CHAPTER XVIII. 



Methods of Testing Disinfectants and Antiseptics Experiments Illustrating 

 the Precautions to be Taken Experiments in Skin-disinfection. 



DETERMINATION OF DISINFECTANT PROPERTIES. 



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 different 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 

 methods of work. 



By the first of these processes 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 disin- 

 fectant is to be tested. I say "pure culture," because it is 

 always desirable in testing a substance to determine its 

 germicidal value for several different resistant species of 



(331) 



