Page:Popular Science Monthly Volume 64.djvu/496

492 conclusion that such substances are continually projecting particles with enormous velocities, for if these particles are projected from all the molecules of the active substance, it would be expected that the temperature of the mass of the substance would rise under this unceasing internal bombardment. But whence comes this energy which is represented in the projected particles, and of which this heat and light are the ultimate manifestation?

The answer to this last question has not yet been fully given. This much, however, can be said, that, thanks to the splendid work of Rutherford and Soddy, of McGill University, of Sir William Crookes, of the Curies and Becquerel in Paris, and of one or two German physicists, a fairly satisfactory answer is at least in sight. Whatever be the cause of this ceaseless emission of particles by radio-active substances, it is certain that it is not due to any ordinary chemical reactions, such as those with which we have heretofore been familiar; for Madame Curie showed, when she originally discovered the activity of thorium, that the activity of all the active substances is proportional simply to the amount of the active element present and has nothing whatever to do with the nature of the chemical compound in which that element is found. Thus, thorium may be changed from a nitrate to a chloride, or from a chloride to a sulphide, or it may undergo any sort of a chemical change, without any change whatever being noticeable in its activity. Furthermore, radio-activity has been found to be absolutely independent of all changes in physical as well as chemical condition. A radio-active substance may be subjected to the lowest temperatures known, or to the highest temperature obtainable, without showing in either case any alteration whatever in the amount of its activity. Radio-activity seems therefore to be as unalterable a property of the atom of the radio-active substances as is weight itself. It is certainly something which is entirely beyond the range of ordinary molecular forces. This is strong evidence in favor of the view that radio-active change, i. e., the change, whatever it be, which is responsible for the expulsion of the alpha and beta particles, involves a change in the nature of the atom itself. This is the first time in the history of science that any subatomic store of energy has been tapped by man, although, as stated above, the possibility of breaking up the atom was first proved by the study of cathode rays.

The view that radio-activity consists in some change going on in the nature of the atom has received powerful support from a series of discoveries which were started in 1900 by an experiment performed" by Sir William Crookes. He found that if uranium nitrate were precipitated by ammonium carbonate and then enough of the ammonium