the ratio of the difference of the absolute temperatures at the source and at the sink, to the absolute temperature at the source. Calorimetric measurements have shown that the efficiency of the human machine is about one-fifth, _i.e._ it can transform 20 per cent. of the energy absorbed. The ordinary temperature of muscle is 38deg C., or 311deg absolute. We have {108} therefore (T - 311) / T = .20, or T = 388.75deg absolute, _i.e._ 115.75deg C. Thus, in order to obtain an efficiency of 20 per cent. with an ordinary thermic transformer, having a temperature of 38deg at the sink, we should need a temperature of over 115deg C. at the source. Such a temperature would be quite incompatible with the integrity of living tissues, and we may therefore conclude that the human organism is not a heat engine. We are indeed completely ignorant of the mode of transformation of chemical into kinetic energy in the living organism; we know only that muscular contraction is accompanied by a change of form; at the moment of transformation the combustion of the muscle is increased, and during contraction the stretched muscular fibre tends to acquire a spherical shape. It is this shortening of the muscular fibre which produces the mechanical movement. The step which we do not as yet fully understand is the physical phenomenon which intervenes between the disengagement of chemical energy and the occurrence of muscular contraction. Professor d'Arsonval supposes that this missing step is a variation in the surface tension of the liquid in the muscular fibre. The surface tension of a liquid is due to the unbalanced forces of cohesion acting on the surface layer of molecules. Under the attraction of cohesion the molecules within the liquid are in a state of equilibrium, being equally attracted in all directions, but those at the surface of the liquid are drawn towards the centre. The resultant of these attractive forces is a pressure normal to the surface, which is mechanically equivalent to an elastic tension tending to diminish the surface. In consequence of this surface tension the liquid has a tendency to assume the form in which its surface area is a minimum, _i.e._ the spherical form. If such a sphere is stretched into a cylinder or fibre by mechanical tension, it will shorten itself when released; and if by any means we increase the surface tension of such a liquid fibre it will tend to assume a spherical form and contract just as a muscular fibre does. The surface tension of a liquid varies with its chemical composition; the slightest chemical modification of a liquid alters the force of {109} this tension. We may therefore explain the mechanism of muscular contraction by supposing that a nervous impulse alters in some way the rate of combustion in a muscular fibre, that this alteration produces a momentary change in the chemical composition of the muscular cell, and that this change of