IN a recent communication in NATURE (July 17, 1943, p. 76) I applied the Boltzmann distribution law to the Donnan membrane equilibrium between two compartments, compartment (1) containing colloidal (or non-diffusible) ions plus diffusible ions, compartment (2) containing only diffusible ions. From the equations obtained, it follows that at the same pH. of the solution the ratio of the membrane potential (E'm) for a dibasic acid to the membrane potential (Em) for a monobasic acid should be where y is the concentration of positive, y + z that of negative ions incompartment (1), x the concentration of positive and negative ions in compartment (2), N* = ½ (N1 + N2), N1 is the valency of the cation, N2 is the valency of the anion, and z is small. At the same pH. value of the solution, z in the case of sulphuric acid is not equal to z in the case of hydrochloric acid.