On the Theory of Surface Forces. 209 



investigation. I do not therefore either affirm or deny his 

 conclusions. I do affirm that they are supported by a rope 

 of argument to the strength of which the one strand which I 

 have tested contributes nothing. It is difficult to believe in 

 discontinuities which their discoverer himself abandons in the 

 heat of argument by showing that his original solution may 

 be replaced by another in which three may be entire curves 

 and parts of two others replaced by two only. 



XX. On the Theory of Surface Forces. — II. Compressible 

 Fluids. By Lord Rayleigh, Sec. R.S.* 



IN the first part of the paper published under the above title 

 (Phil. Mag., Oct. and Dec. 1890) the theory of Young 

 and Laplace was considered, and further developed in certain 

 directions. The two leading assumptions of this theory are 

 (1) that the range of the cohesive forces, though very small 

 in comparison with the dimensions of ordinary bodies, is 

 nevertheless large in comparison with molecular distances, so 

 that matter may be treated as continuous ; and (2) that the 

 fluids considered are incompressible. So far as I am aware, 

 there is at present no reason to suppose that the applicability 

 of the results to actual matter is greatly prejudiced by imper- 

 fect fulfilment of (1) ; but, on the other hand, the assumption 

 of incompressibility is a somewhat violent one, even in the 

 cases of liquids, and altogether precludes the application of 

 the theory to gases and vapours. In the present communica- 

 tion an attempt is made to extend the theory to compressible 

 fluids, and especially to the case of a liquid in contact with 

 its own vapour, retaining the first assumption of continuity, 

 or rather of ultimate homogeneity. There will not be two 

 opinions as to the advantage of the extension to compressible 

 fluids ; but some may perhaps be inclined to ask whether it is 

 worth while to spend labour upon a theory which ignores the 

 accumulated evidence before us in favour of molecular struc- 

 ture. To this the answer is that molecular theories are 

 extremely difficult, and that the phenomenon of a change of 

 state from vapour to liquid is of such extreme importance as 

 to be worthy of all the light that can be thrown upon it. We 

 shall see, I think, that a sufficient account can be given 

 without introducing the consideration of molecules, which on 

 this view belongs to another stage of the theory. 



If p denote the ordinary hydrostatical pressure at any point 

 in the interior of a self-attracting fluid, p the density, and V 

 the potential, the equation of equilibrium is 

 * Coimnunicated by the Author. 



