In the latter year the physicist E. Rutherford and the chemist F. Soddy, while investigating the radioactivity of thorium in the laboratories of McGill University, Montreal, were forced to recognize that thorium continuously gives rise to new kinds of radioactive matter differing from itself in chemical properties, in stability, and in radiant energy. They concurred in the view held by all the most prominent workers in this subject, namely, that radioactivity is an atomic phenomenon. It is not molecular decomposition. They declared that the radioactive substances must be undergoing a spontaneous transformation. The daring nature of this hypothesis and its likelihood to revolutionize physical science is brought home to one by recalling that three decades previously an eminent physicist had said that "though in the course of ages catastrophes have occurred and may yet occur in the heavens, though ancient systems may be dissolved and new systems evolved out of their ruins, the molecules [atoms] out of which these systems are built--the foundation stones of the material universe--remain unbroken and unworn." In 1903 Rutherford and Soddy stated definitely their hypothesis, generally known as the "Transformation Theory," that the atoms of radioactive substances suffer spontaneous disintegration, a process unaffected by great changes of temperature (or by physical or chemical changes of any kind at the disposal of the experimenter) and giving rise to new radioactive substances differing in chemical (and physical) properties from the parent elements. The radiations consist of α particles (atoms of helium minus two negative electrons), β particles, or electrons (charges of negative electricity), and γ rays, of the nature of Röntgen rays and light but of very much shorter wave length and of very great penetrating power. It is by the energy inherent in the atom of the radioactive substance that the radiations are ejected, sometimes, in the case of the γ rays, with velocity sufficient to penetrate two feet of lead. It is through these radiations that spontaneous transformation takes place. After ten years of further investigation Rutherford stated that this hypothesis affords a satisfactory explanation of all radioactive phenomena, and gives unity to what without it would seem disconnected facts. Besides accounting for old experimental results it suggests new lines of work and even enables one to predict the outcome of further investigation. It does not really contradict, as some thought might be the case, the principle of the conservation of energy. The atom, to be sure, can no longer be considered the smallest unit of matter, as the mass of a β particle is approximately one seventeen-hundredths that of an atom of hydrogen. Still the new hypothesis is a modification and not a contradiction of the atomic theory.