Page:Radio-activity.djvu/487

 *mediary substance between uranium and radium. If it could be shown that the amount of actinium in radio-active minerals is, like radium, proportional to the amount of uranium, this would afford indirect proof of such a connection. It is not so simple to settle this point for actinium as for radium, since actinium gives out a very short-lived emanation, and the methods adopted to determine the content of radium in minerals cannot be applied without considerable modifications to determine the amount of actinium present.

The experimental data, so far obtained, do not throw much light upon the origin of the primary active matter in thorium. Hofmann and others (section 23) have shown that thorium separated from minerals containing uranium is always more active the greater the quantity of uranium present. This would indicate that the active substance in thorium also may be derived from uranium.

While much work still remains to be done, a promising beginning has already been made in determining the origin and relation of the radio-elements. We have seen that the connection between polonium, radio-tellurium, and radio-lead with radium has already been established. Radium itself is now added to the list, and it is probable that actinium will soon follow.

While the experiments undoubtedly show that there is a definite relation between the amount of uranium and radium present in the ordinary radio-active minerals, Danne has recently called attention to a very interesting apparent exception. Considerable quantities of radium were found in certain deposits in the neighbourhood of Issy-l'Evêque in the Saône-Loire district, although no trace of uranium was present. The active matter is found in pyromorphite (phosphate of lead), in clays containing lead, and in pegmatite, but the radium is usually present in greater quantities in the former. The pyromorphite is found in veins of the quartz and felspar rocks. The veins are always wet owing to the presence of a number of springs in the neighbourhood. The content of uranium in the pyromorphite varies considerably, but Danne considers that about a centigram of radium is present per ton. It seems probable that the radium found in this locality