The explanation of the cause of the emission of these radiations by the various radioactive substances is also not yet clear, although a great step has been made in advance by Ruther ford and Soddy of :McGill University. They have shown that if to a salt of thorium (tho rium nitrate, for instance) ammonia be added, a precipitate is formed; and, if this be separated from the solution by filtration, the resulting filtrate, called thorium X, is found to he ex tremely radioactive, while, on the other hand, the precipitate is at first but slightly radio active. The thorium X, which can be sepa rated from the filtrate by evaporating the am monia, is found to lose its radioactivity with time, and to set fret the emanation simul taneously; while the precipitate referred to above, which is at first radioactive to only a slight degree, rapidly increases in activity. It is found further that, as the precipitate gains in activity, chemical changes go on of such a nature that, if it be now dissolved in ammonia, it will be seen that thorium X has been formed in the process. This means that starting with the almost inactive thorium, owing to the chem ical changes thorium X is formed in it as a definite substance, which may later be separated from it, and that coincident with these chemical changes the a and p radiations are emitted. In short, the radioactivity of thorium, and of the other substances also, is maintained by the continued production of new kinds of matter which have well-defined chemical properties en abling them to be separated from the original susbtance, and which possess temporary ac tivity. In a similar manner the thorium emarw tion is formed from the thorium X, and the thorium excited activity from the thorium emanation, etc. The final product, as stated above, and as was first conjectured by Rutherford, is the gas known as helium.
It has been shown that the changes just described do not take place simultaneously, but in the order given, and that the radioactivity of each of the compounds, for instance, thorium X, is connected not with the change in which it was produced, but with that by which the next substance is produced from it. It follows from this that a body which is radioactive must of necessity be undergoing change; and, therefore, it is not possible for any of the new forms of radioactive matter, for instance, thorium X, the emanations, etc., to be identical with any of the known elements. They remain in existence un changed for but a short time, and the decay of their radioactivity is an indication of their dim inishing quantity.
As has been described above, there is a definite series of changes by which thorium produces thorium X, thorium X produces thorium emana tion, and so on. This same series of steps holds to a certain extent with the other radiations so far as studied, although the series is in no other case so complete. Radium has been shown
to produce a radium emanation, and this to produce an excited activity, and so on the small amount of radium so far available for investigation has made it impossible to learn whether there is an intermediate product, radium X, between radium and its emanation. Uranium has been shown to produce uranium X; but an emanation from it has not yet been discovered. Both kinds of radiation, the a and p,. are not present to the same extent in all the transformations which occur in these various cases; and in some only the a radia tions have been detected and in others only the p radiations. It seems probable, however, that the production of the a radiations goes On quite independently of the p radiations, which is a secondary phenomenon, the a radia tions playing by far the most important part in the changes which take place in radioactive matter.
One of the most important questions con nected with radioactivity is that of the origin of the energy which is manifested by the ex pulsion of these rapidly moving particles. The Amounts of this energy have been measured ap proximately, and various theories have been advanced based upon obvious assumptions in regard to the nature of the atom and the mo tion in it of its parts, which are to a certain degree satisfactory. The atom is thought to consist of a definite space within which are moving many thousand particles negatively charged, and which are identical with the p radiations. If the atom is unstable, it is not difficult to see that some of these particles might be from time to time thrown nit, and a consistent theory of matter along these lines has been devised by Professor J. J. Thomson. One of the most interesting evidences of the intense energy furnished by these radioactive bodies is the fact that M. Curie has shown that in certain experiments radium sets free in a con tinuous manner energy in such a form as to main tain itself at a temperature higher than that of the surrounding bodies, and of such an amount that one gram of radium would raise the tempera ture of one hundred grams of water one degree Centigrade in an hour. A noteworthy fact is that the radioactive substances uranium, thorium, and radium are of very high atomic weights, standing at the end of their groups of elements in the Periodic Table; and that there fore they may mark a condition in which mat ter is on the verge of instability under present conditions of pressure and temperature.
For the more detailed statements in regard to the properties of radioactive bodies, refer ence should be made to articles by Rutherford in the Philosophical Magazinc, and to a popular article, "Some Recent Advances in Radio activity," by Professor Soddy, in the Contem porary Review, May, 1903.