for some time made an approximate estimate of the age of the oceans by making determinations of the amount of salt which they contain. By analyzing the waters of rivers flowing into the ocean for the salt which they contain and determining the total annual outflow of all the rivers into the ocean, and supposing these constants to have been practically constant during the past, it is easy to make an estimate of the approximate age of the oceans. Now if radium and uranium always exist in a constant proportion, the present radium content of the ocean would have been supplied by the rivers in a comparatively short length of time. For this and other reasons Joly believes that uranium and radium are not always to be found associated together. Now we know that radium has a short period of decay, so that it must constantly be supplied from somewhere. Joly believes that the source is at least partly outside of the earth. This radium is gradually being brought down to the surface. This would account for the ionization over the ocean and the wide distribution of radium over the earth. Elster and Geitel's theory of the escape of the emanation from the upper layers of the soil would still hold true. If radium exists outside the earth, it would be expected that the upper layers of the earth's atmosphere would be highly ionized by the rays. This highly ionizing radiation would serve to explain some of the phenomena of atmospheric electricity. According to C. T. R. Wilson, the positive potential of the atmosphere is largely to be attributed to the carrying down of negative charges by raindrops and snowflakes. The upper layers of the atmosphere, being highly ionized and quite good conductors, would conduct the remaining positive charge to places of lower potential and would thus always aid in equalizing the potential of wet and dry regions.
Radioactivity of the Metals
After the discovery that several of the elements were radioactive, it was natural to ask if radioactivity was a universal property of all the elements. Madame Curie's work showed that if the ordinary elements are radioactive at all, they must possess this property to but a very slight degree. In order to detect any possible radioactivity, it was necessary to have very sensitive instruments. It was found by Wilson and Geitel that there is a leakage of electricity through a gas in a closed vessel and that this leak could be measured very accurately by means of an electroscope. Now either the ions are produced spontaneously in the gas, by a radiation which is capable of penetrating the sides of the electroscope or by radiations from the walls of the electroscope itself. Rutherford, Cooke and McClennan have shown that some thirty or forty per cent, is due to a very penetrating radiation supposed to be the rays emitted by the radium and thorium products in the air and ground. By using lead screens around the electroscope, they were able to decrease the rate of leak to a certain limiting value
