72 BACTERIOLOGf. 



in bacteriological work. The arfimal gelatin liquefies 

 at a much lower temperature, and likewise solidifies at 

 a very much lower temperature, than does the agar- 

 agar. Ordinary gelatin, in the proportion commonly 

 used in this work, liquefies at about 24° C, and 

 becomes solid at from 8°-10° C. It may be employed 

 for those organisms which do not require a higher tem- 

 perature for their development than 22° C. Agar-agar, 

 on the other hand, does not liquefy until the tempera- 

 ture has reached about 98°-99° C. It remains fluid 

 ordinarily until the temperature has fallen to 38°-39° 

 C, when it rapidly solidifies. For our purposes, only 

 that form of agar-agar can be used which remains fluid 

 at from 38°-40° C. Agar-agar which remains fluid 

 only at a temperature above this point would be too 

 hot, when in a fluid state, for use ; many of the organ- 

 isms 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 which 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 (a process of peptoniza- 

 tion), in which state it remains. There are no known 

 organisms that bring about such a change in agar-agar. 



As a rule, the colony- formations seen upon gelatin 

 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. 



