SPECIAL MEDIA. 103 



tion special media have been suggested; a few of them 

 will be mentioned. 



Milk. Fresh milk should be allowed to stand over 

 night in the ice-chest, the cream then removed, and the 

 remainder of the milk pipetted into test-tubes, about 

 8 c.c. to each tube, and sterilized by the intermittent 

 process, at the temperature of steam, for three succes- 

 sive days. 



The separation of the cream may be accelerated and 

 rendered more complete by one sterilization of the milk 

 in the cylinder before it is placed in the ice-chest. 



The cream is best separated from the milk by the use 

 of a cylindrical vessel with stopcock at the bottom, by 

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

 drawn off. A Chevalier creamometer with stopcock 

 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 the ordinary nutri- 



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. 



