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L. On the Nature of the Transition Layer between Two 

 Adjacent Phases. By Wm. 0. McC. Lewis, M.A., D.Sc, 

 Physical Chemistry Laboratory, University College, London *. 



IT is well known that the " internal pressure " or " molecular 

 pressure " in a liquid is included in the van der Waals' 



equation as the correction term -g . Obviously the numerical 



values obtained for the internal pressure by the calculation 

 of a and v will be average values obtaining throughout the 

 bulk of the liquid. Let us denote such values by K m . For 

 the particular case of water at 0° C. van der Waals himself 

 has calculated K m to be 10500-10700 atmospheres f. There 

 is another method, however, first proposed by Dupre (ThSorie 

 MScanique de la Chaleur, Paris, 1869), viz., that K is the 

 work required to remove unit volume from the surface layer 

 in the form of very thin laminae, and carry them outside the 

 range of their mutual attraction. In other words, K is the 

 internal work required to vaporize unit volume of the liquid 

 at the given temperature. Lord Rayleigh in his work on 

 "The Theory of Surface Forces" (Phil. Mag. xxx. p. 285, 

 1890 ; Scientific Papers, vol. iii. p. 396), states that "this 

 view appears to be substantially sound." Assuming that the 

 volume of one gram of water is approximately the same in 

 the bulk of the liquid and in the surface layer one finds that 

 for water at 0° C. the value for the internal pressure comes 

 out to be about 25,000 atmospheres, i. e., there is a large 

 discrepancy between K m and Kpupre- Other substances show 

 the same peculiarity, viz. : — 



Ether K m = 1300-1430 atm. K Duprd = 2426 atm. 



Ethyl alcohol K TO = 2100-2400 „ K Dupre =7266 „ 



Carbon disulphide K m = 2890-2900 „ K Dupr6 = 4704 „ 



The differences are so large that they can hardly be 

 regarded as accidental, so that one is forced to the conclusion 

 that the internal pressure in the surface (call it K s ) is con- 

 siderably greater than the average bulk value K m . This 



* Communicated by the Author. 



f An attempt at calculating how K m varies with temperature is ren- 

 dered very difficult owing to the fact that van der Waals' " a " is anything 

 but constant. Thus, taking as our units atmosphere, litre, grammole, 

 for pressure, volume, and mass respectively, the value of a at 0° C. is 

 3-467; at 100° C. a = 3'29 (according to Traube), and at the critical 

 temp. 362°4 C. a = 5*77 as calculated from the critical data. The 

 inconstancy of a 6" is even more marked, but does not concern us here. 



