VOLCANOS AND RADIOACTIVITY 547 



would be impossible for water vapor to lift its covering and force a 

 way to the surface unless it had a temperature greatly exceeding 

 1,200° C. It would have to be heated to a considerably higher tem- 

 perature to do it. But with increasing temperature the heat is con- 

 ducted away more and more rapidly until the loss of heat is equal to 

 the quantity generated, and thereafter there is no increase of tempera- 

 ture. The generation of radioactive heat is a slow process, and the 

 only method of its escape is by conduction away from the radioactive 

 source. The rate of heat generation is constant and independent of 

 the temperature, but the rate of loss increases rapidly with the tem- 

 perature. Ultimately, as the temperature rises, a point would be 

 reached at which the loss of heat becomes equal to the gain. 



If an eruption from a deep source, say five or six miles, were to 

 occur, we should expect that the temperature of the lava would be very 

 high — probably a white heat — and that its mass would be very great. 

 Its consequences might be disastrous beyond all precedent. 



That volcanism is caused by the generation of heat near the surface 

 was a belief which I expressed over twenty years ago in a chapter of 

 the work on Hawaiian volcanos. Long study of the volcanic problem, 

 in which every other theory failed and went to pieces under criticism, 

 and this alone not only survived but grew more probable and in accord- 

 ance with the facts, led me to the hazardous step of venturing to ex- 

 press it. At that time, no cause could be cited for the increase of heat, 

 and the proposition met with no response, and no doubt justly. Geolo- 

 gists continued to look for the explanation of volcanos in the gradu- 

 ally waning remnants of the earth's internal heat. Within the last 

 five or six years, however, physical science has made discoveries of a 

 wonderful nature, which open a new field, indeed, a new world, in our 

 views of the constitution of matter, and may throw a flood of light on 

 the very subject of our inquiry. 



The subject of radioactivity is so new and so surprising that it has 

 had time only to establish a very few of the fundamental principles 

 which lie at the basis of it. But so hotly is the matter pursued by 

 many of the ablest specialists that each year shows a large increase in 

 our knowledge. As this is familiar to all physicists, I shall allude 

 here briefly only to such as are essential to our discussion. We have to 

 regret that some of the most fundamental questions concerning radio- 

 activity are as yet unsolved, though we can not expect that a new 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 



