RADIOACTIVITY— RUTHERFORD Jgg 



at a constant rate in the thorium and was converted into emanation. 

 The constant activity due to thorium-X was shown to be the result 

 of an equilibrium process in which the decay of the active matter was 

 balanced by its continuous production. This process of production 

 and decay was found to be a universal property of the radioactive 

 bodies. 



A study of the chemical properties showed that the emanation of 

 both thorium and radium must be chemically inert and correspond 

 to the group of gases of the helium-argon family. We now know 

 that the radioactive emanations are isotopic representatives of the 

 last of the inert gases. Finally, the material nature of the emana- 

 tions was definitely established by proving they could be condensed 

 in a spiral surrounded by liquid air. It is a noteworthy example of 

 the delicacy and certainty of the methods of detection of radioactive 

 matter that the chemical nature of the emanations and their condensa- 

 tion at low temperatures could be definitely established with almost 

 infinitesimal amounts of active matter, far too small to be seen or 

 weighed or detected by the spectroscope. 



The experiments with thorium-X and the emanation gave us for 

 the first time a clear idea of radioactive processes and led us to 

 put forward in 1902-03 the transformation theory of radioactive 

 elements. Although the results were substantiated and extended by 

 investigations with other radioactive substances, time does not allow 

 me to refer to them, and I must pass on at once to consider the 

 importance of these new ideas on transformation. 



THE TRANSFORMATION OF RADIO-ELEMENTS 



The proofs that radioactivity was a sign and measure of the insta- 

 bility of atoms and that the radio-elements were undergoing spon- 

 taneous transmutation were contributions to our knowledge of out- 

 standing importance. The long series of radioactive changes in 

 uranium, thorium, and actinium were with few exceptions made clear 

 during the next few years. There were thus brought to light more 

 than 30 radio-elements, each of which showed distinctive radio- 

 active behavior and broke up according to a simple and definite law. 

 In most cases, in the process of transformation, the radio-element 

 emitted either a swift a-particle, now known to be a charged atom 

 of helium, or a fast y3-particle (negative electron). The transforma- 

 tion process is distinguished from an ordinary chemical reaction, not 

 only because the disintegi'ation appears to be spontaneous and unalter- 

 able by the forces at our command, but, most important of all, by the 

 enormous amount of energy emitted from each exploding atom. This 

 energy is for the most part emitted in the kinetic form of a swift 

 a- or )8-particle, but in some cases a part of the energy is emitted in 



