THE MYSTERY OP MATTER. 



181 



are also produced under various other conditions. They travel 

 at a most astounding rate of speed, and their velocity varies 

 according to the conditions under which they are emitted. 

 Their speed may be taken to be from 10,000 to 90,000 miles 

 a second. At the latter rate they would travel round the 

 earth more than three times in one second. The mass of a 

 corpuscle is small beyond conception : it has been calculated 

 not to exceed i-oVo part of the mass of a hydrogen atom. 

 A beam of corpuscles is bent by a magnetic force acting at right 

 angles to its path. The corpuscles cause phosphorescence in 

 bodies which they strike ; they raise the temperature of 

 substances which they strike ; and they communicate mechanical 

 motion to bodies against which they impinge. From whatever 

 source the corpuscles are derived they are all exactly similar to 

 each other in every respect, and they constitute actual parts of 

 the matter from which they are emitted. 



The positive ions, on the other hand, are of atomic dimensions, 

 comparable in size to a hydrogen or helium atom, perhaps 2,000 

 or 3,000 times as large as the negative ions. They carry a charge 

 of positive electricity ; are very slightly deflected or bent by 

 magnetism ; and their velocity is very much less than that of the 

 negative corpuscles. 



Let us consider next the phenomena which accompany 

 electrical discharges in high vacua. When a discharge passes 

 through the air at ordinary atmospheric pressure, a narrow 

 spark is seen to flash between the electrodes of the electrical 

 machine ; but when the electrodes are placed in an air-tight 

 receptacle, and the air withdrawn by means of a mercurial air- 

 pump, the spark becomes broader and more ill-defined as the 

 pressure is reduced. If a cylindrical glass or vacuum-tube be 

 used and the pressure be reduced to 2^0 of an atmosphere, 

 a glow of light is seen to spread out from the electrodes and to 

 fill the tube. This glow is mostly reddish in colour, but sur- 

 rounding the negative electrode, or cathode as it is called, there 

 is a blue glow. When the exhaustion is carried still further, 

 until the pressure is reduced to no more than touoo part of 

 an atmosphere, a patch of green fluorescent light is seen on 

 the side of the tube opposite the negative electrode. Faint 

 streaks of blue luminosity are seen stretching from the cathode 

 to this phosphorescent spot, and these faint streaks have received 

 the name of the cathode rays. These rays are very penetrating 

 and will pass through thin plates of metal. They are absorbed 

 by all substances inversely in proportion to the density of each 



