306 PROCEEDINGS OF THE AMERICAN ACADEMY. 



that capable of being performed by tlie chemical affinity alone. The 

 result would be a cooling reaction and an excess of free energy. This 

 behavior would not be a contradiction of the second law of thermody- 

 namics, because the law states only that heat cannot of itself do work at 

 constant temperature. 



Thus the hypothesis of compressible atoms not only is consistent with 

 the ordinary applications of the two laws of energy, but also affords 

 a conceivable picture of the cause of the newly discovered relation 

 ^dU dA 



°^ Jt '^ Tr 



On the basis of the present data it is unprofitable to attempt the calcu- 



dU dA 

 lation of the mathematical relation of tttt, to >,>,„ or to probe further into 



the mechanism of the change. Wlien more exact data have been ob- 

 tained, it may well be possible to arrive at more definite conclusions. 

 If the contraction of volume on combination could only be interpreted in 

 the light of accurate determination of the compressibilities involved, it 

 seems reasonable that this contraction might be a more exact measure 

 of the affinity than either the free or the total energy change. 



The forecfoinij facts and lo^ic seem to lead to the conclusion that the 

 change of free energy of a process does not really represent the attrac- 

 tive energy at work in the process, unless the heat capacity of the 

 system remains unchanged during the reaction. If the heat capacity 

 diminishes during the reaction, the free energy is less than the affinity, 

 and vice versa. 



This conclusion is contrary to the common belief. If warranted, it 

 shows that the free energy change is a no more satisfactory guide to the 

 affinity than is the heat evolved in the reaction, even when no concen- 

 tration effect is present. The free energy seems to represent rather the 

 remainder left after a resisting energy has been subtracted from the 

 attracting energy. Only when the heat capacity does not change during 

 the reaction may we suppose that the attracting energy, the change of 

 free energy, and the heat of reaction are equal. A consequence of this 

 equality is that the attracting energy, like the free and total energy 

 changes, cannot change with the temperature when the heat capacity is 

 invariable. 



It is of course true that the change of free energy is the total resultant 

 effect which determines whether or not the reaction will take place. To 

 put the question in another way, the present reasoning seems to afford 



