Experiments on air

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By HENRY CAVENDISH, Efq. F.R.S and AS.

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Read at tbe ROYAL SOCIETY, June 2, 178;.

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LONDON:

Printed by J. N I C H O L S*
MDCCLXXXV.

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EXPERIMENTS, &c.

I N a Paper, printed in the laft volume of the Philofophical
Tranfa&ions, in which I gave my reafons for thinking that
the diminution produced in atmofpheric air by phlogiftication
is not owing to the generation of fixed air, l faid it feemed moft
likely, that the phlogiftication of air by the ele&ric fpark was
vowing to the burning of fome inflammable matter in the appa-
ratus ; and that the fixed air, fuppofed to be produced in that
procefs, was only feparated from that inflammable matter by
the burning. At that time, having made no experiments on
the fubjeft myfelf, I was obliged to form my opinion from
thofe already publifhed ; but I now find, that though I was

right in fuppofing the phlogiftication of the air does not pro-
ceed from phlogifton communicated to it by the electric fpark,
and that no part of the air is converted into fixed air ; yet that
the real caufe of the diminution is very different from what I
fufpefted, and depends upon the conversion of phlogifticated
air into nitrous acid.

The apparatus ufed in making the experiments was as follows.
The air through which the fpark was intended to be pafled, was
confined in a glafs tube M, bent to an angle, as in fig. i. (tab.
XV.) which, after being filled with quickfilver, was inverted
into two glafles of the fame fluid, as in the figure. The air to

A 2 be

4 Mr. Cavendish's ‘Experiments on Air,

be tried was then introduced by means of a fmall tube, fuch as-
is u fed for thermometers, bent in the manner reprefented by
ABC (fig. 2.) the bent end of which, after being previoufly
filled with quickfilver, was introduced, as in the figure, under
the gTafs DBF, inverted into water, and filled with the proper
kind of air, the end C of the tube being kept hopped by the
finger; then, on removing the finger from C, the quickfilver
in the tube defcended in the leg BC, and its place was fupplied
with air from the glafs DEF.. Having thus got the proper
quantity of air into the tube ABC, it was held with the end G
uppermoft, and hopped with the finger ; and the end A, made
l'maller for that purpofe, being introduced into one end of the
bent tufiq M, (fig. i.) the air, on removing the finger from C,
was forced into that tube by the prehure of the quickfilver in
the leg BC. By thefe means I was enabled to introduce the
exa& quantity I pleated of any kind of air into the tube M>
and, by the fame means, I could let up any quantity of foap-
lees, or any other liquor which I wanted to be in contact with
the air..

In one cafe, however, in which I wanted to introduce air
into the tube many times in the fame experiment, I ufed the
apparatus reprefented in. fig. 3. confihing of a tube AB of a
fmall bore, a ball C, and a tube DE of a larger bore. This
apparatus was firft filled with quickfilver; and then the ball
C, and the tube AB, were filled with air, by introducing the
end A under a glafs inverted into water, which contained tlie
proper kind of air, and drawing out the quickfilver from the
leg ED by a fyphon. After being thus furnifhed with air, the
apparatus was weighed, and the end A introduced into one end
of the tube M, and kept there during the experiment r the
way of forcing air out of this apparatus into the tube being by

thrufting

Mr. Cavendish’s Experiments on Air. 5

tfirufting down the tube ED a wooden cylinder of fuch a fize
as almoft to fill up the whole bore, and by occafionally pouring
quickfilver into the fame tube, to fupply the place of that
pufhed into the ball C. After the experiment was finifhed, the
apparatus was weighed again, which Ihewed exaCtly how much
air had been forced into the tube M during the whole experi-
ment ; it being equal in bulk to a quantity of quickfilver, whofe
weight was equal to the increafe of weight of the apparatus.

The bore of the tube M ufed in moft of the following ex-
periments, was about one-tenth of an inch ; and the length of
the column of air, occupying the upper part of the tube, was
in general from if to f of an inch.

It is fcarcely neceflhry to inform any-, one ufed to eleCfcrical
experiments, that in order to force an electrical fpark through
the tube, it was neceflary, not to make a communication be-
tween the tube and the conductor, but to place an infulated ball
at fuch a diftance from the conductor as to receive a fpark from
it, and to make a communication between that ball and the
quickfilver in one of the glaffes, while the quickfilver in the
other glafs communicated with the ground.

' • ‘ • . . i »

I now proceed to the experiments.

When the eleCtric fpark was made to pafs through common
air, included between fihort columns of a folution of litmus,
the folution acquired a red colour, and the air was diminilhed,
conformably, to what wasobferved by Dr. Priestley.

When lime-water was ufed in (lead of the folution of litmus,
and the fpark was continued till the air could be no further di-
minifhed, not the leaf! cloud could be perceived in the lime-
water; but the air was reduced to two- thirds of its original
bulk ; which is a greater diminution than it could have futfered
by mere phlogiftication, as that is very little more than one-
fifth of the whole. T he j

6 Mr. Cavendish’s Experiments' on Air.

The experiment was next repeated with fome impure de*
phlogifticated air. The air was very much diminifhed, but
without the lead: cloud being produced in the lime-water. Nei-
ther was any cloud produced when fixed air was let up to it ;
but. on the further addition of a little cauftic volatile alkali, a
brown fediment was immediately perceived.

Hence we may conclude, that the lime-water was faturated
by fome acid formed during the operation ; as in this cafe it is
evident, that no earth could be precipitated by the fixed air
alone, but that cauftic volatile alkali, on being added, would
abforb the fixed air, and thus becoming mild, wrould imme-
diately precipitate the earth ; whereas, if the earth in the lime-
water had not been fatu rated with an acid, it would have been
precipitated by the fixed air. As to the brown colour of the
fediment, it moft likely proceeded from fome of the quickfilver
having been diflolved.

It muft be obferved, that if any fixed air, as well as acid, had
been generated in thefe two experiments with the lime-water,
a cloud muft have been at firft perceived in it, though that
cloud would afterwards difappear by the earth being re-diffolved
by the acid;; for till the acid produced was fufficient to ditfolve
-the whole of the earth, fome of the remainder would be pre-
cipitated by the fixed air ; fo that we may fafely conclude, that
no fixed air was generated in the operation.

When the air is confined by foap-lees, the diminution pro-
ceeds rather fafter than when it is confined by lime-water ; for
which treafon, as well as on account of their containing fo
much more alkaline matter in proportion to their bulk, foap-
lees feemed better adapted for experiments defigned to invefti-
gate the nature of this acid, than lime-water. I accordingly
made fome experiments to determine what degree of purity the

air

Mr. Cavendish’s Experiments on Air. 7

air fliould be pf, m order to be diminifhed moft readily, and to
the greateft degree; and I found, that, when good dephlogifti-
cated air was ufed, the diminution was but fmall ; when per-
fectly phlogifticated air was ufed, no fenfible diminution took
place ; but when five parts of pure dephlogifticated air were
mixed with three parts of common, air, almoft the whole of
the air was made to difappear.

It muft be confidered, that common air confifts of one part
of dephlogifticated air, mixed with four of phlogifticated ; fo
that a mixture of five parts of pure dephlogifticated air, and
three of common air, is the fame thing as a mixture of feven
parts of dephlogifticated air with three of phlogifticated.

Having made thefe previous trials, I introduced into the tube
a little foap-lees, and then let up fome dephlogifticated and
common air, mixed in the above-mentioned proportions, which •
rifing to the top of the tube M, divided the foap-lees into its -
two legs. As faft as the air was diminifhed by the eleCtric:
fpark, I continued adding more of the fame kind,, till no fur--
ther diminution took place : after which a little pure dephlo-
gifticated air, and after that a little common air, were added,,
in order to fee whether the ceffation of diminution was notr
owing to fomc iaipci fcCliou in the proportion of the two kinds'
of air to each other ; but without effect *. The foap-lees being
then poured out of the tube, and feparated from the quick* -

* From what follows it appears, that the reafon why the airceafed to diminish ?
was, that as the foap-lees were then become neutralized, no alkali remained to ab-
forb the acid formed by the operation, and in confequence fcarce any air was turned '
into acid. The fpark, however, was not continued long enough after the appa- -
rent ceffation of diminution, to determine with certainty, whether it was only
that the diminution went on remarkably flower than before, or that it was almoft 1
come to a ftand, and could not have been carried much further, though I had i
perfifted in paffing the fparks.

filver, .

8 Mr. Cavendish's Experiments on Air.

filver, Teemed to be perfectly neutralized, as they did not at aH
difcolour paper tinged with the juice of blue flowers. Being
evaporated to drynefs, they left a final! quantity of .fait, which
was evidently nitre, as appeared by the manner in which paper,
impregnated with a folution of it, burned.

For more fatisfa&ion, I tried this experiment over again on a
larger fcale. About five times the formenquantity of foap-lees
were now let up into a tube of a larger bore ; and .a mixture of
dephlogifticated and common air, in the fame proportions as
before, being introduced by the apparatus reprefented in fig. 3.
the fpark was continued till no more air could be made to difap-
pear. The liquor, when poured out of the tube, fmelled evidently
of phlogifticated nitrous acid, and being evaporated to drynefs,
yielded 1 gr. of fait, which is pretty exadlly equal in weight to
the nitre which that quantity of foap-lees would have afforded if
faturated with nitrous acid. Thisfalt was found, by the man-
ner in which paper dipped into a -folution of it burned, to be
true nitre. It appeared, by the teft-of terra ponderofa falita , to
contain not more vitriolic acid than the foap-lees themfelves
contained, which was exceftively little; and there as no reafon
to think that any other acid entered into it, except the nitrous.

A circumstance, however, occurred, which at firft: feemed
to (hew, that this ’fait contained fome marine acid; namely,
an evident precipitation took place when a folution of filver
was added to fome of it diflolved in water ; though the foap-
lees ufed in its formation were perfectly free from marine acid,
and though, to prevent all danger of any precipitate being
formed by an excefs of alkali in it, fome purified nitrous acid
had been added to it, previous to the addition of the folution
of filver. On confideration, however, 1 (ufpedted, that this
precipitation might arife from the nitrous acid in it being phlo-
gifticated ;

Mr. Cavendish's Experiments on Air . 9

gifticated ; and therefore I tried whether nitre, much phlogifti-
cated, would precipitate filver from its lolution. For this pui>
pole I expol'ed fome nitre to the fire, in an earthen retort, till it
had yielded a good deal of dephlogifticafed air ; and then, hav-
ing difl'olved it in water, and added to it fome well purified
i'pirit of nitre till it was fenfibly acid, in order to be : certain
that the alkali did not predominate, I dropped into it fome
lolution of lilver, which immediately made a very copipus pre-
cipitate. This lolution, however, being deprived of fome of
its phlogifton by evaporation to drynefs, and expofure for a few
weeks to the air, loft the property of precipitating filver from
its lolution ; a proof that this property depended only on its
phlogiftication, and not on its having abforbed fea-falt from
the retort, or by any other means.

Hence it is certain, that nitfe, when much phlogifticated, is
capable of making a precipitate with a folution of filver; and
therefore there is no reafon to think, that the precipitate,
which our fait occafioned with a folution of lilver, proceeded
from any other caufe than that of its being phlogifticated ;
efpecially as it appeared by the fmell, both on firft taking it
out of the tube, and on the addition of the fpirit of nitre,
previous to dropping in the folution of filver, that the acid in
it was much phlogifticated. This property of phlogifticated
nitre is worth the attention of chemifts ; as otherwife they may
fometimes be led into miftakes, in inveftigating the prefence of
marine acid by a folution of filver.

In the above-mentioned Paper I faid, that^-when nitre is deto-
nated with charcoal, the acid is converted into phlogifticated
air; that is, into a fubftance which, as far as I could perceive,
pofiefles all the properties of the phlogifticated air of our at-
mofphere; from which I concluded, that phlogifticated air is

nothing

j© Mr. Cavendish’s 'Experiments on Air.

nothing elfe than nitrous acid united to phlogifton. According
to this conclufion, phlogifticated air ought to be reduced to
nitrous acid by being deprived of its phlogifton. But as de-
phlogifticated air is only water deprived of phlogifton, it h
plain, that adding dephlogifticated air to a body, is equivalent
to depriving it of phlogifton, and adding water to it; and
therefore, phlogifticated' air ought alfo to be reduced to nitrous
acid, by being made to unite to, or form a chemical combina-

* • , i f 't'*

tion with, dephlogifticated air ; only the acid formed this way
will be more dilute, than if the phlogifticated air was fimply
deprived of phlogifton. ' . . -

This being premifed, we may fafely conclude, that in the
prefent experiments the phlogifticated air was enabled, by
means of the electrical fpark, to unite to, or form a chemical
combination with, the dephlogifticated air, and' was thereby
reduced to nitrous acid, which united to the foap-lees, and1
formed a folution of nitre; for ill thefe experiments thofe two*
airs adually disappeared, and nitrous acid was actually formed
in their room ; and as, moreover, it has juft been {hewn, from-

f . t • • .)•

other circumftances, that phlogifticated air muft form nitrous acid.

when combined with dephlogifticated air, the above-mentioned’
opinion feems to be fufficiently eftablifhed. A further confirma-
tion of it is, that, as far as I can perceive, no diminution of air is-

produced when the eleCtric fpark is pafted either through pure

• • •

dephlogifticated air, or through perfectly phlogifticated air;
which indicates the neceftity of a combination of thefe two

f • • ►

airs to produce the acid. Moreover, it was found in the laft
experiment, that the quantity of nitre procured was the lame
that the foap-lees would have produced if faturated with ni-
trous acid; which Ihews, that the production of the nitre was
not owing to any decompofition of the foap-lees.

It

Mr. Cavendish’s Experiments on Air. 1 1

It may be worth remarking, that whereas in the detonation,
of nitre with inflammable fubftances, the acid unites to phlo-
gifton, and forms phlogifticated air, in tliefc experiments the
reverfe of this procefs was carried on ; namely, the phlogifti-
cated air united to the dephlogifticated air, which is equivalent
to being deprived of its phlogifton, and was reduced to nitrous
acid.

In the above-mentioned Paper I alfo gave my reafons for
thinking, that the frnall quantity of nitrous acid, produced by
the explolion of dephlogifticated and inflammable air, pro-
ceeded from a portion of phlogifticated air mixed with the de-
phlogifticated, which I luppofed was deprived of its phlogifton,
and turned into nitrous acid, by the adtion of the dephlogifti-
cated air on it, aflifted by the heat of the explofton. This
opinion, as muft appear to every one, is confirmed in a re-
markable manner by the foregoing experiments ; as from them
it is evident, that dephlogifticated air is able to deprive phlo-
gifticated air of its phlogifton, and reduce it into acid, when
aflifted by the eledtric fpark ; and therefore it is not extraordi-
nary that it ftiould do fo, when aflifted by the heat of the
explolion.

The foap-lees ufed in the foregoing experiments were made
from fait of tartar, prepared without nitre ; and were of fuch
a ftrength as to yield one-tenth of their weight of nitre when
faturated with nitrous acid. The dephlogifticated air alfo was
prepared without nitre, that ufed in the firft experiment with
the foap-lees being procured from the black powder formed by
the agitation of quickfilver mixed with lead*, and that ufed

* This air was as pure as any that can be procured by moil procefles. I pro*
pofe giving an account of the experiment, in which it was prepared, in a future
Paper.

12 21/*/'. CrtVENDisr-i’s Experiments cn Air .

in the latter from turbith mineral. In the fird experiment, the
quantity of foap-lees ufed was 3.^ meafures, each of which was
equal in bulk to one grain of quicklilver ; and that of the air
ablorbed was 416 fucli meafures of phlogidicated air, and 914.
of dephlogidicated. In the fecond experiment, 178 meafures
of foap-lees were ufed, and they ablorbed 192a of phlogidi-
cated air, and 4860 of dephlogifticated. It mud: be obfervedr
however, that in both experiments feme air remained in the
tube uncondenfed, whofe degree of purity I had no way of
trying ; fo that the proportion of each fpecies of air abforbed is-
not known with much exa&nefs.

As far as the experiments hitherto, publifhed extend, we
fcarcely know more of the nature of the phlogidicated part of
our atmofpherc, than that it is not diminifhed by lime-water,
caudic alkalies, or nitrous air ; that it is unfit to fupport fire*
or maintain life in animals ; and that its fpecifi c gravity is
not much lefs than that of common air : fo that, though the
nitrous acid, by being united to phlogidon, is converted into,
air poffcfTed of thele properties, and confequently, though itr
was reafonable to fuppofe, that part at lead of the pldogidi-
cated air of the atmolphere confids of this acid united to phlo-
gidon, yet it might fairly be doubted whether the whole is of
this kind, or whether there are not in reality many different
dvbdances confounded together by us under the name of phlo-
gidicated air. I therefore made an experiment to determine,
whether the whole of a given portion of the phlogidicated ai-r
of the atmolphere could be reduced to nitrous acid, or whether s
there was not a part of a different nature from the red, which
would refufe to undergo that change. Tire foregoing experi-
ments indeed in feme meafure decided this point, as much the
greated part of the air let up into the tube lod its elafticify ;

yet,

Mr . Cav-endish’s Experiments on Air . tj

yet, ns fome remained unabforbed, it did not appear for certain
whether that was of the fame nature as the reft or not. For this
purpofe 1 diminifhed a ii mi la r mixture of dephlogifticated and
common air, in the fame manner as before, till it was reduced to a
fmall part of its original bulk. I then, in order to decompound
as much as I could of the phldgrfticated air which remained in
the tube, added fome dephlogifticated air to it, and continued the
fpark till no further diminution took place* Having by thefe
means condenfed as much as I could of the phlogifticated air, I
let up tome folution of liver of fulphur to abforb the dephlo-
gifticated air ; after which only a fmall bubble of air remained
unabforbed, which certainly was not more than of the
bulk of the phlogifticated air let up into the tube; fo that if
there is any part of the phlogifticated air of our atmofphere
which differs from the reft, and cannot be reduced to nitrous
acid, we may fafely conclude, that it is not more than -4^
part of the whole.

The foregoing experiments (hew, that the chief caufe of
the diminution which common air, or a mixture of common
and dephlogifticated air, fuifers by the define fpark, is the
converfton of the air into nitrons acid ; but yet it leemed not
unlikely, that when any liquor, containing inflammable mat-
ter, was in contact with the air in the tube, fome of this
matter might be burnt by the fpark, and thereby diminish the
air, as I fuppofed in the above-mentioned Paper to be the cafe.
The heft way which occurred to me of difeovering whether
this happened or not, was to- pafstrhe fpark through dephlogi'fti-'
eated air> included between different liquors: for then, if the
diminution proceeded folely from the converfion of air into
nitrous acid, it is plain that, when the dephlogifticated air was
perfectly pure, no diminution would take place ; but when it

contained

*4 Mr* Cavendish’s Experiments on *iir%

contained any phlogifticated air, all this phlogiftjcated air,
joined to as much of the dephlogifticated air as mud unite to it
in order to reduce it into acid, that is, two or three times its
bulk, would difappear, and no more; fo that the whole dimi-
nution could not exceed three or four times the bulk of the'
phlogifticated air : whereas, if the diminution proceeded from
the burning of the inflammable matter, the purer the dephlo-
gifticated air was, the greater and quicker would be the dimi-
nution.

The refult of the experiments was, that when dephlo-
gifticated air, containing only of its bulk of phlo-
gifticated air (that being the pureft air I then had), was con-
flned between Ihort columns of foap lees, and the fpark palled
through it till no further diminution could be perceived, the
air loft -PPtt of its bulk ; which is not a greater diminution than
might very likely proceed from the firft-mentioned caufe ; as
the dephlogifticated air might eafily be mixed with a little
common air while introducing into the tube.

When the lame dephlogifticated air was confined between
columns of diftilled water, the diminution was rather greater
than before, and a white powder was formed on the furface of
the quickfilver beneath ; the reafon of which, in all probabi-
lity, was, that the acid produced in the operation corroded
the quickfilver, and formed the white powder ; and that the
nitrous air, produced by that corrofion, united to the dephlo-
gifticated air, and caufed a greater diminution than would
otherwife have taken place.

When a folution of litmus was ufed, inftead of diftilled
water, the folution foon acquired a red colour, which grew
paler and paler as the fpark was continued, till at laft it be-
came quite colourlefs and tranfparent. The air was diminilhed

Mr . Cavendish’s Experiments on Air . 15

by almoft half, and I believe might have been ftill further di-
minifhed, had the fpark been continued. When lime-water
was let up into the tube, a cloud was formed, and the air was
further diminifhed by about one-fifth. Thu remaining air was
good dephlogifticated air. In this experiment, therefore, the
litmus was, if not burnt, at leaf! decompounded, fo as to lofe
entirely its purple colour, and to yield fixed air ; fo that, though
foap-lees cannot be decompounded by this procefs, yet the folu-
tion of litmus can, and fo very likely might the folutions of
many other combuflible fubflances. But there is nothing, in any
of thefe experiments, which favours the opinion of the air being
at all diminifhed by means of phlogifton communicated to it
by the ele&ric fpark.

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