XI] AND OF ORDINARY MATERIALS 355 
activity on the wire and 2 is the constant of decay of the excited 
activity. Since the activity of a wire after removal falls to half 
value in about 45 minutes, the value of ~ 1s 0°92 with one hour as 
the unit of time. Some experiments made by Allan are in rough 
agreement with the above equation. Accurate comparative results 
are difficult to obtain on account of the inconstancy of the radio- 
activity of the open air. After an exposure of a wire for several 
hours, the activity reached a practical maximum, and was not 
much increased by continued exposure. 
A wire charged to a high potential in the open air abstracts 
the positive radio-active carriers from a large volume of air. Very 
little excited activity, in comparison, is produced in a closed vessel 
or by drawing a rapid current of the outside air through a cylinder 
in the centre of which a negatively charged rod is placed. In one 
experiment a current of air of 500 cms. per second was drawn 
through a cylinder of 141 litres capacity. The amount of activity 
produced on the negative electrode was only a few per cent. of the 
amount observed on the same electrode charged to the same 
potential in the open aur. 
The amount of excited activity produced on a wire, supported 
some distance from the surface of the earth, should increase steadily 
with the voltage, for the greater the potential, the greater the 
volume of air from which the radio-active carriers are abstracted. 
The presence of radio-active matter in the atmosphere will 
account for a portion of the ionization of the air observed near 
the earth. It seems unlikely, however, that the whole of the 
ionization observed in the air is due to this cause alone. 
211. Radio-activity of freshly fallen rain and snow. 
C. T. R. Wilson! tried experiments to see if any of the radio- 
active material from the air was carried down by rain. For this 
purpose a quantity of freshly fallen rain was collected, rapidly 
evaporated to dryness in a platinum vessel, and the activity of the 
residue tested by placing the vessel in an electroscope. In all 
cases, the rate of discharge of the electroscope was considerably 
increased. From about 50 c.c. of rain water, an amount of activity 
was obtained sufficient to increase the rate of discharge of the 
1 Proc. Camb. Phil. Soc. 11, Pt. vi. p. 428, 1902. 
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