SPECIAL MEDIA. 99 



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 successive 

 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. 



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 may be seen that dif- 

 ferent 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- 

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

 the disadvantage of not being transparent, and can there- 

 fore 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 



