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.
While the proof of the identity of the active constituent in polonium is not so definite as for radio-tellurium, I think there can
