them by the electrometer, widely variable quantitative results have been obtained, but in every instance the amount of radioactivity indicated much exceeds the amount required to compensate the loss of heat by the earth by conduction and radiation into space. For instance, Professors Elster and Geitel, of Berlin, who have made many discoveries and contributed many observations on radioactivity, placed 3,300 c.c. of garden soil within a closed vessel with an electroscope to determine the conductivity of the enclosed gas. Allowing it to stand for several days, the conductivity of the air became constant at three times the normal amount. This increase of conductivity, Professor Eutherford estimates, would be equivalent to that produced by the emanation from 7 X 10-10 grams of radium. If the density of the soil be taken as two, this corresponds to the emanation from 10-13 grams of radium per gram of clay. Now Professor Rutherford computes that the earth's loss of heat by conduction and radiation is equivalent to what would be supplied by 4.1 X 10-14 grams of radium per cubic centimeter of its mass. According, then, to the results obtained by Elster and Geitel, twice as much heat would be supplied by radioactivity as is lost by conduction and radiation into space.
This experiment with a small quantity of soil taken up in somebody's back yard will hardly be regarded as an accurate determination of such a quantity as the earth's supply of radioactive heat. But the question has been tested by many observers, whose results vary considerably, yet all are of the same order of magnitude. By sinking a pipe into the ground anywhere and sucking up a sample of the air from the soil, it is found to possess a much higher degree of radioactivity than the free air at the surface. It also has a marked degree of conductivity; and this conductivity falls to half of its initial value in a little less than four clays, which is regarded as proving that it is due to radium emanation. The air of caves and cellars has been observed to have a marked degree of ionization, greatly exceeding the open atmosphere and the air in closed vessels. This is attributable only to the presence of radium emanation diffused from surrounding rocks or soils. Many common well-waters give satisfactory tests of the presence of radium emanation, which is soluble in water—more so than most gases.
The most pronounced occurrence of radium is in hot springs. Their waters always give evidence of its presence, and sometimes in quantities many times exceeding the air taken from the soil or cellars. Hon. E. J. Strutt, of Trinity College, has devoted much attention to the springs of Bath, and finds not only radium emanation in their waters, but actual radium in the deposits of the springs. The hot springs of Baden-Baden have been found to contain radium salts. M. Curie has tested a large number of the mineral springs of central and southern France and finds radium emanation in nearly all of them.
