PHOSPHOBUS 13 



Characteristics of chemical elements. — The distinguish- 

 ing feature of a chemical element is that it cannot be 

 resolved by any known means into a simpler substance. 

 Thus while it is possible to resolve a substance like com- 

 mon salt into two substances known respectively as sodium 

 and chlorine, no one has ever been able to separate sodium 

 or chlorine into anything simpler, and these two sub- 

 stances are, therefore, called elements. Iron, sulphur, 

 gold, silver, are familiar examples of elements. Since the 

 chemical elements are the substances out of which all 

 matter is formed, it will be well for us to examine a few 

 of them to learn their properties and the manner in "which 

 they combine to form compounds. Wc shall select for 

 examination those which are particularly concerned in 

 the composition of our body materials. 



Phosphorus. — (See Ex. III.) This element forms a 

 large part of our bones; these are, in fact, composed mainly 

 of a compoimd of this element with calcium and oxygen. 

 In its pure state phosphorus has a yellow color and a 

 waxy consistency. "When exposed to the air it gives off 

 white fumes which have a peculiar odor. These fumes 

 are a compound of phosphorus with the oxygen of the 

 air, and to keep phosphorus from combining with oxygen 

 it is necessary to cover it with ^\'ater. 



These fumes teach us of a peculiar property of phos- 

 phorus, namely, its power to combine ^^'ith another element. 

 Such power on the part of an element is called its chemi- 

 cal affinity. All elements possess this affinity for certain 

 other elements, but the strength of the affinity varies 

 with the element and with the external conditions. 

 To this affinity of elements all chemical compounds are 

 due. 



