PLANTS WHICH 'J A USE DECAY 381 



those which change milk-sugar into lactic acid and so 

 cause the souring of milk, and (c) those which pro- 

 duce disease. It must not be supposed that these 

 three groups can be sharply separated, for all bacteria 

 cause fermentation and decay in a certain sense. Still, 

 the above division may be used for convenience; let us 

 proceed to examine the first two classes a little more 

 in detail. 



Decay is necessary in order that life may exist; 

 hence the bacteria of decay do an indispensable work in 

 decomposing the dead bodies of animals and plants into 

 such substances as may again be taken up by plants as 

 food and so eventually made available to animals. 



Hardly is an animal dead before the bacteria com- 

 mence the work of decomposition. The proteids, fats, 

 sugars, etc., are rapidly split up into simpler com- 

 pounds, with the result that finally they are nearly all 

 converted into ammonia, carbon dioxide, water and 

 hydrogen sulphide (a gas familiar as the source of the 

 characteristic odor of rotten eggs) . In the decay of 

 plants the same thing occurs, with the addition that 

 certain special bacteria decompose the cellulose and 

 woody fiber into carbon dioxide and water with some 

 evolution of hydrogen and marsh gas. 



The action of these bacteria is further illustrated 

 in the decomposition of manure and sewage. Two 

 methods of purifying sewage by bacterial action are 

 in extensive use. 



