DISINFECTANTS 483 



Caution the assistant, if one is employed, to avoid 

 unnecessary movement or speech. 



b. Contact Temperature, 15-18 C. 



This is the temperature at which contact between the 

 germicide and the test germ takes place, and is of 

 importance, since some germicides (e. g., Phenol) appear 

 to be more powerful at high temperatures. 18 C. 

 practically the ordinary room temperature is a tem- 

 perature at which the multiplication of B. coli is a com- 

 paratively slow process, but variation of a degree above 

 this temperature or of two or three degrees below is of 

 no moment. If the room temperature is below 15 C. 

 when the experiments are in progress, arrange a water- 

 bath regulated at 18 C. for the reception of the tubes 

 containing the mixture of germ and germicide ; if above 

 19 C. immerse the tubes in cold water, to which small 

 pieces of ice are added from time to time to prevent the 

 temperature rising above 18 C. 



c. Relative Proportional Bulk of Test Germ and 

 Germicide, 50 : 1 . 



Five cubic centimetres is a convenient amount of 

 germicidal solution to employ, and to this o.i c.c. of 

 the emulsion of test germ should be added. 



d. Bulk of Sample Removed from Germ -{-Germicide 

 Mixture at Each of the Time Periods, 0. 1 c.c. 



This is sufficient to afford a fair sample of the germ 

 content of the mixture, and at the same time is insuffi- 

 cient to exert any inhibitory action when transferred to 

 the subculture medium. 



e. Subculture Medium. Bile Salt Broth. 



A fluid medium is essential in order to obtain imme- 

 diate dilution of the germicide carried over ; at the same 

 time it is advantageous to employ a selective medium 

 which favours the growth of the test germ to the 



