THE MANURE HEAP AND SEWAGE. I1Q 



which bear so closely upon agriculture that at least a brief 

 consideration is necessary. The problem of the bacteria in 

 sewage is very closely related to that which we have just been 

 considering ; for sewage composition and sewage disposal in- 

 volve all of the subjects of decomposition and putrefaction 

 which have such an intimate relation to soil fertility. Indeed, 

 the most recent method of sewage disposal depends upon the 

 very same principles of chemical decomposition by bacteria 

 which have been considered in the last chapter. 



Composition of Sewage. By sewage we commonly under- 

 stand the material which collects in the sewerage system of 

 our larger communities and which has no exact counterpart on 

 the farm. It always contains the products of the life of men 

 and animals, which are thrown away as no longer useful ; also 

 large quantities of both animal and vegetable foods which have 

 passed through the alimentary canal of men and animals unas- 

 similated. It contains a large amount of urea which has come 

 from the animal metabolism ; it contains woody matter, cellu- 

 lose, fatty matter, starchy matter, and an indefinite series of 

 other organic bodies. Almost anything which enters the city 

 may find its way eventually into the sewers where, mixed with 

 large amounts of water, it contributes to the sewage. The 

 sewage thus contains exactly the same sort of material found 

 in the manure heap and the compost pile. Evidently the 

 problem of the various steps of decomposition of this material 

 will be nearly identical with that already considered at length. 



But the practical application of the matter is quite different. 

 This is due partly to the fact that the chief aim is to get rid 

 of sewage rather than to utilize it, and partly to the great 

 modification produced by the presence of such large amounts 

 of water. Upon the farm the problem of sewage is of little 

 importance. The material, which in the city enters the sewage, 

 is separated into several portions on the farm, each of which is 

 differently utilized. Part of it goes to the manure heap, part 



