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 temperature to do it. But with increasing temperature the heat is conducted away more and more rapidly until the loss of heat is equal to the quantity generated, and thereafter there is no increase of temperature. 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 temperature. 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 accordance with the facts, led me to the hazardous step of venturing to express 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. Geologists continued to look for the explanation of volcanos in the gradually 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 radioactivity 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