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 [alpha] rays only, some emit [beta] rays, some emit two of the possible rays, as for instance, [beta] and [gamma], 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 [alpha] 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: 1. That there is a constant production of fresh radio-active matter by the radio-active body. 2. That the activity of the matter so formed decreases according to an exponential law with the time from the moment of its formation. These hypotheses agree with the experimental results. The decrease and rise of activity, for example, of uranium and uranium _X_, and also of thorium and thorium _X_, have been measured, plotted, and the equations worked out.