[Eve] ABSORPTION BY AIR OF THE BETA RAYS 65 
Discussion. 
So far as I am aware, no previous measurements have been made 
of the value of the coefficient of absorption of rays in air. 
Rutherford* has given an estimate for the value for the # rays from 
uranium, namely -0066. This is based on the value of #/density, for 
various metal sheets, which is about 5. It is not surprising that my 
value for air is less than this, since “/D decreases with D; thus, for lead 
10, silver 7, aluminium 5. My value for #/D for air is between 2-6 
and 3-4, according to the experimental conditions. 
It is remarkable, and contrary to expectation, that the above 
described experiments give # = -0033 for short ranges and 4 — -0048 
for long ranges. With metal screens the reverse order is usually ob- 
served, the less penetrating rays are first absorbed, and then / decreases 
for the more penetrating rays. Moreover, in the present case it is 
possible that some of the # rays of radium D at the shorter distance 
may reach the electroscope. Some large values of*/# found with the 
capsule, not recorded here, may find their true explanation in this fact. 
Two different causes may be suggested to account for the paradox. 
Either the # rays ionize more efficiently towards the end of their 
path when their velocities have been somewhat reduced, or at the longer 
ranges the scattering of the particles by air has caused the distances, 
which were measured on the straight, to be largely in defect of the actual 
total distances traversed by the electrons in their zig-zag flights. 
The first explanation suggests a relative increase of I at the longer 
ranges, due to increased ionization of lower speed 8 particles. The 
second explanation suggests that any point on the curve should have 
its abscissa r multiplied by some function of r, f (r), which increases 
with r; whilst the corresponding ordinate should have added to it 
2 log, f (r). 
Without entering further into these obscure points, it may be stated 
that the curves and values obtained represent the facts and results 
usually most required, and that the # rays are absorbed by a law almost 
exponential, but with / increasing, probably gradually, from -0033 
em. | to -0045 cm.—!, as r increases from 60 to 600 em. 
If a mean -004 is selected, it is possible to estimate the value of 
the coefficient of absorption #’ by air of the y rays from radium C 
Thus for aluminium, with 8 rays from radium C, Kovarik*! found 
p—13 cm.~!, and with y rays McClelland finds “’ = -103. If it is 
* “Radioactivity,” p 197. 
*Phil. Mag. Nov. 1910. 
