393 



point, which coincided with the value found by tlie exti'a|)olation 

 of the vapour pressure curve of the solid pliase to that of tlie 

 liquid phase (35,8 aim.)- 



The Heats of Vaporization and Sublimation of Arsenic. 



Heat of vaporization is, of course, a teuiperature function, but 

 its temperature coefficient dQ/dt is generally negative, so that 

 T log p — t curve should be concave to the straight line given by 

 the expression 



which was deduced from the assumption that (2 is a constant. On the 

 contrary, the present experimental results show that the T log p — t 

 curve is somewhat convex to the said expression of pressure, so 

 that we can see that the deviation of the assumption, that Q is a 

 constant, is smaller than the total effect of deviations from other 

 assumptions, so that we may say that the temperature coefficient of 

 the heat of vaporization is comparatively small. It is, therefore, 

 possible to calculate the heat of vaporization from the expression 



3- /".^ P = - i;|j + CT". 



which was found to hold good for comparatively low temperatures. 

 For the molecular heat of sublimation we have 



hence, 



QsG = 33.6 Kg. cal. 

 For the molecular heat of vaporization for the liquid phase, we have 



uï-, = '''' 



hence, 



QzG = 11.2 kg. cal. 



From the difference of the above two heats of vaporization, we 

 have the moleculai' heat of fusion 



QsL = 22.4 kg. cal. 



According to Tkouton's law, Le Chatelier showed, that the quotient 

 Q/T, where Q is the heat of sublimation at sublimation temperature 

 under one atmosphere and 7' is the sublimation temperature, would 

 be 30 for all substances. In the case of arsenic, the temperature of 

 sublimation is 616° C. or 889 in absolute unit, then 



