The testing vessel and system of plates were waxed to a lead plate _P_ so that the rays entered the vessel _V_ only through the aluminium foil. It is necessary in these experiments to have a steady stream of gas passing downwards between the plates in order to prevent the diffusion of the emanation from the radium upwards into the testing vessel. The presence in the testing vessel of a small amount of this emanation, which is always given out by radium, would produce great ionization and completely mask the effect to be observed. For this purpose, a steady current of dry electrolytic hydrogen of about 2 c.c. per second was passed into the testing vessel; it then streamed through the porous aluminium foil, and passed between the plates carrying the emanation with it away from the apparatus. The use of a stream of hydrogen instead of air greatly simplifies the experiment, for it _increases_ the ionization current due to the α rays in the testing vessel, and at the same time greatly _diminishes_ that due to the β and γ rays. This is caused by the fact that the α rays are much more readily absorbed in air than in hydrogen, while the rate of production of ions due to the β and γ rays is much less in hydrogen than in air. The intensity of the α 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 × 2·50 cms. Strength of field between pole-pieces 8370 units. Apparatus of 25 parallel plates of length 3·70 cms., width ·70 cm., 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 8·33 (2) With magnetic field 1·72 (3) Radium covered with thin layer of 0·93 mica to absorb all α rays (4) Radium covered with mica and 0·92 magnetic field applied The mica plate, ·01 cm. thick, was of sufficient thickness to absorb completely all the α rays, while it allowed the β rays and γ rays to pass through without appreciable absorption. The difference between (1) and (3), 7·40 volts per minute, gives the rate of discharge due to the α rays alone; the difference between (2) and (3), 0·79 volts per minute, that due to the α rays not deviated by the magnetic field employed.