Page:Popular Science Monthly Volume 71.djvu/537

As the potential of the earth is negative compared with that of the air, the active deposit is dragged down to the surface of the ground and upon the leaves and branches of plants and trees. A hill or mountain top concentrates the earth's field and so receives a greater amount of the active deposit. In this way Elster and Geitel explain the greater ionization on hills and mountains. Experiments show that the active deposit tends to collect on dust particles. These dust particles serve as nuclei for the condensation of raindrops and snowflakes. The deposit resulting from evaporating rain and snow should be very radioactive. This was found to be true by Wilson and Allen. Again, a big rain or snow should carry down most of the active deposit, and as the emanation does not emit ${\displaystyle \gamma }$ rays, the amount of ${\displaystyle \gamma }$ radiation from the radioactive matter in the air should be very much decreased. The penetrating radiation, if it consists mainly of ${\displaystyle \gamma }$ rays, should then become very small. This has been found to be borne out by experiments made by the writer. It must be remembered that the emanation is insoluble in water and as this does not seem to be carried down by water or snow, the products radium C and thorium C would soon be in equilibrium again after the rain or snow.