Now, these are ordinary chemical processes for the separation of distinct chemical individuals. The results, therefore, lead naturally to the conclusions: (1) it would seem that uranium and thorium are themselves inactive and the activity is due to some other substance formed by these elements; (2) this active substance is produced by some transformation in those elements, for on standing the activity is regained. This latter conclusion is startling, for it indicates a change in the atom which, up to the time of this discovery, was deemed unchangeable under the influence of such physical and chemical changes as were known to us.
Search for New Radio-active Bodies
The search for new radio-active bodies and the study of their characteristics has been systematically and successfully carried on. The bodies obtained in the above experiments were named uranium X and thorium X, respectively. Further, it became clear from the investigation of uranium minerals that radium, polonium, actinium, and ionium originated from uranium. From thorium minerals a body was separated called mesothorium, which was analogous to radium. Both thorium and radium were found to give off a radio-active gas. The first lost half of its activity in less than one minute. The second was more stable and lost half of its activity in about four days. The name radium emanation was given to the latter and it was found chemically and physically to belong to the class of monatomic or noble gases, such as helium, argon, neon, etc., which had been discovered by Ramsay. In some cases the chemical action was determined and these new bodies were found analogous to well-known elements, as radium to barium, polonium to bismuth. The physical properties were investigated and, where possible, spectra were mapped and atomic weights determined.
It is clear, therefore, that these bodies are elemental in character and as such are made up of distinct, similar atoms, just as the commonly recognized elements are believed to be. In this way more than thirty new elements have been added to the list. These new elements are called radio-active elements, but it is an open question whether all atoms do not possess this property in greater or less degree. Certainly, it is possessed in varying degree by four of the old elements widely separated in the Periodic System, namely, uranium, thorium, rubidium, and potassium. The last two, while feebly active themselves, do not form any secondary radio-active substance so far as is known. Only two of the elements, then, can definitely be said to go through these transformations. It is just possible that radio-activity may be found to be a common property of all atoms and of all matter.
Methods of Investigation
It is important to know how these new bodies were discovered and distinguished from one another. Two properties are relied upon. One is the nature of the rays emitted and the other is the duration of the activity. Of course, knowledge of the physical and chemical properties is also of great importance whenever obtainable.
Nature of the Radiations
The nature of the radiation is a distinguishing characteristic, though similarity here does not prove identity of substances. Some emit α rays only, some emit β rays, some emit two of the possible rays, as for instance, β and γ, and some emit all three. The rays may also differ in the velocity with which they are emitted by different radio-active substances. Thus, in the case of one substance the α rays may have a slightly greater or less penetrating power than those emitted by some other substance, and this may be true also of the other rays.
Life Periods
The duration of the activity is called the life period. This is absolutely fixed for each body and furnishes the most important mode of differentiating among them. It measures the relative stability and is the time which must elapse before their activity is lost and they, changing into something else, entirely disappear. The measure usually adopted is the half-value period. Two hypotheses are made use of: