And what are the substances that perform the office of thus reconciling the two adverse parties?
MRS. B.
The most common mordant is sulphat of alumine, or alum. Oxyds of tin and iron, in the state of compound salts, are likewise used for that purpose.
_Tannin_ is another vegetable ingredient of great importance in the arts. It is obtained chiefly from the bark of trees; but it is found also in nut-galls, and in some other vegetables.
EMILY.
Is that the substance commonly called _tan_, which is used in hot-houses?
MRS. B.
Tan is the prepared bark in which the peculiar substance, tannin, is contained. But the use of tan in hot-houses is of much less importance than in the operation of _tanning_, by which skin is converted into leather.
EMILY.
Pray, how is this operation performed?
MRS. B.
Various methods are employed for this purpose, which all consist in exposing skin to the action of tannin, or of substances containing this principle, in sufficient quant.i.ties, and disposed to yield it to the skin. The most usual way is to infuse coa.r.s.ely powdered oak bark in water, and to keep the skin immersed in this infusion for a certain length of time. During this process, which is slow and gradual, the skin is found to have increased in weight, and to have acquired a considerable tenacity and impermeability to water. This effect may be much accelerated by using strong saturations of the tanning principle (which can be extracted from bark), instead of employing the bark itself. But this quick mode of preparation does not appear to make equally good leather.
Tannin is contained in a great variety of astringent vegetable substances, as galls, the rose-tree, and wine; but it is nowhere so plentiful as in bark. All these substances yield it to water, from which it may be precipitated by a solution of isingla.s.s, or glue, with which it strongly unites and forms an insoluble compound. Hence its valuable property of combining with skin (which consists chiefly of glue), and of enabling it to resist the action of water.
EMILY.
Might we not see that effect by pouring a little melted isingla.s.s into a gla.s.s of wine, which you say contains tannin?
MRS. B.
Yes. I have prepared a solution of isingla.s.s for that very purpose. --Do you observe the thick muddy precipitate? --That is the tannin combined with the isingla.s.s.
CAROLINE.
This precipitate must then be of the same nature as leather?
MRS. B.
It is composed of the same ingredients; but the organisation and texture of the skin being wanting, it has neither the consistence nor the tenacity of leather.
CAROLINE.
One might suppose that men who drink large quant.i.ties of red wine stand a chance of having the coats of their stomachs converted into leather, since tannin has so strong an affinity for skin.
MRS. B.
It is not impossible but that the coats of their stomachs may be, in some measure, tanned, or hardened by the constant use of this liquor; but you must remember that where a number of other chemical agents are concerned, and, above all, where life exists, no certain chemical inference can be drawn.
I must not dismiss this subject, without mentioning a recent discovery of Mr. Hatchett, which relates to it. This gentleman found that a substance very similar to tannin, possessing all its leading properties, and actually capable of tanning leather, may be produced by exposing carbon, or any substance containing carbonaceous matter, whether vegetable, animal, or mineral, to the action of nitric acid.
CAROLINE.
And is not this discovery very likely to be of use to manufactures?
MRS. B.
That is very doubtful, because tannin, thus artificially prepared, must probably always be more expensive than that which is obtained from bark.
But the fact is extremely curious, as it affords one of those very rare instances of chemistry being able to imitate the proximate principles of organised bodies.
The last of the vegetable materials is _woody fibre_; it is the hardest part of plants. The chief source from which this substance is derived is wood, but it is also contained, more or less, in every solid part of that plant. It forms a kind of skeleton of the part to which it belongs, and retains its shape after all the other materials have disappeared. It consists chiefly of carbon, united with a small proportion of salts, and the other const.i.tuents common to all vegetables.
EMILY.
It is of woody fibre, then, that the common charcoal is made?
MRS. B.
Yes. Charcoal, as you may recollect, is obtained from wood, by the separation of all its evaporable parts.
Before we take leave of the vegetable materials, it will be proper, at least, to enumerate the several vegetable acids which we either have had, or may have occasion to mention. I believe I formerly told you that their basis, or radical, was uniformly composed of hydrogen and carbon, and that their difference consisted only in the various proportions of oxygen which they contained.
The following are the names of the vegetable acids:
The _Mucous Acid_, obtained from gum or mucilage; _Suberic_ - - - from cork; _Camphoric_ - - - from camphor; _Benzoic_ - - - from balsams; _Gallic_ - - - from galls, bark, &c.
_Malic_ - - - from ripe fruits; _Citric_ - - - from lemon juice; _Oxalic_ - - - from sorrel; _Succinic_ - - - from amber; _Tartarous_ - - - from tartrit of potash: _Acetic_ - - - from vinegar.
They are all decomposable by heat, soluble in water, and turn vegetable blue colours red. The _succinic_, the _tartarous_, and the _acetous acids_, are the products of the decomposition of vegetables; we shall, therefore, reserve their examination for a future period.
The _oxalic acid_, distilled from sorrel, is the highest term of vegetable acidification; for, if more oxygen be added to it, it loses its vegetable nature, and is resolved into carbonic acid and water; therefore, though all the other acids may be converted into the oxalic by an addition of oxygen, the oxalic itself is not susceptible of a further degree of oxygenation; nor can it be made, by any chemical processes, to return to a state of lower acidification.
To conclude this subject, I have only to add a few words on the _gallic acid_. . . . .
CAROLINE.
Is not this the same acid before mentioned, which forms ink, by precipitating sulphat of iron from its solution?
MRS. B.
Yes. Though it is usually extracted from galls, on account of its being most abundant in that vegetable substance, it may also be obtained from a great variety of plants. It const.i.tutes what is called the _astringent principle_ of vegetables; it is generally combined with tannin, and you will find that an infusion of tea, coffee, bark, red-wine, or any vegetable substance that contains the astringent principle, will make a black precipitate with a solution of sulphat of iron.
CAROLINE.
But pray what are galls?