METHODS OF ISOLATION. 93 



trypsin in other instances. AV^hen bacteria endowed 

 with this physiological property are cultivated upon 

 bone gelatin their growth is accompanied by the pro- 

 gressive 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 gelatin. 

 This striking difference between the two gelatins under 

 the influence of bacterial activity is one of the most im- 

 portant and commonly employed differential 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. 



