202 REVIEW OF PRECEDING THEORIES. 



the diminution or increase of the mineral substances conveyed to 

 it in manure. 



The formation of the constituents of the blood, and of the vege- 

 table substances containing nitrogen existing in cultivated plants, 

 depends upon the presence of certain substances contained in the 

 soil. When these ingredients are absent, nitrogen will not be 

 assimilated, however abundantly it may be supplied. The am- 

 monia of animal excrements exerts a favorable influence only 

 because it is accompanied by other substances necessary for its 

 conversion into the constituents of blood. When these conditions 

 are furnished with ammonia, the latter becomes assimilated. But 

 when the ammonia is absent from the manure, the plants extract 

 their nitrogen from the ammonia of the air ; to which it is again 

 restored by the decay and putrefaction of dead animal and vege- 

 table remains. 



Ammonia accelerates and favors the growth of plants on all 

 kinds of soil, in which exist the conditions for its assimilation ; 

 but it is quite without action upon the production of the consti- 

 tuents of the blood, when these conditions do not exist. 



It is possible to conceive that asparagin (the active ingredient 

 of asparagus) and the ingredients so rich in nitrogen and sulphur, 

 of mustard and of all Cruciferae, could be generated without the 

 co-operation of the ingredientsof the soil. But if it were possible 

 to form the organic constituents of blood existing in plants, with- 

 out the aid of the inorganic ingredients of the blood, such as 

 potash, soda, phosphates of soda and of lime, they would be of 

 very little use to animals, and could not fulfil the purposes for 

 which they were destined by the wisdom of the Creator. Blood, 

 milk, and muscular fibre cannot be formed without the aid of 

 alkalies and of phosphates ; and bones cannot be produced without 

 phosphate of lime. 



In urine, and in the solid excrements of animals, and in guano, 

 we furnish ammonia, and therefore, nitrogen, in our plants. 

 This nitrogen is accompanied by the mineral food of plants, and 

 actually in the same proportion as both exist in the plants which 

 served the animals for food ; or, what is the same thing, in the 

 same proportion in which both are capable of being applied for a 

 new generation of plants. 



