CHAPTER IX.
THEORY OF SUCCESSIVE CHANGES.
193. Introduction. We have seen in previous chapters
that the radio-activity of the radio-elements is always accompanied
by the production of a series of new substances with some distinctive
physical and chemical properties. For example, thorium
produces from itself an intensely radio-active substance, Th X,
which can be separated from the thorium in consequence of its
solubility in ammonia. In addition, thorium gives rise to a gaseous
product, the thorium emanation, and also to another substance
which is deposited on the surface of bodies in the neighbourhood
of the thorium, where its presence is indicated by the phenomenon
known as "excited activity."
A close examination of the origin of these products shows that they are not produced simultaneously, but arise in consequence of a succession of changes originating in the radio-element. Thorium first of all gives rise to the product Th X. The Th X produces from itself the thorium emanation, and this in turn is transformed into a non-volatile substance. A similar series of changes is observed in radium, with the exception that there is no product in radium corresponding to the Th X in the case of thorium. Radium first of all produces an emanation, which, like thorium, is transformed into a non-volatile substance. In uranium only one product, Ur X, has been observed, for uranium does not give off an emanation and in consequence does not produce excited activity on bodies.
As a typical example of the evidence, from which it is deduced that one substance is the parent of another, we will consider the connection of the two products Th X and the thorium emanation. It has been shown (section 154) that after the separation of Th X
