indicates that an enormous store of latent energy is resident in the radio-atoms themselves. This store of energy has not been observed previously, on account of the impossibility of breaking up into simpler forms the atoms of the elements by the action of the chemical or physical forces at our command.
On this theory we are witnessing in the radio-active bodies a veritable transformation of matter. This process of disintegration was investigated, not by direct chemical methods, but by means of the property possessed by the radio-active bodies of giving out specific types of radiation. Except in the case of a very active element like radium, the process of disintegration takes place so slowly, that hundreds if not thousands of years would be required before the amount transformed would come within the range of detection of the balance or the spectroscope. In radium, however, the process of disintegration takes place at such a rate that it should be possible within a limited space of time to obtain definite chemical evidence on this question. The recent discovery that helium can be obtained from radium adds strong confirmation to the theory; for helium was indicated as a probable disintegration product of the radio-active elements before this experimental evidence was forthcoming. Several products of the transformation of the radio-active bodies have already been examined, and the further study of these substances promises to open up new and important fields of chemical enquiry.
In this book the experimental facts of radio-activity and the connection between them are interpreted on the disintegration theory. Many of the phenomena observed can be investigated in a quantitative manner, and prominence has been given to work of this character, for the agreement of any theory with the facts, which it attempts to explain, must ultimately depend upon the results of accurate measurement.
The value of any working theory depends upon the number of experimental facts it serves to correlate, and upon its power of suggesting new lines of work. In these respects the disintegration theory, whether or not it may ultimately be proved to be correct, has already been justified by its results.
2. Radio-active Substances. The term “radio-active” is now generally applied to a class of substances, such as uranium, thorium, radium, and their compounds, which possess the property of spontaneously emitting radiations capable of passing through plates of metal and other substances opaque to ordinary light. The characteristic property of these radiations, besides their penetrating power, is their action on a photographic plate and their power of discharging electrified bodies. In addition, a strongly radio-active body like radium is able to cause marked phosphorescence and fluorescence on some substances placed near it. In the above respects the radiations possess properties analogous to Röntgen rays, but it will be shown that, for the major part of the radiations emitted, the resemblance is only superficial.
The most remarkable property of the radio-active bodies is their power of radiating energy spontaneously and continuously at a constant rate, without, as far as is known, the action upon them of any external exciting cause. The phenomena at first sight appear to be in direct contradiction to the law of conservation of energy, since no obvious change with time occurs in the radiating material. The phenomena appear still more remarkable when it is considered that the radio-active bodies must have been steadily radiating energy since the time of their formation in the earth’s crust.
Immediately after Röntgen’s discovery of the production of X rays, several physicists were led to examine if any natural bodies possessed the property of giving out radiations which could penetrate metals and other substances opaque to light. As the production of X rays seemed to be connected in some way with cathode rays, which cause strong fluorescent and phosphorescent effects on various bodies, the substances first