318 APPLICATION OF METHODS OF BACTERIOLOGY 



drop directly to the fresh medium, add it to 10 or 12 c.c. 

 of sterihzed salt-solution (0.6-0.7 per cent, of NaCl in dis- 

 tilled water) or distilled water, and after thoroughly shaking 

 add a drop of this to the medium in which the power of 

 development 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 exposed 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 con- 

 taining sand or finely divided particles of glass; after grow- 

 ing for a sufificient 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 con- 

 tamination 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. 65. 



This tube, after being plugged at the bottom with glass- 

 wool (a. Fig. 65), and at its wide extremity with cotton- 

 wool, is placed vertically, small end down, into an Erlen- 



