Iv] NATURE OF THE RADIATIONS 119 
instead of air greatly simplifies the experiment, for it increases the 
ionization current due to the a rays in the testing vessel, and at 
the same time greatly diminishes that due to the @ and y¥ rays. 
This is caused by the fact that the a rays are much more readily 
absorbed in air than in hydrogen, while the rate of production of 
ions due to the @ and ¥ rays is much less in hydrogen than in air. 
The intensity of the « rays after passing between the plates is 
consequently greater when hydrogen is used; and since the rays 
pass through a sufficient distance of hydrogen in the testing vessel 
to be largely absorbed, the total amount of ionization produced by 
them is greater with hydrogen than with air. 
The following is an example of an observation on the magnetic 
deviation :— _ 
Pole-pieces 1:90 x 2°50 cms. 
Strength of field between pole-pieces 8370 units. 
Apparatus of 25 parallel plates of length 3°70 cms., width 
‘70cm., with an average air-space between plates of 
‘042 cm. 
Distance of radium below plates 1:4 cm. 
Rate of discharge 
of electroscope in 
volts per minute 
(1) Without magnetic field ... 608 she Be 8°33 
(2) With magnetic field 200 + a eg 
(3) Radium covered with thin layer a mica to 
absorb all a rays... 0:93 
(4) Radium covered with mica ana taenete field 
applied ae oes ase bo re 0°92 
The mica plate, ‘Ol cm. thick, was of sufficient thickness to 
completely absorb all the a rays, but allowed the @ rays and y rays 
to pass through without appreciable absorption. The difference 
between (1) and (3), 7-40 volts per minute, gives the rate of dis- 
charge due to the a rays alone; the difference between (2) and (8), 
0:79 volts per minute, that due to the a rays not deviated by the 
magnetic field employed. 
The amount of « rays not deviated by the field is thus about 
11°/, of the total. The small difference between (8) and (4) 
measures the small ionization due to the @ rays, for they would 
