ANTISEPTICS AND DISINFECTANTS, 165 
Restrains. Kills. 
Hydrochloric acid... ...... 0.2. oe eee tas 1.2100 1: 800 
Sulphuric acid sees: wow sesiertveidine auneperice aes 1: 1550 1:500 
Sil Ver Wibrate: soe eaccsseansoetaus; oacd mao aia sees 1 : 50000 1: 4000 
Sodium arseniate........... ....0... eee eters 1: 6000 1: 250 
Carole AGids cice~ ie eased o8 se nhee Keg anadnind er 1 : 400 1: 200 
Method of Determining Antiseptic Value.—To determine the 
restraining or antiseptic power of an agent for a particular micro- 
érganism, 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 microérganism, 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 toa 
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 
