POLYMORPHISM AT HIGH PRESSURES. 151 



noteworthy example of any that I have found of complete isopoly- 

 morphism. Not only are the three substances so similar that they 

 will form mixed crystals with each other, but the three phase diagrams 

 of the pure substances are very similar, except for the absolute value 

 of the temperature of transition at atmospheric pressure. These 

 cases of polymorphism are also unique in another particular; the high 

 and low temperature forms of each substance belong not only to the 

 same crystalline group, the cubic, but also to the same sub-group, the 

 pentagonikositetrahedric. 



In large features the phase diagrams are much similar; the transi- 

 tion lines are unusually steep, the changes of volume are unusually 

 large, the curvature of all three lines is unusual in that they are convex 

 toward the pressure axis, and the transitions are all sharp and rapid 

 with a very narrow band of indifference. In finer detail, however, 

 the several diagrams do not show regular variations. The atomic 

 weights of CI, Br, I are approximately 35, 80, and 127. Br is very 

 nearly half way between CI and I and we should expect the properties 

 of the NHjBr transition to be midway between those of NH4CI and 

 NHJ. This regularity does not exist. The transition temperatures 

 at atmospheric pressure are 184.3°, 137.8°, and —17.6°, in order of 

 increasing atomic weight. The Br salt is much nearer the CI end. It 

 is noteworthy that the order of these transition points is the exact 

 reverse of the usual order of the boiling or melting points of homolo- 

 gous halogen compounds — the chlorine compound having the lower 

 boiling point and behaving as if it had less internal cohesion. The 

 slopes of the transition lines at atmospheric pressure do not even 

 follow the order of atomic weight, being 0.0645, 0.0800, and 0.0699. 

 At the same temperature, however, the order is normal, being at 185°, 

 for example, 0.065, 0.098, and 0.153. Again Br is nearer the CI end. 

 The changes of volume at atmospheric pressure are 0.0985, 0.0647, 

 and 0.0561 cm.^ per gm. In this respect Br falls nearer the I end. 

 The changes of volume per gm. molecule, which afford perhaps a 

 juster comparison, are 5.26, 6.34, and 8.14. Br is now much closer to 

 CI. It is rather surprising that the molecular changes of volume of 

 all three salts are not more nearly equal ; the easy isomorphism would 

 lead one to expect it. Finally, the latent heats of transition are 6.98, 

 3.32, and 2.05 respectively in kg. m. per gm., or 374, 327, and 247 kg. 

 m. per gm. mol. According to the first method of comparison Br is 

 closer to I, but by the second, which is more significant, is closer to CI. 



Til is marked with an asterisk because it is known to have two 

 modifications. The transition takes place on heating to about 168°, 



