14 The Feeding of Animals 



Oxygen. — This element is, next to carbon, the most 

 abundant component of vegetable and animal tissues, 

 and it stands second to none in its relation to the vital 

 processes of nearly all forms of life. It is not a sub- 

 stance with which we are familiar by sight, because we 

 ordinarily come in contact with it as a transparent, 

 colorless gas. We live and move in it, for it is an im- 

 portant and uniformly abundant constituent of the at- 

 mosphere. The air is over one-fifth oxygen by volume, 

 the proportion by weight being slightly larger. More 

 than twenty -one million pounds of this element are 

 contained in the air above a single acre of land, a 

 quantity which remains remarkably constant, and which 

 is surprisingly uniform over the entire surface of the 

 globe. While it is being continuously withdrawn from 

 the air for the uses of life and to maintain fuel com- 

 bustion and processes of decay, it is, like carbon, as 

 continuousl}^ returned. 



Vast quantities of oxygen are also contained in 

 water, as this compound, which fills the ocean and 

 lakes, and is abundant in the crust of the earth, is 

 nearly 89 per cent oxygen. It is estimated also that 

 the solids in the ci'ust of the earth are one -half oxy- 

 gen. That which enters directly into the uses of ani- 

 mal life is, however, chiefly that which is derived from 

 the atmosphere and water. 



Not a plant grows or animal lives excepting through 

 the circulation of oxygen, during which it passes into 

 fixed combinations and back again to the free form. 

 The animal uses the free oxygen in breathing and re- 

 turns it to the air in part combined with carbon as car- 



