POLYMORPHISM AT HIGH PRESSURES. 147 



has been made is of necessity more restricted than for the other 

 substances. Since the same substance has sometimes been suggested 

 by more than one clue, cases will be found of the inclusion of the same 

 substance in more than one group. 



The search for new modifications has been made, unless stated to 

 the contrary, up to 12000 or 13000 kgm. at 20° and 200°. For sub- 

 stances melting within the range, search was made out to 12000 at 

 room temperature and the highest temperature of the melting curve. 

 I have not especially mentioned these cases. If no transition is found 

 on either of these isothermals, and if it is certain that there is no 

 transition at atmospheric pressure between 20° and 200°, it is very 

 unlikely that there vdW be found to be a transition at any pressure less 

 than 12000 at any temperature between 20° and 200°. Benzol shows 

 the character of the transition curves of possible exceptions. I have 

 not, except in a few cases, made any examination for polymorphism 

 at atmospheric pressure between 20° and 200°, but have accepted the 

 absence of any mention in the literature of polymorphism as probable 

 evidence that there are no other forms. In several cases, however, 

 I have found forms at atmospheric pressure not previously listed. 

 But it is very probable that if there is a transition at atmospheric 

 pressure the transition line will run across the isothermal at either 20° 

 or 200°, and so be discovered by the run to high pressures. It is not 

 probable that any large transitions have been overlooked. Another 

 restriction to which this investigation is subject is that the change of 

 volume of the transition must be large enough to detect with this 

 apparatus. It is not likely that transitions with a change of volume 

 of much less than 1/100% would have been detected, although if the 

 transition were known to exist, measurements could be made on still 

 smaller transitions. This means that transitions as small as many 

 described by Cohen ^^ in his papers on the allotropy of the metals are 

 beyond the reach of this apparatus. 



It will pay to give a preliminary discussion of the nature of the 

 various clues. The clue of chemical similarity is perhaps the most 

 obvious of all. If one substance shows polymorphism, one may 

 expect others built up on the same chemical scheme also to have poly- 

 morphism. The expectation is especially strong if the substances differ 

 only by the replacement of one element by another which usually 

 stands to it in the relation of isomorphism. In particular the substi- 

 tution of one atom for another would be expected to have less disturb- 



29 Ernst Cohen, Proc. Amst. Acad., numerous papers in 1914 and 1915. 



