38 NOURISHMENT OF PLANTS. 



that pure nitrogen is to them no digestible and be- 

 fitting food. And thus it is in fact. The chemist 

 undertakes to explain this indigestibility of nitrogen, 

 from its natural constitution. One of the distinctive 

 chemical properties of this gas is its disinclination to 

 combine with other bodies ; if this is to be accom- 

 plished, it must be brought about by compulsory 

 measures, which the chemist has frequently to apply 

 in a very circuitous method. This unwillingness to 

 give up its natural freedom is apparently so strong, 

 that the plant does not possess sufficient power, to 

 overcome it. 



Except in the atmosphere, we find azotized com- 

 binations only in the organisms of plants and an- 

 imals ; and this is the nitrogen which benefits 

 plants, when, after the death of the organic matter, it 

 has undergone an alteration by putrefaction and de- 

 cay. These processes perform the same important 

 service to the nourishment of plants, that boiling, 

 roasting, and baking render to the nourishment used 

 by man. The nitrogen is thereby carried over from 

 those proximate constituents of animal and vege- 

 table matter which are composed of four elements 

 (page 26) into a simpler combination. Withdraw- 

 ing itself from two of these elements (carbon and 

 oxygen), it remains in combination with the third 

 (hydrogen), and now furnishes the most important 

 and valuable nutrient of plants, we mean ammonia. 

 In a pure form this substance possesses a very pun- 

 gent odor and exceeding volatility ; for it is a kind 



