CHAPTER XXIII 

 THE CHEMICAL BASIS OF LIFE 



The division of Biology which deals with the activities 

 or functions of the parts of a living organism is known 

 as physiology, as distinguished from morphology which 

 deals with the structure of organisms. The activities 

 of an organism are in part physical and in part chemical. 

 The material world both living and non-living is made up 

 of a limited number (about eighty) of substances, called 

 elements which cannot be separated further into substances 

 of different kinds. Different -elements may be grouped 

 together to form compounds and compounds may be 

 separated into their elements. Thus common salt is a 

 compound composed of two elements sodium and chlorine, 

 but the elements themselves cannot ordinarily be de- 

 composed further. Changes which involve the combina- 

 tion, separation or rearrangement of elements are known 

 as chemical changes. The rusting of iron is a chemical 

 change. Iron, an element, combining with the oxygen 

 of the air, another element, produces a substance, iron 

 oxide, which is very different in appearance and properties 

 from either of its constituents. Other chemical changes 

 are the burning of wood and coal, the action of acid on 

 soda and the fermentation of sugar. In all these cases 

 there are changes between the elements of the substances 

 involved in the process, resulting in the production of 

 very different kinds of substances. Changes which do 

 not involve any alteration of the elementary composition 

 of bodies are called physical. The conversion of water 



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