Qualities.-Nitrous acid, as the term is understood in the Edinburgh Pharmacopoeia, is a yellow or orange-coloured fluid, emitting, when exposed to the air, deep orange-coloured, suffocating fumes. In its chemical affinities and other qualities, it agrees in many respects with nitric acid. It consists of nitrous gas loosely combined with nitric acid and water; and the colour varies according to the proportion of nitrous gas which is present. From experiments made by Sir H. Davy3 on this subject, the following appear to be the proportions in the three states in which nitrous acid is usually procured for pharmaceutical purposes : -

3 Researches, 37.

The Edinburgh College states the specific gravity too high, for it seldom exceeds 1.502.2 When one part by weight of water is added to four parts of yellow nitrous acid, the colour . is altered to a fine green; when equal parts of both are mixed, it becomes blue; and by another addition of water, or by > allowing it to stand exposed to the air, it changes to a very pale straw-colour, or becomes nearly colourless.

Liquid nitric acid is a colourless, or very pale yellow limpid fluid, emitting, when exposed to the air, white, suffocating vapours, and possessing strong acid properties. If the yellow acid be heated in a retort it becomes colourless. It is highly corrosive, and tinges the skin yellow, the tint remaining till the epidermis peel off. When exposed to the air, it attracts moisture and becomes weaker. It unites with water in every proportion, and while mixing heat is evolved. Eighty-one grains of the officinal acid neutralize 217 grains of carbonate of soda. The following table, constructed by Sir H. Davy3, shows the quantity of real acid and water contained in 100 parts of fluid acid of different densities : -

Nitric acid is volatilized by heat, and decomposed by light. It is also decomposed by all the simple combustibles, and by some, namely, phosphorus, charcoal, and sugar, with great violence of action, which is accompanied with the evolution of copious red fumes. When poured on volatile oils, it sets them on fire; it is capable of oxidizing all the metals; and combines with the earths, alkalies, and metallic oxides l, forming nitrates; one fluid ounce of specific gravity 1.500 should dissolve 476 grains of white marble. The constituents of nitric acid, independent of the water, which gives it the fluid form, are 25.93 nitrogen, and 74.07 oxygen in 100 parts; or 1 equivalent of nitrogen = 14.15 + 5 of oxygen (8x5)= 40, making its equivalent = 54.15. The strongest fluid acid (sp. gr. 1.5) contains more than 20 per cent. of water, and 80 of dry acid, or 1 equiv. of dry acid = 54.15 + 2 of water = 18, making the equivalent 72.15. The nitric acid of the shops often contains sulphuric or hydrochloric acid.

1 Anhydrous nitric acid is procured by exposing dry nitrate of lead to a low-red heat. The acid of the nitrate is resolved into nitrous acid and oxygen; if the receiver he kept cool, the former condenses into a liquid.

2 Rouelle states the specific gravity of the strongest nitric acid that can be procured to be 1.583; Kirwan makes it 1.5543 only, at C0° Fahrenheit.

3 Researches, p. 41.

It is incompatible with sulphate of iron, tinctures made with alcohol, oxides, earths, alkalies and their carbonates, and the sulphurets.

Medical properties and uses. - Strong fluid nitric acid is seldom employed except for pharmaceutical purposes; but it has been lately recommended as an escharotic in sloughing phagedenic ulcers by Mr. Welbank. To employ it properly, the surface of the ulcer must be well cleansed and dried, and, after applying a thick coating of lard to protect the surrounding sound skin, a pledget, moistened with the undiluted acid, must be pressed steadily on every point of the diseased surface, which by this treatment soon throws off a slough, and assumes an healthy aspect.2 When extricated in the form of vapour, it is employed for destroying contagion. It is less powerful than the chlorine, but it can be extricated in the chambers of the sick without proving uncomfortable.3 For this purpose f 3 ij. of sulphuric acid may be poured over 3 iv. of coarsely powdered nitre in a china cup, and placed in a pipkin of hot sand. This quantity is sufficient for fumigating a room of ten feet square; and, where a larger portion is required, it is more advisable to multiply the number of pipkins, than to put a larger quantity of the materials into one vessel.

Officinal preparations.-Acidum Nitricum Dilutum, L. E. D. Acidum Nitro-muriaticum, D. Argenti Nitras, L. E. D. Anti-monii Oxydum Nitro-muriaticum, D. Hydrargyri Persulphas, D. Ung. Hydrargyri Nitratis, L. E. D. Hydrargyri Nitrico-oxydum, L. D. Spiritus Aetheris nitrici, L. E. D. Unguentum Acidi nitrosi, E. D.

1 In its concentrated state, it does not act upon the metals at ordinary temperatures; but when slightly diluted, it quickly oxidizes them and dissolves the oxides. This is owing to the decomposition of the water, which yields up its oxygen to the metal, which in a state of oxide is readily soluble in the acid. It does not act on platina or gold.

2 Medico-Chirurg. Trans, vol. ix. p. 69.

3 The Effects of Nitrous Vapour, etc. by J. C. Smyth, M. D.