Formaldehyde.—At present the tendency is to employ formaldehyde gas instead of the sulphur so popular some time ago. The advantages of formaldehyde over sulphur are: (1) its nonpoisonous nature; (2) it is a very good germicide; (3) it has no injurious effect upon fabrics and objects; (4) it does not change colors; and (5) it can be used for the disinfection of rooms with the richest hangings, bric-a-brac, etc., without danger to these. Formaldehyde is evolved either from paraform or from the liquid formalin; formerly it was also obtained by the action of wood-alcohol vapor upon red-hot platinum.
Formaldehyde gas has not very great penetrating power; it is not an insecticide, but kills bacteria in a very short time, and spores in an hour or two.
Paraform (polymerized formaldehyde; trioxymethylene) is sold in pastilles or in powder form, and when heated reverts again to formaldehyde; it must not burn, for no gas is evolved when the heating reaches the stage of burning. The lamps used for disinfection with paraform are very simple in construction, but as the evolution of the gas is very uncertain, this method is used only for small places, and it demands two ounces of paraform for every 1,000 cubic feet of space, with an exposure of twelve hours. Formaldehyde is also used in the form of the liquid formalin either by spraying and sprinkling the objects to be disinfected with the liquid, and then placing them in a tightly covered box, so that they are disinfected by the evolution of the gas, or by wetting sheets with a formalin solution and letting them hang in the room to be disinfected.
The method most frequently employed is to generate the formaldehyde in generators, retorts, and in the so-called autoclaves, and then to force it through apertures into the room.
Of the other gaseous disinfectants used, hydrocyanic acid and chlorine may be mentioned, although they are very rarely used because of their irritating and poisonous character.
Hydrocyanic Acid is frequently used as an insecticide in ships, mills, and greenhouses, but its germicidal power is weak.
Chlorine is a good germicide, but is very irritating, poisonous, and dangerous to handle; it is evolved by the decomposition of chlorinated lime with sulphuric acid. Chlorine gas is very injurious to objects, materials, and colors, and its use is therefore very limited.
Chemicals Used as Disinfectants
Solution of chemicals, in order to be effective, must be used generously, in concentrated form, for a prolonged time, and, if possible, warm or hot. The strength of the solution must depend upon the work to be performed and the materials used. The method of applying the solution differs. It may consist in immersing and soaking the infected object in the solution; or the solution may be applied as a wash to surfaces, or used in the form of sprays, atomizers, etc. The most important solutions of chemicals and the ones most frequently employed are those of carbolic acid and bichloride of mercury.
Carbolic Acid.—In the strength of 1:15,000 carbolic acid prevents decomposition; a strength of 1:1,000 is needed for the destruction of bacteria, and a three per cent to five per cent solution for the destruction of spores. Carbolic acid is used, as a rule, in two per cent to five per cent solutions, and is a very good disinfectant for washing floors, walls, ceilings, woodwork, small objects, etc. The cresols, creolin, lysol, and other solutions of the cresols are more germicidal than carbolic acid, and are sometimes used for the same purposes.