CAPILLARY ACTION. 553 



If this quantity is positive, the surface of contact will tend to contract, 

 and the liquids will remain distinct. If, however, it were negative, the dis- 

 placement of the liquids which tends to enlarge the surface of contact would 

 be aided by the molecular forces, so that the liquids, if not kept separate by 

 gravity, would at length become thoroughly mixed. No instance, however, of 

 a phenomenon of this kind has been discovered, for those liquids which mix 

 of themselves do so by the process of diffusion, which is a molecular motion, 

 and not by the spontaneous puckering and replication of the bounding surface, 

 as would be the case if T were negative. 



It is probable, however, that there are many cases in which the integral 

 belonging to the less dense fluid is negative. If the denser body be solid we 

 can often demonstrate this; for the liquid tends to spread itself over the 

 surface of the solid, so as to increase the area of the surface of contact, even 

 although in so doing it is obliged to increase the free surface in opposition 

 to the surface-tension. Thus water spreads itself out on a clean surface of glass. 



This shews that (x~Xo) P^ v must be negative for water in contact with glass. 



J D 



ON THE TENSION OF LIQUID FILMS. 



The method already given for the investigation of the surface-tension of 

 a liquid, all whose dimensions are sensible, fails in the case of a liquid film, 

 such as a soap-bubble. In such a film it is possible that no part of the liquid 

 may be so far from the surface as to have the potential and density corre- 

 sponding to what we have called the interior of a liquid mass, and measurements 

 of the tension of the film when drawn out to different degrees of thinness 

 may possibly lead to an estimate of the range of the molecular forces, or at 

 least of the depth within a liquid mass, at which its properties become sensibly 

 uniform. We shall therefore indicate a method of investigating the tension of 

 such films. 



Let S be the area of the film, M its mass, and E its energy; a- the 

 mass, and e the energy of unit of area ; then 



M=S<r (11), 



E = Se (12). 



VOL. 1L 70 



