CHAPTER XXVIII. 



Methods of testing disinfectants and antiseptics Experiments illus- 

 trating the precautions to be taken Experiments in skin-disin- 

 fection. 



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 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 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 disinfectant is to be tested. 

 I say " pure culture/ 7 because it is always desirable in 

 testing a substance to determine its germicidal value 



654 



