McCurLttanp—Lonization in Atmospheric Air. 63 
units. ‘Taking the charge on an ion as 34 x 10-” electrostatic units, the observed 
current would mean the using up of 8 x 10° ions per second from the volume of air 
between the plates Z and J, or 680 ions from each c.c., the volume between the 
plates being 1170 c.e. 
If there were only about 15 ions per ¢.c., this would mean a much more rapid 
renewal of the air than could have been taking place. 
To prevent the possibility of ions travelling up to the apparatus under the 
action of the electric field, and thus drawing the supply from a larger volume of 
air, the apparatus was enclosed in a case of perforated tinfoil connected to earth. 
This diminished the electrometer deflections about 25°/.. 
It is intended to make an accurate determination of the number of ions in 
free air, but the above experiments show that the number is usually very much 
greater than that observed in closed vessels, where only the rate of production 
is measured. 
5.— Variations of the Amount of Ionization in Atmospheric Arr. 
A large number of observations of the amount of ionization at different times 
was made. ‘The variations were found to be very considerable, the maximum 
ionization observed being about 21 times the minimum. ‘The greatest ionization 
observed was after several hours’ steady rain. This agrees with the known radio- 
activity of freshly-fallen rain. Wilson* has shown that the residue obtained by 
boiling off some freshly-fallen rain emits a radiation capable of ionizing a gas. 
After several hours’ rain we may, therefore, look upon the surface of the ground 
as strongly radio-active, and the large ionization observed would thus be 
accounted for. 
On the other hand, the smallest ionization observed was after a slight shower ; 
this is probably due to the ions having been removed from the air by the con- 
densation on them of water-vapour. Apart from these extremes of maximum and 
minimum, it is difficult to account for the variations in the amount of ionization. 
Quite rapid variations occur; a 25°/, change in one hour often occurs. Ona 
clear, bright day, with little wind, the amount of ionization is usually very 
steady. 
6.—Causes of Ionization. 
By enclosing air in a vessel, and lining the interior with various substances, 
Strutt} has shown that all the substances tested emit a radiation capable of 
* Proc. Camb. Phil. Soc. Vol, xz., Pt. v1. } Phil. Mag., June, 19038, 
