126 BACTERIOLOGY 



a cylindrical vessel with a stopcock at the bottom, by 

 means of which the milk, devoid of cream, may be drawn 

 off. A Chevalier creamometer with a stopcock at the bottom 

 serves the purpose very well. It should be covered while 

 standing.! 



Milk may be used as a culture-medium without any addi- 

 tion to it, or, before sterilizing, a few drops of litmus tinc- 

 ture 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 propor- 

 tions given for the preparation of ordinary nutrient gelatin 

 or agar-agar. It has, however, in this form the disadvan- 

 tage 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 precipitation 

 of the casein. 



Litmus-whey Milk. — ^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 satisfactory. 



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. 



