The molecules of a solid attract one another with a force \(F(r)=\alpha(l / r)^{5}\). Two semiinfinite solids composed of \(n\) molecules per unit volume are separated by a distance \(d\), that is, the solids fill the whole of the space with \(x \leq 0\) and \(x \geq d\). Calculate the force of attraction, per unit area of the surface, between the two solids.