620 BAGTEBIOLOQY. 



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

 is not as well as that at which it in 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 pres- 

 ence of the antiseptic ; the amount necessary for antisep- 

 sis is then a trifle greater than that used in the last tube. 

 If, for example, there was no develoiDment in the tubes 

 in which the antiseptic was present in the proportion 

 of 1 : 1000, and growth in the one in which it was 

 present in 1 : 1400, the experiment should be repeated 

 with strengths of the antiseptic corresponding to 

 1 : 1000, 1 : 1100, 1 : 1200, 1 : 1300, 1 : 1400, and in 

 this way one ultimately determines the amount by which 

 growth is just prevented ; this rej)resents the antiseptic 

 value of the substance for the organism with which it 

 was tested. 



EXPERIMENTS. 



To each of three tubes containing 10 c.c. — one of 

 physiological salt-solution, another of bouillon, a third of 

 fluid blood-serum — add as much of a culture of staphylo- 

 coccus pyogenes aureus as can be held upon a looped 

 platinum wire. Break this up carefully to eliminate 

 clumps, and then add exactly 10 c.c. of a 1 : 500 solution 

 of corrosive sublimate. Mix thoroughly, and at the 

 end of three minutes transfer a drop from each tube 

 into tubes of liquefied agar-agar, and pour these into 

 Petri dishes. Label each dish carefully and place them 

 in the incubator. Are the results the same in all the 

 plates ? How are the differences to be explained ? To 

 what strength of the disinfectant were the organisms 

 exposed in the experiment? 



