316 SCIENCE PROGRESS 



of an atom is determined by its internal conditions. We have 

 reason for believing from the phenomena of spontaneous radio- 

 activity that atoms possess large stores of internal energy, in 

 amounts which are not readily susceptible to extra-atomic con- 

 ditions (such as temperature), but which probably vary a good 

 deal from atom to atom. If the possession of an abnormal amount 

 of internal energy meant diminished stability then it might easily 

 happen that only abnormal atoms would be ionised : as these 

 form only a very small fraction of the whole, the fewness of the 

 ionised atoms would be accounted for. Thus far, well and good. 

 But, if we now take up the consideration of the experimental 

 evidence which secondary radiation provides, we find it useful, 

 if not necessary, to be able to invoke still further the aid of this 

 atomic energy. 



When ultra-violet light Rontgen and 7 rays fall on metal 

 surfaces they cause the emission of corpuscles. The effects 

 strongly resemble each other, as will appear; for comparison's 

 sake the results are summarised. 



(1) With ultra-violet light the emitted corpuscles from metals 

 have a mean velocity of about 5 x io 7 cms. per second. The 

 intensity of the light controls the number of the particles, but not 

 their velocity, which is, however, according to Ladenburg (1907), 

 directly proportional to the frequency of the ultra-violet light. 



(2) Rontgen rays cause electrons to be emitted from metals 

 with velocities which average about 7 x io 9 cms. per second. 

 The intensity of the rays affects the number of the corpuscles ; 

 their velocity is independent of the intensity of the rays, but 

 increases somewhat with their hardness, and possibly a little 

 with the atomic weight of the radiator. The corpuscles pre- 

 ponderate in the direction of propagation of the Rontgen rays. 

 It should be noticed, in passing, that the speed is much the same 

 as those of the cathode rays in an X-ray tube. 



(3) In the case of the 7 rays (from radium) the expelled 

 ft particles from metals have velocities approaching those of 

 the primary ft rays from radium, which have an average speed 

 of about 2'6 x io 10 cms. per second. The secondary ft particles 

 have a maximum intensity in the direction in which the primary 

 7 rays are moving. Their speed is independent of the nature of 

 the metal. 



The Explosion Theory. — Of these results, that which remarks 

 on the independence of the velocity of the corpuscles on the 



