An account of some experiments and observations on the constituent parts of certain astringent vegetables, and on their operation in tanning

c 233 n

IX. An Account of some Experiments and Observations on the
constituent Parts of certain astringent Vegetables ; and on their
Operation in Tanning. By Humphry Davy, Esq. Professor of
Chemistry in the Royal Institution. Communicated by the Right
Hon. Sir Joseph Banks, Bart. K. B. P. R. S.

Read February 24, 1803.

The discovery made by M, Seguin, of a peculiar vegetable
matter which is essential to the tanning of skin, and which is
possessed of the property of precipitating gelatine from its solu-
tions, has added considerably to our knowledge of the constituent
parts of astringent vegetables.

Mr. Proust has investigated many of the properties of this
substance; but, though his labours, and those of other chemists,
have led to various interesting observations, yet they are far
from having exhausted the subject. The affinities of tannin have
been hitherto very little examined; and the manner in which
its action upon animal matters is modified by combination with
other substances, has been scarcely at all studied.

At the desire of the Managers of the Royal Institution, I
began, in September, 1801, a series of experiments on the sub-
stances employed in the process of tanning, and on the chemical
agencies concerned in it. These experiments have occupied,
ever since, a considerable portion of my leisure hours ; and I now
presume to lay before the Royal Society an account of their ge-
neral results. My chief design was, to attempt to elucidate the

MDCCCIII. H h

234 ’ Davy’s Experimerits and Observations

practical part of the art ; but, in pursuing it, I was necessarily
led to general chemical inquiries concerning the analysis of the
different vegetable substances containing tannin, and their pe-
culiar properties.

I. OBSEx^.VATIONS ON THE ANALYSIS OF ASTRINGENT VEGETABLE

INFUSIONS.

Tlie substances that have been supposed to exist most gene-
rally in astringent infusions are, tannin, gallic acid, and extractive
matter.

Tlie presence of tannin in an infusion, is denoted by the pre-
cipitate it forms with the solution of glue, or of isinglass. And,
when this principle is wholly separated, if the remaining liquor
gives a dark colour w'ith the oxygenated salts of iron, and an
immediate precipitate with the solutions of alum and of muriate
of tin, it is believed to contain gallic acid, and extractive matter.

The experiments of MM. Fourcroy, Vauouelin, and
Seguin, have shown that many astringent solutions undergo
a change by exposure to the atmosphere ; an insoluble matter
being precipitated from them, A precipitation is likewise occa-
sioned in them by the action of heat ; and these circumstances
render it extremely difficult to ascertain, with any degree of
precision, the quantities of their constituent parts, as they exist
in the primitive combination.

Affer trying several experiments on different methods of
ascertaining the quantity of tannin in astringent infusions, I was
induced to employ the common process of precipitation by gela-
tine, as being the most accurate.

This process, however, requires many precautions. The tan-
ning principle in different vegetables, as will be seen hereafter,
demands for its saturation different proportions of gelatine; and

on the constituent Pai'ts of astrmgent Vegetables. 235

the quantity of the precipitate obtained by filtration, is not always
exactly proportional to the quantities of tannin and gelatine in
solutions, but is influenced by the degree of their concentration.
Thus, 1 found that 10 grains of dry isinglass, dissolved in two
ounces of distilled water, gave, with solution of galls in ex-
cess, a precipitate weighing, when dry, 17 grains; whilst the
same quantity, dissolved in six ounces of water, produced, all
other circumstances being similar, not quite 1 5 grains. With
more diluted solutions, the loss was still greater; and analogous
effects took place, when equal portions of the same solution of
isinglass were acted on by equal portions of the same infusion
of galls diluted in different degrees with water; tlie least
quantity of precipitate being always produced by the least con-
centrated liquor. In all cases, when the weak solutions ^^■ere
used, it was observed, that the residual fluid, though passed two
or three times through the filtre, still remained more or less
turbid and opaque; so that it is most likely that the deficiency
arose from the continued suspension of some of the minutely
divided solid matter in the liquid mass.

The solutions of gelatine, for the purposes of analysis, should
be employed only when quite fresh, and in as high a state of
saturation as is compatible with their perfect fluidity. I have
observed, that in cases when they approach towards the state of
jelly, their power of acting upon tannin is materially altered,
and they produce only a very slight precipitation. As the de-
gree of fluidity of solutions of gelatine is influenced by their
temperature, I have found it expedient, in all comparative ex-
periments, to bring them and the astringent infusions on which
they are designed to act, as nearly as possible to a common
degree of heat. My standard temperature has been between 6’o

H h 3

*3^ Davy’s Experiments a?id Observations

and 70° Fahrenheit; and the solutions of gelatine that I have
used, were made by dissolving 120 grains of isinglass in 20
ounces of water.

In ascertaining the proportions of tannin in astringent infu-
sions, great care must be taken to prevent the presence of any
excess of gelatine; for, when this excess exists, I have found
that a small portion of the solid compound formed is redissolved,
and the results of the experiment otherwise affected. It is not
difficult to discover the precise point of saturation, if the solution
of isinglass be added only in small quantities at a time, and if
portions of the clear liquor be passed through a filtre at different
periods of the process. The properties of these portions will
indicate the quantities of the solution of gelatine required for
the completion of the experiment. _

That the composition of any precipitate containing tannin and
gelatine may be known with a tolerable degree of precision, it
is necessary that the isinglass employed in the solution, and the
new compound formed, be brought as nearly as possible to the
same degree of dryness. For this purpose, I have generally ex-
posed them, for an equal time, upon the low^er plate of a sand-
bath, which was seldom heated to more than 150°. This method
I have found much better than that of drying at the temperatures
of the atmosphere, as the different states of the air, with regard
to moisture, materially influence the results.

Mr. Hatchett has noticed, in his excellent Paper on Zoo-
phytes, &c.* that isinglass is almost wholly composed of gelatine.

I have found, that 100 grains of good and dry isinglass contain
rather more than 98 grains of matter soluble in water. So that,
when the quantity of isinglass, in any solution employed for

• Phil. Trans, for 1800, page 327.

on the constituent Parts of astringent Vegetables. 237

acting upon an astringent infusion, is compared with the quan-
tity of the precipitate obtained, the difference between them will
indicate the proportion of tannin, as it exists in the combination.

After the tannin has been separated from an astringent in-
fusion, for the purpose of ascertaining its other component parts,
I have been accustomed to evaporate the residual liquor very
slowly, at a temperature below 200°.* In this process, if it
contains extractive matter, that substance is in part rendered
insoluble, so as to fall to the bottom of the vessel. When the
fluid is reduced to a thick consistence, I pour alcohol upon it.
If any gallic acid or soluble extractive matter be present, they
will be dissolved, after a little agitation, in the alcohol; whilst
the mucilage, if any exist, will remain unaltered, and may be
separated from the insoluble extract, by lixiviation with water.

I have made many experiments, with the hope of discovering
a method by which the respective quantities of gallic acid and
extractive matter, when they exist in solution in the alcohol,
may be ascertained ; but without obtaining success in the results.
It is impossible to render the whole of any quantity of extrac-
tive matter insoluble by exposure to heat and air, without at the
same time decomposing a portion of the gallic acid. That acid
cannot be sublimed, without being in part destroyed ; and, at
the temperature of its sublimation, extractive matter is wholly
converted into new products.

Ether dissolves gallic acid; but it has comparatively little

* M. Deyeux has shewn, ( Annales de Cbimie, Tome XVII. page 36,) that in
tlie pirocess of evaporating solutions of galls, no gallic acid is carried over by the
water, at a temperature below that of ebullition. Many astringent infusions, however,
lose a portion of their aromatic principle, even in cases when they are not made to
boil ; but this substance, though evident to the smell, in the water that comes over,
cannot be detected by chemical reagents.

238 M}\ Davy’s Experiments and Observations

action upon extractive matter, I have been able, in examining
solutions of galls, to separate a portion of gallic acid by
means of ether. But, when the extractive matter is in large
quantities, tliis method does not succeed, as, in consequence of
that affinity which is connected with mass,* the greatest part of
the acid continues to adliere to the extract,

Alumine has a strong attraction for extractive matter; but
comparatively a weak one for gallic acid.-f When carbonate of
alumine is boiled for some time with a solution containing ex-
tractive matter, the extractive matter is wholly taken up by the
earth, with which it forms an insoluble compound; but, into
this compound, some of the gallic acid appears likewise to enter;
and the portion remaining dissolved in the solution is always
combined with alumine.

I have not, in any instance, been able to separate gallic acid
and extractive matter perfectly from each other; but I have
generally endeavoured to form some judgment concerning their
relative proportions, by means of the action of the salts of alu-
mine, and the oxygenated salts of iron. Muriate of alumine
precipitates much of the extractive matter from solutions, without
acting materially upon gallic acid; and, after this precipitation,
some idea may be formed concerning the quantity of the gallic
acid, by the colour it gives with the oxygenated sulphate of iron.
In this process, however, great care must be taken not to add
the solution of the sulphate of iron in excess ; for, in this case,
the black precipitate formed with the gallic acid will be redis-
solved, and a clear olive-coloured fluid only will be obtained.

* See Berthollet, Recherches sur les Lois de V Affiniie Mem. de I'Listitul ISfa^
tional. Tome 111. p. 5.

f See Fiedler, Journal de Cbimie, par ]. B. Van-mons, Tome I. pag. 85.

on the constituent Parts of astringent Vegetables. 239

The saline matters in astringent infusions, adhere so strongly
to tlie vegetable principles, that it is impossible to ascertain their
nature with any degree of accuracy, by means of common re-
agents. By incineration of the products obtained from the eva-
poration of astringent infusions, I have usually procured carbo-
nate of lime and carbonate of potash.

In the different analyses, as will be seen from the results
given in the following sections, I have attended chiefly to the
proportions of the tanning principle, and of the principles pre-
cipitable by the salts of iron, as being most connected with
practical applications.

With regard to the knowledge of the nature of the different
substances, as they exist in the primitive astringent infusion, we
can gain, by our artificial methods of examination, only very
imperfect approximations. In acting upon them by reagents,
we probably, in many cases, alter their nature; and very few
of them only can be obtained in an uncombined state. The
comparisoh, however, of the products of different experiments
with each other, is always connected with some useful conclu-
sions ; and the accumulation of facts with regard to the subject,
must finally tend to elucidate this obscure but most interesting
part of chemistry.

II. EXPERIMENTS ON THE INFUSIONS OF GALLS.

I have been very much assisted in my inquiries concerning
the properties of the infusions of galls, by the able Memoir of
M. Deyeux, on galls.*

The strongest infusion of galls that I could obtain, at 5^°
* Annales de Chimie, Tome XVII, pag. 1.

240 Davy’s Experiments and Observations

Fahrenheit, by repeatedly pouring distilled water upon the
best Aleppo galls broken into small pieces, and suffering it to
remain in contact witli them till the saturation was complete,
was of the specific gravity i.o58. Four hundred grains of it pro-
duced, by evaporation at a temperature below 200°, fifty-three
grains of solid matter ; which, as well as I could estimate, by the
methods of analysis that have been just described, consisted of
about ^ of tannin, or matter precipitable by gelatine, and
of gallic acid, united to a minute portion of extractive matter.

100 grains of the solid matter obtained from the infusion,
left, after incineration, nearly 4^ grains of ashes ; which were
chiefly calcareous matter, mixed with a small portion of fixed
alkali. The infusion strongly reddened paper tinged with litmus.
It was semitransparent, and of a yellowish-brown colour.
Its taste was highly astringent.

When sulphuric acid was poured into the infusion, a dense
whitish precipitate was produced; and this effect was constant,
whatever quantity of the acid was used. The residual liquor,
when passed through the filtre, was found of a shade of colour
deeper than before. It precipitated gelatine, and gave a dark
colour with the oxygenated sulphate of iron.

The solid matter remaining on the filtre, slightly reddened
vegetable blues ; and, when dissolved in warm water, copiously
precipitated the solutions of isinglass. M. Proust,* who first
paid attention to its properties, supposes that it is a compound
of the acid with tannin ; but I suspect that it also contains
gallic acid, and probably a small portion of extractive matter.

* The fact of the precipitation of solution of galls by acids, was noticed by M.
Dize'. See Annales de Chimie, Tome XXXV. p. 37,

on the constituent Parts of astringent Vegetables. 241

This last substance, as is well known, is thrown down from its
solutions by sulphuric acid ; and I found, in distilling the pre-
cipitate from galls by sulphuric acid, at a heat above 212°, that
a fluid came over, of a light yellow colour, which was ren-
dered black by oxygenated sulphate of iron ; but which was not
altered by gelatine.

Muriatic acid produced, in the infusion, effects analogous to
those produced by sulphuric acid ; and two compounds of the
acid and the vegetable substances were formed ; the one united
to excess of acid, which remained in solution ; the other con-
taining a considerable quantity of tannin, which was precipi-
tated in the solid form.

When concentrated nitric acid was made to act upon the
infusion, it was rendered turbid; but the solid matter formed was
immediately dissolved with effervescence, and the liquor then be-
came clear, and of an orange colour. On examining it, it was
found that both the tannin and the gallic acid were destroyed ;
for it gave no precipitate, either with gelatine or the salts of iron,
even after the residual nitric acid was saturated by an alkali.
By evaporation of a portion of the fluid, a soft substance was
obtained, of a yellowish-brown colour, and of a slightly sourish
taste. It was soluble in water, and precipitated the nitro-muriate
of tin, and the nitrate of aluraine; so that its properties approached
to those of extractive matter ; and it probably contained oxalic
acid, as it rendered turbid a solution of muriate of lime.

When a very weak solution of nitric acid was mixed with the
infusion, a permanent precipitate was formed ; and the residual
liquor, examined by the solution of gelatine, was found to con-
tain tannin.

A solution of pure potash was poured into a portion of the

MDCCCIII. I i

24,2 Mr. Davy’s Experiments and Observations

infusion. At first, a faint turbid appearance was perceived; but,
by agitation, the fluid became clear, and its colour changed from
yellow brown to brown red ; and this last tint was most vivid
on the surface, where the solution was exposed to the atmo-
sphere. The solution of isinglass did not act upon the infusion
modified by the alkali, till an acid was added in excess, when a
copious precipitation was occasioned.

The compound of potash and solution of galls, when eva-
porated, appeared in the form of an olive-coloured mass, which
h.ad a faint alkaline taste, and which slowly deliquesced when
exposed to the air.

Soda acted upon the infusion in the same manner as potash;
and a fluid was formed, of a red-brown colour, which gave no
precipitate to gelatine.

Solution of ammonia produced the same colour as potash and
soda, and formed so perfect an union with the tannin of the
infusion, that it was not acted upon by gelatine. When the
compound liquor was exposed to the heat of boiling water, a
part of the ammonia flew off, and another part reacted upon
the infusion, so as to effect a material change in its properties.
A considerable quantity of insoluble matter was formed ; and
the remaining liquor contained little tannin and gallic acid, but
a considerable portion of a substance that precipitated muriate
of tin, and the salts of alumine.

When the experiment on the ebullition of the com.pound of
the infusion and ammonia was made in close vessels, the liquor
that came over was strongly impregnated with ammonia ;
its colour was light yellow, and, when saturated with an acid,
it was very little altered by the salts of iron. The residual fluid,
after the process had been continued for some time, as in the

on the constituejit Parts of astringent Vegetables. 243

other case, precipitated gelatine slightly, but the salts ofalumine
copiously ; and it gave a tinge of red to litmus paper.

When solution of lime, of strontia, or of barytes, was poured
in excess into a portion of the infusion, a copious olive-coloured
precipitate was formed, and the solution became almost clear,
and of a reddish tint. In this case, the tannin, the gallic acid, and
the extractive matter, seemed to be almost wholly carried down
in the precipitates; as the residual fluids, when saturated by an
acid, gave no precipitate to gelatine, and only a very slight tint
of purple to oxygenated sulphate of iron.

When the solutions of the alkaline earths were used only in
small quantities, the infusion being in excess, a smaller quantity
of precipitate was formed, and the residual liquor was of an
olive-green colour ; the tint being darkest in the experiment
with the barytes, and lightest in that with the lime. This fluid,
when examined, was found to hold in solution a compound of
gallic acid and alkaline earth. It became turbid when acted on
by a little sulphuric acid ; and, after being filtrated, gave a black
colour with the solutions of iron, but was not acted upon by
gelatine.

When a large proportion of lime was heated for some time
with the infusion, it combined with all its constituent principles,
and gave, by washing, a fluid which had the taste of lime-water,
and which held in solution only a very small quantity of ve-
getable matter. Its colour was pale yellow ; and, when saturated
with muriatic acid, it did not precipitate gelatine, and gave only
a slight purple tinge to the solutions of the salts of iron. The
lime in combination with the solid matter of the infusion, was of
a fawn colour. It became green at its surface, where it was ex-
posed to the air; and, wdien washed with large quantities of water,

I i 2

244 Davy’s Experhnents and Observations

it continued to give, even to the last portions, a pale yellow
tinge.

Magnesia was boiled in one portion of the infusion for a few
hours ; and mixed in excess with another portion, which was
suiFered to remain cold. In both cases, a deep green fluid was
obtained, which precipitated the salts of iron, but not the solu-
tions of gelatine; and the magnesia had acquired a grayish-
green tint. Water poured upon it became green, and acquired
the properties of the fluid at first obtained. After long washing,
the colour of the magnesia changed to dirty yellow ; and the
last portions of water made to act upon it were pale yellow, and
altered very little the solutions of iron. >

When the magnesia was dissolved in muriatic acid, a brownish
and turbid fluid was obtained, which precipitated gelatine and
the oxygenated salts of iron. So that there is every reason to
believe, that the earth, in acting on the astringent infusion, had
formed two combinations ; one containing chiefly gallic acid,
which was easily soluble in water; the other containing chiefly
tannin, which was very diflicultly soluble.

Alumine boiled with the infusion became yellowish-gray, and
gave a clear white fluid, which produced only a tinge of light
purple in the solutions of iron. When the earth* was employed
in very small quantity, however, it formed an insoluble com-
pound only with the tannin and the extract; and the residual liquor
was found to contain a gallate of alumine with excess of acid.

The oxides of tin and of zinc, obtained by nitric acid, were
boiled with separate portions of the infusion for two hours. In
both cases, a clear fluid, which appeared to be pure water, was

* Mr. Fiedler, I believe, first observed the action of alumine upon tannin. Van-
Mgns’s Journal, Vol. I, page 86.

on the constituent Parts of astringent Vegetables. 245

obtained ; and the oxides gained a tint of dull yellow. A part of
each of them was dissolved in muriatic acid. The solution ob-
tained was yellow: it copiously precipitated gelatine; and gave
a dense black with the salts of iron. Mr. Proust,* who first
observed the action of oxide of tin upon astringent infusions,
supposes that portions of tannin and gallic acid are decomposed
in the process, or converted, by the oxygen of the oxide, into
new substances. These experiments do not, however, appear to
confirm the supposition.

M. Deyeux observed, that a copious precipitation was
occasioned in infusion of galls, by solutions of the alkalis
combined with carbonic acid. Mr. Proust has supposed that
the solid matter formed is pure tannin, separated from its so-
lution by the stronger affinity of the alkali for water ; and he
recommends the process, as a method of obtaining tannin.

In examining the precipitate obtained by carbonate of potash
fully combined with carbonic acid, and used to saturation, I
have not been able to recognise in it the properties which are
usually ascribed to tannin : it is not possessed of the astringent
taste; and it is but slightly soluble in cold water, or in alcohol.
Its solution acts very little upon gelatine, till it is saturated with
an acid; and it is not possessed of the property of tanning skin.

In various cases, in which the greatest care was taken to use
no excess, either of the astringent infusion or of the alkaline
solution, I have found the solid matter obtained possessed of
analogous properties ; and it has always given, by incineration,
a considerable portion of carbonate of potash, and a small quan-
tity of carbonate pf lime.

The fluid remaining after the separation Tf the precipitate,

* Annales de Cbimic, Tome XLII. p. 69.

2^6 Mr. Davy’s Experime?iis and Observations

was of a dark-brown colour, and became green at the surface,
when it was exposed to the air. It gave no precipitate to solu-
tion of gelatine ; and afforded only an olive-coloured precipi-
tate with the salts of iron.

When muriatic acid was poured into the clear fluid, a violent
effervescence was produced ; the fluid became turbid; a precipi-
tate was deposited ; and the residual liquor acted upon gelatine and
tlie salts of iron, in a manner similar to the primitive infusion.

M. Deyeux, in distilling the precipitate from infusion of galls
by carbonate of potash, obtained crystals of gallic acid. In fol-
lowing his process, I had similar results ; and a fluid came over,
which reddened litmus-paper, and precipitated the salts of iron
black, but did not act upon gelatine.

When the precipitate by carbonate of potash was acted upon
by warm water, applied in large quantities, a considerable por-
tion of it was dissolved ; but a part remained, which could not
in any way be made to enter into solution ; and its properties
were very different from those of the entire precipitate. It was
not at all affected by alcohol ; it was acted on by muriatic acid,
and partially dissolved ; and the solution precipitated gelatine
and the salts of iron. It afforded, by incineration, a considerable
portion of lime, but no alkali.

In comparing these facts, it would seem, that the precipitate
from infusion of galls, consists partly of tannin and gallic acid
united to a small quantity of alkali, and partly of these vegetable
matters combined with calcareous earth ; and it will appear pro-
bable, when the facts hereafter detailed are examined, that both
'the potash and the lime arc contained in these compounds in a
state of union with carbonic acid.

The solutions of carbonate of soda and of carbonate of

071 the co7istituent Pai'ts of astringe7it Vegetables. 24^'/

ammonia, both precipitated the infusion of galls in a manner
similar to the carbonate of potash ; and each of the precipitates,
when acted on by boiling water, left a small quantity of insoluble
matter, which seemed to consist chiefly of tannin and carbonate
of lime.

The entire precipitate by carbonate of soda produced, wlieii
incinerated, carbonate of soda and carbonate of lime. The pre-
cipitate by carbonate of ammonia, when exposed to a heat
sufficient to boil water, in a retort having a receiver attached to
it, gave out carbonate of ammonia, (which was condensed in
small crystals in the neck of the retort,) and a yellowish fluid,
which had the strong smell and taste of this volatile salt. After
the process of distillation, the solid matter remaining was found
of a dark brown colour; a part of it readily dissolved in cold
water, and the solution acted on gelatine.

The residual fluid of the portions of the infusion which had
been acted on by the carbonates of soda and of ammonia, as in
the instance of the carbonate of potash, gave no precipitate with
gelatine, till they were saturated with an acid; so that, in all
these cases, the changes are strictly analogous.

The infusion of galls, as appears from the analysis, contains
in its primitive state calcareous matter. By the action of the
mild alkalis, this substance is precipitated in union with a por-
tion of the vegetable matter, in the form of an insoluble com-
pound. The alkalis themselves, at the same time, enter into
actual combination with the remaining tannin and gallic acid ;
and a part of the compound formed is precipitated, whilst anotiier
part remains in solution.

When the artifleial carbonates of lime, magnesia, and barytes,
were separately boiled with portions of the infusion of galls for

24,8 Mr. Davy’s Experiments and Observations

some hours, they combined with the tannin contained in it, so
as to form with it insoluble compounds ; and, in each case, a
deep green fluid w'as obtained, w'hich gave no precipitate to
gelatine, even when an acid was added, but which produced ti
deep black colour in the solutions of the salts of iron.

Sulphate of lime, when finely divided, whether natural or
artificial, after having been long heated with a small quantity
of the infusion, was found to have combined with the tannin of
it, and to have gained a faint tinge of light brown. The liquid
became of a blue-green colour, and acted upon the salts of iron,
but not upon gelatine ; and there is every reason to suppose,
that it held in solution a triple compound, of gallic acid, sulphuric
acid, and lime.

We owe to Mr. Proust, the discovery that different solutions
of the neutral salts precipitate the infusion of galls ; and he
supposes, that the precipitation is owing to their combining with
a portion of the water which held the vegetable matter in solu-
tion. In examining the solid matters thrown down from the
infusion, by sulphate of alumine, nitrate of potash, acetite of
potash, muriate of soda, and muriate of barytes, I found them
soluble, to a certain extent, in water, and possessed of the power
of acting upon gelatine. From the products given by their inci-
neration, and by their distillation, 1 am however inclined to
believe that they contahi, besides tannin, a portion of gallic acid
and extractive matter, and a quantity of the salt employed in the
primitive solution.

It is well known, that many of the metallic solutions occasion
dense precipitates in the infusion of galls; and it has been gene-
rally supposed, that these precipitates are composed of tannin and
extractive matter, or of those tw'o substances and gallic acid, united

on the constituent Parts oj astringent Vegetables. 249

to the metallic oxide ; but, from the observation of different pro-
cesses of this kind, in which the salts of iron and of tin were
employed, I am inclined to believe, that they contain also a
portion of the acid of the saline compound.

When the muriate of tin was made to act upon a portion of
the infusion, till no more precipitation could be produced in it, the
fluid that passed through the filtre still acted upon gelatine, and
seemed to contain no excess of acid; for it gave a precipitate
to carbonate of potash, without producing effervescence. The
solid compound, when decomposed by sulphuretted hydrogen,
after the manner recommended by Mr. Proust, was found
strongly to redden litmus-paper, and it copiously precipitated
nitrate of silver ; whereas, the primitive infusion only rendered
it slightly turbid ; so that there is every reason to believe, that
the precipitate contained muriatic acid.

By passing the black and turbid fluid, procured by the action
of solution of oxygenated sulphate of iron in excess upon a
portion of the infusion, through finely-divided pure flint, con-
tained in four folds of filtrating paper, I obtained a light olive-
green fluid, in which there was no excess of sulphuric acid, and
which I am inclined to suppose was a solution of the compound
of gallic acid and sulphate of iron, with superabundance of me-
tallic salt. I have already mentioned that gallic acid, when in
very small proportion, does not precipitate the oxygenated salts
of iron ; and Mr. Proust, in his ingenious Paper upon the Dif-
ference of the Salts of Iron, has supposed that, in the formation
of ink, a portion of the oxide of iron in union with gallic acid
is dissolved by the sulphuric acid of the sulphate. This comes
near to the opinion that they form a triple compound : and, in
reasoning upon the general phenomena, it seems fair to

MDCCCIII. K k

2^0 Mr. Davy's Experiments and Observations

conclude, that, in the case of the precipitation of tannin by the
salts of tin and of iron, compounds are formed, of tannin and
the salts ; and that, of these compounds, such as contain tin are
Slightly soluble in water, whilst those that contain iron are
almost wholly insoluble.

In examining the action of animal substances upon the infu-
sion of galls, with the view of ascertaining the composition cf
th.e compounds of gelatine, and of skin, with tannin, I found that
a saturated solution of gelatine, which contained the soluble
matter of 50 grains of dry isinglass, produced from the infusion
a precipitate that weighed nearly 91 grains ; and, in another
instance, a solution containing 30 grains of isinglass, gave about
56 grains ; so that, taking the mean of the tv/o experiments,
and allowing for the small quantity of insoluble matter in
isinglass, we may conclude, that 100 grains of the compound of
gelatine and tannin, formed by precipitation from saturated
solutions, contain about 54, grains of gelatine, and 46 of tannin.

A piece of dry calf-skin, perfectly free from extraneous matter,
that weighed 180 grains, after being prepared for tanning by
long immersion in water, was tanned in a portion of the infu-
sion, being exposed to it for three weeks. When dry, the leather
weighed 295 grains ; so that, considering this experiment as
accurate, leather quickly tanned by means of an infusion of
galls, consists of about bi grains of skin, and 39 of vegetable
matter, in 100 grains.

After depriving a portion of the infusion of all its tanning
matter, by repeatedly exposing it to the action of pieces of skin,
I found that it gave a much slighter colour to oxygenated sul-
piiateof iron, than an equal portion of a similar infusion which
had been immediately precipitated by solution of isinglass;

on the constituent Parts of astringent Fegetables. 25 <

but I am inclined to attribute this effect, not to any absorption .
of gallic acid by the skin, but rather to the decomposition of it
by the long continued action of the atmosphere; for much in-
soluble matter had been precipitated, during the process of
tanning, and the residuum contained a small portion of acetous
acid.

In ascertaining the quantity of tannin in galls, I found that
500 grains of good Aleppo galls gave, by lixiviation with pure
water till their soluble parts were taken up, and subsequent slov/
evaporation, 185 grains of solid matter. And this matter, exa-
mined by analysis, appeared to consist,

Of tannin - - - - 130 grains.

Of mucilage, and matter rendered insoluble by
evaporation - - - - 12

Of gallic acid, with a little extractive matter - 31

Remainder, calcareous earth and saline matter 12
The fluid obtained by the last lixiviation of galls, as M.
Deyeux observed, is pale green ; and I am inclined to believe,
that it is chiefly a weak solution of gallate of lime. The ashes
of galls, deprived of soluble matter, furnish a very considerable
quantity of calcareous earth. And the property which M.
Deyeux discovered in the liquor of the last lixiviations, of be-
coming red by the action of acids, and of regaining tlie green
colour by means of alkalis, I have observed, more or less, in all
the soluble compounds containing gallic acid and the alkaline
earths.

Kk 2

252

Mr. Davy's Experiments and Observations

III. EXPERIMENTS AND OBSERVATIONS ON CATECHU OR TERRA

JAPONICA.

The extract called catechu is said to be obtained from the
wood of a species of the Mimosa,* which is found abundantly
in India, by decoction and subsequent evaporation.

There are two kinds of this extract ; one is sent from Bom-
bay, the other from Bengal ; and they differ from each other
more in their external appearance than in their chemical com-
position. The extract from Bombay is of an uniform texture,
and of a red-brown tint, its specific gravity being generally
about 1.39. The extract from Bengal is more friable, and less
consistent ; its colour is like that of chocolate externally, but,
when broken, its fracture presents streaks of chocolate and of
red-brown. Its specific gravity is about 1.28. Their tastes are
precisely similar, being astringent, but leaving in the mouth a
sensation of sweetness. They do not deliquesce, or apparently
change, by exposure to the air.

The discovery of the tanning powers of catechu, is owing to
the President of the Royal Society, who, concluding from its
sensible properties that it contained tannin, furnished me, in
December, 1801, with a quantity for chemical examination.

In my first experiments, I found that the solutions of catechu
copiously precipitated gelatine, and speedily tanned skin ; and,
in consequence, I began a particular investigation of their
properties.

The strongest infusions and decoctions of the two different
kinds of catechu, do not sensibly differ in their nature, or in
their composition. Their colour is deep red-brown, and they

* See Kerr. Medical Observations, Vol. V. page 155.

on the constituent Parts of astringent Vegetables. 253

communicate this tinge to paper ; they slightly redden litmus-
paper; their taste is highly astringent, and they have no per-
ceptible smell.

The strongest infusions tliat I could obtain from the two
kinds of catechu, at 48° Fahrenheit, were of the same specific
gravity, 1.037. decoction, I procured solutions of

1.102, which gave, by evaporation, more than of their weight
of solid matter.

Five hundred grains of the strongest infusion of catechu from
Bombay, furnished only 41 grains of solid matter; which,
from analysis, apjieared to consist of 34 grains of tannin, or
matter precipitable by gelatine, and 7 grains that were chiefly
a peculiar extractive matter, the properties of which will be
hereafter described. The quantity of solid matter given by the
strongest infusion of the Bengal catechu, was the same, and
there was no sensible difference in its composition. Portions
of these solid matters, when incinerated, left a residuum which
seemed to be calcareous ; but it was too small in quantity to be
accurately examined, and it could not have amounted to more
than of their original weights.

The strongest infusions of catechu acted upon the acids and
pure alkalis in a manner analogous to the infusion of galls.
With the concentrated sulphuric and muriatic acids, they gave
dense light fawn coloured precipitates. With strong nitrous
acid they effervesced ; and lost their power of precipitating tiie
solutions of isinglass, and the salts of iron. The pure alkalis
entered into union with their tannin, so as to prevent it from
being acted upr n by gelatine.

When the solutions of lime, of strontia, or of barytes, were

254) I^avy’s Experiments and Observations

poured into the infusions, copious precipitates, of a shade of light
brown, were formed; and the residual fluid assumed a paler
tint of red, and was found to have lost its power of precipitating
gelatine.

After lime had been boiled for some time with a portion of
the infusion, it assumed a dull red colour. The liquor that
passed from it through the filtre had only a faint tint of red,
did not act upon gelatine, and seemed to contain only a very
small portion of vegetable matter. Pure magnesia, when heated
with the infusion, acted upon it in an analogous manner; the
magnesia became light red, and the residual fluid had only a
very slight tinge of that colour. With carbonate of magnesia,
the infusion became deeper in colour, and lost its power of pre-
cipitating gelatine; though it still gave, with oxygenated sulphate
of iron, a light oliv-e precipitate.

The carbonates of potash, of soda, and of ammonia, in their
concentrated solutions, produced only a slight degree of turbid-
ness in the infusion of catechu; they communicated to them a
darker colour, and deprived them of the power of acting upon
gelatine ; tiiough this power was restored by the addition of an
acid.

After the mixture of the solution of carbonate of potash and
the infusions had been exposed to the atmosphere for some
hours, a brown crust was found to have formed upon its surface,
and a slight precipitation had taken place.

The salts of alumine precipitated the infusions, but less co-
piously than they precipitate the infusion of galls. A similar
effect was produced by nitrate of potash, sulphate of magnesia,
prussiate of potash, and many other neutral salts.

on the constituent Parts of astringent Vegetables. 255

The nitrate, or acetite, of lead, in concentrated solution, when
poured into the infusion, produced in it a dense light brown
precipitate, which gave to the fluid a gelatinous appearance.
After this effect, there was no free acid found in it; and both
the tannin and the extractive matter seemed to have been car-
ried down, in union w ith a portion of the metallic salt.

The solution of muriate of tin, acted upon the infusion of
catechu in a manner similar to that in which it acts upon the
infusion of galls.

The least oxygenated sulphate of iron produced no change in
the infusion. With the most oxygenated sulphate it gave a dense
black precipitate, which, when diffused upon paper, appeared
rather more inclined to olive than the precipitate from galls.

Tl’.e infusions w’ere precipitated by the solution of albumen.

The precipitates by gelatine had all a pale tint of red-browm,
wdiich became deeper w'hen they were exposed to tlie air. The
compound of gelatine and the tannin of the strongest infusions
of catechu appeared, by estimation of the quantit}^ of isinglass
ill the solutions used for their precipitation, to consist of about
■41 parts of tannin, and 59 of gelatine.

Of two pieces of calf-skin which w^eighed, when dry, 132
grains each, and w’hicli Iiad been prepared for tanning, one was
immersed in a large quantity of the infusion of catechu from
Bengal, and the other in an equal portion of the infusion of
that from Bombay. In less than a montirtlicy were found con-
verted into leather. When freed from moisture, by long e.xposure
in the sunshine, tliey were weighed. The first piece had gained
about 34 grains; and the second piece 35I- grains. The leather
was of a much deeper colour than that tanned with galls,

2^6 Mr. Davy’s Experiments and Observations

and on the upper surface was red-brown. It was not acted on
by hot or cold water ; and its apparent strength was the same
as that of similar leather tanned in the usual manner.

In examining the remainder of the infusions of catechu, in
which skin had been converted into leather, I found in them
much less extractive matter than I had reason to expect, from
the comparative analysis of equal portions of the unaltered in-
fusions made by solutions of gelatine. At first, I was inclined
to suppose that the deficiency arose from the action of the
atmosphere upon- the extractive matter, by which a part of it
was rendered insoluble. But, on considering that there had been
very little precipitation in the process, I was led to adopt the
supposition, that it had entered into union with the skin, at the
same time with the tannin ; and this supposition was confirmed
by new experiments.

Both kinds of catechu are almost wholly soluble in large
quantities of water; and, to form a complete solution, about
i8 ounces of water, at 52°, are required to 100 grains of extract.
The residuum seldom amounts to of the original weight of
the catechu ; and, in most cases, it is found to consist (ihiefly of
calcareous and aluminous earths, and of fine sand, which, by
accident or design, had probably been mixed with the primitive
infusion at the time of its evaporation.

A considerable portion of both kinds of catechu is soluble in
alcohol ; but, after the action of alcohol upon it, a substance
remains, of a gelatinous appearance and a light brown colour,
which is soluble in water, and is analogous in its properties
to gum or mucilage.

The peculiar extractive matter in the catechu, is much less

on the constituent Parts of astringent Vegetables. 257

soluble in water than the tanning principle; and, when a small
quantity of water is used to a large quantity of catechu, the
quantity of tanmn taken up, as appears from the nature of the
'strongest infusion, is very much greater than that of the extrac-
tive matter

The extractive matter is much more soluble in warm water
than in cold water ; and, when saturated solutions of catechu
are made in boiling water, a considerable quantity of extrac-
tive matter, in its pure state, falls down, as the liquor becomes
cool.

The peculiar extractive matter of the catechu may be like-
wise obtained, by repeatedly lixiviating the catechu, when in
fine powder, till the fluids obtained cease to precipitate gelatine ;
the residual solid will then be found to be the substance in
question.

The pure extractive matter, whether procured from the
Bombay or Bengal catechu, is pale, with a faint tinge of red-
brown. It has no perceptible smell ; its taste is slightly astrin-
gent; but it leaves in the mouth, for some time, a sensation of
sweetness, stronger than that given by the catechu itself.

Its solution in water is at first yellow-brown ; but it gains a
tint of red by exposure to the air. Its solution in alcohol does
not materially change colour in the atmosphere ; and it is of an
uniform dull brown.

The extractive matter, whether solid or in solution, was not
found to produce any change of colour upon vegetable blues.

It became of a brighter colour by the action of the alkalis ; but
it was not precipitated from its solution in water Dy these bodies,
nor by the alkaline earths.

L 1

MDCCCIII.

2^8 Mr. Davy’s Experiments and Observations

The aqueous solution of it, when mixed \vith solutions of
nitrate of alumine and of muriate of tin, became slightly turbid.

To nitrate of lead, it gave a dense light brown precipitate.

It was not perceptibly acted upon by solution of gelatine;
but, when solution of sulphate of alumine was added to the
mixture of the two fluids, a considerable quantity of solid matter,
of a light brown colour, was immediately deposited.

To the solution of oxygenated sulphate of iron, it communi-
cated a fine grass-green tint; and a green precipitate was de-
posited, which became black by exposure to the air.

It was not precipitated by the mineral acids.

Linen, by being boiled in the strongest solution of the ex-
tractive matter, acquired a light red-brown tint. The liquor
became almost colourless ; and, after this, produced very little
change in the solution of oxygenated sulphate of iron.

Raw skin, prepared for tanning by being immersed in the
strong solution, soon acquired the same kind of tint as the linen.
It united itself to a part of the extractive matter; but it was not
rendered by it insoluble in boiling water.

The solid extractive matter, when exposed to heat, softened,
and became darker in its colour, but did not enter into fusion.
At a temperature below that of ignition, it was decompounded.
The volatile products of its decomposition were, carbonic acid,
hydrocarbonate, and water holding in solution acetous acid and
a little unaltered extractive matter. There remained a lig-ht and
very porous charcoal.

In considering the manner in which the catechu is prepared,
it would be reasonable to conclude, that different specimens of
that substance must differ in some measure in their composition,

071 the constituent Parts of astringent V egetables. 259

even in their pure states ; and, for the purposes of Commerce,
they are often adulterated to a considerable extent, with sand
and earthy matter.*

In attempting to estimate the composition of the purest cate-
chu, I selected pieces from different specimens, with which I
was supplied by the President, and reduced them together into
powder ; mixing, however, only those pieces which were from
catechu of the same kind.

Two hundred grains of the pow'der procured in this way,
from the catechu of Bombay, afforded by analysis,

Grains.

Tannin - - - - -509

Peculiar extractive matter - - - 68

Mucilage - - - - _

Residual matter; chiefly sand and calcareous earth 10

The powder of the Bengal catechu gave, by similar methods
of analysis, in 200 grains,

Grains.

Tannin _ _ « « «

Peculiar extractive matter - - “73

Mucilage - - - - - 16

Residual matter; sand, with a small quantity of calca-
reous and aluminous earths - - - 14,

In examining those parts of the catechu from Bengal which
were differently coloured, I found the largest proportion of
tannin in the darkest part of the substance ; and most extractive
matter in the lightest part. It is probable that the inequality of
composition in this catechu, is owing to its being evaporated

• One specimen that I examined, of the terra japonica of commerce, furnished, by
Incineration, | of sand and earthy matter ; and another specimen, nearly i.

LI 2

26o Mr. Davy’s Experiments and Observations

and formed without much agitation ; in consequence of which,
the constituent parts of it that are least soluble, being first pre-
cipitated, appear in some measure distinct from the more soluble
parts, which assume the solid form at a later period of the
process.

From the observations of Mr. Kerr,* it would appear, that
the pale catechu is that most sought after in India ; and it is
evidently that which contains most extractive matter. The ex-
tractive matter seems to be the substance that gives to the
catechu the peculiar sweetness of taste which follows the impres-
sion of astringency ; and it is probably this sweetness of taste
which renders it so agreeable to the Hindoos, for the purpose
of chewing with the betle-nut.

IV. EXPERIMENTS AND OBSERVATIONS ON THE ASTRINGENT IN-
FUSIONS OF BARKS, AND OTHER VEGETABLE PRODUCTIONS.

The barks that I examined were furnished me by my friend
Samuel Purkis, Esq. of Brentford; they had been collected in
the proper season, and preserved with care.

In making the infusions, I employed the barks in coarse
powder; and, to expedite the solution, a heat of from loo to
120° Fahrenheit was applied. -

Tlie strongest infusions of the barks of the oak, of the Lei-
cester willow, and of the Spanish chesnut, were nearly of tiie
same specific gravity, 1.05. Tlieir tastes were alike, and strongly
astringent; they all reddened litmus-paper; the infusion of the
Spanish chesnut bark producing the highest tint; and that of
the Leicester willow bark the feeblest tint.

Two hundred grains of each of the infusions were submitted

^ Medical Observations, Vol. V, page 155.

on the constituent Parts of astringent Vegetables. 261

to evaporation ; and, in this process, tlie infusion of the oak
bark furnished 17 grains of solid matter; that of the Leicester
willow about i6f grains; and that of the Spanish chesnut nearly
an equal quantity.

The tannin given by these solid matters was, in that from the
oak bark infusion, 14 grains; in that from the willow bark
infusion 14^ grains; and in that from the Spanish chesnut bark
infusion 13 grains.

The residual substances of the infusions of the Spanish chesnut
bark, and of the oak bark, slightly reddened litmus- paper, and
precipitated the solutions of tin of a fawn colour, and those of
iron black. The residual matter of the infusion of the willow
bark, did not perceptibly change the colour of litmus ; but it
precipitated the salts of iron of an olive colour, and rendered
turbid the solution of nitrate of alumine.

The solid matters produced by the evaporation of the infu-
sions, gave, by incineration, only a very small quantity of ashes,
which could not have been more than of their original
weights. These ashes chiefly consisted of calcareous earth and
alkali ; and the quantity was greatest from the infusion of
chesnut bark.

The infusions were acted on by the acids, and the pure alkalis,
in a manner very similar to the infusion of galls. With the so -
lutions of carbonated alkalis, they gave dense fawn-coloured
precipitates. They were copiously precipitated by the solutions of
lime, of strontia, and of barytes; and, by lime-water in excess,
the infusions of oak and of chesnut bark seemed to be deprived
of the whole of the vegetable matter they held in solution.

By being boiled for some time with alumine, lime, and mag-
nesia, they became almost colourless, and lost their power of

2^2 Mr, Davyds Experiments and Observations

acting upon gelatine and the salts of iron. After being heated
with carbonate of lime and carbonate of magnesia, they were
found deeper coloured than before; and, though they had lost
their power of acting on gelatine, they still gave dense olive-
coloured precipitates with the salts of iron.

In all these cases, the earths gained tints of brawn, more or
less intense.

When the compound of the astringent principles of the in-
fusion of oak bark with lime, procured by means of lime-water,
was acted on by sulphuric acid, a solution was obtained, which
precipitated gelatine, and contained a portion of the vegetable
principles, and a certain quantity of sulphate of lime; a solid
fawn-coloured matter was likewise formed, which appeared
to be sulphate of lime, united to a little tannin and extractive
matter.^

The solutions were copiously precipitated by solution of
albumen.

The precipitates they gave with gelatine were similar in their
appearance ; their colour, at first, was a light tinge of brown, but
they became very dark by exposure to the air. Their composition
was very nearly similar ; and, judging from the experiments
on the quantity of gelatine employed in forming them, the
compound of tannin and gelatine from the strongest infusion
of oak bark, seems to consist, in the lOO parts, of 59 parts of

* M. Merat Guillot proposes a method of procuring pure tannin, ( Annales de
Chim'ie, Tome XLI. p. 325.) which consists in precipitating a solution of tan by
lime-water, and decomposing it by nitric or muriatic acid. The solution of the
solid matter obtained in this way in alcohol, he considers as a solution of pure tannin ;
but, from the experiments abovementioned, it appears, that it must contain, besides
tannin, some of the extractive matter of the bark ; and it may likewise contain saline
matter.

on the constituent Parts of astringent Vegetables. 2^53

gelatine and 41 of tannin; that from the infusion of Leicester
willow bark, of 57 parts of gelatine and 43 of tannin ; and that
from the infusion of Spanish chesnut bark, of 61 parts of gelatine
and 39 of tanniiv.

Two pieces of calf-skin, which weighed when dry 120 grains
each, were tanned; one in the strongest infusion of Leicester
willow bark, and the other in the strongest infusion of oak bark.
The process was completed, in both instances, in less than a
fortnight ; when the weight of the leather formed by the tannin
of the Leicester willow bark was found equal to 161 grains;
and that of the leather formed by the infusion of oak bark was
equal to 164 grains.

When pieces of skin were suffered to remain in small quan-
tities'of the infusions of the oak bark, and of the Leicester willow
bark, till they were exhausted of their tanning principle, it was
found, that though the residual liquors gave olive-coloured pre-
cipitates with the solutions of sulphate of iron, yet they were
scarcely rendered turbid by solutions of muriate of tin ; and
there is every reason to suppose, that a portion of their extractive
matter had been taken up with the tannin by the skin.

I attempted, in different modes, to obtain uncombined gallic
acid from the solid matter produced by the evaporation of the
barks, but without success. When portions of this solid matter
were exposed to the degree of heat that is required for the pro-
duction of gallic acid from Aleppo galls, no crystals were formed;
and the fluid that came over gave only a brown colour to the
solution of salts of iron, and W'as found to contain much acetous
acid and empyreumatic oil.

When pure water was made to act, in successive portions,
upon oak bark in coarse p owder, till all its soluble parts were

26*4 Mr. Davy’s Experiments and Observatmis

taken up, the quantities of liquor last obtained, though they did
not act much upon solution of gelatine, or perceptibly redden
litmus-paper, produced a dense black with the solution of sul-
phate of iron : by evaporation, they furnished a brown matter,
of which a part was rendered insoluble in water by the action of
the atmosphere; and the part soluble in water was not in any
degree taken up by sulphuric ether ; so that, if it contained gallic
acid, it was in a state of intimate union with extractive matter.

Two pieces of calf-skin, which weighed when dry 94 grains
each, were slowly tanned ; one by being exposed to a weak in-
fusion of the Leicester willow bark, and the other by being acted
upon by a weak infusion of oak bark. The process was completed
in about three months ; and it was found, that one piece of skin
had gained in weight 14 grains, and the other piece about
grains. This increase is proportionally much less than that which
took place in the experiment on the process of quick tanning.
The colour of the pieces of leather was deeper than that of the
pieces which had been quickly tanned ; and, to judge from the
properties of the residual liquors, more of the extractive matters
of the barks had been combined with them.

The experiments of Mr. Biggin* have shown, that similar
barks, when taken from trees at different seasons, differ as to
the quantities of tannin they contain : and 1 have observed, that
the proportions of the astringent principles in barks, vary con-
siderably according as their age and size are different; besides,
these proportions are often influenced by accidental circum-
stances, so that it is extremely difficult to ascertain their distinct
relations to each other.

In every astringent bark, the interior white bark (that is, the

• Phil. Trans, for 1799, p. 299.

on the constituent Parts of astringent Vegetables. 265

part next to the alburnum) contains the largest quantity of
tannin. The proportion of extractive matter is generally greatest
in the middle or coloured part : but the epidermis seldom fur-
nishes either tannin or extractive matter.

The 'vdiite cortical layers are comparatively most abundant in
young trees ; and hence their barks contain, in the same weight,
a larger proportion of tannin than the barks of old trees. In
barks of the same kind, but of different ages, which have been
cut at the same season, the similar parts contain always very
nearly the same quantities of astringent principles; and the
interior layers afford about equal portions of tannin.

An ounce of the white cortical layers of old oak bark, fur-
nished, by lixiviation and subsequent evaporation, 108 grains of
solid matter; and, of this, 72 grains were tannin. An equal
quantity of the white cortical layers of young oak produced 111
grains of solid matter, of which 77 were precipitated by gelatine.

An ounce of the interior part of the bark of the Spanish
chesnut, gave 8g grains of solid matter, containing 63 grains of
tannin.

The same quantity of the same part of the bark of the Lei-
cester willow, produced 117 grains, of which 79 were tannin.

An ounce of the coloured or external cortical layers from the
oak, produced 43 grains of solid matter, of which 19 were tannin.

From the Spanish chesnut, 41 grains, of which 14 were
tannin.

And, from the Leicester willow, 34 grains, of which 16 were
tannin.

In attempting to ascertain the relative quantities of tannin in
the different entire barks, I selected those specimens which
appeared similar with regard to the proportions of the external

MDcccni. M m

266' Mr. Davy’s Experiments and Observations

and internal layers, and which were about the average thickness
of the barks commonly used in tanning, namely, y an inch.

Of these barks, the oak produced, in the quantity of an ounce,
6i grains of matter dissolved by water, of which 29 grains were
tannin.

The Spanish chesnut 53 grains, of which 21 were tannin.

And the Leicester willow 71 grains, of which 33 were tannin.

The proportions of these quantities, in respect to the tanning
principle, are not very different from those estimated in Mr.
Biggin’s table.*

The residual substances obtained in the different experiments,
differed considerably in their properties ; but certain portions of
them were, in all instances, rendered insoluble during the process
of evaporation. The residuum of the chesnut bark, as in the
instance of the strongest infusion, possessed slightly acid pro-
perties ; but more than \ of its weight consisted of extractive
matter. All the residuums in solution, as in the other cases,
were precipitated by muriate of tin ; and, after this precipitation,
the clear fluids acted much more feebly than before on the salts
of iron; so that there is great reason for believing, that the
power of astringent infusions to precipitate the salts of iron
black, or dark coloured, depends partly upon the agency of
the extractive matters they contain, as well as upon that of the
tanning principle and gallic acid.

In pursuing the experiments upon the different astringent
infusions, I examined the infusions of the bark of the elm and
of the common willow. These infusions were acted on by re-
agents, in a manner exactly similar to the infusions of the other
barks : they were precipitated by the acids, by solutions of the

* Phil. Trans, for 1799, p. 263.

on the constituent Parts of astringent Vegetables. 267

alkaline earths, and of the carbonated alkalis ; and they formed,
with the caustic alkalis, fluids not precipitable by gelatine.

An ounce of the bark of the elm, furnished 13 grains of
tannin.

The same quantity of the bark of the common willow, gave
11 grains.

The residual matter of the bark of the elm, contained a con-
siderable portion of mucilage; and that of the bark of the
willow, a small quantity of bitter principle.

The strongest infusions of the sumachs from Sicily and
Malaga, agree with the infusions of barks, in most of their pro-
perties ; but they differ from all the other astringent infusions
that have been mentioned, in one respect ; they give dense pre-
cipitates with the caustic alkalis. Mr. Proust has shown, that
sumach contains abundance of sulphate of lime ; and it is pro-
bably to this substance that the peculiar effect is owing.

From an ounce of Sicilian sumach, I obtained 165 grains of
matter soluble in water, and, of this matter, 78 grains were
tannin.

An ounce of Malaga sumach, produced 156 grains of soluble
matter, of which 79 appeared to be tannin.

The infusion of Myrobalans* from the East Indies, differed
from the other astringent infusions chiefly by this circumstance,
that it effervesced with the carbonated alkalis ; and it gave with
them a dense precipitate, that was almost immediately redis-
solved. After the tannin had been precipitated from it by
gelatine, it strongly reddened litmus-paper, and gave a bright
black with the solutions of iron. I expected to be able to procure

• The Myrobalans used in these experiments are the fruit of the Terminalia Che-
bula. Retz. Obs. Bolan. Fasc. V. p. 31.

Mm2

268 Mr. Davy’s Experiments and Observations

gallic acid, by distillation, from the Myrobalans; but in this
I was mistaken ; they furnished only a pale yellow fluid, which
gave merely a slight olive tinge to solution of sulphate of
iron.

Skin was speedily tanned in the infusion of the Myrobalans ;
and the appearance of the leather was similar to the appearance
of that from galls.

The strongest infusions of the teas are very similar, in their
agencies upon chemical tests, to the infusions of catechu.

An ounce of Souchong tea, produced 48 grains of tannin.

The same quantity of green tea, gave 41 grains.

Dr. Maton has observed, that very little tannin is found in
cinchona, or in the other barks supposed to be possessed of
febrifuge properties. My experiments tend to confirm the ob-
servation. None of the infusions of the strongly bitter vegetable
substances that I have examined, give any precipitate to gelatine.
And the infusions of quassia, of gentian, of hops, and of cha-
momile, are scarcely affected by muriate of tin ; so that they
likewise contain very little extractive matter.

In all substances possessed of the astringent taste, there is
great reason to suspect the presence of tannin ; it even exists in
substances which contain sugar and vegetable acids. I have
found it in abundance in the juice of sloes; and my friend Mr.
Poole, of Stowey, has detected it in port wine.

V. GENERAL OBSERVATIONS,

Mr. Proust has supposed, in his Paper upon Tannin and
its Species,* that there exist different species of the tanning
principle, possessed of different properties, and different powers

* Annales dc Cb'mie, Tome XLI. p. 332.

on the constituent Parts of astringent Vegetables. 26g

of acting upon reagents, but all precipitable by gelatine. This
opinion is sufficiently conformable to the facts generally known
concerning the nature of the substances which are produced in
organised matter; but it cannot be considered as proved, till the
tannin in different vegetables has been examined in its pure or
insulated state. In all the vegetable infusions which have been
subjected to experiment, it exists in a state of union with other
principles; and its properties must necessarily be modified by
the peculiar circumstances of its combination.

From the experiments that have been detailed it appears, that
the specific agencies of tannin in all the different astringent
infusions are die same. In every instance, it is capable of en-
tering into union with the acids, alkalis, and earths ; and of
forming insoluble compounds with gelatine, and with skin. The
infusions of the barks aftect the greater number of reagents in a
manner similar to the infusion of galls ; and, that this last fluid
is rendered green by the carbonated alkalis, evidently depends
upon the large proportion of gallic acid it contains. The infu-
sion of sumach owes its characteristic property, of being pre-
cipitated by the caustic alkalis, to the presence of sulphate of
lime ; and, that the solutions of catechu do not copiously pre-
cipitate the carbonated alkalis, appears to depend upon their
containing tannin in a peculiar state of union with extractive
matter, and uncombined with gallic acid or earthy salts.

In making some experiments upon the affinities of the tanning
principle, I found that all the earths were capable of attracting
it from the alkalis : and, so great is their tendency to combine
with it, that, by means of them, the compound of tannin and
gelatine may be decomposed without much difficulty ; for,
after pure magnesia had been boiled for a few hours with

syo Mr. Davy’s Experiments and Observations

this substance diffused through water, it became of a red-brown
colour, and the fluid obtained by filtration produced a distinct
precipitate with solution of galls. The acids have less affinity for
tannin than for gelatine; and, in cases where compounds of the
acids and tannin are acted on by solution of gelatine, an equi-
librium of affinity is established, in consequence of which, by
far the greatest quantity of tannin is carried down in the inso-
luble combination. The different neutral salts have, compara-
tively, feeble powers of attraction for the tanning principle ; but,
that the precipitation they occasion in astringent solutions, is
not simply owing to the circumstance of their uniting to a por-
tion of the water which held the vegetable substances in so-
lution, is evident from many facts, besides those which have
been already stated. The solutions of alum, and of some
other salts which are less soluble in water than tannin, pro-
duce, in many astringent infusions, precipitates as copious as
the more soluble saline matters ; and sulphate of lime, and other
earthy neutral compounds, which are, comparatively speaking,
insoluble in water, speedily deprive them of their tanning prin-
ciple.

From the different facts that have been stated, it is evident
that tannin may exist in a state of combination in different sub-
stances, in which its presence cannot be made evident by means
of solution of gelatine; and, in this case, to detect its existence,
it is necessary to have recourse to the action of the diluted acids.

In considering the relations of the different facts that have
been detailed, to the processes of tanning and of leather-making,
it will appear sufficiently evident, that when skin is tanned in
astringent infusions that contain, as well as tannin, extractive
matters, portions of these matters enter, with the tannin, into

on the constituent Parts of asiringetit Vegetables. ^71

chemical combination with the skin. In no case is there any
reason to believe that gallic acid is absorbed in tliis process ;
and M, Seguin’s ingenious theory of the agency of this sub-
stance, in producing the deoxygenatioii of skin, seems sup-
ported by no proof’s. Eveii in the formation of glue from
skin, there is no evidence which ought to induce us to suppose
that it loses a portion of oxygen ; and the effect appears to be
owing merely to the separation of the gelatine, from the small
quantity of albumen with which it was combined in the orga-
nised form, by the solvent powers of water.

The different qualities of leather made with the same kind of
skin, seem to depend very much upon tlie different quantities
of extractive matter it contains. The leather obtained by means
of infusion of galls, is generally found harder, and more liable
to crack, than the leather obtained from the infusions of barks ;
and, in all cases, it contains a much larger proportion of tannin,
and a smaller proportion of extractive matter.

When skin is very slowly tanned in weak solutions of the
barks, or of catechu, it combines with a considerable proportion
of extractive matter; and, in these cases, though the increase
of weight of the skin is comparatively small, yet it is rendered
perfectly insoluble in water ; and is found soft, and at the same
time strong.

The saturated astringent infusions of barks contain much less
extractive matter, in proportion to their tannin, than the weak
infusions; and, when skin is quickly tanned in them, common
experience shows that it produces leather less durable than the
leather slowly formed.

Besides, in the case of quick tanning by means of infusions
of barks, a quantity of vegetable extractive matter i$ lost to the

27J2 Mr. Davy’s Experiments and Observations

manufacturer, which might have been made to enter into the
composition of his leather. These observations show, that there
is some foundation for the vulgar opinion of workmen, con-
cerning what is technically called the feeding of leather in the
slow method of tanning ; and, though the processes of the art
may in some cases be protracted for an unnecessary length of
time, yet, in general, they appear to have arrived, in consequence
of repeated practical experiments, at a degree of perfection
which cannot be very far extended by means of any elucidations
of theory that have as yet been made known.

On the first view it appears singular that, in those cases of
tanning where extractive matter forms a certain portion of the
leather, the increase of weight is less than when the skin is
combined with pure tannin ; but the fact is easily accounted for,
when we consider that the attraction of skin for tannin must be
probably weakened by its union with extractive matter; and,
whether we suppose that the tannin and extractive matter enter
together into combination with the matter of skin, or unite with
separate portions of it, still, in either case, the primary attraction
of tannin for skin must be, to a certain extent, diminished.

In examining astringent vegetables in relation to their powers
of tanning skin, it is necessary to take into account, not only
the quantity they contain of the substance precipitable by gela-
tine, but likewise the quantity, and the nature, of the extractive
matter; and, in cases of comparison, it is essential to employ
infusions of the same degree of concentration.

It is evident, from the experiments detailed in the Illd section,
that of all the astringent substances which have been as yet
examined, catechu is that which contains the largest propor-
tion of tannin ; and, in supposing, according to the common

on the constituent Parts of astringent Vegetables. 273

estimation, that from four to five pounds of common oak bark
are required to produce one pound of leather, it appears, from
the various synthetical experiments, that about half a pound of
catechu would answer the same purpose.*

Also, allowing for the difference in the composition of the
different kinds of leather, it appears, from the general detail of
facts, that one pound of catechu, for the common uses of the
tanner, would be nearly equal in value to 2^ pounds of galls,
to 7^ pounds of the bark of the Leicester willow, to 11 pounds
of the bark of the Spanish chesnut, to 18 pounds of the bark of
the elm, to 21 pounds of the bark of the common willow, and
to 3 pounds of sumach.

Various menstruums have been proposed for the purpose of
expediting and improving the process of tanning, and, amongst
them, lime-water and the solutions of pearl-ash : but, as these
two substances form compounds with tannin which are not de-
composable by gelatine, it follows that their effects must be
highly pernicious ; and there is very little reason to suppose, that
any bodies will be found which, at the same time that they
increase the solubility of tannin in water, will not likewise di-
minish its attraction for skin.

• This estimation agrees very well with the experiments lately made by Mr. Purkis,
upon the tanning powers of Bombay catechu in the processes of manufacture, and which
he has permitted me to mention. Mr. Purkis found, by the results of different
accurate experiments, that one pound of catechu was equivalent to seven or eight of
oak bark.

MDCCCin.

Nn

L 274 ]

X, Appendix to Mr. William Henry’s Paper, on the Qiianliiy of

Gases absorbed by Water, at differeiit Temperatures, and under

different Pressures. (See Page 29).

Since my Paper was printed, I have found that the numbers
assigned in it, as indicating the quantities taken up by water,
of some of the more absorbable, and of all the less absorb-
able gases, are rather below the truth. The accuracy of these
numbers I was led to doubt, by a suspicion that due atten-
tion had not always been paid, in my former experiments, to
the quality of the unabsorbed residuum. For, the theory which
Mr. Dalton has suggested to me on this subject, and which
appears to be confirmed by my experiments, is, that the absorp-
tion of gases by water is purely a mechanical effect, and that its
amount is exactly proportional to the density of the gas, consi-
dered abstractedly from any other gas with which it may acci-
deptally be mixed. Conformably to this theory, if the residuary
gas contain or any other proportion, of foreign gas, the

quantity absorbed by water will be &c. short of the

maximum. The proof of these propositions would lead me into
a minuteness of detail, not suited to the present occasion ; I
therefore hasten to communicate the results of my latest expe-
riments.

The report which I have already given, of the quantity of
Carbonic Acid Gas, absorbed under the ordinary pressure
of the atmosphere, I find no reason to correct; but, of Sul- /-'
PHURETTED Hydrogen Gas, I liave effected a larger absorption