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 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.
Hofmann and Strauss have observed a peculiar action of the cathode rays on the active lead sulphate separated by them. They state that the activity diminishes with time, but is recovered by exposure of the lead for a short time to the action of cathode rays. No such action is shown by the active lead sulphide. This effect is due most probably to the action of the cathode rays in causing a strong phosphorescence of the lead sulphate and has nothing to do with the radio-activity proper of the substance.
23. Is thorium a radio-active element? The similarity of the chemical properties of actinium and thorium has led to the suggestion at different times that the activity of thorium is not due to thorium itself, but to the presence of a slight trace of actinium. In view of the difference in the rate of decay of the emanations of thorium and actinium, this position is not tenable. If the activity of thorium were due to actinium, the two emanations, as well as the other products obtained from these substances, should have identical rates of decay. Since there is not the slightest evidence that the rate of decay of activity of the various products can be altered by chemical or physical agencies, we may conclude with confidence that whatever radio-active substance is responsible for the activity of thorium, it certainly is not actinium. This difference in the rate of decay of the active products is of far more weight in deciding the question whether two bodies contain the same radio-active constituent than differences in chemical behaviour, for it is quite probable that the active material in each case may exist only in minute quantity in the matter under examination, and, under such conditions, a direct chemical examination in the first place is of little value.
Recent work of Hofmann and Zerban and of Baskerville, however, certainly tends to show that the element thorium is itself non-radio-active, and that the radio-activity observed in ordinary thorium compounds is due to the admixture with it of an unknown radio-active element. Hofmann and Zerban[[42]] made a systematic examination of the radio-activity of thorium obtained from different mineral sources. They found generally that thorium, obtained from minerals containing a large percentage of uranium, were more active than those obtained from minerals nearly free from uranium. This indicates that the radio-activity observed in thorium may possibly be due to a transformation product of uranium which is closely allied chemically to thorium and is always separated with it. A small quantity of thorium obtained from the mineral gadolinite was found by Hofmann to be almost inactive, whether tested by the electric or by the photographic method. Later Baskerville and Zerban[[43]] found that thorium obtained from a Brazilian mineral was practically devoid of activity.
In this connection the recent work of Baskerville on the complexity of ordinary thorium is of interest. By special chemical methods, he succeeded in separating two new and distinct substances from thorium, which he has named carolinium and berzelium. Both of these substances are strongly radio-active, and it thus seems probable that the active constituent observed in ordinary thorium may be due to one of these elements.