44 HOW CROPS GROW. 
finities 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 sufliciently long to the air, or immedi- 
ately, on burning, this element unites with oxygen, form- 
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 is 
acid to the taste. 
In nature phosphorus is usually found in the form of 
phosphates, which are compounds of metals with phos- 
phorie 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 ina fire tntil it becomes white, or 
nearly so. The bone loses about half its weight. What remains is 
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. 
Exp. 20.—Pour into each of two wine or test glasses a small quantity 
