of Hamburg, undoubtedly loses its activity with time. The radio-tellurium is obtained in the form of a thin radio-active deposit on a polished bismuth rod or plate. A bismuth rod was found to have lost half its activity in about 150 days, and a similar result has been recorded by other observers.
The two substances are thus similar in both radio-active and chemical properties, and there can be no reasonable doubt that the active constituent present in each case is the same. The evidence is discussed in detail in chapter XI and it will there be shown that the active substance present in the radio-tellurium of Marckwald is a slow transformation product of radium.
22. Radio-active lead. Several observers early noticed that the lead separated from pitchblende showed strong radio-active properties, but considerable difference of opinion was expressed in regard to the permanence of its activity. Elster and Geitel38 found that lead sulphate obtained from pitchblende was very active, but they considered that the activity was probably due to an admixture of radium or polonium with the lead, and, by suitable chemical treatment, the lead sulphate was obtained in an inactive state. Giesel39 also separated some radio-active lead but found that its activity diminished with the time. On the other hand, Hofmann and Strauss40 obtained lead from pitchblende whose activity seemed fairly permanent. They state that the radio-active lead resembled ordinary lead in most of its reactions, but showed differences in the behaviour of the sulphide and sulphate. The sulphate was found to be strongly phosphorescent. These results of Hofmann and Strauss were subjected at the time of their publication to considerable criticism, and there is no doubt that the lead itself is not radio-active but contains a small quantity of radio-active matter which is separated with it. In later work41, it has been shown that radio-lead contains several radio-active constituents which can be removed temporarily from it by suitable chemical methods.