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 from it, provided an interval of several months is allowed to elapse.

It will be calculated later that in all probability the radio-lead would lose half of its activity in an interval of 40 years.

The constituent present in radio-lead has not yet been separated, but it will be shown that, in the pure state, it should have an activity considerably greater than that of radium itself. Sufficient attention has not yet been paid to this substance, for, separated in a pure state, it should be as useful scientifically as radium. In addition, since it is the parent of polonium, it should be possible to obtain from it at any time a supply of very active polonium, in the same way that a supply of the radium emanation can be obtained at intervals from radium.

Hofmann and Strauss have observed a peculiar action of the cathode rays on the active lead sulphate separated by them. They state that the activity diminishes with time, but is recovered by exposure of the lead for a short time to the action of cathode rays. No such action is shown by the active lead sulphide. This effect is due most probably to the action of the cathode rays in causing a strong phosphorescence of the lead sulphate and has nothing to do with the radio-activity proper of the substance.

23. Is thorium a radio-active element? The similarity of the chemical properties of actinium and thorium has led to the suggestion at different times that the activity of thorium is not due to thorium itself, but to the presence of a slight trace of actinium. In view of the difference in the rate of decay of the emanations of thorium and actinium, this position is not tenable. If the activity of thorium were due to actinium, the two emanations, as well as the other products obtained from these substances, should have identical rates of decay. Since there is not the slightest evidence that the rate of decay of activity of the various products can be altered by chemical or physical agencies, we may conclude with confidence that whatever radio-active substance is responsible for the activity of thorium, it certainly is not actinium. This difference in the rate of decay of the active products is of far more weight in deciding the question whether two bodies contain the same radio-active constituent than differences in chemical