78 THE MANURE HEAP AND SEWAGE. 
material may be obtained from seaweeds, if they are at hand. In- 
deed, any abundant vegetable substances may thus be heaped into a 
mass, and, if moistened sufficiently by rains, the bacteria will be 
sure to work within it, gradually transforming the nitrogens, and 
converting them finally into nitrates for plant food. Into his 
garbage heap, then, the farmer may throw all sorts of organic 
debris, animal or vegetable, with the confidence that his bacterial 
aids will in time place the nitrogenous material at his service as a 
fertilizer. Thus, by the aid of his invisible allies the agriculturist 
will be able to make use of the wastes on his farm and in time return 
to his soil a considerable portion of the nitrogen. 
SEWAGE AND ITS TREATMENT. 
Composition of Sewage. By sewage we ordinarily understand 
the material which collects in the sewerage system of our larger com- 
munities and which has no exact counterpart on the farm. It al- 
ways contains the products of the life of men and animals, which are 
no longer useful; also large quantities of both animal and vegetable 
foods which have passed through the alimentary canals of men and 
animals unassimilated. It contains a large amount of urea which 
has come. from the animal metabolism; and also woody matter, 
cellulose, fat, starch, 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 quantities of 
water, it contributes to the sewage. The sewage thus contains 
exactly the same sort of material as that 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. 
TREATMENT OF CITY SEWAGE. 
As cities have grown, the matter of disposing of their sewage be- 
comes more and more difficult. In small communities the digging 
of cess-pools is satisfactory; but as larger numbers of people con- 
