F. Re[348] recently advanced a very general theory of matter with a special application to radio-active bodies. He supposes that the parts of the atom were originally free, constituting a nebula of extreme tenuity. These parts have gradually become united round centres of condensation, and have thus formed the atoms of the elements. On this view an atom may be likened to an extinct sun. The radio-active atoms occupy a transitional stage between the original nebula and the more stable chemical atoms, and in the course of their contraction give rise to the heat emission observed. Lord Kelvin in a paper to the British Association meeting, 1903, has suggested that radium may obtain its energy from external sources. If a piece of white paper is put into one vessel and a piece of black paper into an exactly similar vessel, on exposure of both vessels to the light the vessel containing the black paper is found to be at a higher temperature. He suggests that radium in a similar manner may keep its temperature above the surrounding air by its power of absorption of unknown radiations. Richarz and Schenck[349] have suggested that radio-activity may be due to the production and breaking up of ozone which is known to be produced by radium salts. =255. Discussion of Theories.= From the survey of the general hypotheses advanced as possible explanations of radio-activity, it is seen that they may be divided broadly into two classes, one of which assumes that the energy emitted from the radio-elements is obtained at the expense of the internal energy of the atom, and the other that the energy is derived from external sources, but that the radio-elements act as mechanisms capable of transforming this borrowed energy into the special forms manifested in the phenomena of radio-activity. Of these two sets of hypotheses the first appears to be the more probable, and to be best supported by the experimental evidence. Up to the present not the slightest experimental evidence has been adduced to show that the energy of radium is derived from external sources. J. J. Thomson (_loc. cit._) has discussed the question in the following way:— “It has been suggested that the radium derives its energy from the air surrounding it, that the atoms of radium possess the faculty of abstracting the kinetic energy from the more rapidly moving air molecules while they are able to retain their own energy when in collision with the slowly moving molecules of air. I cannot see, however, that even the possession of this property would explain the behaviour of radium; for imagine a portion of radium placed in a cavity in a block of ice; the ice around the radium gets melted; where does the energy for this come from? By the hypothesis there is no change in the air-radium system in the cavity, for the energy gained by the radium is lost by the air, while heat cannot flow into the cavity from the outside, for the melted ice round the cavity is hotter than the ice surrounding it.”