CHAPTER XXVIII. 



Methods of testing disinfectants and antiseptics — Experiments illustrating 

 the precautions to be taken — Experiments in sliin disinfection. 



There are several ways of determining the germicidal 

 value of chemical substances, the most common being 

 to expose organisms dried upon bits of silk thread to 

 the disinfectant for different lengths of time, and then, 

 after removing, and carefully washing the threads in 

 water, to place them in nutrient media at a favorable 

 temperature, and notice if any growth appears. If no 

 growth results, the disinfection is presumably successful. 

 Another method is to mix fluid cultures of bacteria with 

 the disinfectant in varying proportions, and, after dif- 

 ferent intervals of time, to determine if disinfection is 

 in progress by transferring a portion of the mixture to 

 nutrient media, just as in the other method of work. 



By the former process the bits of thread, usually 

 about 1 to 2 cm. long, are placed in a dry test-tube 

 provided with a cotton plug and carefully sterilized, 

 either by the dry method or in the steam sterilizer, 

 before using. They are then immersed in a pure 

 bouillon culture or in a salt solution suspension of the 

 organism upon which the disinfectiint is to be tested. 

 I say ''pure culture" because it is always desirable in 

 testing a new germicide to determine its value as such 

 on several different resistant species of bacteria, both 

 in the vegetating and in the spore stage. After the 

 threads have remained in the culture or suspension for 



