This acid, united to the mineral alkali, makes _Rochelle salt_.
Every kind of wood, when distilled, or burned, yields a peculiar acid; and it is the vapour of this acid that is so offensive to the eyes in the smoke of wood.
A peculiar acid is obtained from most vegetable substances, especially the farinaceous ones, and from sugar, by distillation with the nitrous acid. This seizes upon the substance with which the acid was united, and especially the phlogiston adhering to it, and then the peculiar _acid of sugar_ crystallizes. Thus with three parts of sugar, and thirty of nitrous acid, one part of the proper acid of sugar may be obtained. By the same process an acid may be procured from camphor.
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.