760 Sir J. J. Thomson on 



The preceding considerations show 1 think that we may 

 divide the positive rays into the following classes : — 



1. The undeflected rays, i. e. rays which are not affected 

 by electric or magnetic forces; we cannot determine directly 

 the velocity or the value o£ elm for these rays. 



2. Secondary rays produced by the rays (1). As the rays 

 of the first type pass through a gas and collide against the 

 molecules they produce secondary rays ; whether they do 

 this by splitting up themselves or by dissociating the 

 molecules against which they strike, is uncertain. The rays 

 of this class have a constant velocity 2 x 10 8 cm,/scc. 

 roughly; whatever may be the potential difference between 

 the electrodes, they have a constant maximum value of 

 ejm = 10 4 . At the higher pressures and when the discharge- 

 tube is small, these rays predominate and swamp the others ; 

 they get fainter and fainter as the pressure is reduced below 

 a certain amount. We shall call the rays of this type 

 secondary positive rays. 



3. Rays characteristic of the gases in the tube. These are 

 seen at low pressures, they produce bright spots on the 

 screen ; with each spot a thin parabolic band of luminosity 

 is connected, the separate bands forming a kind of spectrum 

 characteristic of the gases in the tube. The velocity of 

 these rays depends on the potential difference between the 

 electrodes, and the value of ejm is inversely proportional fo 

 the atomic weight of the gas from which they are derived. 

 Their kinetic energy is that due to the potential difference 

 between the negative glow and the cathode, in a mixture of 

 gases the electrostatic deflexion of the rays from each g;is is 

 the same. 



The retrograde rays which start from the cathode and 

 travel away from it in the same direction as the cathode rays 

 belong to classes (1) and (2). I have never seen the bright 

 spots characteristic of class 3 in the retrograde rays. 



In addition to the positively charged rays there are 

 negatively charged ones of type 2 and in some cases of 

 type 3. The different gases show great variations in the 

 brightness of the negative tails connected with the rays 

 peculiar to the atoms of the element, some elements show 

 the negative tail readily while I have never seen it with 

 others. 



If we suppose that the undeflected rays are formed by the 

 recombination of positive and negative particles and that 

 these by collision with the molecules of the gas through 

 which they pass form rays of .type (2), either by splitting up 

 themselves or by dissociating the molecules against which 



