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 rays, the amount of radiation from the radioactive matter in the air should be very much decreased. The penetrating radiation, if it consists mainly of 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.
The effect of the presence of radioactive matter in the atmosphere upon ordinary phenomena is perhaps very great, though at present little is known. It has been found that deep wells and hot springs contain considerable radium. From this Elster and Geitel suggest that the curative effect of thermal springs and the physiological action of the air at high levels may be related to the large amount of radioactive matter present. The presence of radioactive matter, and therefore of ionization, in the air probably plays a very important role in the growth of plants. It has been found that vegetables grown in an atmosphere electrified positively are much above those grown in normal fields both in quantity and in quality. The ionization and nucleation produced by radioactive matter in the air are very essential for the condensation of rain and hail, and serve to explain the enormous accumulation of static electricity during thunderstorms. Simpson and others have measured the activity of the air which has blown over the sea and have found it small. Now if most of the radium and thorium emanations come from the pores of the soil and underground cavities, the results obtained by the above investigators would be expected, for, as will be seen from Table II., the radium content of ocean water is very small. Eve has recently measured the ionization over the ocean and has found it to be the same as the ionization over the land, a rather unexpected result. In this state the matter rests at present. A crucial test would be to expose negatively charged wires far out in the ocean and find whether there was any active deposit and to test for the presence of a penetrating radiation. According to J. Joly, the distinguished geologist, Eve's results can easily be explained. Geologists have
