Mr Crowther, On the Transmission of ^-rays. 449 



the table gives the percentage of the incident rays absorbed in the 

 aluminium sheet used ; and it will be seen that while 71 7o of the 

 softer rays are absorbed in the aluminium screen used, the change 

 of velocity is only of the order of about 2°/^. Allowing for the 

 change in mass of the /S-corpuscles with change in velocity, which 

 is fairly rapid at these high velocities, this corresponds to a loss 

 of energy by the /3-rays of about 10°/^. Only a small portion 

 of the whole absorption of the rays can therefore be due to 

 this gradual decrease in velocity. For the main causes of absorp- 

 tion we must seek elsewhere. However, as we shall see later, the 

 gradual change in velocity of the rays may have a very appreciable 

 effect in modifying the shape of the absorption curves. 



Glass screens gave very similar results to aluminium. Platinum 

 on the other hand behaved quite differently. The curves obtained 

 between the intensity of ionization and the strength of the second 

 magnetic field were low, showed no decided maxima, and were 

 very much broadened out in both directions, but particularly in 

 the direction of the softer rays. On account of the very small 

 amount of radiation transmitted for any given field it was not 

 possible to determine the exact shape of the curves at all accu- 

 rately. Some of the broadening may have been due to the rays 

 on the whole emerging from the platinum more obliquely than 

 from aluminium or glass, but it seems too great to be altogether 

 explained on this assumption, and more probable that a homo- 

 geneous pencil of /S-rays after passing through a platinum screen 

 emerges with a very considerable range of velocities. This effect 

 must be connected with some secondary radiation excited in the 

 platinum. We shall return to this point later. 



§ 3. On the absorption of homogeneous l3-rays. 



The absorption of the yS-rays from a single radio-active sub- 

 stance, such as uranium X, or one of the radium products, may be 

 measured by interposing screens of absorbing material of different 

 thicknesses between the radiating layer and an ionization chamber, 

 and measuring the change in ionization produced. If the absorb- 

 ing material is of low atomic weight the absorption curve can be 

 represented accurately by the equation 



where / is the intensity of the radiation after passing through a 

 thickness d of absorbing material, and A, is a constant. If the 

 absorbing substance is of high atomic weight, there is a short 

 initial steeper portion before the curve assumes the true expo- 

 nential form. 



This law has been tested by many observers for practically all 



