PLANTS WHICH CAUSE DECAY 383 



infusion which has a thick film of bacteria over it and 

 pass a current of air into it by means of the apparatus 

 shown in Fig. 158. Observe the rapidity with which 

 the water becomes clear. 



From these illustrations we may gain some idea of 

 the useful work done by the bacteria, which have been 

 appropriately called the scavengers of the world. 



The decomposition of manure is effected by bac- 

 teria, the most important product from a practical 

 standpoint being ammonia gas. The ammonia gas is 

 in turn acted on by a special class of bacteria, the 

 nitrifying bacteria. Of these there are two kinds, — 

 the nitrous bacteria, which convert ammonia gas into 

 nitrous acid (and nitrites) ; and the nitric bacteria, 

 which are so sensitive to ammonia gas that they 

 cannot begin to work till it has all disappeared, but 

 which have the power of converting nitrous acid (and 

 nitrites) into nitric acid (and nitrates), in which form 

 it can be used by green plants. In order to carry on 

 their work the nitrifying bacteria must have plenty of 

 air. Hence it would seem to be useful to allow a 

 circulation of air in the manure -heap: it is found, 

 however, that there are denitrifying bacteria which 

 convert nitric acid into free nitrogen, which escapes 

 into the air ; it is better, therefore, to keep the heap 

 closed to the air until the denitrifying bacteria have 

 ceased their growth, after which air may be admitted 

 to stimulate the work of the nitrifying bacteria (see 



