87 



But if we turn from the theoretical to the experimental 

 investigation we find a much more encouraging prospect. 

 The experiments of Lenard are practically a complete graphi- 

 cal solution of the question. (See Taf. iv., Wied. Ann., Bd. 

 51). We know that an assemblage of atoms behaves just the 

 same in respect to these radiations, when it is condensed in 

 a solid or spread out as a gas. Thus the sketches which 

 Lenard gives us showing the way in which the cathode rays 

 diverge from a small window and scatter in going through 

 various gases at different densities must be quite applicable to 

 solids also. 



Lenard found that his results could be accounted for on 

 the supposition that there was an absorption according to 

 an exponential law, over and above the weakening due to 

 spreading from a centre. 



li a f3 particle or cathode j^article were liable to com- 

 plete absorption by an atom which it entered, such an ex- 

 ponential law would result at once. As a matter of fact, it 

 looks as if several violent deflections might take place 

 before the final disappearance of the particle's activity. It 

 looks, also, I think, as if deflections were usually not at all 

 great during the progress of the particle through the atom, 

 but were apt to be severe when they did happen, as if, in 

 fact, the field of force which deflected the jDarticle was strong 

 but circumscribed. This would happen if the positives and 

 negatives were arranged in doublets. When a particle is 

 deflected from a beam crossing a thin plate, it starts off on a 

 new path which leads much less directly to the open air, and 

 its velocity is somewhat diminished. It may be, therefore, 

 that the infrequency but severity of the particle's encounters 

 makes it possible to look upon each encounter as an absolute, 

 or at least a definite, loss to the stream, so that an exponential 

 law results. 



Certainly the application of this law to the interpretation 

 of experiments has had very great success, both in respect 

 to cathode and to /3 and y rays. As examples of the latter 

 we may take Riitherford's determination of the absorption of 

 the /3 rays of uranium, and Godlewski's similar determination 



cation to have any meaning, the proper factor should have been 

 greater than that adopted in the proportion of T' to- u, for in 

 advancing a distance ^x along the axis of .t the corpuscle moves 

 a distance \5xju, not 5 x. If this change is made, the expo- 

 nential form disappears from the answer. But, apart from this, 

 it does not seem that the step is justifiable at all. It is tanta- 

 mount to putting the corpuscle liack in its old track after each 

 encounter, and is equivalent to neglecting the existence of the 

 function mentioned above, and the absolute necessity of finding it. 



