520 BACTERIOLOGY. 



tacks, 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. 



In this test sterile media are employed and are usu- 

 ally arranged in two groups : the one to remain normal 

 in composition and to serve as controls, while to the 

 other is to be added the substance to be tested in dif- 

 ferent but known strengths. It is customary to employ 

 test-tubes each containing 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, they 

 may then be ' sterilized. After this they are to be in- 

 oculated with the organism upon 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 ' i 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 prevent- 

 ing 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 of the antiseptic ; its antiseptic strength 

 then lies a trifle above the amount present in this tube. 



