y-Rays of Thorium and Actinium. 141 



and 0'5 cm. of lead over the preparation, the absorbing plates 

 of lead may be placed anywhere between the preparation and 

 the base, the value of X is constant at 0'50 (cm.) -1 . These 

 two facts are extraordinarily difficult to reconcile. The first 

 indicates that each successive thickness of lead traversed 

 modifies the nature of the radiation remaining, continuously 

 without limit, while the second indicates that the effect of 

 each successive thickness is to absorb a definite fraction of 

 the radiation without modification of the nature of the 

 remainder. 



This difficulty suggested the possibility that a heterogeneous 

 beam, consisting of two types of about equal energy but 

 different penetrating power, might be transformed by a layer 

 of the absorbent so as to act subsequently as a homogeneous 

 beam. A heterogeneous beam of 7-rays was made by placing 

 two point sources, one of radium and the other of meso- 

 thorium, side by side. The activity of the mesothorium was 

 40 per cent, of the total, measured through 1 cm. of lead. 

 Lead laid over the preparations was used as absorbent. 

 X for lead for radium for this disposition is O50, and for 

 mesothorium 0*62. The differences are thus of the same 

 order as that produced in the value of X (zinc) by hardening 

 the y-rays of radium with lead. The absorption curve for 

 the heterogeneous beam, over the 5 cm. of lead investigated, 

 was not exponential and by no allowance for experimental 

 errors could it be made so, the curve being plainly convex to 

 the origin *. The theoretical values of the ionization of the 

 beam for the different thicknesses of lead were calculated 

 from the data known, and they agreed perfectly with the 

 observed values. The only way of escape from the difficulty 

 seems to be to deny that an exponential absorption curve 

 necessarily means the stopping by the metal of a definite 

 proportion of the incident radiation without modification of 

 the fraction emerging. A similar state of things exists with 

 regard to the /3-rays. It has been fairly conclusively shown 

 that an exponential absorption occurs with a heterogeneous 

 beam, and that the part unabsorbed suffers reduction in 

 velocity and penetrating power. But a true exponential 

 curve for the /3-rays has never been obtained by anyone over 

 any considerable range, whereas with they-rays under proper 

 conditions the absorption curve never departs measurably 

 from the exponential form. 



Initial Part of the Absorption Curves. — The deviations 



* This experiment suggests a possible means of detecting- the adul- 

 teration of radium by mesothorium without opening the tube. 



