THE MANURE HEAP AND SEWAGE. 121 



may commonly be left to care for itself. The amount of solid 

 material in such water is very slight, and it can be allowed 

 to run out on the soil where, generally, it is rapidly absorbed 

 and decomposed without any undue pollution. The organic 

 matter undergoes the same type of decomposition to which all 

 organic bodies are subjected under the influence of bacteria, 

 and becomes eventually converted into plant food and incor- 

 porated into the soil. The drainage which comes from the 

 large dairy or creamery may, however, be considerable, too 

 much to be disposed of by such a simple manner. In this 

 case some means must be adopted for disposing of it. The 

 problem thus presenting itself for the disposal of this drainage 

 is precisely the same as that presented to the city for disposing 

 of its sewage, and the same means are to be used in each case. 

 Where the amount of sewage becomes large, as in the case 

 of a large city, it offers quite a different series of problems and 

 presents some important questions for the sanitary engineer. 

 The disposal of the sewage of a city is also becoming a matter 

 of some importance to agriculture. The chief reason for the 

 difficulty which this problem presents is the large amount of 

 water which is present in the sewage. It is true that sewage 

 contains almost every conceivable kind of organic substance 

 washed into it from the city, and the total amount of organic 

 matter is very large, but this is always mixed with an ex- 

 tremely large amount of water, especially during rains. Hence, 

 the total result is an extremely dilute but very abundant mix- 

 ture. One who is not actually familiar with sewage can hardly 

 realize its extreme dilution, especially in American cities. 



SEWAGE FARMS. 



The fact that sewage contains nitrogen, which is known to be 

 of such use to vegetation, has led to a general feeling that in 

 this waste product there is a most valuable amount of plant 

 food which is ordinarily thrown away. To prevent this waste 



