Page:Popular Science Monthly Volume 82.djvu/149

as they do that of uncharged molecules. The ions formed by the dissociation of any electrolyte have as a rule unequal diffusion-velocities, and presumably unequal solubilities and other physical properties, in correspondence with their chemical differences; and hence we may infer that they possess unequal abilities to pass through membranes. If this is so, a membrane separating two electrolyte-solutions becomes the seat of a potential-difference; i. e., a potential-difference, which may be considerable, will exist between its opposite faces. This suggestion, first made by Ostwald in 1890, has formed the basis of the chief prevailing view—the so-called "membrane-theory" of the nature of the bioelectric processes. Ostwald's suggestion, modified to suit the conditions Fig. 2. Illustrating the supposed conditions of polarization of the plasma membrane. The electrolytes are lactic and carbonic acids; the membrane is supposed to be permeable only to H-ions. in cells, was essentially as follows. Imagine the cell enclosed in a plasma-membrane freely permeable to the cations (positive ions, e. g., hydrogen ions or potassium ions) and impermeable to the anions (negative ions) of a certain electrolyte (which we may suppose to be lactic or carbonic acid) contained in the protoplasm (Fig. 2). The cations then pass outward, carrying their positive charges, while the anions remain behind; this will proceed until the potential-difference thus arising is sufficient to compensate the diffusion-tendency (equivalent to the osmotic pressure) of the cations. A condition of equilibrium with outer surface positive and inner negative thus results. The membrane becomes the seat of an electrical polarization (normal or physiological polarization) which is dependent on its impermeability to anions. If the permeability of such a membrane were to increase sufficiently to transmit the anions, a fall of the potential-difference between the exterior and the interior of the cell would at once follow. An effect of just this kind is seen in muscle and nerve during stimulation, and is attributed by Bernstein and other upholders of the membrane-theory to the changing ionic permeability of the membrane. The selective permeability to ions of different sign, on which the potential-difference between exterior and interior depends, disappears along with the general increase in permeability accompanying stimulation: hence a negative electrical variation is always associated with this process. The precise arrangement imagined by Ostwald has not yet been satisfactory realized, although, according to Brünings, precipitation-membranes of copper ferrocyanide show sorre of the properties required by this theory. But certain natural membranes present a much closer approach to the theoretical requirements; thus the surface-membranes of apples, which Beutner and Loeb have recently studied, behave as if