546 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the effect of pressure being merely to smooth out the variation of 

 curvature. Now this is distinctly what one would not expect at first. 

 The usual explanation of the normalizing effect of pressure is to sup- 

 pose that the amount of polymerization, the cause of the irregularities, 

 is decreased by rising pressure. This is the assumption made by 

 Rontgen ^^ and Sutherland. ^^ The effect of this evidently would be 

 to displace the region of abnormality from high to low temperatures, 

 which is what we have seen does not happen. The explanation is 

 rather to be found in supposing that the pressure merely reduces the 

 effects of polymerization uniformly at every temperature without 

 necessarily reducing the amount. This means that the difference of 

 volume between the double molecules and the two single molecules 

 becomes rapidly less at higher pressures ; in other words, that the 

 double molecules possess an abnormally high compressibility. This 

 seems an entirely plausible hypothesis in view of the abnormally large 

 volume of the double molecules. At high pressures, then, the poly- 

 merization, even if it occurs, is unable to produce volume effects, and 

 might as far as we are concerned be entirely neglected. There may be 

 effects on the specific heats, which cannot be detected from the pres- 

 ent data. The explanation of the various pressure effects on this 

 basis is simple. The anomalous decrease of compressibility with rising 

 temperature is due to the fact that at the higher temperatures the 

 double molecules with abnormally high compressibility are becoming 

 fewer. When pressure has become so high that there is no longer dis- 

 tinction between the associated and the dissociated molecules, the 

 behavior of the liquid becomes normal. This explanation leaves en- 

 tirely open the question as to whether pressure actually increases or 

 decreases the amount of polymerization. One would naturally expect 

 an increase. The explanation also leaves open the possibility of the 

 polymerization being to several different groups, more complicated than 

 doublets or triplets. This possibility is suggested by the appearance 

 of the several allotropic forms of ice. 



It is to be noticed that in assuming that the molecules are compres- 

 sible we have not by any means assumed that the actual change of 

 size of the molecules under pressure is the chief factor in the change 

 of volume of the substance as a whole under pressure. In fact it is 

 almost certainly true that the greater part of the total change of 

 volume is due to the closing up of the spaces between the molecules. 

 The substance as a whole may have a greater or less compressibility 



" Rontgen, Wied. Ann., 45, 91-97 (1892). 

 "^Sutherland, Phil. Mag., 50, 460-489 (1900). 



