548 POPULAR SCIENCE MONTHLY 



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, Pro- 

 fessors 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 Eutherford 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 some- 

 body'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 consid- 

 erably, 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 conduc- 

 tivity; 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 at- 

 mosphere 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. 



