132 
On the Alpha Particles of Radium, and their Loss 
OF Range in passing through various 
Atoms and molecules. 
By W. H. Bragg, M.A., Elder Professor of Mathematics and 
Physics in the University of Adelaide, and R. Kleeman, 
B.Sc. 
[Read June 6, 1905.] 
Abstract. 
In a previous paper laid before the Royal Society of 
South Australia on September 6, 1904 (see Vol. xxviii., p. 
298; also The Fhilosoyhical Magazine, December, 1904), the 
authors had adduced theoretical and experimental evidence 
in support of the following propositions : — 
1. The alpha particle moves always in a rectilinear course, 
spending its energy as it traverses atoms of matter, 
until its velocity becomes so small that it cannot 
ionise, and there is in consequence no further evi- 
dence of its motion. 
2. Each particle possesses, therefore, a definite range in 
a given medium, the length of which depends on the 
initial velocit}?' of the particle and the nature of the 
medium. 
3. The alpha particles of radium which is in radio-active 
equilibrium can be divided into four groups, each 
group being produced by one of the first four radio- 
active changes in which alpha particles are emitted. 
4. All the particles of any one group have the same 
initial velocity and the same range. 
The present paper could be regarded as a continuation of 
the previous communication. Its contents were arranged 
under the following heads : — 
1. Improvements in the apparatus used for measuring 
the ranges and relative strengths of the four groups of rays. 
2. Results of experiments with the new apparatus, giving 
the following values of the ranges in air at 76 cm. pressure 
and 20° C. : — 
Radium, 3'50. 
Emanation or (^ ^.90 
Radium A I 
Radium A 01 . .,q„ 
Emanation 
I 
Radium C, 7-06 
These were probably correct to 05 cm. 
It also appeared that the four groups were alike in all respects 
save that of initial velocity, and that the alpha particle spent 
