26 ELEMENTS OF AGRICULTURE 



oxygen in our food. We could not live for a moment 

 without it. 



Oxygen alone is too active to support properly the 

 growth of plants and animals. In the air it is mixed 

 with a slow, inactive sort of gas called nitrogen. 



25. Nitrogen. — In its habits this element is very 

 much the reverse of oxygen. It makes up nearly four- 



' fifths of the volume of the air, or 77 per cent by weight, 

 but does not readily combine with other substances. It 

 is, however, very useful in agriculture, as no plant or 

 animal can grow without a supply of nitrogen. But a 

 few plants have the power of obtaining nitrogen from 

 the air, and animals obtain their nitrogen only from 

 plants. Small quantities of oxygen and nitrogen are 

 found in the air, combined with two other elements 

 called hydrogen and carbon. 



26. Hydrogen. — This element is a gas, and is the 

 lightest of all known substances. All of the hydrogen 

 found in the air is combined with other elements. Com- 

 bined with oxygen it forms water; combined with 

 nitrogen it forms ammonia. 



27. Carbon. — This element is a solid substance, and 

 is well known in the shape of mineral coal, which is 

 nearly all carbon. The diamond is also very nearly pure 

 carbon, and so is the lead of an ordinary lead pencil. 

 Carbon when heated in excess of air unites very readily 

 with oxygen, forming the compound, carbon dioxide. 

 Thus when coal is burned most of it disappears into 

 the atmosphere as a gas, carbon dioxide, leaving behind 

 only a few ashes. A large part of all plant and animal 

 bodies is made up of carbon, 



