THE CHEMICAL ELEMENTS 17 



and sometimes below it. The large quantity found in the 

 air is inert for animal uses, and is ignored by a large 

 majority of plants. Much of that in the soil is also un- 

 available. Its immediately useful compounds on the farm 

 are constantly subject to loss through fermentations which 

 the farmer can not wholly prevent and through soil losses 

 which are to some extent beyond control. The sale of 

 crops removes from the farm much nitrogen. The sources 

 of supply to balance this outgo are the nitric acid and 

 ammonia of the rainfall, the free nitrogen captiu-ed by a 

 class of plants known as legumes, that which is secured 

 through purchase of fertilizers and the residues of animal 

 foods. These facts relate primarily to plant production, 

 but they also sustain an essential relation to the main- 

 tenance of animal life. 



14. Uses of nitrogen. — ^Physiologically, the nitrogen 

 compounds of foods stand in the front rank. These com- 

 pounds are necessary building-material for the funda- 

 mental tissues of the animal, and are intimately related to 

 prominent chemical changes which are involved in growth 

 and in the maintenance of life. 



Nitrogen compoimds have come to have an important 

 place in commerce. It is the most costly ingredient of 

 fertihzers and the value of commercial cattle foods is in 

 part dependent upon their content of these compounds. 

 For all these reasons, the partial control which the farmer 

 might now assume over the income and outgo of nitrogen 

 compounds has become an important feature of farm 

 economics. (See Par. 59.) 



15. Argon. — ^Argon exists in the atmosphere to the 

 extent of about nine-tenths of 1 per cent. So far as known, 

 argon does not function in vegetable and animal life. 



16. Sulfur. — ^This common and familiar substance is 



