The camphoric acid has not been used in medicine, and the properties of its neutral salts are little known. Its crystals resemble the muriat of ammonia, and are, with difficulty, soluble in water. It is not, perhaps, very different from the benzoic acid. It burns without any residuum; does not precipitate lime from lime water, nor produce any change in the sulphuric solution of indigo.

Two animal acids yet remain, the phosphoric and prussic; for the uric acid we shall not again consider, unless additional information lead us to resume the sub-jest. See Calculus.

Phosphoric acid is produced from phosphorus (see Inflammables) by burning, but is not in that process saturated with oxygen. It is in a more perfect state when prepared, as usual, by nitric acid. The phosphoric acid when pure is not corrosive, and has no smell; its specific gravity is three times that of water, and it may be concentrated to dryness, when it is styled the glacial acid of phosphorus. It combines with alkalis, and the alkaline earths; but its salts are seldom soluble. With magnesia, if the acid is in excess, the salt is soluble; and were not phosphorus a suspicious medicine, we should recommend this salt to a cautious trial. The only neutral employed by the physician is the phosphat of soda; a salt almost tasteless, though sufficiently active as a cathartic for children, perfectly safe, and highly useful. Its crystals are rhomboidal, and they effloresce in the air. It is decomposed by alkalis, the mineral acids, tartarous neutrals, alum, muriated lime, Epsom salt, muriated barytes, and almost every metallic salt.

The prussic acid is an ingredient in the vital fluid, and may be obtained by distilling blood with the nitric. acid. It has an acid taste and suffocating smell; combines with alkalis and metals; and has lately been discovered in the vegetable kingdom as a component part of bitter almonds, the cherry, peach, and apricot kernels, and perhaps laurel and peach leaves. Berthollet supposes its basis to be composed of hydrogen, nitrogen, and carbon; but the proportions are not known. It has in no form been yet employed in medicine, except in the (now neglected) black cherry water.

Two anomalous neutrals yet remain, which, to complete the subject, we shall here consider, viz. the hydro-sul/i/iurat of ammonia, introduced into the last edition of the Edinburgh Dispensatory, and the sal polycrestum Glaseri. Tromsdort seems first to have remarked that sulphurated hydrogen combines with different bases like an acid. The hydrosulphural of ammonia is prepared by decomposing sulphurat of iron with muriatic acid. When the acid separates the iron from the sulphur, the latter, with the oxygen of the water, forms sulphuric acid; while another portion of the sulphur forms the hydrogen, forming sulphurated hydrogen gas, which is afterwards combined with the ammonia. This is the preparation of the salt recommended by Mr. Cruick-shanks in diabetes, which he styles hepatised ammonia; and though experience has not established its utility in this complaint, there is great reason for supposing that it may be useful in some others, particularly in phthisis. It must be, however, employed with caution, since it produces vertigo and other unpleasant symptoms.

The sal polycrest of Glaser is only a combination of an imperfect sulphureous acid with potash. It is easily decomposed; and though it was once a celebrated remedy in Germany, is now rarely used.

Alkalis are salts of an opposite nature, and very rarely found native; never, perhaps, pure. They are either the vegetable, the mineral, or the volatile: but the two former are not exclusively derived from that kingdom which gives them their distinguishing appellation; as the vegetable alkali has been found in some granites of a very early formation, if not primordial, and the mineral in numerous vegetables.

Alkalis were long supposed to be elementary substances; but a few years since M. de Morveau announced that he had discovered the vegetable alkali to consist of hydrogen and lime; the mineral of the same principle and magnesia. His few experiments were, however, inconclusive; and would, perhaps, not have occurred to our notice, but for the facts pointed out when we spoke of the decomposition of the muriatic acid. The whole is strongly prepossessing from its simplicity.

The alkalis in the earlier chemical works were by no means such as the modern chemistry recognises by this title. They are, in the modern systems, considered as imperfect neutrals, neutralised by the carbonic acid or fixed air, which, as we have hinted, does not deprive them of their alkaline properties; nor, indeed, when supersaturated with this acid and become acidulous, do they wholly lose their alkaline or urinous taste. We have nothing further to add on this subject. See Alkali.

Earths. Chemistry has lately been enriched by numerous additions to this class of bodies, and medicine has also made a few acquisitions. Though earths should not appear to be the bases of alkalis, yet many approach alkalis very nearly in numerous properties. In general, earths are insoluble in water, at least when joined with carbonic acid; and in this state also they are without taste and smell. They are fixed, and unalterable in fire, assuming the form of a dry powder, and their specific gravity rarely exceeds 4.9. The five earths employed in medicine are, the barytes, strontia, lime, magnesia, alumine; the five others are the yttria, glucina, zircona, agustina, and silica. These are arranged nearly in the order of what may be styled their alkalinity.

Barytes is a greyish white porous body, possessing even a greater pungency than lime. It tinges blue colours green, and, as we have said, destroys animal substances. Its specific gravity is nearly 4, and it is not affected by the strongest heat. It is slaked in the air, and by water, like lime; and when dissolved in hot water it crystallizes on cooling, in needle-like crystals, composed of 53 parts of water, and 47 of barytes. Water dissolves 0.05 of this earth, and resembles in taste and properties lime water. Boiling water dissolves half its weight. It will not unite with oxygen, azote, hydrogen, carbon, or charcoal, but joins readily sulphur and phosphorus. (See Barytes).