Carbon 13 



cent, and as the weight of a column of air one inch 

 square is fifteen pounds, it follows that over every acre 

 of land there is 28.2 tons of carbon dioxid, or 7.7 tons 

 of carbon. As we know that plants draw their supply 

 of this element from the atmosphere, and as vegetable 

 tissue is its only source to the animal, we are able to 

 assert, with confidence, that the carbon in the tissues 

 of animal life was once floating in space. 



A long time ago, Boussingault determined the aver- 

 age yearly amount of carbon which was withdrawn from 

 the air by the crops grown on a particular field during 

 a period of fiv^e years, and found it to be 4,615 pounds. 

 This is no more than is acquired by a large crop of 

 maize. As a matter of fact, plants, as well as animals, 

 contain a larger proportion of this element than of any 

 other, and the amount of this substance which enters into 

 the processes of growth and decay in the vegetable and 

 animal kingdoms is almost beyond comprehension. It 

 is natural to wonder whether the atmospheric supply is 

 equal to the demand. Any anxieties we may have con- 

 cerning this should be removed by learning that during 

 many years the percentage of atmospheric carbon has 

 not changed appreciably. The processes of decay on 

 the earth's surface, the combustion of wood and coal 

 as fuel and of carbon compounds by animal life are re- 

 turning carbon to the air as rapidly as it is being with- 

 drawn. This is the round traveled, — from the air to 

 the plant, from the plant to the animal, and from the 

 animal back to the air, — a cycle in which this element 

 has been moving since life began, and in which it will 

 continue to move so long as life exists. 



