ing that the radium is uniformly distributed throughout the earth. To account for this anomaly it is supposed that this latter assumption is not the case, and it has been suggested that the radium is all confined to a superficial shell 45 miles in thickness, and that the matter inside the earth is entirely free from radium. It is interesting to note that it has been deduced by Milne1 from considerations of the velocity of earthquakes that the interior of the earth is composed of homogeneous matter surrounded by a crust of a different nature about 30 miles thick; a conclusion which is in agreement with the deduction drawn by Strutt from a study of the distribution of radium in the earth. 1 Milne, Bakerian Lecture, Royal Society, 1906. CHAPTER IX THE RADIOACTIVE TRANSFORMATIONS OF URANIUM It has been pointed out in a previous chapter that the bodies radium, thorium, and actinium give rise to emanations, but that repeated and most careful searches have failed to reveal any analogous product from uranium. Now the emanations are radioactive gases, and it can hardly be regarded as more than a matter of chance that the three substances, radium, thorium, and actinium, should all give rise to products which happen to exist in the gaseous state at ordinary temperatures. It need therefore afford no real cause of surprise that such a product should not exist in the case of uranium. If, however, our views as to the nature of radio-activity are correct, we must regard uranium as being transformed into some other form of matter, and it becomes necessary to inquire whether any evidence of such a transformation exists. The evidence on this point was supplied by Sir William Crookes1 soon after the discovery of the phenomenon of radio-activity. It is well known that uranium can be precipitated from a solution of a salt by the addition of ammonium carbonate; the precipitate is, however, dissolved when an excess of the reagent is added. But if the experiment is performed with sufficient care it is found that there is a small light brown residue which remains undissolved. This residue was collected by Crookes and found on examination to be highly radioactive-indeed, more so than an equal quantity of uranium itself. Just as the activity of radium is temporarily diminished by the separation of the emanation, so the activity of the uranium from which the brown precipitate had been separated was diminished in activity. The activity of the uranium, however, gradually increased with time, whereas that of the 1 Crooke's Proc. Roy. Soc., LXVI, 409, 1900. brown precipitate, which Crookes called Uranium X, gradually lost its activity. The phenomenon will therefore be seen to be exactly analogous to the separation of radium emanation from radium, except that in the present case the product, uranium X, separated from the uranium, does not happen to be gaseous. The rate of decay of uranium X was measured and found to lose half its activity in twenty-two days. It gives out only B and y rays, whereas the uranium from which it has been separated gives out only a rays. Direct experiments have up till the present failed to reveal with certainty the presence of any other product from uranium, but there is considerable indirect evidence pointing to the conclusion that uranium X ultimately gives rise to radium and the series of radioactive bodies which we have already studied. This most important matter will be more fully considered in a later chapter. THORIUM AND ACTINIUM The important discovery made both by Hahn and Blanc of a highly radioactive sub stance which has been called radio-thorium, exhibiting all the radioactive properties of thorium, has already been mentioned in chapter III, p. 50. Long before this discovery, however, it was shown by Rutherford and Soddy' that a body analogous to uranium X could be separated from thorium. The separation was effected by precipitating the thorium from a solution of a salt by means of ammonia. The thorium so precipitated was found to have lost much of its activity, whereas the solution when evaporated to dryness was found to be strongly radioactive. This active residue, which by analogy with uranium was called thorium X, emitted only a rays, and was found to decay to half its activity in about four days. It had the property of giving off thorium emanation, whereas the freshly precipitated thorium had temporarily lost that power. It appears, therefore, that unlike radium, thorium does not give rise directly to an emanation, but that the formation of this gas is preceded by two changes, namely, from 1 Rutherford and Soddy, Journal of the Chemical Society, LXXXI, p. 837, 1902. |