METHODS OF ISOLATION. 75 



the coast of Japan, where it is employed as an article of 

 diet by the natives. 



Aside from these differences in origin of the two forms 

 of gelatin employed, their behavior toward heat and 

 toward bacteria renders them of different application 

 in bacteriological work. The animal gelatin liquefies 

 at a much lower temperature, and also requires a lower 

 temperature for its solidification, than does the agar- 

 agar. Ordinary gelatin, in the proportion commonly 

 used in this work, liquefies at about 24-26 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 22-24 C. Agar- 

 agar, on the other hand, does not liquefy until the tem- 

 perature 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 is for use in those cases in 

 which the cultivation must be conducted at a tempera- 

 ture above the melting point of gelatin. 



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 



