"The correlation of forces," said he, "was prominently introduced and considered by a painstaking scientific writer named Joule, in several papers that appeared between 1843 and 1850, and he was followed by others, who engaged themselves in experimenting and theorizing, and I may add that Joule was indeed preceded in such thought by Mayer. This department of scientific study just now appears of unusual interest to scientists, and your questions embrace problems connected with some phases of its phenomena. We believe that light, heat, and electricity are mutually convertible, in fact, the evidences recently opened up to us show that such must be the case. These agencies or manifestations are now known to be so related that whenever one disappears others spring into existence. Study the beautiful experiments and remarkable investigations of Sir William Thomson in these directions." "And what of gravitation?" I asked, observing that Prof. Vaughn neglected to include gravitation among his numerous enumerated forces, and recollecting that the force gravitation was more closely connected with my visitor's story than perhaps were any of the others, excepting the mysterious mid-earth illumination. "Of that force we are in greater ignorance than of the others," he replied. "It affects bodies terrestrial and celestial, drawing a material substance, or pressing to the earth; also holds, we believe, the earth and all other bodies in position in the heavens, thus maintaining the equilibrium of the planets. Seemingly gravitation is not derived from, or sustained by, an external force, or supply reservoir, but is an intrinsic entity, a characteristic of matter that decreases in intensity at the rate of the square of the increasing distance, as bodies recede from each other, or from the surface of the earth. However, gravitation neither escapes by radiation from bodies nor needs to be replenished, so far as we know, from without. It may be compared to an elastic band, but there is no intermediate tangible substance to influence bodies that are affected by it, and it remains in undying tension, unlike all elastic material substances known, neither losing nor acquiring energy as time passes. Unlike cohesion, or chemical attraction, it exerts its influence upon bodies that are out of contact, and have no material connection, and this necessitates a purely fanciful explanation concerning the medium that conducts such influences, bringing into existence the illogical, hypothetical, fifth ether, made conspicuous by Aristotle." "What of this ether?" I queried.