EXPERIMENTS. 45I 



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, and its antiseptic strength 

 falls a trifle above the amount present in this tube. 

 If, for example, there was development in the tubes in 

 which the antiseptic was present in the proportion of 

 1:1000 and no growth in the one in which it was 

 present in 1:1400, the experiment would be repeated 

 with strength of the antiseptic corresponding to 1:1000, 

 1:1100, 1:1200, 1:1300, 1:1400, and in this way one 

 gradually strikes the point at which growth is just pre- 

 vented. This point represents the antiseptic value of 

 the substance used for the organism upon which it has 

 been tested. 



EXPERIMENTS. 



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

 normal salt solution, another of bouillon, a third of 

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

 staphylococcus pyogenes aureus as can be held upon 

 the looped platinum needle. Mix this thoroughly, so 

 that no clumps exist, and then add exactly 10 c.c. of 

 1 : 500 solution of corrosive sublimate. Mix it thor- 

 oughly, and at the end of three minutes transfer a drop 

 from each tube into a tube of liquefied agar-agar, and 

 pour this into a Petri dish. Label each dish carefully 

 and place them in the incubator. Are the results the 

 same in all the plates ? How are the difierences to be 

 explained ? To what strength of the disinfectant were 

 the organisms exposed in the experiment? 



