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II. On the Coefficient of Absorption by Air of the Beta 

 Rays from Radium C. By A. S. Eve, M.A., B.Sc, 

 McGill University, Montreal *. 



npHE object of this paper is to describe a new method of 

 JL finding the value of /jl the coefficient of absorption 

 of the /3 rays from radium C in their passage through air 

 at atmospheric pressure and at room temperature ; and to give 

 the value of /jl obtained in a series of experiments. 



The general method employed is simple. A very thin- 

 walled electroscope was suspended by fine wires at a 

 considerable distance from surrounding objects. A test- 

 tube containing radium bromide was also suspended at 

 various distances from the electroscope. The ionization 

 current in the electroscope, measured in divisions per minute 

 of the observing microscope, was due to the joint effects 

 of the /3 rays, 7 rays, and natural leak. The /3 rays were 

 then cut off by screens, or by a strong magnetic field, 

 or, better still, by both methods, and the ionization due to 

 7 rays and natural leak measured. As the natural leak is 

 known, by subtraction measurements are obtained of the 

 ionization due to the /3 rays alone, I, and to the y rays 

 alone, I'. 



As the distance r between the radium and electroscope is 

 increased, the value of I falls off sharply, not only on 

 account of the law of inverse squares, but also on account of 

 the absorption by air of the complex ft rays of radium C 

 which, as these experiments will show, obey closely the 

 exponential law. Thus w 7 e shall see that the experiments 

 justify the assumption that the ionization in the electroscope 

 I varies inversely as r 2 e? r or 



Ir 2 = Ae-» r , 



where A is a constant, and /jl is the coefficient of absorption 

 by air. 



If the ionization current, multiplied by the square of the 

 distance, is plotted against the distance, an exponential curve 

 is determined. Or, on taking logarithms, 



log e L< 2 = B— /*r, 



and when this is plotted with log e Ir 2 as ordinate, and r as 

 abscissa, a straight line is determined, whose slope is /jl. 



It will be observed that the multiplication by r 2 is 

 equivalent to a handicap for loss of distance. 



* Communicated by the Author. 



