86 PROCEEDINGS OF THE AMERICAN ACADEMY. 



separation of the curves for 20° and 40° at 4000 kgm., while ethyl 

 iodide shows a similar, but smaller separation between the 20°-40° 

 and also the 40°-60° curves. The halogens all show reversals at the 

 high pressures. 



The occasional reversal of compressibility with temperature, that 

 is, a smaller compressibility at a higher temperature, has ready 

 explanation if we adopt the hypothesis of molecules with shape. We 

 have seen that in this event, because of the varying completeness 

 with which the molecules interlock,' it may sometimes happen that 

 the increase of volume with temperature results in a decrease of the 

 free space open to the thermal agitation of the molecules. In this 

 case the compressibility is less at high temperatures than at low. 

 The same argument would admit also the possibility of compressi- 

 bility increasing with increasing pressure if the effect of pressure is 

 to produce a better fitting together of the molecules and so greater 

 free space. It may be, therefore, that the effect found for carbon 

 bisulphide is genuine, and not to be explained away by experimental 

 error. 



As a general rule, the result found for water applies to these twelve 

 liquids also, namely that compressibility when plotted against volume 

 is less at the higher temperatures. There are, however, a few excep- 

 tions. Carbon bisulphide and ethyl chloride, for example, show a 

 reverse effect at the higher pressures. A possible explanation of this 

 is to be found in the opposition of two effects. In general we think 

 of the effects of lowering the temperature and of increasing the pres- 

 sure as the same, namely to increase the degree of interlocking of 

 the molecules. If now we compare two states of the liquid, each 

 occupying the same volume, but one at a higher temperature than 

 the other, we see that the high temperature condition differs from the 

 low in that the molecules are less interlocked so that they have less 

 free space at their disposal. An increase of pressure will tend to 

 produce a smaller change of volume at the high temperature there- 

 fore, because of the smaller free space which the molecules possess. 

 But on the other hand, an increase of pressure will tend to produce 

 a greater change of volume because the effect of increased pressure 

 is to increase the amount of interlocking, and so to decrease the 

 volume. As one or the other of these effects predominates, we shall 

 have smaller or greater compressibility at the higher temperatures 

 with constant volume. We have seen that the compressibility is 

 usually smaller. 



We turn now to the diagram (Figure 45) in which are collected 



