854 Sir E. Rutherford and Dr. E. N. da C. Andrade on 



higher value) the room (positive or negative) which 1 gram 

 of matter takes up in the aether is certainly not more than 

 1/45 that of the total bulk of electrons, and is probably very 

 much less. 



The cost of the experimental work above referred to was 

 partly defrayed by a Government grant received from the 

 Royal Society. 



Boars Hill, Oxford, 

 Feb. 1914. 



XC1V. The Wave-Length of the Soft 7 Rays from Radium B. 

 By Sir Ernest Rutherford, F.R.S., and E. X. da C. 

 Andrade, B.Sc, Ph.D., John Harling Fellow, University 

 of Manchester * . 



[Plate XII.] 



DURING the last few years, a large amount of attention 

 has been directed to the absorption of the 7 rays 

 emitted by radioactive bodies. At first, the nature of the 

 absorption by matter of the very penetrating 7 rays emitted 

 by the products radium G, mesothorium 2, thorium D, and 

 uranium X, was carefully examined, and it was found that 

 all these types of radiation were absorbed by light elements 

 very nearly according to an exponential law over a large 

 range of thickness, but with different constants of absorption 

 for each radiation. In order to explain the emission of 

 homogeneous groups of j3 rays from a number of products, 

 Rutherford suggested that the 7 rays emitted by the radio- 

 active products must be regarded as the " characteristic " 

 radiations excited in the radioelements by the escape of 

 j3 particles from them. These "characteristic" radiations 

 were supposed to be analogous to one or more of the groups 

 of characteristic radiations observed by Barkla to be excited 

 in different elements by X rays. It was suggested that the 

 emission of homogeneous groups of /3 rays was directly con- 

 nected with the emission of different types of characteristic 

 7 rays from each element, and that the energy of the 

 escaping j3 particle was diminished by multiples of definite 

 units depending on the energy required to set the electronic 

 system of the atom in a definite form of vibration. 



In order to test this point of view, Rutherford and 

 Richardson j" analysed in detail the 7 rays emitted by a 



* Communicated by the Authors. 



t Rutherford and Richardson, Phil. Mag. May 1913, p. 722; August 

 1913, p. 325; Feb. 1914, p. 252. 



