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 there be shown that polonium, that is, the radio-active constituent mixed with the bismuth, is a distinct chemical substance, which is allied in chemical properties to bismuth, but possesses some distinct analytical properties which allow of a partial separation from it.
The polonium, if obtained in a pure state, should initially be several hundred times as active as pure radium. This activity, however, is not permanent; it decays with the time, falling to half value in about six months.
The absence of any new lines in the spectrum of radio-active bismuth is to be expected, for, even in the most active bismuth prepared, the active matter exists in a very small proportion.
21. The discussion of the nature of polonium was renewed by the discovery of Marckwald36 that a substance similar to polonium can be separated from pitchblende; the activity of this substance, he stated, did not decay appreciably with the time. The method of separation from the bismuth chloride solution, obtained from uranium residues, was very simple. A rod of bismuth or antimony, dipped in the active solution, rapidly became coated with a black deposit which was intensely active. This process was continued until the whole of the activity was removed from the solution. The active deposit gave out only easily absorbed rays, and in that respect resembled the polonium of Mme Curie.
The active substance was found to consist mainly of tellurium, and for this reason Marckwald gave it the name of radio-tellurium. In later work, however, Marckwald37 has shown that the active constituent has no connection with tellurium, but can always be separated completely from it by a simple chemical process.