PRESENT PROBLEMS OF RADIOACTIVITY 171 



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

 matter every second becomes unstable and breaks up with explosive 

 violence. In most cases, the explosion is accompanied by the ex- 

 pulsion of an a particle, in a few cases, by only a /? particle, and in 

 others by a and /? particles together. On this view, there is at any 

 time present in a radioactive body a proportion of the original 

 matter which is unchanged 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 radium remains unchanged with its age. 



The expulsion of an a or /? 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, obtained at the expense of 

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

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

 energy of the products of the changes, when the process 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 disagreement with the modern views of the electronic con- 

 stitution of matter, which have been so ably developed by J. J. 

 Thomson, Larmor, and Lorentz. A simple calculation shows that the 

 mere concentration of the electric charges, which on the electronic 

 theory are supposed 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-elements is quite insignificant. 



Since the energy emitted from the radio-elements is for the most 

 part kinetic in form, it is necessary to suppose that the a and /? parti- 

 cles were originally in rapid motion in the atoms from which they are 

 projected. The disintegration theory supposes that it is the atoms 

 and not the molecules which break up. Such a view is necessary to 

 explain the independence of the rate of disintegration of radioactive 

 matter, of wide variations of temperature, and of the action of chemi- 

 cal and physical agents at our command. This must be conceded if the 

 term atom is used in the ordinary chemical sense. It is, however, 

 probable that the atoms of the radio-elements are in reality complex 

 aggregates of known or unknown kinds of matter, which break up 

 spontaneously. This aggregate behaves like an atom and cannot be 

 resolved into simpler forms by external chemical or physical agencies. 

 It breaks up, however, spontaneously with an evolution of energy 

 enormous compared with that released in ordinary chemical changes. 

 This question is further considered in section viu of this paper. 



