92 BACTERIOLOGY. 



The bulbs are easily blown, and after having been 

 sealed at the point and plugged with cotton can be kept 

 ■ on hand just as are sterilized test-tubes. 



Special Media. — The media just described — bou- 

 illon, nutrient gelatin, nutrient agar-agar, potato, and 

 blood-serum — are those in general use in the laboratory 

 for purposes of isolation and study of the ordinary forms 

 of bacteria. For the finer points of differentiation 

 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 successive 

 days. 



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 



