146 Prof. 0. W. Richardson on the 



cycle of, in general, similar events. The condition for disrup- 

 tion is that the energy of the electron should be an integral 

 multiple of hv, where v is the frequency of the radiation and 

 A = 6"55xl0~ 27 erg sec. In a large number of cases it is 

 probable that integral multiples other than unity may be 

 left out of consideration without serious error. According 

 to a theory of black body radiation recently developed by 

 Planck *, this is always the case when the intensity of the 

 primary radiation is sufficiently small. I shall suppose 

 the act of disruption to result in the liberation of an electron 

 to such an extent that it no longer forms part of the 

 dynamical system to which it originally belonged. In 

 favourable cases the electron may be expelled from the 

 matter and appear as an emitted electron, or secondary 

 radiation of the j3 or electronic type. This will invariably 

 be the case if v is sufficiently large and the layer of matter 

 which absorbs the radiation is sufficiently thin. I shall 

 assume that up to the instant of disruption no part, either of 

 the momentum or of the energy which any particular 

 electron receives from the radiation, is communicated to the 

 rest of the matter. The acquisition of energy and momentum 

 by the matter as a 'whole then takes place through the acts 

 of the disrupted electrons. This assumption may not 

 invariably be fulfilled, but it will probably lead to a correct 

 estimate of the maximum amount of asymmetry to be found 

 in the most favourable cases. 



Consider the case of X radiation or light, incident 

 normally on a thin slab of absorbing material. In general, 

 absorption may occur through the operation of processes of 

 very different nature, for example, conduction as opposed to 

 resonance effects ; but we shall suppose that the only type 

 of absorption which we need to consider is that which 

 results finally in electronic disruption. We shall fix our 

 attention on the state of things which exists after the slab 

 has been illuminated for some time ; so that there is no 

 further accumulation, in the slab, of energy abstracted from 

 the incident beam. Under these circumstances the energy 

 absorbed from the incident radiation will appear, at any 

 rate in the first instance, as the kinetic energy hv which the 

 disrupted electrons possess at the instant of disruption. 

 Thus if there are N of them disrupted in unit time, the 

 energy absorbed from the radiation is NAv. We are here 

 supposing that some suitable means of return is provided for 

 the electrons, so that the number present in the slab at any 

 instant preserves the same average value. It is clear also 

 * Ann. der Phys. vol. xxxvii. p. 642 (1912). 



