14 CHEMICAL COMPOSITION OF LIVING MATTER 



Chemical compounds distinguished from physical mix- 

 tures. — Most of the substances found in nature are 

 mixtures of several substances. If we examine a piece 

 of wood we can pick out several different kinds of matter 

 which are intimately united in the compound wood. 

 Similarly a piece of flesh shows pieces of fiber, and bits of 

 fat, blood, and muscle, which are all intimately mixed 

 together, but each of which possesses properties peculiar 

 to itself. If we examine a bit of common salt and com- 

 pare it with the elements sodium and chlorine, we find 

 that the salt has properties which are distinct from either 

 the sodium or the chlorine. In short the sodium and 

 chlorine which have combined to form salt have lost their 

 identity and individual properties, and the compound 

 resulting from their union has entirely new properties. 



These examples illustrate the distinction between a 

 mechanical mixture and a chemical compound. The 

 flesh is a mechanical mixture of fat, blood, etc. The salt 

 is a chemical compound. 



Whenever two or more elements combine, losing their 

 individual properties and producing a substance with 

 entirely new properties, the combinaton is called a chemi- 

 cal compound. The white fumes which result from the 

 union of phosphorus with the oxygen of the air are entirely 

 distinct in properties from the phosphorus and oxygen. 

 In their formation, therefore, we have an example of the 

 formation of a chemical compound. If the phosphorus 

 is exposed to the air in a dark room a dull glow of light 

 may be seen. This glow is due to the heat generated 

 in the act of union of the phosphorus and oxygen. A 

 certain amount of heat is always generated when chemical 

 elements combine. 



