Origin

This acid, according to Dr. Percira, was known to Geber in the seventh century. In commerce it is usually denominated aquafortis, and in technical language sometimes azotic acid. It is prepared by heating together a mixture of nitrate of potassa and sulphuric acid, and condensing in a receiver the vapours which are given off. For an account of its composition, chemical relations, the tests of its purity, etc., the reader is referred to the U. S. Dispensatory. It is here to be considered mainly in its direct medical relations.

Properties

Two forms of the acid are kept in the shops, distinguished as the nitrous and nitric acids.

Nitrous acid of the shops is characterized by its orange colour, which is sometimes very deep, and by the orange-coloured fumes which it gives off. This peculiarity is owing to its impregnation with nitric oxide or deutoxide of nitrogen, by reaction between which and a portion of the nitric acid, the proper chemical nitrous acid is generated, which imparts its colour to the mixture. But, when the liquid acid is diluted with water, the orange-coloured nitrous acid is decomposed again into nitric acid and nitric oxide, the latter of which escapes, assuming an orange colour when in contact with the air, and leaving a colourless diluted nitric acid; and, as the medicine must be diluted before being administered, it follows that the nitrous acid of the shops has nothing to distinguish it, in relation to medical effect, from the purer form. This observation is necessary, as peculiar virtues have been ascribed to it.

Nitric acid, when quite pure, is a colourless liquid, but, as often kept, is slightly yellowish. If duly concentrated, it gives out white fumes. As directed by the U. S. Pharmacopoeia, it has the sp. gr. 1.42; but it is of variable strength as found in the shops. It has a peculiar odour, and, when so far diluted as to be borne in the mouth, an intensely sour taste. When in contact with the skin, it stains the cuticle yellow; and the colour remains until the cuticle itself is gradually removed. a property which serves to distinguish the stain from that produced by iodine and bromine. The colour, moreover, may be distinguished by becoming brighter under the application of ammonia or soap.

Effects on the System

Nitric acid, in the smallest medicinal doses, excites the appetite, promotes digestion, and secondarily invigorates the general nutritive process. It is, therefore, a tonic, acting specially on the stomach and bowels, and in this respect resembles sulphuric acid, from which, however, it differs in being without astringency. It is said also to be refrigerant, and to have alterative properties which render it useful in peculiar morbid states of the stem. On these points more will be said directly. It is thought to have sometimes induced ptyalism; but, at best, this result is very rare.

Too largely taken, and in the ordinary medicinal doses if continued too long, it is apt to cause disturbance of the stomach, gastric pain or spasm, and sometimes severe attacks of intestinal colic. Swallowed very copiously, and even in smaller quantities if concentrated, it produces poisonous effects so much like those resulting from sulphuric acid that it is unnecessary to repeat an account of them. (See page 357.) With the nitric acid, however, the inside of the mouth is stained yellowish instead of whitish; and yellow stains on the skin of the face will often serve to distinguish the poison. Two instances have recently occurred of fatal poisoning from inhalation of the fumes of nitric acid.

Arising from the fall and breaking of a jar containing the acid, as it was carried across a room. Timely efforts to save the patients were made but unavailingly. (Pharm. Journ. and Trans., a.d. 1863, iv. 475.) The antidotes and remedial treatment are absolutely the same as in the case of poisoning from sulphuric acid. When poisoning results from inhalation, as in the cases just referred to, the use of the recently invented atomizer obviously suggests itself. By means of it, a perfectly safe solution of one of the bicarbonated alkalies might be thrown, in the form of spray, into the upper passages, and thus made to neutralize any acid there present, leaving only inflammation to be combated, which would require the copious use of leeches preceded by the lancet. Gaseous ammonia is itself so irritating, that there might be fear of aggravated inflammation from its use.

Mode of Operating

Nitric acid is a direct stimulant to the alimentary mucous membrane, becoming irritant in over-doses. It is probably never absorbed in the acid state; but combines in the stomach and bowels with the albumen, and the salifiable bases which it always encounters there, and in this state of combination may enter the circulation, in order to be thrown off immediately by the kidneys. Now the alkaline salts of nitric acid are remarkably refrigerant and sedative to the circulation when absorbed, especially the nitrates of potassa and soda; so that, as one of these salts will be likely to be formed by the nitric acid in the bowels, we may readily account for the refrigerant effect asserted to be produced by the acid. As to its supposed alterative action, so far as such an effect has been experienced, it may be explained in a somewhat similar manner. Muriatic acid frequently exists in the stomach. It is possible that, by reaction with this, the nitric acid may sometimes generate that peculiar combination called nitromuriatic acid, which undoubtedly has an important alterative action on the system. This view is rendered the more probable, as it is only occasionally that nitric acid is found to exercise the alterative influence ascribed to it; and it is only occasionally that the reactions can occur which produce nitromuriatic acid. We may explain in the same way the reported occurrence of pty-alism in some instance;as one of the admitted characteristic properties of nitromuriatic acid is frequently to act on the gums.

In the concentrated state, nitric acid decomposes the tissues through its chemical affinities, and thus causes the death of the part, acting as an escharotic.