96 PROCEEDINGS OF THE AMERICAN ACADEMY. 



on here. In general, the curves for the higher temperatures tend to 

 draw apart, that is, the decrease of internal energy is greater at the 

 higher temperature, but there are several well pronoimced exceptions 

 to this rule. Amyl alcohol, ethyl chloride, and ethyl iodide, for 

 example, are exceptional at the same temperatures where we found 

 exceptional behavior with respect to the work of compression. 



The change of internal energy is to be thought of as brought about 

 by the counterplay of two opposing sets of forces, and it is significant 

 because of what it can tell us about these forces. When a substance 

 is compressed, the molecules are brought closer together, the attrac- 

 tive forces between the molecules do work, the potential energy of 

 the attractive forces decreases, and the internal energy decreases. 

 But at the same time, the molecules become compressed by mutual 

 contact, energy is stored up inside the molecule by the external 

 forces in the form of potential energy of strain, and the internal 

 energy increases. Now these two sets of forces play very different 

 roles at different stages of the compression. At low pressures, where 

 the molecules may be thought of as possessing a free path, no poten- 

 tial energy of strain can be permanently stored up in the molecule, 

 because during the motion in the free path it has been entirely con- 

 verted into translational kinetic or temperature energy. This is 

 what takes place in a gas. But as the volume of the liquid becomes 

 less, the length of the free path rapidly becomes smaller, and at any 

 instant an increasingly large number of molecules is not describing 

 part of any free path at all, but is merely being handed on directly 

 from one collision to the next. At this stage, potential energy of 

 strain can be permanently stored up within the molecule. At still 

 higher compressions, when the molecules are practically in continu- 

 ous contact, there is still greater possibility of storing up internal 

 energy of strain. The possibility is limitless, provided only that the 

 molecule never becomes incapable of further compression. The loss 

 of internal potential energy by the attractive forces cannot proceed 

 beyond a certain limit, however, imposed by the least distance of 

 approach of the molecules. 



We should expect, therefore, that at low pressures the internal 

 energy would decrease with rising pressure, the attractive forces 

 being in the ascendant, but at the higher pressures, where the forces 

 resisting compression have become dominant, that the energy would 

 increase with rising pressure. For the two liquids previously investi- 

 gated in this respect, mercury and water, the energy was found to 

 continue to decrease over a pressure range of 12000 kgm. The direc- 



