44 HOW CROPS GROW. 



Unities for oxygen that it never occurs naturally in the 

 free state, and when prepared by art, is usually obliged to 

 be kept immersed in water to prevent its oxidizing, or 

 even taking fire. It is known to the chemist in the solid 

 state in two distinct forms. In the more commonly occur 

 ring form, it is colorless or yellow, translucent, wax-like in 

 appearance ; is intensely poisonous, inflames by moderate 

 friction, and is luminous in the dark, hence its name, de 

 rived from two Greek words signifying light-bearer. The 

 other form is brick red, opaque, far less inflammable, and 

 destitute of poisonous properties. Phosphorus is exten 

 sively employed for the manufacture of friction matches. 

 For this purpose yellow phosphorus is chiefly used. 



When exposed sufficiently long to the air, or immedi- 

 1 J ately, on burning, this element unites with oxygen, form- 

 j ing a body of the utmost agricultural importance, viz. . 

 | phosphoric acid. 



EXP. 18. Burn a bit of phosphorus under a bottle as in Exp. 8, omit 

 ting the water on the plate. The snow-like cloud of phosphoric acid 

 gathers partly on the sides of the bottle, but mostly on the plate. It 

 attracts moisture when exposed to the air, and hisses when put into wa 

 ter. Dissolve a portion of it in water, and observe that the solution ia 

 acid to the taste. 



/ In nature phosphorus is usually found in the form of 

 ( phosphates, which are compounds of metals with phos 

 phoric acid. 



In plants and animals, it exists for the most part as 

 phosphates of lime, magnesia, potash, and soda. 



The bones of animals contain a considerable proportion 

 (10 per cent) of phosphorus mainly in the form of phos 

 phate of lime. It is. from them that the phosphorus em 

 ployed for matches is largely procured. 



EXP. 19. Burn a piece of bone in a fire nntil it becomes white, or 

 nearly so. The bone lo?es about half its weight. What remains ia 

 bone-earth or bone-ash, and of this 90 per cent is phosphate of lime. 



Phosphates are readily formed by bringing together so 

 lutions of various metals with solution of phosphoric acid. 



RXP. 20. Pour into each of two wine or test glasses a small quantity 



