filO Mr. 1\. D. Kleeman on Different Kinds of 7 Rays 
/3 rays. The 7 rays fell in part on the radiator B, which in 
consequence emitted secondary (3 and secondary 7 rays. 
Some of the secondary rays penetrated into the ionization 
chamber D, ionizing the air that it contained. The selective 
absorption of the secondary 7 rays was investigated by 
placing successively screens of different substances at C, and 
measuring in each case the ionization in the chamber D, the 
ionization in the chamber being produced by the secondary 
rays not absorbed by the screen. By using radiators and 
screens of different substances the properties of the secondary 
radiation from different substances could thus be investigated. 
The screen served also to screen the ionization-chamber from 
the secondary /5 rays emitted by the radiator B. This was 
necessary since the ionization-chamber was made of thin 
sheet iron which would have been penetrated to some extent 
by the ft rays. The object of having the ionization-chamber 
of thin material was to introduce as little absorption of the 
secondary 7 rays as possible other than that due to the screen. 
The screen C and the ionization-chamber were screened from 
the action of the primary rays by the lead block E 16*5 cm. 
long, 9*5 cm. broad, and 5*5 cm. thick. 
But since the 7 rays possess great penetrating power, an 
object can be screened approximately only from the action 
of 7 rays, and the leak in the ionization-chamber was there- 
fore caused in part by the primary 7 rays which penetrated 
the lead block. It was therefore found convenient to partially 
compensate the leak in the chamber by a leak in the opposite 
direction in another chamber F. The air in this chamber 
was ionized by a layer of uranium oxide, and the chamber 
placed at a distance of one metre from the radiator B and 
screened from its secondary 7 rays by a lead plate 1*5 cm. 
thick. The leak in the chamber was therefore not affected 
when the radiator was replaced by another, and was there- 
fore constant. The chamber F was connected to a negative 
potential of 200 volts, while the chamber D was connected 
to a positive potential of the same magnitude. 
On the right-hand side of fig. 2 a horizontal section of 
part of the apparatus is shown. It will be seen that the 
radiator was placed so that it made an angle of about 45° with 
the screen. This was done because the amount of radiation 
received from the radium was greatest in this position. 
On the left-hand side of the figure the arrangement is 
shown by means of which the radiator or screen was kept in 
a fixed position. It consisted in each case of three fixed 
blocks of wood provided with slits whose width was slightly 
greater than the thickness of the radiator or screen. Both 
radiator and screen were kept in a fixed position by being 
