58 BACTERIOLOGY 



98°-99° C. It remains fluid ordinarily until the tem- 

 perature has fallen to 38°-39° C, when it rapidly 

 solidifies. For our purposes, only that form of agar- 

 ao-ar can be used which remains fluid at from 38°- 

 40° C. Agar-agar which remains fluid only at a tem- 

 perature above this point would be too hot, when in a 

 fluid state, for use ; many of the organisms which would 

 be introduced into it would either be destroyed or 

 checked in their development by so high a temperature. 

 Agar-agar, therefore, is for use in those cases in whicli 

 the cultivation must be conducted at a temperature above 

 that at which gelatin remains solid. 



In addition to the differences toward temperature, 

 the relations of these two gelatins to bacteria are differ- 

 ent. Many bacteria bring about alterations in gelatin 

 which cause it to become liquid (probably a process of 

 peptonization), in which state it remains. There are no 

 known organisms wliich bring about such a change in 

 the agar-agar. 



As a rule, the colony-formations seen upon gelatine 

 are much more characteristic than those which develop 

 on agar-agar, and for this reason gelatin is to be pre- 

 ferred when circumstances will permit. Both gelatin 

 and agar-agar may be used in the preparation of plates 

 and Esmarch tubes, subsequently to be described. 



