€60 A Vapour Pressure Equation. 



be the best one. Data on the densities oE liquids at tempe- 

 ratures up to the critical temperature are not available for 

 the proper calculation of the constants A and B. 



Since no liquid can exist at or beyond the critical tempe- 

 rature, the average energy of a molecule at this temperature 

 should be the critical energy required for the penetration of 

 the surface. The average kinetic energy of any molecule 

 at 0° C. is 5 - 5 x 10 ~ 14 erg ; therefore at the critical tempe- 

 rature 6 C it will be 



1 8 



- mc Y 2 = -~- x 5-5 x 10~ u erg. 



■So that 



2 R 273 



B = 1 "^ = A x 5-5 x iQ2 (taking R= 1Q-") 



In the following table approximate values of log 10 A' and B 

 obtained from vapour-pressure data are given for some very 

 different liquids. The values of B calculated from the critical 

 temperature, where this is known, are also given, and in the 

 last column the ratios of B (vap. press, equation) to B 

 (obtained from crit. temp.) are shown. 



B 



Liquid. log 10 A'. B. B (crit. temp.). » 



^ ^ 10 . r B (crit. temp.) 



Hydrogen 4-77153 128 777 17 



Oxygen G'34550 9:38 313 3-0 



Nitrogen 6-24185 787 256 31 



Niton 7-96974 3050 760 4-0 



Sulphur dioxide 7'12622 3253 864 3-8 



Ammonia 7-56507 3229 811 4-0 



Ether 7-03000 3731 947 39 



Benzene 703208 4286 1130 38 



Bromine 703776 4029 1155 35 



Mercury 7-18980 7818 ? — 



Water 8-09470 5495 1285 4-2 



Aniline 671996 4011 1407 2-8 



Nickel carbonyl 673336 3600 ? 



The values of B obtained from the critical temperature are 

 not identical with those obtained from the vapour-pressure 

 equation, but it is rather striking to find that they are 

 generally from one-third to one-quarter of the equation 



values. 



Newcastle-upon-Tj ne, 

 August 1919. 



