CAPILLARY ACTION. 555 



da- _ de _ de 



Tc~ p " dc~ x p< " '^ = Xo> 



pd v , 



(20). 



Hence the tension of a thick film is equal to the sum of the tensions of 



its two surfaces as already calculated (equation 7). On the hypothesis of 



uniform density we shall find that this is true for films whose thickness 

 exceeds e. 



The symbol x * s defined as the energy of unit of mass of the substance. 

 A knowledge of the absolute value of this energy is not required, since in 

 every expression in which it occurs it is under the form x~Xo> that is to 

 say, the difference between the energy in two different states. The only cases, 

 however, in which we have experimental values of this quantity are when the 

 substance is either liquid and surrounded by similar liquid, or gaseous and sur- 

 rounded by similar gas. It is impossible to make direct measurements of the 

 properties of particles of the substance within the insensible distance e of the 

 bounding surface. 



When a liquid is in thermal and dynamical equilibrium with its vapour, 

 then if p and x' are the values of p and x for the vapour, and p a and x 

 those for the liquid, 



where J is the dynamical equivalent of heat, L is the latent heat of unit of 

 mass of the vapour, and p is the pressure. At points in the liquid very near 

 its surface it is probable that x is greater than XD, and at points in the gas 

 very near the surface of the liquid it is probable that x i s l ess than x'. but 

 this has not as yet been ascertained experimentally. We shall therefore en- 

 deavour to apply to this subject the methods used in Thermodynamics, and 

 where these fail us we shall have recourse to the hypotheses of molecular 



physics. 



702 



