Forces in surface films. Part I.—Theoretical considerations. Part II.—Experimental observations and calculations. Part III.—The charge on colloids

In an earlier paper on the adsorption of vapours, the author was led to make the assumption that the specific volume of the last portion of fluid adsorbed from the saturated vapour phase was the same as the specific volume of the liquid in equilibrium with the vapour. This led to a consideration of the nature of the surface film, and certain consequences of the variation in density there from the density of the liquid in bulk were immediately obvious, and are discussed below. Experimental proof of one of these was easily obtained by the author, and, recently, certain other observers, approaching the subject from a different aspect, have published evidence with a promise of further work. Let us consider the determination of the density of a finely divided solid by the usual immersion method, assuming that all gases such as air have been removed from the solid and the pure liquid. Let us assume that upon the surface of the solid there is a film amounting to ∝ grm. liquid per gramme solid where the mean specific volume of the liquid is v2instead of v1, the value for the liquid in bulk. If the true specific volume of the solid is v, and v0 is the volume ordinarily observed and calculated without regard to ∝, then, from a consideration of the total volume occupied by a gramme of the solid and the accompanying film