Most of the highly oxidized mineral acids and their salts have disinfecting and preservative powers, through their oxidizing property. Losing one or more equivalents of oxygen, they are reduced to acids and salts of lower oxidation; nitric acid, for example, and the nitrates passing into nitrous acid and the nitrites; and sulphuric acid and the sulphates, into sulphurous acid and the sulphites. Comparatively few of the acids are actually employed as disinfectants; and none are equal in efficiency to those already treated of. Their saline compounds are almost uniformly preferred, as they possess scarcely inferior power to the acids, while they are at once more safe and convenient. The substances more or less used at present are the sulphates of iron, zinc, and copper, the nitrate of lead, and the subnitrate of bismuth.

Between the three sulphates mentioned, there seems to be little choice in reference to efficiency, and the selection must be made on the grounds of expediency. They have all been treated of in other parts of this work, and it is necessary here to say nothing except in reference to this special application. it is probable that, in their action as disinfectants, the sulphuric acid gives so much of its oxygen as to pass into the state of sulphurous acid, which then reacts with its special powers, adding greatly to the efficiency of the several agents. As will be more fully explained hereafter, sulphurous acid has not only much efficiency as a chemical agent, but is still more powerful through its parasinoidal influence; and thus may be explained the superiority which experience has established for the sulphates over most other metallic salts. it is probable that sulphate of lime, though I have placed it among the absorbents, is more efficient as a disinfectant through this action of its acid.

Sulphate op iron {copperas or green vitriol) has been particularly described among the chalybeate tonics, in the first volume of this work (page 452). it is generally preferred to the other sulphates on account of its great cheapness in the commercial form of copperas. For some purposes it may not be well fitted, in consequence of the black colour it causes, and the stains it is apt to leave behind. But, in the use most made of it, this is of little account. it is a complete purifier in all cases of animal putrefaction; and hence it is employed chiefly as an addition to the contents of water-closets, privies, etc., and for the disinfection of slaughter-houses, or other deposits of putrefying animal substances, masses of manure, sewers, etc. it is used in solution.

Sulphate of Zinc (see vol. I p. 412) is not less efficient; but, being more expensive, it is much less used than the salt of iron upon a large scale. For the disinfection of close-stools, and other recipients of filth in the sick chamber, it is preferable on account of the absence of discoloration in its action. This also should be used in solution.

Sulphate of Copper (see vol. i. p. 406), with perhaps equal efficiency, wants the special recommendation of the salt of iron on the score of cheapness, and of the salt of zinc on that of colour, and is therefore little used. One objection to it, moreover, is its much greater causticity. it is, however, an ingredient in a disinfectant known as Larnaudes' anti-mephitic liquid, which consists of 22 parts of sulphate of zinc, 2 of sulphate of copper, and 975 of water, in 1000 parts. (Bridges, Am. Journ. of Med. Sci., July, 1867, p. 160.) This is stated to be superior in efficiency to the sulphate of iron; and it is possible that there may be greater power in the two metallic oxides than in the single one of the chalybeate. it is a good general rule to use more than one disinfectant at the same time, where there is no incompatibility between them; as each may operate with a peculiar influence, and effects may thus be obtained which could not result from the use of either singly.

Nitrate of Lead (see vol. i. p. 169) is probably now used less than formerly. At one time it enjoyed considerable reputation under the name of Ledoyen's disinfecting liquid, which was thought to possess some influence over poisonous miasms. But it has been shown only to have the power of deodorizing offensive animal matters, similar to that possessed by the preceding salts, but inferior. Dr. Harris, of New York, however, speaks favourably of it, stating that practical considerations placed it at the head of the colourless disinfectants used by the U. S. Sanitary Commission for certain local applications, as the deodorizing of close apartments, and the bedding, etc. of sick persons, by means of a solution distributed in shallow vessels, or upon saturated cloths. (B. and F. Medico-chir. Rev., Oct. 1864, p. 537.) The nitrates of zinc and of iron have similar properties; but are seldom if ever used.

Subnitrate of Bismuth has been fully described already {vol. i. p. 426), and requires little to be said additionally. it may be remarked here, however, that, in the operation of the nitrates, the acid probably gives up so much of its oxygen as to be converted into nitric oxide, which has a strong affinity for oxygen; and this, being still greater in the nascent state, enables it to act with a considerable deoxidizing power on the products of decomposition, though, so far as known, it wants the peculiar dynamic property of sulphurous acid. The subnitrate of bismuth is strongly recommended by Riemslach for its cicatrizing and disinfectant powers, when used as a dressing for foul ulcers and suppurating wounds. He applies it in fine powder, to the thickness of about a line, over the diseased surface, and keeps it in place by an adhesive plaster. The fetid odour ceases in a day, and the diseased surface soon begins to cicatrize. {Journ. de Pharm., 3e sér., xliii. 224.)