POLYMORPHISM AT HIGH PRESSURES. 161 



modifications at low temperatures. It was somewhat of a surprise 

 that there were no new forms under pressure. 



Sulfur is known to have a number of modifications, both stable and 

 unstable, several of them more or less obscure in character, but there 

 are at least two well defined forms that stand to one another in the 

 relation of enantiotropy. I found no other well defined forms- 

 to 12000 at 20°, 100°, or 200°. It is possible, however, that a small 

 percentage of the form insoluble in CS2 was formed by pressure, 

 because the specimen which had been subjected to pressure was not 

 completely soluble. One might possibly expect a new modification 

 of sulfur by an irreversible transition, like that of black phosphorus, 

 because of the proximity of the two elements in the periodic table. 

 On one occasion a piece of sulfur was kept at 12500 kgm. and 200°' 

 for six hours, but with no permanent change. 



The runs on selenium were started with the amorphous variety, 

 which had been fused immediately before the experiment. It showed 

 no transition to 12000 at room temperature. Pressure was main- 

 tained on the amorphous selenium at 7000 for 16 hours with no effect. 

 At 7000 kgm. the selenium was then heated to 200°. There was a 

 transition with decrease of volume somewhere between 20° and 200°, 

 which was doubtless due to the formation of the crystalline phase. 

 At 200°, the new phase showed no transition to 12000 kgm., and also 

 showed none between atmospheric pressure and 12000 after cooling 

 again to room temperature. On releasing pressure the selenium was 

 found to have a density of 4.69. The density of the metallic modi- 

 fication of Se is given as 4.79, and that of the amorphous as 4.29 by 

 Saunders.^® (It may be mentioned incidentally that several wildly in- 

 accurate values for the density have found their way into some Tables. 

 The Chemiker Kakendar, for instance, gives the density of the amor- 

 phous variety as 5.68, and that of the crystalline form as 6.5). There 

 seems little reason to doubt that the substance formed above was 

 metallic selenium, the somewhat small density might easily be due to 

 fissures or occluded kerosene. It is known that ordinary amorphous 

 selenium will crystallize slowly at atmospheric pressure at 200°; 

 the effect of high pressures seems, therefore, to be to lower somewhat 

 the temperature of crystallization. This is as one would expect. 

 Before trying the experiment it seemed plausible to me to expect 

 a new form stable at atmospheric pressure analogous to black phos- 

 phorus, because of the similar position of these two elements in the 

 periodic table. 



36 A. P. Saunders, Jour. Phys. Chem., 4, 491 (1900). 



