x1] AND OF ORDINARY MATERIALS 357 
Schmauss! has observed that drops of water falling through air 
ionized by Réntgen rays acquire a negative charge. This effect is 
ascribed to the fact that the negative ions in air diffuse faster 
than the positive. On this view the drops of rain and flakes of 
snow would acquire a negative charge in falling through the air. 
They would in consequence act as collectors of the positive radio- 
active carriers from the air. On evaporation of the water the 
radio-active matter would be left behind. 
212. Radio-active emanations from the earth. Elster 
and Geitel observed that the air in caves and cellars was, in most 
cases, abnormally radio-active, and showed very strong ionization. 
This action might possibly be due to an effect of stagnant air, by 
which it produced a radio-active emanation from itself, or to a 
diffusion of a radio-active emanation from the soil. In order to 
test if this emanation was produced by the air itself, Elster and 
Geitel shut up the air for several weeks in a large boiler, but no 
appreciable increase of the activity or ionization was observed. In 
order to test if the air imprisoned in the capillaries of the soil was 
radio-active Elster and Geitel*® put a pipe into the earth and sucked 
up the air into a testing vessel by means of a water pump. 
The apparatus employed to test the ionization of the air is 
shown in Fig. 59. C is an electroscope connected with a wire net, 
Z. The active air was introduced into the large bell-jar of 27 litres 
capacity, the inside of which was covered with wire netting, MM’. 
The bell-jar rested on an iron plate AB. The electroscope could 
be charged by the rod S. The rate of discharge of the electro- 
scope, before the active air was introduced, was noted. On allowing 
the active air to enter, the rate of discharge increased rapidly, 
rising in the course of a few hours in one experiment to 30 times 
the original value. They found that the emanation produced 
excited activity on the walls of the containing vessel. The air 
sucked up from the earth was even more active than that observed 
in caves and cellars. There can thus be little doubt that the 
abnormal activity observed in caves and cellars is due to a radio- 
active emanation, present in the earth, which gradually diffuses to 
the surface and collects in places where the air is not disturbed. 
1 Drude’s Annal, 9, p. 224, 1902, 2 Phys. Zeit. 3, p. 574, 1902. 
