BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. 113 



greater at lower than at high temperatures. Association has been 

 discussed in greater detail on page 104. 



The results have exhibited one striking feature which has been 

 frequently emphasized, namely that at high pressures all twelve liquids 

 become more nearly like each other. This suggests that it might 

 be useful in developing a theory of liquids to arbitrarily construct a 

 "perfect liquid" and to discuss its properties. Certainly the con- 

 ception of a "perfect gas" has been of great service in the kinetic 

 theory of gases; and the reason is that all actual gases approximate 

 closely to the "perfect gas." In the same way, at high pressures all 

 liquids approximate to one and the same thing, which may be called 

 by analogy the "perfect liquid." It seems to offer at least a promis- 

 ing line of attack to discuss the properties of this "perfect liquid," 

 and then to invent the simplest possible mechanism to explain them. 



Summary of Results. 



These measurements have disclosed an unexpectedly complicated 

 state of affairs at high pressures, in many respects the exact opposite 

 of what we would expect from the behavior at low pressures. The 

 compressibility may decrease with increasing temperature, or in a 

 few cases may increase with increasing pressure. The thermal ex- 

 pansion also may decrease with increasing temperature or increase 

 with increasing pressure. The peculiarity of thermal expansion with 

 respect to temperature is possessed by all liquids above 3000 kgm. 

 This has been shown to have a bearing on previous theories. Of the 

 other thermodynamic properties, perhaps the most important is the 

 internal energy. This passes through a minimum and then increases 

 again with increasing pressure. The reason is that beyond a certain 

 pressure the attractive forces do less work than is done by the external 

 forces in compressing the molecule. 



Among considerations which would seem to be of importance for 

 a theory of liquids at high pressures, that of the shape of the mole- 

 cules is worthy of attention. It is inconceivable that the molecules 

 should not have shape, and it is natural to suppose that the shape 

 will play an important part when the molecules are forced into close 

 contact. It is shown in detail that considerations of this sort offer 

 possible explanations of the complicated effects actually found. 

 Other modifications of the ordinary conceptions of liquids that may 

 be necessary have to do with our ideas of the kinetic origin of pres- 



