the drop is concerned, then, equilibrium depends on a proper balance between the energy, per unit area, which is resident in its own two surfaces, and that which is external thereto: that is to say, if we call _E__{_bc_} the energy at the surface between the two fluids, and so on with the other two pairs of surface energies, the condition of equilibrium, or of maintenance of the drop, is that _E__{_bc_} < _E__{_ab_} + _E__{_ac_}. If, on the other hand, the fluid _A_ happens to be oil and the fluid _B_, water, then the energy _per unit area_ of the water-air surface is greater than that of the oil-air surface and that of the oil-water surface together; i.e. _E__{_wa_} > _E__{_oa_} + _E__{_ow_}. Here there is no equilibrium, and in order to obtain it the water-air surface must always tend to decrease and the other two interfacial surfaces to increase; which is as much as to say that the water tends to become covered by a spreading film of oil, and the water-air surface to be abolished. {295} The surface energy of which we have here spoken is manifested in that contractile force, or “tension,” of which we have already had so much to say[341]. In any part of the free water surface, for instance, one surface particle attracts another surface particle, and the resultant of these multitudinous attractions is an equilibrium of tension throughout this particular surface. In the case of our three bodies in contact with one another, and within a small area very near to the point of contact, a water particle (for instance) will be pulled outwards by another water particle; but on the opposite side, so to speak, there will be no water surface, and no water particle, to furnish the counterbalancing pull; this counterpull, [Illustration: Fig. 100.] [Illustration: Fig. 101.]