212 COMMON SUBSTANCES 



soils. In many places decayed plant and animal sub- 

 stances have mixed with the soil, making more or less 

 change in its composition. 



The soil is very necessary to plant life. It allows the 

 roots a good support ; it holds moisture which the 

 plant may slowly absorb ; and it supplies small amounts 

 of mineral matter, which is dissolved by the water and 

 so taken into the plant structure. 



248. Earthenware and Porcelain. Clay is a kind of 

 soil that contains a large quantity of aluminium silicate 



( 234). Clay may be moistened slightly and then 

 molded into different shapes. If it is then baked in a 

 furnace for some time the silicate becomes hard so 

 that the vessel will keep its shape. In this way brick 

 and vessels of earthenware (Fig. 138) are made. 



Sometimes the aluminium silicate is found pure. If 

 this be treated similarly to the clay, a finer grade of 

 ware will be made ; this is called porcelain. 



249. Glass. Grlass is a mixture of silicates of two or 

 more metals, usually Ca, Na, K, Pb, Al, or Fe. It is 



