y-Rai/s of Thorium and Actinium, 



137 



of the electroscope will come in. According to the expe- 

 riments of Kovarik and Wilson (Phil. Mag. 1910, xx. 

 p. 849 & p. 866), this reflexion increases with the penetrating 

 power of the rays tip to a maximum, at X (cm.) -1 aluminium 

 = 20, and then again diminishes. This effect for most of the 

 rays therefore would oppose that of scattering. An attempt 

 to evaluate the absorption in the thin aluminium foil covering 

 the preparation led to the unexpected result that the ionization 

 was slightly greater with the foil than without. This was 

 at the time ascribed to a possible generation in the foil of 

 secondary /S-rays, an effect which had frequently been looked 

 for previously but never actually obtained. The same effect 

 is recorded by Kovarik (loc» cit.), who made a closer exami- 

 nation of it than we have done, and ascribed it to an effect of 

 scattering. None of the effects considered affect the order 

 of the <y/j3 ratio to a serious extent, and tbey are relatively 

 unimportant. Probably from the theoretical point of view 

 the original ratios obtained with the preparation at a great 

 distance from the electroscope are to be preferred. For 

 practical purposes the /3 4 position may be selected, as here 

 the distance, 13 cm., is a very usual one. The ratios, relatively 

 to that of radium C, are for these positions : — ■ 





*/&• 



7i//3 4 . 



yJh. 



YMr 



Mesothorium 2... 



1-036 



0-800 







Thorium D 



0-600 



0510 







Actinium C 



0-077 



0065 



0-128 



0105 



The y/{3 ratios are thus of the same order for radium C. 

 mesothorium 2, and thorium D; while for actinium C it is 

 only about one-eighth to one-sixteenth of radium C and 

 therefore more nearly approaches the order previously found 

 for uranium X. 



Experiments ivith Thorium Minerals. — Before any meso- 

 thorium or radiothorium preparations were ready, Mrs. Soddy 

 had carried out a comparison of the y/j3 ratio for Ceylon 

 thorite (containing about 66 per. cent, of thoria) and 

 Joachimsthnl pitchblende (containing about 40 per cent, of 

 uranium oxide). The former mineral contains practically no 

 uranium, and the latter no thorium, so that they represent 

 respectively the thorium and uranium disintegration series in 

 equilibrium. The result may be briefly referred to here. The 

 y//3 ratio of these two minerals was practically the same 



