148 BRIDGMAN. 



ing effect on polymorphic relations when it takes place In a complex 

 than in a simple molecule. Evidently crystalline form is an important 

 factor in the matter of chemical replacement; the chances are much 

 greater for polymorphic similarity in those cases where the known 

 forms belong to the same systems. In the following list of substances 

 the crystalline system is given in all those cases where it is known. 



There are some cases where the existence of mixed crystals suggests 

 the existence of other forms. If one salt, A, crystallizes isomorphously 

 with another, B, in a form which pure A does not show, then one may 

 expect that possibly this strange form may correspond to a modifica- 

 tion of A stable at higher pressures. 



Salts with water of crystallization might possibly be expected to 

 show other forms under pressure, since the water molecules are a much 

 less tenacious part of the compound and might be compelled to assume 

 new bonds by the high pressure. Similar reasoning suggests that pos- 

 sibly double salts will assume other forms under pressure. 



Substances which crystallize in unstable forms from the melt, like a 

 number of organic compounds, would seem to be promising material, 

 because the molecules are known to have the capability of being built 

 up into several arrangements. In the same way, one might hope for 

 polymorphism among those inorganic compounds which are known to 

 exist in several forms as minerals, but whose relationship of monotropy 

 or enantiotropy is not known. And with some plausibility one might 

 expect that in some cases the transition lines of substances with transi- 

 tions at higher temperatures than 200° at atmospheric pressure might 

 be brought down by pressure to the region of this investigation. 



Finally a suggestion may be mentioned growing out of Tammann's ^° 

 theory of polymorphism. He has suggested that according to his 

 theory there is a particularly good chance of polymorphism among 

 those substances whose melts are associated. It is known that most 

 organic acids are associated in the liquid, and accordingly one would 

 expect frequent polymorphism here. A number of this class of sub- 

 stances were investigated. 



In the following, the substances investigated under each group will 

 be first of all simply enumerated, those showing polymorphic forms 

 being marked with an asterisk. In the text comments are [made on 

 the various substances as they are called for. 



30 G. Tammann, Gott. Nach., 1912, p. 1. 



