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XXIII. An Attempt to explain tlie Radioactivity of Radium. 

 By Lord Kelvin *. 



■§ 1. i^VNE chief action concerned in radioactivity is the 

 \J shooting out of electrions from a non-electrified 

 solid or liquid body. In the equilibrium of kinetic averages 

 in any solid or liquid, every individual electrion must occa- 

 sionally have so high a velocity that it is shot out of the 

 body. Hence every solid or liquid body has something of 

 radioactivity. 



§ 2. The Radium atom must, so far as we can at present 

 judge, be assumed to have a special property of being adapted 

 to store enormously more energy, by an electrion within it, 

 than the atom of any other substance hitherto known to us. 

 In a short article, published in the Phil. Mag, of Dec. 1905, 

 I explained the plan of an atom by the purely Boscovichian 

 assumption of mutual force in the line between the centre 

 of the atom and an electrion anywhere within it, according 

 to which there is for the electrion one position of stable 

 equilibrium, near the boundary of the atom, with very small 

 potential energy ; and another position of stable equilibrium 

 at the centre of the atom, with very great potential energy. 

 For brevity I shall call the atom " loaded/' when there is an 

 electrion at its centre, or anywhere within the range of the 

 stability of the central position : and I shall call the atom 

 "unloaded/' when there is no electrion within the central 

 range of stability. 



§ 3. In a solid crystal of Bromide or Chloride of Radium 

 we may suppose the Bromine or Chlorine, and the Helium 

 which Ramsay and Soddy produced from it, to be not 

 directly concerned in the marvellous radioactivity, which the 

 crystal presents. For brevity at present I shall assume that the 

 radioactivity depends primarily on the Radium atoms in the 

 compound. Suppose now the crystal to be given with every 

 Radium atom in it unloaded. In a very short time of pro- 

 gress towards equilibrium of kinetic averages, perhaps the 

 millionth of a second, or the millionth of the millionth of a 

 second, some of the atoms will become loaded. As time 

 advances a greater and greater proportion of the Radium 

 atoms will become loaded until, perhaps in the course of a 

 few months, a permanent average of loadings and unloadings 

 will be reached. The energy of the work done in loading 

 the Radium atoms is taken from the energy of thermometric 

 heat in the crystal. A cooling is thus experienced by the 



* Communicated by the Author. 



