392 



were represenled quite well by Tloij p plotted against tempei-atiire 

 as a straight line as seen in the table 11. 



The Melting Point of Arsenic. 



The direct measurement of the melting point of aisenic was im- 

 possible in the course of this experiment, because the thernio-eleinent 

 was placed outside of the indicator. 



As shown in figure I, the observed pressures *) of the gray modifica- 

 tion very near this melting point (represented by a dotted curve) 

 were always a little lower than the extrapolated pressure curve and 

 at the temperatures a few degrees liigher than the melting point the 

 indicator showed the right pressure of the liquid phase. For determining 

 the melting point I have therefore extrapolated the pressure curve of 

 the solid phase as that of the liquid phase and it was fouiui between 

 817° C. — 818° 0. which agreed well with the value given by Goubkau*) 

 and Rassow"). The corresponding pressure is 35.8 atm. Of course, 

 we could find also the value of the melting point by the intersection 

 of the two T log p — t lines of solid and liquid phases, but in this 

 case we find the following values. 



T Q/4.571 p 



822° C. 1710 36.5 



These values of the triple-point are certainly too high due to the 

 deviation of the expression used, as was already mentioned. 



As to the pressure of the triple-point, we can measure it directly 

 with a certain degree of accuracy. Arsenic shows a very large 

 effect of supercooling, sometimes more than 30 degrees in the authors 

 experiments. In the case of a sudden crystallisation of such a super- 

 cooled liquid, its temperature rose very quickly to the melting point ; 

 consequently the pressure rose also suddenly sevei-al atmospheres, so 

 that it was almost impossible to follow tliis sudden change of the pres- 

 sure, applying the pressure outside the spring of the indicator so that, 

 the spring broke. But if the temperature of melted arsenic was kept 

 a few degrees below that of the melting point, and if the change of 

 the pressure was constantly watched for a long while, sometimes 

 longer than two hours, then it was possible to follow the sudden 

 change of the pressure by crystallisation. In this case the pressure 

 remained constant during the crystallisation This adjustment of 

 the pressure, however, requires much skill, otherwise the spring 

 will break. In this way we could read the pressure at the melting 



•) These values were not given in the table. 

 «) loc. cit. 



