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, when the amount of ionization in the air was small. This result, however, is not necessarily contradictory 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, more excited activity should 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 diffusing 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 from the capillaries of the earth into the atmosphere. This, however, need not necessarily be the case if the conditions of the escape of the emanation into the atmosphere are altered by the variation of the position of underground water or by a heavy fall of rain.
The amount of excited activity to be derived from the air on the Baltic Coast was only one-third of that observed inland at
