METHODS OF TESTING DISINFECTANTS. 513 



from five to ten minutes they are removed under anti- 

 septic precautions and carefully separated and spread 

 out upon the bottom of a sterilized Petri dish. This is 

 then placed either in the incubator at a temperature not 

 exceeding 38 C. until the excess of fluid has evapor- 

 ated, or in a desiccator over sulphuric acid, calcium 

 chloride, or any other drying agent, but they are not 

 left there until absolutely dry, only until the excess of 

 moisture has disappeared. When sufficiently dry they 

 can then be employed in the test. This is done by 

 immersing them in solutions of the disinfectant of dif- 

 ferent but known strengths for a fixed interval of time, 

 say one or two hours, after which they are removed, 

 rinsed off in sterilized distilled water to remove the 

 excess of disinfectant adhering to them, and placed in 

 fresh sterilized culture media, which is then placed in 

 the incubator at from 37 to 38 C. If after twenty- 

 four, forty-eight, or seventy-two hours a growth occurs 

 at or about the bit of thread, and this growth consists 

 of the organism upon which the test was made, mani- 

 festly there has been no disinfection; if no growth 

 occurs after, at most, ninety-six hours, it is safe to pre- 

 sume that the bacteria have been killed, unless our 

 efforts at rinsing off the excess of disinfectant from 

 the thread have not been successful, and a small 

 amount of disinfectant is now active in preventing 

 development i.e., is acting as an antiseptic. 



By the latter process, in which cultures or suspen- 

 sions of the organisms are mixed with different but 

 known strengths of the disinfectant, a small portion of 

 the mixture, usually a loopful or a drop, is transferred 

 at the end of a definite time to the fresh medium 

 which is to determine whether the organisms have 



