68 . bridgman. 



Data. 



The data for the individual substances follow. 



Potassium sulfocyanide. This was Kahlbaum's purest, "zur 

 Analyse," used without further purification. Two sets of readings 

 were made; one consisted of five points on the transition curve and 

 two on the melting curve at higher pressures, and the other was of one 

 point on the transition curve at low pressures. The transition from 

 one solid to the other is sharp, so that apparently the solid does not 

 form mixed crystals with whatever impurity is present. There is 

 some impurity present, however, as shown by a rounrling of the corners 

 of the melting curve, but it must be small in amount, because the 

 melting and freezing ran unusually rapidly and approached closely 

 the same values from alcove and below, nearly independently of the 

 amount melted. The impurity is most probably a small amount of 

 moisture. For the determination at low pressures the sul)stance was 

 carefully dried in vacuum for seven hours at 100°. The value for 

 AV found here lies on a smooth curve with the values found at higher 

 pressures, so the effect of the impurity must be very slight. 



For the determination at high pressures, during which melting was 

 to occur, the KSCN in the form of fine crystals was hammered com- 

 pactly into a steel shell, open only at the top. For the determination 

 at low pressures, without melting, the fine crystals were hammered 

 into a steel shell open at both ends and perforated on the sides with 

 numerous small drill holes. The somewhat greater restraint offered 

 by the steel shell in the first run seemed to have no harmful effect. 

 The quantity used was about 47.5 gm. Pressure was transmitted 

 directly by kerosene. 



The experimental results are shown in Figures 4 and 5 and the 

 computed values of AH and AE in Figure 6. Table I gives numerical 

 values at even pressure intervals. It is especially to be noticed that 

 the values given for the melting curve are only approximate; since 

 no great effort was made to get these very accurately. Only two 

 points were determined, so the curvature can not be stated. The 

 transformation curve and the AJ' curve for the transition solid-solid 

 are both like the corresponding curves for liquids. The AH curve 

 falls, however, whereas for the majority of liquids it rises. 



The effect of pressure on the transition and melting does not seem 

 to have been previously determined, so there are no values for compari- 



