7 o SCIENTIFIC AGRICULTURE. 



quantities are prepared in Europe which command 

 a good price, but in this country it is used to a 

 very limited extent. The efforts thus far made 

 to utilize the sewage of cities have in some in- 

 stances proved successful, but there are difficulties 

 yet to be removed before its profitable use can 

 become general. 



157. Animal manures are richer than vegetable 

 because they contain more nitrogen, which in the 

 process of fermentation and decay unites with hy- 

 drogen to form ammonia, and because they also con- 

 tain a larger proportion of valuable inorganic ele- 

 ments. The effect of respiration and digestion, the 

 two great vital functions of the animal system, is to 

 extract carbon and hydrogen from the food through 

 the blood and lungs where they escape as carbon 

 dioxide and water. The water in large quantities 

 passes through the pores of the skin, leaving the 

 other constituents in a more concentrated form. 



CHAPTER VIII. 



MINERAL FERTILIZERS. 



158. THE list of mineral fertilizers embraces 

 a large number of substances, only a few of which, 

 however, will be mentioned, such as lime, marl, gyp- 

 sum, salt, and ashes. 



