Iv] NATURE OF THE RADIATIONS 117 
The writer was led independently to the same view by a mass 
of indirect evidence which received an explanation only on the 
hypothesis that the rays consisted of matter projected with great 
velocity. Preliminary experiments with radium of activity 1000 
showed that it was very difficult to determine the magnetic devia- 
tion of the a rays. When the rays were passed through slits 
sufficiently narrow to enable a minute deviation of the rays to be 
detected, the ionizing effect of the issuing rays was too small to 
measure with certainty. It was not until radium of activity 19,000 
was obtained that it was possible to detect the deviation of these 
rays in an intense magnetic field. How small the magnetic devia- 
tion is may be judged from the fact that the a rays, projected at 
right angles to a magnetic field of 10,000 c.G.s. units, describe the 
are of a circle of about 39 cms. radius, while under the same con- 
ditions the cathode rays produced ina vacuum tube would describe 
a circle of about ‘Olcm. radius. It is therefore not surprising 
that the a rays were for some time thought to be non-deviable in 
a magnetic field. 
82. Magnetic deviation of the a rays. ‘The general 
method employed? to detect the magnetic deviation of the a rays 
was to allow the rays to pass through narrow slits and to observe 
whether the rate of discharge of an electroscope, due to the issuing 
rays, was altered by the application of a strong magnetic field. 
Fig. 25 shows the general arrangement of the experiment. The 
rays from a thin layer of radium of activity 19,000 passed upwards 
through a number of narrow slits G, in parallel, and then through 
a thin layer of aluminium foil, 00034 cm. thick, into the testing 
vessel V. The ionization produced by the rays in the testing 
vessel was measured by the rate of movement of the leaves of a 
gold-leaf electroscope B. The gold-leaf system was insulated inside 
the vessel by a sulphur bead C, and could be charged by means of 
a movable wire J), which was afterwards earthed. The rate of 
movement of the gold-leaf was observed through small mica 
windows in the testing vessel by means of a microscope provided 
with a micrometer eye-piece. 
In order to increase the ionization in the testing vessel, the 
1 Rutherford, Phil. Mag. Feb. 1903. Phys, Zeit. 4, p. 235, 1902. 
