On the Absorption of j Rays. 601 



applicable without further discussion, and here 1 per cent, 

 difference in the atomic weights should produce a difference 

 of J per cent, in the diffusion constant. 



Summary". 



(1) The valency of fifteen radio-elements has been deter 

 mined by measuring their diffusion constants in acid solution. 

 The valency thus measured directly agrees without exception 

 with what would be expected from the character of the 

 respective radio-elements determined by their non-separa- 

 bility from an element of known properties. 



(2) Actinium sends triple charged ions into solution. 

 This fact supports the view that actinium is the subsequent 

 homologue of lanthanum. 



(3) In alkaline solutions the short and long lived products 

 of the emanation are present in a colloidal state, in agreement 

 with the views of Paneth and Godlewski. Polonium, in very 

 weakly acid and in neutral solution, is also present in a 

 colloidal condition. 



(4) The particles of the radio-colloids contain, in some 

 cases, only two or three atoms. 



(5) The possibility of separating chemically identical 

 elements has been discussed. 



My best thanks are due to Professor Sir Ernest Rutherford 

 for the continued kind interest which he has taken in this 

 research. 



Manchester, August 1913. 



LXYII. The Absorption of <y Rays. By S. Oba, 

 Hon. Research Fellow, University of Manchester *. 



IT is well known that with the usual experimental arrange- 

 ments the absorption coefficient of the <y rays from 

 radium decreases with increased thickness of absorber. This 

 result has generally been ascribed to the heterogeneity of 

 the primary rays. The softer constituents are supposed to 

 suffer a greater amount of absorption and scattering, so that 

 the beam as a whole becomes harder. It has also been 

 observed that the 7 rays scattered by matter are softer than 

 the primary rays, and this result was explained by Florance f 

 on the supposition that the softer constituents of the original 

 7 rays are more easily scattered. Gray J, however, has 



* Communicated by Sir Ernest Rutherford, F.R.S. 

 t Florance, Phil. Mag. xx. p. 921 (1910). 

 t Gray, Phil. Mag. xxvi. p. 611 (1913). 



