80 ROYAL SOCIETY OF CANADA 
This gives the ratio of the charge of an ion to its mass in the 
electro-magnetic system of units. 
So far no assumptions have been made as to the actual value of 
the charge of the mass of an ion. Rough approximations to the 
values of these quantities can be obtained from considerations based 
on the kinetic theory of gases, but, as will be seen later, the evidence 
does not rest on the actual values of the mass but only on the value 
of the ratio © 
mt 
Sir William Crookes first drew attention to a remarkable phe- 
nomenon which showed itself when an electric discharge was passed 
through a highly exhausted vacuum tube. Below a certain pressure 
of the gas in the tube, a peculiar kind of rays are shot off from the 
cathode. These “cathode” rays travel in straight lines and produce 
brilliant phosphorescent effects on the walls of the tube and also on 
many other substances placed directly in their path. Crookes showed 
the path of the rays could be bent by a magnet. In a strong mag- 
netic field these rays can be made to trace out spirals round the 
direction of the lines of magnetic force. He showed that they pro- 
duced strong heating effects by their impact and a considerable 
mechanical pressure on vanes placed in their path. 
For a long time two rival theories held the ground as to the 
explanation of these effects. The German school of physicists took 
the view that the cathode rays were ether waves of some kind. The 
English view, as voiced by Crookes, held that they were in reality 
projected particles travelling with high velocity. On the latter view 
most of the effects observed by Crookes received a simple explanation. | 
The phosphorescent, heating, and mechanical effects were due to the 
bombardment of material particles, driven off from the cathode by a 
strong eleciric field. The curvature of the path of the rays by a 
magnetic field was due to the fact that a moving charge acts like a 
current. 
The presence of two rival theories led to a large amount of 
investigation of the discharge in vacuum tubes. 
Hertz tried if the rays were deviated by a strong electric field 
but failed to get any effect. Lenard, in 1895, showed that the 
cathode rays were able to pass through thin windows of glass, mica, 
or metal foil. He was thus able to examine the cathode rays outside 
the vacuum tube. He showed that the absorption of the rays by 
matter was independent of its chemical constitution and depended 
only on its density. This was true whether the matter was in the 
state of solid, liquid or gas. The fact that these rays could pass 
through solid matter, together with the absorption results, pointed 
