a Sie 
54 IONIZATION THEORY OF GASES [CH. 
units. This apparent mass increases with the speed as the velocity 
of light is approached (see section 76). 
The ions which are produced in gases at ordinary pressure have 
an apparent size, as determined from their rates of diffusion, large 
compared with the molecule of the gas in which they are produced. 
The negative ion consists of an electron with a cluster,of molecules 
attached to and moving with it. The positive ion consists of a 
molecule from which an electron has been expelled, with a cluster 
of molecules attached; at low pressures under the action of an 
electric field the electron does not form a cluster. The positive ion 
is always atomic in size, even at low pressure of the gas. Each of 
the ions carries a charge of value 3-4 x 107 electrostatic units. 
41. Ions produced by collision. The greater part of the 
radiation from the radio-active bodies consists of a stream of charged 
particles travelling with great velocity. Of this radiation, the « 
particles, which cause most of the ionization observed in the gas, 
consist of positively charged bodies projected with a velocity about 
one-tenth the velocity of hight. The § rays consist of negatively 
charged particles, which are identical with the cathode rays pro- 
duced in a vacuum tube and travel with a speed about one-half 
the velocity of lhght (chapter Iv.). Each of these projected 
particles possesses such great kinetic energy that it is able to 
produce a large number of ions by collision with the gas molecules 
in its path. No defimite experimental evidence has yet been. 
obtamed of the number of ions produced by a single particle, or 
of the way the ionization varies with the speed, but there is no 
doubt that each projected body produces many thousand ions m 
its path before its. energy of motion is destroyed. 
It has already been mentioned (section 29) that at low pressures 
ions moving under the action of an electric field are able to pro- 
duce fresh ions by collision with the molecules of the gas. At low 
pressures the negative ion is identical with the electron produced 
in a vacuum tube, or emitted by a radio-active substance. 
The mean free path of the ion is inversely proportional to the 
pressure of the gas. Consequently, if an ion moves in an electric 
field, the velocity acquired between collisions increases with diminu- 
tion of the pressure. Townsend has shown that fresh ions are 
