172 PHYSICS OF THE ELECTRON 



The disintegration theory assumes that a small fraction of the 

 atoms break up in unit time, but no definite explanation is, as yet, 

 forthcoming of the causes which lead to this explosive disruption 

 of the atom. The experimental results are equally in agreement with 

 the view that each atom contains within itself the potentiality of 

 its final disruption, or with the view that the disintegration is pre- 

 cipitated by the action of some external cause that may lead to the 

 disintegration of the atom in the same way that a detonator is neces- 

 sary to start certain explosions. The energy set free is, however, not 

 derived from the detonator, but from the substance on which it acts. 

 There is another general view which .may possibly lead to an explana- 

 tion of atomic disruption. If the atom is supposed to consist of elec- 

 trons or charged bodies in rapid motion, it tends to radiate energy 

 in the form of electromagnetic waves. If an atom is to be permanently 

 stable, the parts of the atom must be so arranged that there is no 

 loss of energy by electromagnetic radiation. J. J. Thomson has in- 

 vestigated certain possible arrangements of electrons in an atom 

 which radiate energy extremely slowly, but which ultimately must 

 break up in consequence of the loss of internal energy. According to 

 present views, it is not such a matter of surprise that atoms do break 

 up as that atoms are so stable as they appear to be. This question 

 of the causes of disintegration is fundamental, and no adequate 

 explanation has yet been put forward. 



VII. Radioactive Products 



Rutherford and Soddy showed that the radioactivity was always 

 accompanied by the appearance of new types of active matter 

 which possessed physical and chemical properties distinct from the 

 parent radio-element. The radioactivity of these products is not 

 permanent, but decays according to an exponential law with the 

 time. The activity I t and at any time t is given by I t =/ e~ A * where 

 I is the initial activity and ^ a constant. Each radioactive pro- 

 duct has a definite change-constant which distinguishes it from all 

 other products. These products do not arise simultaneously, but in 

 consequence of a succession of changes in the radio-elements; for 

 example, thorium in breaking up gives rise to Thorium X, which 

 behaves as a solid substance soluble in ammonia. This in turn breaks 

 up and gives rise to a gaseous product, the thorium emanation. The 

 emanation is again unstable and gives rise to another type of matter 

 which behaves as a solid and is deposited on the surface of the vessel 

 containing the emanation. It was found that the results would be 

 quantitatively explained on the assumption that the activity of any 

 product at any time is the measure of the rate of production of the 

 next product. This is to be expected, since the activity of any sub- 



