The bark of oak, and some other vegetable substances, especially nut-galls, contain a substance which has obtained the name of the astringent principle; the peculiar property of which is, that it precipitates solutions of iron in the form of a black powder, and in this manner ink is made. But by solution in water and evaporation, crystals, which are a proper acid of galls, may be obtained.
Amber is a hard semitransparent substance, chiefly found in Prussia, either dug out of the earth, or thrown up by the sea. It is chiefly remarkable for its electrical property; but by distillation in close vessels there sublimes from it a concreted acid, soluble in 24 times its weight of cold water. Amber seems to be of vegetable origin, and to consist of an oil united to this peculiar acid.
The acids I shall mention next are of a mineral origin; but being of a less perfect nature as acids, I shall only just note them here.
Borax is a substance chiefly found in a crystallized state in some lakes in the East Indies. It consists of the mineral alkali and a peculiar acid, which may be separated, and exhibited in white flakes, by putting acid of vitriol to a solution of it in water. This acid has been called sedative salt, from its supposed uses in medicine. It is an acid that requires fifty times its weight of water to dissolve it.
Several other mineral substances, as arsenic, molybdena, tungsten, and wolfram, in consequence of being treated as the preceding vegetables ones, have been lately found to yield peculiar acids. They are also produced in a concrete state, and require a considerable proportion of water to make them liquid; but as the water in which they are dissolved turns the juice of litmus red, and as they also unite with alkalis, they have all the necessary characteristics of acids.
LECTURE XV.
Of the Phosphoric Acid.
The most important acid of animal origin, though it has lately been found in some mineral substances, is the phosphoric.
Phosphorus itself is a remarkable substance, much resembling sulphur, but much more inflammable. It has been procured chiefly, till of late, from urine, but now more generally from bones, by means of the vitriolic acid, which unites with the calcareous earth of which bones consist, and sets at liberty the phosphoric acid, or the base of that acid, with which it was naturally combined. The acid thus procured, mixed with charcoal, and exposed to a strong heat, makes phosphorus.