Secrion III, 1911. [59] À Trans, R.S. C. 
On the Coefficient of Absorption by Air of the Beta Rays from Radium C. 
By A. S. Eve, M.A., D.Se., McGill University, Montreal. 
The object of this paper is to describe a new method of finding the 
value of #4, the coefficient of absorption of the # rays from radium C in 
their passage through air at atmospheric pressure and at room tempera- 
ture; and to give the value of # 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 ff rays, 
yrays, and natural leak. The B rays were then cut off by screens, or 
by a strong magnetic field, or, better still, by both methods, and the 
ionization due to y rays and natural leak measured. As the natural 
leak is known, by subtraction measurements are obtained of the ioniza- 
tion due to the rays alone, I, and to the 7 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 f 
rays of radium C which, as these experiments will show, obey closely 
the exponential law. Thus we shall see that the experiments justify 
the assumption that the ionization in the electroscope I varies inversely 
as re or Ir?=Ae—/, 
where A is a constant, and 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, h’—B—pr 
and when this is plotted with log,Ir’ as ordinate, and r as abscissa, a 
straight line is determined, whose slope is /. 
It will be observed that the multiplication by r° is equivalent to a 
handicap for loss of distance. 
A similar process for the ionization due to the y rays gives ['r? a 
constant, because of the minute absorption which the y rays undergo, 
over a range of a few metres in air. 
Sec. III., 1911. 5. 
