106 BACTERIOLOGY 



In practice two gelatinous substances are employed — 

 the one an animal or bone gelatin, the ordinary table gelatin 

 of good quality; the other a vegetable gum, known as 

 agar-agar, the native name for Ceylon moss or Bengal 

 isinglass, which is obtained from a group of marine algte 

 found 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 re- 

 quires 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 temperature for their develop- 

 ment 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 employed in those cases in which the cultivation must be 

 conducted at a temperature above the melting-point of 

 gelatin. 



In addition to their thermal reactions, these two gelati- 

 nous substances are affected very differently by different 



