DETERMINATION OF DISINFECTANT PROPERTIES 315 



By the process in which cultures or suspensions 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 or not. This is commonly a 

 tube of fluid agar-agar, which is poured into a Petri dish, 

 allowed to solidify, and placed in the incubator, as in the 

 preceding method. 



After the minimum strength of disinfectant necessary to 

 destroy the vitality of the organisms with which we are 

 working has been determined for any fixed time, it remains 

 for us to decide what is the shortest time in which this strength 

 will have the same effect. We then work with a constant 

 dilution of the disinfectant, but with varying intervals of 

 exposure — one, five, ten minutes, etc. — until we have 

 decided not only the minimum amount of disinfectant 

 required for the destruction of the bacteria, but the shortest 

 time necessary for this under known conditions. 



A factor not to be lost sight of is the temperature at 

 which these experiments are conducted, for it must always 

 be borne in mind that the action of a disinfectant is usually 

 more energetic at a higher than at a lower temperature. 



Now in both of these methods it is easy t9 see that unless 

 special precautions are taken a minute portion of the disin- 

 fectant may be carried along with the thread, or drop, into 

 the medium which is to determine whether 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 disinfectant — the amount of 

 certain disinfectants that is necessary to restrain growth 



