152 



ANTISEPTIC AND GERMICIDAL POWER 



To 5 c.c. of a particular dilution of the disinfectant in 

 sterilized water 5 drops of the broth culture are added. After 

 shaking, subcultures are taken every two and a half minutes up 

 to fifteen minutes. Care must be taken to use the same sized 

 platinum loop so as to remove, as far as possible, the same 

 amount of disinfectant and the same number of bacilli. 



The subcultures are made into broth and incubated for at 

 least forty-eight hours at 37 C. Those with a growth are then 

 entered in the tables. 



Care must, of course, be taken at every stage to avoid 

 extraneous bacterial contamination. The purity of the growth 

 in the positive broth tube from the greatest dilution of the 

 disinfectant after the longest time should be carefully tested. 



A number of different dilutions of the disinfectant under 

 examination, and also several dilutions of the carbolic acid, are 

 tested at the same time, and under precisely similar conditions 

 of temperature, amount of disinfectant solution used, quantity of 

 typhoid broth culture added, etc. A dilution of the disinfectant 

 which possesses the same germicidal efficiency as the standard 

 carbolic acid dilution is obtained and the efficiency of the dis- 

 infectant is expressed in multiples of carbolic acid performing 

 the same work. The ratio obtained by dividing the former by 

 the latter is called the " carbolic acid coefficient." The results 

 are conveniently recorded in tables of which the following is an 

 illustration : 



B. typhosus Twenty-Four Hours' Broth Culture at 

 37 C. Room Temperature 15 to 18 C. 



Carbolic acid coefficient = # = 0*87. 



