EQUILIBRIA AND THEIR DISPLACEMENT 263 



and hence the temperature. Similarly, in the case of 

 the reduced volume, there is a decrease in the molecu- 

 lar potential energy, which is thereby converted into 

 kinetic energy and thus increases the pressure exerted 

 by the vapor molecules. This change or reaction is 

 also such as to oppose the action. 



In Newton's third law of motion we have the expres- 

 sion of the physical fact that every action is accom- 

 panied by an equal and opposite reaction. We are 

 accustomed, however, to consider this generalization 

 as referring specifically to the interactions of matter in 

 bulk. In an earlier chapter we met the same general- 

 ization, as expressed by Lenz, for the interactions of 

 moving electrons. In the case of molecular reactions 

 this important idea awaited expression until the work 

 of Le Chatelier in 1884 and Braun in 1887. 



Their principle is, that any external action upon a 

 system produces a change of such a character (in such 

 a sense) that the resistance which the system offers 

 to the external action is increased. In quantitative 

 form this principle is embodied in the second law of 

 thermodynamics. The latter, however, involves some 

 of the most difficult abstract reasoning to be found 

 in physical science. Its mathematical expression also 

 does not lend itself to simple verbal interpretation hi 

 elementary treatments of science. In the principle of 

 Le Chatelier-Braun, however, we are fortunate in 

 having a non-mathematical expression of marked 

 simplicity which indicates the character of the reac- 

 tions, the quantitative relations of which are expressed 

 by the mathematical forms of the second law. This 

 principle is not restricted by any assumptions as to 



