VOLCANOS AND RADIOACTIVITY 265 



activity are as yet unsolved, though we cannot expect that a new and 

 and far-reaching science should in six years have accomplished all of 

 its immense possibilities. 



A good many efforts have been made, by the use of the extremely 

 sensitive quadrant electrometer, to ascertain by measurement the 

 quantity of radioactive substances in the accessible portions of the earth. 

 By taking samples of earth from varying depths and testing them by the 

 electrometer^ widely variable quantitative results have been obtained, 

 but in every instance the amount of radioactivity indicated much ex- 

 ceeds the amount required to compensate the loss of heat from the earth 

 by conduction and radiation into space. For instance, Professors 

 Elster and Geitel, of Berlin, who have made many discoveries and con- 

 tributed many observations in radioactivity, placed 3,300 ^'^ of garden 

 soil within a closed vessel with an electroscope to determine the con- 

 ductivity of the inclosed 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 Rutherford esti- 

 mates, 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 2, 

 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 

 days, 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 



