AIR AND OXYGEN 27 



Activity and Stability. A substance which enters into 

 combination easily, is called active, so that oxygen is spoken of as 

 an active element, nitrogen as a relatively inactive or indifferent 

 element. An active element, since it combines greedily, holds 

 tenaciously to that with which it has combined. An active 

 element means, therefore, also, one which is in general difficult to 

 liberate from combination. Its compounds are in general rela- 

 tively stable, or inactive. The compounds of more indifferent 

 elements, on the other hand, are in general relatively easy 

 to decompose. In other words, they are unstable or active. 



Law of the Influence of Heating. Even oxygen, active 

 as it is, does not combine visibly with tin, when both are cold. 

 Lead rusts very slowly at the ordinary temperature. Iron rusts 

 very much faster when heated than when cold. In every chemical 

 change we find that raising the temperature hastens the process 

 very considerably. Other things being equal, it causes a greater 

 quantity of reacting material to undergo the change in a given 

 interval of time. 



As exceptions to this law, radioactive disintegrations (see p. 545) 

 must be noted. The rate at which reactions of this type proceed 

 is in every case absolutely independent of the temperature. The 

 speed of a photochemical reaction (a chemical change induced by 

 the action of light; see p. 498) is also, in general, very little 

 affected by raising the temperature. For normal chemical 

 changes, however, the influence of temperature is very marked, 

 a rise of only ten degrees (approximately) causing most reactions 

 to proceed at twice their original rate. 



Oxygen. We cannot do better than begin the more systematic 

 study of chemistry with oxygen, for it is a most interesting as well 

 as useful substance. It is the active component of the air. We 

 depend upon it for life, since in its absence we suffocate, for heat, 



