The standard of the efficacy of a chemical disinfectant is the so-called carbolic coefficient. Roughly, the test is made by adding a definite amount of a culture of typhoid bacilli to definite amounts of definite concentrations of the chemical to be tested. At intervals of from 21 to 121 minutes the solutions are tested to determine if any germs are alive.
If the chemical under test kills the culture in a higher solution than carbolic acid, its coefficient is above 1. If it falls below, the coefficient is less than 1. Thus, if the culture of germs is killed by a 1 to 90 carbolic acid solution, and by a 1 to 900 solution of the test chemical, the carbolic coefficient of the tested disinfectant is said to be 10.
Carbolic acid is used as the standard only for convenience, not because it is necessarily the best or the strongest disinfectant, just as the freezing point of water is used as a standard of temperature, not because it is the ideal temperature.
It is amusing to read the official reports on some of the widely touted germ killers. Many of them are so weak that their coefficient cannot be measured.
The best disinfectants are inexpensive and familiarcarbolic acid, lysol, creosol, bichloride of mercury, lime, the hypochlorites, chlorin, formaldehyde, alcohol and iodine. Each is adapted to a special use: and their uses cover all the various practical purposes of disinfection.
Bichloride of mercury, for instance, while one of the most potent of disinfectants, has the disadvantage that it corrodes metals, coagulates living tissue, and is extremely poisonous.
The uses of lime are many. It is one of the cheapest and best disinfecting substances we have. Lime added to water is very caustic and destructive to organic matter as well as germs. A 3 per cent solution kills typhoid bacilli in one hour. A 20 per cent solution added to human excreta and filth will disinfect them in an hour.
Chiorid of lime, or chlorinated lime, is made by passing nascent chlorin gas over slightly moist calcium hydroxide. After preparation it takes up water from the air and becomes pasty, losing also some of its chlorin: it thus loses much of its germicidal properties when exposed. Freshly prepared chlorid of lime should have a slight odor of chlorin. A strong odor indicates that deterioration is taking place. On account of these chemical changes it should be kept in an air-tight receptacle.
Six ounces to the gallon of water can be used to mop floors, or the dry powder can be scattered in damp corners, in cellars, to deodorize privies and disinfect water closets. An extremely small amount of chlorinated lime can be safely used to disinfect drinking water. It may be used to disinfect the bath water of typhoid fever patients or patients with communicable diseases.