POLYMORPHISM AT HIGH PRESSURES. 173 



that nearly one quarter of all the transition curves are of the ice type. 

 The persistence of the curves for AgNOs and Hglo suggests that an 

 ice type of transition may be as capable of continued stability over a 

 wide range of pressure and temperature as an ordinary transition. In 

 the early stages of this work I was inclined to regard the existence of an 

 ice type of transition as a priori evidence that there must be at higher 

 pressures a normal transition to supplant it, as on the melting curve of 

 ice I. This surmise did not prove fruitful. 



A summary of all the transition lines examined is shown in Table 

 XIV. This shows the number of various classes of lines grouped 

 according to important characteristics. Thus, for example, out of 69 

 lines examined, there are 3 rising curves whose direction of curvature 

 is abnormal, and whose direction of variation of Av is also abnormal. 

 In drawing up this table, normal behavior has been called that which 

 is like that on the melting curve. In detail, normal curvature is 

 concavity downward, normal variation of Ay is decrease with rising 

 temperature, a and j8 are normal if the phase of larger volume is the 

 more compressible or expansible, and Cp is normal if the phase stable 

 at the higher temperature has the higher specific heat. In drawing up 

 the table all those lines which are sensibly straight, 33 out of 69, were 

 not tabulated as of either normal or abnormal curvature, but their 

 other properties were tabulated under the normal branch. The results 

 for Hgl2 have not been included at all, because its curve both rises and 

 falls. 



In general the normal type of behavior preponderates, but the 

 possibilities that have been discovered are so numerous that one would 

 be prepared to admit that after extensive search probably representa- 

 tives of every one of the divisions could be found. It is certainly 

 evident that the mechanism of polymorphic transitions in different 

 substances does not possess any one notable characteristic which 

 expresses itself in a common type of behavior on all the transition 

 lines, as is the case for melting. 



Two significant features of the table call for comment. In the first 

 place, abnormal curvature means that the factor by which the change 

 of volume is multiplied to give the change of internal energy becomes 

 smaller at higher pressures. This factor is called by some writers 

 the internal pressure, and is taken as a measure of the internal co- 

 hesion. It is at first surprising that this cohesion should become less 

 as the substance is compressed so as to occupy less volume. It is 

 difficult to imagine the possibility of such an effect in a substance com- 

 posed of spherical molecules. The effect must be due to the configu- 



