Page:Popular Science Monthly Volume 67.djvu/29

Rh The radioactive products derived from each radio-element together with the type of radiation emitted during their disintegration, are shown graphically in Fig. 1.

The radiations from actinium have not been so far examined sufficiently closely to determine the character of the radiation emitted by each product. There is some evidence that a product, actinium X, exists in actinium corresponding to Th X in thorium. It has not, however, been very closely examined.

The question of nomenclature for the successive products is important. The names Ur X, Th X have been retained and also the term emanation. The emanation from the three radio-elements in each case gives rise to a non-volatile type of matter which is deposited on the surface of the bodies. The matter initially deposited from the radium emanation is called radium A. Radium A changes into B and B into C, and so on. A similar nomenclature is applied to the further products of the emanation of thorium and actinium. This notation is simple and elastic and is very useful in mathematical discussion of the theory of successive changes. In the following table a list of the products is given, together with the nature of the radiation and the most marked chemical and physical properties of each product. The time T for each of the products to be half transformed is also added.

The changes which occur in the active deposits from the emanation of radium, thorium and actinium have been difficult to determine on account of their complexity. For example, in the case of radium, the active deposit obtained as a result of a long exposure to the emanation contains quantities of radium A, B and C. The changes occurring in the active deposit of radium have been determined by P. Curie, Danne and the writer. The value of T for the three successive changes is 3, 21 and 28 minutes, respectively. Radium A gives only a rays, B gives out no rays at all, while C gives out α, β and γ rays. These results have been deduced by the comparison of the change of activity with time, with the mathematical theory of successive changes. The variation of the activity with time depends upon whether the activity is measured by the α, β or γ rays. The complicated curves are very completely explained on the hypothesis of three successive changes of the character already mentioned.

The activity of a vessel in which the radium emanation has been stored for some time rapidly falls to a very small fraction after the emanation is withdrawn. However, there always remains a slight residual activity. The writer has recently examined the activity in