170 PHYSICS OF THE ELECTRON 



that the atoms of the radio-elements contain a great store of latent 

 energy, which only manifests itself when the atom breaks up. There 

 is no direct evidence in support of the view that the energy of the 

 radio-elements is derived from external sources, while there is much 

 indirect evidence against it. Some of this evidence will now be con- 

 sidered. There is now no doubt that the a and /? rays consist of parti- 

 cles projected with great speed. In order for the a particle to acquire 

 the velocity with which it is expelled, it can be calculated that it 

 would be necessary for it to move freely between two points differ- 

 ing in potential by about five million volts. It is very difficult to 

 imagine any mechanism which could suddenly impress such an 

 enormous velocity on one of the parts of an atom. It seems much 

 more reasonable to suppose that the a and /? particles were originally 

 in rapid motion in the atom, and, for some reason, escaped from the 

 atomic system with the velocity they possessed at the instant of their 

 release. There is now undeniable evidence that radioactivity is 

 always accompanied by the production of new kinds of active mat- 

 ter. Some sort of chemical theory is thus required to explain the 

 facts, whether the view is taken that the energy is derived from the 

 atom itself or from external sources. The "external" theory of the 

 origin of the energy was initially advanced to explain only the heat 

 emission of radium. We have seen that this is undoubtedly con- 

 nected with the expulsion of a particles from the different disintegra- 

 tion products of radium, and that the radium itself only supplies one 

 quarter of the total heat emission, the rest being derived from the 

 emanation and its further products. On such a theory it is neces- 

 sary to suppose that in radium there are a number of different active 

 substances, whose power of absorbing external energy dies away 

 with the time, at different but definite rates. This still leaves the 

 fundamental difficulty of the origin of these radioactive 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 cannot be from the air itself, 

 for radium gives out heat inside an ice calometer. It cannot be any 

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

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

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

 dence, 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, 

 advanced by Rutherford and Soddy, not only offers a satisfactory 

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

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



