22 THE PRINCIPLES OF AGRICULTURE. 



Silicon is the most abundant solid substance known. 

 It forms about one fourth of the solid parts of the earth. 

 It is commonly combined with oxygen in the form of sil- 

 ica (Si0 2 ). Quartz rock is a variety of silica. It forms a 

 large part of granite rock, sandstone, and common sand. 

 It serves, like an acid, to unite with bases, forming what 

 are called silicates. Common glass is a mixture of dif- 

 ferent silicates. Clay is chiefly composed of silicate of 

 alumina. 



Silica is found in plants, particularly in the grasses. 

 It gives to the stalks and branches greater firmness and 

 hardness. 



Sulphur is a familiar substance, commonly known as 

 brimstone, or flowers of sulphur. With hydrogen and 

 oxygen it forms sulphuric acid (H 2 S0 4 ), one of the most 

 common acids. This acid forms a great variety of useful 

 salts, as sulphate of potash (K 2 S0 4 ), sulphate of lime 

 (CaS0 4 ), etc. 



Sulphur is always found in plants and animals. The 

 strong flavor of such vegetables as turnips and -onions is 

 due to the presence of sulphur. 



Phosphorus is a soft, yellow substance, which unites 

 with the oxygen of the air and takes fire so easily that 

 it can only be kept under water. It is used in manu- 

 facturing matches. It combines with hydrogen and 

 oxygen, forming phosphoric acid (H 3 P0 4 ), a substance 

 of great importance in fertilizers. It forms about ten 

 per cent, of the bones of animals, in the form of calcium 

 phosphate. 



Chlorine is a gas which is found only in combination 

 with other elements. With- hydrogen it produces hydro- 

 chloric or muriatic acid (HC1) ; with sodium, sodium 

 chloride or common salt (NaCl). 



