COMPOSITION OF MATTER 



25 



30. Distribution of the Chemical Elements. The different 

 chemical elements are very unequally distributed in the earth's 

 crust. Some, like radium, are extremely rare, and occur only 

 in combination with other elements. Others are very common 

 and occasionally occur in the pure or " native" state. Gold, 

 silver, copper, and some of the other metals may be found 

 native. The oxygen and nitrogen in the air are uncombined, 

 but usually these elements are combined with others, and in the 



FIG. 3. Proportion of various elements in the earth. 



latter state may be found in vast deposits. About 95 per cent, 

 of the earth is made up of the five elements, oxygen, silicon, 

 aluminum, calcium, and iron. Oxygen is the most abundant of 

 the elements and itself forms nearly one-half the earth. It 

 forms one-fifth of the air, eight-ninths of the water, and enters 

 into a great many combinations with other elements. Carbon 

 is found in practically all organic substances; that is, in all 

 substances formed by living things. Silicon is the essential 



