160 MANURE. 



224. All fats and oils exposed to air give off a 

 great quantity of carbonic acid, and end by becom- 

 ing themselves acids. As their ultimate elements 

 are the same as those of plants, it may be inferred, 

 that under the influence of growing plants, fats and 

 oils are decomposed and become vegetable food. 

 But there is another action of fats and oils on sili- 

 cates ; they not only let loose the alkali of silicates 

 by the carbonic acid, which they evolve, but the 

 oils now become acids, immediately combine with 

 this alkali, and imperfect soaps are formed. Soaps 

 are truly chemical salts, and hence we have at once 

 a clue to the action of oil and fat. 



225. Among the most powerful of manures in the 

 class composed of geine and salts, is soot. There 

 is no one substance so rich in both. Its composi- 

 tion allies it to animal solids, and is as follows : 



100- 



