EXPERIMENTS. 261 



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

 is not as well as that at which it w 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 development 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 represents 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 

 mierococcus 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? 



