246 BACTERIOLOGICAL METHODS 



elasticity, producing an undesirable space between the cotton and 

 the constricted part of the tube. 



II. Culture Medium. 



500 grams chopped beef. 

 20 grams peptone (Witte). 

 5 grams sodium chloride. 

 1000 cc. water. 



The beef is digested at 50° C. for M hr., then boiled, strained, 

 the other ingredients added, then boiled again, filtered and ad- 

 justed to + I reaction. 



III. The dilutions of sample and standard are made either by 

 weight or volume, depending on the character of the disinfectant 

 to be tested. In the case of liquids such as the coal-tar disin- 

 fectants, both sample and standard should be diluted by volume. 



The Sample. — Dilutions of an emulsive coal-tar product 

 should be made by adding water gradually to the measured 

 quantity of disinfectant. The reason for this is that in some 

 cases the character of the emulsion is greatly altered by the 

 method of making the dilution. 



An emulsion is less likely to break if it is made as follows: 

 To make a i per cent, solution, moisten the measuring flask or 

 cylinder with about 2 cc. water. With a capacity i cc. pipette 

 measure the disinfectant and mix it with the water, using this 

 mixture for a partial cleaning of the pipette. Then add more 

 water, stirring just enough to mix but not to make the mixture 

 foam. Wash out the pipette by drawing up and expelling the 

 dilution, then make up to the mark. 



This method requires more care in measuring the final dilu- 

 tion if the meniscus is obscured by the emulsion. If the last 

 addition of water is made by carefully running it down the side 

 of the container, the surface liquid will not greatly obscure the 

 reading. 



This I per cent, solution is further diluted to the desired ex- 



