tion of hydrogen peroxide is poured over the surface of pure mercury, a film of peroxidate at once forms over the surface of the metal. Its formation alters the surface-tension of the mercury by changing the potential-difference between the metal and the solution. Consequently, the form of the mercury-surface changes. Under appropriate conditions this deformation causes a mechanical rupture of the film at some portion of its surface; there follows on this an electrolytic decomposition of the peroxidate at the margin of the fissure, an effect which spreads over the whole surface and involves the dissolution of the film, and its reduction to metallic mercury, together with the liberation of oxygen. The film then reforms, and the process is repeated. Thus a regular rhythm, involving a form-change, a chemical decomposition, and a change of electrical polarization, is started and continues automatically. The rate of rhythm may be altered, just as in organic processes, by altering the chemical character of the medium, e. g., by changing its alkalinity, or by the addition of various other chemical substances. The velocity with which the film is laid down and dissolved may thus be influenced, and the whole rhythm correspondingly affected. Graphic records showing the variation in the rate of oxygen-liberation present a marked resemblance to the records of rhythmical organic processes like the heart-beat. Now the general conditions determining the rhythm in this phenomenon are strikingly like those which, on the foregoing theory of stimulation, determine the physiological rhythms. The surfacefilm of peroxidate may be compared to the plasma-membrane. Its rupture is equivalent to a local increase of permeability. This change is the direct condition both of the chemical change and of the electro-motor change, on which last depends the variation of surface-tension conditioning the form-change. While the living system is indefinitely more complex than the mercury-peroxide system, yet in its rhythmical character and in the essential nature of the controlling conditions this automatic rhythmical catalysis bears an undeniable and striking resemblance to the action of living tissues like the heart, in which a rhythmical autostimulation is the distinguishing characteristic. In both cases an alteration of a surface-film is the critical change; and the rate of this change determines the rate of the other rhythmical events of the cycle. We may infer that if we could control the condition of the plasma-membranes of cells we could control the entire range of cell-processes. But I do not wish to prejudge these questions; I make the above comparison chiefly in order to suggest possibilities, and to indicate the desirability of devoting more careful study to the surface-films of cells. Investigation of the conditions of their formation, their permeability and their physical and chemical nature is certain to lead to results of far-reaching importance for biology.
