Effective mercury intrusion in a wedge-shaped slit is gradual, the intruded depth increasing with applied pressure. The Washburn equation must be modified accordingly. It relates the distance, e, separating the three-phase contact lines on the wedge faces to the hydrostatic pressure, P, wedge half-opening angle alpha, mercury surface tension gamma, and contact angle theta: e = (-2 gamma/P) cos(theta - alpha) if theta - alpha > pi/2. The equations relating the volume of mercury in a single slit to hydrostatic pressure are established. The total volume of mercury V-Hg(tot)(E-0, e) intruded in a of unconnected isomorphous slits (same alpha value) with opening width, E, distributed over interval [E-0, 0], and volume-based distribution of opening width, f(V)(E), is written as V-Hg(tot)(E-0, e) = -integral(E0)(e) f(V)(E)dE + (1 - b)e(2) integral(E0)(e) f(V)(E)dE/E-2 - tan alpha integral(E=e)(0) G(X(E, e))f(V)(E)dE, where G(X) = (sin(-1) X - X root 1 - X-2)/X2 and X(E, e) = -cos(theta -alpha)E/e. The exact relation between total internal surface area and integral pressure work is S-tot = 1/gamma Hg(cos theta + sin alpha) integral(0)(VHgtot) P dV(Hg)(tot). (c) 2004 Elsevier Inc. All rights reserved.