§ =90.= Radium salts possess another very remarkable property, namely, that of continuously emitting light and heat. It seemed, at first, that here was a startling contradiction to the law of the conservation of energy, but the whole mystery becomes comparatively clear in terms of the corpuscular or the electronic theory of matter. The radium-atom is a system of a large number (see § 81) of corpuscles or electrons, and contains in virtue of their motion an enormous amount of energy. But it is known from Chemistry that atomic systems (_i.e._, molecules) which contain very much energy are unstable and liable to explode. The same law holds good on the more interior plane--the radium-atom is liable to, and actually does, explode. And the result? Energy is set free, and manifests itself partly as heat and light. Some free electrons are shot off (the β-rays), which, striking the undecomposed particles of salt, give rise to pulses in the ether (the γ-rays),[107] just as the kathode particles give rise to X-rays when they strike the walls of the vacuum tube or a platinum disc placed in their path. The β- and γ-rays do not, however, result immediately from the exploding radium-atoms, the initial products being the emanation and one α-particle from each radium-atom destroyed. [107] This view regarding the γ-rays is not, however, universally accepted, some scientists regarding them as consisting of a stream of particles moving with very high velocities. “Induced Radioactivity.” § =91.= Radium salts have the property of causing surrounding objects to become temporally radioactive. This “induced radioactivity,” as it may be called, is found to be due to the emanation, which is itself radioactive (it emits α-rays only), and is decomposed into minute traces of solid radioactive deposits. By examining the rate of decay of the activity of the deposit, it has been found that it is undergoing a series of sub-atomic changes, the products being termed Radium A, B, C, &c. It has been proved that all the β- and γ-rays emitted by radium salts are really due to certain of these secondary products. Radium F is thought to be identical with Polonium (§ 87). Another product is also obtained by these decompositions, with which we shall deal later (§ 94). Properties of Uranium and Thorium.