114 THE ROYAL SOCIETY OF CANADA 



100 /,. 100 7t, 

 per cent, transmitted should be constant. That is — — 



■* r -'s 



etc., is a series of equal quantities — each equal to 100 e""^"^ —d. 



These last two deductions provide very simple tests for expon- 

 ential absorption, and judging by these, a glance at the absorption 

 curves given in Figs. 4 and 7 and at the percentages shown in Table I 

 makes it evident that the |S-rays of Ra.E. do not comply with these 

 requirements. 



The explanation has been slowly forthcoming. The loss of 

 intensity in passing through matter is brought about in two ways, i.e., 

 (1) By the particles being slowed dowm or stopped, the energy going 

 probably into ionization and possibly a small part into the production 

 of X-rays. 



(2) By the particles being scattered by collision with the atoms 

 of the absorbing material. It is obvious that the scattering loss will 

 be greater the less the velocity of the particles. 



In 1900 Becquerel showed photographically by magnetic deflection 

 ùhat the /3-rays from radium are not homogeneous. W. Wilson 

 (Proc. R.S., 1909) used this method to isolate an approximately 

 homogeneous beam of ^S-rays of velocity v, by magnetic deflection in a 

 circle of radius R, under a field of strength H, the velocity being given 



m V 



by the well-known relation =H.R. Kis results showed that the 



■' e 



absorption was not exponential but that the rays became more and 

 more absorbable as the thickness of absorbing material was increased 

 He further showed that the absorption increased rapidly as the velocity 

 diminished and in no case could be called exponential. 



One of the best series of experiments on homogeneous /S-rays is 

 that of Crowther (Proc. R.S., 1910). One of his curves showing absorp- 

 tion in aluminium is reproduced in Fig. 2, which indicates that for very 

 thin layers there is practically no absorption (similar to the results 

 obtained with a-particles). The increasingly large scattering effect, 

 however, soon alters the slope of the curve and the relative absorp- 

 tion is seen to increase as the thickness of aluminium is increased 



By putting a thin plate of platinum (.001 cm. thick) over the 

 active material and then absorbing in aluminium Crowther found 

 that the curve obtained was very nearly exponential, showing that the 

 character of the rays had been altered by passage through a substance 

 of such high scattering power as platinum. 



Later experiments of Wilson and von Baeyer showed definitely 

 that /S-rays lose velocity in passing through matter and consequently 



