148 Dr. G. Bakker on the 



p represents the ordinary vapour pressure, Spd{ is the work 

 done by p, and p T £dS that done by p T . So we have : 



ISTow, however, 



H-=(]> N — ^ T )f and u=fS. 

 So we get : 



Tdri=de+ 9 dv-tfdS+pvZd&. 



Further, for the plane capillary layer : 



i p 1 =|> 2 = p=^ N = ordinary vapour pressure. 



Hence Tdrj = d€ + $dv — {fy— p T )fdS, 



or Td v = de + pdv-KdS. ..... (31) 



For the curved capillary layer we may also deduce the 

 equation of energy without having recourse to the equations 

 (27) and (28). 



If as above Sj, S, and S 2 are the total surface, which refer 

 to the radii R T , R, and R 2 (per unit of mass of the capillary 

 layer), we have for the work done by p x and p 2 : 



s 2 p 2 (dR+idz)-SrFi(dn-yn- 



For unchanged R consequently : 

 Further we have : 



The work done by p x and p 2 becomes consequently : 



For values of R of the order of a micron p\—p 2 is of the 

 order yoVo against p. For values of R of the order of a 

 wave-length of light or larger, we may thus put for the 

 work done by p x and p 2 : 



The work done by the pressure p T (in a direction parallel 

 'to the surface of the capillary layer) becomes : 



2 dS lPr dh (l+ g + -^) -«!{£,+ § ?>', (32) 

 pj= y 2 \ p T dh 2 . 



