Original text - Engels - the element of area

Consider an element of area dS at the point P(x,y,z)on the surface of body 1.Provided the radii of curvature of bodies 1 and 2 are large compared to the separation distance H,then the area element can be approximately regarded as a surface element of a plane half-space with the properties of body 1 parallel to and separated a distance D(see fig.4)from another planar half-space with the properties of body 2.If E(D) the energy per unit area of half-space 1 interacting with the half-space 2,then,in this approximation,dV=E(D)dS is the interaction energy of the area element dS of body 1 with body 2.