Mercuric chloride (corrosive sublimate) has been an accepted germicide for some time. But the experiments of Behring, Crookshank, and others have proved that the weaker solutions cannot be relied upon. This is, in part, due to the fact that it forms in albuminous liquids an albuminate of mercury which is inactive. Dilute solutions have the further disadvantage of being unstable. Various authorities recommend a solution of 1–500 as a germicide, and much weaker solutions are, of course, antiseptic. An ounce each of corrosive sublimate and hydrochloric acid in three gallons of water makes an efficient disinfectant.
Potassium permanganate is, of course, the chief substance in Condy's fluid, as zinc chloride is in Burnett's disinfecting fluid. A 5 per cent. of the former and a 2-1/2 per cent. of the latter are germicidal.
Boracic acid is one of the most useful antiseptics with which to wash sore eyes, or preserve tinned foods or milk. It is not a strong germicide, but an unirritating and effective wash. Many cases of its addition to milk have found their way into the law courts, owing to cumulative poisoning, and it should only be used with the very greatest care as a food preservative.
Carbolic acid has come into prominence as an antiseptic since its adoption by Lister in antiseptic surgery. It is cheap, volatile, and effective. One part in 400 is antiseptic, and 1 in 20 germicidal. As a wash for the hands the former is used, and a weaker solution for the body generally. Carbolic soap and similar toilet combinations are now very common. At one time it appeared as if corrosive sublimate would oust carbolic from the first place as an antiseptic solution, but a large number of experiments have confirmed opinion in favour of carbolic. Professor Crookshank found that carbolic acid, 1 in 40, acting for only one minute is sufficient to destroy Streptococcus pyogenes, S. erysipelatis, and Staphylococcus pyogenes aureus, and in the strength of 1 in 20 carbolic acid completely sterilised tubercular sputum when shaken up with it for one minute.
Creosol, a member of the phenol series, is a good disinfectant, and the active element in lysol, Jeye's fluid, creoline, izal, and creosote.
Sulphurous acid is one of the commonest disinfectants employed for fumigation—the old orthodox method of disinfecting a room in which a case of infective disease has been nursed. It is evolved, of course, by burning sulphur. For each thousand cubic feet from one to five pounds of sulphur is used, and the walls may be washed with carbolic acid. Dr. Kenwood carried out some experiments in 1896[105] which appear to support the disinfecting power of sulphur fumes. He found that the Bacillus diphtheriæ was not killed, though markedly inhibited, when the sulphurous gas (SO2) did not much exceed .25 per cent. But the bacillus was killed where the sulphur fumes exceeded .5 per cent. Both these results had reference to the
SO2 in the air in the centre of the room at a height of four feet, and after the lapse of four hours. There can be little doubt that fuming a sealed-up room with sulphur fumes in a moist atmosphere, and leaving it thus for twenty-four hours, is generally, if not always, efficient disinfection. It will kill the bacillus of diphtheria, though not always more resistant germs. Moreover, its simplicity of adoption is greatly in its favour. Anyone can readily apply it by purchasing a few pounds weight of ordinary roll sulphur and burning this in a saucer in the middle of a room which has had all its crevices and cracks in windows and walls blocked up with pasted paper. Nitrous fumes may also be used in this way.
Recently formalin has come much in favour as a room disinfectant. Formalin is a 40 per cent. solution of formaldehyde in water, a gas discovered by Hofmann in 1869. This gas is a product of imperfect oxidation of methyl alcohol, and may be obtained by passing vapour of methyl alcohol, mixed with air, over a glowing platinum wire or other heated metals, such as copper and silver. It is the simplest of a series of aldehydes, the highest of which is palmitic aldehyde. Its formula is CH2O, and it is a colourless gas with a pungent odour, and having penetrating and irritating properties, particularly affecting the nasal mucous membrane and the eyes of those working with it. It is readily soluble in water, and in the air oxidises into formic acid (CH2O2). This latter substance occurs in the stings of bees, wasps, nettles, and various poisonous animal secretions. Formalin is a strong bactericide even in dilute solutions, and, of course, volatile. A solution of 1 to 10,000 is said to be able to destroy the bacilli of typhoid, cholera, and anthrax. A teaspoonful to ten gallons of milk is said to retard souring. When formalin is evaporated down, a white residue is left known as paraform. In lozenge form this latter body is used by combustion of methylated spirit to produce the gas. Hence we have three common forms of the same thing—formalin, formic aldehyde, paraform—each of which yields formic acid, and thus disinfects. The vapour cannot in practice be generated from the formalin as readily as from the paraform.
By a variety of ingenious arrangements formic aldehyde has been tested by a large number of observers during the last two or three years. We may refer to three modes of application. 1. The sprayer (Equifex apparatus) produces a mixture of air and solution for spraying walls, ceilings, floors, and sometimes garments. 2. The autoclave (Trillat's apparatus). In this a mixture of a 30–40 per cent. watery solution of formaldehyde and calcium chloride (4–5 per cent.) is heated under a pressure of three or four atmospheres, and the almost pure, dry gas is conducted through a tube passing through the keyhole of the door into the sealed-up room. 3. The paraform lamp (the Alformant). The principle of this lamp is that the hot, moist products from the combustion of methylated spirit act upon the paraform tablets, converting them into gas. Most of the conclusions derived from experiments with these three different forms of apparatus are the same. It is agreed that the gas is harmless to colours and metal and polished wood. The vapour acts best in a warm atmosphere. As for its action on bacteria, it compares favourably with any other disinfectant. In 1 per cent. solution formalin destroys non-spore-bearing bacteria in thirty to sixty minutes.