Iv] NATURE OF THE RADIATIONS TS 
79. Variation of the amount of radiation with the 
thickness of the layer of radiating material. The radiations 
are sent out equally from all portions of the active mass, but the 
ionization of the gas which is measured is due only to the radiations 
which escape into the air. The depth from which the radiations 
can reach the surface depends on the absorption of the radiation 
by the active matter itself. 
Let 2X be the absorption constant of the homogeneous radiation 
by the active material. It can readily be shown that the intensity 
I of the rays issuing from a layer of active matter, of thickness a, 
is given by 
JE 
I, 
where J, is the intensity at the surface due to a very thick layer. 
This equation has been confirmed experimentally by observing 
the current due to the 8 rays for different thicknesses of uranium 
oxide. In this case J=4 J, for a thickness of oxide corresponding 
to ‘11 gr. per sq. cm. This gives a value of X divided by density of 
63. This is a value slightly greater than that observed for the 
absorption of the same rays in aluminium. Such a result shows 
clearly that the substance which gives rise to the @ rays does not 
absorb them to a much greater extent than does ordinary matter 
of the same density. 
The value of X will vary, not only for the different active 
substances, but also for the different compounds of the same 
substance. 
=e 
PART III. 
THE a RAYS. 
80. Thearays. The magnetic deviation of the 8 rays was 
discovered towards the end of 1899, at a comparatively early stage 
in the history of radio-activity, but it was not until three years later 
that the true character of the « rays was disclosed. It was natural 
that great prominence should have been given in the early stages 
of the subject to the @ rays, on account of their great penetrating 
8—2 
