Page:Popular Science Monthly Volume 74.djvu/488

484 of a substance connotes the intensity with which it tends to expand, its temperature the intensity with which it tends to part with heat, while the potential of a given chemical component represents (in Maxwell's acute interpretation) the intensity with which it tends to expel itself from the mass or compound containing it. Mathematically the Gibbsian potential, which Maxwell thought " likely to become very important in the history of chemistry," has been identified by Larmor with the marginal available energy per unit mass of substance at constant temperature, depending upon the percentage composition of the substance rather than its actual quantity. The chemical potentials may be regarded, not unlike the potentialities of an individual, as definite intensities which set things going, and as such their close relationship to the surface energies and surface tensions of biological science is obvious. As to the ultimate nature of the forces bound up with these potentials, whether due in the last analysis to electronic stresses or rotational stresses in the ether simply, we know little or nothing. Thermodynamic (or "energetic") doctrine rests upon the simple idea that mechanical, thermal, chemical and electric forces are different modes of energy, continually changing and passing into one another in an apparently elusive way, and is more concerned with their dynamic effects than with their actual nature.