92 BRIDGMAN. 



mvestigated to date to 37, including 34 new phases whose existence 

 was not known before. In addition to substances which do show 

 polymorphism, 94 other substances have been examined to 12000 kgm. 

 at 20° and 200° without result. These substances are enumerated 

 here. 



This paper brings to a close, at least for the present, this series of 

 investigations. The reason for this is not at all that«all polymorphic 

 substances have been studied, or that all interesting cases have been 

 exhausted. The field is merely opened by these results. The reason 

 is a practical one. Most of the substances that now suggest them- 

 selves for examination are not common chemicals that can be supplied 

 by the large chemical houses, but require special preparation, for which 

 I have not the facilities. 



The number of substances investigated is nevertheless probably 

 large enough to justify our pausing a little for examination of the 

 entire field. The purpose of this paper, l^eyond the presentation of 

 new data, is to coordinate somewhat this mass of results. To this end 

 all the substances examined have been collected into various groups, 

 chemical or not, according to the various clues which suggested an 

 examination of them, and part of the discussion is concerned with 

 these possible clues. The discussion also takes up a number of other 

 general considerations connected A\dth polymorphism, and tries to 

 picture what ma^• be the mechanism of a polymorphic change. It 

 must be emphasized, however, that we cannot hope to get a complete 

 explanation or description of polymorphism from data such as these. 

 Data of many other kinds, particularly crystallographic data, are 

 needed before the picture is complete. 



New Data for Individual Substances. 



Acetic Acid. — Several attempts M'ere made before this substance 

 could be obtained commercially in sufficient purity to show a good 

 freezing point. The sample finally used was from the J. T. Baker 

 Chemical Co., and by analysis was better than 99.9% pure. It is 

 necessary to keep the acid as much as possible from contact with the 

 air, because it rapidly absorbs moisture; to this end the glass stop- 

 pered bottle was kept sealed with paraffins. 



The acetic acid was placed in the nickel steel cup, inverted under 

 mercury. After prolonged contact with the steel at high temperatures 

 and pressures there seems to be some slight chemical action; the 



