132 BACTERIOLOGY. 



at the bottom serves the purpose very well. It should 

 be covered while standing. 1 



Milk may be used as a culture-medium without any 

 addition to it, or, before sterilizing, a few drops of 

 litmus tincture may be added, just enough to give it a 

 pale-blue color. By this means it will be seen that 

 different organisms bring about different reactions in 

 the medium : some producing alkalies, which cause the 

 blue color to be intensified ; others producing acids, which 

 change it to red ; while others bring about neither of 

 these changes. Similarly litmus solution is often added 

 to gelatin and agar-agar for the same purpose. 



Milk may also be employed as a solid culture-medium 

 by the addition to it of gelatin or agar-agar in the pro- 

 portions given for the preparation of ordinary nutri- 

 ent gelatin or agar-agar. It has, however, in this form 

 the disadvantage of not being transparent, and can 

 therefore best be used for the study of those organisms 

 which grow upon the surface of the medium without 

 causing liquefaction. 



Nutrient gelatin and agar-agar can also be prepared 

 from neutral milk-whey, obtained from milk after pre- 

 cipitation of the casein. 



Litmus-milk-whey. An important differential medium 

 is milk-whey to which litmus tincture has been added. 

 A number of methods for its preparation are in use, but 

 the one employed by Durham seems to be the most sat- 

 isfactory. Briefly it is as follows : fresh milk, free 

 from antiseptic adulterations, is gently warmed and 

 clotted with essence of rennet. The whey is strained 



1 For some time past we have been using what is technically known 

 as " separator milk " i. e., the fluid left after milk has been deprived 

 of its fat (cream) by centrifugal force. 



