THE VOLATILE PART OF PLANTS. 29 



phosphates, which are phosphoric acid whose hydrogen 

 has been partly or entirely replaced by metals. 



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

 phosphates of calcium (or lime), magnesium (or mag- 

 nesia), potassium (or potash), and sodium (or soda). 



The bones of animals contain a considerable proportion 

 (10 per cent.) of phosphorus, mainly in the form of cal- 

 cium phosphate. It is from this that the phosphorus 

 employed for matches is largely procured. 



EXP. 19. Burn a piece of bone in a fire until 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 calcium phosphate. 



Phosphates are readily formed by bringing together 

 solutions of various metals with solution of phosphoric 

 acid. 



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

 of the solution of phosphoric acid obtained in Exp. 18. To one, add 

 some lime-water (see note p. 19) until a white cloud or precipitate is per- 

 ceived. This is a calcium phosphate. Into the other portion drop solu- 

 tion of alum. A translucent cloud of aluminium phosphate is immedi- 

 ately produced. 



In soils and rocks, phosphorus exists in the state of 

 phosphates of calcium, aluminium, and iron. 



The tissues and juices of animals and plants generally 

 contain small proportions of several highly complex " or- 

 ganic compounds" in which phosphoric acid is associated 

 with the elements carbon, oxygen, hydrogen and nitrogen. 

 Such substances are chlorophyll, lecithin and nuclein, 

 to be noticed hereafter. 



We have thus briefly considered the more important 

 characters of those six bodies which constitute that part 

 of plants, and of animals also, which is volatile or de- 

 structible at high temperatures, viz. : carbon, hydrogen, 

 oxygen, nitrogen, sulphur, and phosphorus. 



Out of these substances, which are often termed the 

 organic elements of vegetation, are chiefly compounded all 

 the numberless products of life to be met with, either in 

 the vegetable or animal world. 



