150 Messrs. A. S. Russell and F. Soddy on the 



The results are collected in the following table : — 



1. 



Thorium D. 



Radium C. 



Mesothorium 2. 



Uranium X. 



Lead 1 092 



1-00 



1-24 



1-45 





2. 



Class II. 1 0-82 



1-00 



1-06 



118 



3. 



Lead 2 A \ 079 



1-00 



1-24 



1-27 



4. 



5. 



Lead 2 B 82 



1-00 



1-27 



— 



Class II. 2 A... 1 0-88 



1-00 



1-03 



[1-25]* 



6. 



7. 



Class II. 2 B... 0-92 



100 



1-045 



— 



Copper 3 0-80 



100 



L035 



1-26 



* Not exactly the same disposition. 



Thus the value of X for thorium D is from 8 to 21 per 

 cent, less, and for mesothorium 2 from 4 to 25 per cent, 

 greater than for radium O. Disposition 1 might well be 

 adopted as a standard disposition for 7-ray comparison of 

 the intensity of radioactive preparations. Preparations of 

 widely differing activity might be compared by means 

 of carefully prepared lead blocks of known thickness, 

 adopting, for the value of X, 0'500 (cm.) -1 . 



The y-rays of Actinium. — Through the kindness of Messrs. 

 Buchler and Co., of Braunschweig, who most generously 

 lent for the purpose a very fine preparation of actinium, 

 weighing 1*5 grams and equivalent in 7-activity through 

 3 mm. of lead to 023 mg. of radium bromide, it has been 

 possible to make a more extended examination of the 7-ray s 

 of this substance than has hitherto been done. The prepa- 

 ration was in a sealed tube for these measurements, and 

 subsequently the tube was opened and the preparation used 

 to obtain the active deposit for the determination of the y//3 

 ratio described in Section I. Godlewski (Phil. Mag. [6] x. 

 p. 378, 1905) measured the absorption of the actinium 7-rays 

 over the range from to 3'5 mm. of lead and to 10 mm. 

 of iron and of zinc, and obtained nearly exponential curves, 

 the value of X(cm.) _1 for lead being 4*54, for iron 1*23, and 

 for zinc 1*24. He thus showed clearly the markedly less 

 penetrating power of these rays as compared with those o£ 

 radium and the abnormally high absorption in lead. Eve 

 (Phys. Zeit. viii. p. 185, 1907) with a stronger preparation 

 found for X 4*1 over a similar range of lead. At 3 mm. 

 thickness a sudden change to X = 2'7 took place and continued 



