POLYMORPHISM AT HIGH PRESSURES. 171 



in what region a transition will become so viscous that it cannot be 

 observed, because the behavior of different substances is very different. 

 As a general rule, however, the viscous resistance to transition becomes 

 greater at greater distances from, the liquid phase. Substances with 

 high melting points would be expected to show less frequent poly- 

 morphism. Now in all the above list of substances with polymorphic 

 forms, the highest melting point is 628°. Not one of the substances 

 examined which has a higher melting point shows polymorphism in my 

 range. The only example among substances which I did not examine 

 is CuoS with a melting point of 1100°. There are, however, numerous 

 examples known of polymorphism at higher temperatures; several of 

 the substances examined above belong here. It is therefore likely that 

 many of the substances would show polymorphism if examined over a 

 wider range. It is, furthermore, significant that the nitrates, among , 

 which polymorphism is widely prevalent, are all low melting, as are 

 also the iodides. The organic compounds all have low melting points; 

 in the above list there are 39 organic compounds, of which 11 are 

 polymorphic. This does not include substances with unstable forms. 

 Of the inorganic substances with known melting points below 650°, 

 25 out of 42 are polymorphic. Polymorphism seems of more frequent 

 occurrence with inorganic compounds. As a general average, perhaps 

 one out of three substances are polymorphic. 



AYe next examine the relative frequency of occurrence of the differ- 

 ent crystalline systems. I have not yet been able to determine the 

 system of any of the new forms stable at higher pressures; we cannot 

 yet tell whether all substances tend to any one simple type under high 

 pressures. The known forms include 17 cubic, 3 (or 4) tetragonal, 8 

 trigonal, 11 rhombic, 4 monoclinic, and 1 triclinic. The relatively 

 high frequency of the rhombic system is perhaps surprising. The 

 number of cases in which the cubic form, which has the highest sym- 

 metry, is of greater volume than a neighboring more unsymmetrical 

 form is striking. It would perhaps be natural to expect that the forms 

 stable at the higher temperatures, with the greater energy of tempera- 

 ture agitation, and in many cases the greater volume, would have 

 fewer elements of symmetry. However, in 10 of the above 17 cases, 

 the cubic crystal may be transformed by proper change of pressure 

 and temperature to a phase of smaller volume and also of lower sym- 

 metry. It is evident that the cubic arrangement in these cases can- 

 not be the arrangement of closest packing. There is, of course, no 

 especial reason to expect it when the crystal is built up of different 

 kinds of atoms. Out of the five cases above in which a trigonal form 



