METHODS OF TESTING DISINFECTANTS. 255 



the organisms do or do not possess the power of growth, 

 and there have a restraining or antiseptic action. For 

 organisms in their normal condition that is, those 

 which have never been exposed to the action of a dis- 

 infectant the amount of certain disinfectants that is 

 necessary to restrain growth is very small indeed ; but 

 for organisms that have already been exposed for a 

 time to such agents this amount is very much less. 

 It is plain, then, that if the test is to be an accurate 

 one, precautions must be taken against admitting this 

 minute trace of disinfectant to the medium with which 

 we are to determine whether the bacteria exposed to the 

 disinfectant were killed or not. The precautions hitherto 

 taken to prevent this accident have been, when the threads 

 were employed, washing them in sterilized distilled 

 water and then in alcohol ; or, where fluid cultures were 

 mixed with the disinfectant in solution, an effort was 

 usually made to dilute the amount of disinfectant car- 

 ried over, to a point at which it lost its inhibiting 

 power. 



While such precautions are sufficient in many cases, 

 they do not answer for all. Certain chemicals have the 

 property of combining so firmly with the threads upon 

 which the bacteria are located as to require other special 

 means of ridding the threads of them ; and in solutions 

 in which proteid substances are present along with the 

 bacteria a similar union between them and the disin- 

 fectant may likewise take place. In both instances this 

 amount of disinfectant adhering to the threads or in 

 combination with the proteids must be gotten rid of, 

 otherwise the results of the test may be fallacious. A par- 

 tial solution of the problem is given through studies that 

 have been made upon corrosive sublimate in its various 



