314 APPLICATION OP METHODS OP BACTERIOLOGY 



bacteria, both in the vegetating and in the spore stage, and 

 also because 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 manipulation. 



After the threads have remained in the culture or suspen- 

 sion for from five to ten minutes they are removed under 

 aseptic precautions and carefully separated and spread 

 upon the bottom of a sterilized Petri dish, which is then 

 placed either in the incubator at a temperature not exceed- 

 ing 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 different 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 excess 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, manifestly there 

 has been no disinfection; if no growth occurs after, at most, 

 ninety-six hours, it is safe to presume 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 still active in preventing 

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



