METHODS OF TESTING DISINFECTANTS. 253 



for several different resistant species of bacteria, both 

 in the vegetating and in the spore stage, and also be- 

 cause it is only by the use of pure cultures of familiar 

 species that it is possible to distinguish between the 

 colonies growing from the individuals that have not 

 been destroyed by the disinfectant under investigation 

 and those of unknown species that may appear upon 

 the plate as contaminations occurring during the manip- 

 ulation. 



After the threads have remained in the culture or 

 suspension for from five to ten minutes they are re- 

 moved under antiseptic precautions and carefully sepa- 

 rated and spread upon the bottom of a sterilized Petri 

 dish, which is then placed either in the incubator at a 

 temperature not exceeding 38 C. until the excess of fluid 

 has evaporated, or in a desiccator over sulphuric acid, 

 calcium chloride, or any other drying-agent. The threads 

 are not left there until absolutely dry, but only until the 

 excess of moisture has evaporated. When sufficiently dry 

 they are immersed 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 in sterilized distilled water to remove the ex- 

 cess of disinfectant adhering to them, and placed in 

 fresh, sterile culture-media, which are then placed in 

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

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

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

 of the organism with 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 



