264 THE REALITIES OF MODERN SCIENCE 



molecular structure, and like the second law is per- 

 fectly general. It serves admirably either in lieu of 

 this law for students whose mathematical training is 

 insufficient, or as an introduction to the law. 



From our study of continuity of state we recognize 

 that the molecular state of a body or system is deter- 

 mined by various conditions, of which temperature, 

 pressure, and volume are the three so far considered. 1 

 These conditions, or parameters, are variable at the 

 will of the experimenter. The behavior of the mole- 

 cules of the substance is not directly controllable, 

 however, but only indirectly as one or the other of 

 these factors may be varied by an external action. 

 The principle of Le Chatelier-Braun enables us to pre- 

 dict in any case what the character of the effect upon 

 the molecules will be when one of these parameters 

 is varied. 



For example, consider the case of a gas under a given 

 pressure and occupying a definite volume at a definite 

 temperature. If the external pressure is increased, the 

 effect is to decrease the volume. The equilibrium 

 which the moment before existed between the external 

 pressure and the opposing pressure of the gas molecules 

 has been disturbed. The volume is reduced. Now, 

 the principle tells us the compensating change which 

 occurs within the system. The change in V is ac- 

 companied by a change in one of the other variables, 

 in this case T, of such a character as to oppose the 

 change in V. That gases expand with increased tem- 



1 Electromotive force, that is, a difference in electrical potential 

 energy, is obviously another factor. The ions which take part in 

 chemical reactions do so as the result of the potential energy of the 

 electronic systems which they form with one another. 



