METHODS OF ISOLATION. 97 



or Bengal isinglass, which is obtained from a group of 

 algae growing in the sea along the coast of Japan, China, 

 and many parts of the East, where it is employed as an 

 article of diet by the natives. 



The behavior of the two gelatinous substances 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 to 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 to 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 employed in those cases in 

 which the cultivation must be conducted at a tempera- 

 ture above the melting-point of gelatin. 



In addition to their thermal reactions, these two 

 gelatinous substances are affected very differently by 

 different species of bacteria. As we were told above, 

 and shall soon see for ourselves, certain bacteria elabo- 

 rate in the course of their growth digestive enzymes 



or ferments that in their action upon proteid matters 



7 



