BACTERIA 



off a characteristic smell or impart a well-marked 

 colour to their food material; a few even cause the 

 gelatine to become luminous. These easily seen 

 characters are quite as typical of the various 

 bacteria which bring them about as are the micro- 

 scopic characters ; in fact it sometimes happens 

 that only by a combination of the two is it possible 

 to be certain of what one has obtained. 



106 



