Transition Layer of a Liquid on its Surface Tension, 877 



the process of separation, and A the amount of new surface 

 formed, the surface-tension X 2 is given by 



w 



X *=A W 



If the distribution of the molecules in the new surfaces 

 formed is the same as in the interior of the liquid, and the 

 law of molecular attraction were known, the work done 

 could at once be calculated. But the distribution of the 

 molecules in the surface of a liquid is not the same as in 

 the interior, a transition layer is formed in which the density 

 changes continuously from that of the liquid on one side of 

 the layer to that of the vapour on the other side. 



The nature of the effect of the formation of a transition 

 layer on the surface tension can be easily investigated. 

 Suppose that on separation of the two slabs of liquid in the 

 foregoing process, no change takes place in the distribution 

 of the molecules in the surfaces of the slabs. The work 

 done, as before, is W. Now suppose the transition layers to 

 be formed. This will require that the surface layer of the 

 liquid undergoes expansion in different degrees in different 

 parts. Since the complete process of increase of the surface 

 of a liquid is a reversible one, we must suppose that the 

 formation of the transition layer takes place in such a way 

 that external work is done during the process, and that it is 

 a reversible one. We may, for example, suppose that the 

 liquid is contained in a cylinder in which the piston is in 

 contact with the liquid surface in question exerting a 

 pressure tending to prevent the formation of the layer. 

 This work is done at the expense of the heat supplied since 

 the temperature has to be kept constant during the process. 

 Let w denote the amount of work in this case. The actual 

 surface-tension of a liquid is then given by 



*-*? w 



Thus we see that the production of a transition layer on the 

 surface of a liquid has the effect of decreasing the magnitude 

 of the surface tension. 



A formula will be developed in this paper which 

 expresses W in terms of quantities which can be measured. 

 Since \ ± can also be measured, the value of w can be cal- 

 culated by means of equation (2). 



Phil Maq. S. 6. Vol. 24. No. 144. Dec. 1912. 3 M 



