atoms known. Thus the atomic weight of uranium is 240, that of thorium 233, that of radium 225 or, according to a recent spectroscopic test, by Runge, 256. There is no other property in which these three substances are at all alike. In their chemical characteristics they are extremely different. Now, according to our modern mechanical theory of heat, the atoms of all substances are in extremely rapid rotation. It appears, therefore, that these rapidly rotating systems of heavy atoms, such as characterize radio-active substances, not infrequently become unstable and project off a part of their mass. These particles which are first projected were found to be the alpha particles, and this process of projecting the alpha particles is the first stage of radio-activity. The mass which is left behind, namely, the uranium X, the thorium X, or the emanation, according as the original atom was uranium, thorium or radium, is itself unstable, and projects still other particles. The remainder, at least in the case of thorium and radium, is still unstable, and another particle is projected. Thus we were able to follow the disintegration of the atoms through at least four (according to Eutherford, five) successive stages. How many more stages there may be no one can tell, but as soon as the stable condition is reached and no more particles are projected the product is of course no longer radioactive, and its presence can no longer be detected by the delicate test of radio-activity. It is then only after it has accumulated in sufficient quantity to be capable of detection by the ordinary methods, namely, by spectroscopic or chemical analysis, that it could be expected to be found.
The Birth of Helium.
More than two years ago Rutherford, with this picture of the mechanism of radio-activity in mind, made a prediction which has recently been most remarkably verified. The history of science scarcely affords a more striking instance of the fulfilment of scientific prophecy. Since 'helium' (the element which was first discovered in the sun, by means of a line in the solar spectrum which did not agree with the lines of any of our known elements, and which was discovered on the earth only a few years ago by Lord Rayleigh and Professor Ramsay) is found in nature only in connection with radio-active minerals, i. e., in connection with those minerals which contain uranium, thorium or radium, Rutherford predicted that helium would one day be found to be one of the ultimate products of the disintegration of the radio-active elements. A year later, it may be remembered, Rutherford himself found that the alpha particle, which is certainly one of the products of radio-active change, had about the same mass as the helium atom. This pointed still more strongly to the confirmation of his original prophesy. Last July Professor Ramsay and Mr. Soddy actually saw the spectrum of helium grow out of the emanation of radium. They collected the emanation from fifty milligrams of radium
