14 THE FEEDING OF ANIMALS 



we safely reach the conclusion that the carbon in the 

 tissues of animal life was once floating in space. 



Boussingault determined the average yearly amount of 

 carbon withdrawn from the air by the crops grown on a 

 particular field during a period of five years to be 4,615 

 potmds. A large crop of maize or alfalfa would acquire 

 even more than this. Such large drafts upon the atmos- 

 pheric supply of carbon raise the question whether this 

 supply remains constant. Investigation has shown that 

 it does. The processes of decay through oxidation of 

 vegetable and animal substance on the earth's surface, 

 the combustion of wood and coal as fuel and of food com- 

 pounds by animal life are all returning carbon to the air 

 as carbon dioxid and it would appear that a balance is 

 being maintained. The round traveled in the circula- 

 tion of carbon is from the air to the plant, from the plant 

 to the animal, and from the animal back to the air — a 

 cycle of movement that has always existed and which will 

 continue so long as life exists. 



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

 abundant component of the vegetable and animal tissues. 

 It is second to none in the importance of its relations to the 

 vital processes of nearly all forms of life. We are not 

 familiar with this substance by sight because it is a trans- 

 parent, colorless gas. The air is over one-fifth oxygen by 

 volume. More than 21,000,000 pounds of this element are 

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

 tity which remains remarkably constant and is practically 

 uniform over the entire surface of the globe. Like carbon, 

 it is being continuously withdrawn from the air for pur- 

 poses of combustion and is, like carbon, as continuously 

 retiu-ned. Water contains nearly 89 per cent of oxygen, 

 and it is estimated that the crust of the earth is one-half 



