METHODS IN ISOLATION. 103 



in some and trypsin in other instances. When bacteria 

 endowed with this physiological property are cultivated 

 upon bone gelatin their growth is accompanied by the 

 progressive digestion (liquefaction) of the gelatin, which 

 liquefied gelatin cannot again be brought to a solid con- 

 dition. We know of no bacteria capable of producing 

 a similar liquefaction of agar-agar or vegetable gum. 

 This striking difference between the two gelatinous sub- 

 stances under the influence of bacterial activity is one 

 of the most important and commonly employed differ- 

 ential reactions in the identification of species. 



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. 



