BACTERIAL FERMENTATIONS IN SUGARS. 85 



carbon dioxide, hydrogen, and methane). For the estima- 

 tion of the two former groups complicated chemical pro- 

 cedure may be necessary. The formation of gases is, 

 however, usually taken as the criterion of the possession of 

 fermentative properties. Generally speaking, it is reliable, 

 and the methods to be pursued are simple. It must not 

 be forgotten, however, that some organisms give rise to 



l^VY/ 

 }'*< 



& 



FIG. 35. Tubes for demonstrating gas-formation by bacteria. 



#, tube with "shake" culture. 



b, Durham's fermentation tube. 



c, ordinary form of fermentation tube. 



sulphuretted hydrogen by breaking up the proteid present. 

 The formation of this gas can be detected by the blackening 

 of lead acetate when it is added to the gas-containing 

 medium. The following are the chief methods for detecting 

 the formation of gas : 



(i) Gelatine Shake Cultures (Fig. 35 a). The gelatine 

 in the tube is melted as for making plates ; while liquid it 

 is inoculated with the growth to be observed, and shaken 

 to distribute the organisms throughout the jelly. It is then 

 allowed to solidify, and is set aside at a suitable tempera- 



