In the course of a detailed study of the radio-activity of thorium, Rutherford and Soddy[340] found that it was necessary to suppose that thorium was continuously producing from itself new kinds of active matter, which possess temporary activity and differ in chemical properties from the thorium itself. The constant radio-activity of thorium was shown to be the result of equilibrium between the processes of production of active matter and the change of that already produced. At the same time, the theory was advanced that the production of active matter was a consequence of the disintegration of the atom. The work of the following year was devoted to an examination of the radio-activity of uranium and radium on similar lines, and it was found that the conclusions already advanced for thorium held equally for uranium and radium[341]. The discovery of a condensation of the radio-active emanations[342] gave additional support to the view that the emanations were gaseous in character. In the meantime, the writer[343] had found that the rays consisted of positively charged bodies atomic in size, projected with great velocity. The discovery of the material nature of these rays served to strengthen the theory of atomic disintegration, and at the same time to offer an explanation of the connection between the α rays and the changes occurring in the radio-elements. In a paper entitled “Radio-active Change,” Rutherford and Soddy[344] put forward in some detail the theory of atomic disintegration as an explanation of the phenomena of radio-activity, and at the same time some of the more important consequences which follow from the theory were discussed. In a paper announcing the discovery of the heat emission of radium, P. Curie and Laborde[345] state that the heat energy may be equally well supposed to be derived from a breaking up of the radium atom or from energy absorbed by the radium from some external source. J. J. Thomson in an article on “Radium,” communicated to _Nature_[346], put forward the view that the emission of energy from radium is probably due to some change within the atom, and pointed out that a large store of energy would be released by a contraction of the atom. Sir William Crookes[347], in 1899, proposed the theory that the radio-active elements possess the property of abstracting energy from the gas. If the moving molecules, impinging more swiftly on the substance, were released from the active substance at a much lower velocity, the energy released from the radio-elements might be derived from the atmosphere. This theory was advanced again later on to account for the large heat emission of radium, discovered by P. Curie and Laborde.