On the Photographic Action of a, ß and y Rays emitted from Eadioactive Substances. ]^5 



C. T. R. Wilson in liis well-known condensation experiment may, 

 though not quite , similar, serve as an analogy. While an a 

 ray track would still 1;)e detectable, if the number of water drops 

 per unit length were reduced to one-thousandth, say, in a ß ray 

 track this would l)e far from being the case. It must be remem- 

 Ijered that /9 particles will suffer greater deflexions in gelatine films 

 than in air. A direct evidence for the above conclusion is found 

 in the fact that it is possible to obtain a halo due to the a particles 

 from radium C, outside of which no perceptible track of any sort 

 is present, although the source emits simultaneously ß particles 

 as well and the most of them travel far beyond the boundary of 

 the halo. 



The photographic action of [i particles should l)e explained by 

 an hypothesis of the kind we have made in the case of « particles; 

 for, in the latter the l)asis of the hypothesis is the ionisation of 

 halide molecules, which is the efïect also common to /? particles. 

 We shall consider that a halide grain becomes capal)le of develop- 

 ment when it is encountered by a (i particle, but only when the, 

 encounter takes place under certain favourable conditions. This 

 seems plausible, because a ß particle, while it would encounter on 

 a path per unit length as many halide grains as an « particle does, 

 renders developable silver grains of only a small part compared 

 with those similarl}^ affected by an « particle. Since it is very 

 likely that, whenever a halide grain is encountered by a /? particle, 

 some halide molecules in it, however few in number, are ionised, 

 we may assume that the process of development in the grain cannot 

 start unless the number of initially ionised halide molecules exceeds 

 a certain value. An analogous phenomenon is met with in the case 

 of an electric discharge between two electrodes. The discharge is 

 facilitated and starts at a lower but definite potential when the inter- 

 posed gas is initially ionised beyond a certain degree. The above 

 assumption apparently contradicts what has been said in the case 

 of a particles; but it seems quite possible to an « particle, which in 

 gases shows an ionising action several thousand times stronger than 

 that of a ß particle, to ionise a number of halide molecules over the 

 threshold value, whenever it strikes a halide grain. 



