Mr Crowther, On the Transmission of /3-rays. 457 



present paper on the absorption by platinum, and the results of 

 Sadler already alluded to, on the absorption of secondary cor- 

 puscular radiation, show that if this is the case a similar distri- 

 bution of velocities occurs also when secondary /8-rays are emitted 

 under the influence either of homogeneous ^-rays or of homoge- 

 neous Rontgen rays ; while the experiments of Schmidt, described 

 in the earlier part of this paper, show that this distribution is 

 not disturbed during the passage of the rays through very con- 

 siderable thicknesses of absorbing material. 



A law so fundamental, applying equally to the emission of 

 /3-rays under radio-active disintegrations, under the action of 

 homogeneous /^-radiation, and under the action of homogeneous 

 Eontgen rays, should be capable of some simple explanation in 

 terms of the properties of the /3-rays and of the atom. Until 

 further experiments have been performed however it seems pre- 

 mature to frame any suggestion as to its cause, or even as to 

 its precise nature. A knowledge of the true law of absorption 

 of homogeneous rays would enable us to determine the initial 

 distribution of velocities necessary to produce an exponential law 

 of absorption, and Wilson, assuming that the law of absorption is 

 linear, has calculated one such possible distribution. No theoretical 

 evidence in favour of the distribution which he arrived at has yet 

 been given, and the results of the present experiments seem to 

 show that the "linear law" of absorption is only an approximation 

 even for aluminium, and is probably the result of an interaction 

 of the different causes producing and influencing the absorption 

 rather than a fundamental law. 



One fact alone seems perfectly clear. The absorption of the 

 yS-rays is a far more complicated phenomenon and is influenced 

 by a far greater variety of causes than had previously been 

 'imagined. It is only by disentangling the different phenomena 

 involved and considering each of them separately that we shall 

 arrive at any satisfactory idea as to the nature of the processes 

 involved. 



Summary. 



Experiments have been made on the velocity of a beam of 

 homogeneous yS-rays before and after their passage through an 

 absorbing medium. It was found that aluminium caused a small 

 but perceptible diminution in velocity of the rays, without any 

 appreciable disturbance of the homogeneity of the beam. On the 

 other hand a homogeneous beam of /3-rays after passing through 

 platinum emerged with a fairly wide range of velocities. 



