METHODS OF TESTING DISINFECTANTS. 465 



to ten minutes they are removed under antiseptic pre- 

 cautions 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 evaporated, 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 different 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 disin- 

 fectant adhering to them, and placed in fresh steril- 

 ized culture media, which is then placed in the incu- 

 bator 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, 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 dis- 

 infectant 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 been killed 



