DESIGN IN NATURE 279 



pounds that compose the crust of the earth, so that 

 there would be no free water, and life could not ex- 

 ist. If it had existed in much greater quantity than 

 at present, it would have combined with all the free 

 oxygen of the air, thus forming more water, and ren- 

 dering animal life impossible. 



Carbon is a third element that is necessary in every 

 living being. It exists largely in carbonates, of which 

 limestone is the chief; also, in coal, coal-oil, and 

 "natural gas." One of its most important com- 

 pounds is carbon dioxide, or carbonic acid, which ex- 

 ists in small quantity in the air, and from this gas 

 plants obtain their supply of carbon. 



If the quantity of carbon had been largely increased 

 it would have combined with all the free oxygen of 

 the air, converting it into carbon dioxide, thus ren- 

 dering life impossible. 



If the quantity had been much smaller than it is, 

 then none would have been stored up in the form of 

 coal for the use of man, and the supply in the atmos- 

 phere would long since have been exhausted by plants, 

 thus rendering the earth lifeless. 



Nitrogen is a fourth necessary element. Free nitro- 

 gen comprises about four-fifths of the volume of the 

 air. It also helps to form certain compounds which 

 exist in small quantities in the soil, and which are nec- 

 essary food for plants. These compounds do not 

 accumulate in large quantities in the soil, owing to 

 the fact that they are very soluble and are conse- 

 quently carried away by running water into the oceans. 

 Besides, their presence in large quantities in the soil 

 would be destructive to vegetation. 



To keep up the necessary supply of nitrogen com- 

 pounds in the soil, it is claimed that the free nitrogen 

 of the air is, by the process of nitrification, being 

 slowly converted into food for plants. The supply of 



