PRESENT PROBLEMS IN RADIOACTIVITY. 19 



products unexplained. Unless there is some unknown source of energy 

 in the medium which the radioactive bodies are capable of absorbing, 

 it is difficult to imagine whence the energy demanded by the external 

 theory can be derived. It certainly can not be from the air itself, 

 for radium gives out heat inside an ice calorimeter. It can not be any 

 type of rays such as the radioactive bodies emit, for the radioactivity 

 of radium, and consequently its heating effect is unaltered by her- 

 metically sealing it in a vessel of lead several inches thick. The 

 evidence, as a whole, is strongly against the theory that the energy is 

 borrowed from external sources and, unless a number of improbable 

 assumptions are made, such a theory is quite inadequate to explain the 

 experimental facts. On the other hand, the disintegration theory, ad- 

 vanced by Rutherford and Soddy, not only offers a satisfactory ex- 

 planation of the origin of the energy emitted by the radio-elements, 

 but also accounts for the succession of radioactive bodies. On this 

 theory, a definite, small proportion of the atoms of radioactive matter 

 every second become unstable and break up with explosive violence. 

 In most cases, the explosion is accompanied by the expulsion of an a 

 particle ; in a few cases by only a ft particle, and in others by a and ft 

 particles together. On this view, there is at any time present in a 

 radioactive body a proportion of the original matter which is un- 

 changed and the products of the part which has undergone change. 

 In the case of a slowly changing substance like radium, this point of 

 view is in agreement with the observed fact that the spectrum of ra- 

 dium remains unchanged with its age. 



The expulsion of an a or ft particle, or both, from the atom leaves 

 behind an atom which is lighter than before and which has different 

 chemical and physical properties. This atom in turn becomes un- 

 stable and breaks up, and the process, once started, proceeds from 

 stage to stage with a definite and measurable velocity in each case. 



The energy radiated is, on this view, derived at the expense of the 

 internal energy of the radio-atoms themselves. It does not contra- 

 dict the principle of the conservation of energy, for the internal energy 

 of the products of the changes, when the process of change has come 

 to an end, is supposed to be diminished by the amount of energy emitted 

 during the changes. This theory supposes that there is a great store 

 of internal energy in the radio-atoms themselves. This is not in dis- 

 agreement with the modern views of the electronic constitution of 

 matter, which have been so ably developed by J. J. Thomson, Larmor 

 and Lorentz. A simple calculation shows that the mere concentra- 

 tion of the electric charges, which on the electronic theory are sup- 

 posed to be contained in an atom, implies a store of energy in the 

 atom so enormous that, in comparison, the large evolution of energy 

 .from the radio-element is quite insignificant. 



