24 Elementary Biology. 



Water is a simple compound ; it is stable, and it is found 

 in by far the greatest abundance in the inorganic environ- 

 ment, but it constitutes ninety-three per cent, of the turnip, 

 for example, among vegetals, and forms about seventy per 

 cent, of the weight of the human body. Salts of potash, 

 lime, and soda, common salt, and other substances found 

 in the environment in prodigious quantities, are nevertheless 

 present in the organism in no insignificant amount. So also 

 many oils and carbohydrates (or compounds of carbon, oxy- 

 gen, and hydrogen), although probably the result of vital 

 processes in some bygone period in the world's history, are 

 nevertheless found existing abundantly, undecomposed, in 

 the environment. 



Many comparatively simple compounds are extremely 

 unstable, such for instance as the compound known as iodide 

 of nitrogen, which, although it consists of only four atoms, 

 explodes at once on being touched. Many other examples 

 will be noted in the sequel. 



