The most recent results in the study of radioactivity point to the conclusion that elements which differ in atomic weight and radioactive properties may be chemically identical, or at least chemically inseparable; such elements have been termed isotopes. The end product of the thorium series of radio-elements should have an atomic weight of about 208·4, and it has been suggested that the element actually produced in this series of changes may be bismuth. The latest results, however, rather point to the conclusion that disintegration in the thorium series gives rise to an isotope of lead. If this hypothesis be true, the lead derived from a mineral rich in thorium and poor in uranium should have an atomic weight appreciably higher than that of ordinary lead. Experiments to test this conclusion have recently been carried out by Soddy and Hyman.[145]
[145] Proc. Chem. Soc. 1914, 30, 134.
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.