438 Prof. W. H. Bragg on the Properties and 



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. 



If a /3 particle or cathode particle were liable to complete 

 absorption by an atom which it entered, such an exponential 

 law would result at once. Asa matter of fact, it looks as 

 if several violent deflexions might take place before the 

 final disappearance of the particle's activity. It looks, 

 also, I think, as if deflexions 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 particle 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 abso- 

 lute, 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 Rutherford's determination of the absorption of 

 the j3 rays of uranium, and Grodlewski's similar determination 

 for actinium (Jahrbuch der Bad. und Elek. Bd. iii. Heft 2, 

 p. 159) . In experiments of this kind the radiating material 

 is spread evenly on a level surface, and sheets of absorbing 

 material are placed upon it. The ionization produced in the 

 space above the sheets is compared with the thickness of the 

 sheets ; and the two variables are found to be connected 

 together more or less exactly by an exponential law. There 

 is some difficulty whether such measurements give more 

 nearly the number or the energy of the stream of particles 

 which emerges from the plate, as Rutherford ('Radioactivity,' 

 2nd ed. p, 134) and Thomson (' Conduction through Grases,' 

 2nd ed. p. 375) have pointed out. The point was also dis- 

 cussed in my address to Section A of the Austr. Assoc, for 

 the Adv. of Science, Dunedin, 1904, p. 69. There is also 

 an uncertainty due to the application of a formula to radia- 

 tion from an assemblage of points which is really only appli- 

 cable to a plane wave, or a stream moving normally to the 

 plate. If a point source of radiation is placed below an 

 absorbing plate of thickness d. and there is a true coefficient 



