as uranium and radium, the activity of which appeared fairly permanent. This difference in behaviour is, however, one of degree rather than of kind. We shall show later that there is present in pitchblende a number of radio-active substances, the activity of which is not permanent. The time taken for these bodies to lose half of their activity varies in different cases from a few seconds to several hundreds of years. In fact, this gradual loss of activity is an essential feature of our theory of regarding the phenomena of radio-activity. No radio-active substance, left to itself, can continue to radiate indefinitely; it must ultimately lose its activity. In the case of bodies like uranium and radium, the loss of activity is so slow that no sensible alteration has been observed over a period of several years, but it can be deduced theoretically that the activity of radium will eventually decrease to half value in a period of about 1000 years, while in the case of a feebly radio-active substance like uranium, more than a 100 million years must elapse before the diminution of the activity becomes appreciable.
It may be of interest here to consider briefly the suggestions advanced at various times to account for the temporary character of the activity of polonium. Its association with bismuth led to the view that polonium was not a new active substance, but merely radio-active bismuth, that is, bismuth which in some way had been made active by admixture with radio-active bodies. It was known that a body placed in the vicinity of thorium or radium became temporarily active. The same action was supposed to take place when inactive matter was in solution with active matter. The non-active matter was supposed to acquire activity by "induction," as it was called, in consequence of its intimate contact with the active material.
There is no proof, however, that such is the case. The evidence points rather to the conclusion that the activity is due, not to an alteration of the inactive body itself, but to an admixture with it of a very small quantity of intensely active matter. This active matter is present in pitchblende and is separated with the bismuth but differs from it in chemical properties.
The subject cannot be considered with advantage at this stage, but will be discussed later in detail in chapter XI. It will
