158 REVIEW OF THE PRINCIPLES OF NUTRITION. 



water, which served merely for its conveyance, and to assimilate it into 

 food capable of being organized into cells and their various contents. 



And it is proper in this place also to notice the effects of this vast 

 machinery upon the -constitution of the atmosphere and its relation to 

 the animal kingdom. 



825. CARBONIC ACID gas is dissolved in the atmosphere and some- 

 what uniformly diffused throughout its whole extent in the proportion 

 of about 4 parts in 10,000, or ^jo- This gas flows, and is ever flow- 

 ing into the air from decaying animal and vegetable substances, from 

 combustion, and from the breath of all living animals. The quantity 

 thus added to the atmosphere annually is estimated at 100 billions Ibs., 

 or nearly one tenth of the whole amount of carbon, and yet it does not 

 accumulate. 



826. THE DEMAND AND SUPPLY. Were we able ~to compute in 

 pounds the annual growth of the entire plant world, and the proportion 

 of solid carbon which enters into that amount, we should doubtless find 

 that the grand total of the demand equals this grand total supply. 



A POISONOUS ATMOSPHERE. And further ; not only are the necessi- 

 ties of the plant met by this wonderful circulation, but the necessities 

 of animal existence also. Carbonic acid is poisonous, and should it be 

 left to accumulate unchecked, it would gradually corrupt the air, and 

 within a few centuries extinguish all animal life. 



828. ANIMALS AND PLANTS MUTUALLY DEPENDENT. Thus are the 

 two kingdoms of the organic world mutually, through the inorganic, 

 dependent upon each other. The plant furnishes the oxygen which 

 the animal consumes, the animal the carbonic acid which the plant 

 consumes, while each would perish in an atmosphere of its own pro- 

 duction. " Great and marvelous are thy works, O Lord of Hosts ! in 

 wisdom hast thou made them all." 



CHAPTER VII. 



KEVIEW OF THE PRINCIPLES OF NUTRITION. 



829. THE FOUR ORGANOGENS. It has already appeared in the pre- 

 ceding chapters that plants consist chiefly of four simple organic ele- 

 ments, viz. : carbon, oxygen, hydrogen, nitrogen. The first exists in a 

 larger proportion, the last in a smaller than either of the others. 

 Unitedly these four elements constitute about 94 per cent, of all vege- 

 table matter. 



