18 AGBICULTURAL CHEMISTRY. 



20. Silicon is the basis of sand, flint, and quartz. 

 It enters largely into all soils, and constitutes proba- 

 bly about 1-5 of tbe solid globe. In its pure state it 

 is a dark brown powder. Combined with oxygen, it 

 forms the flinty stones so common everywhere ; also 

 sand, which is flint stone reduced to different degrees 

 of fineness. 



21. Nitrogen. — A gas, tasteless, colorless, inodorous, 

 and a little lighter than common air. Mixed with 

 oxygen, it constitutes 4-5 of the atmosphere. It en^ 

 ters into the composition of all animals, and of nearly 

 all plants. It constitutes, with oxygen, nitric acid ; 

 and forms a part of all those salts called nitrates. 



22. Hydrogen is a tasteless, colorless, inodorous gas, 

 14 times lighter than air, and used on this account for 

 filling balloons. It constitutes 1-9 of water, and a 

 part of all vegetable and animal substances. Oxygen 

 is a supporter of combustion (ca^j^ses other bodies to 

 burn) ; Hydrogen is combustible (burns) ; Nitrogen is 

 neither a supporter of combustion nor a combustible. 

 Oxygen is also a supporter of respiration, as well as 

 of combustion. Nitrogen is neither. No fire can 

 burn nor animal breathe in it. And though Hydro- 

 gen burns, yet it is not a supporter of combustion, 

 A burning body is extinguished if immersed in it. 



23. Iron. — A well-known metal ; cheap, because 

 plenty; but, beyond doubt, the most useful of all 

 metals. 



