THE CHEMICAL ELEMENTS . 13 



found to no appreciable extent outside of plants and 

 animals except as fixed compounds in water and in the 

 crust of the earth. These fifteen elementary substances 

 are nearly all absolutely essential to the existence of 

 animal life as now constituted. 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 are greatly more 

 important than the others. 



THE ELEMENTS AND THEIR SOURCES 



7. Carbon.^-This is a familiar siibstance in common 

 life. Coal and charcoal consist chiefly of carbon, while 

 graphite, much used in lead-pencils and diamonds, is 

 pure carbon. Carbon makes up a large proportion of 

 vegetable and animal tissue. This is made evident when 

 vegetable and animal tissues become black through heat- 

 ing to a temperature which causes them to decompose, 

 leaving the carbon as a residue while the elements with 

 which it was associated are driven out. The dark humus 

 bodies of the soil have undergone somewhat the same 

 change. 



8. Carbon in the air. — ^An immense quantity of carbon 

 exists in the air, combined with oxygen as carbon dioxid. 

 The average proportion of this compound in the atmos- 

 phere by weight is approximately .06 per cent. As the 

 weight of a column of air over 1 inch square of the earth's 

 surface is fifteen pounds, it follows that over every acre 

 of land there is an average of 28.2 tons of carbon dioxid, 

 or 7.7 tons of carbon. As we know that plants draw their 

 supply of carbon from the atmosphere and as vegetable 

 tissue is the primary source of this element to the animal. 



