aluminium or mica of ·01 cms. or a sheet of ordinary writing-paper is sufficient to absorb completely all the α rays. With such a screen over the active material, the effects are due only to the β and γ rays, which pass through with a very slight absorption. Most of the β rays are absorbed in 5 mms. of aluminium or 2 mms. of lead. The radiation passing through such screens consists very largely of the γ rays. As a rough working rule, it may be taken that a thickness of matter required to absorb any type of rays is inversely proportional to the density of the substance, i.e. the absorption is proportional to the density. This rule holds approximately for light substances, but, in heavy substances like mercury and lead, the radiations are about twice as readily absorbed as the density rule would lead us to expect.
PART II.
The β or Cathodic Rays.
75. Discovery of the β rays. A discovery which gave
a great impetus to the study of the radiations from active bodies
was made in 1899, almost simultaneously in Germany, France, and
Austria. It was observed that preparations of radium gave out
some rays which were deviable by a magnetic field, and very
similar in character to the cathode rays produced in a vacuum tube.
The observation of Elster and Geitel that a magnetic field altered
the conductivity produced in air by radium rays, led Giesel[1] to
examine the effect of a magnetic field on the radiations. In his
experiments, the radio-active preparation was placed in a small
vessel between the poles of an electromagnet. The vessel was
arranged to give a pencil of rays which was approximately perpendicular
to the field. The rays caused a small fluorescent patch
on the screen. On exciting the electromagnet, the fluorescent
zone was observed to broaden out on one side. On reversing the
field, the extension of the zone was in the opposite direction. The
deviation of the rays thus indicated was in the same direction and
of the same order of magnitude as that for cathode rays.
S. Meyer and Schweidler[2] also obtained similar results. They
