among tha alloys of copper resembling brass, delta metal, invented by A. Dick
(London) is largely used (since 1883). It contains 65 p.c. Cu, and 41 p.c. Zn, the
Remaining 4 p.c. being composed of iron (as much as 3J p.c., which is first alloyed with
.zinc), or of cobalt, and manganese, afcd certain other metals. The sp. gr. of delta metal
is &'4. It melts at 950°, and then becomes so fluid that it fills up all the cavities in a
'mould and forms excellent castings. It has a tensile strength of 70 kilos per sq. mm.
• (gun metal about 20, phosphor bronze about 80). It is very soft, especially when heated
to 600°, but after forging and rolling it becomes very liard ; it is more difficultly acted
Upon by air and water than other kinds of brass, and preserves its golden yellow colour
for any length of time, especially if well polished. 'It is used for making bearing's, screw
propellers, valves, and many other articles. In general the alloys of Cu and Zn con-
taining about | p.c. by weight of copper were for a long time almost exclusively made io
Sweden and England (Bristol, Birmingham). These alloys for the most part are cheaper,
Larder, and more fusible than copper alone, and form good castings. The alloys con-

COPPEB, SILVER, AND GOLD 415

colour of the latter to a considerable degree / with a certain amount of
zinc the colour of the copper becomes yellow, and with a still larger
proportion of zinc an alloy is formed w hich has a greenish tint In those
alloys of zinc and copper which contain a larger Amount of zine than
of copper, the yellow colour disappears and is replaced by a greyish
colour. But when the amount of zinc is diminished to about 20 p.c.j
the alloy is red and hard, and is called ' tombac.' A contraction takes
place in alloying copper with zinc, so that the volume of the alloy is
less than that of either metal individually. The zinc volatilises on
prolonged heating at a high temperature and the excess of metallic
copper remains behind. When heated in the air, the zinc oxidises
before the copper, so that all the zinc alloyed with copper may be
removed from the copper by this means. An important property of
brass containing about 30 p.c. of zinc is that it is soft and malleable in:
the cold, but becomes somewhat brittle when heated. We may also
mention that ordinary copper coins contain, in order to render them
hard, tin, zinc, and iron (Cu = 95 p.c.) ; that it is now customary to add
a small amount of phosphorus to copper and bronze, for the same pur-
pose ; and also that copper is added to silver and gold in coining, &c.
to render it hard ; moreover, in Germany, Switzerland, and Belgium,
and other countries, a silver- white alloy (melchior, German silver, &c.),
for base coinage and other purposes, is prepared from brass and nickel
(from 10 to 20 p.c. of nickel ; 20 to 30 p.c. zinc ; 50 to 70 p.c. copper),
or directly from copper and nickel, or, more rarely, from an alloy con-
taining silver, nickel, and copper. I2bis

Copper, in its cuprous compounds, is so analogous to silver, that

taining 45-80 p.c. Cu crystallise in cubes if slowly cooled (Bi also gives crystals). By
washing the surface of brass with dilute sulphuric acid, Zn is removed and the article
acquires the colour of copper. The alloys approaching Zn^Cuj in their composition
exhibit the greatest resistance (under other equal conditions ; of purity, forging, rolling,
&c.) The addition of 3 p.c. Al, or 5 p.c. Sn, improves the quality of brass. Bespecting
aluminium bronze see Chapter XVII. p. 88.

12 bte Ball (also Kamensky), 1888, by investigating the electrical conductivity of the
alloys of antimony and copper with lead, came to the conclusion that only two definite
compounds of antimony and copper exist, whilst the other alloys are either alloys of these
two together or with antimony or with copper. These compounds are Cu2Sb and
Cu4Sb — one corresponds with the maximum, and the other with the minimum, electrical
resistance. In general, the resistance offered to an electrical current forms one of the
methods by which the composition of definite alloys (for example, PbaZn7) is often
established, whilst the electromotive force of alloys affords (Laurie, 1888) a still more
accurate method— for instance, several definite compounds were discovered by thia
method among the alloys of copper with zinc and tin ; but we will not enter into any
details of this subject, because we avoid all references to electricity, although the reader
is recommended to make himself acquainted with this branch of science, which has many
points in common with chemistry. The study of alloys regarded as solid solutions should,
in my opinion, throw much light upon the question of solutions, which is still obscure .-
in many aspects and in many branches of chemistry.
*G

416 PRINCIPLES OF CHEMISTKY

were thefe* no cupric compounds, or if silver gave stable compounds
of the higher oxide, AgO, the resemblance would be as close as that
between chlorine and bromine or zinc and cadmium ; but silver
compounds corresponding to AgO are quite unknown. Although
silver peroxide — which was regarded as AgO, but which Berthelot
^1880) recognised as the sesquioxide Ag2O3 — is known, still it does
not form any true salts, and consequently cannot be placed along
with cupric oxide. In distinction to copper, silver as a metal does
not oxidise under the influence of heat ; and its oxides, Ag2O and
Ag2O3, easily lose oxygen (see Note 8 tri). Silver does not oxidise in
air at the ordinary pressure, and is therefore classed among the
so-called noble metals. It has a white colour, which is much purer
than that of any other known metal, especially when the metal is chemi-
cally pure. In the arts silver is always used alloyed, because chemi-
cally-pure silver is so soft that it wears exceedingly easily, whilst when
fused with a small amount of copper, it becomes very hard, without
losing its colour.13

15 There are not many soft metals ; lead, tin, copper, silver, iron, and fold are some-
what soft, and potassium and sodium very soft. The metals of the alkaline earths
are sonorous and hard, and many other metals are even brittle, especially, bismuth
and antimony. But the very slight significance which these properties have in
determining the fundamental chemical properties of substances (although, however, of
immense importance in the practical applications of metals) is seen from the example
shown by zinc, which is hard at the ordinary temperature, soft at 100°, and brittle
at 200°.

As the value of silver depends exclusively on its purity, and as there is no possibility,
of telling the amount of impurities alloyed with it from its external appearance, it is
customary in most countries to mark an article with the amount of pure silver ifc contains
after an accurately-made analysis known as the assay of the silver. In France the

FJG. 95.— Cupel for silver assaying. FIG. 96.— Clay muffle.

assay of silver shows the amount of pure silver in 1,000 parts by weight ; in Russia the
amount of pure silver in 96 parts— that is, the assay shows the number 'of zolotniks
(4'26 grams) of pure silver in one pound (410 grams) of alloyed silver. Russian silver is
generally 84 assay— that is, contains 84 parts by weight of pure silver and 12 parts of
Copper and other metals. French money contains 90 p.c. (in the Russian system this
Will be 86'4 assay) by weight of silver [English coins and jewellery contain 92'5 p.c. of
silver] ; the silver rouble is of 83£ assay— that is, it contains 86'8 p.c. of silver— and the
smaller Russian silver coinage is of 48 assay, and therefore contains 50 p.c. of silver.
Silver ornaments and articles are usually made in Russia of 84 and 72 assay. As
the alloys of silver and copper, especially after being subjected to the action of heat, are
not so white as pure silver, they generally undergo a process known as ' blanching ' (or

COPPER, SILVER, AND GOLD

417

Silver occurs in nature, both in a native state and in certain com-
pounds. Native silver, however, is of rather rare occurrence. A far

' pickling ') after being worked up. This consists in removing the copper from the surface
of the article by subjecting it to a dark-red heat and then immersing it in dilute acid.
During the calcination the copper on the surface is oxidised, whilst the silver remains
unchanged ; the dilute acid then dissolves the copper oxides formed, and pure silver is
left on the surface. The surface is dull after this treatment, owing to the removal of a
portion of the metal by the acid. After being polished the article acquires the desired
lustre and colour, so as to be indistinguishable from a pure silver object. In order to
test a silver article, a portion of its mass must be taken, not from the surface, but to a
certain depth. The methods of assay used in practice are very varied.. The commonest
and most often used is that known as cupellation. It is based on the difference in the
oxidisability of copper, lead, and silver. The cupel is a porous cup with thick sides,

FIG. 97.— Portable muffle furnace

made by compressing bone ash. The porous mass of bone ash absorbs the fused oxides,
especially the lead oxide, which is easily fusible, but it does not absorb the unoxidised
metal. The latter collects into a globule under the action of a strong heat in the cupel,
and on cooling solidifies into a button, which may then be weighed. Several cupels are
placed in a muffle. A muffle is a semi-cylindrical clay vessel, shown in the accompanying
drawing. The sides of the muffle are pierced with several orifices, which allow the access
of air into it. The muffle is placed in a furnace, where it is strongly heated. Under the
action of the air entering the muffle the copper of the silver alloy is oxidised, but as the
oxide of copper is infusible, or, more strictly speaking, difficultly fusible, a certain quan-
tity of lead is added to the alloy ; the lead is also oxidised by the air at the high tem-
perature of the muffle, and gives the very fusible lead oxide. The copper oxide then
fuses with the lead oxide, and is absorbed by the cupel, whilst the silver remains as a

41B PRINCIPLES OF CHEMISTRY

greater quantity of silver occurs in combination with sulphur, and
especially in the form of silver sulphide^ Ag2S, with lead sulphide
or copper sulphide, or the ores of various other metals. The largest
amount of silver is extracted from the lead in which it occurs. If this
lead be calcined in the presence of air, it oxidises, and the resultant
lead oxide, PbO (* litharge ' or ' silberglatte/ as it is called), melts into
a mobile liquid, which is easily removed. The silver remains in an
unoxidised metallic state.14 This process is called cupellation.

bright white globule. If the weight of the alloy taken and of the silver left on the cupel
be determined, it is possible tp calculate the composition of the alloy. Thus the essence
of cupellation consists in the separation of the oxidisable metals from silver, which does
not oxidise under the action of heat. A more accurate method, based on the precipitation
of silver from its solutions in the form of silver chloride, is described in detail in works
on analytical chemistry.

14 In America, whence the largest amount of silver is now obtained, ores are worked
containing not more than £ p.c. of silver, whilst at £ p.c. its extraction is very profitable.
Moreover, the extraction of silver from ores containing not more than O'Ol p.c. of this
metal is sometimes profitable. The majority of the lead smelted from galena contains
silver, which is extracted from it.- Thus near Arras, in France, an ore is worked
which contains about 65 parts of lead and 0'088 part of silver in 100 parts of ore, which
corresponds with 186 parts of silver in 100,000 parts of lead. At Freiberg, in Saxony, the
ore used (enriched by mechanical dressing) contains about 0'9 of silver, 160 of lead, and
2 of copper in 10,000 parts. In every case the lead is first extracted in the manner
described in Chapter XVIII., and this lead will contain all the silver. Not unfrequently
other ores of silver are mixed with lead ores, in order to obtain an argentiferous lead as
the product. The extraction of small quantities of silver from lead is facilitated by the
fact (Pattinson's process) that molten argentiferous- lead in cooling first deposits
crystals of pure lead, which fall to the bottom of the cooling vessel, whilst the propoiv.
tion of silver in the unsolidified mass increases owing to the removal of the crystals'
of lead. The lead is enriched in this manner until it contains ^ part of "silver, and'
is then subjected to cupellation on a larger scale. According to Park's process, zinc is-
added to the molten argentiferous lead, and the alloy of Pb and Zn, which first separates
out on cooling, is collected. This alloy is found to contain all the silver previously con-
tained in the lead. The addition of O'i> p.c. of aluminium to the zinc (Bossier and Edelman)
facilitates the extraction of the Ag from the resultant alloy besides preventing oxida-
tion ; for, after re-melting, nearly all the lead easily runs off (remains, fluid), and
leaves an alloy containing about 80 p.c. Ag and about 70 p.c. Zn. This alloy may be used
as an anode in a solution of ZnCl2, when the Zn is deposited on the cathode, leaving the
silver with a small amount of Pb, &c. behind. The silver can be easily obtained pure by
treating it with dilute acids and cupelling.

The ores of silver which contain a larger amount of it are : silver glance, Ag2S (sp.
gr. 7'2) ; argentiferous-copper glance, CuAgS ; horn silver or chloride of silver, AgCl ;
argentiferous grey copper ore; polybasite, M9RS6 (where M=Ag, Cu4 and R=Sb, As),
and argentiferous gold. The latter is the usual form in which gold is found in alluvial
deposits and ores. The crystals of gold from the Berezoffsky mines in the Urals contain
•90 to 95 of gold and 5 to 9 of silver, and the Altai gold contains 50 to 65 of gold and 86 to
88 of silver. The proportion of silver in native gold varies between these limits in other
localities. Silver ores, which generally occur in veins, usually contain native silver and
Various sulphur compounds. The most famous mines in Europe are in Saxony (Frei-
fcerg), which has a yearly output of as much as 26 tons of silver, Hungary, and Bohemia
{41 tons). In Russia, silver is extracted in the Altai and at Nerchinsk (17 tons). The
richest silver mines known are in America, especially in Chili (as much as 70 tons),
Mexico (200 tons), and more particularly in the Western States of North America. The

COPPER, SILVER, AND GOLD 419

Commercial silver generally contains copper, and, more rarely, other
metallic impurities also. Chemically pure silver is obtained either by
cupellation or by subjecting ordinary silver to the following treatment.
The silver is first dissolved in nitric acid, which converts it and the
copper into nitrates, Cu(NO3)2 and AgN03 ; hydrochloric acid is then
added to the resultant solution (green, owing to the presence of the
cupric salt), which is considerably diluted with water in order to retain
the lead chloride in solution if the silver contained lead. The copper
and many other metals remain in solution, whilst the silver is precipi-
tated as silver chloride. The precipitate is allowed to settle, and the
liquid is decanted off ; the precipitate is then washed and fused with
sodium carbonate. A double decomposition then takes place, sodium
chloride and silver carbonate being formed ; but the latter decomposes
into metallic silver, because the silver oxide is decomposed by heat :
•A.g2CO3 = Ag2 + O + CO2. The silver chloride may also be mixed
with metallic zinc, sulphuric acid, and .water, and left for some time,
when the zinc removes the chlorine from the silver chloride and pre-
cipitates the silver as a powder. This finely-divided silver is called
1 molecular silver.' l5

richness of these mines may be judged 'from the fact that one mine in the State of
Nevada (Comstock, near Washoe and the cities of Gold Hill and Virginia), which was dis-
covered in 1859, gave an output of 400 tons in 1866. In place of cupellation, chlpri-
nation may also be employed for extracting silver from its ores. The method of
chlorination consists in converting the silver in an ore into silver chloride. This is
either done by a wet or by a dry method, roasting the ore with NaCl. When the silver
chloride is formed, the extraction of the metal is also done by two methods. The first
consists in the silver chloride being reduced to metal by means of iron in rotating
barrels, with the subsequent addition of mercury which dissolves the silver,. but
does not act on the other petals. The mercury holding the silver in solution is distilled,
when the silver remains behind. This method is called amalgamation. The other
method is less frequently used, and consists in dissolving the silver chloride in sodium
chloride or in sodium thiosulphate, and then precipitating the silver from the solution.
The amalgamation is then carried on in rotating barrels containing the roasted ore mixed
with water, iron, and mercury. The iron reduces the silver chloride by taking up the
chlorine from it. The technical .details of these processes are described in works on
metallurgy. The extraction of AgCl by the wet method is carried on (Patera's process)
by means of a solution of hyposulphite of sodium which dissolves AgCl (see Note 28), or
by lixiviating with a 2 p.c. solution of a double hyposulphite of Na and Cu (obtained by
adding CuS04 to NaaS2O3). The resultant solution of AgCl is first treated with soda
to precipitate PbCO3, and then with Na-jS, which precipitates the Ag and Au. The
.process should be carried on rapidly to prevent the precipitation of Cu2S from the solu-
tion of CuS04 and Na2S2O3.

15 There is another practical method which is also suitable for separating the silver
from the solutions obtained in photography, and consists in precipitating the silver by
oxalic acid. In this case the amount of silver in the solution must be known, and 28
grams of oxalic acid dissolved in 400 grams of water must be added for every 60 grains
of silver in solution in a litre of water. A precipitate of silver oxalate, AgaC2O4, is then
obtained, which is insoluble in water but soluble in acids. Hence, if the liquid contain
any free acid it must be previously freed from it by the addition of podium carbonate.

420 PRINCIPLES OF CHEMISTRY

Chemically-pure silver has an exceeding pure white colour, and a
'specific gravity of 10'5. Solid silver is lighter than the molten metal,
and therefore a piece of silver floats on the latter. The fusing -
point of silver is about 950° C., and at the high temperature attained
by the combustion of detonating gas it volatilises.16 By employing
silver reduced from silver chloride by milk sugar and caustic potash,
and distilling it, Stas obtained silver purer than that obtained by any
other means ; in fact, this was perfectly pure silver. The vapour of
silver has a very, beautiful green colour, which is seen when a silver
wire is placed in an oxy hydrogen flame.17

It has long been known (Wohler) that when nitrate of silver,
AgN03, reacts as an oxidising agent, upon citrates and tartrates, it is
able under certain conditions to give either a salt of suboxide of silver
(see Note 19) or a red solution, or to give a precipitate of metallic
silver reduced at the expense of the organic substances. In 1889 Carey
Lea, in his researches on this class of reactions, showed that soluble

The resultant precipitate of silver oxalate is dried, mixed with an equal weight of dry
sodium carbonate, and thrown into a gently-heated crucible. The separation of the
silver then proceeds without an explosion, whilst the silver oxalate if heated alone
decomposes with explosion.

According to Stas, the best method for obtaining silver from its solutions is by the
reduction of silver chloride dissolved in ammonia by means of an ammoniacal solution of
cuprous thiosulphate ; the silver is then precipitated in a crystalline form. A solution of
ammonium sulphite may be used instead of the cuprous salt.

16 Silver is very malleable and ductile; it may be beaten into leaves 0'002 mm. in
thickness. Silver wire may be made so fine that 1 gram is drawn into a wire 2£ kilo-,
metres long. In this respect silver is second only to gold. A wire of 2 mm. diameter
breaks under a strain of 20 kilograms.

17 In melting, silver absorbs a considerable amount of oxygen, which is disengaged on
solidifying. One volume of molten silver absorbs as much as 22 volumes of oxygen. In
solidifying, the silver forms cavities like the craters of a volcano, and throws qff metal,
owing to the evolution of the gas } all these phenomena recall a volcano on a miniature
scale (Dumas). Silver which contains a small quantity of copper or gold, &c., does not
.show this property of dissolving oxygen.

The absorption of oxygen by molten silver is, however, an oxidation, but it is at the
same time a phenomenon of solution. One cubic centimetre of molten silver can
dissolve twenty-two cubic centimetres of oxygen, which, even at 0°, only weighs 0'08
gram, whilst 1 cubic centimetre of silver weighs at least 10 grams, and therefore it is
impossible to suppose that the absorption of the oxygen is attended by the formation of
any definite compound (rich in oxygen) of silver and oxygen (about 45 atoms of silver to
1 of oxygen) in any other but a dissociated form, and this is the state in which sub-
stances in solution must be regarded (Chapter I.)

Le Chatelier showed that at '300° and 15 atmospheres pressure silver absorbs so
much oxygen that it may be regarded as having formed the compound Ag4O, or a
mixture of Ag2 and Ag2O. Moreover, silver oxide, Ag20, only decomposes at 800° under,
low pressures, whilst at pressures above 10 atmospheres there is no decomposition at
800° but only at 400°.

Stas showed that silver is oxidised by air in the presence of acids. V. d. Pfordten
.confirmed this, and showed that an acidified solution of potassium permanganate rapidly
dissolves silver in the presence of air.

COPPER, SILVER, AND GOLD 421,

silver is here formed, which he called allotropic silver. It may be
obtained by taking 200 c.c. of a 10 per cent, solution of AgNO3 and
quickly adding a mixture -(neutralised with NaHO) of 200 c.c. of a
80 per cent, solution of FeSO4 and 200 c.c. of a 40 per cent, solution
of sodium citrate. A lilac precipitate is obtained, which is collected
on a filter (the precipitate becomes blue) and washed with a solution of
NH4NO3. It then becomes soluble in pure water, forming a red
perfectly transparent solution from which the dissolved silver is preci-
pitated on the addition of many soluble foreign bodies. Some of the
latter — for instance, NH4N03, alkaline sulphates, nitrates, and citrates
— give a precipitate which redissolves in pure water, whilst others — for
instance, MgS04, FeS04, K2Cr2O7, AgNO3, Ba(N03)2 and many others-
convert the precipitated silver into a new variety, which, although no
longer soluble in water, regains its solubility in a solution of borax
and is soluble in ammonia. Both the soluble and insoluble silver are
rapidly converted into the ordinary grey-metallic variety by sulphuric
acid, although nothing is given off in the reaction ; the same changd
takes place on ignition, but in this case CO2 is disengaged ; the latter
is formed from the organic substances which remain (to the amount of
3 per cent.) in the modified silver (they are not removed by soaking in
alcohol or water). If the precipitated silver be slightly washed and
laid in a smooth thin layer on paper or glass, it is seen that the soluble
variety is red when moist and a fine blue colour when dry, whilst the in-
soluble variety has a blue reflex. Besides these, under special conditions 18

18 When solutions of AgNO'3, FeSO4, sodium citrate, and NaHO are mixed together
in the manner described above, they throw down a precipitate of a beautiful lilac
colour; when transferred to a filter paper the precipitate soon changes colour, and
becomes dark blue. To obtain the, substance as pure as possible it is washed with a
5-10 p.c. solution of ammonium nitrate; the liquid is decanted, and 150 c.c. of water
poured Over the precipitate. It then dissolves entirely in the water. A small quantity of
a saturated solution of ammonium nitrate is added to the solution, and the silver
in solution again separates out as a precipitate. These alternate solutions and
precipitations are repeated seven or eight times, after which 'the precipitate is trans-
ferred to a filter and washed with 95 jp.c. alcohol until the filtrate gives no residue oil
evaporation. An analysis of the substance so obtained showed that it contained from
97'18 p.c. to 97'31 p.c. of metallic silver. It remained to discover what the remaining
2-3 p.c. were composed of. Are they merely impurities, or is the substance sonje com-
pound of silver with oxygen or hydrogen, or does it contain citric acid in combination
which might account for its solubility ? The first supposition is set aside by the fact'
that no gases are disengaged by the precipitate of silver, either under the action of gases
or when heated. The second supposition is shown to be impossible by the fact that
there is no definite relation between the silver and citric acid. A determination of the
amount of silver in solution showed that the amount of citric acid varies greatly for one
and the same amount of silver, and there is no simple ratio between them. Among
other methods of preparing soluble silver given by Carey Lea, we may mention the
method published by him in 1891. AgNO3 is added to a solution of dextrine in caustic
soda or potash ; at first a precipitate of brown oxide of silver is thrown down, but the

422 PEINCIPLES OF CHEMISTKY

a golden yellow variety may be obtained, which gives a brilliant golden,
yellow coating on glass ; but it is easily converted into the ordinary
grey- metallic state by friction or trituration. There is no doubt 18 bis
that there is the same relation between ordinary silver which is per-
fectly insoluble in water and the varieties of silver obtained by Carey
Lea 18 tri as there is between quartz and soluble silica or between

brown colour then changes into a reddish chocolate, owing to the reduction of the silver
by the dextrine, and the solution turns a deep red. A few drops of this solution turn
water bright red, and give a perfectly transparent liquid. The dextrine solution is pre-
pared by dissolving 40 grams of caustic soda and the same amount of ordinary brown
dextrine in two litres of water. To this solution is gradually added 28 grams of AgNOs
dissolved in a small quantity of water.

The insoluble allotropic silver is obtained, as was mentioned above, from a solution
of silver prepared in the manner described, by the addition of sulphate of copper,
iron, barium, magnesium, &c. In one experiment Lea succeeded hi obtaining the
insoluble allotropic Ag in a crystalline form. The red solution, described above, after
standing several weeks, deposits crystals spontaneously in the form of short black
needles and thin prisms, the liquid becoming colourless. This insoluble variety, when
rubbed upon paper, has the appearance of bright shining green flakes, which polarise
light.

The gold variety is obtained in a different manner to the two other varieties. A
solution is prepared containing 200 c.c. of a 10 p.c. solution of nitrate of silver, 200 c.c.
of a 20 p.c. solution of Rochelle salt, and 800 c.c. of water. Just as in the previous" case
the reaction consisted in the reduction of the citrate of silver, so in this case it consists
in the reduction of the tartrate, which here first forms a red, and then a black precipitate
of allotropic Ag, which, when transferred to the, filter, appears of a beautiful bronze
colour. After washing and drying, this precipitate acquires the lustre and colour
peculiar to polished gold, and this is especially remarked where the precipitate comes
into contact with glass or china. An analysis of the golden variety gave a percentage
composition of 98'750 to 98'749 Ag. Both the insoluble varieties (the blue and gold)
.have a different specific gravity from ordinary silver. Whilst that of fused silver is 10'50,
and of finely-divided silver 10'62, the specific gravity of the blue insoluble variety is 9'58,
and of the gold variety 8'51r The gold variety passes into ordinary Ag with great ease.
This transition may even be remarked on the filter in those places which have acciden-
tally not been moistened with wate^r. A simple shock, and therefore friction of one
particle upon another, is enough to convert the gold variety into normal white silver.
Carey Lea sent samples of the gold variety for a long distance by rail packed in three
.tubes, in which the silver occupied about the quarter of their volume ; in one tube only
he filled up this space with cotton-wool. It was afterwards found that the shaking of
the particles of Ag had completely converted it into ordinary white silver, and that only
the tube containing the cotton-wool had preserved the golden variety intact.

The soluble variety of Ag also passes into the ordinary state with great ease, the
heat of conversion being, .as Prange showed in 1890, about + 60 calories.

18 bio The opinion of the nature of soluble silver given below was first enunciated in
the Journal of the Russian Chemical Society, February 1, 1890, Vol. XXII., Note 78.
This view is, at the present tune, generally accepted, and this silver is frequently known
AS the ' colloid ' variety. \ may add that Carey Lea observed the solution of ordinary
molecular silver in ammonia without the access of air.

18 trt It is, however, noteworthy that ordinary metallic lead has long been considered
soluble in water, that boron has been repeatedly obtained in a 'brown solution, and that
observations upon the development of, certain bacteria have shown that the latter die in
water which has been for some time in contact with metals. This seems to indicate the
passage of small quantities of metals into water (however, the formation of peroxide of
hydrogen may be supposed to have eorae influence in these oases)

COPPER, SILVER, AND GOLD 42 8/

CuS and As2S2 in their ordinary insoluble forms and in the state of
the colloid solution of their hydrosols (see Chapter I., Note 57, and
Chapter XVII.,. Note 25 bis). Here, however, an important step in
advance has been made in this respect, that we are dealing with the
solution of a simple body, and moreover of a metal — i.e. of a particu-
larly characteristic state of matter. And as boron, gold, and certain
other simple bodies have already been obtained in a soluble (colloid)
form, and as numerous organic compounds (albuminous substances,
gum, cellulose, starch, &c.) and inorganic substances are also known in
this form, it might be said that the colloid state (of hydrogels and hydro-
sols) can be acquired, if not by every substance, at all .events by sub<-
stances of most varied chemical character under particular conditions
of formation from solutions. And this being the case, we may hope
that a further study of soluble colloid compounds, which apparently
present various transitions towards emulsions, may throw a new light
upon the complex question of solutions, which forms one of the problems
of the present epoch of chemical science. Moreover, we may remark that
Spring (1890) clearly proved the colloid state of soluble silver by means
of dialysis as it did not pass through' the membrane.

As regards the capacity of silver for chemical reactions, it is
remarkable for its small capacity for combination with oxygen, and for
its considerable energy of combination with sulphur, iodine, and cer-
tain kindred non-metals. Silver does not oxidise at any temperature,
and its oxide, Ag20, is decomposed by heat. It is also a very impor-
tant fact that silver is not oxidised by oxygen either in the presence of
alkalis, even at exceedingly high temperatures, or in the presence of
acids — at least, of dilute acids — which properties render it a very
important metal in chemical industry for the fusion of alkalis, and also
for many purposes in everyday life ; for instance, for making spoons,
salt-cellars, <fec. Ozone, however, oxidises it. Of all acids nitric acid
has the greatest action on silver. The reaction is accompanied by the
formation of oxides of nitrogen and sjrver nitrate, AgNO3, which
dissolves in water and does not, therefore, hinder the further action of
the acid on the metal. The halogen acids, especially hydriodic acid,
act on silver, hydrogen being evolved ; but this action soon stops,
owing to the halogen compounds of silver being insoluble in water and
only very slightly soluble in acids ; they therefore preserve the remaining
mass of metal from the further action of the acid ; in consequence of
this the action of the halogen acids is only distinctly seen with finely-
divided silver. Sulphuric acid acts on silver in the same manner that
it does on copper, only it must be concentrated and at a higher
temperature. Sulphurous anhydride, and not hydrogen, is then evolved.

424 PEINCIPLES OF CHEMISTRY

but there is no action at the ordinary temperature, even in the presence
of air. Among the various salts, sodium chloride (in the presence of
moisture, air, and carbonic acid) and potassium cyanide (in the presence
of air) act on silver more decidedly than any others, converting it respec-
tively into silver chloride and a double cyanide.

Although silver does not directly combine with oxygen, still three
different grades of combination with oxygen may be obtained indi-
rectly from the salts of silver. They are all, however, unstable, and
decompose into oxygen and metallic silver when ignited. These three
oxides of silver have the following composition : silver suboxide,
Ag4O,19 corresponding with the (little investigated) suboxides of the
alkali metals ; silver oxide, Ag2O, corresponding with the oxides of the
alkali metals and the ordinary salts of silver, AgX ; and silver peroxide,
AgO,19 bis or, judging from Berthelot's researches, Ag2O3. Silver oxide
is obtained as a brown precipitate (which when dried does not contain
water) by adding potassium hydroxide to a solution of a silver salt —
for example, of silver nitrate. The precipitate formed seems, however,

19 Silver suboxide (Ag40) or argentous oxide is obtained from argentic citrate by
heating it to 100° in a stream of hydrogen. Water and argentous citrate are then
formed, and the latter, although but slightly soluble in water, ' gives a reddish'
brown solution of colloid silver (Note 18), and: when boiled this solution becomes
colourless and deposits metallic silver, the argentic salt being again formed. Wb'hler,
who discovered this oxide, obtained it as a black precipitate by adding potassium
hydroxide to the above solution of argentous citrate. With hydrochloric acid the
suboxide gives a brown compound, Ag2Cl. Since the discovery of soluble silver the
above data cannot b§ Regarded as perfectly trustworthy ; it is probable that A mixture of
Ag2*and Ag20 was. being dealt with, so that the actual existence of Ag4O is' now
doubtful, but there can be no doubt as to. the formation of a subchloride, Ag2Cl (see
Note 25), corresponding to the suboxide. The same compound is obtained by the action
of light on the higher chloride. Other, acids do .not combine with silver suboxide, but
convert it into an argentic salt and metallic silver. In this respect cuprous oxide
presents a certain .resemblance to these suboxides. But copper forms a suboxide of
the composition Cu4O, which is obtained by the action of an alkaline solution of
etannous oxide on cupric hydroxide, and is decomposed by acids into cupric salts and
metallic copper. The problems offered by the suboxides, as well as by the peroxides,
cannot be considered as fully solved.

19 bls Silver peroxide, AgO or Ag2O3, is obtained by the decomposition of a dilute
(10 p.c.) solution of silver nitrate by the action of a galvanic current (Hitter). On the
positive pole, where oxygen is usually evolved in the decomposition of salts, brittle grey
needles with a metallic lustre, which occasionally attain a somewhat considerable size,
are then formed. They are insoluble in water, and decompose with the evolution of
oxygen when they are dried.. and heated, especially up to 160°, and, like lead dioxide,
barium peroxide, &c., their action is strongly oxidising. When treated with acids, oxygen
is evolved and a salt of the oxide formed. Silver peroxide absorbs sulphurous anhydride
and forms silver sulphate. Hydrochloric acid evolves chlorine ; ammonia reduces the
silver, and is itself oxidised, forming water and gaseous nitrogen. Analyses of the above-
mentioned crystals show that they contain silver nitrate, peroxide, and water. According
to Fisher, they have the composition 4AgO,AgN05,H8O, and, according to Bertbelot,

COPPER, SILVER, AND GOLD 425

to be an hydroxide, AgHO, t.e. AgN03 + KHO = KNO3 + AgHO,
and the formation of the anhydrous oxide, 2AgHO = Ag2O + H2O,
may be compared with the formation of the anhydrous cupric oxide by
the action of potassium hydroxide on hot cupric solutions. Silver
hydroxide decomposes into water and silver oxide, even at low
.temperatures ; at least, the hydroxide no longer exists at 60°, bub
'forms the anhydrous oxide, Ag2O.19tri Silver oxide is almost
.insoluble in water ; but, nevertheless, it is undoubtedly a rather
powerful basic oxide, because it displaces the oxides of many metals
from their soluble salts, and saturates such acids as nitric acid,
forming with them neutral salts, which do not act on litmus paper.20
Undoubtedly water dissolves a small quantity of silver oxide,
which explains the possibility of its action on solutions of salts — for
example, on solutions of cupric salts. Water in which silver oxide
is shaken up has a distinctly alkaline reaction. The oxide is dis-
tinguished by its great instability when heated, so that it loses all its
oxygen when slightly heated. Hydrogen reduces it at about 80°.20 bis
The feebleness of the affinity of silver for oxygen is shown by the fact
that silver oxide decomposes under the action of light, so that it must be
kept in opaque vessels. The silver salts are colourless and decompose
when heated, leaving metallic silver if the elements of the acid are
volatile.20 tri They have a peculiar metallic taste, and are exceedingly
poisonous ; the majority of them are acted on by light, especially in
the presence of organic substances, which are then oxidised. The
alkaline carbonates give a white precipitate of silver carbonate,
•A.g2C03, which is insoluble in water, but soluble in ammonia and
ammonium carbonate. Aqueous ammonia, added to a solution of a
normal silver salt, first acts like potassium hydroxide, but the precipitate
dissolves in an excess of the reagent, like the precipitate of cupric

19 trl According to Carey Lea, however, oxide of silve? still retains water even at 100°,
and only parts with it together with the oxygen. Oxide of silver is used for colouring
glass yellow.

20 The reaction of Pb(OH)2 upon AgHO in the presence of NaHO leads to the
formation of a compound of both oxides, PbOrtAg3O,' from which the oxide of lead
cannot be removed by alkalies (Wbhler, Leton). t Wb'hler, Welch, and others obtained
crystalline double salts, EsAgX3, by the action of strong solutions of EX of the halogen
salts of the alkaline metals upon AgX, where R = Cs, Eb, K.

20t>i» According to Miiller, ferric oxide is reduced by hydrogen" (see Chapter XXII.,
Note 5) at 295° (into what ?), cupric oxide at 140°, NL,Os«t, 150° ; nickelous oxide, NiO,
js reduced to the suboxide, Ni^O, at 195°, and to nickel at 270° ; zinc oxide requires so
high a temperature for its reduction that the glass tube ia which Miiller conducted the
experiment did not stand the heat ; antimony oxide requires a temperature of 215° for
its reduction ; yellow mercuric oxide is reduced at 180° and the red oxide at 230° ; silver
oxide at 85°, and. platinum oxide even at the ordinary temperature.

20 tH A silica compound, Ag2OSiO2 is obtained by fusing AgNO5 with eilica; this salt
is able to decompose with the evolution of oxvgen, leaving Ag + SiOa.

426 PRINCIPLES OF CHEMISTRY

hydroxide.21 Silver oxalate and the halogen compounds of silver are
insoluble in water; hydrochloric acid and soluble chlorides give,
as already repeatedly observed, a white precipitate of silver chloride
in solutions of silver salts. Potassium iodide gives a yellowish
precipitate of silver iodide. Zinc separates all the silver in a metallic.'
form from solutions of silver salts. Many other metals and reducing
agents — for example, organic substances — also reduce silver from the
solutions of its salts.

Silver nitrate, AgNO3, is known by the name of lunar caustic
(or lapis infernalis) ; it is obtained by dissolving metallic silver
in nitric acid. If the silver be impure, the resultant solution will
contain a mixture of the nitrates of copper and silver. If this mixture'
be evaporated to dry ness and the residue carefully fused at an\
incipient red heat, all the cupric nitrate is decomposed, whilst the
greater part of the silver nitrate remains unchanged. On treating
the fused mass with water the latter is dissolved, whilst the cupric
oxide remains insoluble. If a certain amount of silver oxide be added
to the solution containing the nitrates of silver and copper, it displaces
all the cupric oxide. In this case it is of course not necessary to take"
pure silver oxide, but only to pour off some of the solution and to add
potassium hydroxide to one portion, and to mix the resultant pre-
cipitate of the .hydroxides, Cu(OH)2 and AgOH, with the remaining
portion.22 By these methods all the copper can be easily jremoved and

21 If a solution of a silver salt be precipitated by sodium hydroxide, and aqueous
ammonia is added drop by drop until the precipitate is completely dissolved, the
liquid when evaporated deposits a violet mass of crystalline silver oxide. If moist silver
oxide be left in a strong solution of ainm6nia it gives a black mass, which easily decom-
poses with a loud explosion, especially when struck. This black substance is called
fulminating silver. Probably this is a compound like the other compounds of oxides
with ammonia, and in exploding the oxygen of the silver oxide forms water with the
hydrogen of the ammonia, which is naturally accompanied by the evolution of heat and
formation of g&seous nitrogen, or, as Raschig states, fulminating silver contains NAg3 or
o&e of the amides (for instance, NHAg2=NH3 + Ag2O — H2O). Fulminating silver is also
formed when potassium hydroxide is added to a solution of silver nitrate in ammonia.
The dangerous explosions which are produced by, this compound render it heedful that,
great care be taken when salts of silver come into contact with ammonia and alkalis*
{see Chapter XVI., Note 26).

•' w So that we here encounter the following phenomena : copper displaces silver from
jbhe solutions of , its salts, and silver oxide displaces copper oxide from cupric salts.
Guided by the conceptions enunciated in Chapter XV., we can account for this in the
following manners The atomic volume of silver =10-8, and of copper = 7'2, of silver
oxide ='82, and of copper oxide = 18. A greater contraction has 'taken place in the for*
iqiaition of cupric oxide, CuO, than in the formation of silver oxide, Ag2O, since in the
former (18—7 = 6) the volume after combination with the oxygen has increased by very
little, whilst the volume of silver oxide is considerably greater than that of the metal it
contains [82 - (2 x ID'S) = 11*4]. Hence silver oxide is less compact than cupric oxide,
and is therefore less stable ; but, on the other hand, there are greater intervals
between.the atoms in silver oxide than in cupric oxide, and therefore silver oxide is able to

COPPER, SILVER, AND GOLD 427

pure silver nitrate obtained (its solution is colourless, while the presence
of Cu renders it blue), which may be ultimately purified by crystallisa-
tion. It crystallises in colourless transparent prismatic plates, which
are not acted on by air. They are anhydrous. Its sp. gr. is 4'34 ; it
dissolves in half its weight of water at the ordinary temperature. 22bis
The pure salt is nob acted on by light, but it easily acts in an oxidising
manner on the majority of organic substances, which it generally
blackens. This is due to the fact that the organic substance is oxidised
by the silver nitrate, which is reduced to metallic silver ; the latter is
thus obtained in a finely-divided state, which causes the black stain.
This peculiarity is taken advantage of for marking linen. Silver nitrate
is for the same reason used for cauterising wounds and various growths
•on the body. Here again it acts by virtue of its oxidising capacity in
destroying the organic matter, which it oxidises, as is seen from the
separation of a coating of black metallic powdery silver from the part
^cauterised.22 tri From the description of the preparation of silver nitrate
it will have been seen that this salt, which fuses at 218°, does not

give more stable compounds than those of copper oxide. This is verified by the figures
and data of their reactions. It is impossible to calculate for cupric nitrate, because this
salt has not yet been obtained in. an anhydrous state ; but the sulphates of both oxides
are known. The specific gravity of copper sulphate in an anhydrous state is 3'53, and of
silver sulphate 5'36; the molecular volume of the former is 45, and of the latter 58.
The group S03 in the" copper occupies, as it were, a volume 45-18 = 82, and in the silver
palt a volume 58 — 82 = 26 ; hence a smaller contraction has taken place in the formation
of the copper salt from the oxide than in the formation of the silver salt, and conse-
quently the latter should be more stable than the former. Hence silver oxide is
able to decompose the salt of copper oxide, whilst with respect to the metals both salts
have been formed with an almost identical contraction, since 58 volumes of the silver
salt contain 21 volumes of metal (difference = 87), and 45 volumes of the copper salt
contain 7 volumes of copper (difference = 38). Besides which, it must be observed that
copper o'xide displaces iron oxide'/ just .as silver, oxide displaces copper oxide. Silver,
.copper, and iron, in the form of oxides, displace each other in the above order, but in the
'form of metals in a reverse order (iron, copper, silver). The cause of this order of the
displacement of the oxides lies, amongst other things, in their composition. They have
the composition Ag20, Cu2O2, Fe2O3 ; the oxide containing a less proportion of oxygen
; displaces that containing a larger proportion, because the basic character diminishes
with the increase of contained oxygen.

Copper also displaces mercury from its salts. It may here be remarked that Spring
(1888), on leaving a mixture of dry mercurous chloride and copper for two hours,
observed a distinct reduction, which belongs to the category of those phenomena which
demonstrate the existence of a mobility of parts (i.e. atoms and molecules) in solid- sub-
stances.

22 bis The reaction of 1 part' by weight of AgN03 requires (according to Kremers) the
following amounts of water : at 0°, 0'82 part, at 19°'5, 0'41 part, at 54°, 0'20 part,
at 110°, 0-09 part, and, according to Tilden, at 125°, 0-0617 part, and at 188°, 0'0515
.part.

»2trl It may be remarked that the black stain produced by the reduction of metallic
silver disappears under the action of a solution of. mercuric chloride or of potassium
Cyanide, because these salts act on finely-divided silver,

428 PRINCIPLES OF CHEMISTRY

decompose at an incipient red heat ; when cast into sticks it is usually
employed for cauterising. On further heating,. the fused salt undergoes
decomposition, first forming silver nitrite and then metallic .silver.
With ammonia, silver nitrate forms, on evaporation of the solution,
colourless crystals containing AgNO3,2HN3 (Marignac). In general
the salts of silver, like cuprous, cupric, zinc, &c. salts, are able to give
several compounds with ammonia j for example, silver nitrate in a dry
state absorbs three molecules (Rose). The ammonia is generally easily
expelled from these compounds by the action of heat.

Nitrate of silver easily forms double salts like AgN032NaNO3 and
AgNO3KNO3. Silver nitrate under the action of water and a halogen
gives nitric acid (see Vol. I. p* 280, formation of N2O5), a halogen salt of
silver, and a silver salt of an oxygen acid of the halogen. Thus, for
example, a solution of chlorine in watery when mixed with a solution of
silver nitrate, gives silver chloride and chlorate. It is here evident that
the reaction of the silver nitrate is identical with the reaction of the
caustic alkalis, as the nitric acid is all set free and the silver oxide x>nly
reacts in exactly the same way in which aqueous potash acts on free
chlorine. Hence the reaction may be expressed in the following
manner : 6AgNO3 + 3C12 + 3H20 = SAgCl + AgC103 + 6NH03.

Silver nitrate, like the nitrates of the alkalis, does not contain any
water of crystallisation. Moreover the other salts of silver almost
always separate out without any water of crystallisation. The silver
salts are further characterised by the fact that they give neither
basic nor acid salts, owing to which the formation of silver salts
generally forms the means of determining the true composition of
acids — thus, to any acid HnX there corresponds a salt AgnX — for
instance, Ag3P04 (Chapter XIX., Note 15).

Silver gives insoluble and exceedingly stable compounds with the
halogens. They are obtained by double decomposition with great
facility whenever a silver salt comes in contact with halogen salts.
Solutions of nitrate, sulphate, and all other kindred salts of silver give
a precipitate of silver chloride or iodide in solutions of chlorides and
iodides and 6f the halogen acids, because the halogen salts of silver are
insoluble both in water 23 and in other acids. Silver chloride, AgCl, is

25 Silver chloride is almost perfectly insoluble in water, but is somewhat- soluble i»
water containing sodium chloride or hydrochloric acid, or other chlorides, and many salts,
in solution. Thus at 100°, 100 parts of water saturated with sodium chloride dissolve
0'4 part of silver chloride. Bromide and iodide of silver are less soluble in this respect,
as also in regard to other solvents. It should be remarked that silver chloride dissolves
in solutions of ammonia, potassium cyanide, and of sodium thiosulphate, Na2S2O5.
Silver bromide is almost perfectly analogous to the chloride, but silver iodide is nearly
insoluble in a solution of ammonia. Silver ehloride even absorbs dry ammonia g»%

COPPER, SILVER, AND GOLD 429

then obtained as a white flocculent precipitate, silver bromide forms a
yellowish precipitate, and silver iodide has a very distinct yellow
colour. These halogen compounds sometimes occur in nature ; they
are formed by a dry method — by the action of halogen compounds on
silver compounds, especially under the influence of heat. Silver chlo-
ride easily fuses at 451° on cooling from a molten state ; it forms
a somewhat soft horn-like mass which can be cut with a knife
and is known as horn silver. It volatilises at a higher tempera-
ture. Its ammoniacal solution, on the evaporation of the ammonia,
deposits crystalline chloride of silver, in octahedra. Bromide and
iodide of silver also appear in forms of the regular system, so that in
this respect the halogen salts of silver resemble the halogen salts of the
alkali metals;24

forming very unstable ammoniacal compounds. When heated, these compounds (Vol. I.
p. 250, Note 8) evolve the ammonia, as they also do under the action of all acids. Silver
chloride enters into double decomposition with potassium cyanide, forming a soluble
double cyanide, which we shall presently describe ; it also forms a soluble double salt,
NaAgS2O3, with sodium thiosulphate.

Silver chloride offers different modifications in the structure of its molecule, as is seen
in the variations in the consistency of the precipitate, and in the differences in the action
of light which partially decomposes AgCl (see Note 25), Stas and Carey Lea investigated
this subject, which has a particular importance in photography, because silver bromide
also gives photo-salts. There is still much to be discovered in this respect, since Abney
showed that perfectly dry AgCl placed in a vacuum in the dark is not in the least acted
upon when subsequently exposed to light.

24 Silver bromide and iodide (which occur as the minerals bromite and iodite)
resemble the chloride in many respects, but the degree of affinity of silver for iodine is
greater than that for chlorine and bromine, although less heat is evolved (see Note 28 Bis),
Deville deduced this fact from a number of experiments. Thus silver chloride, when
treated with hydriodic acid, evolves hydrochloric acid, and forms silver iodide. Finely-
divided silver easily liberates* hydrogen when treated with hydriodic acid ; it produces
the same decomposition with hydrochloric acid, but in a considerably less degree and
only on the surface. 'The difference between silver chloride and iodide is especially
remarkable, sinco the formation of the former is attended with a greater contraction
than that of the latter. The volume of AgCl = 26 ; of chlorine 27, of silver 10, the sum
= 87, hence a contraction has ensued j and in the formation of silver iodide an expansion
takes place, for the volume of Ag is 10, of I 26, and of Agl 89 instead of 86 (density,
AgCl, 5'59 ; Agl, 5'67). The atoms of chlorine have united with the atoms of silver
without moving asunder, whilst the atoms of iodine must have moved apart in
combining with, the silver. It is otherwise with respect to the metal ; the distance
between its atoms in the metal = 2-2, in silver chloride' =8'0, and in silver iodide
= 8'5 ; hence its atoms have moved asunder considerably in both cases. It is also very
remarkable, as Fizeau observed, that the density of silver iodide increases with a rise oi
temperature — that is, a contraction takes place when it is heated and an expansion wheq
it is cooled.

In order to explain the fact that in silver compounds the iodide is more stable than
the chloride and oxide, Professor N. N. Beketoff, in his '.Researches on the Phenomena
of Substitutions ' (Kharkoff, 1865), proposed the following original hypothesis, which we
will give in almost the words of the author:— In the case of aluminium, the oxide, Al90g,
is more stable than the chloride, A12C16, and the iodide, A12I0. In the oxide the amount
of the metal is to the amount of the element, combined with it as 54*8 (Al» 87'8) iff to 48»

430 PRINCIPLES QE CHEMISTRY

Silver chloride may be decomposed, with the separation of silver
oxide, by heating it with a solution of an alkali, and if an organic

or in the ratio 112 : 100 ; for the chloride the ratio is = 55 : 100 ; for the iodide it = 7 : 100
In the case of silver the oxide (ratio = 1350 : 100) is less stable than the chloride (ratio
= 804 : 100), and the iodide (ratio of the weight of metal to the weight of the halogen
= 85 : 100) is the most stable. From these and similar examples it follows that the most
stable compounds are those in which the weights of the combined substances are equal.
This may be partly explained by the attraction of similar molecules even after their
having passed into combination with others. This attraction is proportional to the
product of the acting masses. In silver oxide the attraction of Ag2 for Ag2 = 216x 216
= 46,656, and the attraction of Ag2 for O = 216 x 16 = 3,456. The attraction of like mole-
cules thus counteracts the attraction of the unlike molecules. The former naturally
does not overcome the latter, otherwise there would be a. disruption, but it nevertheless
diminishes the stability. In the case of an equality or proximity of the magnitude of
the combining masses, the attraction of the like parts will counteract the stability of the
compound to the least extent — in other words, with an inequality of the combined masses,
the molecules have an inclination to return to an elementary state, to decompose, which
does riot exist to such an extent where the combined masses are equal. There is, there-
fore, a tendency for large masses to combine with large, and for small masses to combine
with small. Hence' Ag2O + 2KI 'gives K2O + 2AgI. The influence of an equality of
masses on the stability is seen particularly clearly in the effect of a rise of temperature.
Argentic, mercuric, auric and other oxides composed of unequal masses, are somewhat
readily decomposed by heat, whilst the oxides of the lighter metals (like water) are not so
easily decomposed by heat. Silver chloride and iodide approach the condition of
equality, and are not decomposed by heat. The most stable oxides under the action of
heat are those of magnesium, calcium, silicon, and aluminium, since they also approach
the condition of equality. For the same reason hydriodic acid decomposesrwith greater
facility than hydrochloric acid. Chlorine does not act on magnesia or alumina, but it
acts on lime and silver oxide, &c. This is partially explained by the fact that by con-
sidering heat as a mode of motion, and knowing that the atomic heats of the free elements
are equal, it must be supposed that the amount of the motion of atoms (their vis viv.a) is
equal, and as it is equal to the product of the mass (atomic weight) into the square of the
velocity, it follows that the greater the combining weight the smaller will be the square
of the velocity, and if the combining weights be nearly equal, then the velocities also will
be nearly equal. Hence the greater the difference between the weights of the combined
atoms the greater will be the difference between their velocities. The difference between
the velocities will increase with the temperature, and therefore the temperature of de-
composition will be the sooner attained the greater be. the original difference— that is,
the greater the difference of the weights of the combined substances. The nearer these
weights are to each other, the more analogous the motion of the unlike atoms, and con-
sequently, the more stable the resultant compound.

The instability of cupric chloride and nitric oxide, the 'absence of compounds of fluorine
with oxygen, whilst there are compounds of oxygen with chlorine, the greater stability of
the oxygen compounds of iodine than those of chlorine, the stability of boron nitride, and
the instability of cyanogen, and a number of similar instances, T> here, j udging from the above
argument, one would expect (owing to the closeness of the atomic weights) a stability,
show that Beketoff's addition to the mechanical theory of chemical phenomena is still
far from sufficient for explaining the true relations of affinities. Nevertheless, in his
mode of explaining the relative stabilities of compounds,, we find an exceedingly interest-
ing treatment of questions of primary importance. Without such efforts it would be
unpossible;.to generalise the complex data of experimental knowledge.

Fluoride of silver, AgF, is obtained by dissolving Ag.2O or Ag.2CO3 in hydrofluoric acid.
It differs from the other halogen salts of silver in being soluble in water (1 part of salt in
0'55 of water). It crystallises from its solution in prisms, AgFH2O (Marignac), or AgF2H2O
(Pfaundler), which lose their water in vacuo. Guntz (1891), by electrolising a saturated

COPPER, SILVER, AND OO.LD 431

substance be added to the alkali the chloride can easily be reduced to
metallic silver, the silver oxide being reduced in the oxidation of the
organic substance. Iron, zinc, and many other metals reduce silver
chloride in the presence of water. Cuprous and mercurous chlorides
and many organic substances are also able to reduce the silver from
chloride of silver. This shows the rather easy decomposability of the
halogen compounds of silver; Silver iodide is much more stable in this
respect than the chloride. The same is also observed with respect to
the action of light upon moist AgCL White silver chloride soon acquires
a violet -colour when exposed to the action of light, and especially
under the direct action of the sun's rays. After being acted upon
by light it is no longer entirely soluble in ammonia, ,but leaves
metallic silver undissolved, from which it might be assumed that the
action of light consisted in the decomposition of the silver chloride
into chlorine and metallic silver and in fact the silver chloride be-
comes in time darker and darker. Silver bromide and iodide are much
more slowly acted on by light, and, according to certain observations,
when pure they are even quite unacted on ; at least they do not change
in weight,24 bi* so that if they are acted on by light, the change they
undergo must be one of a change in the structure of their parts and not
of decomposition, as it is in silver chloride. The silver chloride under
the action of light changes in weight, which indicates the formation of
a volatile product, and the deposition of metallic silver on dissolving
in ammonia shows the loss of chlorine. The change does actually
occur under the action of light, but the decomposition does not go
as far as into chlorine and silver, but only to the formation of a sub-
chloride of silver, Ag201, which is of a brown colour and is easily de-
composed into metallic silver and silver chloride, Ag2Cl = AgCl + Ag.
This change of the chemical composition and structure of the halogen
salts of silver under the action of light forms the basis of photography,
because the halogen compounds of silver, after having been exposed to
light, give a precipitate of finely-divided silver, of a -black colour,
when treated with reducing agents.25

solution of Ag2F, obtained poly fluoride of silver, Ag2F, which is decomposed by water
into AgF + Ag. It is also formed by the action of a strong solution of AgF upon finely-
divided (precipitated) silver.

24 bu The changes brought about by the action of light necessitate distinguishing the
photo-salts of silver.

35 In photography these are called ' developers.' The most common developers are :
solutions of ferrous sulphate, pyrogallol, ferrous oxalate, hydroxylamine, potassium sul-
phite, hydroquinone (the last acts particularly well and is very convenient to use), &c. The
chemical processes of photography are of great practical and theoretical interest ; but it
would be impossible in this work to enter into this special branch of chemistry, which has as
yet been very little worked out from a theoretical point of view. Nevertheless, we .will pause

482 . PRINCIPLES OF CHEMISTRY

The insolubility of the halogen compounds of silver forms tlid
basis of many methods used in practical chemistry. Thus by means of
this reaction it is possible to obtain salts of other acids from a halogen
salt of a given metal, for instance, RC12 + 2AgN03=R(N03)2 4- 2AgCl. ,
The formati6n of the halogen compounds of silver is very frequently
used in thes investigation of organic substances ; for example, if any
product of metalepsis containing iodine or chlorine be heated with a
silver salt or silver oxide, the silver combines with the halogen and
gives a halogen salt, whilst the elements previously combined with the
silver replace thq halogen. For instance, ethylene dibromide, C2H4Br2,
is transformed^ into ethylene diacetate, C2H4(C2H3O2)2l and silver

to consider certain^ aspects of this subject which are of a purely chemical interest, and
especially the facts concerning subchloride of silver, AgoCl (see -Note 19), and the photo-
salts (Note 28). There is no doubt that under the action of light, AgCl becomes darker
in .colour, decreases iu weight, and probably forms a mixture of AgCl, Ag2Cl, and Ag.
But the isolation of the subchloride has' only been recently accomplished by Giintz by
means of the Ag2F, discovered by him (see Note 24). Many chemists (and among them
.Hodgkinson) assumed that an oxychloride of silver was formed by the decomposition of
AgCl under the action of light. Carey Lea's (1889) and A. Richardson's (1891) experiments
showed that the product formed does not, however, contain any oxygen aft all, and the
change in colour produced by the-, action of light upon AgCl is most probably due to the
formation of Ag2Cl. This substance was isolated by Giintz (1891) by passing HC1 over
crystals of AgoF. He also obtahied Ag2I in a similar manner by passing HI, and Ag2S
by passing H2S over AgoF. Ag2Cl is best prepared by the action of phosphorus tri-
chloride upon Ag2F. At the temperature of its formation Ag2Cl has an easily changeable
tint, with shades of violet red to violet black. Under the action of light a similar
fisomeric) substance is obtained, which splits up into AgCl + Ag when heated. With
potassium cyanide Ag2Cl gives Ag + AgCN + KCl, whence it is possible to calculate the
heat of formation of Ag2Cl ; it = 29:7, whilst the heat of formation of AgCl = 29'2 — i.e. the
reaction 2AgCl = Ag2Cl + Cl corresponds to an absorption of 28-7 major calories. If we
admit the formation of such a compound by the action of light, it is evident that the energy
of the light is consumed in the above reaction. Carey Lea (1892) subjected AgCl, AgBr, and
Agl to a pressure (of course in the dark) of 3,000 atmospheres, and to trituration with'
water in a mortar, and observed a change of colour indicating incipient decomposition,
which is facilitated under the action of light by the molecular currents set up (Lermontoff,
Egoroff). The change of colour, of the halogen salts of silver under the action of light,
and their faculty o£ subsequently giving a visible photographic image under the action of
' developers,' must now be regarded as connected with the decomposition of AgX, leading
to the formation of AgoX, and the different tinted photo-salts must be considered as
systems containing such AgoX^s. Carey Lea obtained photo-salts of this kind not only by
the action of light but also in many other ways, which we will enumerate to prove that
they contain the products of an incomplete combination of Ag with the halogens, (for the
salts Ag2X must be regarded as. such). The photo-salts have been obtained (1) by the
imperfect chlorination of silver ; (2) by the incomplete decomposition of Ag2O or Ag2COs
by alternately heating and treating with a halogen acid ; (3) by the action of nitric acid
or Na2S2O3 upon Ag->Cl; (4) by mixing a solution of AgNO3 with the hydrates of FeO,
MnO and CrO, and precipitating by HC1 ; (5) by the action of HC1 upon the product
obtained by the reduction of citrate of silver in hydrogen (Note 19), and (6) by the action
of milk sugar upon AgNO3 together with soda and afterwards acidulating with HC1. All
these reactions should lead to the formation of products of imperfect combination with
the halogens and give photo-salts of a similar diversity of colour to those produced
by the action of developers upon the halogen salts of silver after exposure to light.

COPPER, SILVER, AND GOLD 483

bromide by heating it with silver acetate, 2C2H3O2 Ag. The insolubility
of the halogen compounds of silver is still more frequently taken ad-
vantage of in determining the amount of silver and halogen in a given
solution. If it is required, for instance, to determine the quantity of
chlorine present in the form of a metallic chloride in a given solution,
a solution of silver nitrate is added to it so long as it gives a pre-
cipitate. On shaking or stirring the liquid, the silver chloride easily
settles in the form of heavy flakes. It is possible in this way to
precipitate the whole of the chlorine from a solution, without adding
an excess of silver nitrate, since it can be easily seen whether the
addition of a fresh quantity of silver nitrate produces a precipitate in
the cleap liquid. In this manner it is possible to add to a solution
containing chlorine, as much silver as is required for its entire precipi-
tation, and to calculate the amount of chlorine previously in solution
from the amount of the solution of silver nitrate consumed, if the
quantity of silver nitrate in this solution has been previously deter-
mined.25 bis The atomic proportions and preliminary experiments with
a pure salt — for example, with sodium chloride — will give the amount
of chlorine from the quantity of silver nitrate. Details of these
methods will be found in works on analytical chemistry. 25trl

25 bis jn oraer to determine when the reaction is at an end, a few drops of a solution
of K2CrO4 are added to the solution of the chloride. Before all the chlorine is precipitated
as AgCl, the precipitate (after shaking) is white (since Ag2CrO4 with 2RC1 gives 2AgCl) ;
but when all the chlorine is thrown down Ag2CrO4 is formed, which colours tte precipi-
tate reddish-brown. In order to obtain accurate results the liquid should be neutral
to litmus.

25 tri Silver cyanide, AgCN, is closely analogous to the haloid salts of silver. It is
obtained, in similar manner to silver chloride, by the addition of potassium cyanide to
silver nitrate. A white precipitate is then formed, which is almost insoluble in boiling
water. It is also, like silver chloride, insoluble in dilute acids. However, it is dissolved
when heated with nitric acid, and both hydriodic and hydrochloric acids act on it, con-
verting it into silver chloride and iodide. Alkalis, however, do not act on silver cyanide,
although they act on the other haloid salts of silver. Ammonia and solutions of the
cyanides of the alkali metals dissolve silver cyanide, as they do the chloride. In the
latter case double cyanides are formed — for example, KAgC2N2. This salt is obtained in
a crystalline state on evaporating a solution of silver cyanide in potassium cyanide. It
is much more stable than silver cyanide itself. It has a neutral reaction, does not
change in the air, and does not smell of hydrocyanic acid. Many acids, in acting on a
solution of this double salt, precipitate the insoluble silver cyanide. Metallic silver dis-
solves in a solution of potassium cyanide in the presence of air, with formation of
the same double salt and potassium hydroxide, and when silver chloride dissolves in
potassium cyanide it forms potassium chloride, besides the salt KAgC2N2. This double
salt of silver is used in silver plating. For this purpose potassium cyanide is added to
its solution, as otherwise silver cyanide, and not metallic silver, is deposited by the
electric current. If two electrodes — one positive (silver) and the other negative (copper)—
be immersed in such a solution, silver will be deposited upon the latter, and the
silver of the positive electrode will be dissolved by the liquid, which will thus preserve
the same amount of metal in solution as it originally contained. If instead of the
negative electrode a copper object be taken, well cleaned from all dirt, the silver

434 PRINCIPLES OF CHEMISTRY

Accurate experiments, and more especially the researches of
Stas at Brussels, show the proportion in which silver reacts with
metallic chlorides. These researches have led -to the determina-
tion of the combining weights of silver, sodium, potassium, chlorine,
bromine, iodine, and other elements, and are distinguished for their
model exactitude, and we will therefore describe them in some detail.
As sodium chloride is the chloride most generally used for the pre-
cipitation of silver, since it can most easily be obtained in a pure state,
we will here cite the quantitative observations made by Stas for show-
ing the co-relation between the quantities of chloride .of sodium and
silver which react together. In order to obtain perfectly pure sodium

will be deposited in an even coating ; this, indeed, forms the mode of silver plating by
the wet method, which is most often used in practice. A solution of one part of silver
nitrate in 80 to 50 parts of 'water, and mixed with a siifficient quantity of a solution of
potassium cyanide to redissolve the precipitate of silver cyanide formed, gives a dull
coating of silver, but if twice as much water be used the same mixture gives a bright
coating.

.Silver plating in the wet way has now replaced to a considerable extent the old
process of dry silvering, because this process, which consists in dissolving .silver in
mercury and applying the amalgam to the surface of the objects, and then vaporising
the mercury, offers the great disadvantage of the poisonous mercury fumes. Besides
these, there is another method of silver plating, based on the direct displacement of
silver from its salts by other metals — for example, by copper. The copper reduces the
silver from its compounds, and the silver separated is deposited upon the copper. Thus
a solution of silver chloride in sodium thiosulphate deposits a coating of silver upon a
strip of copper immersed in it. It is best for this purpose to take pure 'silver sulphite.
This is prepared by mixing a solution of silyer nitrate with an excess of ammonia, and
adding a saturated solution of sodium sulphite, and then alcohol, which precipitates
silver sulphite from the solution. The latter and its solutions are very easily decomposed
by copper. Metallic iron produces the same decomposition, and iron and steel articles
may be very readily silver-plated by means of the thiosulphate solution of silver chloride.
Indeed, copper and similar metals may even be silver-plated by means of silver chloride ;
if the chloride of silver, with a small amount of acid, be rubbed upon the surface of the
copper, the latter becomes covered with a coating of silver, which it has reduced.

Silver plating is not only applicable to metallic objects, but also to glass, china, &c.
Glass is silvered for various purposes — for example, glass globes silvered internally are
used for ornamentation, and have a mirrored surface. Common looking-glass silvered
upon one side forms a mirror which is better than the ordinary mercury mirrors, owing
to the truer colours of the image due to the whiteness of the silver. For optical in-
struments—for example, telescopes— concave mirrors are now made of silvered glass,
which has first been ground and polished into the required form. . The silvering of glass
is based on the fact that silver which is reduced from certain solutions deposits itself uni-
formly in a perfectly homogeneous and continuous but very thin layer, "forming a bright
reflecting surface. Certain organic substances have the property of reducing silver in this
form. The best known among these are certain aldehydes— :for instance, ordinary
acetaldehyde, C2H4O, which easily oxidises in the air and formff acetic acid, C2H4O3.
This oxidation also easily takes place at the expense of silver oxide, when a certain amount
of ammonia is added to the mixture. The oxide of silver gives up its oxygen to the
aldehyde, and the silver reduced from it is deposited in a metallic state in a uniform
bright coating. The same action is produced by certain saccharine substances and
certain organic acids, such as tartaric acid, &c.

COPPER, SILVER, AND GOLD 435

chloride, he took pure rock salt, containing only a small quantity of
magnesium and calcium compounds and a small amount of potassium
salts. This salt was dissolved in water, and the saturated solution
evaporated by boiling. The sodium chloride separated out during the
boiling, and the mother liquor containing the impurities was poured
off. Alcohol of 65 p.c. strength and platinic chloride were added
to the resultant salt, in order to precipitate all the potassium and
a certain part of the sodium salts. The resultant alcoholic/ solution,
containing the sodium and . platinum chlorides, was then mixed with a
solution of pure ammonium chloride in order to remove the platinic
chloride. After this precipitation, the solution was evaporated in a
platinum retort, and then separate portions of this purified sodium
chloride were collected as they crystallised. The same salt was pre-
pared from sodium sulphate, tartrate, nitrate, and from the platino-
chlpride, in order to have sodium chloride prepared b# different methods
and from different sources, and in this manner ten samples of sodium
chloride thus prepared were purified and investigated in their relation
tK? silver. After being dried, weighed quantities of all ten samples
of sodium chloride were dissolved in water and mixed with a solution
in nitric acid of a weighed quantity of perfectly pure silver. A
slightly greater quantity of silver was taken than would be required
for the decomposition of the sodium chloride, arid when, after pour-
ing in all the silver solution, the silver chloride had settled, the
amount of silver remaining in excess was determined by means of a
solution of sodium chloride of known strength. This solution of
sodium chloride was added so long as it formed a precipitate. In this
manner Stas determined how many parts of sodium chloride corre-
spond to 100 parts by weight of silver. The result of ten determina-
tions was that for the entire precipitation of 100 parts of silver,
from 54-2060 to 54-2093 parts of sodium chloride were required. The
difference is so inconsiderable that it has no perceptible influence
on the subsequent calculations. The mean of ten experiments was
that 100 parts of silver react with 54*2078 parts of sodium chloride.
In order to learn from this the relation between the chlorine and
silver, it was necessary to determine the quantity of chlorine contained in
54-2078 parts of sodium chloride, or, what is the same- thing, the quantity
of chlorine which combines with 100 parts of silver. For this purpose
Stag made a series of observations on the quantity of silver chloride
obtained from 100 parts of silver. Four syntheses were made by him
for this purpose. The first synthesis consisted in the formation of
silver chloride by the action of chlorine on silver at a red heat. This
experiment showed that 100 parts of silver give 132-841, 132-843 and

436 PRINCIPLES OF CHEMISTRY

132-843 of silver chloride. The second method consisted in dissolving
a given quantity of silver in nitric acid and precipitating it by means
of gaseous hydrochloric acid passed over the surface of the liquid ; the
resultant mass was evaporated in the dark to drive off the nitric acid
and excess of hydrochloric acid, and the remaining silver chloride was
fused first in^an atmosphere of hydrochloric acid gas and then in air.
In this process the silver chloride was not washed, and therefore there
could be no loss from solution. Two experiments made by this
method showed that 100 parts of silver give 132-849 and 132-846
parts of silver chloride. A third series of determinations was also
made by precipitating a solution of silver nitrate with a certain
excess of gaseous hydrochloric acid. The amount of silver chloride
obtained was altogether 132-848. Lastly, a fourth determination was
made by precipitating dissolved silver with a solution of ammonium
chloride, when it was found that a considerable amount of silver
(0-3175) had passed into solution in the washing; for 100 parts
of silver there was obtained altogether 132-8417 of silver chloride.
Thus from the mean of seven determinations it appears that 100
parts of silver give 132 '8445 parts of silver chloride — that, is, that
32-8445 parts of chlorine are able to combine with 100 parts of
silver and with that quantity of sodium which is contained in
54-2078 parts of sodium chloride. These observations show that
32-8445 parts of chlorine combine with 100 parts of silver and
with 21-3633 parts of sodium. From these figures expressing the
relation between the combining weights of chlorine, silver, and sodium,
it would be possible to determine their atomic weights— that is, the
combining quantity of these elements with respect to one part by
weight of hydrogen or 16 parts of oxygen, if there existed a series of
similarly accurate determinations for the reactions between hydrogen
or oxygen and one of these elements — chlorine, sodium, or silver. If
we determine the quantity of silver chloride which is obtained from
silver chlorate, AgClO3, we shall know the relation between the
combining weights of silver chloride and oxygen, so that, taking the
quantity of oxygen as a constant magnitude, we can learn from this
reaction the combining weight of silver chloride, and from the preced-
ing numbers the combining weights of chlorine and silver. For this
purpose it was first necessary to obtain pure silver chlorate. This
Stas did by acting on silver oxide or carbonate, suspended in water,
with gaseous chlorine.26

26 The phenomenon which then takes place is described by Stas as follows, in a manner
which is perfect in its clearness and accuracy : if silver oxide or carbonate be suspended
in water, and an excess of water saturated with chlorine Jbe added, all the silver

COPPEE, SILVER, AND GOLD 437

The decomposition of the silver chlorate thus obtained was accom-
plished by the action of a solution of sulphurous anhydride on
it. The salt was first fused by carefully heating it at 243°. The solution
of sulphurous anhydride used was one saturated at 0°. Sulphurous
anhydride in dilute solutions is oxidised at the expense of silver
chlorate, even at low temperatures, with great ease if the liquid be
continually shaken, sulphuric acid "and silver chloride being formed :
AgC103 + 3S02 + 3H20==AgCl + 3H2SO4. After decomposition, the
resultant liquid was evaporated, and the residue of silver chloride
weighed. Thus the process consisted in taking a known weight of
silver chlorate, converting it into silver chloride, and determining
the weight of the latter. "The analysis conducted in this manner gave
the following results, which, like the preceding, designate the weight
in a vacuum calculated from the weights obtained in air : In the
first experiment it appeared that 138*7890 grams of silver chlorate
gave 103-9795 parts of silver chloride, and in the second experiment

is converted into chloride, just as is the case with oxide or carbonate of mercury,
and the water then contains, besides the excess of chlorine, only pure hypochlorous
acid without the least trace of chloric or chlorous acid. If a stream of chlorine be
passed into water containing an excess of silver oxide or silver carbonate while the
liquid is continually agitated, the reaction is the same as the preceding; silver
chloride and hypochlorous acid are formed. But this acid does not long remain in a free
state-: it gradually acts on the silver oxide and gives silver hypochlorite, i.e. AgClO.
If, after some time, the current of chlorine be stopped but the shaking continued,
the liquid loses its characteristic odour of hypochlorous acid, while preserving its
energetic decolourising property, because the silver hypochlorite which is formed is easily
soluble in water. In the presence of an excess of silver oxide this salt can be kept for
several days without decomposition, but it is exceedingly unstable when no excess of
silver oxide or carbonate is present. So long as the solution of silver hypochlorite 19
shaken up with the silver oxide, it preserves its transparency and bleaching property,
but directly it is allowed to stand, and the silver oxide settles, it becomes rapidly cloudy
and deposits large flakes of silver chloride, so that- the black silver oxide which had
settled becomes covered with the white precipitate. The liquid then loses its bleaching
properties and contains silver chlorate, i.e. AgClO3, in solution, which has a slightly
alkaline reaction, owing to the presence of a small amount of dissolved oxide. In this
manner the reactions which are consecutively accomplished may be expressed by the
equations :

6C12 + 3Ag2O + 8H2O = GAgCl + 6HC1O ; 6HC1O + 3Ag2O = 3H20 + 'GAgClO ;

6AgC10 = 4AgCl + 2AgClO3.

Hence, Stas gives the following method for the preparation of silver chlorate : A slow
current of chlorine is caused to act on oxide of silver, suspended in water which is kept
in a state of continual agitation. The shaking is continued after the supply of chlorine
has been stopped, in order that the free hypochlorous acid should pass into silver
hypochlorite, and the resultant solution of the hypochlorite is drawn off from the
sediment of the excess of silver oxide. This solution decomposes spontaneously into
silver chloride and chlorate. The pure silver chlorate, AgClO3, does not change under
the action of light. The salt is prepared for further use by drying it in dry air at 150°.
It is necessary during drying to prevent the access of any organic matter ; this is done by
filtering the air through cotton wool, and, passing it over & layer of red-hot copper oxide.

488 PRINCIPLES OF CHEMISTRY

that 259-5287 grams of chlorate gave 194-44515 grams of silver
chloride, and after fusion 194-4435 grams. The mean result of both
experiments, converted into percentages, shows that 100 parts of silver
chlorate contain 74*9205 of silver chloride and 25-0795 parts of oxygen.
From this it is possible to calculate the combining weight of silver
chloride, because in the decomposition of silver chlorate there are
obtained three atoms of oxygen and one molecule of silver
chloride : AgC103 s= AgCl + 3O. Taking the weight of an atom
of oxygen to be 16, we find from the mean result that the equi-
valent weight of silver chloride is equal to 143-395. Thus if O=16,
AgCl= 143-395, and as the preceding experiments show that silver
chloride contains 32-8445 parts of chlorine per 100 parts of silver,
the- weight of the atom of silver 26bis must be 107*94 and that
of chlorine 35*45 The weight of the atom of sodium is determined
from the fact that 21*3633 parts of sodium chloride combine with
32-8445 parts of chlorine ; consequently Na=23-05. This conclusion,
arrived at by the analysis of silver chlorate, was verified by means
of the analysis of potassium chlorate by decomposing it by heat
and determining the weight of the potassium chloride formed, and alsd
by effecting the same decomposition by igniting the chlorate in a
stream of hydrochloric acid. The combining weight of potassium
chloride was thus determined, and another series of determinations
confirmed the relation between chlorine, potassium, and silver, in the
.same manner as the relation between sodium, chlorine, and silver was
determined above. Consequently, the combining weights of sodium,
chlorine, and potassium could be deduced by combining these data with
the analysis of silver chlorate and the synthesis of silver chloride. The
agreement between the results showed that the determinations made
by the last method were perfectly correct, and did not depend in any
considerable degree on the methods which were employed in the pre-
ceding determinations, as the combining weights of chlorine and silver
obtained were the same as before: There was naturally a difference,
but so small a one that it undoubtedly depended on the errors inciden-
tal to every process of weighing and experiment. The atomic weight
of silver was also determined by Stas by means of the synthesis of
silver sulphide and the analysis of silver sulphate. The combining
weight obtained by this method .was 107-920. The synthesis of silver
iodide and the analysis of silver iodate gave the figure 107-928. The

ae tu The results given by Stas' determinations have recen.tly been recalculated and
certain corrections have been introduced. We give in the context the average results of
van der Plaat's and Thomseh's calculations, as well as in Table HE. neglecting' the
doubtful thousandths.

COPPER, SILVER, AND GOLD 439

synthesis of silver bromide with the analysis of silver bromate gave the
figure 107-921. The synthesis of silver chloride and the analysis of
silver chlorate gave a mean result of 107-937. Hence there is no
doubt that the combining weight of silver is at least as much as 107*9
— greater than 107-90 and less than 107-95, and probably equal to the
mean= 107-92. Stas determined the combining weights of many other
elements in this manner, such as lithium, potassium, sodium, bromine,
chlorine, iodine, and also nitrogen, for the determination of the
amount of silver nitrate obtained from a given amount of silver
gives directly the combining weight of nitrogen. Taking that
of oxygen as 16, he obtained the following combining weights
for these elements : nitrogen 14-04, silver 107 '93, chlorine 35-46,
bromine 79-95, iodine 126-85, lithium 7*02, sodium 23-04, potassium
39*15. These figures differ slightly from those which are usually
employed in chemical investigations. They must be regarded as the
result of. the best observations, whilst the figures usually used in
practical chemistry are only approximate — are, so to speak, round
numbers for the atomic weights which differ so little from the exact
figures (for instance, for Ag 108 instead of 107-92, for Na 23 instead
of 23-04) that in ordinary determinations and calculations the
difference falls within the limits of experimental error inseparable from
such determinations.

The exhaustive investigations conducted by Stas on the atomic
weights of the above-named elements have great significance in
the solution of the problem as to whether the atomic weights of the
elements can be expressed in whole numbers if the unit taken be the
atomic weight of hydrogen. Prout, at the beginning of this century,
stated that this was the case, and held that the atomic weights of the
elements are multiples pf the atomic weight of hydrogen. The subse-
quent determinations of Berzelius, Penny, Marchand, Marignac, Dumas,
and more especially of Stas, proved this conclusion to be untenable ;
since a whole series of elements proved to have fractional atomic
weights—for example, chlorine, about 35-5. On account of this,
Marignac and Dumas stated that the atomic weights of the elements
are expressed in relation to hydrogen, either by whole numbers
or by numbers with simple fractions of the magnitudes ^ and J. But
Stas's researches refute this supposition also. Even between the com-
bining weight of hydrogen and oxygen, there is not, so far as is yet
known, that simple relation which is required by Front's hypothesis^

37 This hypothesis, for the establishment or refutation of which so many researches
have been made, is exceedingly important, and fully deserves the attention which has
been given to it. Indeed, if it appeared that the atomic weights of all the elements could
*H

440 PRINCIPLES OP CHEMISTRY

i.e.y taking 0=16, the atomic weight of hydrogen is equal not to 1 bufc
to a greater number somewhere between 1-002 and 1-008 or mean-

be expressed in whole numbers with reference to hydrogen, or if they at least proved to
be commensurable with one another, then it could be affirmed with confidence that the
elements, with all their diversity, were formed of one material condensed or grouped in
various manners into the stable, and, under known conditions, undecomposable groups
which we call the atoms of the elements. At first it was supposed that all the elements
were nothing else but condensed hydrogen, but when it appeared that the atomic weights
of the elements could not be expressed in whole numbers in relation to hydrogen,
it was still possible to imagine the existence of a certain material from which both hydro-
gen and all the other elements were formed. If it should transpire that four atoms of this
material form an atom of hydrogen, then the atom of chlorine would present itself as
consisting of 142 atoms of .this substance, the weight of whose atom would be equal to
0'25. But in this case the atoms of all the elements should be "expressed in whole
numbers with respect to the weight of the atom of this original material. Let us sup-
pose that the atomic weight of this material is equal to unity, then all the atomic weights
should be expressible in whole numbers relatively to this unit. Thus the atom of one ele-
ment, let us suppose, would weigh m, and of another n, but, as both m and n must be
whole numbers, it follows that the atomic weights of all the elements would be commen-
surable. But it is sufficient to glance over the results obtained by Stas, and to be
assured of their accuracy, especially for silver, in order to entirely destroy, or at least
strongly undermine, this attractive hypothesis. We must therefore refuse our assent to the
doctrine of the building up from a single substance of the elements knqwn to us. This
hypothesis is not supported either by any known transformation (for one element has never
been converted into another element), or by the commensurability of the atomic weights
of the elements. Although the hypothesis of the formation of all the elements from a
single substance (for which Crookes has suggested the name protyle) is most attractive
in its comprehensiveness, it can neither be denied nor accepted for want of sufficient data.
Marignac endeavoured, however, to overcome Stas's conclusions as to the incommensu-
rability oft the atomic weights by supposing that in his, as in the determinations of all
other observers, there were unperceived errors which were quite independent of the mode
of observation — for example, silver nitrate might be supposed to be an unstable substance
which changes, under the heatings, evaporations, and other processes' to which it is sub-
jected in the reactions for the determination of the combining weight of silver. It might
be supposed, for instance, that silver nitrate contains some impurity which cannot be
removed by any means ; it might also be supposed that a portion of the elements of the
nitric acid are disengaged in the evaporation of the solution of silver nitrate (owing to the
decomposing action of water), and in its fusion, and that we have not to deal with normal
silver nitrate, but with a slightly basic salt, or perhaps an excess of nitric acid which
cannot be removed from the salt. In this case the observed combining weight will n»t
refer to an actually definite chemical compound, but to some mixture for which there
does not exist any perfectly exact combining relations. Marignac upholds this proposition
by the fact that the conclusions of Stas and other observers respecting the combining
weights determined with the greatest exactitude very nearly agree with the proposition
of the commensurability of the atomic weights— for example, the combining weight of
silver was shown to be equal to 107'98, so that it only differs by 0'08 from the whole
number 108, which is generally accepted for silver. The combining weight of iodine
proved to be equal to 126'85 — that is, it differs from 127 by 0'15. The combining weights
of sodium, nitrogen, bromine, chlorine, and lithium are still nearer to the whole or round
numbers which are generally accepted. But Marignac's proposition will hardly bear
criticism. Indeed if we express the combining weights of the elements determined by
Stas in relation to hydrogen, the approximation of these weights to whole numbers
disappears, because one part of hydrogen in reality does not combine with 16 parts of
oxygen, but with 15'92 parts, and therefore we shall obtain, taking H = 1, not the above-
cited figures, but for silver 107-38, for bromine 79*55, magnitudes which are still further

COPPER, SILVER, AND GOLD 441

1 005. Such a conclusion arrived at by direct experiment cannot but
be regarded as having greater weight than Prout's supposition
(hypothesis) that the atomic weights of the elements are in multiple
proportion to each other, which would gi\ e reason for surmising (but not
asserting) a complexity of nature in the elements, and their com-
mon origin from a single primary material, and for expecting their
mutual conversion into each other. All such ideas and hopes must

removed from whole numbers. Besides which, if Marignac's proposition were true the
combining weight of silver determined by one method — e.g. .by the analysis of silver
chlorate combined with the synthesis of silver chloride — would not agree well with the
combining weight determined by another method— e.g. by means of the analysis of silver
iodate and the synthesis of silver iodide. If in one case a basic salt could be obtained,
in the other case an acid salt might be obtained. Then the analysis of the acid salt
would give different results from that of the basic salt. Thus Marignac's arguments
cannot serve as a support for the vindication of Prout's hypothesis.

In conclusion, I think it will not be out of place to cite the following passage from a
paper" I read before the Chemical Society of London in 1889 (Appendix II.), referring to
the hypothesis of the complexity of the elements recognised in chemistry, owing to the
fact that many have endeavoured to apply the periodic law to the justification of this
idea 'dating from a remote antiquity, when it was found convenient to admit the existence
of many gods but only one matter.

' When we try to explain the origin of the idea of a unique primary matter, we easily
trace that, in the absence of deductions from experiment, it derives its origin from the
scientifically philosophical attempt at discovering some kind of unity in the immense
diversity of individualities which we see around. In classical times such a tendency
could only be satisfied by conceptions about the immaterial world. As to the material
world, our ancestors were compelled to resort to some hypothesis, and they adopted the
idea of unity in the formative material, because they were not able to evolve the concep-
tion of any other possible unity in order to connect the multifarious relations of matter.
Responding to the same legitimate scientific tendency, natural science has discovered
throughout the universe a unity of plan, a unity of forces, and a unity of matter ; and
the convincing conclusions of modem science compel every one to admit these kinds of
unity. But while we admit unity in many things, we none the less must also explain
the individuality and the apparent diversity which we cannot fail to trace everywhere.
It was said of old " Give us a fulcrum and it will become easy to displace the earth."
So also we must say, "Give us something that is individualised, and the apparent
diversity will be easily understood." Otherwise, how could unity result in a multitude

' After a long and painstaking research, natural science has discovered the individu-
alities of the chemical elements, and therefore it is now capable, not only of analysing,
but also of synthesising ; it can understand and grasp generality and unity, as well as
the individualised and multifarious. The general and universal, like time and space, like
force and motion, vary uniformly. The uniform admit of interpolations, revealing every
intermediate phase ; but the multitudinous, the individualised— such as ourselves, or the
chemical elements, or the members of a peculiar periodic function of the elements, or
Dalton's multiple proportions— is characterised in another way. We see in it— side by
Bide with a general connecting principle— leaps, breaks of continuity, points which escape
from the analysis of the infinitely small— an absence of complete intermediate linka.
Chemistry has found an answer to the question as to the causes of multitudes, and while
retaining the conception of many elements, all submitted to the discipline of a general
law, it offers an escape from the Indian Nirvana— the absorption in the universal— re-
placing it by the individualised. However, the place for individuality is so limited by
the all-grasping, all-powerful universal, that it is merely a point of support for the under«
standing of multitude in unity.'

442 PRINCIPLES OF CHEMISTRY

now, thanks more especially to Stas, be placed in a region void of any
experimental support whatever, and therefore not subject to the dis-
cipline of the positive data of science.

Among the platinum metals ruthenium, rhodium, and palladium,
by their atomic weights and properties, approach silver, just as iron
and its analogues (cobalt and nickel) approach copper in all respects.
Gold stands in exactly the sarnie position in relation to the heavy
platinum metals, osmium, iridium, and platinum, as copper and
silver do to the two preceding series. The atomic weight of gold is
nearly equal to their atomic weights ; 28 it is dense like these metals.
It also gives various grades of oxidation, which are feeble, both in
a basic and an acid sense. Whilst near to osmium, iridium, and pla-
tinum, gold at the same time is able, like copper and silver, to form
compounds which answer to the type RX — that is, oxides of the compo-
sition R20. Cuprous chloride, CuCl, silver chloride, AgCl, and aurous
chloride, AuCl, are substances which are very much alike in their
physical and chemical properties.28 bis They are insoluble in water,
but dissolve in hydrochloric acid and ammonia, in potassium cyanide,

88 It might be expected from the periodic law and analogies with the series iron, cobalt,
nickel, copper, zinc, that the atomic weights of the elements of the series osmium,
iridium, platinum, gold, mercury, would rise in this order, and at the time of the esta-
blishment of the periodic law (1869), the determinations of Berzelius, Rose, and ofchera
gave the following values for the atomic weights : Os = 200, Ir = 197, Pt = 198, Au = 196,
Hg = 200. The fulfilment of the expectations of the periodic law was given in the first
place by the fresh determinations (Seubert, Dittmar, and Arthur) of the atomic weight of
platinum, which proved to be nearly 196, if O = 16 (as Marjgnac, Brauner, and others
propose) ; in the second place, by the fact that Seubert proved that the atomic weight of
osmium is really less than that of platinum, and approximately Os = 191 ; and, in the
third place, by the fact that after the researches of Kriiss, Thorpe, and Laurie there was
no doubt that, the atomic weight- of gold is greater than that of platinum— namely,
nearly 197.

88 t>u in Chapter XXII., Note 40, we gave the thermal data for certain of the com-
pounds of copper of the type CuXa ; we will now cite certain data for the cuprous
compounds of the type CuX, which present an analogy to the . corresponding compounds
AgX and AuX, some of which were investigated by Thomson in his classical work,
' Thermochemische Untersuchungen ' (Vol. iii., 1883). The data are given in the same
manner as in the above-mentioned note :

R = Cu Ag Au

R + C1 +33 +29 +6

RfBi +25 +23 0

R + I +16 +14 -6

R + O +41 +6 -?

Thus we see in the first place that gold, which possesses a much smaller affinity than Ag,
evolves far less heat than an equivalent amount of copper, giving the same compound, and
in the second place that the combination of copper with one atom of oxygen disengages
more heat than its combination with one atom of a halogen, whilst with silver the reverse
is the case. This is connected with the fact that Cu2O is more stable under the action
of beat than Ag2O.

COPPER, SILVER, AND GOLD 443

sodium thiosulphate, &c. Just as copper forms a link between the iron
metals and zinc, and as silver unites the light platinum metals with
cadmium, so also gold presents a transition from the heavy platinum
metals to mercury. Copper gives saline compounds of the types CuX
and CuX2, silver of the type AgX, whilst gold, besides compounds of-
the type AuX, very easily and most frequently forms those of the type
AuCl3. The compounds of this type frequently pass into those of the
lower type, just as PtX4 passes into PtX2, and the same is observable
in the elements which, in their atomic weights, follow gold. Mercury
gives HgX2 and HgX, thallium gives T1X3 and T1X, lead gives
PbX4 and PbX2. On the other hand, gold in a qualitative respect
differs from silver and oopper in the extreme ease with which all its com-
pounds are reduced to metal by many means. This is not only accom-
plished by many reducing agents, but also by the action of heat. Thus
its chlorides and oxides lose their chlorine and oxygen when heated;
and, if the temperature be sufficiently high, these elements are entirely
expelled and metallic gold alone remains. Its compounds, therefore,
act as oxidising agents.29

In nature gold occurs in the primary and chiefly in quartzose rocks,
and especially in quartz veins, as in the Urals (at Berezoffsk), in
Australia, and in California. The native gold is extracted from these
rocks by subjecting them to a mechanical treatment consisting of
crushing and washing.29 Ws Nature has already accomplished a similar

* Heavy atoms and molecules, although they may present many points of analogy, are
more easily isolated ; thus CigH^, although, like C2H4, it combines with Br2, and has a
similar composition, yet reacts with much greater difficulty than C2H4) and in this it resem-
bles gold ; the heavy atoms and molecules are, so to say, inert, and already saturated by
themselves. Gold in its higher grade of oxidation, Au2O3, presents feeble basic pro-
perties and weakly-developed acid properties, so that this oxide of gold, Au2O3, may be
referred to the class of feeble acid oxides,, like platinic oxide. This is not the case in the
highest known oxides of copper and silver. But in the lower grade of oxidation, aurous
oxide, AujO, gold, like silver and copper, presents basic properties, although they ar^
not very pronounced. In this respect it stands very close in its properties, although
not in its types of combination (AuX and AuX3), to platinum (PtX2 and PtX4) and its
analogues.

As yet the general chemical characteristics of gold and its compounds have not been
fully investigated. This is partly due to the fact that very few researches have been
undertaken on the compounds of this metal, owing to its inaccessibility for working
in large quantities. As the atomic weight of gold is high (Au = 197), the preparation of
its compounds requires that it should be taken in large quantities, which forma ah
obstacle to its being fully studied. Hence the facts concerning the history of this metal
are rarely distinguished by that exactitude with which many facts have been established
concerning other elements more accessible, and long known in use.

Wbl» Sonstadt (1872) showed that sea water, besides silver, always contains gold,
Munster (1892) showed that the water of the Norwegian fiords contains about 5 milli-
grams of gold per ton (or 5 milliardths) — i.e a quantity deserving practical attention, and
I think it may be already said that, considering the immeasurable amount of sea water,
in time means will be discovered for profitably extracting gold from eea water by

444 PRINCIPLES OF CHEMISTRY

disintegration of the hard rocky matter containing gold.30 These dis-
integrated rocks, washed by rain and other water, have formed gold-
bearing deposits, which are known as alluvial gold deposits. Gold-
bearing soil is sometimes met with on the surface and sometimes under

bringing it into contact with substances capable of depositing gold upon their surface.
The first efforts might be made upon the extraction of salt from sea water, and as the
total amount of sea water maybe taken as about 2,000,000,000,000,000,000 tons, it follows
that it contains about 10,000 million tons of gold. The yearly production of gold, is about
200 tons for, the whole world, of which about one quarter is extracted in Eussia. It ia
supposed that gold is dissolved in sea water owing to the presence of iodides, which, under
the action of animal organisms, yield free iodine. It is thought (as Professor Konova-
loff mentions in his work upon 'The Industries of the United States,' 1894) that
iodine facilitates the solution of the gold, and the organic matter its precipitation.
These facts and considerations to a certain extent explain the distribution of gold in
veins or rock fissures, chiefly filled with quartz, because there is sufficient reason for
supposing that these rocks once formed the ocean bottom. R. Dentrie, and subse-
quently Wilkinson, showed that organic matter— for instance, cork — and pyrites are able
to precipitate gold from its solutions in that -metallic form and state in which it occurs
in quartz veins, where (especially in the deeper parts of vein deposits) gold is frequently
found on the surface of pyrites, chiefly arsenical pyrites. Kazantseff (in Ekaterinburg,
1891) eve^n supposes, from the distribution of the gold in these pyrites, that it occurred
in solution as a compound of sulphide of gold and sulphide of arsenic when it penetrated
'into the veins. It is from such considerations that the origin of vein and pyritic gold
is, at the present time, attributed to the reaction of solutions of this metal, the remains
of which are seen in the gold still present in sea water.

30 However, in recent times, especially since about 1870, when chlorine (either -as a
solution of the gas or as bleaching powder) and bromine began to be applied to the extrac-
tion, of finely-divided gold from poor ores (previously roasted in order to drive off arsenio
and sulphur, and oxidise the iron), the extraction of gold from quartz and pyrites,
by the wet method, increases from year to year, and begins to equal the amount
extracted from alluvial deposits. Since the nineties th,e cyanide process (Chapter
XIII., Note 18 bis) has taken an important place among the wet methods for
extracting gold from its ores. It consists in pouring a dilute solution of cyanide of potas-
sium (about 500 parts of water and 1 to 4 parts of cyanide of potassium per 1.000 parts
of ore, the amount of cyanide depending upon the richness of the ore) and a mixture
of it with NaCN, (see Chapter XIII., Note 12) over the crushed ore (which need not be
roasted, whilst roasting is indispensable in the chlorination process, as otherwise the
chlorine is used up in oxidising the sulphur, arsenic, &c.) The gold is dissolved
very rapidly even from pyrites, where it generally occurs on the surface in such
fine and adherent particles that it either cannot be mechanically washed away, or,
more frequently is carried away by the stream of water, and cannot be caught by
mechanical means or by the mercury used for catching the gold In the sluices.
Chlorination had already given the possibility of- extracting the finest particles of gold ;
but the cyanide process enables such pyrites to be treated as could be scarcely worked
by other means. The treatment of the crushed ore by the KCN is carried on in simple
wooden vats (coated with paraffin or tar) with the greatest possible rapidity (in order that
the KCN solution should not have time to change) by a method of systematic lixiviation,
and is completed in 10 to 12 hours. The resultant solution of gold, containing AuK(CN)8>
is decomposed either with freshly-made zinc filings (but when the gold settles on the
Zn, the cyanide solution reacts upon the Zn with the evolution of EL, and formation of
ZnH^Oj) or by sodium amalgam prepared at the moment of reaction by the action of an
electric current upon a solution of NaHO poured into a vessel partially immersed in
mercury (the NaCN is renewed continually by this means). The silver in the ore passes
into solution, together with the gold, as in amalgamation.

COPPER, SILVER, AND GOLD 445

the upper soil, but more frequently along the banks of dried-tip water-
courses and running streams. The sand of many rivers contains,
however, a very small amount of gold, which it is not profitable to
work ; for example, that of the Alpine rivers contains 5 parts of gold
in 10,000,000 parts of sand. The richest gold deposits are those of
Siberia, especially in the southern parts of the Government of Yeniseisk,
the South Urals, Mexico, California, South Africa, and Australia,
and then the comparatively poorer alluvial deposits of many countries
(Hungary, the. Alps, and Spain in Europe). The extraction of the
gold from alluvial deposits is based on the principle of levigation ; the
earth is washed, while constantly agitated, by a stream of water,
which carries away the lighter portion of the earth, and leaves the
coarser particles of the rock and heavier particles of the gold, together
with certain substances which accompany it, in the washing apparatus.
The extraction of this washed gold only necessitates mechanical ap-
pliances,31 and it is not therefore surprising that gold was known to
savages and. in the most remote period of history. It sometimes occurs
in crystals belonging to the regular system, but in the majority of cases

51 But the particles of golcLare sometimes so small that a large amount is lost during
the washing. It is then profitable to have' recourse to the extraction by chlorine and
KCN (Note 80).

In speaking of the extraction of gold the following remarks may not be out of
place:

In California advantage is taken of water supplied from high altitudes in order to
have a powerful head of water, with which the rocks are directly washed away, thus
avoiding the greater portion of the mechanical labour required for the exploitation of
these deposits.

The last residues of gold are sometimes extracted from sand by washing them with
mercury, which dissolves the gold. The sand mixed with water is caused to come into
contact with mercury during the washing. The mercury is then distilled.

Many sulphurous ores, even pyrites, contain a small amount of gold. Compounds of
gold with bismuth, BiAuj, tellurium, AuTe2 (calverite), &c., have been found, although
rarely.

Among the minerals which accompany gold, and from which the presence of gold may
be expected, we may mention white quartz, titanic and magnetic iron ores, and also the
following, which are of rarer occurrence : zircon, topaz, garnet, and such like. The con-
centrated gold washings first undergo a mechanical treatment, and the impure gold
obtained is treated for pure gold by various methods. If the gold contain a considerable
amount of foreign metals, especially lead and copper, it is sometimes cupelled, like silver,
so that the oxidisable metals may be absorbed by the cupel in the form of oxides, but in
every case the gold is obtained together with silver, because the latter metal also is" not
oxidised. Sometimes the gold is extracted by means of mercury, that is, by amalgama-
tion (and the mercury subsequently driven off by distillation), or by smelting it with
lead (which is afterwasds removed by oxidation) and processes like those employed for
the extraction of silver, because, gold, like silver, does not oxidise, is dissolved by lead
and mercury, and is non-volatile. If .copper or any other metal contain gold and it be
employed as an anode, pure copper will be deposited upon the cathode, while all the
gold will remain at the anode as a slime. This method often amply repays the whole
cost of the process, since it gives, besides the gold, a pure electrolytic copper.

446 PRINCIPLES OF CHEMISTRY

in nuggets or grains of greater- or less magnitude. It always contains
silver (from very small quantities up to 30 p.c., when it is called
* electrum ') and certain other metals, among which lead and rhodium
are sometimes found.

The separation of the silver from gold is generally carried on with
great precision,, as the presence of the silver in the gold does not
increase its value for exchange, and it can be substituted by other
less valuable metals, so that the extraction of the silver, as a precious
metal, from its alloy with gold, is a profitable operation. This
separation is conducted by different methods. Sometimes the argenti-
ferous gold is melted in crucibles, together with a mixture of common
salt and powdered bricks. The greater portion of the silver is' thus
converted into the chloride, which fuses and is absorbed by the slags,
from which it may be extracted by the usual methods. The silver is
also extracted from gold by treating it with boiling sulphuric acid,
which does not act on the gold but dissolves the silver. But if the
alloy does not contain a large proportion of silver it cannot be extracted
by this method or at B,ll events the separation will be imperfect, and
therefore a fresh amount of silver is added (by fusion) to the gold, in
such quantity that the alloy contains twice as much silver as gold.
The silver which is added is preferably such as contains gold, which is
very frequently the case. The alloy thus formed is poured in a thin
stream into water, by which means it is obtained in a granulated
form j it is then boiled with strong sulphuric acid, three parts of
acid' being used to one part of alloy. The sulphuric acid extracts
,all the silver without acting on the gold. It is best, however, to'
pour off the first portion of the acid, which has dissolved the silver,
and then treat the residue of still imperfectly pure gold with a fresh
quantity of sulphuric acid. The gold is thus obtained in the form
of powder, which is washed with water until it is quite free from
silver. The silver is precipitated from the solution by means of
copper, so that cupric sulphate and metallic silver are obtained. This
process is carried out in many countries, as in Russia, at the Govern-
ment mints.

Gold is generally used alloyed with copper ; since pure gold,
like pure silver, is very soft, and therefore soon worn away. In
assaying or determining the amount of pure gold in such an alloy
it is usual to add silver to the gold in order to make up an alloy
containing three parts of silver to one of gold (this is known as
quartation because the alloy contains £ of gold), and the resultant
alloy is treated with nitric acidi If the silver be not in excess over
the gold, it is not all dissolved by the nitric acid, and this is the reason

COPPER, .SILVER, AtfD GOLD 447

for the quartation. The amount of pure gold (assay) is determined by
weighing the gold which remains after this treatment. English gold
(= 22 carats) coinage is composed of an alloy containing 91-66 p.c. of
gold, but for many articles gold is frequently used containing a larger
lamount of foreign metals.

Pure gold may be obtained from gold alloys by dissolving in aqua
regia, and then adding ferrous sulphate to the solution or heating it
•with a» solution of oxalic acid. These deoxidising agents reduce the
gold, but not the other metals. The chlorine combined with the gold
then acts like free chlorine. The gold, thus reduced, is precipitated as
ftn exceedingly fine brown powder.31 bis It is then washed with water,
and fused with nitre or borax. Pure gold reflects a yellow light, and
in the form of very thin sheets (gold leaf), into which it can be
hammered and rolled,31 trl it transmits a bluish-green light. The
specific gravity of gold is about 19'5, the sp. gr. of gold coin is about
17'1. It fuses at 1090° — at a higher temperature than silver — and can
be drawn into exceedingly fine wires or hammered into thin sheets.
With its softness and ductility, gold is distinguished for its tenacity,
and a gold wire two millimetres thick breaks only under a load of 68
kilograms. Gold vaporises even at a furnace heat, and imparts a
greenish colour to a flame passing over it in a furnace. Gold , alloys
with copper almost without changing its volume.32 In its chemical

si t>is Schottlander (1893) obtained gold in a soluble colloid form (the solution is violet)
by the action of a mixture of solutions of cerium acetate .and NaHO upon a solution of'
AuClj. The gold separates out from such a solution in exactly~the same manner as Ag-
does from the solution of colloid, silver mentioned above. • There always remains »
certain amount of a higher oxide of cerium, CeO2, in the. solution — i.e. the gold is
reduced by converting -the cerium into a higher grade 'of oxidation. Besides ., which
Kriiss and Hofmann show'ed that sulphide of gold precipitated by the -action of H2S upon
a solution of AuKCyg mixed with HC1 easily passea into a colloid solution after being
properly washed (like As2S3, CuS, &c., Chapter I., Note 57).

31 trl Gold-leaf is used for gilding wood (leather, cardboard, and suchlike, upon which
it is glued by means of varnish,, &c.), and is about O'OOS millimetre thick. It is obtained
from thin sheets (weighing at first about £ grm. to a square inch), rolled between gold
rollers, by gradually hammering them (in packets of a number at once) between sheets
of moist (but not wet) parchment, and then, after cutting them into four pieces, between
a specially prepared membrane, which, when at the right degree of moisture, does not
tear or stick together under the blows of the hammer.

32 The. formation of the alloys Cu + Zn, Cu + Sn, Cu + Bi, Cu + Sb, Pb-f Sb, Ag + Pb,
Ag+.S3, "A'u'4-Zn, Au + Sn, &c., is accompanied by a contraction (and evolution of heat).
The formation of the alloys Fe + Sb, Fe + Pb, Cu + Pb, Pb + Sn, Pb+Sn, Pb + Sb,
Zn + Sb, Ag + Cu, Au + Cu, 'Au + Pb, takes place with a certain increase in volume.
With regard to the alloys of gold,' it may be mentioned that gold is only slightly
dissolved by mercury (about 0'06 p.c., Dudley, 1890) j the remaining portion forms a
granular alloy, whose composition has not been definitely determined. Aluminium (and
silicon) also have the capacity of forming alloys with gold. The presence of a small
amount of aluminium lowers the melting point of gold considerably (Roberts- Austen,
1692) ; thus the addition of 4 p.c. of aluminium lowers it by U0>28, the addition of 10 P.O.

448 PRINCIPLES OF CHEMISTRY

aspect, gold presents, as is already seen from its general characteristics
given above, an example of the so-called noble metals — i.e. it is
incapable of being oxidised at any temperature, and its oxide is
decomposed when calcined. Only chlorine and bromine combine
directly with it at the ordinary temperature, but many other metals
and non-metals combine with it at a red heat — for example, sulphur,
phosphorus, and arsenic. Mercury dissolves it with great ease. It
dissolves in potassium cyanide in the presence of air ; a .mixture of
sulphuric acid with nitric acid dissolves it with the aid of heat,
although in small quantity. It is also soluble in aqua regia and in
selenic acid. Sulphuric, hydrochloric, nitric, and hydrofluoric acids
and the caustic alkalis do not act oh gold, but a mixture of hydro-
chloric acid with such oxidising agents as evolve chlorine naturally
dissolves it like aqua regia.32 bi*.

As regards the compounds of gold, they belong, as was said
above, to the types AuX3 and AuX. Auric chloride or gold tri-
chloride, AuCl3, which is formed when gold is dissolved in aqua regia,
belongs to the former and higher of these types. The-solution of this
substance in water has a yellow colour, and jt may be obtained pure by
evaporating the solution in aqua regia to dryness, but not to the point
of decomposition. If the evaporation proceed to the point of crystal-
lisation, a compound of gold chloride and hydrochloric acid, AuHCl4, is
obtained, like the allied compounds of platinum ; but it easily parts
with the acid and leaves auric chloride, which fuses into a red-brown
liquid, and then solidifies to a crystalline mass. If dry chlorine be
passed over gold in powder it forms a mixture of aurous and auric
chlorides, but the aurous chloride is also decomposed by water into
gold and auric chloride. Auric chloride crystallises from its solutions
as AuCl3,2H2O, which easily, loses water, and the dry chloride loses
two-thirds of its chlorine at 185°, forming aurous chloride, whilst

Al by 410<7. The latter alloy is white. The alloy AuAl2 has a characteristic purple
colour, and its melting point is 32°'5 above that of gold, which shows it to be a definite
compound of the two metals. The melting points of alloys richer in Al gradually fall
to 660°— that is, below that of aluminium (665°).

Heycock and Neville (1892), in studying the triple alloys of Au, Cd, and Sn, observed
a tendency in the gold to give compounds with Cd, and by sealing a mixture of Au and Cd
in a tube, from which the air had been exhausted, and heating it, they obtained a grey
crystalline brittle definite alloy AuCd.

53 bis Calderon (1892), at the request of some jewellers, investigated the cause of a
peculiar alteration sometimes found on the surface of dead-gold articles, there appearing
brownish and blackish spots, which widen and alter their form in course of time. He
came to the conclusion that these spots are due to the appearance and development of
peculiar micro-organisms. (Aspergillus niger and Micrococcus cimbareus) on the gold,
spores of which were found in abundance on the cotton-wool in which the gold articles
had been kept.

COPPER, SILVEB, ANE GOLD 449

above 300° the latter chloride also loses its- chlorine and leaves
metallic gold. Auric chloride is the usual form in which gold occurs in
solutions, and in which its salts are used in the arts and for chemical
purposes. It is soluble in water, alcohol, and ether. Light has a reduc-
ing action on these solutions, and after a time metallic gold is deposited
upon the sides of vessels containing the solution. Hydrogen when
nascent, and even in a gaseous form, reduces gold from this solution
to a metallic state. The reduction is more conveniently and usually
effected by ferrous sulphate, and in general by the action of ferrous
salts.38

If a solution of potassium hydroxide be added to a solution of auric
chloride, a precipitate is first formed, which re-dissolves in an excess of
the alkali. On being evaporated under the receiver of an air-pump,
this solution yields yellow crystals, which present the same composition
as the double salts AuMCl4, with the substitution of the chlorine by
oxygen — that is to say, potassium aurate, AuK02, is formed in crystals
containing 3H2O. The solution has a distinctly alkaline reaction.
Auric oxide, Au2O3, separates when this alkaline solution is boiled with
'an excess of sulphuric acid. But it then still retains some alkali ; how-
ever, it may be obtained in a pure state as a brown powder by
dissolving in nitric acid and diluting with water. The brown powder
decomposes below 250° into gold and oxygen. It is insoluble in water
and in many acids, but it dissolves in alkalis, which shows the acid
character of this oxide. An hydroxide, Au(OH)^ may be obtained as a
brown powder by adding magnesium oxide to a solution of auric chlo-
ride and treating the resultant precipitate of magnesiu no aurate with
nitric acid. This hydroxide loses water at 100°. and gives auric oxide.34

53 Stannous chloride as a reducing agent also acts on auric chloride, and gives a red
precipitate known as purple of Cassius. This substance, which probably contains a
mixture or compound of aurous oxide and tin oxide, is used as a red pigment for china
\and glass. Oxalic acid, on heating, reduces metallic gold from its salts, and this property
may be taken advantage of for separating it from its solutions. The oxidation which
then takes place in the presence, pf water may be expressed by the following equation :
5AuClj + 8C2H.2O4 = 2Au + 0HCl+ 6CO2. Nearly all organic substances have a reducing
action on gold, and solutions of gold leave a violet stain on the skin.

Auric chloride, like platinic chloride, is distinguished for its clearly-developed
property of forming double salts. These double salts, as a rule, belong to the type
AuMCl4. The compound of auric Jchloride with hydrochloric acid mentioned above
evidently belongs to the same type. The compounds 2KAuCl4,5H2O, NaAuCl4,2H2O,
A«NH4C14,H2O, Mg'AuCl4)2,2H2O, and jthe like are. easily crystallised in well-formed
crystals*. Wells, Wheeler, and Penfield "(1892) obtained RbAuCl4 (reddish yellow) and
CsAuCl4 (golden yellow), and corresponding bromides (dark coloured). AuBr3 is ex-
tremely like the chloride. Auric cyanide is obtained easily in the form of a double salt
of potassium, KAu(CN)4, by mixing saturated and hot solutions pf potassium cyanide*
with auric chloride1 and then cooling.

34 If ammonia be 'added to a solution of auric chloride, it forms a yellow precipitate

450 PRINCIPLES OF CHEMISTRY

The starting-point of the compounds of the type AuX 35 is gold
monochloride or aurous chloride, AuCl, which is formed, as mentioned
above, by heating auric chloride at 185°. A'urous chloride forms a
yellowish -white powder ; this, when heated with water, is decomposed
into metallic gold and auric chloride, which passes into solution *
3 AuCl = AuCl3 -f 2 Au. This decomposition is accelerated by the action
of light. Hence it is obvious that the compounds corresponding with
aurous oxide are comparatively unstable. But this only refers to the
simple compounds AuX ; some of the complex compounds, on the
contrary, form the most stable, compounds of gold. Such, for ex-
ample, is the cyanide of gold and potassium, AuK(CN)2. It is formed,
for instance, when finely-divided gold dissolves in the presence of
air in a solution of potassium cyanide : 4KCN + 2 Au + H2O + O
= 2KAu(CN)2-f 2KHO (this reaction also proceeds with solid pieces
of gold, although very slowly). The same compound is formed in
solution when many compounds of gold are mixed with potassium
cyanide, because if 'a higher compound of gold be taken, it is reduced

of the so-called fulminating gold, which contains gold, chlorine! hydrogen, nitrogen,
and oxygen, but its formula is not known with certainty. It is probably a sort of am-
Tttonio-metallic compound, AuaO3,4jJH3, or amide (like the- mercury compound). This
precipitate explodes at 140°, but when left in the presence of solutions containing am-
monia it loses all its chlorine and becomes non-explosive. In this form the composition
Au.iO3,2NH5,H2O is ascribed to it, but this is- uncertain. Auric sulphide, Au2S3, is
obtained by the action of hydrogen sulphide on a solution of auric chloride, and also
directly by fusing sulphur with gold. It has an acid character, and therefore dissolves
in sodium arid ammonium sulphides.

35 Many double salts of suboxide.of gold belong to the type AuX— for instance, the
cyanide corresponding to the type AuKX2, like PtK2X4, with which we became acquainted
in the last chapter. We will enumerate several of the representatives of this class of
compounds. If auric chloride, AuCl3, be mixed with a solution of sodium thiosulphate,
the gold passes into a colourless solution, which deposits colourless crystals, con-
taining a double thiosulphate of gold and sodium, which are easily soluble in water
but are precipitated by alcohol. The composition of this salt is Na3Au(S2O3)2,2H2O.'
If the sodium thiosulphate be represented as NaS2O5Na, the double salt in question
will be AuNa(S2O3Na)2,2H2O, according to the type AuNaX2. The solution of this
colourless and easily crystallisable salt has a sweet taste, and the gold is not separated
from it either by ferroms sulphate or oxalic acid. This salt, which is known as Fordos
and Gelis's salt, is used in medicine .and photography In general, aurous oxide
exhibits a distinct inclination to the formation of similar double salts, as we Baw also
with PtX2 — for example, it forms similar salts with sulphurous acid. Thus if a solution
of sodium sulphite be gradually added to a solution of oxide of gold in sodium
hydroxide, the precipitate at first formed re-dissolves to a colourless solution, which
contains the double salt Na3Au(SO5)2=AuNa(SO3Na)2, The solution of this salt>,
when mixed with barium chloride, first forms a precipitate of barium sulphite, and
then a red barium double salt which corresponds with the above sodium salt.

The oxygen compound of the type AuX, aurous oxide, Au2O, is obtained as a greenish
violet powder on mixing aurous chloride with potassium chloride in the cold. With
hydrochloric acid this oxide gives gold and auric chloride, and when heated it easily
splits up into oxygen and metallic gold.

COPPER, SILVER, AND GOLD 451

T>y the potassium cyanide into aurous oxide, which dissolves in potas-
sium cyanide and forms KAu(CN)2. This substance is soluble in
water, and gives a colourless solution, which can be kept for a long
time, and is employed in electro-gilding — that is, for coating other
metallic objects with a layer of gold, which is deposited if the object
be connected with the negative pole of a battery and the positive pole
consist of a gold plate. When an electric current is passed between
them, the gold from the latter will dissolve, whilst a coating of gold
from the solution will be deposited on the object.

APPENDIX I

AN ATTEMPT TO APPLY TO CHEMISTRY ONE OF THE
PRINCIPLES OF NEWTON'S NATURAL PHILOSOPHY

BY PKOFESSOB MENDELEEFF

A LECTURE DELIVERED AT THE ROYAL INSTITUTION OF GREAT BRITAIN
ON FRIDAY, MAY 31, 1889

NATURE, inert to the eyes of the ancients, has been revealed to us as full of
life and activity. The conviction that motion pervaded all things, which was
first realised with respect to the stellar universe, has now extended to the
unseen world of atoms. No sooner had the human understanding denied to
the earth a fixed position and launched it along its path in space, than it was
sought to fix immovably the sun and the stars. But astronomy has demon-
strated that the sun moves with unswerving regularity through the star- set
tiniverse at the rate of about 50 kilometres per second. Among the so-called
iixed stars are now discerned manifold changes and various orders of move-1
ment. Light, heat, electricity— like sound--have been proved to be modes
of motion; to the realisation of this fact modern science is indebted for
powers which have been used with such brilliant success, and which have been
expounded so clearly at this lecture table by Faraday and by his successors,
As, in the imagination of Dante, the invisible air became peopled with spiritual
beings, so before the eyes of earnest investigators, and especially before those
of Clerk Maxwell, the invisible mass of gases became peopled with particles :
their rapid movements, their collisions, and impacts became so manifest that
it seemed almost possible to count the impacts and determine many of
'the peculiarities or laws of their collisions. The fact of the existence of
these invisible motions may at once be made apparent by demonstrating the
difference in the rate of diffusion through porous bodies of the light and
rapidly moving atoms of hydrogen and the heavier and more sluggish par-
ticles of air. Within the masses of liquid and of solid bodies we have been
forced to acknowledge the existence of persistent though limited motion of
their ultimate particles, for otherwise it would be impossible to explain, for
example, the celebrated experiments of Graham on diffusion through liquid
and colloidal substances. If there were, in our times, no belief in the

454 PRINCIPLES OF CHEMISTRY

molecular motion in solid bodies, could the famous Spring have hoped to
attain any result by mixing carefully-dried powders of potash, saltpetre and,
sodium acetate, in order to produce, by pressure, a chemical reaction between
these substances through the interchange of their metals, and have derived,
for the conviction of the incredulous, a mixture of two hygroscopic though
solid salts — sodium nitrate and potassium acetate ?

In these invisible and apparently chaotic movements', reaching from the
stars to the minutest -atoms, there reigns, however, a harmonious order which
is commonly mistaken for complete rest, but which is really & consequence
of the conservation of that dynamic equilibrium which was first discerned
by the genius of Newton, and which has been traced by his successors in the
detailed analysis of the particular consequences of the great generalisation,
liamely, relative immovability in the midst of universal and active movement.
But the unseen world of chemical changes is closely analogous to the
visible world of the heavenly bodies, since our atoms form distinct portions
of an invisible world, as planets, satellites, and comets form distinct portions
of the astronomer's universe ; our atoms may therefore be compared to the
solar systems, or to the systems of double or of single stars: for example,
ammonia (NH3) may be represented in the simplest manner by supposing
the sun, nitrogen, surrounded by its planets of hydrogen ; and common salt
(NaCl) may be looked on as a double star formed of sodium and chlorine.
Besides, now that the indestructibility of the elements has been acknow-
ledged, chemical changed cannot otherwise be explained than as changes of
motion, and the production by chemical reactions of galvanic qurrents, of
light, of heat, of pressure, or of steam power, demonstrates visibly that the
processes of chemical reaction are inevitably connected with enormous though
unseen displacements, originating in the movements of atoms in molecules*
Astronomers and natural philosophers, in studying the visible motions of the
heavenly bodies .and of matter on the earth, have Tinderstood and have esti-
mated the valuer of this store of energy. But the chemist has had to pursue
a contrary course. Observing in the physical and mechanical phenomena
which accompany chemical reactions the quantity of energy manifested by
the atoms and molecules, he is constrained to acknowledge that within the
molecules there exist atoms in motion, endowed with an energy which, like
matter itself, is neither being created nor capable of being destroyed. There-
fore, in chemistry, we must seek dynamic equilibrium not only between the
molecules, but also in their midst among their component atoms. Many
conditions of such equilibrium have been determined, but much remains to be
done, and it is not uncommon, even in these days, to find that some chemists
forget that there is the possibility of motion in the interior of molecules, and
therefore represent them as being in a condition of death-like inactivity;

Chemical combinations take place with so much ease and rapidity,
possess so many special characteristics, and are so numerous, that their sim-
plicity and order were for a long time hidden from investigators. Sympathy,
relationship, all the caprices or all the fancifulness of human intercourse,
seemed to have found complete analogies in chemical combinations, but with
this difference, that the characteristics of the material substances — such as
eilver, for example, or of any other body — remain unchanged in every sub-

APPENDIX I. 455

division from the largest masses to the smallest particles, and consequently
these characteristics must be properties of the particles. But the world of
heavenly luminaries appeared equally fanciful at man's first acquaintance
With it, so much so, that the astrologers imagined a connection between the
individualities of men and the conjunctions of planets. Thanks to the genius
of Lavoisier and of Dalton, man has been able, in the unseen world of che-
mical combinations, to recognise laws of the same simple order as those
which Copernicus and Kepler proved to exist in the planetary universe. Man
discovered, and continues every hour to discover, what remains unchanged
in chemical evolution, and how changes take place in combinations of the
unchangeable. He has learned to predict, not only what possible combina-
tions may take place, but also the very existence of atoms of unknown elemen-
tary substances, and has besides succeeded in making innumerable practical
applications of his knowledge to the great advantage of his race, and has
accomplished this notwithstanding that notions of sympathy and affinity
still preserve a strong vitality in science. At present we cannot apply
Newton's principles to chemistry, because the soil is only being now prepared.
The invisible world of chemical atoms is still waiting for the creator of che-
mical mechanics. For him our age is collecting a mass of materials, the
inductions of well-digested facts, and many-sided inferences similar to those
which existed for Astronomy and Mechanics in the days of Newton. It is
well also to remember that Newton devoted much time to chemical experi-
ments, and while considering questions of celestial mechanics, persistently
kept in view the mutual action of those infinitely small worlds which are
concerned in chemical evolutions. For this reason, and also to maintain the
unity of laws, it seems to me that we must, in the first instance, seek to
harmonise the various phases of contemporary chemical theories with the
immortal principles of the Newtonian natural philosophy, and so hasten the
advent of true chemical mechanics. Let the above considerations serve as
my justification for the attempt Which I propose to make to act as a champion
of the universality of the Newtonian principles, which I believe are com-
petent to embrace every phenomenon in the universe, from the rotation of
the fixed stars to the interchanges of chemical atoms.

In the first place I consider it indispensable to bear in mind that, up to
quite recent times, only a one-sided affinity has been recognised in chemical
reactions. Thus, for example, from the circumstance that red-hot iron de-
composes water with the evolution of hydrogen, it was concluded that oxygen
had a greater affinity for iron than for hydrogen. But hydrogen, in presence
of red-hot iron scale, appropriates its oxygen and forms water, whence an
exactly opposite conclusion may be formed.

During the last ten years a gradual, scarcely perceptible, but most
important change has taken place in the views, and consequently in the
researches, of chemists. They have sought everywhere, and have always,
found, systems of conservation or dynamic equilibrium substantially similar
to those which natural philosophers have long since discovered in the visible
world, and in virtue of which the position of the heavenly bodies in the
universe is determined. There where one-sided affinities only were at first
•detected, not only secondary or lateral ones have been found, but even those

456 PRINCIPLES OF CHEMISTRY

Which are diametrically opposite ; yet among these, dynamical equilibrium
establishes itself not by excluding one or other of the forces, but regulating
them all. So the chemist finds in the flame of the blast furnace, in the
formation of every salt, and, with especial clearness, in double salts and in
the crystallisation of solutions, not a fight ending in the victory of one side,
as used to be supposed, but the conjunction of forces ; the peace of dynamic
equilibrium resulting from the action of many forces and affinities. Car-
bonaceous matters, for example, burn at the expense of the oxygen of the
air, yielding a quantity of heat, and forming products of combustion, in
which it was thought that the affinities of the oxygen with the combustible
elements were satisfied. But it appeared that the heat of combustion was
competent to decompose these products, to dissociate the oxygen from the
combustible elements, and therefore to explain combustion fully it is neces-
sary to take into account the equilibrium between opposite reactions, between,
those which evolve and those which absorb heat.

In the same way, in the case of the solution of common salt in water, it
is necessary to take into account, on the one hand, the formati6n of compound
particles generated by the combination of salt with water, and, on the other,
the disintegration or scattering of the new particles formed, as well as of
these originally contained. At present we find two currents of thought,
apparently antagonistic to each other, dominating the study of solutions :
according to the one, solution seems a mere act of building up or association ;
according to the. other, it is only dissociation or disintegration. The truth
lies, evidently, between these views ; it lies, as I have endeavoured to prove
by my investigations into aqueous solutions, in the dynamic equilibrium of
particles tending to combine and also to fall asunder. The large majority of
chemical reactions which appeared to act victoriously along one line have
been proved capable of acting as victoriously even along an exactly opposite
line. Elements which utterly decline to combine directly may often be
formed into comparatively stable "compounds by indirect means, as, for ex-
ample, in the case of chlorine and carbon ; and consequently the sympathies
and antipathies which it was thought 'to transfer from human relations to
those of atoms should be laid aside until the mechanism of chemical rela-
tions is explained. Let us remember, however, that chlorine, which does not
form with carbon the chloride of carbon, is strongly absorbed, or, as it were,
dissolved, by carbon, which leads us to suspect incipient chemical action even
in an external and purely surface contact, and involuntarily gives rise to
conceptions of that unity of the' forces of nature which has been so ener-
getically insisted on by Sir William Grove and formulated in his famous
paradox. Grove noticed that platinum, when fused in the oxyhydrogen
flame, during which operation water is formed, when allowed to drop into
water decomposes the latter and produces the explosive oxyhydrogen mixture.
The explanation of this paradox, as of many others which arose during the
period of chemical renaissance, has led, in our time, to the promulgation by
Henri Sainte-Claire Deville of the conception of dissociation and of equili-
brium, and has recalled the teaching of Berthollet, which, notwithstanding its
brilliant confirmation by Heinrich Rose and Dr. Gladstone, had not, up to
that period, been included in received chemical views.

APPENDIX I. 457

Chemical equilibrium in general, and dissociation in particular, are now
being so fully worked out in detail, and supplied in such various ways, that I
do not allude to them to develop, but only use them as examples by which
to indicate the correctness of a tendency to regard chemical combinations
from points of view differing from those expressed by the term hitherto ap-
propriated to define chemical forces, namely, ' affinity.' Chemical equilibria,
dissociation, the speed of chemical reactions, thermochemistry, spectroscopy,
and, more than all, the determination of the influence of masses and the
search for a connection between the properties and weights of atoms and
molecules — in one word, the vast mass of the most important chemical re-
searches of the present day--clearly indicate the near approach of the time
when chemical doctrines will submit fully and completely to the doctrine
which was first announced, in the Principia of Newton.

In order that the application of these principles may bear fruit it is evi-
dently insufficient? to assume that statical equilibrium reigns alone in chemical
systems or chemical molecules: it is necessary to grasp the conditions of
possible states of dynamical equilibria, and to apply to them kinetic prin-
ciples. Numerous considerations compel us to renounce the idea of statical
equilibrium in molecules, and the recent yet strongly- supported appeals to
dynamic principles constitute, in my opinion, the foundation of the modern
teaching relating to atomicity, or the valency of the elements, which usually
forms the basis of investigations into organic or carbon compounds.

This teaching has led to brilliant explanations of very many chemical
relations and to cases of isomerism, or the difference in the properties of
substances having the same composition. It has been so fruitful in its many
applications and in the foreshadowing of remote consequences, especially
respecting carbon compounds, that it is impossible to deny its claims to be
ranked as a great achievement of chemical science. Its practical application
to the synthesis of many substances of the most complicated composition
entering into the structure of organised bodies, and to the creation of an un-
limited number of carbon compounds, among which the colours derived from
coal tar stand prominently forward, surpass the synthetical powers of Nature
itself. Yet this teaching, as applied to the structure of carbon compounds,
is not on the face of it directly applicable to the investigation of other ele-
ments, because in examining the first it is possible to assume that the atoms
of carbon have always a definite and equal number of affinities, whilst in the
combinations of other elements this is evidently inadmissible. Thus, for
example, an atom of carbon yields only one compound with four atoms of
hydrogen and one with four atoms of chlorine in the molecule, whilst the
atoms of chlorine and hydrogen unite only in the proportions of one to one.
Simplicity is here evident, and forms a point of departure from which it is
easy to move forward with firm and secure tread. Other elements are of a
different nature. Phosphorus unites with' three and with five atoms of
chlorine, and consequently the simplicity and sharpness of the application of
structural conceptions are lost. Sulphur unites only with two atoms of
hydrogen, but with oxygen it enters into higher orders of combination. The
periodic relationship which exists among all the properties of the elements-
such, for example,, as their ability to enter into various combinations— and

458 PRINCIPLES OF CHEMISTRY

their,atomic weights, indicate that this variation in atomicity is subject to
one perfectly exact and general law, and it is only carbon and its near
analogues which constitute cases of permanently preserved atomicity. It ia
impossible to recognise as constant and fundamental properties of atoms,
powers which, in substance, have proved to be variable. But by abandoning
the idea of permanence, and of the constant saturation of affinities— that is
to say, by acknowledging the possibility of free affinities— many retain a
comprehension of the atomicity of the elements * under given conditions ; '
and on this frail foundation they build up structures composed of chemical
molecules, evidently only because the conception of manifold affinities gives,
at once, a simple statical method of estimating the composition of the most
complicated molecules.

I shall enter neither into details, nor into the various-consequences follow-
ing from these views, nor into the disputes which have sprung up respecting
them (and relating especially to the number of isomerides possible on the
assumption of free affinities), because the foundation or origin of theories of
this nature suffers from the radical defect of being in opposition to dynamics.
The molecule, as even Laurent expressed himself, is represented as an archi-
tectural structure, the style of which is determined by the fundamental
arrangement of a few atoms, whilst the decorative details, which are capable
of being varied by the same forces, are formed by the elements entering into
the combination. It is on this account that the term ' structural ' is so appro-
priate to the contemporary views of the above order, and that the ' struc-
turalists' seek to justify the tetrahedric, plane, or prismatic disposition of
the atoms' of carbon in benzene. It is evident that the consideration relates
•to the statical position of atoms and molecules and not to their kinetic rela-
tions. The atoms of the structural type are like the lifeless pieces on a chess
board : they are endowed but with the voices of living beings, and are not
those living beings themselves ; acting, indeed, according to laws, yet each
possessed of a store of energy which, in the present state of our knowledge,
must be taken into account.

In the days of Haiiy, crystals were considered in the same statical and
structural light, but modern crystallographers, having become more tho-
roughly acquainted with their physical properties and their actual formation,
have abandoned the earlier views, and have made their doctrines dependent
on dynamics.

The immediate object of this lecture is to show that, starting with
Newton's third law of motion, it is possible to preserve to chemistry all the
advantages arising from structural teaching, without being obliged to build
up. molecules in solid and motionless figures, or to ascribe to atoms definite
limited valencies, directions of cohesion, or affinities. The wide extent of
the subject obliges me to treat only a smalfportion of it, namely otsubstitU'
tions, without specially considering combinations and decompositions, and
even then limiting myself to the simplest examples, which, however, will
throw open prospects embracing all the natural complexity of chemical rela-
tions. For this reason, if it should prove, possible to form groups similar, for
example, to H4 or CH6 as the remnants of molecules CH4 or C2H7 we shall
not pause to consider them, because, as far as we know, they fall asunder into

APPENDIX I. 459

two parts, H2 + H2 or CH4 + H^as soon as they are even temporarily formed,
and are incapable of separate existence, and therefore can take no part in
the elementary act of substitution. With respect to the simplest molecules
which we shall select— that is to say, those of whicn the parts have no sepa-
rate existence, and therefore cannot appear in substitutions— we shall con<
eider them according to the periodic law, arranging them in direct dependence
on the atomic weight of the elements.

Thus, for example, the molecules of the simplest hydrogen compounds-—

HF H40 H3N H4C

hydrofluoric acid water ammonia methane

correspond with elements the atomic weights of which decrease consecutively
F = 19, 0 = 10, N = 14, C = 12.

Neither the arithmetical order (1, 2, 3, 4 atoms of hydrogen) nor the total
information we possess respecting the elements will permit us to interpolate
Into this typical series one more additional element ; and therefore we have
here, for hydrogen compounds, a natural base on which are built up thos&
simple chemical combinations which we take as typical. But even they arfc
competent to unite with each other, as we see, for instance, in the property
which hydrofluoric acid has of forming a hydrate— that is, of combining with
water ; and a similar attribute of ammonia, resulting in the formation of a
caustic alkali, NH3,H,0, or NH4OH.

Having made these indispensable preliminary observations, I may now
,attack.the problem itself and attempt to explain the so-called structure or
•father construction, of molecules -r- that is to say, their constitution and trans-
"formations — without having recourse to the teaching of ' structuralists,' but on
Newton's dynamical principles.

Of Newton's three laws of motion, only the third can be applied directly
to chemical molecules when regarded as systems of atoms among which it
must be supposed that there exist common influences or forces, and resulting
compounded relative motions. Chemical reactions of every kind are un-
doubtedly accomplished by changes in these internal movements, respecting
the nature of which nothing is known at present, but the existence of which
the mass of evidence collected in modern times forces us to acknowledge as
forming part of the common motion of the universe, and as a fact further
established by the circumstance that chemical reactions are always charac-
terised by changes of volume .or the relations between the atoms or the
molecules. Newton's third law, which is applicable to every system, declares
that, ' action is also associated with reaction, and is equal to it.' The
brevity of conciseness of this axiom was, however, qualified by Newton in-
a more expanded statement, 'the action of bodies one upon another ate
always equal, and in opposite directions.' This simple fact constitutes tb.0
point of departure for explaining dynamic equilibrium— that is to say, systems
of conservancy. Jt is capable of satisfying even the dualists, and of explain-
ing, Without additional assumptions, the preservation of those chemical types
.which Dumas, Laurent, and Gerhardt created unit types, and those views of
atomic combinations which the structuralists express by atomicity or the

460 PRINCIPLES OF CHEMISTRY

valency of the elements, and, in connection with them, the various numbers
of affinities. In reality, if a system of atoins or a molecule he g'ven, then in
it, according to the third law of Newton, each portion of atoms acts on the
remaining portion in the same manner, and with the same force as the
second set of atoms acts on the first. We infer directly from this considera-
tion that both sets of atoms, forming a molecule, are not only equivalent with,
regard to themselves, as they must be according to Dalton's law, but also that
they may, if united, replace each other. Let there be a molecule containing
.atoms A B C, it is clear that, according to Newton's law, the action of A on
B C must be equal to the action of B C on A, and if the first action is directed
on B C, then the second must be directed on A, and consequently then, where
A can exist in dynamic equilibrium, B C may take its place and act in a like
manner. In the same way the action of C is equal to the action of A B. In
one word every two sets of atoms forming a molecule are equivalent to each
other, and may take each other's place in other molecules, or, having the
power of balancing each other, the atoms or their complements are endowed
with the power of replacing each other. Let us call this consequence of an
evident axiom ' the principle of substitution,' and let us apply it to those
typical forms of hydrogen compounds which we have already discussed, and
which, on account of their simplicity and regularity, have served as starting--
points of chemical argument long before the appearance of the doctrine of
structure.

In the type of hydrofluoric acid, HF, or in systems of double stars, are
included a multitude of the simplest molecules. It will be sufficient for our
purpose to recall a few : for example, the molecules of chlorine, Cl^, and of
hydrogen, H^, and hydrochloric acid, HC1, which is familiar to all in aqueous
solution as spirits of salt, and which has many points of resemblance with
HF, H Br, HI. In these cases division into two parts can only be made in
one way, and therefore the principle of substitution renders it probable that
exchanges between the chlorine and the hydrogen can take place, if they are
competent to unite with each other. There was a time when no chemist
would even admit the idea of any such action ; it was then thought that the
power of combination indicated •o, polar difference of the molecules in com-
bination, and this thought set aside all idea of the substitution of one com-
ponent element by another.

Thanks to the observations and experiments of Dumas and Laurent fifty
years ago, such fallacies were dispelled, and in this manner the principle
of substitution was exhibited. Chlorine and bromine acting on many
hydrogen compounds, occupy immediately the place of their hydrogen, and
the displaced hydrogen, with another atom of chlorine or bromine, forms
hydrochloric acid or bromide of hydrogen. This takes place in all typical
hydrogen compounds. Thus chlorine acts on this principle on gaseous
hydrogen — reaction, under the influence of light, resulting in the formation
of hvdrochloric acid. Chlorine acting on the alkalis, constituted similarly to
water, and even o'n water itself — only, however, under the influence of light
and only partially because of the instability of HC1O— forms by this principle
bleachirg salts, which are the same as the alkalis, but with their hydrogen
replaced by chlorine. In ammonia and in methane, chlorine can also replace

APPENDIX 1 461

the hydrogen. From ammonia is formed in this manner the so-called
chloride of nitrogen, NC13, which decomposes very readily with violent explo-
sion on account of the evolved gases, and falls asunder as chlorine and
nitrogen. Out of marsh gas, or methane, CH4, may be obtained consecu-
tively, by this method, every possible substitution, of which chloroform,
CHC13, is the best known, and carbon tetrachloride, CC14, the most instruc-
tive. But by virtue of the fact that chlorine and bromine act, in the manner
shown, on the simplest typical hydrogen compounds, their action on the
more complicated ones may be assumed to be the same. This can be easily
demonstrated. The hydrogen of benzene, CtiH6, reacts feebly under the influ-
ence of light on liquid bromine, but Gustavson has shown that the addition
of the smallest quantity of metallic aluminium causes energetic action and
the evolution of large volumes of hydrogen bromide.

If we pass on to the second typical hydrogen compound — that is to say,
water — its molecule, HOH, maybe split up in two ways : either into an atom
of hydrogen and a semi-molecule of hydrogen peroxide, HO, or into oxygen,
O, and two atoms of hydrogen, H ; and therefore, according to the principle
of substitution, it is evident that one atom of hydrogen can exchange
with hydrogen oxide, HO, and two atoms of hydrogen, H, with one atom of
oxygen, 0.

Both these forms of substitution will constitute methods of oxidation—
that is to say, of the entrance of oxygen into the -compound— a reaction
which is so common in nature as well as in the arts, taking place at the
expense of the oxygen of the air or by the aid of various oxidising sub-
stances or bodies which part easily with their oxygen. There is no occasion
to reckon up the unlimited number of cases of such oxidising reactions. It
is sufficient to state that in the first of these oxygen is directly transferred,
and the position, the chemical function, which hydrogen originally occupied,
is, after the substitution, occupied by the hydroxyl. Thus ammonia, NH3,
yields hydroxylamine, NH2(OH), a substance which retains many of the
properties of ammonia.

Methane and a number of other hydrocarbons yield, by substitution of
the hydrogen by its oxide, methyl alcohol, CH3(OH), and other alcohols. The
substitution of one atom of oxygen for two atoms of hydrogen is equally
common with hydrogen compounds. By this means alcoholic liquids con»
taining ethyl alcohol, or spirits of wine, C.2H-,(OH), are oxidised until they
become vinegar, or acetic aci>d, C2H30(OH). In the same way caustic
ammonia, or the combination of ammonia with water, NH3,H20, or NH4(OH),
which contains a great deal of hydrogen, by oxidation exchanges four atoms
of hydrogen for two atoms of oxygen, and becomes converted into nitric acid,
NO,(OH). This process of conversion of ammonium salts into saltpetre goes
on in the fields every summer, and with especial rapidity hi tropical countries.
The method by which this is accomplished, though complex, though involving
the agency of all-permeating micro-organisms, is, in substance, the same as
that by which alcohol is converted into acetic acid, or glycol, C2H4(OH)2, into
oxalic acid, if we view the process of oxidation in the light of the Newtonian
principles.

But while speaking of the application of the principle of substitution to

462 PRINCIPLES OF CHEMISTRY

water, we need not multiply instances, but must turn our attention to two>
special circumstances which are closely connected with the very mechanism,
of substitutions.

In the first place, the replacement of two; atoms of hydrogen by one atom
of oxygen may take place in two ways, because the hydrogen molecule is
composed of two atoms, and therefore, under the influence of oxygen, the
molecule forming water may separate before the o. gen has time to take its
place. It is for this reason that we find, during the conversion of alcohol
into acetic acid, that there is an interval during which is formed aldehyde*
C2H40, which, as its very name implies, is « alcohol dehydrogenatum,' or
alcohol deprived of hydrogen. Hence aldehyde combined with hydrogen
yields alcohol ; and united to oxygen, acetic acid.

For the same reason there should be, and there actually are, intermediate
products between ammonia and nitric acid, N08(HO), containing either less
hydrogen than ammonia, less oxygen than nitric acid, or less water than
caustic ammonia. Accordingly we find, among the products" of the deoxida-
tion of nitric acid and the oxidation of ammonia, not only hydroxylamine,
but also nitrous oxide, nitrous and nitric anhydrides. Thus, the production
of nitrous acid results from the removal of two a'toms of hydrogen from
caustic ammonia and the substitution .of the oxygen for the hydrogen,
NO(OH) ; or by the substitution, in ammonia, of three atoms of hydrogen by
hydroxyl, N(OH)3, and by the removal of water: N(OH)3-HiO = NO(OH).
The peculiarities and properties of nitrous acid — as, for instance, its action on
ammonia and its conversion, by oxidation, into nitric acid— are thus clearly
revealed

On the other hand, in speaking of the principle of substitution as applied
to water, it is necessary to observe that hydrogen and hydroxyl, H and OH,
are not. only competent to unite, but also to form combinations with them-
selves, and thus become H^ and Hj0.2 ; and such are hydrogen and the
peroxide thereof. In general, if a molecule A B exists, then molecules A A
and BB can exist also. A direct reaction of this kind does not, however,
take place in water, therefore undoubtedly, at the moment of formation,
hydrogen reacts on hydrogen peroxide, as we can show at once by
experiment; and further because hydrogen peroxide, HjOa, exhibits a
-structure containing a molecule of hydrogen, H2, and one of oxygen, 02,
«ither of which is capable of separate existence. The fact, however, may
now be taken as thoroughly established, that, at the moment of combustion
of hydrogen or of the hydrogen compounds, hydrogen peroxide, is always
formed, and not only so, but hi all probability its formation invariably pre-
cedes the formation of water. This was to be expected as a consequence of
the law of Avogadro and Gerhardt, which leads us to expect this sequence
in the case of equal interactions of volumes of vapours and gases ; and in
hydrogen peroxide we actually have such equal volumes of the elementary

The. instability of hydrogen peroxide— that is to say, the ease with
which it decomposes into water and oxygen, even at the mere contact of
porous substances— accounts for the circumstance that it does not form a per-
manent product of combustion, and is not produced during the decomposition

APPENDIX I. 463

of water. I may mention this additional consideration that, with respect
to hydrogen peroxide, we may look for its effecting still farther substitu-
tions of hydrogen by means of which we may expect to obtain still more
highly oxidised water compounds, such as Hj03 and H,,O4. These Schonbein
and Bunsen have long been seeking, and Berthelot is investigating 'them
a,t present. It is probable, however, that the reaction will stop at the
last compound, because we find that, in a number of cases, the addition of
four atoms of oxygen seems to form a limit. Thus, Os04, KC104, KMn04,

4, Na.,P04, and such like, represent the highest grades of oxidation.1
As for the last forty years, from the times of Berzelius, Dumas, Liebig,
Gerhardt, Williamson, Frankland, Kolbe, Kekule*, and Butleroff, most theo-
retical generalisations have centred round organic or carbon compounds,
we will, for the sake of brevity, leave out the discussion of ammonia deriva-
tives, notwithstanding their simplicity with respect to the doctrine of substi-
tutions ; we will dwell more especially on its application to carbon compounds,
starting from methane, CH4, as the simplest of the hydrocarbons, containing
in its molecule one atom of carbon. According to the principles enumerated
we may derive from CH4 every combination of the form CH3X, CB^X,,
CHX3, and CX4, in which X is an element, or radicle, equivalent to hydrogen—
that is to say, competent to take its place or to combine with it. Such are
the chlorine substitutes already mentioned, such is wood-spirit, CHa(OH), in
which X is represented by the residue of water, and such are numerous other
carbon derivatives. If we continue, with the aid of hydroxyl, further substi-
tutions of the hydrogen of methane we shall obtain successively CH2(OH)8,
CH(OH)3, and C(OH)4. But if, in proceeding thus, we bear in mind that
<5H2(OH)2 contains two hydroxyis in the same form as hydrogen peroxide,
HjO2 or (OH)*, contains them — and moreover not only in one molecule, but
together, attached to one and the same atom of carbon — so here we must
look for the same decomposition as that which we find in hydrogenperoxide,
And accompanied also by the formation of water as an independently
existing molecule ; therefore CH,j(OH)8 should yield, as it actually does, im-
mediately water and the oxide of methylene, CH.^0, which is methane with

1 Because more than four atoms of hydrogen never unite with one atom of the ele«
ments, and because the hydrogen compounds (e.g. HC1, HjS, H3P, H4Si) always form
their highest oxides with four atoms of oxygen, and as the highest forms of oxides (Os04,
Eu04) also contain four of oxygen, and eight groups of the periodic system, corresponding
to the highest basic oxides B-jO, BO, B2O3, BO8, RjOs, R03, B207, and RO4, imply the
above relationship, and because of the nearest analogues among the elements— such as
Mg, Zn, Cd, and Hg ; or Cr, Mo, W, and U ; or Si, Ge, Sn, and Pt; or P, Cl, Br, and I,
and so forth— not more than four are known, it seems to me that in these relationships
there lies a deep interest and meaning with regard to chemical mechanics. But because,
to my imagination, the idea of unity of design in Nature, either acting in complex
celestial systems or among chemical molecules, is very attractive, especially because the
atomic teaching at once acquires its true meaning, I will recall the following facts re-
lating to the solar system. There are eight major plarets, of which the four inner ones
are not only separated from the -four outer by asteroids, but differ from, them in many
respects, as, for example, in the smallness of their diameters and their greater density.
Saturn with his ring has eight satellites, Jupiter and Uranus have each four. It is evi-
dent that in the solar systems also we meet with these higher numbers four and eight
which appear in the- combination of chemical molecules.

*1

464 PRINCIPLES Off CHEMISTRY

oxygen substituted for two atoms of hydrogen. Exactly in the same manner
qut of CH(OH)3 are formed water and formic acid, CHO(OH), and out of
C(OH)4 is produced water an i carbonic acid, or directly carbonic anhydride,
CQ2? which will therefore be nothing else than methane with the double re-
.placement of pairs of hydrogen by oxygen. As nothing leads to the supposi-
tion that the four atoms of hydrogen in methane differ one from the other,
so it does not matter by what means we obtain any one of the combinations
indicated— they will be identical ; that is to say, there will be no case of
actual isomerism, although there may easily be such cases of isomerisui as
have been distinguished by the term metamerism.

Formic acid, for example, has two atoms of hydrogen, dne attached to the<
Carbon left from the methane, and the other attached to the oxygen which
ha's entered in the form of hydroxyl, and if one of them be replaced by some
substance X it is evident that we shall obtain substances of the same composi-
tion, but of different construction, or of different orders of movement among
the molecules, and therefore endowed with other properties and reactions. If
X be methyl, CH3— -that is to say, a group capable of replacing hydrogen
because it is actually contained with hydrogen hi methane itself— then by
substituting this group for the original hydrogen we obtain acetic acid,
CCILjO(OH), out of formic, and by substitution of the hydrogen in its oxide or
hydroxyl we obtain methyl formate, CHO(OCH3). These substances differ so
much from each Other physically and chemically that at first sight it is hardly
possible to admit that they contain the same atoms in identically the same
proportions. Acetic acid, for example, boils at a higher temperature than
water, and has a higher specific gravity than it, whilst its metameride,
methyl formate, is lighter than water, and boils at 80° — that is {o say, it
evaporates very easily.

Let us now turn to carbon compounds containing two atoms of carbon to
the molecule, as. in acetic acid, and proceed to evolve them from methane by
the principle of substitution. This principle declares at once that methane
can only be split up in the four following ways : —

1. Into a group CH3 equivalent with H. Let us call changes of this
nature methylation.

2. Into a group CBLj and H.J. We will call this order of substitutions
methylenation.

3. Into CH and H3, which commutations we will call acetylenatioru

4. Into C and H4, which may be called carbonation.

It is evident that hydrocarbon compounds containing two atoms of carbon
can only proceed from, methane, CH4, which contains four atoms of hydrogen
by the first three methods of substitution ; carbonation would yield free carbon
if it could take place directly, and if the molecule of free carbon— which is in
reality very complex, that is to say strongly polyatomic, as I have long since
been proving by various means — could contain only C2 like the molecules
O..,, H^; N2, and so on.

By methylation we should evidently obtain from marsh gas, ethane,

By methylenation — that is, by substituting group CH,, for H, — methane
forms ethylene, CHjCH., = CaH4.

APPENDIX I. 465

By acetylenation-s-that is, by substituting three atoms of hydrogen, H3, in
methane — by the remnant CH, we get acetylene, CHCH^O^ELj.

If we have applied the principles of Newton correctly, there should not be
any other hydrocarbons contain ing two atoms of carbon in the molecule.
All these combinations have long been known, and in each of them we can
not only produce those substitutions of which an example has been given in
the case of methane, but also all the phases of other substitutions, as we shall
find frorn'a few more instances, by the aid of which I trust that I shall be
able to show the great complexity of those derivatives which, on the principle
of substitution, can be obtained from each hydrocarbon. Let us content our-
Selves with the case of ethano, CHgCHj, and the substitution of the hydrogen
*by hydroxyl. The following are the possible changes :—

1. CH3CH9(OH) : this is nothing more than spirit of wine, or ethyl
alcohol, C8H5(OH) or C8HeO.

2. CH^OEOCSiOH) : this is the glycol of Wtirtz^ which has shed so
much light on the history of alcohol. Its isomeride may be CH3CH(OH)a,
but as we have seen in the case of CH(OH),j, it decomposes, giving off water,
and forming aldehyde, CH3CHO, a substance" capable of yielding alcohol by
uniting with hydrogen, and of yielding acetic acid by uniting with oxygen.

If glycol, CE,(OH)GH..(OH), loses its water, it may be seen at once that

it will not now yield aldehyde, CH8CHO, but its isomeride, CH2^5Ha' the

oxide of ethylene. I have here indicated in a special manner the oxygen
which has taken the place of two atoms of 'the hydrogen of ethane taken
from different atoms of the carbon.

8. CH3C(OH)8 decomposed as CH(OH)3, forming water and acetic acid,
CH3CO(OH). It is evident that this acid is nothing else than formic acid,
CHO(OH), with its hydrogen replaced by methyl. Without examining
further the vast member of possible derivatives, I will direct your attention
to the circumstance that in dissolving acetic acid in water we obtain the
maximum contraction and the greatest viscosity when to the molecule
<CH3CO(OH) is added a molecule of water, which is the proportion which
would form the hydrate CHgC(OH)g. It is probable that the doubling of
the molecule of acetic acid at temperatures approaching its boiling-point
has some connection with this power of uniting with one molecule of
water.

4. CH2(OH)C(OH)8 is evidently an alcoholic acid, and indeed this com-
pound, after losing water, answers to glycolic acid, CH2(OH)CO(OH).
Without investigating all the possible isomerides, we will note only that the
hydrate CH(QH)8CH(OH)3 has the same composition as CH^OHJCfOH)^
and although corresponding to glycol, and,being a symmetrical substance, it
becomes, on parting with its water, the aldehyde of oxalic acid, or the glyoxal
of Debus, CHOCHO.

6. CHCOH^CCOHa), from. the tendency of all the preceding, eorresporrdfli
with glyoxylio acid,, an aldehyde acid, CHOCO(OH), because th.e group
CO (OH), or carboxyl, enters into the compositions of organic acids, and the)
group CHO defines the aldehyde function.

& C(OH)3C(OH)8 through the loss of 2H,Q yields the bibaeio oxalic aci4

466 PRINCIPLES OP CHEMISTRY

CO(OH)CO(OH), which generally .crystallises with 2H,0, following thus the
normal type of hydration characteristic of ethane.8

Thus, by applying the principle of substitution, we can, in the simplest
manner, derive not only every kind of hydrocarbon compound, such as the
alcohols, the aldehyde-alcohols, aldehydes, alcohol-acids, and the acids, but
also combinations analogous to hydrated crystals which usually are dis»
regarded.

But even those unsaturated substances, of which ethylene, CH,CH,, and
acetylene, CHCH, are types, may be evolved with equal simplicity. With
respect to the. phenomena of isomerism, there are many possibilities among
the hydrocarbon compounds conta;ning two atoms of carbon, and without
going into details it will be sufficient to indicate that the following formulae,
though not identical, will be isomeric substantially among themselves : —
CH8CHX2 and CH^XCH^X, although both contain C.2H4X3 ; or CH,CX, and
CHXCHX, although both contain CjELjXj, if by X we indicate chlorine or
generally an element capable of replacing one atom of hydrogen, or capable
of uniting with it. To isomerism of this kind belongs the case of aldehyde
and the oxide of ethylene, to which we have already referred, because both
have the composition C2H40.

What I have said appears to me sufficient to show that the principle of
substitution adequately explains the composition, the isomerism, and all the
diversity of combination of the hydrocarbons, and I shall limit the further
development of these views to preparing a complete list of every possible
hydrocarbon compound containing three atoms of carbon in the molecule.
There are eight in all, of which only five are known at present.3

Among those possible for C3H6 there should be two isomerides, propylene
and trimethyleue, and they are both already known. For C3H4 there should
be three isomerides : allylene and allene are known, but the third has not
yet been discovered ; and for C3H,j there should be two isomerides-, though
neither of them is. known as yet. Their composition and structure are easily

9 One more isomeride, CH2CH(OH), is possible — that is, secondary vinyl alcohol,
which is related to etbylene, CH3CH2, but derived by the principle of substitution from
CH4. Other isomerides, of the composition C^O, such, for example, AS CCH3(OH),,
are impossible, because it would correspond with the hydrocarbon CHCH3=C2H4, which
is isomeric with ethylene, and it cannot be derived from methane. If such an isomeride
existed it would be derived from CH^but such products are, up to the present, unknown.
In such cases the insufficiency of the points of departure of the statical structural teach-
ing is shown. It first admits constant atomicity and then rejects it, the facts serving to
establish either one or the other view ; and. therefore it seems to me that we must come
to the conclusion that the structural method of reasoning, having done a service to
science, has outlived the age, and must be regenerated, as in their time was the teaching
of the electro-chemists, the radicalists, and the adherents of- the doctrine of types. As
we cannot now lean on the views above stated, it is time to abandon the structural
theory They will all be united in chemical mechanics, and the principle of substitution
must be looked on only as a preparation for the coming epoch in chemistry, where
such cases as the isomerism of f umaric and maleic acids, when explained dynamically, ae
proposed by Le Bel and, Van't Hoff, may yield points of departure.

5 Conceding variable atomicity, the structuralists must expect an incomparably larger
Dumber of isomerides, and they cannot now decline to acknowledge the change of
atomicity, were it only for the examples HgCl and HgCla, CO and COa, PC13 and PCfe,

APPENDIX I. 467

deduced from ethane, ethylene, and acetylene, by methylation, by methylena*
tion, by acetylenation and by carbonation.

1. C3H8 o CHjCHaCH^ out of CH8CH3 by methylation. This hydro-
carbon is named propane.

2. C3Hfl « CH3CHCH, out of CH3CH3 by methylenation. This eub»
stance is propylene.

8. C9He - CHjCH^CHj out of CHaCH3 by methylenation. This sub.
stance is trimethylene.

4. C3H4 » CH3CCH out of- CH3CH3 by acetylenation or from CHCH by
methylation. This hydrocarbon is named allylene.

6. C3H4 » out of CH3CH3 by acetylenation, or from CHjCHa by

methylenation, because C^H « C^?H This body is as yet unknown.

6. C8H4 = CHjCCHj out of CHjCH, by methylenation. This hydro*
carbon is named allene, or iso-allylene.

fTTPTT

7. C8Hj e ^ Qa out of CH3CH3 by symmetrical carbonation, or out

of CH^CHj by acetylenation. This compound is unknown.
i~i 1-1

8. CgH^ «= ^ out of CH3CH3 by carbonation, or out of CHCH by

methylenation. This compound is unknown.

If we bear in mind that for each hydrocarbon serving as a type in the
above tables there are a number of corresponding" derivatives, and that every
compound obtained may, by further methylation, methylenation, acetylena*
tion, and carbonation, produce new hydrocarbons, and these may be followed
by a numerous suite of derivatives and an immense number of isomerio
substances, it is possible to understand the limitless number of carbon com*
pounds, although they all have the one substance, methane, for their origin*
The number of substances is so enormous that it is no longer a question of
enlarging the possibilities of discovery, but rather of finding some means of
testing them analogous to the well-known two which for a long time have
served as gauges for all carbon compounds.

I refer to the law of even numbers and to that of limits, the first enunciated
by Gerhardt some forty years ago, with respect to hydrocarbons, namely,
that their molecules always contain an even number of atoms of hydrogen.
But by the method which I have used of deriving all the hydrocarbons from
methane, CH4, this law may be deduced as a direct consequence of the
principle of substitutions. Accordingly, in methylation, CH3 takes the place
of H, and therefore CH4 is added. In methylenation the number of atoms of
hydrogen remains unchanged, and at each acetylenation it is reduced by two,
and in carbonation by four, atoms— that is to say, an even number of atoms/
of hydrogen is always added or removed. And because the fundamental
hydrocarbon, methane, CH4, contains an even number of atoms of hydrogen,
all its derivative hydrocarbons will also contain even numbers of hydrogen,
and this constitutes the law of even numbers.

The principle of substitutions explains with equal simplicity the conception
of the limiting compositions of hydrocarbons C«H.,,,+2, which I derived, in

$68 PRINCIPLES t)p CHEMISTRY'

1861,4 in an empirical manner from accumulated materials available at that?
time, and on the basis of the limits to combinations worked out by Dr. Frank-
land for other elements.

Of all the various substitutions the highest proportion of hjdrogen is
yielded by methylation, because in that operation alone does the quantity of
hydrogen increase; hence, taking methane as a point of departure, if we
imagine methylation effected (n - 1) time ,^.we obtain hydrocarbon compounds-
containing the highest quantities of hydrogen,- It is evident that they will
contain CH4 + (n - ^CH^ or C«H,jn + 2, because methylation leads to the addi-
tion of CH2 to the compound.

It will thus be seen that by the principle of substitution — that is to say,
by the third law of Newton— we are able to deduce, in the simplest manner,
not only the individual composition, the isomerism, and relations of sub-
stances, but also the general laws which govern their most complex combina-
tions without having recourse either to statical constructions, to the definition
of atomicities, to the exclusion of free affinities, or to the recognition of those
single, double or treble bonds which are so indispensable to structuralists in the
explanation of the composition and construction of hydrocarbon compounds.
And yet, by the application of the dynamical principles of Newton, we can
attain to that chief and fundamental object, the comprehension of isomerism
in hydrocarbon compounds, and the forecasting of the Existence of combina-
tions as yet unknown, by which the edifice raised by structural teaching is
strengthened and supported. Besides — and I count this for a circumstance
of special importance — the process which I advocate, will make no difference
in those special cases which have been already so .well worked out, such as,
for example, the isomerism of the hydrocarbons and alcohols, even to the
extent of not interfering with the nomenclature which has been adopted, and
the structural system will retain all the glory of having worked up, in a
thoroughly scientific manner, the store of information which Gerhardt had
accumulated about the middle of the fifties, and the still higher glory of
establishing the rational synthesis of organic substances. Nothing will be
lost to the structural doctrine except its statical origin ; and as soon as it
will embrace the dynamic^ principles of Newton, and suffer itself to be guided
by them, I believe that we shall attain for chemistry that unity of principle,
which is now wanting. Many an adept will be attracted to that brilliant and
fascinating enterprise, the penetration .into the unseen worjd of the kinetid
relations of atoms, to the study of which the last twenty-five years have con-
tributed so much labour and such high inventive faculties.

£)' Alembert found in mechanics that if inertia be taken to represent force,
dynamic equations may be applied to statical questions, which are thereby
rendered more simple and more easily understood.

The structural doctrine in chemistry has unconsciously followed the same
course, and therefore its terms are easily adopted ; they may retain their
present forms provided that a truly dynamical— that is to say, Newtonian-?-
meaning be ascribed to them.

Before finishing my task and demonstrating the possibility of adapting

Essai d'une thgorie sur les limites des combinaisons organiques,' par D. MendelSeff,
$/ll aotit 1861, Bulletin de VAcadimie i. d. Sc. de St. Pttersbourg, t. v

APPENDIX I, 469

structural doctrines to the dynamics of Newton, I consider it indispensable
to touch on one question which naturally arises, and which I have heard
discussed more than once. If bromine, the atom of which is eighty times
heavier than that of 'hydrogen, takes the place of hydrogen, it would eeem
that the whole system of dynamic equilibrium must be destroyed.

Without entering into the minute analysis of this question, I think it
mil be sufficient to examine it by the light of two well-known phenomena,
one of which will be found in the department of chemistry and the other in
that of celestial mechanics, and both will serve to demonstrate the existence
of that unity in the plan of creation which is a consequence of the Newtonian
doctrines. Experiments demonstrate that when a heavy element is substi-
tuted for a light one in a chemical compound — for example, for magnesium,
in the oxide of that metal, an atom of mercury, which is 8£ times heavier—-
the chief chemical characteristics or properties are generally, though not
always, preserved.

The substitution of silver for hydrogen, than which it is 108 times heavier,
does not affect all the properties of the substance, though it does some.
Therefore chemical substitutions of this kind— -the substitution of light for
heavy atoms— need not necessarily entail changes in the original equilibrium ;
and this point is still further elucidated by the consideration that the periodic
law indicates the degree of influence of an increment of weight in the atom
as affecting the possible equilibria, and also what degree of increase hi the
weight of the atoms reproduces some, though not all, of the properties of the
substance.

This tendency to repetition — these periods — may be likened to those
annual or diurnal periods with which we are so familiar on the earth. Days
and years follow each other, but, as they do BO, many things change ; and in
like manner chemical evolutions, changes in the masses of the elements,
permit of much remaining undisturbed, though many properties undergo
alteration. The system is maintained according to. the laws of conservation
in nature» but the motions are altered in consequence of the change of parfe.

Next, let us take an astronomical case — such, for example, as. the earth and
the moon-— and let us imagine that the mass of tne latter is constantly
increasing. The question is, what will then occur ? The path of the moon
in space is a wave-line similar to that which geometricians have named epi-
cycloidal, or the locus of a point in a circle rolling round another circle. But
in consequence of the influence of the moon it is evident that the path of the
earth itself cannot be a geometric ellipse, even supposing the sun to be im-
movably fixed ; it must be an epicycloidal curve, though not very far removed
from the true ellipse — that is to say, it will be impressed with but faint un»
dulatiohs. It is only the common centre of gravity of the earth and the
moon which describes a true ellipse round the sun. If the moon were to
increase, the relative undulations of the earth's path would increase in ampli-
tude, those of the moon would also change, and when the mas.s of the moon
had increased to an equality with that of the earth, the path would consist of
epicycloidal curves crossing each other, and having opposite phases. Bu$ a
eunilar relation exists between the sun and the earth, because the former is
also moving in space. "We way apply these views to the world of atoms, and

470 PRINCIPLES OF CHEMISTRY

'flupposs that in their movements, when heavy ones take the place of those
that are lighter, similar changes take place, provided that the system or the
molecule is preserved throughout the change.

It seems probable that in the heavenly systems, during incalculable
astronomical periods, changes have taken place and are still going on similar
ta those which pass rapidly before our eyes during the chemical reaction of
molecules, and the progress of molecular mechanics may — we hope will— in
course of time permit us to explain those changes in the stellar world which
have more than once been noticed by astronomers, and which are now so
carefully studied. A coming Newton, will discover the laws of these changes.
Those laws, when applied to chemistry, may exhibit peculiarities, but these
will certainly be mere variations on the grand harmonious theme which
reigns in nature. The discovery of the laws which produce this harmony in
chemical evolution will only be possible, it seems to me, under the banner
of Newtonian dynamics, which has so. long waved over the domains of
mechanics, astronomy, and physics. la calling chemists to take their stand
under its peaceful and catholic shadow I imagine that I am aiding in estab-
lishing that scientific union which the managers of the Boyal Institution
wish to effect, who have shown their desire to do so by the flattering invita-
tion which has given me— a Bussian — the opportunity of laying before the
countrymen of Newton an attempt to apply to chemistry one of his immortal
principles.

APPENDIX II

THE PERIODIC LAW OF THE CHEMICAL ELEMENTS

BY PROFESSOR MENDELEEFF

FARADAY LECTURE DELIVERED BEFORE THE FELLOWS OF
THE CHEMICAL SOCIETY IN THE THEATRE OF THE ROYAL INSTITUTION,
ON TUESDAY, JUNE 4, 1889

THE high honour bestowed by the Chemical Society in inviting me to pay a
tribute to the world-famed name of Faraday by delivering this lecture has
induced me to take for its subject the Periodic Law of the Elements — this
being a generalisation in chemistry which has1 of late attracted much
attention.

While science is pursuing a steady onward movement, it is convenient
from time to time to cast a glance back on the route already traversed, and
especially to consider the new conceptions which aim at discovering the
general meaning of the stock of facts accumulated from day to day in our
laboratories. Owing to the possession of laboratories, modern science now
bears a new character, quite unknown, not only to antiquity, but even to the
preceding century. Bacon's and Descartes' idea of submitting the mechanism
of science simultaneously to experiment and reasoning has been fully realised
in the case of chemistry, it having become not only possible but always
customary to experiment. Under the all-penetrating control of experiment,
a new theory, even if crude, is quickly strengthened, provided it be founded
on a sufficient basis ; the asperities are removed, it is amended by degrees,
and soon loses the phantom light of a shadowy form or of one founded on
mere prejudice ; it is able to lead to logical conclusions, and to submit to ex«
perimental proof. Willingly or not, in science we all must sttbmit not to what
seems to us attractive from one point of view or from another, but to what
represents an agreement between theory and experiment ; in other words, to
demonstrated generalisation and to the approved experiment. Is it long
since many refused to accept the generalisations involved in the law of Avo-
gadro and Ampere, so widely extended by Gerhardt ? We still may hear the
voices of its opponents ; they enjoy perfect freedom, but vainly will their
voices rise so long as they do not use the language of demonstrated facts

172 PRINCIPLES OF CHEMISTRY

The striking observations with the spectroscope which have permitted us to
analyse the chemical constitution of distant worlds, seemed, at first, appli-
cable to the task of determining the nature of the atoms themselves ; but the
working out of the idea in the laboratory soon demonstrated that the charac-
ters of spectra are determined, not directly by the atoms, but by the mole-
cules into which the atoms are packed ; and so it became evident that more
verified facts must be collected before it will be possible to formulate new
generalisations capable of taking their place beside those ordinary ones based
upon the conception of simple substances and atoms. But as the shade of the
leaves and roots of living plants, together with the relics of a decayed vege-
tation, favour the growth of the seedling and serve to promote its luxurious
development, in like manner sound generalisations— together with the relics
of those which have proved to be untenable-— promote scientific productivity,
and ensure the luxurious growth of science under the influence of rays ema-
nating from the centres of scientific energy. Such centres are scientific
associations and societies. Before one of the oldest and most powerful of
these I am about to take the liberty of passing in review the twenty years' life
of a generalisation which is known under' the name of the Periodic Law. It
vf&s in March 1869 that I ventured to lay before the then youthful Kussian
Chemical Society the ideas upon the same subject which I had expressed in
my just written ' Principles of Chemistry.'

Without entering into details, I will give the conclusion* I then arrived
at in the very worfls I used :—

• 1. The elements, if arranged according 'to their atomic weights, exhibit
on evident periodicity of properties.

4 2. Elements which are similar as regards their chemical properties have
atomic weights which are either of nearly the Same value (e.g. platinum,
iridium, osmium) or which increase regularly (e.g. potassium, rubidium,
caesium).

' 8. The arrangement of the elements, or of groups of elements, in the
order of their atomic weights, corresponds to their so-called valencies as well
as, to some extent, to their distinctive chemical properties— as is apparent,
among other series, in that of lithium, beryllium, barium, carbon, nitrogen,
oxygen, and iron.

• 4. The elements which are the most widely diffused have small atomic
weights.

*6. The magnitude of the atomic weight determines the character of the
dement, just as the magnitude of the molecule determines the character of
6 compound.

4 6. Wo must expect the discovery of many -yet wnknown elements— for
example, elements analogous to aluminium and silicon, whose atomic weight
would be between 65 and 75.

4 7. The atomic weight of an element may sometimes be* amended by a
knowledge of those of the contiguous elements. Thus, the atomic weight of
tellurium must lie between 123 and 126, and cannot be 128.

« 8, Certain characteristic properties of the elements can be foretold from
their atomic weights.

APPENDIX II. 473

1 The aim of this communication will be fuHy attained if I succeed in.
drawing the attention of investigators to those relations which exist between!
the atomic weights of dissimilar elements, which, So far as I know, have;
hitherto been almost completely neglected. I believe that the solution of
eome of the most important problems of our science lies in researches of this]
kind.'

To-day, twenty years after the above conclusions were formulated, they
may still be considered as expressing the essence of the now well-known
periodic law.

Reverting to the epoch terminating with the sixties, it is proper to indi-
cate three series of data without the knowledge of which the periodic law
could not have been discovered, and which rendered its appearance natural
and intelligible.

In the first place, it was at that time that the numerical value of atomic
weights became definitely known. Ten years earlier such knowledge did not!
exist, as may be gathered from the fact that in 1860 chemists from all parts'
of the world met at Karlsruhe in order to come to some agreement, if not
with respect to views relating to atoms, at any rate as regards their definite-
representation. Many of those present probably remember how vain were!
the hopes of coming to an understanding, and how much ground was gained1
at that Congress by the followers of the unitary theory so brilliantly repre-
sented by Cannizzaro. I vividly remember the impression produced by his
speeches, which admitted of no compromise, and seemed to advocate truth
itself, based on the conceptions of Avogadro, Gerhardt, and Regnault, which
>Bt that time were far from being generally recognised. And though no
understanding could be arrived at, yet the objects of the meeting were attained,,
for the ideas of Cannizzaro proved, after a few years, to be the only ones
which could stand criticism,- and which represented an atom as— 'the
smallest portion of an element which enters into a molecule of its compound.*
Only such real atomic weights— not conventional ones— could afford a basis
for generalisation. It is sufficient, by way of example, to indicate the
following cases in which the relation is seen at once and is perfectly clear : —

K = 39 Rb = 85 Cs=rl33

Ca = 40 Sr=87 Ba = 137

whereas with the equivalents then in use—

K =39 Rb = 85 Cs =133

Ca = 20 Sr =43'5 Ba = 68'5

the consecutiveness of change in atomic weight, which with the true values
is so evident, completely disappears.

Secondly, it had become evident during the period 1860-70, and even
during the preceding decade, that the relations between the atomic, weights
of analogous elements were governed by some general and simple laws.
Cooke, Cremers, Gladstone, Gmelin, Lenssen, Petfcenkofer, and especially
Dumas, had already established many facts bearing on that view. Thus
Dumas compared the following groups of analogous elements with organic
radicles : —

474

PRINCIPLES OF CHEMISTRY

Dift Diff. Difl. Ditf.

Mg=12j8 *=31J44 0-818

Li » 7) Ca =20v Ae = 75\ S =16,

16 3*8 44 3x8

Na*23 8r =44 Bb = 119 Se-40

16 13x8 12x44 18x8

K =39' Ba =68) ABi=207) Te = 64)

and pointed dut some really striking relationships, such as the following:—
F o!9.

Cl «= 86-5 =19 + 1^*6
Br=80 =19 + 2x16-5 + 28.
I = 127 = 2 x.19 + 2 x l6'5 -f 2 x 28.

A. Strecker, in his work • Theorien und Experimente zur Bestimmung
der Atomgewichte der Elemente ' (Braunschweig, 1859), after summarising
the data relating to the subject, and pointing out the remarkable series of
equivalents—

Cr,-26-2 Mn-27-6 Fe~28 Ni = 29 Co»80 Cu«=31-7

remarks that: It is hardly probable that all the above-mentioned relations
between the atomic weights (or equivalents) of chemically analogous elements
are merely accidental. We must, however, leave to the future tbe discovery
of the law of the relations which appears in these figures.' l

In such attempts at arrangement and in such views are to be recognised
the real forerunners of the periodic law ; the ground was prepared for it
between I860 and 1870, and that it was not expressed in a determinate form
before the end of the decade may, I suppose, be ascribed to the fact that only
analogous elements had been compared. The idea of seeking for a relation
between the atomic weights of all the elements was foreign to the ideas then
current, so that neither the vis tellurique of Be Chancourtois, nor the law of
octaves of Newlands, could secure anybody's attention. And yet both De
Chancourtois and Newlands like Dumas 'and Strecker, more than Lenssen
and Pettenkofer, had made an approach to the periodic law and had dis-
covered its germs. The solution of the problem advanced but slowly, because
the facts, but not the law, stood foremost in all attempts*, and the law could
not awaken a general interest so long as elements, having no apparent con-
nection with each other, were included in the same octave, as for example : —

1st octave of

Newlands . .

7th Ditto

Cl

Fe

Co&Ni
Se

Br

Bh'&Ru

Pd
Te

Au

Pt&Ir
Os or Th

Analogies of the above order seemed quite accidental, and the more so as
the octave contained occasionally ten elements instead of eight, and when two

1 * Ea ist wohl kanm anztmehmen, dass alle ii» Vorhergehfcnden hervorgehobenen
Beziehangen zwischen den Atomgewichten (oder Aequi valenten) in chemischen Verh51t«
nissen einander ahnJjche Elemente bloss zufallig sind. T)ie Auffindung der in dieseo
Zablen gesetzlichen Beziehungen mttssen wir jedoch dex Zqkunft iiberlassen.'

APPENDIX II. 47$

ench elements as Ba and V, Co and N.i, or Rh and Ru, occupied one place in
the octave,3 Nevertheless, the fruit was ripening, and I now see clearly thafr
Strecker, De Chancourtois, and Newlands stood foremosf in the way towards
the discovery of the periodic law, and that they merely wanted the boldness
necessary to place the whole question at such a height that its reflection on
the facts could be clearly seen.

A third circumstance which revealed the periodicity of chemical elements

, ivas the accumulation, by the end of the sixties, of new information respecting

' the rare elements, disclosing their many-sided relations to the other elements
and to each other. The researches of Marignac on niobium, and those of

, Roscoe on vanadium, were of special moment. The striding analogies between
vanadium and phosphorus on the one hand, and between vanadium and
chromium on the other, which became so apparent in the investigations con-
nected with that element, naturally induced the comparison of V = 51 with
Cr = 62, Kb = 94 with Mo = 96, and Ta = 192 with W = 194; while, on the
other hand, P = 31 could be compared with S « 32, As » 75 with Se - 79, and
Sb • 120 with Te« 125. From such approximations there remained but one
Step to the discovery of the law of periodicity.

The law of periodicity was thus a direct outcome of the stock of generali-
sations and established facts which had accumulated by the end of the decade
1860-1870 : it is an embodiment of those data in a more or less systematio
expression. \Vhere, then, lies the secret of the special importance which has
since been attached to the periodic law, and has raised it to the position of a
generalisation which has already given to chemistry unexpected aid, and
which promises to be far more fruitful in the future and to impress upon
several branches of chemical research a peculiar and original stamp? The
remaining part of my communication will be an attempt to answer this
Question.

In the first place we have the circumstance that, as soon as the law made
its appearance, it demanded a revision of many facts which were considered
by chemists as fully established by existing experience. I shall return, later
on, briefly to this subject, but I wish now to remind you that the periodic
law, by insisting on the necessity for a revision of supposed facts, exposed
itself at once to destruction in its very origin. Its first requirements, how-
ever, have been almost entirely satisfied during the last 20 years ; the sap-
posed facts have yielded to the law, thus proving that the law itself was a
legitimate induction from the verified facts. But our inductions from data

'bave often to do with such- details of a s6ience so rich in facts, that only
generalisations which cover a wide range of important phenomena can attract
general attention. What were the regions touched on by the periodic J»w ?
This is what we shall now consider.

The most important point to notice is, that periodic functions, used for
the purpose of expressing changes which are dependent on variations of time
find space, have been long known. Tbey are familiar to the mind when we
have to deal with motion in closed cycles, or with any kind of deviation from

* To judge from J. A. R. Newlands's work, OH the Discovery of the Periodic Lawt
London, 3884, p. 149; 'Oo the Law of Octaves' (from the Chemical News, 12, 83,
. 1S65J.

476 PRINCIPLES OF CHEMISTRY

a stable position, such as occurs in pendulum-oscillations. A like periodic
function became evident in the case of the elements, depending on the mass
of the atom. The primary conception of the masses of bodies, or of the masses
of atoms, belongs to a category which the present state of science forbids us
to discuss, because as yet we have no means of dissecting or analysing the
conception. All that was known of functions dependent on masses derived
its origin from Galileo and Newton, and indicated that such functions
either decrease or increase with the increase of mass, like the attraction of
celestial bodies. The numerical expression of the phenomena was always
found to be proportional to 'the mass, and in no case was an increase of mass
followed by a recurrence of properties such as is disclosed by the periodic law
of the elements. This constituted such a novelty in the study of the phenomena
of nature that, although it did not lift the veil which conceals the true concep-
tion of mass» it nevertheless indicated that the explanation of that conception
must be searched for in the masses of the atoms ; the more so, as all masses
are nothing but aggregations, or additions, of chemical atoms which would be
best described as chemical individuals. Let me remark, by the way, that
though the Latin word ' individual ' is merely a translation of the Greek word
• atom,' nevertheless history and custom have drawn a sharp distinction
between the two words, and the present chemical conception of atoms is
nearer to that defined by the Latin word than by the Greek, although this
latter also has acquired a special meaning which was unknown to the classics.
The periodic law has shown that our chemical individuals display a harmonic
periodicity of properties dependent on their masses. Now natural science
has long been accustomed to. deal with periodicities observed in nature, to
seize them with the vice of mathematical analysis, to submit them to the>
rasp of experiment. And these instruments' of scientific thought wouldL
surely, long since, have mastered the problem connected with the chemical
elements, were it not for a new feature which was brpught to light by the
periodic law, and which gave a peculiar and original character to the periodic
function.

If we mark on an axis of abscissa) a series of lengths proportional to
angles, and trace ordinates which are proportional to eines or other trigono-
metrical functions, we get periodic curves of a harmonic character. So it
might seem, at first sight, that with the increase of atomic weights the funct
tionof the properties of, the elements should also vary in the same harmonioui
way. But in this case there is no such continuous change as in th& curves
just referred to, because the periods do not contain the infinite number of
points constituting a curve, but a finite number only of such points. An
example will Better ffltfstrate this view. The atomic weights—

Ag = 108 Cd = 112 In = 113 Sn = 118 Sb-120

Te = 125 1 = 127

steadily 'increase, and their increase is accompanied by a modification of
many properties which constitutes the essence of the periodic law. Thus,
for example, the densities of the above elements decrease, steadily, being
respectively—

10-5 8-6 7'4 7'2 6-7 6'4 4-9

APPENDIX II. 477

•while their oxides contain an increasing quantity of oxygen—

Ag20 CdA InA SiifcO, Sb20a Te209 1,0,

But to connect by a. curve the summits of the ordinates expressing any
of these properties would involve the rejection of Dalton's law of multiple
proportions. Not only are there no intermediate elements between silver,
which gives AgCl, and cadmium, which gives CdCl2, but, according to the
very essence of the periodic law, there can be none ; in fact a uniform curve
would be inapplicable in such a case, as it would lead us to expect elements
possessed of special properties at any point of the curve. The periods of the
/elements have thus a character very different from those which are so simply
represented by geometers. They correspond to points, to numbers, to sudden
changes of the masses, and not to a continuous evolution. In these sudden
-changes destitute of intermediate steps or positions, in the absence of
elements intermediate between-, say, silver and cadmium, or aluminium
and silicon, we must recognise a, problem to which no direct application
of the analysis of the infinitely small can be made. Therefore, neither the
trigonometrical functions proposed by Ridberg and Flavitzky, nor the pen.
dulum-oscillations suggested by Crookes, nor the cubical curves of the Bev,
Mr. Haughton, which have been proposed for expressing the periodic law,
from the nature of the case, can represent the periods of the chemical
elements. If geometrical analysis is to be applied to this subject, it will re»
quire to be modified in a special manner. It must find the means of repre-
senting in a special way, not only such long periods as that comprising

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br,

but short periods like the following :—

Na Mg Ai Si P 8 Cl.

In the theory of numbers only do we find problems analogous to ours,
and two attempts at expressing the atomic weights of the elements by alge-
braic formulae seem to be deserving of attention, although neither of them
can be considered as a complete theory, nor as promising finally to solve the
problem of the periodic law. The attempt of E. J. Mills (1886) does not
even aspire to attain this end. He considers that all' atomic weights can be
expressed by a logarithmic function,

15(71-0-9375'),

in which the variables n and t are whole numbers. Thus, for oxygen, n = 2,
and t = 1, whence its atomic weight is = 15'94 ; in the case of chlorine,
bromine, and iodine, n has .respective values of 8, 6, and 9, whilst t = 7, 6,
and 0 ; in the case of potassium, rubidium, and caesium, n = 4, 6, and 9, and
t = 14, 18, and 20.

Another attempt was made in 1888 by B. N. Tchitche"rra. Its author
places the problem of the periodic law in the first rank, but as yet he has
investigated the alkali "metals only Tchitche'rin first noticed the simple

478 FB.INCIPLES OP CHEMISTRY

relations existing between the atomic volumes of all alkali metals;
can be expressed, according to his views, by the formula

A(2^- 0-00535 Aw),

where A is the atomic weight, and n is equal to 8 for lithium and sodium, to
4 for potassium, to 3 for rubidium, and to 2 for caesium. If n remained equal
to 8 during the increase of A, the volume would become zero at A = 40$,
and it would reach its maximum at A - 23£. The close approximation of
the number 46§ to the differences between the atomic weights of analogous
elements (such as Cs — Bb, I — Br, and so on) ; the close correspondence of
the number 23£ to the atomic weight of sodium ; the fact of n being neces*
earily a whole number, and several other aspects of the question, induce
Tchitcherin to believe that they afford a clue to the understanding of the
nature cf the elements ; we must, however, await the full develcpment of
his theory before pronouncing judgment on it. What we can at present only
be certain of is this : that attempts like the two above named must be re-
peated and multiplied, because the periodic law has clearly shown that the
masses of the atoms increase abruptly, by steps, which are clearly connected
in some way with Dalton's law of multiple proportions ; and because the
periodicity of the elements finds expression in the transition from BX to
BX2, EX3, BX4, and so on till RXd, at which point, the energy of the com-
bining forces being exhausted, the series begins anew from BX £o BX.,, and
so on.

While connecting by new bonds the theory of the chemical elements with
Dalton's theory of multiple proportions, or atomic structure of bodies, the
periodic law opened for natural philosophy a new and wide field for specula*
tion. Kant said that there are in the world ' two things which never cease
to call for the admiration and reverence of man : the" moral law within
ourselves, and the stellar sky above us.' But when we turn our thoughts
towards the nature of the elements and the periodic law, we must add a third
subject, namely, ' the nature of the elementary individuals which we discover
everywhere around us.* Without them the stellar sky itself is inconceiv-
able ; and in tho atoms we see at once then* peculiar individualities, the in-
finite multiplicity of the individuals, and the submission of their seeming
freedom to the general harmony of Nature.

Having thus indicated a new mystery of Nature, which does not yet yield
to rational conception, the periodic law, together with the revelations of
spectrum analysis, have contributed to again revive an old but remarkably
long-lived hope— that of discovering, if not by experiment, at least by a
mental effort, the primary matter— -which had its genesis in the minds of
the Grecian philosophers, and has been transmitted, together with many
other ideas of the classic period, to the heirs of their civilisation. Having
grown, during the times of the alchemists up to the period when experimental
proof was required, the idea has rendered good service; it induced those
•careful observations and experiments which later on called into being the
works of Scheele, Lavoisier, Priestley, and Cavendish. It then slumbered
awhile, but was soon awakened by the attempts either to confirm or to refute
the ideas of Prout as to the multiple proportion relationship of the atomic

APPENDIX. IL 479

^reighteof all the elements, And once again the inductive or experimental
method of studying Nature gained a direct advantage from the old Pytha-
gorean 'ide'a : because atomic weights were determined with an accuracy
formerly unknown. But again the idea could not stand the ordeal of expert-
tnental test, yet the prejudice remains and has not been uprooted, even by
'Stas; nay, it has gained a new vigour, for we see that all which is imperfectly
worked out, new and unexplained, from the still scarcely studied rare metals
to the hardly perceptible nebulae, have been used to justify it. As soon da
spectrum analysis appears as a new and powerful weapon of chemistry, the
idea of a primary matter is immediately attached to it. From all sides we
see 'attempts to constitute -the imaginary substance helium,9 the, so much
longed for primary matter.. No attention is paid to the circumstance that
the helium line is only seen in the spectrum of the solar protuberances, so
that its universality in Nature remains, as problematic as the primary matter
itself ; nor to the fact that the helium line is wanting amongst the Fraun-
hofer lines of the solar spectrum, and thus does not answer to the brilliant
fundamental conception which gives its real force to spectrum analysis.

And finally, no notice is even taken of the indubitable fact that the bril-
liancies of the spectral lines of the simple substances vary under different tem-
peratures and pressures ; so that all probabilities are in favour of the helium
line simply belonging to some long since known, element placed under such
conditions of temperature, pressure, and gravity as have not yet been realised
in our experiments. Again, the idea that the excellent investigations of
Lockyer of the spectrum o£ iron can be interpreted in favour of the compound
nature of that element, evidently, must have arisen from some misunder-
standing. The spectrum of a compound certainly does not appear as a
sum of the spectra of its components ; and therefore the observations of
Lockyer can oe considered precisely as a- proof that iron undergoes no other
changes at the temperature of the sun than those which it experiences in the
•voltaic arc— provided the spectrum of iron is preserved. As to the shifting
of some of the lines -of the spectrum of iron while the other lines maintain
their positions, it can be explained, as shown by M. Klelbetf ('Journal -of the
Russian Chemical and Physical Society, 1885, 14*7), by the relative motion
of the various strata of the sun's atmosphere, and by Zollner's laws of the
relative brilliancies of different lines of the spectrum. Moreover, it ought
not to be forgotten that if iron were really proved to consist of two or more,
unknown elements, we should simply have an increase in the number of our
elements^-not a reduction, and still less a reduction of all of them to one
single primary matter.

Feeling that spectrum analysis will not yield a support to the Pythagorean
conception, its. modern promoters are so bent upon its being confirmed by
the periodic law, that the illustrious Berthelot, in his work * Les origines de
i'Alchjmie,' 1885, 318, has simply mixed up the fundamental idea of the law
of periodicity with the ideas of Prout, the alchemists, and Democritus about
primary matter,4 But the periodic law, based as it is on the solid and whole-

8 That is, a substance having a wave-length equal to 0*0005875 millimetre,
* He maintains (on p. 800) that the periodic law requires two new analogous
elements, having atomic Weights of. 48 and 64, occupying positions between sulphur

480, PRINCIPLES OF CHEMISTRY

some ground of experimental research, has been evolved Independently of
any conception as to the nature of the elements; it does not in the least
Originate in the idea of a unique matter ; and it has no historical connec-
tion with that relic of the torments of classical thought, and therefore it
affords no more indication of the unity of matter or of the compound character,
of our elements, than the law of Avogadro, or the law of specific heats, or
even the conclusions of spectrum analysis. None of the advocates of a
unique matter have ever tried to explain the law from the standpoint of ideas
taken from a remote antiquity when it vyas found convenient to admit the
existence of many gods— and. of a unique matter.

When we try to explain the origin of the idea of a unique primary
matter, we easily trace that in the absence of inductions from experiment it
derives its origin from the scientifically philosophical attempt at discovering
eome'kind of unity in the immense diversity of individualities which we see
around. In classical times such a tendency could only be satisfied by con-
ceptions about the immaterial world. As to the material world, our ancestors
were compelled to resort to some hypothesis, and they adopted the idea of
unity in the formative material, because they were not able to evolve the
conception of any other possible unity in order to connect the multifarious
relations of matter. Responding to the same legitimate scientific tendency,
natural science has discovered throughout the universe a unity of plan, a
unity of forces, and a unity of matter, and the convincing conclusions of
modern science compel every one to admit these kinds of unity. But while
we admit unity in many things, we none the less must also explain the
individuality and the apparent diversity which we cannot fail to trace every-
where. It has been said of old, ' Give us a fulcrum, and it will become easy to
displace the earth.' So also we must say, ' Give us something that is individu-
alised, and the apparent diversity will be easily understood.' Otherwise, how
could unity result in a multitude ?

After a long and painstaking research, natural science has discovered the
individualities of the chemical elements, and therefore it is now capable not
only of analysing, but also of synthesising ; it can understand and grasp
generality and unity, as well as the individualised and the multifarious.
The general and universal, like time and space, like force and motion, vary uni-
formly ; the uniform admit of interpolations, revealing every intermediate
phase. But the multitudirious, the individualised— such as ourselves, or the
chemical elements, or the members of a peculiar periodic function of the
Clements, or Dalton's multiple proportions — is characterised in another
way : We see in it, side by side with a connecting general principle, leaps,
breaks of continuity, points which escape from the analysis of the infinitely
small— an absence- of complete intermediate links. Chemistry has found an
answer to the question as to the causes of multitudes ; and while retaining
the conception of many elements, all submitted to the discipline of a general
law, it offers an esc.ape from the Indian Nirvana — the. absorption in the
universal, replacing it by the individualised. However, the place for indi-

and selenium, although nothing of the kind results from any of the different readings of
the law.

A£PE;NDIX a 481

viduality is so limited by the all-grasping, all-powerful 'universal, that it. Is
merely a point of support for the understanding of multitude in unity.

Having touched upon the metaphysical bases of the conception of a
unique matter which is supposed to enter into the composition of all bodies
I think it necessary to dwell upon another theory, akin to the above concep-
tion—the theory of the compound character of the elements now admitted by
some—and especially upon one particular circumstance which, being related
to tfie periodic law, is considered to be an argument in favour of that hypo-
thesis.

Dr. Pelopidas, in 1883, made a communication to the Russian Chemical
and Physical Society on the periodicity of the hydrocarbon radicles, pointing
out the remarkable parallelism which was to be noticed in the change of
properties of hydrocarbon radicles and elements when classed in groups.
Professor Carnelley, in 1686, developed a similar parallelism. The idea of
M. Pelopidas will be easily understood if we consider the series of hydro-'
carbon radicles which contain, say, 6 atoms of carbon :—

i. n. in. iv. v. vi. vnt vni.

C6H13 C6H13 C6Hn C0H1Q CaHa 06H8 06H7

The first of these radicles, like the elements of the 1st group, combines with,
Cl, OH, and so on, and gives the derivatives of hexyl alcohol, CeH13(OH) ;
but, in proportion as the number of hydrogen atoms decreases, the capacity
of the radicles of combining with, say, the halogens increases. 06HW already
combines, with 2 atoms of chlorine ; C0HU with 3 atoms, and so on. The
last members of the series comprise the radicles of ae.ids : thus C6H8, which
belongs to the 6th group, gives, like sulphur, a bibasio acid, 09H8O2(OH)2,
which is homologous with oxalic acid. The parallelism can be traced still
further, because C6H5 appears as a monovalent radicle of benzene,- and with
it begins a new series of aromatic derivatives, so analogous to the derivatives of
the aliphatic series. Let me also mention another example from among those
which have been given by M. Pelopidas. Starting from the alkaline radicle
of monomethylammonium, N(CH3)H3,- or -NCH6, which presents many
analogies with the alkaline metals of the 1st group, he arrives, by successively
diminishing the number of the atoms of hydrogen, at a 7th group which
contains cyanogen, CN, which lias long since been compared to the halogens
of the 7th group.

The most important consequence which, in my opinion, can be drawn
from the above comparison is that the. periodic law, so apparent in the
elements, has a wider, application than might appear at first sight ; it opens
tip a new vista of chemical evolutions. But, while admitting the fullest
parallelism between the periodicity of the elements and that of the compound
radicles, we must not forget that in the periods of the hydrocarbon radicles
we have a* decrease of mass as We pass from the representatives of the first
group to the1 next, while in the periods of the elements the mass increases
during the progression. It thus becomes evident that we cannot speak of an
identity of periodicity in both, cases, unless we put aside the ideas of mass
and attraction* which are the real corner-stones of the whole of natural
science, and even enter into those very conceptions of simple substances which

482 PRINCIPLES OF CHEMISTRY

came to light a foil hundred years later than the immortal principles of
Newton.5

From the foregoing, as well as from the failures of so many attempts at
finding in experiment and speculation a proof of the compound character of
the elements and of the existence of primordial matter, it is evident, in my
opinion, that this theory must be classed among mere Utopias. But Utopias
can only be combated by freedom of opinion, by experiment, and by new
Utopias. In the republic of scientific theories freedom of opinions is guaran-
teed. It is precisely that freedom which permits me to criticise openly the
widely-diffused idea as to the unity of matter in the elements. Experiments
and attempts at confirming that idea have been so numerous that it really
would be instructive to have them all collected together, if only to serve as a
warning against the repetition of old failures. And now as to new Utopias
which may bo helpful in the struggle against the old ones, I do not think it
quite useless to mention & phantasy of one of my students who imagined that
the weight of bodies does not depend upon their mass, but upon the character
of the motion of then* atoms. The atoms, according to this new Utopian, may
all be homogeneous or heterogeneous, we know not which ; we know them
in motion only, and that motion they maintain with the same persistence as
the stellar bodies maintain theirs. The weights of atoms differ only in con-
sequence of their various modes and quantity of motion ; the heaviest atoms
may be much simpler than the lighter ones : thus an atom of mercury may
be simpler than an atom of hydrogen— the manner in which it moves causes
it to be heavier. My interlocutor even suggested that the view which
attributes the greater complexity to the lighter elements finds confirmation
in the fact that the hydrocarbon radicles mentioned by Pelopidas, while
becoming lighter as they lose hydrogen, change their properties periodically
in the same manner as the elements, change theirs, according as the atoms
grow heavier.

The French proverb, La critique est facile, mais I'art est difficile, how-
ever, may well be reversed in the case of all such ideal views, as it is much
easier to formulate than to criticise them. Arising from the virgin soil of
newly-established facts, the knowledge relating to the elements, to their
masses, and to the periodic changes of their properties has given a motive
for the formation of Utopian hypotheses, probably because they could not be
foreseen by the aid of any of the various metaphysical systems, and exist,
like the idea of gravitation, as an independent outcome of natural science,
requiring the acknowledgment of general laws, when these have been estab-
lished with the same degree of persistency as is indispensable for the accept-
ance of a thoroughly established fact. Two centuries have elapsed since the
theory of gravitation was enunciated, and although we do not understand its
cause, we still must regard gravitation as a fundamental conception of natural
philosophy, a conception which has enabled us to perceive much more than
the metaphysicians did or could with their seeming omniscience. A hundred

5 It is noteworthy that the year in which Lavoisier was born (1748) — the author of
the idea of elements and of the indestructibility of matter — is later by exactly one
century than the year in which the author of the theory of gravitation and mass was born
(1643 N.3.). The affiliation of the ideas of Lavoisier and those of Newton is beyond doubt.

APPENDIX H. 483

years later the conception of the elements arose; it made chemistry what it
now is ; and yet we have advanced as little in our comprehension of simple
substances since the times of Lavoisier and Dalton as we have in our und*r-
istanding of gravitation. The periodic law of the elements is only twenty
years old; it is not surprising, therefore, that, knowing nothing about tho.
causes of gravitation and mass, or about the nature of the elements, we do
not comprehend the rationale of the periodic law. It is only by collecting
established laws — that is, by working at the acquirement of truth — that \\ o
can hope gradually to lift the veil which, conceals from us the causes of the
mysteries of Nature and to discover their mutual dependency. Like the
telescope and the microscope, laws founded on the basis of experiment are
the instruments and means of enlarging our mental horizon.

In the remaining part of my communication I shall endeavour to show,
and as briefly as possible, in how far the periodic law contributes to enlarge
our range of vision. Before the promulgation of this law the chemical
elements were mere fragmentary, incidental facts in Nature ; there was no
special reason to expect the discovery of new elements, and the new ones
which were discovered from time to time appeared to be possessed of quite
novel properties. The law of periodicity first enabled us to perceive undis-
covered elements at a distance which formerly was inaccessible to chemical
vision ; and long ere they were discovered new elements appeared before oui"
eyes possessed of a number of well-defined properties. We jnow know three
cases of elements whose existence and properties were foreseen by the instru-
mentality of the periodic law. I need but mention the brilliant discovery of
ffaltwm, which proved to correspond to eka-aluminium of the periodic law, by
Lecoq de Boisbaudran ; of scandium, corresponding to ekaboron, by Nilson ;
.and of germanium, which proved to correspond in all respects to ekasilicon,
by Wlnkler. "When, in 1871, 1 described to the Russian Chemical Society
the properties, clearly defined by the ^eripdio law, which such elements
ought to possess, I never hoped that I should live to mention their discovery
to the Chemical Society of Great Britain as a confirmation of the exactitude
and the generality of the periodic law. Now that I have had the happiness
of doing so, I unhesitatingly say that, although greatly enlarging our vision,
even now the "periodic law needs further improvements in order that it may
become a trustworthy instrument in further discoveries.9

I will venture to allude to some other matters which chemistry has dis-
•cemed by mjsaus of its new instrument, and which it could not have made

• I foresee 'some more new elements, but not w,ith the same certitude as before. I
shall give one example, and yet f do not see it quite distinctly. In the series which con-
tains Hg «= 204, Pb =» 206, and Bi •** 208, we can imagine the existence (at the place VI— 11)
of an element analogous to tellurium, which we can describe as dvi-tellurium, Dt, having
fin atomic weight of 212, and the property of forming the oxide DtOj. If this element
really exists, it ought in the free state to be an easily fusible, crystalline, non- volatile
metal of. a grey colour, having a density pf about 9-8, capable of giving a dioxide, DtO?,
equally endowed with feeble acid and' basic properties. This dioxide must give on active
! oxidation an unstable higher oxide, DtOj, which should resemble in its properties PbO9
and Bi205. Dvi-tellurium hydride, if it be found to exist, will be a less stable compound
than even HjTe. The compounds of dvi-tellurium will be easily reduced, and it will form
characteristic defcute alloys with other metals.

484 PRINCIPLES OF CHEMISTRY

out without a knowledge of the law of periodicity, and I will confine myself
to simple substances and to oxides.

Before the periodic law was formulated the atomic weights of the elements
were purely empirical numbers, so that the magnitude of the equivalent, and'
the atomicity, or the value in substitution possessed by an atom, could only,
be tested by critically examining the methods of determination, but never
directly by considering the numerical values themselves ; in short, we wer#
compelled to move in the dark, to submit to the facts, instead of being masters
of them. I need not recount the methods which permitted the periodic law
at last to master the facts relating to atomic weights, and I would merely
call to mind that it compelled us to modify the valencies of indium and
cerium, and to assign to their compounds a different molecular composition*
Determinations of the specific heats of these two metals fully confirmed the
change. The trivalency of yttriii/ml which makes us now represent its oxide ^
as Y203 instead of as YO, was also foreseen (in 1870) by the periodic law, and
it has now become so probable that Cleve, and all other subsequent investi-
gators of the rare metals, have not only adopted it, but have also applied it
without any new demonstration to substances so imperfectly known as thosa-
of the cerite and gadolinite group, especially since Hillebrand determined the
specific heats of lanthanum and didymium and confirmed the expectations
suggested by the periodic law. But here, especially hi the case of didymium,
we meet with a series of difficulties long since foreseen through the periodic
law, but only now becoming evident, and chiefly arising from the relative
rarity and insufficient knowledge of the elements which usually accompany
didymium.

Passing to the results obtained in the case of the rare elements leryllium%.
scandium, and thorium, it is found that these have many points of contact
with the periodic law. Although Avd^eff long since proposed the magnesia
formula to represent beryllium oxide, yet there was so much to be said in
favour of the alumina formula, on account of the specific heat of the metals
and the isomorphism of the two oxides, that it became generally adopted
and seemed to be well established. The periodic law, however, as Brauner
repeatedly insisted ('Berichte,' 1878,872; 1881, 53), was against the formula
Be203 ; it required the magnesia formula BeO — that is, an atomic weight!
of 9— because there was no place in the system for an element like beryllium
having an atomic weight of 18'5. This divergence of opinion lasted for
years, and I often heard that the question as to the atomic weight of beryllium
threatened to disturb the generality of the periodic law, or, at any rate, to
require some important modifications of it. Many forces were operating in
the controversy regarding beryllium, evidently because a much more im-
portant question was at issue than merely that involved in the discussion of
the atomic weight of a relatively rare element: and during the controversy
the periodic law became better understood, and the mutual relations of the
elements became more apparent than ever before. It is most remarkable that
the victory of the periodic law was won by the researches of the very observers
who previously had discovered a number of facts in support of the tri-
valency of beryllium. Applying the higher law of Avogadro, Nilson and
Petterson have finally shown that the density of the vapour of the beryl-

APPENDIX II. 485

Hum chloridey/BeClj, obliges us to regard beryllium as bivalent in
conformity witji the periodic law.7 I consider the confirmation of Avde*efFs
and Braunery view as important in the history of the periodic law as the
discovery of scandium, which, in Nilson's hands, confirmed the existence of
ekaboron.

The circumstance that thorium proved to be quadrivalent, and Th = 232,
in accordance with the views of Chydenius and the requirements of thl
periodic law", passed almost unnoticed, and was accepted without opposition,
and yet both thorium and uranium are of great importance in the periodic
system, as they are its last members,. and have the highest atomic weights of
all the elements.

The alteration of the atomic weight of uranium from U = 120 into U = 240
attracted more attention, the change having been made on account of the
periodic law, and for no other reason. Now that Roscoe, Rammelsberg,
Zimmerman^) and several others have admitted the various claims of the
periodic law in the case of uranium, its high atomic weight is received .with-
out objection, and it endows that element with a special interest.

While thus demonstrating the necessity for modifying the atomic weights
of several insufficiently known elements, the periodic law enabled us also to
detect errors in the determination of the atomic weights of several elements
whose valencies and true position among other elements were already well
known. Three such cases are especially noteworthy: those of tellurium,
titanium and platinum. Berzelius had determined the atomic weight of
tellurium to be 128, while the periodic law claimed for it an atomic weight
below that of iodine, which had been fixed by Stas at 126*5, and which was
certainly not higher 'than 127. Brauner then undertook the investigation,
and he has shown that the true atomic weight of tellurium is lower than that
of iodine, being near to 125. For titanium the extensive researches of
Thorpe have confirmed the atomic weight of Ti - 48, indicated by the law,
and already foreseen by Rose, but contradicted by the analyses of Pierre and
several other chemists. An equally brilliant confirmation of the expectations
based on the periodic law has been given in the case of the series osmium,
iridium, platinum, and gold. At the time of the promulgation of the periodic
law, the determinations of Berzelius, Rose, and many others gave the follow-
ing figures :—

Os = 200; Ir = 197; Pt = 198; Au = 196.

7 Let me mention another proof of the bivalency of beryllium which may have passed
unnoticed, as it was only published in the Russian chemical literature. Having remarked"
(in 1884) that the density of such solutions of chlorides of metals, MCln, as contain 200
mols. of water (or a large and constant amount of water) regularly increases as the mole-
cular weight of the dissolved salt increases, I proposed to one of our young chemists,
M. Burdakoff, that he should investigate beryllium chloride. If its molecule be BeGlj
its weight must be =80; and in such a case it must be heavier than the molecule of
KC1 = 74-5, and lighter than that of MgCl2 = 93. On the contrary, if beryllium chloride is
a trichloride, BeCl3 = 120, its molecule must be heavier than that of CaCl2 = lll, ind
lighter than that of MnCl2 = 126. Experiment has shown the correctness of the fonrier
formula, the solution BeCl2 + 200H2O having (at 15°/4°) a density of T0138, this being a
lugher density than that of the solution KC14-200H2O ( = 1'0121), and lower than that of
MgCl2+200H2O ( = 1-0203). The bivalency of beryllium was thus confirmed in the case
both of the dissolved and the vaporised chloride.

486 PEINCIPLES Off CHEMISTRY

The expectations of the periodic law8 have been confirmed, first, by n,e>
determinations of the atomic weight of ptatvmm (by Seubert, Dittmar, and
W Arthur, which proved to be near. to 196 (taking 0 = 16, as proposed by
Marignac, Brauner, and others) j secondly, by Seubert having proved that
the atomic weight of osmwm is really lower than that of platinum, being
near to 191; and thirdly, by the investigations of Krtiss, Thorpe and
Laurie, proving that the atomic weight of gold exceeds that of platinum,
and approximates to 197. The. atomic weights .which were thus found to
require correction were precisely those, which the periodic law had indicated
as affected with errors ;. and it has been proved/ therefore, that the periodic
law affords a means of testing experimental results. If we succeed in dis-
covering the exact character of the periodic relationships between the
increments in atomic weights of allied elements discussed by Eidberg in
1885, and again by Bazaroff in 1887, we may expect that our instrument
will give us the means of still more closely controlling .the experimental data,
relating to atomic weights.

Let me next call to mind that, while disclosing the variation of chemical
properties,9 the periodic law has also enabled us to systematically discuss
many of the physical properties of elementary bodies, and to show that these
properties are also subject to the law of periodicity. At the Moscow Congress
of Bussian Naturalists in August, 1869, I dwelt upon the relations which
existed between density and the atomic weight of the elements. The follow-
ing year Professor Lothar Meyer, in his well-known paper,10 studied the
same subject in more detail, and thus contributed to spread information
about the periodic law. Later on, Carnelley, Laurie, L. Meyer, Roberts*
Austen, and several others applied the periodic system to represent the order
in the changes of the magnetic properties of the elements, their melting
points, the heats of formation of their haloid compounds, and even of such
mechanical properties as the co-efficient of elasticity, the breaking stress, &c.,
Ac. These deductions, which have received further support in the discovery
of new elements endowed not only with chemical but even with physical
properties, which were foreseen by the law of periodicity, are well known ;
eo I need not dwell upon the subject, and may pass to the consideration of
oxides.11

8 I pointed them out in the Liebig's Annalen, Supplement Band* viii. 1871, p. 211.

9 Thus, in the typical small period of

Li,Be,B,C,N,0,F,

we see at once the progression from the alkali metals to the acid non-metals, such as
are the halogens.

»» LieUg's Annalen, Supplement Band., vii 1870.

11 A distinct periodicity can also be discovered in the spectra of the elements. Thua
the researches of Hartley, Ciamician, and others have disclosed, first, the homology
of the spectra of analogous elements: secondly, that the alkali metals have simpler
epectra than the metals of the following groups; and thirdly, that there is a certain like-
ness between the complicated spectra of manganese and iron on the one hand, and the»
no less complicated spectra of chlorine and bromine on the other hand, and their likeness
corresponds to the degree of analogy between those elements which is indicated by the>
periodic law.

APPENDIX n. 487

In indicating that the gradual increase of the power of elements of com-
bining with oxygen ia accompanied by a corresponding decrease in their
power of combining with hydrogen, the periodic law has shown that there is
a limit of oxidation, just as there is a well-known limit to the capacity of
elements for combining with hydrogen. A single atom of an element com-
bines with at most four atoms of either hydrogen or oxygen ; and while CH4
and SiH4 represent the highest hydrides, so Bu04 and OsO4 are the highest
oxides. We are thus led to recognise types of oxides, just as we have had to
recognise types of hydrides.12

The periodic law has demonstrated that the maximum extent to which
different non-metals enter into combination with oxygen is determined by the
extent to which they combine with hydrogen, and that the sum of the number
of equivalents of both must be equal to 8. Thus chlorine, which combines
with 1 atom or 1 equivalent of hydrogen, cannot fix more than 7 equivalents
of oxygen, giving C^Of ; while sulphur, which fixes 2 equivalents of hydrogen,
cannot combine with more than 6 equivalents or 3 atoms of oxygen. It thua
becomes evident that we cannot recognise as a fundamental property of the
elements the atomic valencies deduced from their hydrides ; and that we
must modify, to a certain extent, the theory of atomicity if we desire to raise
it to the dignity of a general principle capable of affording an insight into the
constitution of all compound molecules. In other words, it is only to carbon,
which is quadrivalent with regard both to oxygen and hydrogen, that we can
apply the theory of constant valency and of bond, by means of which so many
still endeavour to explain the structure of compound molecules. But I should
go too far if I ventured to explain in detail the conclusions which can be
drawn from the above considerations. Still, I think it necessary to dwell
upon one particular fact which must be explained from the point of view of
the periodic law in order to clear the way to its extension in that particular
direction.

The higher oxides yielding salts the formation of which was foreseen by
the periodic system— for instance, in the short series beginning with sodium—

Na,0, MgO, A1A, Si02, P205> S03, CIA.

must be clearly distinguished from the higher degrees of oxidation which cor-
respond to hydrogen peroxide and bear the true character of peroxides. Per-
oxides such as Na3Oa, BaO2, and the like have long been known, Similar

12 Formerly it was supposed that, being a bivalent element, oxygen can enter into any
grouping of the atoms, and there was no limit foreseen as to the extent to which it could
further enter into combination. We couldnot explain why bivalent sulphur, which forms.
compounds such as

could not also form oxides such

while other elements, as, for instance, chlorine, form compounds such as—

Cl-O-Q— 0-0— K
*J

488 PRINCIPLES OF CHEMISTRY

peroxides have also recently become known in the case of chromium, sulphur,
titanium, and many other elements, and I have sometimes heard it said that
discoveries of this kind weaken the conclusions of the periodic law in so far
as it concerns the oxides. I do not think so in the least, and I may remark,
in the first place, that all these peroxides are endowed with certain properties
obviously common to all of them, which distinguish them from the actual,
Higher, salt-forming oxides, especially their easy decomposition by means of
simple contact agencies ; their incapability of forming salts of the common
type ; and their capability of combining with other peroxides (like the faculty
which hydrogen peroxide possesses of combining with barium peroxide, dis-
covered by Schoene). Again, we remark that some groups are especially
characterised by their capacity of generating peroxides. Such is, for instance,
the case in the sixth group, where we find the well-known peroxides of
sulphur, chromium, and uranium ; so that further investigation of peroxides
will probably establish a new periodic function, foreshadowing that molyb-
denum and tungsten will assume peroxide forms with comparative readiness.
To appreciate the constitution of such peroxides, it is enough to notice that
the peroxide form of sulphur (so-called persulphuric acid) stands in the same
relation to sulphuric acid as hydrogen peroxide stands to water : —

H(OH), or ItjO, responds to (OH)(OH), or H202,
and so also —

H(HSOJ, or H2S04, responds to (HS04)(HS04), or H.2S208.

Similar relations are seen everywhere, and they correspond to the principle
of substitutions which I long since endeavoured, to represent as one of the
chemical generalisations called into life by the periodic law. So also
sulphuric acid, if considered with reference .to hydroxyl, and represented as
follows—

HO(S02OH),

has its corresponding compound in dithionio acid—
(S02OH)(S02OH), or H2S206.

Therefore, also, phosphoric acid, HO(POH20S), has, in the same sense, its
corresponding compound in the subphosphoric acid of Saltzer : —

), or H4Pa06;

and we must suppose that the peroxide compound corresponding to phosphoric
acid, if it be discovered, will have the following structure : —

(H.,P04)2 or H4P208 = 2H.O + 2P03.13
So far as is known at present, the highest form of peroxides is met with in

is Jn this sense, oxalic acid, (COOH)2, also corresponds to carbonic acid, OH(COOH),
in the same way that ditliionic acid corresponds to sulphuric acid, and subphosphoric
acid to phosphoric; hence, if a peroxide corresponding to carbonic acid be obtained,
.it will have the structure of (HCO3)2, or H2C206=H2O + C806, So .also lead must have
a real peroxide, Pb2O6.

APPENDIX II. 489

the peroxide of uranium, U04, prepared by Fairley ; 14 while Os04 is the
highest oxide giving salts. The line of argument which is inspired by the
periodic law, so far from being weakened by the discovery of peroxides, is
thus actually strengthened, and we must hope that a further exploration of
the region under consideration will confirm the applicability to chemistry
generally of the principles deduced from the periodic law.

Permit me now to conclude my rapid sketch of the oxygen compounds by
the observation that the periodic law is especially brought into evidence hi
the case of the oxides which constitute the immense majority of bodies at our
disposal on the surface of the earth.

The oxides are evidently subject to the law, both as regards their chemical
and their physical properties, especially if we take into account the cases of
polymerism which are so obvious when comparing C02 with Si»02n. In order
to prove this I give the densities s and the specific volumes v of the higher
oxides of two short periods. To render comparison easier, the oxides are all
represented as of the form E.20n. In the column headed A the differences
are given between the volume of the oxygen compound and that of the parent
element, divided by n — that is, by the number of atoms of oxygen in the
Compound : — 15

Na20

s. v.
. 2-6 24

A

-22

K00...

8. V.

2-7 35

A

-55

Mg,00...

3-6 22

-3

Ca2O

3-15 36

-7

ALO,

4-0 26

+ 1-3-

Sc,0o .,

3-86 35

0

SLO,

2-65 45

5-2

Li 0

. . 4-2 38

+ 5

PnO

2-39 59

6-2

V 0.

.. . . 3*49 52

6-7

S00,

,. 1-96 82

8-7

Cr^CX .,

.. 2-74 73

9-5

I have nothing to add to these figures, except that like relations appear in
other periods as well. The above relations were precisely those which made
it possible for me to be certain that the relative density of ekasilicon oxide
would be about 4-7 ; germanium oxide, actually obtained by Winkler, proved,
m fact, to have the relative density 4-703.

The foregoing account is far from being an exhaustive one of all that has
already been discovered by means of the periodic law telescope in the bound-
less realms of chemical evolution. Still less is it an exhaustive account of all
that may yet be seen, but I trust that the little which I have said will account

14 The compounds of uranium prepared by Fairley seem to me especially instructive
in understanding the peroxides. By the action jof hydrogen peroxide on uranium oxide,
TJO3, a peroxide of uranium, UO4,4H2O, is obtained (U = 240) if the solution be .acid; but
if hydrogen peroxide act on uranium oxide in the presence of caustic soda, a crystalline
deposit is obtained which has the composition Na4UO8,4H2O, and evidently is a combina-
tion of sodium peroxide, Na^O^ with uranium peroxide, UO^ It is possible that the
former peroxide, UO4,4H2O, contains the elements of hydrogen peroxide and uranium
peroxide, XJ2O7, or even U(OH)6,H2O2, like the peroxide of tin recently discovered by
Spring, which has the constitution Sn206,H2O2.

15 A thus represents the average increase of volume for each atom of oxygen con-
tained in the higher salt-forming oxide. The acid oxides give, as a rule, a higher value
of A, while in the case of the strongly alkaline oxides its value is usually negative.

490 PKINCIPLES OF CHEMISTRY

for the philosophical interest attached in chemistry to this law. Although
but a recent scientific generalisation, it has already stood the test of laboratory
verification, and appears as an instrument of thought which has not yet been
compelled to undergo modification ; but it needs not only new applications,
but also improvements, further development, and plenty of fresh energy. All
this will surely come, seeing that such an assembly of men of science as the
Chemical Society of Great Britain has expressed the desire to have the his-
tory of the periodic law described in a lecture dedicated to the glorious name
of Faraday.

491

APPENDIX III

ARGON, A NEW CONSTITUENT OF THE ATMOSPHERE

WBITTEN BY PEOFESSOE MENDELEEFF IN FEBEUAEY 1895.

THE remarks made in Chapter V., Note 16 bis respecting the newly discovered,
constituent of the atmosphere are here supplemented by data (taken from
the publications of the Boyal Society of London) given by the discoverers
Lord Bayleigh and Professor Kamsay in January 1895, together with obser-
vations made by Crookes and Olszewsky upon the same subject,

This gas, which was discovered by Bayleigh and Bamsay -in atmo-
spheric nitrogen, was named argon l by them, and upon the supposition of
its being an element, they gave it the symbol A. But its true chemical
nature is not yet fully known, for not only has no compound of it been yet
obtained, but it has not even been brought into any reaction. From all that
is known about it at the present time, we may conclude with the discoverers
that argon belongs to those gases which are permanent constituents of the
atmosphere, and that it is a new element. The latter statement, however^
requires confirmation. We shall presently see, however, that the negative
chemical character of argon (its incapacity to react with any substance), and
the small amount of it present in the atmosphere (about 1£ per cent, by
volume in the nitrogen of air, and consequently about 1 per cent, by volume
in air), as well as the recent date of its discovery (1894) and the difficulty
pf its preparation, are quite sufficient reasons for the incompleteness of the
existing knowledge respecting this element. But since, so far as is yet known,
we are dealing with a normal constituent of the atmosphere * WS the

» From the Greet Ap-ybv— inert,

t bia In Note 16 bis, Chapter V., I mentioned that, judging from the specific gravity
of argon, it might possibly be polymerised nitrogen, N5, bearing the same relationship to
nitrogen, N^, that ozone, Oj, bears to ordinary oxygen. If this idea were confirmed, stilt.
one would not imagine that argon was formed from the atmospheric nitrogen by those;
reactions by which it was obtained by Eayleigh and Ramsay, but rather that it arises
from the nitrogen of the atmosphere under natural conditions. Although this proposition
is not quite destroyed by the more recent results, still it is contradicted by the fact that
the ratio of the specific heats of argon was found to be T66, which, as far as is now known,
could not be the case for a gas containing 8 atoms in its molecule, since such gases (sea
Chapter XIV., Note 7) give the ratio approximately l'8-(for example, CO,j). La abstain.
ing from further conclusions, for they must inevitably be purely conjectural, I consider,
it advisable to suggest, that in conducting further researches upon argou it ought be well.

492 PRINCIPLES OF CHEMISTRY

existing data, notwithstanding their insufficiently definite nature, should
find a place even in such an elementary work as the present, all the more as
the names of Rayleigh, Ramsay, Crookes and Olszewsky, who have worked
upon argon, are among the highest in our science, and their researches among
ithe most difficult.2 These researches, moreover,- "were directed straight to
the goal, which was only partly reached owing to the unusual properties of
argon itself.

When it became known (Chapter V., Note 4 bis) that the nitrogen obtained
from air (by removing the oxygen, moisture and C02 by various reagents)
has a greater density than that obtained from the various (oxygen, hydrogen
and metallic) compounds of nitrogen, it was a plausible explanation that the
latter contained an admixture of hydrogen, or of some other light gas lower-
ing the density of the mixture. But such an assumption is refuted not only
by the fact that the nitrogen obtained from its various compounds (after
purification) has always the same density (although the supposed impurities
mixed with it should vary), but also by Rayleigh and Ramsay's experiment
of artificially adding hydrogBn to nitrogen, and then passing the mixture over
red-hot oxide of copper, when it was found that the nitrogen regained its
original density, i.e. that the whole of the hydrogen was removed by this
treatment. Therefore ths difference in the density of the two varieties of
nitrogen had to be explained by the presence of a heavier gas in admixture
with the nitrogen obtained from the atmosphere. This hypothesis was con-
firmed by the fact that Rayleigh and Ramsay having obtained purified nitrogen
(by removing the 0.2, CO., and H20), both from ordinary air and from air
which had been previously subjected Jo atmolysis, that is which had been
passed through porous tubes (of burnt clay, e.g. pipe-stem), surrounded by a
rarefied space, and so deprived of its lighter constituents (chiefly nitrogen),
found that the nitrogen from the air which had been subjected to atmolysis
was heavier than that obtained from- ah- which had not been so treated. This
experiment showed that the nitrogen of ah- contains an admixture of a gas
which, being heavier than nitrogen itself,3 diffuses more slowly than nitrogen

to subject it to as high a temperature as possible. And the possibility of nitrogen
polymerising is all the more admissible from the fact that the aggregation of its atoms
in the molecule is not at all unlikely, and that polymerised, nitrogen, judging from many
.examples, might be inert if the polymerisation were accompanied by the evolution of
neat. In' the following footnotes I frequently return to this hypothesis, not only because
I have not yet met any facts definitely contradictory to it, but also because the chief'
properties of argon agree with it to a certain extent.

2 The chief difficulty in investigating argon lies in the fact that its preparation requires
the employment of a large quantity of air, which has to be treated with a number of
different reagents, whose perfect purity (especially that of magnesium) will always be
doubtful, and argon hasjnot yet been transferred to a substance in which it could be easily
purified. Perhaps the considerable solubility of argou in water (or in other suitable
liquids, which have not apparently yet been tried) may give the means of doing so, and it
may be possible, by collecting the air expelled from boiling water, to obtain a richer source
of argon than ordinary air.

. 3 It inight also be supposed that this heavy gas is separated by the copper when the
latter absorbs the oxygen of the air; but such a supposition is not only improbable in
itself, but does not agree with the fact that nitrogen may be obtained from air by absorb-
ing the oxygen by various other substances in solution (for instance, by the lower oxides

APPENDIX III; 493

through the porous material.. It remained, therefore-, to separate this im-
purity from the nitrogen. To do this Rayleigh and Ramsay adopted two
methods, converting the nitrogen into solid and liquid substances, either,
by absorbing the nitrogen by heated magnesium (Chapter V., Note 6, and
Chapter XIV., Note 14), with the formation of nitride of magnesium, or else-
by converting it into nitric, acid by the action of electric sparks or the presence
of an excess" of air and alkali, as in Cavendish's method.3 bis In both cases
the nitrogen entered into reaction, while the heavier gas mixed with it
remained inert, and was thus able to be isolated. That is", the argon could be
separated by these means from the excess of atmospheric nitrogen accom-
panying it.4 As an illustration, we will describe how argon was obtained
from the atmospheric nitrogen by means of magnesium.5 To begin with,
it was discovered that when atmospheric nitrogen was passed through a tube
containing metallic magnesium heated to redness,' its specific gravity rose to
14'88. As this showed that part of the gas was absorbed by the magnesium,
a mercury gasometer filled with atmospheric nitrogen was taken, and the
gas drawn over soda-lime, P205» heated magnesium6 and then through
tubes containing red-hot copper oxide, soda-lime and phosphoric anhydride
to a second mercury gasometer.. Every time the gas was repassed through
the tubes, it decreased in volume and increased in density. After repeating

of the metals, like PeO) besides red-hot copper, and that the nitrogen obtained is always
just as heavy. Besides which, nitrogen is also set free from its oxides by copper, and the
nitrogen thus obtained is lighter. Therefore it is not the copper which produces the
.heavy gas — i.e. argon.

8 bis 1$ ja worthy 'of note that Cavendish obtained a small residue of gas in con-
verting nitrogen into nitric acid ; but he paid no attention to it, although probably he
had in.his hands the very argon recently discovered.

4 When in these experiments, instead of atmospheric nitrogen the gas obtained from
its compound was taken, an inert residue of a heavy gas, having the properties of argon,
was-also remarked, but its-amount was very small. Eayleigh and Ramsay ascribe the
formation of this residue to the fact that the gas in these experiments was collected over
water, and $ portion of the dissolved argon in it might have passed into the nitrogen. As
the authors of this supposition did not prove it by any special experiments, it forms a
weak point in their classical research. If it be admitted that argon is N3, the fact of its
being obtained from the nitrogen of compounds might be explained by the polymerisation
of a portion of the nitrogen in the act of reaction, although it is impossible to refute
Eayleigh and Eamsay's hypothesis of its being evolved from the water employed in the
manipulation of the gases. Three thousand volumes of nitrogen extracted from its
compounds gave about three volumes of argon, while thirty volume's were yielded by the
eame amount of atmospheric nitrogen.

6 The preparation of argon by the conversion of nitrogen into nitric acid is complicated
by the necessity of adding a large proportion of. pxygen and alkali, of passing jin electric
discharge through the mixture #>r a long period, and then removing the remaining'
oxygen. AlLthis was repeatedly done by the authors, but this method is far more
complex, both in practice and theory, than the preparation of argon by means of
magnesium. From 100 volumes of air subjected to conversion into HN03, 0'76 volume
of argon were obtained after absorbing the excess of oxygen.

6 In these and the following experiments the magnesium was placed in an ordinary
hard glass tube, and heated in a gas furnace to a temperature almost sufficient to soften
the glass. The current of gas must be very slow (a tube containing a small quantity of
sulphuric acid served as a meter), as otherwise the heat evolved in the formation of tho
Mg3N2 (Chapter XTV., Note 14) will melt the tube.

494 PRINCIPLES OF CHEMISTRY

this for tea days 1,500 c.c. of gas were reduced to 200 cc., and the density
increased to IG'l (if that of EL, - 1 and N2 • 14). Further treatment of the
remainder brought the density up to 19-09. After adding a small quantity
of oxygen and repassing the gas through the apparatus, the density rose to
20-0. To obtain argon by this process Ramsay and Eayleigh (employing a
mercury air pump and mercury gasometers) once treated about 150 litres of
atmospheric nitrogen. On another occasion they treated 7,925 c.c. of air by
the oxidation method and obtained 65 c.c. of argon, which corresponds to
0'82 per cent. The density of the argon obtained by this means was nearly
19-7, while that obtained by the magnesium method varied between 19-09
and 20-88.

Thus the first positive and very important fact respecting argon is that
its specific gravity is nearly 20 — that is, that it is 20 times heavier than
hydrogen, while nitrogen is only 14 times and oxygen 16 times heavier than
hydrogen. This explains the difference observed by Bayleigh between the
-densities of nitrogen obtained from- its compounds and from the atmosphere
(Chapter V., Note 4 bis). At 0° and 760 mm. a litre of the former gas weighs
1-2505 grm., while a litre of the latter weighs 1-2572, or taking H - 1, the
density of the first - 13-916, and of the latter = 13-991. If the density of
argon be taken as 20, it is contained in atmospheric nitrogen to the extent of
about 1'23 per cent, by volume, whilst air contains about 0-97 per cent, by
volume.

When argon had been isolated the question naturally arose, was it a new
homogeneous substance having definite properties or was it a mixture of
gases ? The former may now be positively asserted, namely, that argon is a
peculiar gas previously unknown to chemistry. Such a conviction is in the
first place established by the fact that argon has a greater number of nega-
tive properties, a smaller capacity for reaction, than any other simple or
compound body known. The most inert gas known is nitrogen, but argon
far exceeds it in this respect. Thus nitrogen is absorbed at a red heat by many
metals, with the formation of nitrides, while argon, as is seen in the mode
of its preparation and by direct experiment, does not possess this property.
Nitrogen, under the action of electric sparks, combines with hydrogen in -the
presence of acids and with oxygen hi the presence of alkalis, while argon is
unable to do so, as is seen from the method of separation from nitrogen.
Bayleigh and Ramsay also proved that argon is unable to react with chlorine
(dry or moist) either directly or under the action of an electric discharge, or
with phosphorus or sulphur, at a red heat. Sodium, potassium, and tellurium
may be distilled in an atmosphere of argon without change. Fused caustic
soda, incandescent soda-lime, molten nitre, red-hot peroxide of sodium,
and the polysulphides of calcium and sodium also do not react with argon.
Platinum black does not absorb it, and spongy platinum is unable to excite its
reaction with oxygen or chlorine. Aqua regia, bromine water, and a mixture
of hydrochloric acid and KMn04 were also without action upon argon. Besides
which it is evident from the method of its preparation that it is not acted upon
by red-hot oxide of copper. All these facts exclude any possibility of argon con-
taining any already known body, and prove it to be the most inert of all the
gases known. But besides these negative points, the independency of argon is

APPENDIX III. 495

confirmed by four observed positive properties possessed by it, which
are: —

1. The spectrum of argon observed by Crookes under a low pressure (in
Geissler-Pliicker tubes) distinguishes it; from other gases.7 It was proved
by this means that the argon obtained by means of magnesium is identical
with that which remains after the uconversion of the atmospheric nitrogen
into nitric acid. Like nitrogen, argon presents two spectra produced at
different potentials of the induced current, one being orange-red, the other
steel-blue ; the latter is obtained under a higher degree of rarefaction and
with a battery of Leyden jars. Both the spectra of argon (in contradistinction
to those of nitrogen) are distinguished by clearly defined lines.8 The red
(ordinary) spectrum of argon has two particularly brilliant and characteristic
red lines (not far from the bright red line of lithium, on the opposite side to
the orange band) having wave-lengths 705*64 and 696*56 (see Vol. I.,
p. 565). Between these bright lines there are in addition lines with wave
lengths 603*8, 565*1, 561*0, 555*7, 518*58, 516*5, 450*95, 420*10, 415*95 and
394*85. Altogether 80 lines have been observed in this spectrum and 119 in
the blue spectrum, .of which 26 are common to both spectra.9

2. According to Rayleigh and Ramsay the solubility of argon in water
is approximately 4 volumes hi 100 volumes of water at 13°. Thus argon
is nearly 2£ times more soluble than nitrogen, and its solubility ap-
proaches that of oxygen. Direct experiment proves that nitrogen obtained
from air from boiled water is heavier than that obtained straight from the
atmosphere. This again is an indirect proof of the presence of argon in
air.

3. The ratio & of the two specific heats (at a constant pressure and at

7 The greatest brilliancy of the spectrum of argon is obtained at a tension of 9 mm.,
while for nitrogen it is about 75 mm. (Crookes). In Chapter V., Note 16 bis, it is said
that the same blue line observed in the spectrum of argon is also observed in the spectrum
of nitrogen. This is a mistake, since there is no coincidence between the blue lines of
the argon and nitrogen spectra. However, we may add that for nitrogen the following
moderately bright lines are known of wave-lengths 585, 574, 544, 516, 457, 442,^36, and
426, which are repeated in the spectra (red and blue) of argon, judging by Crookes'
researches (1895) ; but it is naturally impossible to assert that there is perfect identity
until some special comparative work has been done in this subject, which is very desirable,
and more especially for the bluish-violet portion of the spectrum, more particularly
between the lines 442-436, as these lines are distinguished by their brilliancy in both the
argon and nitrogen spectra. The above-mentioned supposition of argon being polymerised
nitrogen (N3), formed from nitrogen (N2), with the evolution of heat, might find some
support should it be found after careful comparison that even a limited number of
spectral lines coincided.

8 At first the spectrum of argon exhibits the nitrogen lines, but after a certain time
these lines disappear (under the influence of the platinum, and also of Al and Mg, but
with the latter the spectrum of hydrogen appears) and leave a pure argon spectrum. It
does not appear clear to me whether a polymerisation here takes place or a simple
absorption. Perhaps the elucidation of this question would prove important in the
history of argon. It would be desirable to know, for instance, whether the volume of
argon changes when it is first subjected to the action of the electric discharge.

9 Crookea supposes that argon contains a mixture of two gases, but as he gives no
reasons for this, beyond certain peculiarities of a spectroscopio character, we will not
consider this hypothesis further.

496 PRINCIPLES OF CHEMISTRY

a constant volume) of argon was determined by Kayleigh and Ramsay "by the
method of the velocity of sound (see Chapter XIV., Note 7 and Chapter VII.,
Note 26) and was found to be nearly 1'GG, that is greater than for those gases
whose molecules contain two atoms (for instance, CO,H2,N2, air, &c., for
which" 7c is nearly 1'4) or those whose molecules contain three atoms (for
instance, C02,N20, &c., for which ft is about 1'3), but closely approximate
to the ratio of the specific heats of mercury vapour (Kundt and Warburg,
fc = 1*67). And as the molecule of mercury vapour contains one atom, so it
may be said that argon is a simple gaseous body whose molecule contains
one atom.10 A compound body should give a smaller ratio. The experi-
ments upon the liquefaction of argon, which we shall presently describe, speak
against the supposition that argon is a mixture of two gases. The import-
ance of the results in question makes one wish that the determinations of the
ratio of the specific heats (and other physical properties) might be confirmed
with all possible accuracy.11 If we admit, as we are obliged to do for the
present, that argon is a new element, its density shows that its atomic weight
must be nearly 40, that is, near to that of K = 39 and Ca = 40, which does
cot correspond to the existing data respecting the periodicity of the properties

10 This portion of Rayleigh and Ramsay's researches deserves particular attention as,
80 far, no gaseous substance is known whose molecule contains but one atom. Were it
not for the above determinations, it might be thought that argon, having a density 20,
has a complex molecule, and may be a compound or polymerised body, for instance, N5
or NXn, or in general Xn ; but as the matter stands, it can only be said that either (1)
argon is a new, peculiar, and quite unusual elementary substance, since there is no
reason for assuming it to contain two simple gases, or (2) the magnitude, k (the ratio of
the specific heats) does not only depend upon the number of atoms contained in the
molecules, but also upon the store of internal energy (internal motion of the atoms in
the molecule). Should the latter be admitted, it would follow that the molecules of very
active gaseous elements would correspond to a smaller k than those of other gases having
an equal number of atoms in their molecule. • Such a gas is chlorine, for which k = T33
(Chapter XIV., Note 7). For gases having a small chemical energy, on the contrary, a
larger magnitude would be expected for k. I think these questions might be partially
8e.ttled by determining k for ozone (O3) and sulphur (S6) (at about 500°). In other words,
I would suggest, though only provisionally, that the magnitude, k = T6, obtained for
argon might prove to agree with the hypothesis that argon is N3, formed from N2 with
the evolution of heat or loss of energy. Here argon gives rise to questions of primary
importance, and it is to be hoped that further research will throw some light upon them.
In making these remarks, I only wish to clear the road for further progress in the study
of argon, and of the questions depending on it. I may also remark that if argon is NS
formed with the evolution of heat, its conversion into nitrogen, N2, and into nitride
Compounds (for instance, boron nitride or nitride of titanium) might only take place at a,
very high temperature.

11 Without having the slightest reason for doubting the accuracy of Rayleigh and
Ramsay's determinations, I think it necessary to say that as yet (February 1895) I am
only acquainted with the short memoir of the above chemists in the ' Proceedings of the
Royal Society,' which does not give any description of the methods employed and results
obtained, while at the end (in the general conclusions) the authors themselves express
some doubt as to the simple nature of argon. Moreover, it seems to me that (Note 10)
there must be a dependence of k upon the chemical energy. Besides which, it is not
clear what density of the gas Rayleigh and Ramsay took in determining k. (If argon be
N5, its density would be near to 21.) Hence I permit myself to express some doubt as to
whether the molecule of argon contains but one atom.

APPENDIX III. 497

of the elements in dependence upon their atomic weights, for there is no
reason on the basis of existing data, for admitting any intermediate elements
between Cl = 35*5 and K =» 39, and all the positions above potassium in the
periodic system are occupied. This renders it very desirable that the velocity
of sound in argon should be re-determined.13

4. Argon was liquefied by Professor Olszewsky, who is well known for his
classical researches upon liquefied gases. These researches have an especial
interest since they show that argon exhibits a perfect constancy in its

w If it should be found that Tc for argon is less than 1-4, or that k is dependent upon
the chemical energy, it would be possible to admit that the molecule of argon contains
not one, but several atoms— for instance, either N3 (then the density would be 21, which
is near to the observed density) or Xg, if X stand for an element with an atomic weight
near to 6*7. No elements are known between H=l and Li = 7, but perhaps they may
exist. The hypothesis A = 40 does not admit argon into the periodic system. If the
molecule of argon be taken as A2 — i.e. the atomic weight as A =20 — argon apparently
finds a place in Group VIII., between, P = 19 and Na = 23 ; but such a position could only
be justified by the consideration that elements of small atomic weight belong to the
category of typical elements widen offer many peculiarities in their properties, as is
seen on comparing N with the other elements of Group V., or O with those of Group VI.
Apart from this there appears to me to be little probability, in the light of the periodic
law, in the position of an inert substance like argon in Group VIET., between such active
elements as fluorine and sodium, as the representatives of this group by their atomic
weights and also by their properties show distinct transitions from the elements of the
last groups of the uneven series to the elements of the first groups of the even series— for
instance,

Group vi. vn. vm. i. n.

Cr Mn Fe,Co,Ni Cu Zn

While if we place argpn in a similar manner,

vi. vn. VHI. i. n.

0 = 16 F = 19 A=20 Na=23 Mg = 24

although from a numerical point of view there is a similar sequence to the gjbove, still
from a chemical and physical point of view the result is quite different, as there is no
such resemblance between the properties of O, F and Na, Mg, as between Cr, Mn, and
Cu, Zn. I repeat that only the typical character of the elements with small atomic
weights can justify the atomic weight A =20, and the placing of argon in Group VIII
amongst the typical elements ; then N, O, F, A are a series of gases.

It appears to me simpler to assume that argon contains N3, especially as argon is
present in nitrogen and accompanies it, and, as a matter of fact, none of the observed
properties of argon are contradictory to this hypothesis.

These observations were written by me in the beginning of February 1895, and on
the 29th of that month I received a letter, dated February 25, from Professor Eamsay
informing me that ' the periodic classification entirely corresponds to its (argon's) atomic
weight, and that it even gives a fresh proof of the periodic law,' judging from the
researches of my English friends. But in what these researches consisted, and how the
above agreement between the atomic weight of argon and the periodic system was arrived
at, is not referred to in the letter, and we remain in expectation of a first publication of
the work of Lord Rayleigh and Professor Ramsey. [For more complete information see
papers read before the Royal Society, January 81, 1895, February 13, March 10, and
May 21, 1896, and a paper published in the Chemical Society's Transactions, 1895,
p. 684. For abstracts of these and other papers on argon and helium, and correspon-
dence, see ' Nature,' 1895 and 1896.]

498 PRINCIPLES OF CHEMISTRY

properties in the liquid and critical states, which almost 13 disposes of the sup-
position that it contains a mixture of two or more unknown gases. As the
first experiments showed, argon remains a gas under a pressure of 100
atmospheres and at a temperature of — 90° ; this indicated that its critical
temperature was probably below this temperature, as was indeed found to
be the case when the temperature was lowered to — 128°'6 14 by means of
liquid ethylene. At this temperature argon easily liquefies to a colourless
liquid under 38 atmospheres. The meniscus begins to disappear at between

13 There only remains the very remote possibility that argon consists of a mixture of
two gases having very nearly the same properties.

14 The following data, given by Olszewsky, supplement the data given in Chapter II.,
JNote 29, upon liquefied gases.

(tc) (pc) t «, s

N2 -146° 85 -194°-4 -214° 0'885

CO -139°'5 85-5 -190° -207 ?

A -121° 60-6 -187° -189°'6 1-5

O2 -118°-8 60-8 -182°'7 ? T124

NO - 93°'5 71-2 -153°'6 -167° ?

CH4 - 81°'8 54-9 -164° -158°'8 0'415

where tc is the absolute (critical) boiling point, pc the pressure (critical) in atmospheres
corresponding to it, t the boiling point (under a pressure of 760 mm.), t\ the melting point,
and s the specific gravity in a liquid state at t.

The above shows that argon in its properties in a liquid state stands near to oxyge^
(as it also does in its solubility), but that all the temperatures relating to it (tc, £, and <t)
are higher than for nitrogen. This fully answers, not only to the higher density of argon,
but also to the hypothesis that it contains N3. And as the boiling point of 'argon differs
from that of nitrogen and oxygen by less than 10°, and its amount is small, it is easy to
understand how Dewar (1894), who tried to separate it from liquid air and nitrogen by
fractional distillation, was unable to do so. The first and last portions were identical,
and nitrogen from air showed no difference in its liquefaction from that obtained from
its compounds, or from that which had been passed through a tube containing incandescent
magnesium. Still, it is not quite clear why both kinds of nitrogen, after being passed
over the magnesium in Dewar's experiments, exhibited an almost similar alteration in
their properties, independent of the appearance of a small quantity of hydrogen in them.

Concluding Remarks (March 81, 1895). — The ' Comptes rendus ' of the Paris
Academy of Sciences of March 18, 1895, contains a memoir by Berthelot upon the reaction
of argon with the vapour of benzene under the action of a silent discharge. In his ex-
periments, Berthelot succeeded in treating 83 per cent, of the argon taken for the
purpose, and supplied to him by Ramsay (37 c.c. in all). The composition of the product
could not be determined owing to the small amount obtained, but in its outward
appearance it quite resembled the product formed under similar conditions by nitrogen.
This observation of the famous French chemist to some extent supports the supposition
that argon is a polymerised variety of nitrogen whose molecule contains N3, while ordinary
nitrogen contains N2. Should this supposition be eventually verified, the interest in
argon will not only not lessen, but become greater. For this, however, we must wait for
further observations and detailed experimental data from Rayleigh and Ramsay.

The latest information obtained by me from London is that Professor Ramsay, by
treating cleveite (containing PbO, TJOj, Y2O3, &c.) with sulphuric acid, obtained argon,
and, judging by the spectrum, helium also. The accumulation of similar data may, after
detailed and diversified research, considerably increase the stock of chemical knowledge
which, constantly widening, cannot be exhaustively treated in these 'Principles of
Chemistry,' although rery probably furnishing fresh proof of the 'periodicity of the
elements.*

APPENDIX III. 499

- 119°-8 and - 121°-6, mean - 121° at a pressure of 50-6 atmospheres. The
vapour tension of liquid argon at - 1280>C, is 38'0 atmospheres, at - 187°
it is one atmosphere, and at — 189°*6 it solidifies to a colourless substance
like ice. The specific gravity of liquid argon at about - 187° is nearly 1-5,
which is far above that of other liquefied gases of very low absolute boiling
point.

The discovery of argon is one of the most remarkable chemical acquisi-
tions of recent times, and we trust that Lord Eayleigh and Professor Ramsay,
who made this wonderful discovery, will further elucidate the true nature of
argon, as this should widen the fundamental principles of chemistry, to which
the chemists of Great Britain have from early times made such valuable
contributions. It would be premature now to give any definite opinions
upon so new a subject. Only one thing can be said ; argon is so inert that
its role in nature cannot be considerable, notwithstanding its presence hi the
atmosphere. But as the atmosphere itself plays such a vast part in the
life of the surface of the earth, every addition to our knowledge of its compo-
sition must directly -or indirectly react upon the sum total of our knowledge
of nature.

INDEX OP AUTHOEITIES

?, i. 75
Abel, ii. 56, 326, 410
Acheson, ii. 107
Adie, ii. 186
Alexeeff, i. 75, 94
Alluard, i. 458
Amagat, i. 132, 135, 140
Amat, ii. 171
Ammennuller, i. 504
Ampere, i. 309
Andr^eff, i. 251
Andrews, i. 136, 203
Angeli, i. 266
Ansdell, i. 451
Arfvedson, i. 575
Arrhenius, i. 89, 92, 889
Aschoff, ii. 313
kskenasy, i. 508
Aubel, ii. 45
Aubin, i. 238
Avdeeff,i. 618; ii. 484
Avogadro, i. 309

BABO, v., i. 9? 900, 203

Bach, i. 39^

Bachmetien, ii. 31

Baeyer, v., i. 507

Bagouski, i. 384

Bailey, i. 449 ; ii. 29, 538

Baker, i. 318, 403

Balard, i. 480, 494, 495, 505

Ball, ii. 414

Bannoff, i. 506

Barfoed, ii. 53

Baroni, i. 331

Barreswill, ii. 282,

Baudrimont, ii. 35

Baume, i. 193

Baumgau -. v' 20

Baumhauer, i, 495

Bayer, ii. 76, 159

Bazaroff, i. 409 ; ii. 24, 68, 486

BOX

Becher, i. 17

Becker, i. 16

Beckmann, i. 91, 496 ; ii. 156

Becquerel, i. 228 ; ii. 97, 220

Beilby, i. 71

Beilstein, i. 373 ; ii. 188

Beketoff, i. 120, 122, 124, 146, 403, 459,

466, 534, 541, 674, 577 ; ii. 87, 102,

289, 429
Bender, i. 476
Benedict, ii. 65
Berglund, ii. 229
Bergman, i. 27, 435 ; ii 100
Berlin, i. 95
Bernouilli, i. 81
Bernthsen, ii. 228
Bert, i. 86, 153
Bertheim, ii. 337
Berthelot, i. 171, 173,189,199,229,230,

258, 264, 266, 267, 272, 283, 289, 351,

372, 393, 394, 405, 415,424, 438, 457,

463, 502, 506, 507, 518, 529, 537, 582 ;

ii. 23, 57, 207, 209, 251, 253, 259, 345,

367

Berthier, ii. 8
Berthollet, i. 27, 31, 105, 433, 434, 459,

470, 502, 609
Berzelius, i. 131, 148, 194, 255, 379 ; ii.

8, 100, 102, 147, 148, 219, 281, 300, 485
Besson, i. 288 ; ii. 67, 70, 105, 179
Beudant, ii. 7, 8

Bineau, i. 100, 271, 452, 504; ii. 239
Binget, i. 75
Blaese, ii. 188
Blagden, i. 91. 428
Blake, ii. 30
Blitz, ii. 184
Blomstrand, ii. 299
Boerwald, ii. 279
Bottger, ii. 595
Bogorodsky, i.
Boilleau, i. 415
Boisbaudran.L. de, i. 97, 102, 672, 600 {

ii. 6, 26, 90, 82, 284, 483

502

PRINCIPLES OF CHEMISTRY

Bornemann, i. 509

Botkin, ii. 30

Bouchardat, ii. 45

Boullay, ii. 55

Bourdiakoff, i. 584, 617

Boussingault, i. 131, 157, 233, 235, 525,

615

Boyle, i. 124
Brand, ii. 150
Brandau, i. 481
Brandes. i. 72
Bravais, i. 233
Brauner, i. 490, 491 ; ii. 26, 59, 94, 96,

97, 134, 144, 194, 271, 483
Brewster, i. 569
Brigham, ii. 193
Brodie, i. 212, 351, 405 : ii. 252
Brooke, ii. 357
Brown, i. 81, 88
Brugellrnann, i. 616
Brunn, ii. 182, 189
Bruyn, i. 262
Biiihl, i. 263, 337

Brunner, i. 124, 146, 263 ; ii. 230, 309, 534
Buchner, i. 615
Buckton, ii. 143
Buff, ii. 103
Bunge, i. 288
Bunsen, i. 43, 69, 78, 117, 180, 465, 568,

575, 576, 577 ; ii. 27, 289
Bussy, i. 75, 594, 619
>Butleroff, i. 143
Bystrom, i. 585

CAONIARD DE LATOUB, i. 135, 345

Cahours, ii. 143, 173

Cailletet, i. 132, 138 ; ii. 45

Callender, i. 134

Calvert, i. 484 ; ii. 45

Cannizzaro, i. 584, 587

Carey-Lea, ii. 420, 424, 425, 432

Carius, i. 69, 481

Carnelley, i. 483, 515, 555 ; ii. 22, 29, 30,

31, 64, 143, 486
Carnot, ii. 294, 361
Cavon, i. 595, 604, 610 ; ii. 336
Carrara, i. 213
Cass, ii. 85

Castner, i. 431, 535, 541
Cavazzi, ii. 160, 172, 182
Cavendish, i. 113, 125, 228 ; ii. 493
Chabrie, i. 229

Chappuis, i. 50, 199, 205, 264
Chapuy, i. 59

Cheltzoff, i. 393, 457 ; ii. 41, 247, 582
Cherikoff, ii. 102
Chertel, ii. 245
Chevillot, ii. 311

DEV

Chevreul, i. 530
Chigoffsky, ii. 62
Christomanos, i. 511
Chroustchoff, i. 353, 444 ; ii. 122
Chydenius, ii. 148, 485
Ciamician, i. 565, 573 ; ii. 486
Clark, i. 26
Classen, ii. 146
Clausius, i. 81, 93, 140, 142, 212, 309,

491

Clement, i. 494
Cleve, ii. 26, 94, 97, 484
Cloez, i. 207, 246, 377
Clowes, i. 242
Calderon, i. 596
Collendar, i. 134
Cbmaille, i. 596
Comb, ii. 81
Connell, i. 508
Coppet, i. 91, 428, 601
Corenwinde*, i. 501
Cornu, i. 565
Courtois, i. 494
Cracow, ii. 380
Crafts, i. 380 ; ii. 80, 83
Cremers, ii. 100
Croissier, i. 251
Crompton, i. 247
Crookes, i. 229, 617; ii. 20, 91, 96,

440, 491

Crum, ii. 79, 311
Cundall, i. 611
Curtius, i. 258, 265

DAHL, ii. 59

Dalton, i. 29, 78, 81, 109, 206, 271, 322

Dana, ii. 8

Davies, i. 484

Davy, i. 37, 114, 195, 255, 364,460,463,
484, 489, 494, 533, 541, 594, 604, 617

Deacon, i. 599

Debray, i. 609 ; ii. 45, 122, 291, 293,
384, 385

De Chancourtois, ii. 20, 26

De Forcrand, ii. 106, 211

De Haen, ii. 189

De Heen, i. 140

Delafontaine, ii. 97, 148, 198

De la Rive, i. 198 ; ii. 226

Del-Rio, ii. 197

De Saussure, i. 235, 240

De Schulten, ii. 48

Deville, St.-Claire, i. 4, 36, 118, 143, 179,
180, 227, 239, 280, 281, 301, 320, 392,
393, 399, 459, 467, 476, 477, 500, 534,
595, 608, 609 ; ii. 48, 80, 83, 85, 102,
147, 156, 198, 289, 309, 321, 352, 373
374, 429

INDEX OF AUTHORITIES

503

De Vries, i. 62, 64, 429

Dewar, i. 3, 5, 135, 139, 163, 297, 563,

565, 569, 585 ; ii. 176, 220
Dick, ii. 414
Dingwall, i. 486
Ditte, i. 72, 403, 430, 457, 509, 539, 618 ;

ii. 64, 65, 85, 189, 249
Dittmar, i. 100, 452 ; ii. 240
Divers, i. 274. 294 ; ii. 54
Dixon, i. 171
Dobereiner, i. 145
Dokouchaeff, i. 344
Donny, i. 534
Dossios, i. 502
Draper, i. 465
Drawe, ii. 161
Drebbel, i. 294
Dulong, i. 131, 148, 437
Dumas, i. 28, 131, 148, 150, 233, 302,

320, 379, 471, 476, 584, 586, 604 ; ii.

22, 37, 62, 101, 156, 420
Dumont, ii. 197

EBELMANN, ii. 65
Eder, i. 566
Edron, ii. 95
Edwards, ii. 311
Egoreff, i. 569
Eissler, i. 553
Elbers, ii. 221
Emich, i. 286 287

Emilianoff, ii
Engel, i. 457
Engelhardt, i

126

ii. 130, 132, 189, 206

530

Eotvos, i. 333
Erdmann, i. 150
Ernst, i. 399
Erofeeff, i. 352
Esson, ii. 314

tard, i. 72, 516, 615 ; ii. 238, 335, 356
Ettinger, i. 53, 312

FAMINTZIN, i. 611

Faraday, i. 134, 177, 296, 385, 463, 464

Favorsky, i. 373

Favre, i. 120, 172, 267 ; ii, 83, 259,284,

380, 582
Fick, i. 62
Fisher, ii. 424
Fizeau, ii. 31, 429
Flavitzky, i. 21
Fleitmann, ii. 170
Foerster, ii. 375, 389
Forchhammer, ii. 311
Fordos, ii. 257
Fortmann, ii. 230, 366
Fourcroy, i. 114

OBO

Fowler, i. 449

Frank, ii. 88

Franke, ii. 311, 313

Frankel, ii. 294

Frankenheim, ii. 7

Frankland, i. 178, 357, 486 ; ii. 16, 143

Fraunhofer, i. 563

Fremy, i. 228, 489, 492 ; ii. 74, 131, 133,

142, 229, 290, 359
Freyer, i. 171, 488
Friedheim, ii. 197, 294
Friedel, i. 353, 472 ; ii. 80, 83, 103,

122

Friedrich, i. 49 ; ii. 144
Fritzsche, i. 94, 285, 600, 612 ; ii. 125,

218, 280, 341
Fromherz, ii. 313
Fiirst, i. 484

GALILEO, i. 7

Garni, i. 582

Garzarolli-Thurnlackh, i. 481

Gattenfiann, i. 596 ; ii. 102, 104

Gautier, i. 585

Gavaloffsky, i. 160

Gay-Lussac, i. 40, 61, 71, 93, 170, 302,

307, 406, 412, 460, 463, 464, 467, 500,

506, 508, 511, 515, 534, 539 ; ii. 8, 50,

256

Geber, i. 17
Ge"lis, ii. 257
Genth, ii. 359
Georgi, ii. 197
Georgiewics, ii. 64
Gerberts, i. 528
Gerhardt, i. 196, 309, 357, 388
Gerlach, i. 525
Gernez, i. 97 ; ii. 205
Geuther, i. 281, 283, 285 , ii. 176
Gibbs, i. 140, 464 ; ii. 293, 410
Girault, i. 498

Gladstone, i. 337, 438, 573 ; ii. 213
Glatzel, ii. 213, 289, 309
Glauber, f. 17, 26, 193, 432
Glinka, i. 607
Goldberg, i. 93
Gooch, i. 484
Gore, i. 489, 492, 493
Graham, i. 62, 63, 98, 143, 155, 388,

429, 518, 601; ii. 77, 114, 131, 163,

170, 296, 307
Granger, ii. 157, 410
Grassi, i. 88
Green, ii. 310
Greshoff, i. 403
Griffiths, i. 135
Grimaldi.i. 537
Groth, ii. 10

504

PRINCIPLES OF CHEMISTRY

ORO

Grouven, i. 615
Grove, i. 118, 119
Grunwald, i. 573
Grutzner, ii. 296
Guckelberger, ii. 84
Guibourt, ii. 53
Guldberg, i. 439, 464
Giintz, i. 575 ; ii. 430
Gustavson, i. 443, 444, 472, 505, 547 ;

ii. 29, 175

Guthrie, i. 99, 428, 601
Guy, i. 136

HABERMANN, ii. 210

Hagebach, i. 573

Hagen, i. 337

Haitinger, i. 593

Hammerl, i. 613

Hanisch, ii. 233

Hannay, i. 352 ; ii. 135

Harcourt, ii. 314

Hargreaves, i. 515

Harris, ii. 52

Hartley, i. 573 ; ii. 486

Hartog, ii. 268

Hasselberg, i. 560

Haiiy, ii. 7

Haughton, ii. 20

Haussermann, i. 483

Hautefeuille, i. 199,-205,.264, 409, 414,

476, 477, 501, 538 ; ii. 102, 122, 379
Hayter, ii. 175
Hemilian, i. 132
Hempel, i. 59, 524
Henkotf, i. 530
Henneberg, ii. 170
Henning, ii. 3
Henry, i. 78, 81
Herard, ii. 191
Hermann, ii. 8, 47, 197
Hermes, i. 529
Hertz, ii. 156
Hess, i. 178, 588
Heycock, i. 537 ; ii. 128, 448
Hillebrand, ii. 26, 93, 94, 484
Hintze, ii. 10
Hirtzel, ii. 55
Hittorf, ii. 155
Hodgkinson, ii. 432
Hoglund, ii. 94

Hofmann, i. 302 ; ii. 146, 218, 447
Holtzmann, i. 505
Hoppe-Seyler, i. 611
Horstmann, i. 408
Houz«au, i. 202
Hughes, ii. 212
Hugo, ii. 21
Humboldt, i. 170

Humbly, i. 493 ; ii. 311
Hutchinson, i. 491
Huth, ii. 20
Kuyghens, i. 569

IKEDA, ii. 152
Ilosva, i. 202

Inostrantzeif, i. 345 ; ii. 4
Isambert, i. 250, 257, 408 ; ii. 41
Ittner, i. 412

JANSSEN, i. 569

Jawein, ii. 170

Jay, i. 258

Jeannel, i. 104

Joannis, i. 251, 255, 405, 537, 559

Jorgensen, i. 498 ; ii. 359, 361, 376

Johnson, ii. 45

Jolly, i. 233

Joly, ii. 384, 385

KAMENSKY, ii. 414

Kamuierer, i. 286, 462, 509 ; ii. 297

Kane, ii. 57

Kapoustin, i. 403

Karsten, i. 427, 428, 541, 599

Kassner, i. 158

Kayander, i. 133, 384; ii. 46

K.eiser, i. 150

Kekule, i. 358, 369, 507 ; ii. 294

Keyser, ii. 33

Khichinsky, i. 440

Kimmins, i. 510

Kirchhoff, i. 567

Kirmann, ii. 268

Kirpieheff, i. 132

Kjeldahl, i. 249 ; -ii. 249

Klaproth, ii. 7, 145, 147, 301

Kleiber, i. 570

Klimenko, i. 465

Klobb, ii. 357

Klodt, i. 426

Knopp, ii. 338

Knox, i. 489

Kobb, ii. 125

Kobell, ii. 197

Koch, i. 44

Kohlrausch, i. 245, 525

Kolbe, i. 506

Konovalofi, i. 39, 65, 90, 93, 100, 140,

142, 172, 322 ; ii. 235, 268
Kopp, i. 586, 587, 612 ; ii. 3, 37
Koucheroff, i. 373
Kolotoff, i. 263

Kournakoff, i. 393 ; ii. 294, 365, 396
Kouriloff, i. 209, 247, 274 ; ii. 41

INDEX OF AUTHOKITIES

505

Kraevitch, i. 133, 134

Kraft, 1. 65, 88, 537

Krafts, i. 393

Kreisler, i. 233

Kremers, i. 87, 443 ; ii. 244, 427

Kreider, i. 484

Kronig, i. 81

Kruger, ii. 282, 284 .

Kriiss, ii. 355, 442, 447, 486

Kubierschky, ii. 213

Kuhlmann, i. 608

Kuhnheim, i. 612

Kundt, i. 328, 589 ; ii. 496

Kvasnik, ii. 57

Kynaston., i. 522

LACHINOFF,!. 116, 457; ii. 410

Ladenburg, ii, 103

Lamy, ii. 91

Landolt, i. 7, 837

Lang, i. 399

Langer, i. 226, 459, 462

Langlois, i. 570 ; ii. 257

Latchinoff (see LachinoS), i. 103, 352

Laurent, i. 28, 196, 388, 471, 526 ; ii. 7,

9, 10, 117, 292

Laurie, i. 106 ; ii. 32, 442, 486
Lavenig, i. 140 •

Lavoisier, i. 7, 29, 49, 114, 131, 155,

379, 459
Leblanc, ii. 8
Le Chatelier, i. 158, 172, 350, 393, 399,

585, 588, 611 ; ii. 51, 65, 420
Le Due, i. 131, 170
Lemery, i. 125
Lemoine, i. 501 ; ii. 155
Lerch, i. 405
Leroy, i. 285
Lescoeur, i. 103
Leton, ii. 425
Levy, ii. 102
Lewes, i. 371
Lewy, i. 232
Lidoff, ii. 209

Liebig, i. 195, 388, 495, 527 ; il. 58
Linder, ii. 223
Lies-Bodart, i. 604, 612
Lisenko, i. 373
Liveing, i. 563, 569
Lockyer, i. 565, 569
Loew, ii. 376

L6wetfi?.525, 600 ; ii. 45, 284, 286
LoewigTi. 528 ; ii. 77
Loewitz, i. 96
Lessen, i. 262
Louget, i. 489
Louginine, i. 360
Louise, ii. 81

Mil

Lovel, i. 515 ; ii. 338
Lubavin, i. 593
Lubbert, ii. 85, 170
Ludwig, i. 463
Luedeking, ii. 194
Luff, ii. 321
Lunge, ii. 244, 246
Liipke, ii. 157
Lvoff, i. 358

MAACK, i. 596

Mac Cobb, i. 612

Mac Laurin, i. 553

McLeod, ii. 180

Magnus, i. 93, 510

Mailfert, i. 199

Malaguti, i. 437,; ii. 300

Mallard, i. 172, 393, 588 ; ii. 4

Mallet, i. 493

Maquenne, i. 349, 620, 621

Marchand, i. 150

Marchetti, ii. 288

Maresca, i. 534

Marguerite, ii. 292

Marignac, i. 198, 233, 428, 430, 453, 454,

518, 525, 600, 601 ; ii. 6, 9, 95, 101,

194, 197, 198, 199, 234, 239, 241, 244,

292, 293, 295, 357, 440, 486
Markleffsky, i. 273
Markovnikoff, i. 373
Maroffsky, ii. 138
Marshall, ii. 253, 365
Matigon, i. 258, 266
Mau'mene, i. 258
Maxwell, i. 81
Mayow, i. 17
Mendeleeff, i. 99, 132, 133, 136, 141, 275,

321, 357, 373, 377, 406, 426, 427, 428,

506, 587, 596 ; ii. 27, 33, 93, 94
Menschutkin, i. 171
Mente, ii. 270
Merme, i. 462
Merz, i. 505
Meselan, i. 463
Metzner, ii. 189
Metchikoff, i. 44
Meusnier, i. 114
Meyer (Lothar), i. 226, 321, 403; ii. 21,

24, 26, 29, 33, 486
Meyer (Victor), i. 135, 171, 294, 303, 320,

427, 459, 462, 467, 488, 506, 508, 558;

ii. 43, 48, 52, 80, 129, 184
Meyerhoffer, ii. 410
Miasnikoff, i. 372
Michaelis, ii. 175
Michel, i. 65, 88
Millon, i. 481, 484, 508
Mills, ii. 20

506

PRINCIPLES OF CHEMISTRY

MIT

Mitchell, i. 156

Mitscherlich, i. 428, 527 ; ii. 1, 5, 8, 156
184, 311, 313

Moissan, i. 202, 349, 353, 490, 564, 585,
621 ; ii. 66, 67, 70, 88, 100, 107, 147,
174, 196, 289, 295, 309, 311, 313, 321

Mond, i. 129, 400, 405; ii. 345, 367-

Monge, i. 114

Monnier, i. 611

Montemartini, i. 279

Moraht, ii. 384

Moreau, ii. 298

Morel, i. 549

Mosander, ii. 97

Miihlhauser, ii. 66, 107

Muir, ii. 193

Mulder, i. 515

Miiller-Erzbach, i. 103

Muller, i. 427 ; ii. 425

Munster, ii. 443

Muntz, i. 238, 241, 420, 553

Muthmann, ii. 273

MyOius, ii. 375, 389

NASCHOLD, i. 483

Nasini, i. 496 ; ii. 156

Natanson, i. 282, 409

Natterer, i. 132, 135, 141, 385

Naumann, i. 399, 408

Nernst, i. 62, 148 ; ii. 3, 50

Nensky, i. 245

Neville, i. 537 ; ii. 128, 448

Newlands, ii. 21, 26

Newth, i. 505

Newton, i. 7, 29

Nickles, ii. 10

Nikolukin, i. 491 ; ii. 144

Nilson, i. 618 ; ii. 26, 37, 80, 83, 0L 94

95,271,378,483
Nordenskiold, i. 241
Norton, i. 76; ii. 94
Nuricsan, ii. 264

ODLINO, ii. 52

Offer, i. 99

Ogier, i. 321, 509 ; ii. 159, 182

Olszewski, i. 139, 569 ; ii. 491, 497

Oppenheim, i. 506

Ordway, ii. 80

Osmond, ii. 326

Ossovetsky, ii. 137

Ostwald, i. 89, 92, 389, 441, 443

Oumoff, i. 62

PALLARD, i. 491 ; ii. 83

Panfeloff, i. 603

Paracelsus, i. 17, 125, 129, 379

<JU1

Parkinson, i. 596,
Pashkoffsky, i. 595
Pasteur, i. 44, 241, 242
Paterno, i. 496 ; ii. 156
Pattison Muir, i. 436
Pebal, i. 315, 484
Pechard, ii. 282, 294, 296, 297
Pekatoros, i. 465
Peligot, ii. 299, 301
Pelopidas, ii. 22, 481
Pelouze, i. 463, 464, 480, 610 ; ii. 22§
Penfield, i. 545 ; ii. 370
Perkin, i. 558 ; ii. 244
Perman, i. 537
Personne, i. 75, 506, 537
Petit, i. 584, 586
Petrieff, i. 440
Pettenkofer, ii. 22
Pettersson, i. 618, 619 ; ii. 37, 80, 83, 91,

Pfaundler, i. 445 ; ii. 241, 430

Pfeiffer, i. 64

Pfordten, V. der, ii. 420

Phipson, i. 596 ; ii. 59

Piccini, ii. 23, 146, 197, 2S8, 298

Pici, ii. 57

Pickering, i. 88, 91, 99, 104, 106,- 272,

333, 452, 517, 525, 529, 613 ; ii. 241,

245, 246, 247

Pictet, i. 81, 129, 137 ; ii. 31, 241
Picton, ii. 223

Pierre, i. 452, 495 ; ii. 226, 485
Pierson, i. 93
Pigeon, ii. 377
Pionchon, i. 585
Pistor, i. 399
Plantamour, ii. 5
Plaset, ii. 289
Plessy, ii. 257
Pliicker, i. 572
Poggiale, i. 427
Poiseuille, i. 355
Poleck, ii. 296
Poluta, ii. 30
Popp, ii. 232
Potilitzin, i. 96, 97, 98, 445, 486, 499.

502, 509, 612 ; ii. 29, 357
Pott, ii. 100
Poulenc, ii. 174, 289
Prange, ii. 422
Prelinger, ii. 310

Priestley, i. 17, 154, 159, 297, 379, 402
Pringsheim, i. 465
Prost, i. 98, 486
Prout, i. 31 ; ii. 439
Puchot, i. 452
Pullinger, ii. 389

QUINCKE, i. 427, 495

INDEX OF AUTHORITIES

507

RAMMELSBERG, i. 430, 510, 525 ; ii. 26,

161, 485
Ramsay, i. 133, 140, 141, 232, 247, 333,

495, 496, 581 ; ii. 128, 491
Bantsheff, ii. 20

Raoult, i. 91, 274, 330, 331, 332, 429
Rascher, ii. 85
Raschig, i. 263 ; ii. 229
Rathke, i. 399
Ray, i. 17

Rayleigh, i. 226, 232, 491
Rebs, ii. 213, 217
Recoura, i. 332 ; ii. 289
Regnault, i. 40, 53, 54, 90, 93, 131, 133,

297, 443, 495, 584, 587, 588 ; ii. 50,

208, 238
Reich, ii. 91
Reiset, i. 238
Remsen, ii. 335
Retgers, ii. 157, 158, 180
Reychler, ii. 65
Reynolds, i. 581

Richards, i. 526, 585 ; ii. 32, 432
Riche, i. 509 ; ii. 127, 292
Richter, i. 193, 194 ; ii. 91
Ridberg, ii. 21, 24, 486
Riddle, i. 135
Rideal, ii. 297
Roberts-Austen, ii. 486
Robinson, i. 515
Rodger, ii. 213, 263
Rodwell, i. 17
Roebuck, i. 294
Roggs, ii. 119
Rohrer, ii. 343
Roozeboom, i. 106, 452, 453, 464, 496,

506, 511, 599, 613 ; ii. 3, 226, 341,

410
Roscoe, i. 80, 100, 101, 379, 452, 463,

485, 486, 568, 572 ; ii. 26, 194, 196,

197, 297, 303, 485
Rose, i. 436, 437, 518, 525, 608, 612 ;

ii. 8, 230, 235, 248, 281,. 363, 428, 485
Rosenberg, ii. 351
Rossetti, i. 428
Rouart, Le, ii. 86

Rousseau, i. 354 ; ii. 337/366, 378
Roux, ii. 81
Rudberg, ii. 136
Riicker, i. 142
Riidorff, i. 91, 428, 598, 601
Rybalkin, i. 455

SABANEEFF, i. 371
Sabatier, L 284 ; ii. 66
Saint Edme", ii. 335
Saint Gilles, i. 431
Sakurai, i. 331

Salzer, ii. 161

Sarasin, ii. 122

Sarrau, i. 140, 142

Saunders, ii. 189

Scharples, i. 576

Scheele, i. 155, 161, 412, 459, 462, 608 ;

ii. 100, 150, 291
Scheffer, i. 453
Scheibler, ii. 292, 296
Scherer, ii. 8
Schiaparelli, ii. 318
Schidloffsky, i. 238
Schiloff, i. 212
Schlamp, i. 332
Schiff, i. 430, 588 ; ii. 106, 267
Schloesing, i. 238, 239, 240, 553, 610
Schmidt, i. 539
Schneider, i. 89
Schone, i. 208, 209, 211, 394, 617 ; ii. 15,

72, 219, 251, 488
Schonebein, i. 198, 202, 208, 212, 509 ;

ii. 228, 463
Schottlander, ii. 447
Schroder, i. 75
Schroederer, ii. 366
Schrotter, ii. 153, 284
Schiitzenberger, i. 511, 579 ; ii. 102, 107,

228, 367, 389
Schuliachenko, i. 608
Schuller, ii. 180
Schultz, i. 518 ; ii. 273
Schulze, i. 98 ; ii. 215
Schuster, i. 572
Schwicker, ii. 227, 230
Scott, i. 405, 537, 558
Sechenoff, i. 80, 86
Seelheim, ii. 379
Sefstrom, ii. 197
Selivanoff, i. 476, 507, 508
Senderens, i. 284
Serullas, i. 485
Setterberg, i. 576
Seubert, ii. 27, 83, 343, 442
Sewitsch, i. 372
Shaffgotsch, i, 555
Shapleigh, ii. 95
Shenstone, i. 611
Shields, i. 333
Shishkoff, i. 276 ; ii. 56
Silbermann, i. 120, 172 ; ii. 259
Sims, ii. 268
Skraup, ii. 346
Smith, i. 271
Smithson, ii. 100
Snyders, ii. 100
Sokoloff, ii. 85, 122
Solet, i. 5Q9
Sonstadt, ii. 443
Sorby, i. 88

508

PRINCIPLES OF CHEMISTRY

BOB

Soret, i. 66, 202, 203, 427
Spring, i. 38, 98, 434, 486; ii. 45, 50,

133, 223, 258, 288, 314, 423, 427
Stadion, i. 485
Stahl, i. 16
Stas, i. 7, 233, 379, 428, 498, 681 ; ii.

420, 434, 485
Staudenmaier, ii. 168
Stcherbakoff, i. 97, 428, 458, 601
Stohmann, i. 359, 360, 396
Stokes, i. 355
Stortenbeker, i. 511
Stromeyer, ii. 47
Struve, i. 208, 612

TAIT, i. 203

Tammann, i. 91, 148 ; ii. 1703, 247

Tanatar, i. 511

Tchitcherin, ii. 21

Terreil, ii. 313

Than, i. 317

Thenard, i. 207, 229, 460, 464, 534, 539 ;

ii. 251

Thillot, ii. 170
Thilorier, i. 385
Thomsen, i. Ill, 120, 124, 131, 173, 189,

267, 359, 339, 396, 441, 453, 466, 472,

494, 502, 515, 529, 555, 582 ; ii. 9, 32,

50, 55, 105, 165, 208, 224, 264, 368,

370, 438, 442
Thorpe, i. 142, 285, 445, 493 ; ii. 27, 160,

173, 213, 259, 263, 268, 301, 313, 442,.

486

Thoune, i. 294, 295
Tiemman, i. 213
Tilden, i. 516
Timeraseeff, i. 170
Timofeeff, i. 78
Tessi6 du Motay, i. '158
Tissandier, i. 78
Titherley, i. 539
Tivoli, ii. 183
Tomassi, ii. 339
Topsoe, i. 506
Tourbaba, i. 88 ; ii. 247
Trapp, i. 511

Traub£, i. 312, 611 ; ii. 270
Troost, i. 64, 274, 281, 320, 409, 414,

500, 538 ; ii. 80, 83, 102, 147, 156,

254, 379

Tscherbacheff, i. 577
Tutton, i. 543 ; ii. 160, 174, 412

UBTOFF, i. 429 . *
Unverdorben, ii. 280
Urlaub, ii. 301

VALENTINE, i. 17
Van der Heyd, i. 599

Van der Plaats, i. 496 ; ii. 438

Van der Waals, i. 82, 140

Van Deventer, i. 599

Van Helmont, i. 379

Van Marum, i. 198

Van 't Hoff, i. 64, 65, 331, 599 ; ii, I

Vare, ii. 55

Vauquelin, i. 114, 619 ; ii. 7

Veeren, i. 612 ; ii. 45

Veley, i. 279

Verneuille, ii. 225

Vernon, ii. 151

Vezes, ii. 391

Viard, ii. 285

Viguon, ii. 126, 131

Villard, i. 106, 296, 297

Villiers, ii. 259

Violette, i. 342, 345

Violle, i. 301

Vogt, i. 611

Volkovitch, ii. 201

Voskresensky, i. 345

WAAGE, i. 439

Wachter, i. 508

Wagner, i. 357

Wahl, ii. 310

Walden, ii. 57

Walker, ii. 143

Walmer, i. 573

Walter-, ii. 256

Walters, ii. 234

Wanklyn, i. 100, 539

Warburg, i. 589 ; ii. 496

Warder, i. 450

Warren, ii. 102

Watson, i. 527 ; ii. 169

Watts, i. 526

Weber, i. 280, 583 ; ii. 83, 129, 131, 186,

230, 233, 234, 249
Weith, ii 502
Weitz, ii. 57
Welch, ii. 425
Weller, ii. 146

Wells, i. 477, 545 ; ii. 57, 370
Welsbach, ii. 96, 97
Weltzien, i. 204, 595
Wenzel, i. 193

Weruboff (see Wyrubofi), ii. 4
Weselski, i. 507
Weyl, i. 255
Wheeler, i. 545
Wichelhaus, ii. 179
Wiedemann, i. 439, 688
Wilhelmj, ii. 315
Willgerodt, L 508; ii. 29
Williamson, ii. 268
Wilm, ii. 376, 388

INDEX OF AUTHORITIES

509

Winkler, i. 78, 79, 577, 594, 621 ; ii. 25,

30, 66, 97, 102, 124, 125, 147. 234,

246, 355, 483
Wischin, ii. 384
Wislicenus, i. 267, 294
Witt, ii. 3
Wohler, i. 410, 619; ii. 85, 103, 107,

146, 285, 289, 420, 425
Wollaston, i. 8
Wreden, i. 507
Wright, ii. 321
Wroblewski, i. 79, 80, 106, 139, 387 ;

ii. 226

Wulfing, ii. 119
Wiillner, i. 91, 572

Wtirtz, i. 301, 476 ; ii. 171, 173, 213,

267
Wyruboff, ii. 4, 9

, i. 134, 136, 140, 141, 247, 49&

496

ZABOTJDSKY, i. 354
Zaencheffsky, i. 140
Zimmermann, ii. 26, 303, 355, 485
Zinin, i. 276
ZSrensen, i. 284
Zorn, i. 295

SUBJECT INDEX

ACI

ACID, acetic sp. gr. of solutions of, i. 59

— arsenic, ii. 181

— bismuthic, ii. 190

— boric, ii. 64

— carbamic, i. 408

— chamber, i. 294

— chloric, i. 482

•*— chloro-platino-phosphorous, ii. 890

— chlorosulphonic, ii. 268

— chlorous, i. 481

— chromic, i. 208 ; ii. 282

— chromo-sulphuric, ii. 288

— cyanic, i. 409

— cyanuric, i. 409

— dithionic, ii. 256

— ferric, ii. 344

— fluoboric, ii. 69

— graphitic, i. 351

— hydriodic, i. 501, 503, 505, 506

— hydro-boro-fluoric, ii. 69

— hydrobromic, i. 80, 503, 505, 506

— hydrochloric, i. 448, 451, 453

— hydrocyanic, i. 406, 411

— hydro-ferro-cyanic, ii. 348

— hydrofluoric, i. 49

— hydrofluosilic, ii. 106

— hydro-platino-cyanic, ii. 386

— hydrosulphurous, ii. 228

— hydro-rutheno-cyanic, ii. 388

— hypochlorous, i. 479, 481

— hyponitrous, i. 265, 294

— hypophosphoric, ii. 161

— hypophosphorous, ii. 172

— iodic, i. 100, 508

— isethionic, ii. 250

— metantimonic, ii. 188

— metaphosphoric, ii. 162, 169

— metastannic, ii. 131

— molybdic, ii. 292

— nitric, i. 268, 272

— Nordhausen, ii. 233

— orthophosphoric, ii. 162
- — osmic, ii. 384

— pentathionic, ii. 257

ALK

Acid, percarbonic, i. 394

— perchloric, i. 484

— periodic, i. 510

— permanganic, ii. 313

— permolybdic, ii. 297

— pernitric, i. 264

— persulphuric, ii. 251

— pertungstic, ii, 297

— phosphamic, ii. 179

— phosphamolybdic, ii. 293

— phosphorous, ii. 171

— polysilicic, ii. 117

— pyrophosphoric, ii/ 169

— pyrosulphuric, ii. 234

— silenic, ii. 272

— silico-tungstic, ii. 295

— stannic, ii. 130

— sulphonic, ii. 249

— sulphuric, i. 76, 77, 89, 111, 290, 294:

ii. 235, 238, 241

— telluric, ii. 272

— tetrathionic, ii. 257

— thiocarbonic, ii. 263

— thiocyanic, ii. 263

— thionic, ii. 255

— thiosulphuric, ii. 230

— trithionic, ii. 257

— tungstic, ii. 292, 294

— vanadic, ii. 196
Acids, i. 185

— avidity of, i. 389, 442

— basicity of, i. 387

— complex, i. 197 ; ii. 293

— fuming, i. 102

— organic, i. 394, 396, 405
Acetylene, i. 372
Actinium, ii. 59

Affinity, chemical, i. 26, 389
Air, i. 131, 231, 233
Alchemy, i. 14
Alcohol, i. 53, 88
Alkali, metals, i. 558, 577

— waste, ii. 204
Alkalis, i. 186

SUBJECT INDEX

511

ALL

Allotropism, i. 207
Alloys, ii. 128, 537
Alumina, ii. 75
Aluminium, ii. 70, 85

— bromide, ii. 84

— bronze, ii. 88

— carbide, ii. 88

— chloride, ii. 80, 83

— double chlorides, ii. 84

— fluoride, ii. 83

— hydroxide, ii. 75

— iodide, ii. 85

— nitrate, ii. 80

— sulphate, ii. 82
Alums, ii. 5, 82, 343
Alunite, ii. 80
Amalgams, ii. 58
Amides, i. 258, 406
Amidogen, i. 258

— hydrate, i. 258
Amines, i. 416
Ammonia, i. 229, 246

— of crystallisation, i. 257

— heat of solution of, i. 74

— in air, i. 240

— liquefaction of, i. 250

— salts, i. 254

— soda process, i. 524

— solutions of, i. 80, 252
Ammonium, i. 254

— amalgam, i. 255

— bicarbonate, i. 527

— carbamate, i. 407, 408

— carbonate, i, 407

— cobalt salts, ii. 359

— dichromate, ii. 279

— molybdate, ii. 292

— nitrate, i. 273, 274

— nitrite, i. 284

— phosphates, ii. 167

— sulphate, ii. 269

— sulphide, ii. 218

Analogy of elements, i. 573, 578
Anthracite, i. 345
Antimoniuretted hydrogen, ii. 189)
Antimony, ii. 186

— chlorides, ii. 189

— oxides, ii. 187, 188

— sulphides, ii. 221
Aqua Begia, i. 467
Aqueous radicle, i. 213
Argon, i. 226, 232 ; App. III.
Arsenic, ii. 179

— anhydride, ii. 181

— sulphides, ii. 221

— tribromide, ii. 181

— trichloride, ii. 180

— trifluoride, ii. 18.1
Arsenious anhydride, ii, 184

*K

Arsenious oxychloride, ii. 180
Arsenites, ii. 185
Arseniuretted hydrogen, ii. 182
Astrakhanite, i. 59
Atmolysis, i. 156
Atomic theory, i. 216

— volumes, ii. 33

— weights, i. 21

Atoms and molecules, i. 322

BARIUM, i. 614, 617

— chlorate, i. 483

— chloride, i. 615

— hydroxide, i. 616

— metatungstate," ii. 295

— nitrate, i. 615

— oxide, i. 616

— peroxide, i. 157, 160, 209, 617

— sulphate, i. 614, 615
Bauxite, ii. 76
Benzalazine, i. 258
Berthollet's doctrine, i. 433
Beryllium, i. 618

— atomic weight of, i. 325, 618

— chloride, i. 584

— oxide, i. 619
Binary theory, i. 195
Bismuth, ii. 189

— nitrates, ii. 192

— oxides, ii. 190, 191
Blast furnace, ii. 324
Bleaching, i. 469

— powder, i. 162, 477
Boiling point, absolute, i. 130
Borates, ii. 63

Borax, ii. 61

Boric anhydride, ii. 64

Boron, ii. 60, 66

— chloride, ii. 69

— fluoride, ii. 67, 68

— iodide, ii. 70

— nitride, i. 227 ; ii. 67
- oxide, ii. 60

— specific heat of, i. 585

— sulphide, ii. 62
Bromides, ii. 32
Bromine, i. 494
Bronze, ii. 127

Butyl alcohol, solubility of, i. 75

CADMIUM, ii. 47

— iodide, ii. 48

— oxide, ii. 48

— sulphide, ii. 47
Caesium, i.. 576
Calcium, i. 590, 604

— carbonate-,.!. 592, 608, 609, 610

512

PRINCIPLES OF CHEMISTRY

Calcium chloride, i. 237, 612
crystallohydrates of, i. 613

— fluoride, i. 491

— hypochlorite, i. 162

— iodide, i. 604

— peroxide, i. 607

— phosphate, ii. 167

— sulphate, i. 611

— sulphide, ii. 220
Calomel, ii. 54
Carbamide, i. 409
Carbides, i. 349, 853
Carbon, i. 338

— bisulphide, ii. 258

— molecule of, i. 354

— oxysulphide, ii. 264

— tetrachloride, i. 473
Carbonic anhydride, i. 379

assimilation of by plants, i. 393

dissociation of, i. 392, 393, 399

in air, i. 238, 242

liquid/i. 385

solutions of, i. 80, 86

specific heat of, i. 393

Carbonic oxide, i. 396

and nickel, i. 405

Carborundum, ii. 107
Carboxyl, i. 395
Carnallite, i. 421, 544, 560
Catalytic phenomena, f. 211
Caustic potash, i. 550

— soda, i. 529
Cements, ii. 122
Cerite metals, ii. 93
Cerium, ii. 93

Chamber crystals,!. 290; ii. 230

Charcoal, i. 343

Chemical change, rate of, ii, 314

— transformations, i. 3
Chloranhydrides, i. 468 ; ii. 174, 175,

177

Chlorates, i. 482
Chlorides, i. 455, 466;. ii. 31
Chlorine, i. 463

— compounds, heat of formation of, 1. 44

— crystallohydrates of, i. 464
"— oxides, i. 479

— preparation of, i. 460

— solubility of, i. 463
Chloroform, i. 473
Chlorophosphamide, ii. 179
Chloryl compounds, i. 476
Chrome alum, ii. 283
Chromic acid, i. 208

— anhydride, ii. 280

— oxide, ii. 284, 285
Chromium, ii. 276, 289

— chlorides, ii. 285

— fluorides, ii. 280, 289

Chromyl chloride, ii. 281
Chryseone, ii. 108
Clay, ii. 70
Coal, i. 345
Cobalt, ii. 353

— dioxide, ii. 366

— fluoride, ii. 358
Cobaltamine salts, ii. 359
Cobaltic oxide, ii. 362
Cobalto-amine, ii. 359
Cobaltous hydroxide, ii. 358
Cohesion of liquids, i. 52
Coke, i. 345

Collodion cotton, i. 275
Colloids, i. 63 ; ii. 77, 423
Combination, chemical, i. 3
Combining weights, i. 21 ; ii. 439
Combustion, imperfect, i. 341

— heat of, i. 172, 176, 399, 400
Compounds, definite and indefinite, i. 31

— types of, ii. 10
Compressibility of solutions, i. 88
Conductivity,, electro-molecular, i. 389
Contact reactions, i. 163, 290
Copper, ii. 400

— carbonate, ii. 411

— complex salts of, ii. 412

— nitrate, ii. 411

— nitride, ii. 4Q9

— sulphate, ii. 413
Corundum, ii. 75
Critical points, i. 141
Cryohydrates, i. 99

Cryoscopic investigations of solutions

i. 90, 332 •
Crystals, i. 51
Crystalline form, ii. 7
Crystallo-hydrates, i. 102
Crystalloids, i. 63
Cupellation, ii. 417
Cyanides, i. 406
Cyanogen, i. 406, 414

— chloride, ii. 176

DECOMPOSITION, chemical, i.- 4
Deliquescence, i. 104
Delta metal, ii. 414
Desiccator, i. 58
Detonating gas, i. 115, 170, 173
Depression of freezing point of solu-
tions, i. 90, 92, 330
Dialysis, i. 63 ; ii. 114
Diamond, i. 350, 353
Didymium, ii. 93
Diffusion, rate of, i. 63
Dimorphism, i. 610, ii. 178
Disinfectants, i. 245
Disodium orthpphosphate, ii. 166

SUBJECT INDEX

518

Dissociation, i. 36, 282,608
Distillation, dry, i. 4, 247, 342
Dust, atmospheric, i. 241

EFFLORESCENCE, i. 103
Ekacadmium, ii. 59
Ekasilicon, ii. 25
Electro-chemical theory, i. 195
Electric energy and thermal units, i.

582

Electrolysis, i. 116
Elements, i. 20

— grouping of, ii. 1

— typical, ii. 19
Emulsions, i. 98
Energy, chemical, i. 29
Equations, chemical, i. 278
Equivalents, law of, i. 194
Equivalent weights, i. 581
Ethane, i. 366

Ether, critical points of, i. 141

Ethylene, i. 370

Ethyl silicates, i. 104

Euchlorine, i. 484

Eudiometer, i. 169

Expansion, linear, of elements, ii. 31

Explosion, rate* of transmission of, i.

171
Explosives, i. 275, 276

FELSPAR, ii. 122

Fermentation, i. 242

Ferric chloride, i. 558 ; ii. 340

— hydrates, ii. 339

— nitrate, ii. 340

— orthophosphate, ii. 342

— oxider ii. 339
Ferrous chloride, ii. 335

— sulphate, ii. 335
solubility of, i. 72

— sulphide, ii. 210
Flame, i. 177, 179
Fluoborates, ii. 69
Fluorides, i. 491, 493
Fluorine, i. 203, 489
Fluorspar, i. 491

Formula, chemical, i. 151, 326
Freezing mixtures, i. 76
Fuel, calorific capacity of, i. 360
Furnace, electrical, i. 352
Fusco-cobaltic salts, ii. 360

GADOLINITE METALS, ii. 93
Gallium, ii. 88, 90
Gas, illuminating, i. 361
— producers, i. 397

Gases, absorption of, i. 348-

— diffusion of, i. 83

— expansion of, i. 133

— liquefaction of, i.. 134, 135, 137

— measurement of, i. 78, 300

— solution of, i. 68, 78, 86

— theory of, i. 81, 83, 140
Germanium, ii. 26, 124

— chloride, ii. 125

— oxide, ii. 125
Glass, i. 123

— soluble, ii. 110
Glauber's salt, i. 517
Glycols, ii. 117
Gold, ii. 442

— alloys, i. 446, 447

— chlorides, ii. 448, 450

— colloid, ii. 447 •

— cyanide, ii. 450

— extraction of, ii. 444, 445

— fulminating, ii. 450

— oxides, ii. 448

— refining, ii. 446
Graduators, i. 424
Graphite, i. 350, 351
Grod' salt, ii. 393
Guignet's green, ii. 285
Gunpowder, i. 557
Gypsum, i. 593, 611

HALOOENS, i. 445, 487, 499
Halogen compounds, heat of formation
of, i. 494, 502 ; ii. 32

boiling-points of, i. 502

Hausrnannite, ii. 10

Helium, i. 570 ; ii. 498

Hemimorphism, ii. 9

Homeomorphism, ii. 8

Homologous compounds, i. 358

Humus, i. 344

Hydrates, i. 109, 185

Hydrazine, i. 258

Hydrides, i. 621 ; ii. 23

Hydrocarbons, i. 355, 359

Hydrogen, i. 123, 129, 130, 142, 143, 146

— pentasulphide, ii. 217

— peroxide, i. 207, 312
Hydrosols, i. 98
Hydroxyl, i. 192, 213
Hydroxylamine. i. 262
Hypochlorit.es, i. 481
Hyponitrites, i. 294

IMIDES, i. 258

Indium, ii. 27, 37, 88, 97

lodates, i. 509

Iodides, ii. .32

— _of nitrogen, i. 507

514

PRINCIPLES OF CHEMISTRY

MOL

Iodine, i. 320, 321, 496, 497, 498

— chlorides of, i. 511
lodosobenzol, i. 508
Iridious oxide, ii. 382
Iridium, ii. 382
Iron, ii. 317, 322, 585

— and carbonic oxide, ii. 345

— cast, ii. 325

• — nitride, ii. 346

— ores, 319

— sulphate, ii. 335
Isethionic acid, ii. '250
Isomorphism, i. 203, 368 ; ii. 1, 4, 8

KAOLIN, ii. 70

LAKES, ii. 77

Lanthanum, ii. 93

Laughing gas, ii. 297

Law of Avogadro-Gerhardt, i. 309

— .— Berthollet, i. 445

— - — Boyle and Mariotte, i. 132

-^ — combining weights, i. 221

• Dulong and Petit, i. 584

— *- equivalents, i. 1 94
even numbers, i. 357

— — Gay Lussac, i. 133, 304, 307

Guldberg and Waage, i. 441

Henry and Dalton, i. 78

— r— indestructibility of matter, i. 6
Kirchoff , i.' 568

limits, i. 357

• maximum work, i, 120

multiple proportions, i. 109, 214

— partial pressures, i. 82

periodic, ii. 17

phases, ii. 410

reversed spectra, i. 568

— — specific heats, i. 584

substitution, i. 260, 365

• volumes, i. 304

Lead, ii. 134

— acetate, ii. 137

— carbonate, ii. 140

— chloride, ii. 139

— chromate, ii. 136, 279
i — dioxide, ii. 142

— nitrate, ii. 139

— oxide, ii. 137

— red, ii. 142

— salts of, i. 491

— tetrachloride, ii. 144

— tetrafluoride, ii. 144

— white, ii. 140
Leucone, ii. 107
Levigation, ii. 72

Light, chemical action of, i. 465

ints of, i. 135

Lime, i. 605

Lie

Lithium, i. 574

— carbonate, i. 575

Litharge, ii. 137

Litmus, i. 185

Lixiviation, methodical, i. 521

Luteo-cobaltic salts, ii. 359

MAGNUS' salt, ii. 392
Magnesia, i. 597
Magnesium, i. 590, 594

— carbonate, i. 592, 602

— chloride, i. 602

— crystallohydrates of, i. 601

— double salts of, i. 597

— nitride, i. 595

— silicide, ii. 102

— sulphate, i. 600
Manganese, ii. 303

— nitrides, ii. 310

— oxides, ii. 306, 307, 308, 313

— peroxide, i. 159 ; ii. 305

— sulphate, ii. 307

Mass, influence of, i. 32, 436
Matches, ii. 154
Matter, primary, ii. 440

— transmutability of, i. 14
Mercury, ii. 48

— ammonia compounds, ii. 57

— basic salts of, ii. 54

— chlorides, ii. 52, 53, 54

— compounds, heat of formation, ii. 50

— cyanide, ii. 55

— fulminating, ii. 56

— iodide, ii. 55

— nitrates, ii. 51

— nitrides, ii. 56

— oxides, ii. 53

— sulphate, ii. 57

— sulphides, ii. 221
Metalepsis, i. 28, 471
Metalloids, i. 23
Metals, i. 23

of alkaline earths, i. 64, 590, 591

of alkalis, i. 543

displacement of, ii. 427
Methane, i. 360

Moisture, determination of, in gases, i.
40

influence upon reaction, i. 403
Molecular volumes, ii. 37

— weight and boiling point, i. 331

coefficient of refraction, i. 336

latent heat, i. 329

— specific gravity of solutions, i.

335
surface tension, i. 334

SUBJECT INDEX

515

Molecules, i. 319, 322
Molybdates, ii. 292
Molybdenum, ii. 290

— anhydride, ii. 291

— fluo-compounds, ii. 298

— sulphides, ii. 297
Monophosphamide, ii. 178
Monosodium orthophosphate, iL 167
Morphotropy, ii. 10

NAPHTHA, i. 373, 377
Nascent state, i. 33, 145, 146
Neodymiurn, ii. 97
Nickel, ii. 353

— alloys, ii. 367

— and carbonic oxide, ii. 867
« — fluoride, ii. 358

• — hydroxide, ii. 358
• — oxide, ii. 365

— sulphate, i. 97 ; ii. 359

— tetra-carboxyl, ii. 367
Niobium, ii. 194, 198, 199
Nitrates, i. 273

Nitres, i. 268, 555
Nitric anhydride, i. 280

— oxide, i. 286
Nitrides, i. 227, 258, 620
Nitriles, i. 406
Nitrites, i. 284
Nitro-cellulose, i. 275
Nitro-compounds, i. 274
Nitrogen, i. 223, 225, 475

— chloride, i. 476

— iodide, i. 507

— oxides of, i. 267, 280, 284, 294, 295
• — sulphide, ii. 270
Nitro-prussides, ii. 351
Nitroso-compounds, i. 288
Nitrosulphates, ii. 229

Nitrosyl chloride, ii. 176
Norwegium, ii. 59

OCCLUSION, i. 143
defiant gas, i. 370
Organo-metallic compounds, i. 356
.Osmium, ii. 372, 382, 384
.Osmotic pressure, i. 64
Osmuridium, ii. 383
,Oxamide, i. 406
Oxidation, i. 16
Oxides, i. 183 ; ii. 36
Oxycobaltamine salts, ii. 359
Oxygen, i. 152, 157, 158, 163

— compounds, heat of formation of, i.

120, 466
'Ozone, i. 198, 229

PALLADIUM, ii. 369

— hydride, i. 143 ; ii. 380

Palladous chloride, ii. 379

— iodide, ii. 379
Paracyanogen, i. 414
Paramorphism, ii. 9
Parasulphatammon, ii. 269
Peat, i. 344

Peligot's salt, ii. 281
Percentage composition, i. 326
Perchloric anhydride, ii. 282
Periodates, i. 510
Permanganic anhydride, ii. 313
Permolybdates, ii. 297
Peroxide, chloric, i. 484
Peroxides, i. 159 ; ii. 15/23
Perstannic oxide, ii. 133
Persulphates, ii. 253
Petroleum, i. 373
Phenol, solubility of, i. 75
Phlogiston, i. 17
Phosgene gas, ii. 175
Phospham, ii. 178
Phosphides, ii. 157
Phosphine, ii. 158, 160
Phosphonium iodide, ii. 159
Phosphoric anhydride, ii. 161
Phosphorous anhydride, ii. 160
Phosphorus, ii. 149

— ammonium compounds, ii. 178

— chlorides, ii. 174

— fluorides, ii. 173

— iodides, i. 505, 506 ; ii. 172

— oxychlorides, ii. 175

— sulphides, ii. 213

— sulpho-chloride, ii. 213

— thermo-chemical data for, ii. 153
Phosphuretted hydrogen, ii. 158, 160
Photography, ii. 431
Photo-salts, ii. 432

Plants, chemical reactions in, i. 547

— and nitrogen, i. 230
Platinic chloride, ii. 377

— hydroxide, ii. 379
Platino-ammonium compounds, ii. 391

— chlorides, i. 467 ; ii. 378

— cyanides, ii. 386

— nitrites, ii. 390

— sulphites, ii. 390
Platinous chloride, ii. 379
Platinum, ii. 376

— alloys, ii. 373

— black, ii. 376

— metals, ii. 369, 375

— oxide, ii. 378
Poly-haloid salts, i. 545
Polymerism, i. 207, 367
Polysulphides, ii. 217
Potassium, i. 544, 558

— aurate, ii. 449

— bromide, i. '550

516

PRINCIPLES OF CHEMISTRY

POT

Potassium carbonate, i. 549

— chlorate, i. 161, 482

— chloride, i. 72, 543

— chromate, ii. 280

— cyanide, i. 412, 551

— dichromate, ii. 278

— ferricyanide, ii. 346

— ferrocyanide, i. 346, 412

— hydrosulphide, ii. 219

— hydroxide, i. 548

— iodide, i. 550

— manto'anate, ii. 310

— nitrate, i. 553

— oxides, i. 559

— permanganate, ii. 311

— stannate, ii. 133

— sulphate, i. 72, 549

— sulphide, ii. 219

— telluride, ii. 274
Praseocobaltic salts, ii. 361
Praseodidymium, ii. 97
Proteid substances, i. 224
Prout's hypothesis, ii. 439
Prussian blue, i. 419 ? ii. 349
Purpureo-cobaltic salts, ii. 361
Purpureo-tetramine salts, ii. 361
Pyrocollodion, i. 275
Pyronaphtha, i. 375
Pyrosulphuryl chloride, i. 321 ; ii. 235

REACTIONS, chemical, i. 3

— — conditions for, i. 34

contact, i. 39

endothermal, i. 30

— — exothermal, i. 30
limit of, i. 437

— — rate of, ii. 152
Eecalescence, ii. 333
Reduction, i. 16
Refraction equivalent, i. 336
Regenerative furnaces, i. 399
Reiset's salts, ii. 394
Respiration, i. 152, 154, 387
Rhodium, ii. 381

Rock salt, i. 421
Roseocobaltic salts, ii. 360
Rosetetramine salts, ii. 361
Rubidium, i. 576
Ruthenium, ii. 372, 382, 384

SALAMMONIAC, i. 248, 318, 457

— solubility of, i. 458

— vapour density of, i. 317
Salts, i. 187, 419

— acid, i. 193, 533

— basic, i. 193, 533 ; ii. 54

— double, i. 598

Salts, electrolysis of, i. 191

— heat of formation, i. 189

— melting points of, i. 135

— pyro, i. 193

— theory of, i. 193
Saponification, i. 530
Scandium, ii. 94
Selenium, ii. 273

— chlorides, ii. 275
Selenious anhydride, ii. 271
Silica, ii. 100 ; ii. 108

— soluble, ii. 113
Silicates, i. 544 ; ii. 116
Silicon, ii. 99

— chloride, ii. 103, 104

— chloroform, ii. 103

— bromide, ii. 104

— fluoride, ii. 105

— hydride, ii. 102, 103

— iodide, ii. 105

— iodoform, ii. 105
Silver, ii. 418

— allotropic varieties of, ii. 421

— bromide, ii: 429
— - chlorate, ii. 437

— chloride, ii. 429

— cyanide, ii. 433

— fluoride, ii. 430

— fulminating, ii. 426

— hyponitrite, i. 294

— iodide, ii. 429

— nitrate, ii. 426

— nitrite, i. 284

— orthophosphate, ii. 164

— oxides, ii. 424

— peroxide, ii. 422

— plating, ii. 434

— soluble, ii. 420

— subchloride, ii. 432
Slags, ii. 323
Smalt, ii. 354
Soaps, i. 531

Soda ash, i. 519

— caustic, i. 527

— manufacture of, i. 459

— waste, i. 522
Sodamide, i. 539
Soda lime, i. 237
Sodium, L 513, 533

— alloys, i. 559

— amalgams, i. 537

— bicarbonate, i. 526

— carbonate, i. 519, 525

• crystallohydrates of, i. 108

manufacture of, i. 523

— • — solutions of, i. 525

— chloride, i. 419

double salts of, i. 430

solutions of, i. 88, 99, 429

SUBJECT .INDEX

517

SOD

Sodium hydride, i. 537

— hydroxide, i. 528, 529

— — solutions of, i. 529

— nitrate, i. 269
solutions of, i. 72

— organo compounds of, i. 540

— oxides, i. 540, 541

— phosphates, ii. 166

— platinate, ii. 378

— pyrosulphate, i. 518

— sesquicarbonate, i. 526

— stannate, ii. 133

— subchloride, i. 540

— sulphate, i. 513

. acid salt, i. 518

— — crystallohydrates of, i. 515

solutions of, i. 73, 515, 516

- - sulphite, ii. 226

— thiosulphate, ii. 230

solutions of, i. 74

*— tungstate, ii. 294
Soils, i. 344 ; ii. 73
Solubility coefficient of, i. 67, 71
Solutions, i. 330

• — aqueous, i. 59

— boiling points of, i. 94, 100
• — crystallisation of, i. 427

— colour of, i. 95

— diffusion of, i. 61, 429

— of double salts, i. 599

— formation of ice from, i. 91, 429

— heat of formation of, i. 74, 75, 76

— of gases, i. 68
• — iSotonic, i. 64

— saturated, i. 65

— specific gravity of, i. 429, 584

— supersaturated, i. 96

— theory of, i. 64, 89, 92, 97,106, 215,
323, 608 ; ii. 3, 164

— vapour tension of, i. 90, 92

— volumes of, i. 87

— Specific heat, i. 585, 586, 688
Spectra absorption, i. 566
Spectrum analysis, i. 560, 561
Stannic chloride, ii. 132

— fluoride, ii. 132

— oxide, ii. 130

— sulphide, ii. 132
Stannous chloride, ii. 130

— oxide, ii. 129

— salts, ii. 129

Steam, vapour tension of, i. 54
Steel, ii. 327, 328, 330
Strontium, i. 615

— chloride, i. 615

— hydroxide, i. 615
— • nitrate, i. 615

— oxide, i. 617
Substitution chemical, i. 6

URA

Sulphamide, ii. 270
Sulphatammon, ii. 269
Sulphates, ii. 248
Sulphides, i. 98 ; ii. 213
Sulphonitrites, ii. 229
Sulphoxyl, ii. 250
Sulphur, ii. 200

— chlorides of, ii. 264
Sulphuretted hydrogen, ii. 208
Sulphuric anhydride, ii. 232

— peroxide, ii. 251
Sulphurous anhydride, ii. 224
Sulphuryl chloride, ii. 268
Superphosphates, ii. 168

TANTALUM, ii: 194, 198
Tellurium, ii. 274

— bromide, ii. 275

— chlorides, ii. 275
Tellurious anhydride, ii. 271
Temperature, critical; i. 131
Test papers, i. 185
Thallium, ii. 88, 91
Thallic oxide, ii. 93
Thallous hydroxide, ii. 92

— oxide, ii. 92
Thiocarbonates, ii. 262
Thionyl chloride, ii. 267
Thiophosgene, ii. 262
Thiophosphoryl fluoride, ii. 263
Theory, atomic, i. 216

— • unitary, i. 195

— vortex, i. 217
Thermochemistry, i. 173
Thorium, ii. 148

Tin, ii. 125

— alloys, ii. 127
Titanium, ii. 144

— chloride, ii. 145

— nitride, ii. 146

— nitrocyanide, ii. 146

— oxides, ii. 145
Tripoli, ii. 110

Trisodium orthophosphate, ii. 166
Tungstates, ii. 292
Tungsten, ii. 290

— anhydride, ii. 291

— nitride, ii. 297

— sulphide, ii. 297
TurnbuU's blue, ii. 350
Types of combination, ii. 10

ULTRAMARINE, ii. 84
Uranium, ii. 30, 297

— atomic weight of, ii. 26

— dioxide, ii.-301

— oxides, ii. 298

518

PRINCIPLES OF CHEMISTRY

Uranium tetrachloride, ii. 301
Urano-alkali compounds, ii. 298
Uranyl, ii. 301

— ammonium carbonate, ii. 300

— nitrate, ii. 300

— phosphate, ii. 300
Urea, i. 409

VALENCY of elements, i. 404, 418, 581
Van der Waal's formula, i. 82, 140
Vanadic anhydride, ii. 196
Vanadium, ii. 194

— oxychloride, ii. 195

Vapour density, determination of, i. 301
Ventilation, i. 244
Viscosity, i. 355
Volumes, molecular, ii. 4

— gases, i. 300

WATER, i. 40

— composition of, i. 114, 118, 148, 169,
305, 333

— compressibility of, i. 53

— of constitution, i. 109

— of crystallisation, i. 95, 510

— dissociation of, i. 118

— expansion of, i. 53

Water gas, i. 129, 400, 401

— hard, i. 47

— hygroscopic, i. 56

— mineral, i. 45

— rain, i. 43

— river, i. 43

— sea, i. 46

— specific heat of, i. 52
gravity of, i. 50

— spring, i. 44
Wave lengths, i. 564
Wood, i. 339

YTTERBIUM, ii. 93-
Yttrium, ii. 93

ZINC, ii. 39

— ammonia-chlorides, ii. '41

— chloride, ii. 40, 41

— compounds, heat of formation of, ii.
51

— oxide, ii. 39, 40

— sulphate, ii. '39
Zirconium, ii. 146

— chloride, ii. 147

— hydroxide, ii. 147

— oxide, ii. 147

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