THE LAWS OF LIFE. 241 



fitted for the more [elevated grade on which it is 

 about to enter. 



The process of digestion, however, presents other 

 points which claim remark. 



In the first place, those substances only which are 

 undergoing, or tending to chemical change, will serve 

 as foocir The food supplies not only the materials 

 for nutrition, and for the production of heat, but, in 

 the form of its decomposing action, the life of the 

 being which consumes it. Without the chemical 

 action in the food, no amount of materials and of 

 heat avails for nutrition. 



Plants live upon carbonic acid and ammonia ; but, 

 as stated by Miiller, carbonate of ammonia, in every 

 form, is a poison to them. 



2ndly. The various kinds of food produce various 

 forms of vital action, quite independently of the 

 materials of which they consist. The food of all 

 animals scarcely varies in its elements. But in 

 the nature and intensity of the chemical changes 

 it undergoes, it presents innumerable and most 

 important variations. And upon these, as one 

 element, the characteristic differences of vital action 



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