118 NATURE OF THE RADIATIONS [CH. 
rays passed through 20 to 25 slits of equal width, placed side by 
side. This was arranged by cutting grooves at regular intervals in 
side-plates into which brass plates were slipped. The width of the 
shit varied in different experiments between ‘042 cm. and ‘1 cm. 


<— 
nflow of Hydrogen 
Aluminiunt foil 
Sieverret octets 
—_> 
Outflow of Hydrogen 
Fig. 25. 
The magnetic field was applhed perpendicular to the plane of the 
paper, and parallel to the plane of the slits. The rays are thus 
deflected in a direction perpendicular to the plane of the slits and 
a very small amount of deviation is sufficient to cause the rays to 
impinge on the sides of the plate where they are absorbed. 
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. per second was 
passed into the testing vessel, streamed through the porous alu- 
minium foil, and passed between the plates carrying the emanation 
with it away from the apparatus. The use of a stream of hydrogen 
