We have already seen (section 21) that Marckwald, by special chemical methods, was able to obtain a few milligrams of very active substance by working over 2 tons of pitchblende. We have already seen (section 239) that this substance, if obtained in the pure state, should be about 400 times as active as radium. Comparative measurements of the activity of this substance with radium will thus indicate the amount of impurity that is present with the former. This method should be of value in purifying radium F for the purpose of determining its spectrum, which has not yet been observed. =241. Polonium.= Since the separation of the active substance by Marckwald, called by him radio-tellurium, there has been some discussion as to whether the active constituent is the same as that present in the polonium of Mme Curie. Both of these substances have similar radio-active and chemical properties, but the main objection to the view that the active constituents were identical has rested on an early statement of Marckwald that the activity of one of his very active preparations did not decay appreciably in the course of six months. This objection is now removed, for we have seen that the activity of radio-tellurium does decay fairly rapidly. It was early recognised that the activity of the polonium, separated from pitchblende by the methods of Mme Curie, was not permanent, but decayed with the time. Observations on the rate of decay have not been very precise, but Mme Curie states that some of her preparations lost half of their activity in about six months but in others the rate of decay was somewhat smaller. It is possible that the initial differences observed in the rates of decay of different specimens of polonium may be due to the presence of some radium D with the polonium. The polonium in my possession lost its activity fairly rapidly, and was reduced to a small portion of its value in the course of about four years. Rough observations of its activity, made from time to time, showed that its activity diminished to half value in about six months. If it is identical with radio-tellurium, the activity should decay to half value in 143 days, and I think there is little doubt that more accurate measurement will prove this to be the case.