CH. IT] IONIZATION THEORY OF GASES 29 
produced will in general depend upon the nature and pressure of 
the gas. In the electric field the positive ions travel towards the 
negative plate, and the negative ions towards the other plate, and 
consequently a current will pass through the gas. Some of the 
ions will also recombine, the rate of recombination being propor- 
tional to the square of the number present. For a given intensity 
of radiation, the current passing through the gas will increase at 
first with the potential difference between the plates, but it will 
finally reach a maximum when all the ions are removed by the 
electric field before any recombination occurs. 
This theory accounts also for all the characteristic properties of 
gases made conducting by the rays from active substances, though 
there are certain differences observed between the conductivity 
phenomena produced by active substances and by X rays. These 
differences are for the most part the result of unequal absorption 
of the two types of rays. Unlike Réntgen rays a large proportion 
of the radiation from active bodies consists of rays which are 
absorbed in their passage through a few centimetres of air. The 
ionization of the gas is thus not uniform, but falls off rapidly with 
increase of distance from the active substance. 
26. Variation of the current with voltage. Suppose that 
a layer of radio-active matter is spread uniformly on the lower of 
two horizontal plates A and B (Fig. 1). The lower plate A is 
connected with one pole of a battery of cells the other pole of which 
is connected with earth. The plate B is connected with one pair of 
quadrants of an electrometer, the other pair being connected with 
earth. 
The current’ between the plates, determined by the rate of 
movement of the electrometer needle, is observed at first to in- 
crease rapidly with the voltage, then more slowly, finally reaching 
a value which increases very slightly with a large mcrease in the 
voltage. This, as we have indicated, is simply explained on the 
ionization theory. 
The radiation produces ions at a constant rate, and, before the 
electric field is applied, the number per unit volume increases 
1 A minute current is observed between the plates even if no radio-active matter 
is present. This has been found to be due mainly to a slight natural radio-activity 
of the matter composing them, (See sections 218—220.) 
