In 1903 Messrs. Curie and Laborde*8* made the remarkable announcement that a crystal of radium is persistently warmer than its surrounding medium; in other words, that it is perpetually giving out heat without apparently becoming cooler. At first blush this seemed to contradict the great physical law of the conservation of energy, but physicists were soon agreed that a less revolutionary explanation of the phenomenon is perfectly tenable. The giving off of heat is indeed only an additional evidence of the dissipation of energy to which the radio-active atom is subjected. And no one now believes that radio-activity can persist indefinitely without actually exhausting the substance of the atom. Even so, the evidence of so great a capacity to give out energy is startling, and has given rise to various theories (all as yet tentative) in explanation. Thus J. Perrin*9* has suggested that atoms may consist of parts not unlike a miniature planetary system, and in the atoms of the radio-elements the parts more distant from the centre are continually escaping from the central attraction, thus giving rise to the radiations. Monsieur and Madame Curie have suggested that the energy may be borrowed from the surrounding air in some way, the energy lost by the atom being instantly regained. Pilipo Re,*10* in 1903, advanced the theory that the various parts of the atom might at first have been free particles constituting an extremely tenuous nebula. These parts gradually becoming collected around condensed centres have formed what we know as the atoms of elements, the atom thus becoming like an extinct sun of the solar system. From this point of view the radio-active atoms represent an intermediate stage between nebulae and chemical atoms, the process of contraction giving rise to the heat emissions. Lord Kelvin has called attention to the fact that when two pieces of paper, one white and the other black, are placed in exactly similar glass vessels of water and exposed to light, the temperature of the vessel containing the black paper is raised slightly higher than the other. This suggests the idea that in a similar manner radium may keep its temperature higher than the surrounding air by the absorption of other radiations as yet unknown.