Some Radioactive Properties of Uranium. 45 



The ft rays of actinium are completely distinct in their 

 character from the ft rays emitted by the other radio-elements, 

 inasmuch as they are completely homogeneous. This fact was 

 established by the measurements of absorption of ft rays in 

 passing through solid bodies. The activity measured by ft rays 

 decreased strictly according to an exponential law with the 

 thickness of matter traversed. The equation I = I <?~ AcZ , where 

 d is the thickness, was applicable even in the case when I 

 was less than 1 per cent, of its original value. 



The ft rays from actinium differ also from the ft rays of 

 other radioactive elements in the absolute value of the ab- 

 sorption constant \, which is about 2*5 times as great with 

 actinium as with uranium. Thus the ft rays of actinium have 

 less than half the penetrating pow r er of those emitted by any 

 other radio- element. 



The existence of the y rays from actinium was also dis- 

 tinctly proved. The absorption measurements showed that 

 the y rays of actinium are fairly homogeneous, and their 

 penetrating power was only about one quarter of that ob- 

 served for the rays from radium. A more complete account 

 of these investigations will be published later on. 



In conclusion it is my most pleasant duty to express my 

 deepest gratitude to Prof. Rutherford for suggesting these 

 investigations, for his kindness in tbe advice he has so freely 

 given to me, and for placing at my disposal all the plentiful 

 resources of his laboratory at McGill University. 



McGill University, Physics Building, 

 February 24, 1905. 



VI. Some Radioactive Properties of Uranium. 

 By T. GODLEWSKI, Ph. I). [Cracow)*. 



1. The Discover (/ of UrX. 



IN 1900 Sir William Grookes f showed that it is possible to 

 separate from uranium by a single chemical operation a 

 small amount of radioactive substance to which he gave the 

 name UrX. This substance was, weight for weight, many 

 hundred times more active photographically than the uranium 

 from which it had been separated. The uranium deprived of 

 this substance was almost inactive. 



Similar results were afterwards observed by Becquerel J, 



* Communicated by Prof. E. Rutherford, F.P.S. Presented before 

 the Academy of Sciences in Cracow at the sitting- of 9th of May, 1905. 

 t Crookes, Proc. Roy. Soc. lxvi. p. 409 (1900). 

 % Becquerel, C. Ji. cxxxi. p. 137 (1900) ; cxxxiii. p. 977 (1901). 



