Iv] NATURE OF THE RADIATIONS 95 
aluminium or mica of ‘01 cms. or a sheet of ordinary writing-paper 
is sufficient to completely absorb all the a rays. With such a 
screen over-the active material, the effects are due only to the 
8 and y rays, which pass through with a very slight absorption. 
Most of the 6 rays are absorbed in 5 mms. of aluminium or 2 mms. 
of lead. The radiation passing through such screens consists very 
largely of the y 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, 7.e. the 
absorption is proportional to the density. This rule holds ap- 
proximately 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. 
PAU ea Tle 
THE 8 oR CATHODIC Rays. 
69. Discovery of the § 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, when it was observed that preparations of radium gave 
out some rays 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’ 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 per- 
pendicular 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? also obtained similar results. They 
1 Wied. Annal. 69, p. 831, 1899. 2 Phys. Zeit. 1, pp. 90, 113, 1899. 
