[In order to express the dependence of the tension at the **interface** of two bodies in terms of the forces exercised by the bodies upon themselves and upon one another, we cannot do better than follow the method of Dupre.

If T12 denote the interfacial tension, the energy corresponding to unit of area of the **interface** b Q FIG.

If 2T'12>T1+T2, T12 would be negative, so that the **interface** would of itself tend to increase.

Now Fresnel's formulae were obtained by assuming that the incident, reflected and refracted vibrations are in the same or opposite phases at the **interface** of the media, and since there is no real factor that converts cos T into cos (T+p), he inferred that the occurrence of imaginary expressions for the coefficients of vibration denotes a change of phase other than 7r, this being represented by a change of sign.

Lord Rayleigh has pointed out that all theories are defective in that they disregard the fact that one at least of the media is dispersive, and that it is probable that finite reflection would result at the **interface** of media of different dispersive powers, even in the case of waves for which the refractive indices are absolutely the same.

For instance, if T31> T12+ T23, the second fluid spreads itself indefinitely upon the **interface** of the first and third fluids.

We are thus led to the important conclusion that according to this hypothesis Neumann's triangle is necessarily imaginary, that one of three fluids will always spread upon the **interface** of the other two.

On the whole, then, the work expended in producing two units of **interface** is 2T1+2T2-4T'12, and this, as we have seen, may be equated to 2T 12.

On the analogy between this case and that of the **interface** between two solutions, Nernst has arrived at similar logarithmic expressions for the difference of potential, which becomes proportional to log (P 1 /P 2) where P2 is taken to mean the osmotic pressure of the cations in the solution, and P i the osmotic pressure of the cations in the substance of the metal itself.