We have seen that the two emanations and the products Ur X, Th X lose their activity with the time according to a simple exponential law, and at a rate that is independent—as far as observation has gone—of the chemical and physical agents at our disposal. The time taken for each of these products to fall to half its value is thus a definite physical constant which serves to distinguish it from all other products.
On the other hand, the variation of the excited activity produced by these emanations does not even approximately obey such a law. The rate of decay depends not only on the time of exposure to the respective emanations, but also, in the case of radium, on the type of radiation which is used as a means of comparative measurement. It will be shown, in succeeding chapters, that the complexity of the decay is due to the fact that the matter in the active deposits undergoes several successive transformations, and that the peculiarities of the curves of decay, obtained under different conditions, can be explained completely on the assumption that two changes occur in the active deposit from both thorium and actinium and six in the active deposit from radium.
195. Nomenclature. The nomenclature to be applied to
the numerous radio-active products is a question of great importance
and also one of considerable difficulty. Since there are at
least seven distinct substances produced from radium, and probably
five from thorium and actinium, it is neither advisable nor convenient
to give each a special name such as is applied to the
parent elements. At the same time, it is becoming more and
more necessary that each product should be labelled in such a
way as to indicate its place in the succession of changes. This
difficulty is especially felt in discussing the numerous changes in
the active deposits from the different emanations. Many of the
names attached to the products were given at the time of their
discovery, before their position in the scheme of changes was
understood. In this way the names Ur X, Th X were applied to
the active residues obtained by chemical treatment of uranium
and thorium. Since, in all probability, these substances are the
first products of the two elements, it may be advisable to retain
