I'OLYMORPHIC TRANSFORMATIONS OF SOLIDS. 



73 



with the other values. For this determination, a new manganin coil 

 was used, which however, had been previously carefully seasoned. 

 Both the values of equilibrium pressure and temperature and AF found 

 with this coil lie on a smooth curve with the other points, bespeaking 

 the accuracy of the manganin coils. Two points were determined 

 with the low pressure apparatus; one by the method of varying 

 temperature at a constant pressure of 77 kgm , and the second by 

 varying pressure at constant temperature, the equilibrium pressure 

 being about 240 kgm. The values found at 77 kgm. did not agree 

 with the other values, and were discarded; the error is apparently 

 an effect of air in the apparatus, which of course becomes vanishingly 

 small at higher pressures. 



The transition between the two solids runs cleanly and sharply, 

 very nearly the same pressure being reached from above and below. 



1000 2000 3000 

 Pressure, kgm. /cm/ 



Ammonium 5ulfocyanide 



Figure 7. Ammonium Sulfocyanide. The observed equilibrium tempera- 

 tures and pressures, and the changes of volume. 



The change of volume of NH4SCN was large enough so that it was 

 possible to measure the time rate of reaction, that is, the time rate of 

 recovery of pressure. These results will be given in another paper. 

 The retardation effects, either superheating or subcooling, were not 

 large; no careful attempt to measure these was made. It is probably 

 safe to say that the reaction ran invariably if the transition line was 

 crossed as much as 300 kgm. in either direction. 



The experimental points are shouTi in Figure 7, the computed values 

 of A// and AE in Figure 8, and a summary of the numerical values is 

 given in Table II. The exceptional curvature of the transition line 

 is to be noted. 



