180 Mr. N. Campbell on the 



by radium C are in fair agreement. It would therefore seem 

 that for every a particle emitted by radium C, one, or possibly 

 two, ft particles are simultaneously emitted. It should, how- 

 ever, be noticed that this conclusion has been reached on the 

 assumption that radium B and radium C, when in radioactive 

 equilibrium, emit the same number of ft particles. 



Conclusions. 



( 1 ) The ft rays are stopped and not merely scattered when 

 they pass through matter. 



(2) The number of ft particles emitted per second by the 

 radium C in radioactive equilibrium with one gram of radium 

 is5'0xl0 10 . 



I am indebted to Professor Rutherford for the interest 

 which he has taken in this investigation and for many 

 valuable suggestions. My thanks are also due to Dr. Beattie 

 for comparing the condensers used in the above experiments 

 with his standards. 



X. The Absorption of ft Rays by Liquids. By Norman 

 Campbell, M.A., Felloiv of Trinity College, Cambridge *. 



1. TF I is the intensity of a pencil of ft rays, I the 



JL intensity of the same pencil when it has passed 



through a uniform layer of absorbing material of thickness <i% 



then the coefficient \ of absorption of the material for the 



rays is defined by the equation : 



If p is the density of the absorbing material, Crowther 

 (Phil. Mag. Oct. 1906, p. 388) has shown that the value 

 of \/p for a compound containing the proportions a;, y, z, . . . 

 of the elements X, Y, Z, can be calculated from the known 

 values of the same quantity for the elements by means of the 

 formula : 



Crowther's work extended only to solid compounds. Some 

 experiments on the ft rays from potassium described in a 

 recent paper (Proc. Camb. Phil. Soc. xiv. 6, 557) indicated 

 that the law was not true for such rays absorbed in solutions 

 of potassium salts : it appeared that the value of \/p for the 

 solution might be greater than either of ihose for the solute 



* Communicated by the Author. 



