radium is to be found only in an outer crust of the earth, at least if the earth is becoming cooler. In making these calculations, the effect of thorium and uranium and the possible radioactivity of ordinary materials is not taken into account. If the heating effect of ordinary materials is of the same order of magnitude as is to be expected from the ionization they produce, the earth's temperature gradient would be many times larger than that observed. Strutt believes this to be an argument that ordinary matter possesses no genuine radioactivity of its own. C. B. Thwing claims, however, that he has been able to find a temperature gradient in small cylinders of the various metals and rocks. At present nothing very definite can be said as to the heating effect of the radioactivity of substances upon the temperature of the earth. Having considered the radioactivity of the various rocks, we will now take up the atmosphere.
Radioactivity of the Atmosphere
It was found after considerable work had been done on ionization that the free air is very considerably ionized. Now in the table of the transformation products of the radio-elements, it will be noticed that several products have the property of condensing on a highly negatively-charged wire. Elster and Geitel and others tried exposing such a wire in the open air and found that there was an active deposit of radium and thorium formed on the wire. The amount of active deposit was found to depend upon the locality and the weather conditions. If the air had been undisturbed for some time as the air in caves and cellars, it was found that the active deposit formed was much greater. Air sucked through the pores of the ground was found to be very active. From these results, Elster and Geitel concluded that the radium and thorium emanations (which behave like gases) ooze up through the ground and percolating waters and have their origin in the radium and thorium in the soil. The emanation then breaks up into the various products as given in Table I. The emanation in this course gives rise to positively charged carriers, which are driven to a negatively charged wire by the electric field. It is to the emanation and its products that the ionization of the air is attributed. Thorium C and radium C give off rays, and, as these are very penetrating, they would be the source of a very penetrating radiation, and this latter has been discovered several years ago. The ionization in a closed electroscope is measured, and thick lead screens are then placed around the electroscope, and the ionization is again measured. The ionization in the latter case is found to be very considerably decreased, the penetrating radiation having been largely cut off. Whether all the penetrating radiation can be explained as due to radium C and thorium C will be taken up later.