ANTISEPTICS AXD DISINFECTANTS. 165 



Method of Determining Antiseptic rahte. To determine the 

 restraining or antiseptic power of an agent for a particular micro- 

 organism, the agent is dissolved in a definite proportion in a suitable 

 culture medium, which is then inoculated with a pure culture of the 

 test organism and placed in favorable circumstances as to tempera- 

 ture for its growth. At the same time a control experiment is 

 made by placing another portion of the same culture medium, inocu- 

 lated with the same microorganism, in the same conditions, but with- 

 out the addition of the antiseptic agent. If development occurs in 

 the control experiment and not in the culture medium containing 

 the antiseptic, the failure to grow must be attributed to the presence 

 of this agent. Having made a preliminary experiment, we are 

 guided by the result in further experiments to determine the exact 

 amount required to restrain development under the same conditions. 

 Or we may make a series of experiments in the first instance. The 

 problem being, for example, to determine the antiseptic value of 

 carbolic acid for the typhoid bacillus, we may add this agent to a 

 definite amount of bouillon in test tubes in the proportion of 1 : 100, 

 1 : 200, 1 : 300, 1 : 400, 1 : 500. In experiments with volatile agents 

 the bouillon, in test tubes or small flasks, must be sterilized in ad- 

 vance, and the antiseptic agent introduced by means of a sterilized 

 pipette with great care to prevent the accidental contamination of 

 the nutrient medium. In experiments with non-volatile agents it will 

 be best to sterilize the culture medium after the antiseptic has been 

 added. Next we inoculate the liquid in each flask with a pure cul- 

 ture of the test organism. The flasks are then placed in an incubat- 

 ing oven at 35 to 37 C. At the same time a control, not containing 

 any carbolic acid, is placed in the oven. At the end of twenty-four 

 hours the control will be found to be clouded, showing an abundant 

 multiplication of the bacillus. Taking the result of Boer above given, 

 we would expect to find all of the solutions clear except that contain- 

 ing 1 : 500. This too might remain clear for some days and finally 

 " break down," for experience shows that when we pass the point at 

 which a permanent restraining influence is exerted there may be a 

 temporary restraint or retardation of development. For this reason 

 we must continue the experiment for a considerable time not less 



