MICHIGAN ACADEMY OF SCIENCE. 



223 



The resistance of B. typhosus to the compound to be tested is determined 

 in the same manner and simultaneously with the phenol test. Beef ex- 

 tract broth is used as the culture medium. The phenol coefficient of the 

 compound tested is then obtained by "dividing the figure indicating the 

 degree of dilution of the disinfectant that kills the organism in a given 

 time by that expressing the degree of dilution of the carbolic acid 

 (phenol) that kills the same organism in the same time under exactly 

 similar conditions. Table No. 1 will make clear the method of determin- 

 ing the coefficient of a disinfectant according to the Rideal-Walker 

 method. 



Table No. 1. Rideal-Walker Method. 



Name, (A). 



Temperature of medication, 20 degrees C. 



Cultures used, B. typhosus, 24 hour, extract broth filtered. 



Proportion of culture and disinfectant, 0.1 cc. plus 5 cc. 



Time culture exposed to action of disin- 

 fectant for minutes. 



X 



X 

 X 



71/2 



X 



X 

 X 



10 



121/2 



If) 



X — — 



X Signifies growth; — signifies no growth. 



This table taken fi-om Bulletin No. 82 Hygienic Laboratory. 



Public health and Marine-Hospital Service of the United States. 



Phenol 

 coefficient. 



100/550 



5.5 

 Coefficient 



Following tliis the Lancet^ Commission, Report 7, advanced a method 

 for standardizing coefficients. As in the Rideal-Walker method phenol 

 at varying strengths is used as the standard solution. B. coli instead of 

 B. typhosus is used as the test organism and McConkey's Bile Salt 

 Medium instead of Beef Bouillon, as the culture medium. The reason 

 for using this as a standard is that the organism is non-pathogenic and 

 the medium will give constant growth characteristics for the organism. 

 Twenty-four hour old cultures were used. The risk of misleading results 

 from accidental contamination was almost entirely eliminated by the use 

 of McConkey's medium. The carbolic (phenol) coefficient is determined 

 by the Lancet method as follows : The figure representing the percentage 

 strength of the weakest killing dilution of the phenol is divided by the 

 figure representing the percentage strength of the weakest killing dilution 

 of the unknown disinfectant, both at one and one-half and thirty minute 

 exposures. The mean resulting figure is taken as the true coefficient. 



By combining the more desirable points of the Rideal-Walker and 

 Lancet methods, Anderson and McClintic of the United States Hygienic 



