50  McCrrnuanp anp Hackerr—TZhe Absorption of B Radium Rays by Matter. 
ratio of « to the density shows quite clearly that, on the whole, this ratio increases 
with the atomic weight. Further, the elements fall into divisions with respect to 
this ratio, the divisions being the same as the chemical periods. Thus, lead and 
platinum are in one period; tin, cadmium, and silver in another; zinc, copper, 
and nickel in a third; and aluminium in a fourth. It would be useful to examine 
more elements ; but the above are sufficient to show the dependence on atomic 
weight of the coefficient » for plates of different elements of equal weight per 
unit area. ‘The numbers given also show clearly enough the divisions correspond- 
ing to the chemical periods ; but more determinations would be necessary to show 
how the coefficient varies from one element to another in the same period. That 
the atomic weight should be the factor that determines the absorption produced 
by equal weight thicknesses of different elements is quite intelligible. The actual 
stoppage of a B particle is, no doubt, brought about by electric force and not by 
mere mechanical impact. If, then, a main distinction between different atoms 
consists in a difference of number and grouping of electrons, the stopping power 
per atom should increase more rapidly than the number of electrons, or than the 
atomic weight, so that few atoms of high atomic weight should be more effective 
than an equal mass of atoms of smaller atomic weight. 
Crowther* has recently determined the coefficient of absorption A of the B rays 
of uranium for a large number of substances. The present paper shows how 
necessary it is to distinguish between A and the true coefficient ~ when the 
secondary radiation is great, as it is for 8 radium rays; but in the case of 
8 uranium rays Crowther finds the secondary radiation so small that he considers 
the distinction between \ and p becomes unimportant. . 
Our results as regards the general connexion between the atomic weight and 
the ratio of w to the density are in agreement with Crowther’s; and his results also 
show the nature of the variation of the ratio of \ to density between elements in 
the same period. 
The comparative unimportance of secondary radiation effects in the case of B 
uranium rays, as found by Crowther, is striking when compared with these effects 
in the case of 8 rays from radium, and is a point that requires investigation. 
* Phil. Mag., Oct. 1906. 
