Alcohol.—Ordinary (ethyl) alcohol (C2H5OH) is largely used as a preservative, also as a disinfectant for the body surface, hands, and arms. Experiments show that alcohol of 70 per cent. strength is most strongly bactericidal and that absolute alcohol is very slightly so.
Soap.—Experimenters have obtained many conflicting results with soaps when tested on different organisms, as is to be expected from the great variations in this article. Miss Vera McCoy in the author’s laboratory carried out experiments with nine commercial soaps—Ivory, Naphtha, Packer’s Tar, Grandpa’s Tar, Balsam Peru, A. D. S. Carbolic, German Green, Dutch Cleanser, Sapolio—and obtained abundant growth from spores of Bacillus anthracis, from Bacterium coli and from Staphylococcus pyogenes aureus in all cases even when the organisms had been exposed twenty-four hours in 5 per cent. solutions. From these results and from the wide variations reported in the literature it is clear that soap solutions alone cannot be depended on as disinfectants. Medicated soaps do not appear to offer any advantages in this respect. The amount of the disinfectant which goes into solution when the soap is dissolved is too small to have any effect.
Formaldehyde.—Formaldehyde (HCHO) is perhaps the most largely used chemical disinfectant at the present time. The substance is a gas but occurs most commonly in commerce as a watery solution containing approximately 40 per cent. of the gas. This solution is variously known as formalin, formol, and formaldehyde solution. The first two names are patented and the substance under these names usually costs more. It is used in the gaseous form for disinfecting closed spaces of all kinds to the exclusion of most other means today. A great many types of formalin generators have been devised. The gas has little power of penetration and all material to be reached should be exposed as much as possible. The dry gas is almost ineffective, so that the objects must be moistened or vapor generated along with the gas. A common method in use is to avoid expensive generators by pouring the formaldehyde solution on permanganate of potash crystals placed in a vessel removed from inflammable objects on account of the heat developed which occasionally sets the gas on fire. The formalin is used in amounts varying from 20 to 32 ounces to 8½ to 13 ounces of permanganate to each 1000 cubic feet of space. This method is expensive since one pint (16 ounces) of formalin is sufficient for each 1000 cubic feet, and since the permanganate is an added expense. Dr. Dixon, Commissioner of Health of Pennsylvania, recommends the following mixture to replace the permanganate, claiming that it works more rapidly and is less expensive and just as efficient:
- 1. Sodium bichromate, ten ounces.
- 2. Saturated solution of formaldehyde, sixteen ounces.
- 3. Common sulphuric acid, one and a half ounces.
Two and three are mixed together and when cool are poured on the bichromate which is placed in an earthenware jar of a volume about ten times the quantity of fluid used. The quantities given are for each 1000 cubic feet of space.
A very simple method is to cause the formalin, diluted about twice with water to furnish moisture enough, to drop by means of a regulated “separator funnel” on a heated iron plate. The dropping should be so regulated that each drop is vaporized as it falls. The plate must have raised edges, pan-shaped, to prevent the drops rolling off when they first strike the plate. Formaldehyde has no corrosive (except on iron) or bleaching action, and is the most nearly ideal closed space disinfectant today. In disinfecting stations it is made use of in closed sterilizers such as were described under steam disinfection particularly in connection with vacuum apparatus. It is also used in solution as a preservative and as a disinfectant. The commonest strength is 2 or 3 per cent. of formalin or 0.8 to 1.2 per cent. of the formaldehyde gas. As an antiseptic it is efficient in dilutions as high as 1 to 2000 of the gas. It is very irritant to mucous membranes of most individuals.
Anilin Dyes.—Some of the anilin dyes show remarkable selective disinfectant and antiseptic action on certain kinds of bacteria with little effect on others. This has been well shown by Churchman in his work on Gentian Violet. This dye inhibits the growth of Gram positive organisms up to a dilution of one part in 300,000 while for Gram negative organisms it is without effect even in saturated solution. This is nicely shown in the accompanying illustration. This inhibiting effect of anilin dyes is taken advantage of in several methods of isolating bacteria ([Chapter XVIII]).
Fig. 110.—The lower half of the plate is plain agar medium, the upper half the same medium plus gentian violet to make one part in 300,000. The Gram positive organism is on the right and the Gram negative on the left. Streak inoculations were made across both media.
In addition to the above-discussed disinfectants a large number of substances, particularly organic, are used in medicine, surgery, dentistry, etc., as more or less strong antiseptics, and the list is a constantly lengthening one.