98 BACTERIOLOGY. 



are strikingly like pepsin 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 can- 

 not again be brought to a solid condition. We know of 

 no bacteria capable of producing a similar liquefaction 

 of agar-agar or vegetable gum. This striking difference 

 between the two gelatinous substances under the influ- 

 ence of bacterial activity is one of the most obvious 

 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. 



