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 2-1/2 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 Phenol 1:80 killed in 2-1/2 minutes Disinfectant “A” 1:375 „ „ „ „ Phenol 1:110 „ „ 15 „ Disinfectant “A” 1:650 „ „ „ „ 375 ÷ 80 = 4.69 650 ÷ 110 = 5.91 ----- 2)10.60 ----- Average = 5.30 = “phenol coefficient.” Standard conditions of temperature, age of culture, medium, reaction, etc., and of making the dilutions and transfers are insisted on. Details may be found in the Journal of Infectious Diseases, 1911, 8, p. 1. This is probably as good a method as any for arriving at the relative strengths of disinfectants and in the hands of any given worker concordant results in comparative tests can usually be attained. Experience has shown that the results obtained by different workers with the same disinfectant may be decidedly at variance. This is to be expected from a knowledge of the factors affecting the action of disinfectants above stated and from the known specific action of certain disinfectants on certain organisms (compare anilin dyes, p. 162). It seems that the only sure way to test the action of such a substance is to try it out in the way it is to be used. It is scarcely wise to adopt the “phenol coefficient” method as a legal standard method as some states have done. PRACTICAL STERILIZATION AND DISINFECTION. The methods for sterilizing in the laboratory have been discussed and will be referred to again in the next chapter.