74 BACTERIOLOGY. 



the cotton into the tube, and their spores have fallen upon 

 the surface of the serum and gone on to develop. 



SPECIAL MEDIA. The media just described bou- 

 illon, nutrient gelatin, nutrient agar-agar, potato, ;m<l 

 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 stop-cock at the bottom, by 

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

 off. A Chevallier creamometer with stop-cock 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 enougir 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. 



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 uutri- 



