It has also been observed that an element having chemical properties identical with those of lead is found in uranium minerals. This ele ment is known as uranio-lead and is supposed to be the final disintegration product of the uranium. Its atomic weight has been found to be approximately 206 and in this respect it dif fers from ordinary lead which has an atomic weight of over 207.
The helium found in radioactive minerals is thus a decomposition product of radium and also of the other radioactive substances present in it. It seems likely • that we shall be able to form a rough estimate of the age of the radio active minerals by measuring the amount of helium and lead in them, since for a given amount of radioactive matter present, the amount of helium and lead will be greater the longer the mineral has been formed.
Although radium is found in the highest proportion in the mineral pitchblende and in all other uranium minerals, it is also found dis tributed in minute quantity over the whole stir face of the earth. This has been clearly brought out by the long series of investigations made by Elster and Geitsel of the radioactivity of dif ferent soils and of the atmosphere. J. J. Thom son, Himstedt, Strutt and many others have found' that the radium emanation is present it springs and in well water. Bumstead and Wheeler found that the air and surface waters of New Haven, Conn., was impregnated with the radium emanation. Strntt and Allan have observed that the famous Bath springs contain a considerable amount of radium. emanation, and the former has also shown that the water contains a minute amount of radium in solution. A similar result has been noted for the waters at Baden and elsewhere. It has been
suggested that the curative properties of these hot-water springs may possibly be due to the presence of radium or the radium emanation. This has, however, by no means been estab lished.
The atmosphere is found to always contain a small quantity of the radium emanation. This does not come from the atmosphere itself but diffuses up from the soil or is carried up by spring water. A large amount of investigation has been carried out to determine the amount of emanation present in the atmosphere at dif ferent localities and under varying atmospheric conditions.
It has been suggested that the internal heat of the earth may be, in part at least, kept up by the heat liberated by the radium and other radioactive matter present in it. Observations, that have so far been made, seem to show that the amount of radium in the surface crust of the earth is sufficient to supply as much heat to the earth as is lost by conduction to the sur face from the hot interior of the earth. It is only necessary that radium should exist in the earth to the extent of four parts in one hun dred million million to account for the mainte 'fiance of the internal heat of the earth. The amount of radium or equivalent amount of other radioactive matter present in the earth cannot be greater than 270,000,000 tons or otherwise the earth would be growing hotter.
It is not possible to discuss here the physio logical actions that are produced by radium rays. A large number of experiments have al ready been made and the literature of this branch of the subject Is already extensive and is very rapidly growing. For bibliography see RADIUM.