=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 Geitel[38] 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. Giesel[39] also separated some radio-active lead but found that its activity diminished with the time. On the other hand, Hofmann and Strauss[40] 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 work[41], it has been shown that radio-lead contains several radio-active constituents which can be removed temporarily from it by suitable chemical methods. There can be no doubt that the lead separated from pitchblende by certain methods does show considerable activity and that this activity is fairly permanent. The radio-active changes occurring in radio-lead are complicated and cannot be discussed with advantage at this stage, but will be considered in detail in chapter XI. It will there be shown that the primary constituent present in lead is a slow transformation product of radium. This substance then slowly changes into the active constituent present in polonium, which gives out only easily absorbed rays. This polonium can be separated temporarily from the lead by suitable chemical methods, but the radio-lead still continues to produce polonium, so that a fresh supply may be obtained from it, provided an interval of several months is allowed to elapse. It will be calculated later that in all probability the radio-lead would lose half of its activity in an interval of 40 years. The constituent present in radio-lead has not yet been separated, but it will be shown that, in the pure state, it should have an activity considerably greater than that of radium itself. Sufficient attention has not yet been paid to this substance, for, separated in a pure state, it should be as useful scientifically as radium. In addition, since it is the parent of polonium, it should be possible to obtain from it at any time a supply of very active polonium, in the same way that a supply of the radium emanation can be obtained at intervals from radium.