The Penetrating Radiwm Rays. 67 
water, so that to obtain comparable cases the velocity through 
air would need to be 10 times greater than through water, 
a remaining unchanged. This condition being satisfied, the 
frictions per unit area would be as pv’, or since water is 
about 1000 times as dense as air, the actual friction would 
be about 10 times greater for the water than for the air. 
According to Dr. Zahm’s experiments, where the velocity 
alone is varied, the form of f is within certain limits deter- 
mined to be | 
ea = 8: 
Within the same limits (1) gives as the complete expression 
for F, 
ea ya yer OO ei) 
May, 1904. 
VIL. The Penetrating Radium Rays. By J. A. McCLeLiann, 
M.A., Professor of Experimental Physics, University 
College, Dublin *. | 
HRE has been some difficulty in deciding as to the 
nature of the very penetrating or y rays given by 
radium; whether they are electromagnetic pulses like Rontgen 
rays, ora flight of charged particles like the 2 and Brays. In 
some respects the y rays act like the a and @ rays, which we 
know to consist of charged particles; while, on the other 
hand, no deflexion of the rays in a magnetic field has been 
detected ; and, further, we should expect to get Réntgen rays 
from a body like radium which is emitting charged particles 
travelling with a great velocity. 
The chief reason for considering the y rays to consist of 
charged particles, lies in the fact that the absorption of these 
rays by different substances is approximately proportional to 
the density of the substance; a law obeyed by a and @ rays 
and by cathode rays, but not obeyed by Roéntgen rays. It 
should be stated here that since the experiments described in 
this paper were commenced, Mr. Eve + has found that when 
only very penetrating Réntgen rays are used, the absorption 
produced by different substances becomes more nearly pro- 
portional to the density of the substance, a result which tends 
to remove the chief difficulty in deciding that the y rays are 
of the same nature as Rontgen rays. | 
The method I have used to decide whether or not the y 
* Communicated by the Author. Read before the Royal Dublin 
Society, 19th April, 1904. 
+ Nature, 10th March, 1904. 
i) 
