METHODS OF TESTING DISINFECTANTS. 613 



the thread have not been successful, and a small amount 

 of disinfectant is still aeti\'c in preventing development 

 — i. e., is acting as an antiseptic. 



By the process in which cultures or suspensions 

 of the organisms are mixed with different but kno\vn 

 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, wliioh 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 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 



