BY THE ACTION OF A RADIO-ACTIVE SUBSTANCE, ETC. 
271 
The density of ionization depends greatly on the state of the zinc surface, which 
has to be polished from time to time in order that the ionization should not be too 
small. In the above experiments the greatest density of electrification occurs in 
the third experiment with dry gas. The mean time, t, spent by any portion of the 
gas in the tubes T x is -g-fy second. The total volume of gas that passed through the 
apparatus per minute was 4260 c.c. 
On standardising the electrometer, it was found that each scale division corre¬ 
sponded to a charge of '0044 electrostatic unit. The mean density of the electri¬ 
fication p was therefore 8'5 X 10 “ 5 electrostatic unit per c.c. We have shown in 
Section I. that the product p X t must be less than 10~ 6 in order that the loss of ions 
due to self-repulsion should be less than 1 per cent, of the loss due to diffusion to 
the sides. I 11 the present case the product pXt is - 9 X 10 — fi , so that 110 correction 
need be made for the loss due to self-repulsion. 
Section IV.— Ions produced by the Point Discharge. 
In order to make the apparatus suitable for experimenting with the point dis¬ 
charge, the changes shown in fig. 4 were made. Two circular holes (1‘6 centims. in 
diameter) were made in the tube A, and two tubes, Q and ft, of the same diameter as 
