Roses 
IV. 
THE ABSORPTION OF 8 RADIUM RAYS BY MATTER. 
By J. A. McCLELLAND, M.A., D.Sc., 
Professor of Experimental Physics, University College, Dublin ; 
AND 
F. HE. HACKETT, M.A., B.Sc. 
[ Published, Marcu 22, 1907. | 
THE emission of secondary £ particles from matter traversed by 6 radium rays 
has been investigated in considerable detail by one of us, and the result has 
been established that the intensity of this secondary radiation from any element 
depends on its atomic weight. The secondary radiation increases as the atomic 
weight increases; the rate of imcrease with atomic weight is not uniform, and 
the form of the curve connecting the intensity of the secondary radiation with 
the atomic weight of the element is such as to establish a division of the elements 
exactly corresponding to the division into periods employed in chemistry. The 
details of this work and its bearing on the question of atomic structure have 
been described and discussed in previous papers.* 
The intensity of this secondary radiation being found to depend so markedly 
on the atomic weight of the substance acted upon by the primary £8 rays, 
suggests that the coefficient of absorption of substances for 6 radiation may 
also be dependent on the atomic weight. ‘This coefficient is of course known 
to be roughly proportional to the density; the division of the coefficient by the 
density is necessary to give the same relative amounts of matter traversed in 
the different cases. The object of the present work is rather to study the 
variation of the ratio of the absorption coefficient to the density in the hope 
that this lime of work would yield further results bearing on atomic structure 
of a character similar to those contained in the previous work. 
* Trans. Roy. Dublin Soc., vol. viii., part xiv., 1905; vol. ix., part i., 1905; vol. ix., part i1., 1906; 
and vol. ix., part 11., 1906. 
TRANS. ROY. DUB. SOC., N.S., VOL. IX., PART IV. H 
