METHODS OF ISOLA TION. 77 



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 under the 

 influence of heat and of bacterial growth 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 solidifica- 

 tion, 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 temperature for their development than 

 22°- 24° C. Agar-agar, on the other hand, does not 

 liquefy until the temperature 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 organisms 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 temperature 

 above the melting-point of gelatin. 



In addition to their differences when under the in- 

 fluence of various temperatures, the relations of these 

 two gelatins to bacteria are quite distinct. Many bac- 

 teria bring about alterations in gelatin which cause it to 

 become liquid (a process analogous to peptonization), in 

 which state it remains. There are no known organisms 

 that bring about such a change in agar-agar. 



