ELEMENTS, COMPOUNDS, AND MIXTURES 249 



twenty of the elements. Twelve of these are always 

 found in living matter, and six more are sometimes found 

 in living matter. The twelve which are always found are : 

 phosphorus, sulphur, carbon, oxygen, hydrogen, nitrogen, 

 chlorine, potassium, sodium, calcium, magnesium, and 

 iron. 



Only about one fourth of the elements are found in 

 uncombined state. The others are found only as com- 

 pounds. 



The following table is an estimate of the plentifulness 

 of the elements. Twelve of them make up 99 per cent 

 of the earth (Figure 221). 



Per Cent 



Oxygen 

 Silicon . 

 Aluminum . 

 Iron 

 Calcium 

 Sodium 



50.00 

 26.00 

 7.25 

 4.10 

 3.15 

 2.30 



Per Cent 



Potassium . 

 Magnesium . 

 Hydrogen 

 Titanium . . 

 Chlorine . . 

 Carbon 



2.30 



2.10 



1.00 



.40 



.20 



.20 



All the other elements, including gold, silver, sulphur, 

 and mercury,, make up the remaining one per cent of the 

 earth. 



