METHODS OF TESTING DISINFECTANTS. 257 



instead of transferring the drop directly to the fresh 

 medium, add it to 10 or 12 c.c. of sterilized salt-solu- 

 tion (0.6-0.7 per cent, of NaCl in distilled water) or 

 distilled water, and after thoroughly shaking add a drop 

 of this to the medium in which the power of develop- 

 ment of the bacteria is to be determined. 



Another important point to be borne in mind in 

 testing disinfectants is the necessity of so adjusting the 

 conditions that each individual organism will be ex- 

 posed to the action of the agent used. When clumps 

 of bacteria exist we are not always assured of this, for 

 only those on the surface of the clump may be affected, 

 while those in the centre of the mass may escape, being 

 protected by those surrounding them. These clumps 

 and minute masses are especially liable to be present 

 in fluid cultures and in suspensions of bacteria, and 

 must be eliminated before the test is begun, if this is to 

 be made by mixing them with solutions of the agent to 

 be tested. This is best accomplished in the following 

 way : the organisms, should be cultivated in bouillon 

 containing sand or finely divided particles of glass; 

 after growing for a sufficient length of time they are 

 to be shaken thoroughly, in order that all clumps may 

 be mechanically broken up by the sand. The culture is 

 then filtered through a tube containing closely packed 

 glass-wool. 



The filtration may be accomplished without fear of 

 contamination of the culture by the employment of an 

 Allihin tube, which is practically a thick-walled test- 

 tube drawn out to a finer tube at its blunt end so as to 

 convert it into a sort of cylindrical funnel. The tube 

 when ready for use has the appearance shown in Fig. 

 52), 



