The Absorption of « Rays. 719 
e is the velocity of the earth in its orbit, R the radius of 
earth’s orbit, and 6 the sun’s declination. 
If A=10 x 10 kilometres, 
vf V= 10-4, R= x 10”, A=) x 10~%, 
sin @=0°7, cos 6=1, then A=0:37. 
To obtain this displacement would require a_ circuit 
40 kilometres in length. 
LXXIL. On the Absorption of a Rays, and on the Classijication 
of the a Rays from Radium. By Professor W. H. Braae, 
M.A., University of Adelaide *. 
[Plate XVIII] 
OTH the @ and the 8 rays of radioactive substances are 
absorbed by their passage through matter; and in each 
case the rate of absorption depends almost entirely upon the 
density of the matter and not upon its nature. Apart from 
this striking similarity, there are important differences 
between the phenomena of the absorption of the two classes 
of rays. In a paper read at the Dunedin meeting of the 
Australasian Association for the Advancement of Science, 
in January 1904, | endeavoured to make the contrast clear, 
and to show that a sufficient explanation could be based on 
the hypothesis that the 8 rays are liable to deflexion through 
collision, whereas the a rays are not. Both kinds of rays 
suffer a continual diminution in speed through the expenditure 
of energy on ionization ; but in the case of the @ rays this is 
the only cause to which their so-called “ absorption ”’ is due. 
This hypothesis has many theoretical considerations in its 
favour, and I showed in the paper referred to that it provides 
a good explanation of many known facts. I have recently 
made a direct attempt to test its truth ; and the results of the 
experiments are satisfactory. Moreover they show that it is 
possible to divide the @ rays into classes: certainly two, and 
probably four. All the rays of the same class have the same 
initial velocity. Before setting out the experimental results, 
it will be well to recapitulate some theoretical considerations. 
The ray is an electron of high speed ; and, as experiment 
shows, it may pierce millions of atoms without suffering 
appreciable deviation. It is well known that an electron 
flying past a similar electron which is stationary undergoes 
a detlexion which depends in part upon the relative velocity. 
* Communicated by the Author. 
3D 2 
