390 



TABLE II. Vapour Pressure of the Liquid Phase. 



Q _ 



4.571 



= 2450 



C = 3.80 



librium between vapour and condensed phase of arsenic is not very 

 quickl}' established but the heating of the furnace was so slow 

 that there was no difficulty to measure the pressure at any desired 

 temperature. The whole measurement of the vapour pressure oftlie 

 gray modification of arsenic to its liquid phase, namely from 400° C. 

 to 850° C, required a continuous work of more than 12 hours. The 

 results of the vapour pressure measurement were tabulated as follows: 

 (See Table I, page 389). 



Starting from Clausius and Ciapayron's equation and assuming that 

 the heat of vaporization Q, is constant, that the vapour of arsenic 

 follows the gas law and that the volume of the condensed phase 

 can be neglected with respect to that of the vapour, the vapour pressure 



would be represented by a straight line T log p = — 



Q 



4.571 



H- CT. 



When the value of Tlogp, from the observed values of pressure, 

 was plotted against temperature, a good straight line was obtained 

 from 550° C. to 700° C. as shown in Fig. If, from which we can 

 easily calculate Q/4.57J and C. The table contains in the last column the 

 values of the pressures calculated from this equation. Above 700° C. 

 however, the Tlog p—i curve shows some deviation, and the calculated 

 values of (^/4.571, assuming as a constant must deviate. The deviations 

 become gradually smaller as the temperature rises. Some of these 

 deviations may, of course, be experimental errors, because at high 

 temperature a little observation error of temperature would have a 

 great effect on the value of pressure, quite the contrary of the case 

 at low temperature, where a little error in the measurement of 

 pressure, owing to small value of pressure, would have a great 

 effect upon the calculated value of Q/4.57J. But such a deviation 



