In 1896, Marconi brought out the first practically successful system of wireless telegraphy, Finsen demonstrated the usefulness of certain rays of the spectrum for treating certain skin diseases, and Becquerel discovered what have since been called the Becquerel rays. In experimenting with X-ray photography, he found that a sensitized plate, though covered with black paper, was acted on not only by X-rays, but also by the metal uranium and certain of its salts; and he also found that the mere presence of uranium made the contiguous air a conductor, as did the X-or Röntgen rays. The amazement caused by the discovery of such undreamed-of properties, especially in so commonplace a substance as uranium had been supposed to be, can easily be imagined; and it is plain why strenuous efforts were made at once by scientific people, to see if other substances did not possess those properties also. As a result, it was soon found that other bodies did possess them. To those bodies that seem to possess the quality of radiating activities of certain kinds, the adjective _radio-active_ has been applied. The most important radio-active elements are uranium, thorium and radium, of which the last is immeasurably the most active and important. Radium was discovered in 1898 by M. and Madame Curie and M. Bémont, while experimenting with the uranium mineral pitchblende. It seemed to some people at the time to challenge the theory of the conservation of energy, and to threaten the destruction of the whole science of Physics, by emanating energy without loss to itself. It has since been found, of course, that radium does give up part of its substance; that it disintegrates in fact, as a result of its emanations. How great an influence the discovery of radium is going to exert, it is now impossible to predict with confidence; but it is manifest that the three successive and allied discoveries of cathode rays, X-rays and radium have introduced a new and growing science into the Machine; and it is seemingly possible that that science may, soon or tardily, ascertain the nature of the atom, and even teach us to divide it. It seems that an atom of radium does actually disintegrate, and by disintegrating give out energy. The energy it gives out is so enormous in proportion to the mass which gives it out, as to suggest to us an almost infinite source of available power, if other substances can be made to disintegrate. It is said that one gramme of radium can emit a quantity of heat of about 100 calories per hour; that is enough heat to raise 100 grammes of water a 1° centigrade in temperature, _by simply existing_. It is true that radium is the most expensive article in the world; but that is only because of the difficulties of obtaining it at present. Now if radium is so potentially powerful and disintegrates so easily, it seems possible that other substances less easily disintegrable could emit greater energy, if (or when) a means is discovered for disintegrating them.