metals by it. The colour produced with iron does not seem to
be a precipitate.
With the hope of obtaining something peculiar from this
solution, a quantity of it was precipitated by nitrite of lead ; a
colourless solution and a yellowish-green precipitate were ob-
tained. The latter, being well washed, was next diffused
through water, and sulphuretted hydrogen passed through it ;
by filtration a deep brown solid was obtained and a yellowish
solution. The precipitate when washed and dried was brittle
and hard ; on platina-foil it at first burnt with flame, swelling
much, and giving out odour of ammonia like animal matter ;
after that sulphurous acid burnt off, and ultimately lead and
oxide of lead remained ; hence it was a combination of sul-
phuret of lead, and a highly azoted substance. Heated in a
tube, it gave out much ammonia ; digested in alcohol, scarcely
a trace of matter was removed.
The sulphuretted hydrogen solution, being boiled and eva-
porated, left a yellow varnish-like substance, not deliquescent,
soluble in water, acid to taste and to litmus, the acid not being
sulphuric : it rendered persulphate of iron green, precipitated
nitrate of lead, and gave no ammonia by heat.
The concentrated solution (a) acted upon by alcohol, had an
insoluble matter thrown down, which being separated and well
washed with alcohol, was afterwards treated with water, a deep
brown aqueous solution (b) was obtained, and a small insoluble
portion left ; this was almost black when dried, tasteless, brittle,
burning with difficulty, and when heated in a tube giving much
180 On pure Caoutchouc. [1826.
The solution (b) was almost tasteless, and when dried left a
green, shining, brittle substance, resoluble in water, and of
course precipitable by alcohol. It colours solution of persul-
phate of iron green ; but if its strong aqueous solution be treated
with muriatic acid, a reddish-brown precipitate is formed,
which, when separated, dissolves in water, does not colour per-
salts of iron, and when evaporated yields a pulverulent sub-
stance, burning, but not with facility, and producing a little
ammonia when heated in a tube.
The alcoholic solution from which these matters had been
separated contained the particular principle which colours per-
salts of iron green. When evaporated it left a brown, brittle,
transparent substance, becoming soft by exposure to moist air.
It is very bitter, soluble in water, &c., slightly acid. When
heated on platina-foil, it does not burn easily, but runs out into
a bulky charcoal, much like animal matter ; at the same time it
does not yield ammonia when heated in a tube per se, though
the smell is very like that of animal substance.
Ether warmed with it dissolved a small portion of matter, and
the solution, upon evaporation, left globules, which, in all their
characters, corresponded with wax ; its quantity was but small.
Nine hundred and eighty-one grains of the original sap were
washed in water several times. The washed caoutchouc, being
coagulated by heat and perfectly dried, weighed 311 grains.
The aqueous solutions, upon being boiled, yielded sufficient of
the heavy precipitate to equal, when dried, 18*6 grains. The
clear solution was now evaporated to dryness and digested in
alcohol, 28'5 grains of insoluble matter were left, and the solu-
tion, upon evaporation, afforded 70 grains of dry matter. Hence
the following are the contents nearly of 1000 parts of the ori-
ginal sap :
Albuminous precipitate 19*0
Peculiar bitter colouring matter, al
highly azotated substance . . >* 71*3
Substance soluble in water, not in alcohol 29*0
Water, acid, &c 563-7
1826.] On pure Caoutchouc. 181
'': Thinking it probable that whilst in its natural state of divi-
sion the caoutchouc would combine more intimately or readily
with fixed and volatile oils than when aggregated, as it gene-
rally is in commerce, an experiment or two were made in con-
sequence. A portion of well-washed milky caoutchouc being
added to olive oil, and the two beaten well together, a singu-
larly adhesive stringy substance was produced, which holding
the water diffused through it, assumed a very pearly aspect,
stiffened, and was almost solid ; upon being heated so as to
drive off the water, it became oily, fluid and clear, and was
then a solution of caoutchouc in the fixed oil. On adding water
and stirring considerably, it again became adhesive as before.
Thus introduced, caoutchouc would probably be a useful ele-
ment in varnishes.
Oil of turpentine being added to a mixture of one volume of
sap and one volume of water, and well agitated with it, was
found to be only imperfectly miscible ; after standing twenty-
four hours, three portions were formed : the lower, the usual
aqueous solution ; the upper, oil of turpentine, holding a little
caoutchouc in solution ; the intervening part a clot or tenacious
mass, soft and adhesive, like bird-lime, consisting of caoutchouc,
with some oil of turpentine. It was very difficult to dry, and
always remained adhesive at the surface ; but experiments of
this kind were not pursued, for want, at that time, of further
quantities of the original sap.
Such is a general view of the nature of the sap from which
the substance is obtained, and of the substance itself. I have
not endeavoured to give an accurate account of the properties
or quantities of the other substances present, because there is
reason to believe that both vary in different specimens, pro-
bably according to the age of the tree, the time of the year,
or the manner in which the sap is drawn ; nor have I dwelt
upon the inaccuracies of former accounts, inasmuch as they
are evidently referable to the impurity of the substance ex-
Those who wish to look to former accounts of the chemical
or physical qualities of this remarkable substance, will perhaps
find the following references useful :
1751. De la Condamine on an Elastic Resin, newly discovered at Cayenne,
by M. Fresneau ; and on the Use of various Milky Saps from Trees of
182 On Sulphuric Acid and Naphthaline. [1826.
Guiane or France Equinoctiale. Mem. de I'Acad. Roy ale, 1751.
pp. 17, 319.
1763. MM. Herissant and Macquer on Solution of Caoutchouc. Mem. de
I* Academic, 1763, p. 49.
1768. Macquer, Memoir on the Means of dissolving the Resin Caoutchouc,
known by the name Elastic Resin of Cayenne, and making it appear
with all its properties. Mem. de r Academic, 1768, pp. 58, 208.
1781. Berniard, Memoir on Caoutchouc, known by the name of Elastic
Gum. Journ. de Physique, xvii. 265.
1790. Fourcroy on the Sap furnishing Elastic Gum. Ann. de Chim, xi. 225;
again, Connaissances Chimiques, viii. 36.
1791. Grossart on the Means of making Instruments of Gum Elastic, with
the Bottles obtained from Brazil. Ann. de Chim. xi. 143.
1791. Fabbroni on Solution of Caoutchouc in repeatedly-rectified Petroleum.
Ann. de Chim. xi. 195 ; xii. 156.
Pelletier on Solution of Elastic Gum in Sulphuric Ether. Mem. de
I'lnstitut, i. 56.
1801. Howison on the Elastic Gum Vine of Prince of Wales's Island, and of
Experiments made on the Milky Juice which it produces, with Hints
respecting the useful Purposes to which it may be applied. Asiatic
Researches, v. 157-
1801 . Roxburg, Dr., Botanical Description of Urceola Elastica, or Caoutchouc
Vine of Sumatra and Pulo-Penang, with an Account of the Properties
of its inspissated Juice, compared with those of the American Caout-
chouc. Asiatic Researches, v. 167.
1803. Gough, Description of a Property of Caoutchouc or Indian Rubber,
with Reflections on the Cause of the Elasticity of this Substance.-^-
Manchester Memoirs, N. S. i. 288.
1805. Simple Method of making Tubes of Elastic Gum Caoutchouc, avoiding
the expense of Ether. Phil. Mag. xxii. 340.
1807. Murray's Chemistry, iv. 177, contains a compendium of what was then
known respecting this substance.
Royal Institution, January 1826.
On the Mutual Action of Sulphuric Acid and Naphthaline*.
[Read February 16, 1826.]
IN a paper "On new Compounds of Carbon and Hydrogen,"
lately honoured by tbe Royal Society with a place in the Philo-
sophical Transactions, I had occasion briefly to notice the
peculiar action exerted on certain of those compounds by sul-
phuric acidf . During my attempts to ascertain more minutely
* Philosophical Transactions, 1826, p. 140. t pp. 160, 163, 164, 170.
1828.] Sulpho-Naphthalic Acid. 183
the general nature of this action, I was led to suspect the
occasional combination of the hydrocarbonaceous matter with
the acid, and even its entrance into the constitution of the
salts which the acid afterwards formed with bases. Although
this opinion proved incorrect, relative to the peculiar hydro-
carbons forming the subject of that paper, yet it led to experi-
ments upon analogous bodies, and amongst others, upon naph-
thaline, which terminated in the production of the new acid
body and salts now to be described.
Some of the results obtained by the use of the oil-gas pro-
ducts are very peculiar. If, when completed, I find them
sufficiently interesting, I shall think it my duty to place them
before the Royal Society, as explicatory of that action of sul-
phuric acid which was briefly noticed in my last paper.
Most authors who have had occasion to describe naphtha-
line, have noticed its habitudes with sulphuric acid. Mr.
Brande several years since * stated that naphthaline dissolved
in heated sulphuric acid " in considerable abundance, forming
a deep violet-coloured solution, which bears diluting with
water without decomposition. The alkalies produce in this
solution a white flaky precipitate, and if diluted the mixture
becomes curiously opalescent, in consequence of the separation
of numerous small flakes." The precipitate by alkali was pro-
bably one of the salts to be hereafter described.
Dr. Kidd observes t, that " it blackens sulphuric acid when
boiled with it ; the addition of water to the mixture having no
other effect than to dilute the colour, neither does any preci-
pitation take place upon saturating the acid with ammonia."
Mr. Chamberlain states J, that sulphuric acid probably de-
composes naphthaline, for that it holds but a very small quan-
tity in solution. The true interpretation of these facts and
statements will be readily deduced from the following experi-
1 . Production and properties of the new acid formed from
Sulphuric Acid and Naphthaline. Naphthaline, which had
been almost entirely freed from naphtha by repeated sublima-
tion and pressure, was pulverized ; about one part with three or
* Quart. Journ. of Science, 1819, viii. 289. f Phil- Trans. 1821, p. 216.
J Annals of Philosophy, 1823, N.S. vi. p. 136.
184 On Sulphuric Acid and Naphthaline. [1826.
four parts by weight of cold sulphuric acid were put into a bottle,
well shaken, and left for thirty-six hours. The mixture then
contained a tenacious deep red fluid, and a crystalline solid; it
had no odour of sulphurous acid. Water being added, all the
liquid and part of the solid was dissolved ; a few fragments of
naphthaline were left, but the greater part was retained in
solution. The diluted fluid being filtered was of a light brown
tint, transparent, and of an acid and bitter taste.
For the purpose of combining as much naphthaline as pos-
sible with the sulphuric acid, 700 grains with 520 grains of oil
of vitriol were warmed in a Florence flask until entirely fluid,
and were well shaken for about 30 minutes. The mixture was
red ; and the flask being covered up and left to cool, was found
after some hours to contain, at the bottom, a little brownish
fluid, strongly acid, the rest of the contents having solidified
into a highly crystalline mass. The cake was removed^ and
its lower surface having been cleaned, it was put into another
Florence flask with 300 grains more of naphthaline, the whole
melted and well shaken together, by which a uniform mixture
was obtained, but opake and dingy in colour. It was now
poured into glass tubes, in which it could be retained and ex-
amined without contact of air. In these the substance was
observed to divide into two portions, which could easily be
distinguished from each other, whilst both were retained in the
fluid state. The heavier portion was in the largest quantity ;
it was of a deep red colour, opake in tubes half an inch in
diameter, but in small tubes could be seen through by a candle,
or sunlight, and appeared perfectly clear. The upper portion
was also of a deep red colour, but clear, and far more trans-
parent than the lower: the line of separation very defined.
On cooling the tubes, the lighter substance first solidified, and
after some time the heavier substance also became solid. In
this state, whilst in the tube, they could with great difficulty
be distinguished from each other.
These two substances were separated, and being put into
tubes, were further purified by being left in a state of repose
at temperatures above their fusing-points, so as to allow of
separation ; and when cold, the lower part of the lighter sub-
stance and the upper, as well as the lower part of the heavier
substance, were set aside for further purification.
1826.] Sulpho-Naphthalic Acid. 185
The heavier substance was a red crystalline solid, soft to
the nail like a mixture of wax and oil. Its specific gravity was
from 1*3 to 1*4, varying in different specimens; its taste sour,
bitter, and somewhat metallic. When heated in a tube, it
fused, forming, as before, a clear but deep red fluid. Further
heat decomposed it, naphthaline, sulphurous acid, charcoal,
&c. being produced. When heated in the air it burnt with
much flame. Exposed to air it attracted moisture rapidly,
became brown and damp upon the surface, and developed a
coat of naphthaline. It dissolved entirely in alcohol, forming
a brown solution. When rubbed in water a portion of naph-
thaline separated, amounting to 27 per cent., and a brown acid
solution was obtained. This was found by experiments to
contain a peculiar acid mixed with a little free sulphuric acid,
and it may conveniently be called the impure acid.
The lighter substance was much harder than the former,
and more distinctly crystalline. It was of a dull red colour,
easily broken down in a mortar, the powder being nearly white,
and adhesive like naphthaline. It was highly sapid, being acid,
bitter, and astringent. When heated in a tube it melted,
forming a clear red fluid, from which by a continued heat
much colourless naphthaline sublimed, and a black acid sub-
stance was left, which at a high temperature gave sulphurous
acid and charcoal. When heated in the air it took fire and
burnt like naphthaline. Being rubbed in a mortar with water,
a very large portion of it proved to be insoluble ; this was
naphthaline ; and on filtration the solution contained the pecu-
liar acid found to exist in the heavier substance, contaminated
with very little sulphuric acid. More minute examination
proved that this lighter substance in its fluid state was a solu-
tion of a small quantity of the dry peculiar acid in naphthaline ;
and that the heavier substance was a union of the peculiar
acid in large quantity with water, free sulphuric acid, and
It was easy by diminishing the proportion of naphthaline to
make the whole of it soluble, so that when water was added to
the first result of the experiment, nothing separated ; and the
solution was found to contain sulphuric acid with the peculiar
acid. But reversing the proportions, no excess of naphthaline
was competent, at least in several hours, to cause the entire
186 On Sulphuric Acid and Naphthaline. [1826.
disappearance of the sulphuric acid. When the experiment
was carefully made with pure naphthaline, and either at com-
mon or slightly elevated temperatures, no sulphurous acid
appeared to be formed, and the action seemed to consist in a
simple union of the concentrated acid and the hydrocarbon.
Hence it appears, that when concentrated sulphuric acid
and naphthaline are brought into contact at common or mode-
rately elevated temperatures, a peculiar compound of sulphuric
acid with the elements of the naphthaline is produced, which
possesses acid properties ; and as this exists in large quantity
in the heavier of the bodies above described, that product
may conveniently be called the impure solid acid. The expe-
riments made with it, and the mode of obtaining the pure acid
from it, are now to be described.
Upon applying heat and agitation to a mixture of one volume
of water and five volumes of impure solid acid, the water was
taken up to the exclusion of nearly the whole of the free naph-
thaline present ; the latter separating in a colourless state from
the red hydrated acid beneath it. As the temperature of the
acid diminished, crystallization in tufts commenced here and
there, and ultimately the whole became a brownish yellow solid.
A sufficient addition of water dissolved nearly the whole of this
hydrated acid, a few flakes only of naphthaline separating.
A portion of the impure acid in solution was evaporated at
a moderate temperature; when concentrated, it gradually
assumed a light brown tint. In this state it became solid on
cooling, of the hardness of cheese, and was very deliquescent.
By further heat it melted, then fumed, charred, &c., and gave
evidence of the abundant presence of carbonaceous matter.
Some of the impure acid in solution was neutralized by pot-
ash, during which no naphthaline or other substance separated.
The solution being concentrated until ready to yield a film on
its surface, was set aside whilst hot to crystallize : after some
hours the solution was filled with minute silky crystals, in
tufts, which gave the whole, when stirred, not the appearance
of mixed solid salt and liquid, but that of a very strong solution
of soap. The agitation also caused the sudden solidification
of so much more salt, that the whole became solid, and felt like
a piece of soft soap. The salt when dried had no resemblance
to sulphate of potash. When heated in the air, it burnt with
1806.] Sulpho-Naphthalic Acid. 187
a dense flame, leaving common sulphate of potash, mixed with
some sulphuret of potassium, resulting from the action of the
carbon, &c. upon the salt.
Some of the dry salt was digested in alcohol to separate
common sulphate of potash. The solution, being filtered and
evaporated, gave a white salt soluble in water and alcohol,
crystalline, neutral, burning in the air with much flame, and
leaving sulphate of potash. It was not precipitated by nitrate
of lead, muriate of baryta, or nitrate of silver.
It was now evident that an acid had been formed peculiar
in its nature and composition, and producing with bases peculiar
salts. In consequence of the solubility of its barytic salt, the
following process for the preparation of the pure acid was
A specimen of native carbonate of baryta was selected, and
its purity ascertained. It was then pulverized, and rubbed
in successive portions with a quantity of the impure acid in
solution, until the latter was perfectly neutralized, during which
the slight colour of the acid was entirely removed. The solu-
tion was found to contain the peculiar barytic salt. Water
added to the solid matter dissolved out more of the salt ; and
ultimately only carbonate and sulphate of baryta, mixed with a
little of another barytic salt, remained. The latter salt being
much less soluble in water than the former, was not removed
so readily by lixiviation, and was generally found to be almost
entirely taken up by the last portions of water applied with
The barytic salt in solution was now very carefully decom-
posed, by successive additions of sulphuric acid, until all the
baryta was separated, no excess of sulphuric acid being per-
mitted. Being filtered, a pure aqueous solution of the peculiar
acid was obtained. It powerfully reddened litmus paper, and
had a bitter acid taste. Being evaporated to a certain degree,
a portion of it was subjected to the continued action of heat ;
when very concentrated, it began to assume a brown colour,
and on cooling became thick, and ultimately solid, and was
very deliquescent. By renewed heat it melted, then began to
fume, became charred, but did not flame ; and ultimately gave
sulphuric and sulphurous acid vapours, and left charcoal.
Another portion of the unchanged strong acid solution was
188 On Sulphuric Acid and Naphthaline. [1826.
placed over sulphuric acid in an exhausted receiver. In some
hours it had hy concentration become a soft white solid, ap-
parently dry ; and after a longer period was hard and brittle.
In this state it was deliquescent in the air, but in close vessels
underwent no change in several months. Its taste was bitter,
acid, and accompanied by an after metallic flavour, like that of
cupreous salts. When heated in a tube at temperatures below
212, it melted without any other change, and on being allowed
to cool, crystallized from centres, the whole ultimately becoming
solid. When more highly heated, water at first passed off, and
the acid assumed a slight red tint ; but no sulphurous acid was
as yet produced, nor any charring occasioned ; and a portion
being dissolved and tested by muriate of baryta, gave but a
very minute trace of free sulphuric acid. In this state it was
probably anhydrous. Further heat caused a little naphtha-
line to rise, the red colour became deep brown, and then a
sudden action commenced at the bottom of the tube, which
spread over the whole, and the acid became black and opake.
Continuing the heat, naphthaline, sulphurous acid, and char-
coal were evolved ; but even after some time, the residuum ex-
amined by water and carbonate of baryta was found to contain
a portion of the peculiar acid undecomposed, unless the tem-
perature had been raised to redness.
These facts establish the peculiarity of this acid, and di-
stinguish it from all others. In its solid state it is generally a
hydrate containing much combustible matter. It is readily
soluble in water and alcohol, and its solution forms neutral
salts with bases, all of which are soluble in water, most of them
in alcohol, and all combustible, leaving sulphates or sulphurets
according to circumstances. It dissolves in naphthaline, oil of
turpentine, and olive oil, in greater or smaller quantities, accord-
ing as it contains less or more water. As a hydrate, when it is
almost insoluble in naphthaline, it resembles the heavier sub-
stance obtained as before described, by the action of sulphuric
acid on naphthaline, and which is the solid hydrated acid,
containing a little naphthaline and some free sulphuric acid,
whilst the lighter substance is a solution of the dry acid in
naphthaline ; the water present in the oil of vitriol originally
used being sufficient to cause a separation of a part, but not of
1826.] Sitlpho-Naphthalic Acid. 189
2. Salts formed by the peculiar acid with bases. These
compounds may be formed, either by acting on the bases or
their carbonates by the pure acid, obtained as already de-
scribed ; or the impure acid in solution may be used, the salts
resulting being afterwards freed from sulphates, by solution in
alcohol. It is however proper to mention that another acid,
composed of the same elements, is at the same time formed
with the acid in question, in small, but variable proportions.
The impure acid used, therefore, should be examined as to
the presence of this body, in the way to be directed when
speaking of the barytic salts ; and such specimens as contain
very little or none of it should be selected.
Potash forms with the acid a neutral salt, soluble in water
and alcohol, forming colourless solutions. These yield either
transparent or white pearly crystals, which are soft, slightly
fragile, feel slippery between the fingers, do not alter by
exposure to air, and are bitter and saline to the taste. They
are not very soluble in water ; but they undergo no change by
repeated solutions and crystallizations, or by long-continued
ebullition. The solutions frequently yield the salt in acicular
tufts, and they often vegetate, as it were, by spontaneous eva-
poration, the salt creeping over the sides of the vessel, and
running to a great distance in very beautiful forms. The
solid salt heated in a tube gave off a little water, then some
naphthaline ; after that a little carbonic and sulphurous acid
gases arose, and a black ash remained, containing carbon,