46 THE EAETH. 



By this list it will be perceived that the greatest number 

 of the elements are metals, but most of these are very rare 

 and met with but in certain localities, making up but a 

 very insignificant part of the earth's surface, while others 

 (never found in nature, as metals) in combination with 

 oxygen forming " earths " compose the greater part of its 

 bulk. But of all the elements, oxygen (a gas) is the most 

 widely diffused, and constitutes rather more than one half 

 of the whole earth, thus it forms ^-th of the air, T \ths of the 

 water, of the various earths and rocks about , and in all 

 organised beings about f . 



Of the metallic elements, those which form the greater 

 bulk of the earth are 



Silicon. Magnesium. 



Aluminium. Potassium. 



Calcium. Sodium. 



These never exist in nature as metals, but combined with 

 oxygen forming the following earths and alkalies : 



Silica 

 Alumina 

 Lime 

 Magnesia . 



EABTHS. ' ^ ; ALKALIES. 



These four " earths " form about eighty-five per cent, of the 

 whole bulk of this globe, the remainder being made up of 

 water (composed of eight parts oxygen and one part 

 hydrogen), of "salt," both as rock or crystal-salt, and 

 dissolved in the water of the sea, of the ores of different 

 metals (metals in union with oxygen, sulphur, &c.), and of 

 the remains of former vegetation, existing as coal. 



Silica is composed of 21 '3 parts of silicon with 24 of 

 oxygen, it is that earth which constitutes a great part of 

 almost every soil, and forms sand, sandstone, and flint, it 

 enters largely also into the formation of granite and the 

 other primitive rocks ; when pure it is called " quartz " or 

 "rock-crystal" and is often found in large and beautiful 

 masses. Silica, in different forms, contaminated and coloured 

 by different metallic oxides, &c., forms that class of stones 



