These authors have made analyses of Ceylon thorite, which they find to contain 0·35 per cent. of lead; from the ratio of thorium to uranium in the mineral, they calculate that the lead should have an atomic weight of 208·2, that of ordinary lead being 207·1. Preliminary comparative experiments on 1 gram of pure lead chloride extracted from the mineral point to an atomic weight for the thorite lead of 208·4, a result surprisingly in accord with theory. More extended experiments on this most interesting question are in progress. The present chapter would be incomplete without a reference to the interesting work of Goldsmidt on radioactivity as an aid in identifying mineral species.[146] He describes a simple method by which the activity of a mineral may be rapidly and easily measured to a sufficient degree of approximation, and shows how the determination enables a line to be drawn on a diagram already mapped out; this line will intersect an area on the diagram which corresponds to the particular mineral. Owing to lack of analytical data, and to the great difficulty of determining with accuracy small quantities of uranium and thorium, the method is at present of scientific interest only; but it is capable of development, and its development would be of undoubted value in the further study of this branch of radioactivity. [146] _Zeitsch. Kryst. Min._ 1907-8, ~44~, 545; _ibid._ 1908, ~45~, 490. In order to make this part of the subject as clear as possible, the chief points in this chapter are summarised as follows: 1. Radioactivity is only observed to an appreciable extent in some rather rare minerals. These minerals as a rule contain radium, uranium, thorium, rare earths, and helium. 2. The helium has been produced during geological time by the degradation of one or more members of the three series of active elements (the Uranium, Actinium, and Thorium series). 3. Radium is a degradation product of uranium, and itself is degraded continuously; the final product of degradation is probably lead. 4. The age of minerals has been calculated from the ratio of lead to uranium; the figures obtained are much greater than those put forward by geologists and physicists. 5. The helium ratio has also been used, but appears less trustworthy, owing to escape of helium, and uncertainty as to geological age of the minerals employed. 6. Some connection between radioactivity and the presence of the yttrium or cerium metals appears highly probable, but no satisfactory theories have been advanced on this point; it has been shown that actinium is very closely allied to lanthanum. PART II THE CHEMISTRY OF THE ELEMENTS CHAPTER IX GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS