DETERMINATION OF ANTISEPTIC PROPERTIES 321 



Second. An animal that has suffered this modified form of 

 the disease enjoys protection, more or less perfect, from 

 future attacks, and if used for a subsequent experiment may, 

 by its immunity from the effects of the pathogenic test- 

 organism, give rise to the mistaken assumption that this 

 had been destroyed by the action of the germicidal agent 

 to which it had been subjected." (Sternberg.) 



DETERMINATION OF ANTISEPTIC PROPERTIES. 



For this purpose sterile media are employed, and are 

 usually arranged in two groups: the one to remain normal 

 in composition and to serve as controls, while to the other the 

 substance to be tested is to be added in different but known 

 strengths. It is customary to employ test-tubes each con- 

 taining an exact amount of bouillon, gelatin, or agar-agar, 

 as the case may be. To each tube a definite amount of the 

 antiseptic is added, and if it is not of a volatile nature or 

 not injured by heat, the tubes may then be sterilized. 

 After this they are to be inoculated with the organism 

 with which the test is to be made, and at the same time one 

 of the "control' '-tubes (one of those to which no antiseptic 

 has been added) is inoculated. They are all then to be 

 placed in the incubator and kept under observation. If 

 at the end of twenty-four, forty-eight, or seventy-two hours 

 no growth appears in any but the " control"-tubes, it is 

 evident that the antiseptic must be added in smaller 

 amounts, for we are to determine the point at which it is 

 not as well as that at which it is capable of preventing 

 development. The experiment is then repeated, using 

 smaller amounts of the antiseptic until we reach a point at 

 which growth just occurs, notwithstanding the presence 

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