12 The Feeding of Animals 



portant ingredients of the atmosphere; viz., carbon, 

 oxygen, hydrogen and nitrogen, and they also exist in 

 the soil in gases, as well as in combination in liquids 

 and solids; the other eleven, though sometimes present 

 in the air in minute quantities, are found to no appre- 

 ciable extent except as fixed compounds in water and 

 in the crust of the earth, or in plants and animals. 

 Nearly all of these elementary substances are absolutely 

 essential to the existence of animal life as now con- 

 stituted. From the standpoint of necessity, they are, 

 therefore, nearly all of equal value, but if we take into 

 consideration the relative ease and abundance of the 

 supply, certain ones rise to a position of supreme im- 

 portance. 



THE ELEMENTS AND THEIR SOURCES 



Carbon. This is a familiar substance in common 

 life. Anthracite coal and charcoal are examples of im- 

 pure carbon. Graphite in lead pencils is also carbon, 

 and so are diamonds. When wood chars or food is 

 burned in an overheated oven the partially decomposed 

 materials become black, revealing the presence of car- 

 bon, the other elements with which it was associated 

 being driven out. The humus of the soil is vegetable 

 matter, which, from other causes, has undergone some- 

 what the same change. 



An immense quantity of carbon exists in the air, 

 combined with oxygen as carbon dioxid or carbonic 

 acid gas. The average proportion by weight of this 

 compound in the atmosphere is stated to be .06 per 



