Radium F, after separation, will give out only α rays. Its activity, after separation, will decrease according to an exponential law, falling to half value in 143 days. Since radium in radio-active equilibrium contains four products which emit α rays, the number of α particles expelled per second from radium F will, weight for weight, be about 800 times as numerous as from new radium in radio-active equilibrium. Since the α particles from radium F produce only about half as much ionization as the α particles from the other radium products, the activity of radium F, measured by the electric method, will be about 400 times that of radium.
240. Origin of radio-tellurium and polonium. It is now necessary to consider whether these products of radium have been previously separated from pitchblende, and known by other names.
We shall first consider the α ray product, radium F. The radio-tellurium of Marckwald and the polonium of Mme Curie both resemble radium F in giving out only α rays, and in being deposited on a bismuth disk from a solution. If the active constituent present in radio-tellurium is the same as radium F, its activity should decay at the same rate as the latter. The writer[[320]] has carefully compared the rates of decay of the activity of radium F and of the radio-tellurium of Marckwald and found them to be the same within the limits of experimental error. Both lose half of their activity in about 143 days[[321]]. A similar value of the rate of decay of radio-tellurium has been obtained by Meyer and Schweidler[[322]].
The experiments on radio-tellurium were made upon the active bismuth plates supplied by Dr Sthamer of Hamburg, which were prepared under Marckwald’s directions.
An additional proof[[323]] of the identity of these two products was obtained by comparing the absorption of the α rays by aluminium foil. The α rays from different products are projected with different velocities, and, in consequence, are unequally absorbed by matter. The absorption of the rays from the two products by aluminium foil agreed very closely, indicating the probable identity of the substances from which they were emitted.
There can thus be no doubt that the active constituent present in the radio-tellurium of Marckwald is identical with the product radium F. This is a very interesting result, and shows how the close examination of the successive transformations of the radio-active bodies may throw light on the origin of the various substances found in pitchblende.
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 be no reasonable doubt that these substances both contain the same active substance, which is the seventh transformation product of radium. Marckwald has noticed some chemical differences in the behaviour of polonium and radio-tellurium, but little weight can be attached to such observations, for it must be remembered that the active constituent in both cases is present in minute quantity in the material under examination, and that the apparent chemical properties of the active substance are much influenced by the presence of impurities. The most important and trustworthy test rests upon the identity of the radiations and the period of decay.
241 A. Origin of radio-active lead. Some experiments will now be discussed which show that the radio-lead first separated from pitchblende by Hofmann ([section 22]) contains the products radium D, E and F. Hofmann has observed that the activity of this substance did not appreciably decay in the course of several years. In some recent experiments, Hofmann, Gonder and Wölfl[[324]] have made a close chemical examination of the radio-active lead, and have shown the presence of two radio-active constituents, which are probably identical with the products radium E and F. The radio-active measurements were unfortunately not very precise, and the periods of change of the separated products have not been examined very closely.