x1] AND OF ORDINARY MATERIALS 305 
simple portable apparatus’ was devised by them and used for the 
whole series of experiments. <A large number of observations were 
taken, extending over a period of twelve months. They found 
that the amount of excited activity obtained was subject to great 
variations. The extreme values obtained varied in the ratio of 
16 to 1. No direct connection could be traced between the amount 
of ionization in the atmosphere and the amount of excited activity 
produced. They found that the greatest amount of excited activity 
was obtained during a fog, while the amount of ionization in the 
air is then small. This result, however, is not necessarily contra- 
dictory to the view that the ionization and activity of the air 
are to a certain extent connected. From the experiments of 
Miss Brooks on the effect of dust in acting as carriers of excited 
activity, it 1s to be expected that more excited activity would be 
obtained during a fog than in clear air. The particles of water 
become centres for the deposit of radio-active matter. The 
positive carriers are thus anchored and are not removed from 
the air by the earth’s field. In a strong electric field, these 
small drops will be carried to the negative electrode and manifest 
their activity on the surface of the wire. On the other hand, the 
distribution of water globules throughout the air causes the ions 
in the air to disappear rapidly in consequence of their diffusion to 
the surface of the drops (see section 31). For this reason the 
denser the fog, the smaller will be the conductivity observed in 
the air. 
Lowering the temperature of the air had a decided influence. 
The average activity observed below 0°C. was 1:44 times the 
activity observed above 0°C. The height of the barometer was 
found to exert a marked influence on the amount of excited activity 
to be derived from the air. The lower the barometer the greater 
was the amount of excited activity in the air. The effect of 
variation of the height of the barometer is intelligible, when it is 
considered that probably a large proportion of the radio-activity 
observed in the air is due to the radio-active emanations which 
are continuously diffusmg from the earth into the atmosphere. 
Elster and Geitel have suggested that a lowering of the pressure 
of the air would cause the air from the ground to be drawn up 
1 Phys. Zeit. 4, p. 522, 1903. 
