III. On the thermal effect of drawing out a film of liquid. By Professor William Thomson, F. R. S., &c., being extract of two letters to J. P. Joule, LL. D., F. R. S

Abstract

A very novel application of Carnot’s cycle has just occurred to me in consequence of looking this morning into Waterston’s paper on Capillary Attraction, in the January Number of the Philosophical Magazine. Let T be the contractile force of the surface (by which in Dr. Thomas Young’s theory the resultant effect of cohesion on a liquid mass of varying form is represented), so that, if II be the atmospheric pressure, the pressure of air within a bubble of the liquid of radius r , shall be 4T/ r + Π. Then if a bubble be blown from the end of a tube (as in blowing soap-bubbles), the work spent, per unit of augmentation of the area of one side of the film, will be equal to 2T. Now since liquids stand to different heights in capillary tubes at different temperatures, and generally to less heights at the higher temperatures, T must vary, and in general decrease, as the temperature rises, for one and the same liquid. If T and T' denote the values of the capillary tension at temperatures t and t ' of our abso­lute scale, we shall have 2(T —T') of mechanical work gained, in allowing a bubble on the end of a tube to collapse so as to lose a unit of area at the temperature t and blowing it up again to its original dimensions after having raised its temperature to t '.