Among the general laws of physical science, none seems more firmly established than that of the conservation and correlation of energy; according to this the various forms of energy that constitute the domain of experimental physics, heat, light, electricity, magnetism and chemical action, have reciprocal dependence and “can not originate otherwise than by devolution from some preëxisting force,” or rather energy. That motion is convertible into heat, heat into light and both the former into electricity are phenomena familiar to every one who uses incandescent bulbs or rides in a trolley, and we do not usually recognize any production of light unaccompanied by heat. True, the little fire-fly is possessed of a mysterious power that enables it to emit light without enough heat to affect Langley’s most sensitive bolometer, but the eminent Secretary of the Smithsonian has to admit that the “cheapest form of light” is produced by “processes of nature of which we know nothing.” This little understood property called phosphorescence is shared by many living organisms, both animal and vegetable, as well as by substances of the mineral kingdom; to the former belong coelenterates, mollusks, crustacea, fishes and insects, and decaying wood, certain mushrooms, etc.; to the latter the Bologna stone, so-called, and the commercial article called ‘Balmain’s paint.’ In the case of the mineral substances, barium or calcium sulfids and the like, the light-giving power is not an innate property, but is set in operation by exposure to the energy of sunlight, the light of burning magnesium or to some other source of actinism; moreover, the power thus acquired by insolation is a fugitive one, the substances exercising it after three or four hours become ‘dead’ and lose their activity. Excepting then these living beings and these phosphorescent bodies, light as commonly known to us is always correlated with heat; within the last four years, however, discoveries have been made in France that seem to modify the position taken by philosophers and to necessitate new views concerning the manifestations of that energy with which the universe is endowed. A group of French savants have found mineral substances that apparently give out light perpetually without any exciting cause, realizing the dream of the alchemists--a perpetual lamp consuming no oil. These substances also emit rays having the penetrating properties of X-rays, other rays affecting a photographic plate, and fourthly, rays causing air to become a conductor of electricity. The history of these discoveries can be briefly given.