484 



the density of solid argon is greater than that of liquid argon, we 

 may conclude, upon the ground of the general thermodjnamical 

 rules ^), about the triple point, that the melting curve from the triple 

 point and in the immediate neighbourhood of it will run in the 

 direction of the higher temperatures and pressures. 



4. Calculation of the heat of evaporation at different temperatures. 



As the density of the liquid and of the saturated vapour is known 

 along by far the greater part of the vapour pressure curve -), we 

 were able to calculate the heat of evaporation for a number of 

 temperatures by the well known equation of Clapeyron-Clausius : 



_ (dp\ 



^licj. vap. — -L I -j^ j (^' vap. ^"lifiJ 



p'or the simplitication of the calculations those temperatures were 

 chosen at which the vapour pressure determinations were made. 

 The derivation of the liquid and vapour densities to these tempera- 

 tures was made by the aid of the formulae that Keesom "') has given 

 for the curve of these densities. They are 



Qii,=Qi,{i-\-A{i-t) + B{i-ty\ 



l>vap=9k{l + ^4(1-0- ^(l-O'j- 



With the constants 



A= + 0.744537 ;B= + 1.76408 ; X = 0.341571 

 and the critical density (>krd = 0.53078 ') they give a very satis- 

 factory correspondence with the observations, except for the vapour 

 densities at the lowest temperatures, where the percentual deviations 

 become very large. As, however, at these temperatures the vapour 

 densities can be calculated by the ordinary gaslaws, this gives no 

 difficulty. 



The values of ( --, ) ^vere calculated by the formula of Rankine- 



\dl /coex. 



BosE already mentioned. 



All quantities were expressed in C. G. S. units, and then the 

 whole expression for the heat of evaporation divided by the mechan- 

 ical equivalent, in order to get the result in calories per degree. 

 The results are shown in table III. 



1) Gf. H. W. Bakhuis Roozeboom, Die heterogenen Gleichgewichte, erstes Heft, p. 94. 



2) E. Mathias, H. Kamerlingh Onnes and G. A. Grommelin, Proceedings Oct. 

 and Dec. 1912 and Jan. 1913. Gomm. No. 131a. 



3) \V. H. Keesom, Proceedings April 1902. Gomm. No. 79. See also J. E. Ver- 

 scHAFFELT, Zittingsversl. June 1896, Gomm. No. 25 and Proceedings April 1900, 

 Gomm. No. 55. 



