EQUILIBRIA AND THEIR DISPLACEMENT 265 



perature, is an experimental fact, independent of any 

 theories as to molecular behavior. If the change hi 

 temperature is to oppose a decrease in volume the 

 temperature must increase. That the temperature of 

 a gas increases as the volume is decreased is the simple 

 observation of every one who has pumped a bicycle 

 or automobile tire. 



We may express this principle, then, by saying that 

 when a parameter, e.g. x, which is one of those deter- 

 mining the equilibrium of a system, is altered, another 

 parameter, e.g. y, changes in such a manner as to 

 diminish the direct effect of the external cause. It is, 

 of course, necessary, if we are to foretell the result 

 of a change in x, that we should know how x and y 

 are related. In the particular case just studied we 

 know that V is proportional to T. The change in 

 T must be that corresponding to a change in V opposite 

 to that which actually takes place. Hence, since V 

 actually decreases, T must increase. 



The principle is not, however, limited in its applica- 

 tions to the phenomena of gases and vapors. Con- 

 sider the case of an elastic wire at a definite tempera- 

 ture and having a certain length under a definite 

 tension. What will be the effect of stretching the wire 

 still further? The parameter which is changed by 

 the external cause is the length. The other parameter 

 is the temperature. Now, we know that metal wires 

 are elongated by an increase in temperature. The 

 increase in length will then be accompanied by a 

 change in temperature in such a direction as would 

 normally produce the opposite effect. Hence the wire 

 is cooled by stretching. 



