SPECIAL MEDIA. 131 



escapes pretty rapidly and exerts no preservative action. 

 In fact, bacteria will grow uninterruptedly in a cotton- 

 stoppered test-tube containing bouillon to which chloro- 

 form has been added. When required for use, the 

 serum is decanted into test-tubes, which are then placed 

 in a water-bath at about 50 C. until all the chloro- 

 form has been driven off; this can be determined 

 by the absence of its characteristic odor. The serum 

 may then be solidified, sterilized by heat, and em- 

 ployed for culture purposes. We have found serum 

 so preserved to answer all requirements as a culture- 

 medium. 



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 differentia- 

 tion special media have been suggested ; a few of them 

 will be mentioned. 



Milk. Fresh milk should be allowed to stand over 

 night in an 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 if the cylinder containing the 

 milk be placed in the steam sterilizer for fifteen minutes 

 before it is placed in the ice-chest. 



The cream is best separated from the milk by the use 

 of 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 



