Page:Popular Science Monthly Volume 87.djvu/128

124 by a permanent alteration of the atoms of matter through which the current passes, although there is little doubt the current is carried in part at least by the electrons liberated from the atoms.

The first definite evidence of the transformation of matter was obtained from a study of the processes occurring in radioactive substances. The writer and Mr. Soddy in 1903 put forward the theory that the radiations from active matter accompanied a veritable transformation of the atoms themselves. The correctness of this theory as an explanation of radioactive phenomena is now generally accepted. As an illustration of these processes, consider the transformation of the radioactive element uranium. The series of substances which arise from the transformation of uranium are shown clearly in the diagram (Fig. 12). The best known of these elements is radium, which will be

taken as a typical example of a radioactive substance. Radium differs from an ordinary element in its power of spontaneously expelling alpha particles with very great speed. This property is ascribed to an inherent instability which is not manifest in the atoms of ordinary elements. A small fraction of the radium atoms—about one in 100,000 million—break up each second with explosive violence expelling a fragment of the atom—the alpha particle—with very great speed. The residue of the atom is lighter than before and becomes the atom of an entirely new substance, which is called the radium emanation. The atoms of the latter are far more unstable than those of radium, for half of them break up in 3.85 days, while half of the radium atoms break up in about 2,000 years. After the loss of an alpha particle, an atom of the emanation changes into an atom of a new substance radium A, which behaves as a solid. Radium A is very unstable, half of it breaking up in 3 minutes with the emission of an alpha particle, and gives rise to radium B. The latter differs from the substances already mentioned in the nature of its radiation, for it emits only beta rays but no alpha rays. Notwithstanding this fact, it is transformed according to the same law as an alpha ray substance, and gives rise to an entirely distinct element,