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 Zerban43 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.
If, as we have suggested, thorium itself is not active, it is certainly a matter of surprise that ordinary commercial thorium and the purest chemical preparations show about the same activity. Such a result indicates that the methods of purification have not removed any of the radio-active constituent originally present.
Whatever the radio-active constituent in thorium may ultimately prove to be, it is undoubtedly not radium nor actinium nor any of the known radio-active substances.
In later chapters, the radio-activity of thorium will, for simplicity, be discussed on the assumption that thorium is itself a radio-active element. The analysis of the changes which occur will thus not refer to thorium itself but to the primary radio-active substance usually found associated with it. The conclusions to be drawn from an examination of the radio-active processes are for the most part independent of whether thorium is itself radio-active or whether the radio-activity is due to an unknown element. If thorium is not radio-active itself, it is not possible to draw any conclusions upon the question of the duration of the primary radio-activity associated with it. Such a deduction cannot be made until the quantity of the radio-active element present in thorium has been definitely determined.
24. If elements heavier than uranium exist, it is probable that they will be radio-active. The extreme delicacy of radio-activity as a means of chemical analysis would enable such elements to be recognized even if present in infinitesimal quantities. It is probable that considerably more than the three or four radio-elements at present recognized exist in minute quantity, and that the number at present known will be augmented in the future. In the first stage of the search, a purely chemical examination is of little value, for it is not probable that the new element should exist in sufficient quantity to be detected by chemical or spectroscopic analysis. The main criteria of importance are the existence or absence of distinctive radiations or emanations, and the permanence of the radio-activity. The discovery of a radio-active emanation with a rate of decay different from those already known would afford strong evidence that a new radio-active body was present. The presence of either thorium or radium in matter can very readily be detected by observing the rate of decay of the emanations given out by them. When once the existence of a new radio-element has been inferred by an examination of its radio-active properties, chemical methods of separation can be devised, the radiating or emanating property being used as a guide in qualitative and quantitative analysis.