The Origin of Bi-Ionic Potentials across Porous Membranes of High Ionic Selectivity. I. The Bi-Ionic Potential and the Mechanism of its Origin; The Various Factors Which Determine the Sign and the Magnitude of the Bi-Ionic Potential; The Simplest Chains in Which Bi-Ionic Potentials Arise: Systems with Critical Ions of the Same Size and of Different Adsorbability.