472 BACTERIOLOGY. 



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 not- 

 withstanding the presence of the antiseptic, and its anti- 

 septic 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 pro- 

 portion 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 staphylo- 

 coccus 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 thoroughly, 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 differences to be explained ? To what 

 strength of the disinfectant were the organisms ex- 

 posed in the experiment ? 



