POLYMORPHISM AT HIGH PRESSURES. 



99 



parent crystals were obtained, | to f of an inch in diameter, showing 

 very well the hexagonal primitive form and the characteristic faces on 

 the ends. The purity was still not perfect as shown by a slight re- 

 maining rounding of the melting curves, but it was nevertheless possi- 

 ble to make satisfactory measurements of the melting. 



The observed equilibrium pressures and temperatures are shown in 

 Figure 4, the observed differences of volume in Figure 5, the calculated 

 latent heats and changes of internal energy in Figure 6, and the numer- 

 ical values are collected in Table III. I did not think it worth while 

 to try for a point on the melting curve nearer 12000 kgm., as rather 



13 4 5 6 7 8 9 

 Pressure, kgm./cm.^ x 10^ 

 Acetamide 



10 U 



Figure 4. Acetamide. The observed equiUbrium pressures and tempera- 

 tures. 



inconvenient manipulation would have been necessary to obtain it, 

 and even then satisfactory measurements of Av would not have been 

 possible without taking the apparatus further beyond 12000 kgm. 

 than I cared to. Furthermore, such a point was not necessary to 

 establish the non-existence of other solid forms, because at 173° I 

 found no new solid form up to beyond 12000. The inconvenience of 

 manipulation referred to is caused by the very great subcooling that the 

 liquid will support; after determining the point at 157.5° and 8800 

 kgm. it was necessary to lower the temperature 20° and raise the 

 pressure to 12700 kgm. before the liquid could be forced to freeze. 

 This means a superpressure of 6200 kgm. or a subcooling of 45°. 



