298 
ON THE ABSORPTION OF a RAYS, AND ON THE CLASSI- 
FICATION OF THE a RAYS OF RADIUM. 
Ву ХУ. Н. BraGG, M.A., Elder Professor of Mathematics 
and Physics in the University of Adelaide. 
[Read September 6, 1904.| 
ABSTRACT, 
In a paper read at the Dunedin meeting of the Austral- 
asian Association for the Advancement of Science, in Janu- 
ary, 1904, the author had compared the phenomena attend- 
ing the absorption of а and ß rays, and had shown that the 
following hypotheses were probably true: — 
(1) The Bray is "absorbed" as it penetrates matter from 
two causes—(«*) deflection due to close encounter 
with electrons of the atoms traversed; (b) expendi- 
ture of energy on ionisation. 
(2) The « ray is "absorbed" from the latter cause alone. 
It followed that the « particle moved through mat- 
ter without deviation until its energy was expended ; 
that no dispersion attended its flight; that it had 
a definite range, depending on its initial velocity ; 
and that, like any atom moving with more than 
ionising speed, it must become  positive—even 
though neutral at first—from collision with ions 
traversed and consequent loss of one electron. 
The author now described a number of experiments in sup- 
port of these hypotheses. The principle of the method em- 
ployed consisted in tbe isolation of a narrow cone or cones of 
a rays by suitable stops, and the examiration of the ionisa- 
tion produced by them at different distances from their 
source. This was accomplished by the use of a shallow ionisa- 
tion chamber. The experiments might be divided into two 
classes, those with thick layers of radium and those with thin 
layers. In the former case he showed that a curve in which 
the ordinates represented distances from radium to ionisa- 
tion chamber, and the abscisse currents as measured in a 
quadrant electrometer—and which he termed the ionisation 
curve— should be a rectilinear polygon, on the following assump- 
tions :— 
(a) Al « particles produced ions in proportion merely 
to distance traversed. 
(b) « particles lost energy in proportion to matter tra- 
versed, so that such as came from lower layers of 
radium would lose speed before emerging. 
