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.