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strongly distinguished from the oils from which they are pro-
duced. Sulphuric acid acts upon them instantly with pheno-
mena already briefly referred to.



Dr. Henry, whilst detailing the results of his numerous and
exact experiments in papers laid before the Royal Society,
mentions in that read February .22, 1821 *, the discovery made
by Mr. Dalton, of a vapour in oil-gas of greater specific
gravity than olefiant gas, requiring much more oxygen for its
combustion, but yet condensable by chlorine. Mr. Dalton
appears to consider all that was condensable by chlorine as a
new and constant compound of carbon and hydrogen; but
Dr. Henry, who had observed that the proportion of oxygen
required for its combustion varied from 4*5 to 5 volumes, and
the quantity of carbonic acid produced, from 2*5 to 3 volumes,
was inclined to consider it as a mixture of the vapour of a
highly volatile oil with the olefiant and other combustible
gases : and he further mentions, that naphtha in contact with

* Phil. Trans, for 1821, p. 136.



1825.] obtained by the Decomposition of Oil. 169

hydrogen gas will send up such a vapour ; and that he has
been informed, that when oil-gas was condensed in Gordon's
lamp, it deposited a portion of highly volatile oil.

A writer in the ' Annals of Philosophy,' N. S. iii. 37, has
deduced from Dr. Henry's experiments, that the substance,
the existence of which was pointed out by Mr. Dalton, was not
a new gas sui generis, " but a modification of olefiant gas, con-
stituted of the same elements as that fluid, and in the same pro-
portions ; with this only difference, that the compound atoms
are triple instead of double :" and Dr Thomson has adopted
this opinion in his f Principles of Chemistry.' This, I believe,
is the first time that two gaseous compounds have been sup-
posed to exist, differing from each other in nothing but density ;
and though the proportion of 3 to 2 is not confirmed, yet the
more important part of the statement is, by the existence of
the compound described at page 163 ; which, though composed
of carbon and hydrogen in the same proportion as in olefiant
gas, is of double the density*.

It is evident that the vapour observed by Mr. Dalton and

* In reference to the existence of bodies composed of the same elements
and in the same proportions, but differing in their qualities, it may be ob-
served, that now we are taught to look for them, they will probably multiply
upon us. I had occasion formerly to describe a compound of olefiant gas and
iodine (Phil. Trans, cxi. 72), which upon analysis yielded one proportional of
iodine, two proportionals of carbon, and two of hydrogen (Quarterly Journal,
xiii. 429). M. Serrulas, by the action of potassium upon an alcoholic solution
of iodine, obtained a compound decidedly different from the preceding in its
properties; yet when analysed, it yielded the same elements in the same
proportions (Ann. de Chimie, xx. 245 ; xxii. 172).

Again: MM. Liebig and Gay-Lussac, after an elaborate and beautiful
investigation of the nature of fulminating compounds of silver, mercury, &c.,
were led to the conclusion that they were salts, containing a new acid, and
owed their explosive powers to the facility with which the elements of this acid
separated from each other (Annales de Chimie, xxiv. 294 ; xxv. 285). The acid
itself, being composed of one proportional of oxygen, one of nitrogen, and two
of carbon, is equivalent to a proportional of oxygen + a proportional of
cyanogen, and is therefore considered as a true cyanic acid. But M. Wohler,
by deflagrating together a mixture of ferroprussiate of potash and nitre, has
formed a salt, which, according to his analysis, is a true cyanate of potash.
The acid consists of one proportional of oxygen, one of nitrogen, and two of
carbon. It may be transferred to various other bases; as the earths, the
oxides of lead, silver, &c, ; but the salts formed have nothing in common with



170 On some new Products [1825.

Dr. Henry must have contained not only this compound, and a
portion of the bicarburet of hydrogen, but also portions of the
other (as yet apparently indefinite) substances ; and there can
be no doubt that the quantity of these vapours will vary from
the point of full saturation of the gas, when standing over
water and oil, to unknown, but much smaller proportions. It
is therefore an object in the analysis of oil and coal-gas, to
possess means by which their presence and quantity may be
ascertained; and this I find may be done with considerable
exactness by the use of sulphuric acid, oil, &c., in consequence
of their solvent power over them.

Sulphuric acid is in this respect a very excellent agent. It
acts upon all these substances instantly, evolving no sulphurous
acid ; and though when the quantity of substance is considerable
as compared with the acid, a body is left undecomposed by or
uncombined with the acid, and volatile, so as constantly to
afford a certain portion of vapour, yet when the original sub-
stance is in small quantity, as where it exists in vapour in
a given volume of gas, this does not interfere, in consequence
of the solubility of the vapour of the new compound produced
by the action of the acid in the acid itself in small quantities :
and I found that when 1 volume of the vapour of any of the
products of the oil-gas liquor was acted upon, either alone, or
mixed with 1, 2, 3, 4, up to 12 volumes of air, oxygen, or
hydrogen, by from half a volume to a volume of sulphuric acid,
it was entirely absorbed and removed.

When olefiant gas is present, additional care is required in
analytical experiments, in consequence of the gradual combi-
nation of the olefiant gas with the sulphuric acid. I found
that 1 volume of sulphuric acid in abundance of olefiant gas,
absorbed about 7 volumes in twenty-four hours in the dull
light of a room; sunshine seemed to increase the action a
little. When the olefiant gas was diluted with air or hydrogen,
the quantity absorbed in a given time was much diminished ;
and in those cases it was hardly appreciable in two hours,

the similar salts of MM. Liebig and Gay-Lussac, except their composition
(Gilbert's Annalen, Ixxviii. 157; Ann. de Chimie, xxvii. 190). M. Gay-Lussac
observes, that if the analysis be correct, the difference can only be accounted
for by admitting a different mode of combination.



1825.] obtained by the Decomposition of Oil. 171

a length of time which appears to be quite sufficient for the
removal of any of the peculiar vapours from oil or coal-gas.

My mode of operating was generally in glass tubes over
clean mercury*, introducing the gas, vapour or mixture, and
then throwing up the sulphuric acid by means of a bent tube
with a bulb blown in it, passing the acid through the mercury
by the force of the mouth. The following results are given as
illustrations of the process :

Oil-gas from a Gasometer.

Dimin.
Sul. Acid. in 8'. in 1 hour. 2 hours, per cent.

188 vol. + 9-5 vol. diminished to 155'0 148'5 146'4 22-12

107 + 13-0 88-5 84-5 82'0 23-33

138 + 5-2 113-7 108-0 106'5 22'82

Oil-gas from Gordons Lamp.

Dimin.
Sul. Acid. 15' 30' 3 hours, per. cent.

214 vol. -f 6-8 vol. diminished to 183'3 180*8 176'0 1775

159 4 5-9 137'5 136-0 130-4 17'98

113 + 12-2 98-0 96-0 92'0 18'58

Coal-gas of poor quality.

548-6 + 27'6 533-3 529-2 529*0 3'57

273-6 + 27-8 267-9 266-0 266-0 2'78

190-6 + 13-1 186-0 184-2 184-1 3'41

Oil may also be used in a similar manner for the separation
of these vapours. It condenses about 6 volumes of the most
elastic vapour at common temperatures, and it dissolves with
greater facility the vapour of those liquids requiring higher
temperatures for their ebullition. I found that in mixtures
made with air or oxygen for detonation, I could readily se-
parate the vapour by means of olive oil ; and when olefiant and
other gases were present, its solvent power over them was
prevented by first agitating the oil with olefiant gas or with a
portion of the gas to saturate it, and then using it for the
removal of the vapours.

* If the mercury contain oxidizable metals, the sulphuric acid acts upon
it, and evolves sulphurous acid gas. It may be cleaned sufficiently by being
left in contact with sulphuric acid for twenty-four hours, agitating it frequently
at intervals.



172 On some new Products [1825.

In the same way some of the more fixed essential oils may
be used, as dry oil of turpentine ; and even a portion of the
condensed liquor itself, as that part which requires a tempera-
ture of 220 or 230 for its ebullition; care being taken to
estimate the expansion of the gas by the vapour of the liquid,
which may readily be done by a known portion of common air
preserved over the liquid as a standard.

With reference to the proportions of the different substances
in the liquid as obtained by condensation of oil-gas, it is
extremely difficult to obtain anything like precise results, in
consequence of the immense number of rectifications required
to separate the more volatile from the less volatile portions ;
but the following Table will furnish an approximation. It con-
tains the loss of 100 parts by weight of the original fluid
by evaporation in a flask, for every 10 in elevation of tempera-
ture, the substance being retained in a state of ebullition.

100 parts at 58 parts. differences.

had lost at 70 ... 1-1 - q

80 ... 3-0 gl

90 ... 5-3 .

100 ... 7-7 "

no ... 10-1 f 7

120 ... 13-2 * *

130 ... 16-1 **

140 .., 19-3

150 ... 22-4' *.

160 . . 25-6 o. 4

170 ... 29-0 1*7

180 ... 44-7 il'

190 ... 68-1

200 ... 84-2 7:

210 , . . 91-6 S7

220 . . . 95-3 ?'

230 ... 96-6

The residue, 3'4 parts, was dissipated before 250 with slight
decomposition. The third column expresses the quantity vola-
tilized between each 10, and indicates the existence of what
has been described as bicarburet of hydrogen in considerable
quantity.



1825.] obtained by the Decomposition of Oil. 173

The importance of these vapours in oil-gas, as contributing
to its very high illuminating powers, will be appreciated, when
it is considered that with many of them, and those of the denser
kind, it is quite saturated. On distilling a portion of liquid,
which had condensed in the pipes leading to an oil-gas gaso-
meter, and given to me by Mr. Hennel, of the Apothecaries'
Hall, I found it to contain portions of the bicarburet of hy-
drogen. It was detected by submitting the small quantity of
liquid which distilled over before 190 to a cold of 0, when the
substance crystallized from the solution. It is evident, therefore,
that the gas from which it was deposited must have been
saturated with it. On distilling a portion of recent coal-gas
tar, as was expected, none could be detected in it ; but the
action of sulphuric acid is sufficient to show the existence of
some of these bodies in the coal-gas itself.

With respect to the probable uses of the fluid from com-
pressed oil-gas, it is evident in the first place, that being thus
volatile, it will, if introduced into gas, which burns with a pale
flame, give such quantity of vapour as to make it brightly illu-
minating ; and even the vapour of those portions which require
temperatures of 170, 180, or higher, for their ebullition, is so
dense as to be fully sufficient for this purpose in small quantities.
A taper was burnt out in a jar of common air over water;
a portion of fluid boiling at 190 was thrown up into it, and
agitated : the mixture then burnt from a large aperture with
the bright flame and appearance of oil-gas, though of course
many times the quantity that would have been required of oil-
gas for the same light was consumed ; at the same time there
was no mixture of blueness with the flame, whether it were
large or small. Mr. Gordon has, I understand, proposed using
it in this manner.

The fluid is also an excellent solvent of caoutchouc, sur-
passing every other substance in this quality. It has already
been applied to this purpose.

It will answer all the purposes to which the essential oils
are applied as solvents, as in varnishes, &c. ; and in some
cases where volatility is required, when rectified it will far
surpass them.

It is possible that, at some future time, when we better un-
derstand the minute changes which take place during the



174 On pure Caoutchouc. [1826.

decomposition of oil, fat, and other substances by heat, and
have more command of the process, that this substance, among
others, may furnish the fuel for a lamp, which remaining a
fluid at the pressure of two or three atmospheres, but be-
coming a vapour at less pressure, shall possess all the advan-
tages of a gas lamp, without involving the necessity of high
pressure.

Royal Institution, June 7, 1835.



On Pure Caoutchouc, and the Substances by which it is accom-
panied in the State of Sap or Juice*.

I HAVE had an opportunity lately, through the kindness of Mr.
Thomas Hancock, of examining the chemical properties of
caoutchouc in its pure form, as well as of ascertaining the
nature and proportions of the other substances with which it is
mixed, when it exudes as sap or juice from the tree. At
present much importance attaches to this substance, in con-
sequence of its many peculiar and excellent qualities, and its
increasing applications to useful purposes. I have thought,
therefore, that a correct account of its chemical nature would
possess some interest.

The extensive uses, both domestic and scientific, to which
Mr. Hancock has applied common caoutchouc, in consequence
of his peculiar mode of liquefying it, are well known. Hence
he was fully alive to the importance of its applications, when in
its original state of division. When he gave me the substance,
he communicated many of his observations upon it, which, with
others of my own, form the present paper.

The fluid, I understood, had been obtained from the southern
part of Mexico, and was very nearly in the state in which it
came from the tree ; it had been altered simply by the formation
of a slight film of solid caoutchouc on the surface of the cork
which closed the bottle. The caoutchouc thus removed was
not a 500th part of the whole. The fluid was a pale yellow,
thick, creamy-looking substance, of uniform consistency. It
had a disagreeable acescent odour, something resembling that

* Quarterly Journal of Science, xxi. 19. ,



1826.] On pure Caoutchouc. 175

of putrescent milk; its specific gravity was 1011*74. When
exposed to the air in thin films, it soon dried, losing weight, and
leaving caoutchouc of the usual appearance and colour, and
very tough and elastic : 202*4 grains of the liquid dried in
a Wedgewood basin, at 100 Fahr., became in a few days
94'4 grains, and the solid piece formed being then removed
from the capsule, and exposed on all sides to the air until
quite dry, became 91 grains : hence 100 parts of sap left nearly
45 of solid matter.

Heat caused immediate coagulation of the sap, the caout-
chouc separating in the solid form, and leaving an aqueous
solution of the other substances existing with it in its first
state.

Alcohol poured into the sap in sufficient quantity caused a
coagulum and a precipitate, both of which were caoutchouc of
considerable purity. The alcohol retained in solution the ex-
traneous matters, which, possessing peculiar properties, will be
hereafter described.

Solution of alkali added to the sap evolved a very fetid
odour, but did not appear to exert any particular action on the
caoutchouc.

The sap, left to itself for several days, gradually separated
into two parts ; the opake portion contracted upwards, leaving
beneath a deep brown, but transparent solution, evidently con-
taining substances very different in their nature from caout-
chouc itself, and which, considering the specific gravity of the
sap and of pure caoutchouc (the latter being lighter than water),
were probably present in considerable quantity.

It was found that, by mixing the sap with water, no other
change took place than mere dilution. The mixture was uni-
form, and had all the properties of a weak or thin sap. Heat,
evaporation, acids, and alkali, produced the same effects, gene-
rally, as before.

When the diluted sap was suffered to remain at rest, a sepa-
ration soon took place, similar to that which occurred with the
native juice, but to a greater extent; a creamy portion rose to
the top, whilst a clear aqueous solution remained beneath.
Hence it was found easy to wash the caoutchouc, and remove
from it other principles which had been generally involved in
it to a greater or smaller extent during its coagulation. For



176 On pure Caoutchouc. [1826.

this purpose a portion of the sap was mixed with about four
volumes of water, and the mixture put into a funnel, stopped
below by a cork ; in the course of eighteen or twenty-four hours,
when the caoutchouc had risen to the top and occupied about
its original volume, the aperture at the bottom of the funnel
was opened and the solution drawn off; more water was then
added to, and mixed with, the caoutchouc, and the operation
repeated, and this was done four or five times, until the water
came away nearly pure. During the latter washings, the caout-
chouc required a longer time to rise to the surface, in conse-
quence of the decreasing specific gravity of the solution in
which it was suspended. This was obviated at times, accord-
ing to the experiments for which the caoutchouc was required,
by performing the first washings with solutions of common salt,
muriatic acid, &c., and ultimately finishing with pure water.

In this way the caoutchouc was purified without any altera-
tion of its original state. It now appeared in its state of mixture
with water perfectly white : portions of it left for a twelvemonth
over water underwent no change in that time, except coagula-
tion and a slight film upon the surface ; the rest was as miscible
with the water as at first, and when coagulated, equally elastic.
The sap or the washed caoutchouc is much more easily pre-
served in the diluted than in the concentrated state.

It produced no particular appearance with the solutions of
iron or other metals.

When evaporated, either on paper, in a capsule, or other-
wise, the caoutchouc was left in its elastic state, and perfectly
unaltered, except with respect to purity. When put on to ab-
sorbent surfaces, as bibulous paper, chalk, or plaster of Paris,
the water was rapidly abstracted, and the caoutchouc almost
immediately united into a mass, retaining the form of the thing
on which it was cast. In this way Mr. Hancock has made
beautiful medallions with the sap. Poured on to a filter, the
water passes through, and the caoutchouc coagulates.

When aggregated in any of these ways, the caoutchouc ap-
pears at first as a soft white solid, almost like curd, which by
pressure exudes much water, contracts, becomes more compact,
has acquired elasticity, but is still soft, white and opake. It
also attains this state without pressure, if time be permitted
for the water to evaporate. The opacity belonging to it is not



1826.] On pure Caoutchouc. 177

an essential property of the body, but due to water enclosed
within its mass ; further exposure to air allows of the gradual
dissipation of this water, and then the caoutchouc appears in
its pure and dry state, as a perfectly transparent, colourless arid
elastic body, except it be in thick masses, when a trace of
colour is perceived. The change from first to last is best seen
by pouring enough of the pure mixture into a Wedgewood or
glass basin, to form ultimately a plate of y^th or -^ th of an inch
in thickness, and leaving it exposed to air at common tempe-
ratures undisturbed.

No appearance of texture can be observed in the pure trans-
parent caoutchouc ; it resembles exactly a piece of clear strong
jelly. All the phenomena dependent upon its elasticity, which
are known to belong to common caoutchouc, are well exhibited
by it. When very much extended, it assumes a beautiful pearly
or fibrous appearance, probably belonging to the effects which
Dr. Brewster has observed elastic bodies to produce, when in
a state of tension, upon light. When it has been extended and
doubled several times, until further extension in the same di-
rection is difficult, it is found to possess very great strength.
Its specific gravity is 0*925, and no reduplication and pressure
of it in a Bramah's press was found permanently to alter it. It
is evidently pervious to water in a slight degree, or otherwise
the interior of a piece of caoutchouc coagulated from the sap
would always remain opake. It is equally evident that water
passes but very slowly through it, from the time it takes to
evaporate that which lies in the middle of a thin cake. It is a
non-conductor of electricity.

The pure caoutchouc has a very adhesive surface, which it
retains after many months' exposure to air. Its fresh cut sur-
faces pressed together also adhere with a force equal to that of
any other part of the piece.

A strip of it boiled in solution of potash, so strong as to be solid
when cold, was not at all affected by it, except that its surface as-
sumed a pearly or tendinous appearance ; no swelling or soften-
ing, above what would have been produced by water, occurred.

The combustibility of caoutchouc is very well known. When
the pure substance is heated in a tube, it is resolved into sub-
stances more or less volatile, with the deposition of only a small
trace of charcoal ; at a higher temperature it is resolved into

N



178 Onpure Caoutchouc. [1826.

charcoal and compounds of carbon and hydrogen ; it yields no
ammonia by destructive distillation, nor any compounds of
oxygen, and my experiments agree with those of Dr. Ure, in
indicating carbon and hydrogen as its only elements. I have
not, however, been able to verify his proportions, which are 90
carbon, 9*11 hydrogen, or by theory nearly 3 proportionals of
carbon to 2 of hydrogen, and have never obtained quite so
much as 7 carbon to 1 hydrogen by weight. The mean of my
experiments gives,

Carbon . . 6'812~\ f8 proportionals nearly.

Hydrogen . 1'OOOJ L7.

No means which have yet been discovered seem competent,
when the caoutchouc has once been aggregated, to restore it to
its pristine state. Previous to its aggregation it may be either
scented or coloured. A solution of camphor in alcohol was added
to water, so as to precipitate the camphor in a flocculent state ;
a little of this was added to some of the pure caoutchouc in
water well agitated, and then coagulation caused by heat or
absorption ; the caoutchouc obtained was highly odorous.

In the trials made to give it colour, the body colours were
found to answer best indigo, cinnabar, chrome-yellow, car-
mine, lake, &c., were rubbed very fine with water ; then mixed
well with the pure caoutchouc, in a somewhat diluted state, and
coagulation induced either upon an absorbent surface or other-
wise. Perfectly-coloured specimens were thus obtained.

The aqueous liquid obtained either by letting the sap stand
for some time, or by the first and second washing, was of a
brown colour, bitter, acid to litmus in consequence of the pre-
sence of acetic acid, due apparently to spontaneous changes in
the substances present. It was difficult to filter. Being boiled,
acid vapours rose, a precipitate fell to the bottom, and now the
solution (a) became clear, either by standing or filtration, and
could be separated from the solid matter.

The precipitate or substance thus obtained was dark brown,
glossy and brittle, much heavier than water, not soluble in
alcohol, ether, water, essential or fixed oils. Weak solution of
alkali dissolved it, forming a deep brown solution, precipitable
by dilute muriatic acid. It burnt upon platina-foil, like animal
matter, with flame, leaving a bulky charcoal. When heated in



1826.] On pure Caoutchouc. 179

a tube, it became charred, yielding much ammonia. It re-
sembles albumen more than any other substance, and is the
source of the nitrogen or ammonia obtained by the distillation
of common caoutchouc.

The brown aqueous solution (a) became frothy on agitation ;
alkali rendered it of a deep yellow colour, and produced a
putrescent odour, similar to that evolved by alkali or quick -lime
from white of egg or blood. It was remarkably distinguished
by the deep-green colour it produced with persalts of iron,
especially when a little alkali was present, and the dense yellow
precipitates it formed with muriate of zinc and nitrate of lead ;
indeed, precipitates were produced in solutions of most of the



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