466 BACTERIOLOGY. 



or not. This is commonly a tube of fluid agar-agar 

 which is poured out into a Petri dish, allowed to solidify, 

 and placed in the incubator, as in the other experiment. 



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 

 then 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 different intervals of exposure one, five, ten 

 minutes, etc. until we have decided not only the 

 minimum amount of disinfectant required for the de- 

 struction of the bacteria, but the shortest time neces- 

 sary for this under known conditions. 



A factor not to be lost sight of is the temperature 

 under which these experiments are conducted, for it 

 must always be borne in mind that the action of a dis- 

 infectant is usually more energetic at a higher than at a 

 lower temperature. 



Now in both of these methods it is easy to see that 

 unless special precautions are taken a minute portion of 

 the disinfectant 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 here 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 necessary to restrain growth, for certain dis- 

 infecting agents, is very small indeed, and for organisms 

 that have already been exposed for a time to such agents 

 this amount is even 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 disinfect- 



