256 BACTERIOLOGICAL METHODS 



disinfectants as alcohol, formalin and acids, and i per cent, 

 solutions of the salts of heavy metals and of soluble substances 

 generally. From these primary stock solutions the following 

 secondary dilutions or substock solutions are made, always in 

 those amounts which will serve the purpose, that is, in amounts 

 for perhaps ten subdilutions for each and every disinfectant to 

 be tested. 



I : 10 (of liquids only.) 



I :ioo 



I : 1000 



I : 10,000 



I : 100,000 



Method of Testing 



1. Phenol Standard. — Pour 5 cc. of the egg albumen solution 

 in a standard test-tube, using a standard 5 cc. pipette having a 

 free outflow. Add to this 5 cc. of the phenol stock solution (5 

 per cent.). Set tube in the standard test rack (with black back- 

 ground made of cardboard covered with black tissue paper). 

 The degree of opacity developed is to serve as the standard of 

 comparison. 



2. Preliminary Testing. — The albumen coagulating power of 

 the disinfectant being unknown, much time and labor can be 

 saved by testing with the four or five substock solutions, adding 5 cc. 

 to 5 cc. of the egg albumen test solution, in order to find that dilu- 

 tion of the disinfectant which fails to show any opacity. We will 

 suppose^ that the i : 1000 substock solution shows very marked 

 opacity or precipitation, then the i : 10,000 solution might be tried, 

 which may also show quite marked opacity, then the i : 100,000 

 may be tried. If this gives negative results then we know that 

 the phenol standard lies between 1 : 10,000 and i : 100,000 with the 

 probabilities that it is nearer i : 10,000. 



3. Concluding Testing. — Going back to the i : 10,000 dilution, 

 make ten subdilutions, increasing the dilutions by a difference of 



