5. The number of bacteria present, since with chemical agents the action is one of relative masses.
6. Nature of the substance in which the bacteria are. Metallic salts, especially bichloride of mercury, are precipitated by albuminous substances and if employed at all must be used in several times the ordinary strength. Solids require relatively more of a given solution than liquids.
7. State of the disinfectant, whether solid, liquid or gas, and whether it is ionized or not. Solutions penetrate best and are therefore more quickly active and more efficient.
8. The solvent. Water is the best solvent to use. Strong alcohol (90 per cent. +) diminishes the effect of carbolic acid, formaldehyde and bichloride of mercury. Oil has a similar effect. The action is probably to prevent the penetration of the disinfectant.
9. Strength of solution. The stronger the solution, the more rapid and more certain the action, for the same reason as mentioned under 5. In fact, every disinfectant has a strength below the lethal at which it stimulates bacterial growth.
10. Addition of salts. Common salt favors the action of bichloride of mercury and also of carbolic acid. Other salts may hinder by precipitating the disinfectant.
11. Temperature. Chemical disinfectants, as a rule, follow the general law that chemical action increases with the temperature, up to the point where the heat of itself is sufficient to kill.
12. Time of action. It is scarcely necessary to point out that a certain length of time is necessary for any disinfectant to act. One may touch a red hot stove and not be burned. All the above-mentioned conditions are influenced by the time of action.
STANDARDIZATION OF DISINFECTANTS—“PHENOL COEFFICIENT.”
Many attempts have been made to devise standard methods for testing the relative strengths of disinfectants. The one most widely used in the United States is the so-called “Hygienic Laboratory” method of determining the “phenol coefficient” of the given substance and is a modification of the method originally proposed by Rideal and Walker in England. In this method as proposed by Anderson and McClintic, formerly of the above laboratory, the strengths of the dilution of the disinfectant to be tested which kills a culture of Bacterium typhosum in 2½ minutes is divided by the strength of the dilution of carbolic acid which does the same; and the dilution which kills in 15 minutes is likewise divided by the corresponding dilution of carbolic acid. The two ratios thus obtained are averaged and the result is the “phenol coefficient.” For example