170 BRIDGMAN. 



more, many cases now classified as monotropically polymorphic would 

 turn out to have larger significance if certain lines of conjecture prove 

 to be justifiable. Many investigators have thought that forms un- 

 stable under atmospheric conditions would become stable at higher 

 pressures. In this case such substances would become as significant 

 as those enantiotropically polymorphic under ordinary conditions. 



The experiments above have shown, however, that in none of the 

 cases examined have forms ordinarily unstable become stable at higher 

 pressux'es. This is, after all, not surprising, because the majority of 

 unstable forms are less dense than the stable forms. Their region 

 of stability, if it exists, is at negative pressures. The example of 

 MgS047H20, mentioned above, is a case on the other hand where the 

 unstable form is more dense, but is not formed at high pressures, even 

 when inoculated. The dense unstable form of MgS047HoO may be 

 obtained in the pure state by crystallizing from the supersaturated 

 solution on inoculation with a crystal of FeS047H20. 



Cases have been found, however, in which a phase unstable at high 

 pressures becomes stable at relatively higher pressures. Water below 

 0° and near 6000 kgm. crystallizes most readily in the form of ice VI, 

 which is unstable, in preference to the stable form, ice V. Ice VI is 

 the denser form and becomes stable at higher pressures. Acetamide 

 is another similar example. The case of o-kresol is of the opposite 

 kind. Above the triple point the unstable modification I is very much 

 more likely to crystallize from the melt than the stable form II. I is 

 the less dense form, and it has a domain of stability at lower pressures, 

 corresponding to negative pressures for ordinary substances. Although 

 there are these examples showing the possibility in some cases of an 

 unstable modification acquiring a region of stability, it is likely that 

 in the majority of cases the unstable forms have no region of stability 

 within experimental reach. 



It seems preferable on the whole, therefore, in this discussion to 

 confine the use of the word polymorphic to those substances with two 

 or more phases which are capable of reversible transitions. It is of 

 interest to inquire what is the frequency of occurrence of polymor- 

 phism. I have already emphasized that an examination like that 

 above of many substances cannot possibly disclose all cases of poly- 

 morphism-, a number of stable forms will not appear because of viscous 

 resistance. The phase diagrams afford several cases where the new 

 phase would not have been discovered if the exploration had been 

 confined to the low temperatures. Examples are KHSO4, Hgl2, and 

 o-kresol. It is not possible to give any general rule that will show 



