Page:EB1911 - Volume 22.djvu/817

From Wikisource
Jump to navigation Jump to search
This page needs to be proofread.

In order, however, to obtain a direct proof of the genetic relation between uranium and radium, it is necessary to show that radium appears after some time in a uranium compound from which all trace of radium has been initially removed. It can readily be calculated that the growth of radium should be easily observed by the emanation method in the course of one week, using a kilogram of uranium nitrate. Experiments of this kind were first made by Soddy (37), but initially no definite evidence was obtained that radium grew in the solution at all. The rate of production of radium, if it took place at all, was certainly less than 1/10.000th part of the amount to be expected if uranium were transformed directly into radium. It thus appeared probable that one or more products of slow period of transformation existed between uranium and radium. Since uranium must be transformed through these intermediate stages before radium appears, it is evident that the initial rate of production of radium under these conditions might be extremely small. This conclusion has been confirmed by Soddy, who has shown that radium does appear in the solution which has been placed aside for several years.

Since the direct parent of radium must be present in radioactive minerals, one of the constituents separated from the mineral must grow radium. This was shown to be the case by Boltwood (38), who found that actinium preparations produced radium at a fairly rapid rate. By the work of Rutherford and Boltwood, it was found that the growth of radium was not due to actinium itself, but to a new substance separated in some cases with the actinium. This new substance, which emits a rays, was separated by Boltwood (38), and called by him “ Ionium.” It has chemical properties-very similar to thorium. Soddy has shown that the period of ionium is probably not less than 20,000 years, indicating that ionium must exist in uranium minerals in not less than ten times the quantity of radium. It has not yet been directly shown that uranium produces ionium, but there can be no doubt that it does do so. Since ionium produces' radium, Boltwood (38) has determined, by direct experiment that radium is half transformed in 2000 years-a number in good agreement with other data on that subject. The constant relation between uranium and radium will only hold for old minerals where there has been no opportunity for chemical alteration or removal of its constituents by the action of percolating water or other agencies. It .is quite possible that altered minerals of no great age will not show this constant relation. It seems probable that this is the explanation of some results of Mlle Gleditsch, where the relation between uranium and radium has been found not to be constant for some mineral specimens.

Connexion of the Radioelements.—We have already seen that a number of slowly transforming radioactive substances, viz. polonium (radium F), radiolead (radium D) and ionium are linked up to the uranium-radium series of transformations. Boltwood (39) has made a systematic examination of the relative activity in the form of very thin films due to each of the products present in the uranium-radium family. The results are shown in the following table, where the activity of pure uranium itself is taken as unity:—

Uranium 1.00
Ionium 0.31
Radium 0.45
Emanation 0.62
Radium A 0.54
Radium B 0.04(?)
Radium C 0.91
Radium F 0.46
Actinium and its products 0.28

Total activity mineral, 4.64 times uranium.

Taking into account the differences in the ionization due to an α particle from the various products, the results indicate that uranium expels two α particles for one from each of the other α ray products in the series of transformations. This indicates either that two particles are expelled during the transformation of the atom of uranium, or that another α ray product is present which has so far not been separated from the uranium.

Although thorium is nearly always present in old uranium minerals and uranium in thorium minerals, there does not appear to be any radioactive connexion between these two elements. Uranium and thorium are to be regarded as two distinct radioactive elements. With regard to actinium, there is still no definite information of its place in the scheme of transformations. Boltwood has shown that the amount of actinium in uranium minerals is proportional to the content of uranium. This indicates that actinium, like radium, is in genetic connexion with uranium. On the other hand, the activity of actinium with its series of α ray products is less than that of radium itself or uranium. In order to explain this anomaly, Rutherford has suggested that at a certain stage of disintegration of the uranium-radium series, the disintegration is complex, and two distinct kinds of matter appear, one in much larger quantity than the other. On this view, the smaller fraction is actinium, so that the latter is a branch. descendant of the main uranium-radium series.

End Products of Transformation.—It is now definitely established that the α particle expelled from any type of radioactive matter is an atom of helium, so that helium is a necessary accompaniment of radioactive changes involving the expulsion of a particles. After the radioactive transformations have come to an end, each of the elements uranium and thorium and actinium should give rise to an end or final product, which may be either a known element or some unknown element of very slow period of transformation. Supposing, as seems probable, that the expulsion of an α particle lowers the atomic weight of an element by four units-the atomic weight of helium-the atomic weights of each of the products in the uranium and radium series can be simply calculated. Since uranium expels two α particles, the atomic weight of the next ray product, ionium, is 238.5-8 or 230.5. The atomic weight of radium comes out to be 266.5, a number in good agreement with the experimental value. Similarly the atomic weight of polonium is 210.5, and that of the final product after the transformation of polonium should be 206.5. This value is very close to the atomic weight of lead, and indicates that this substance is the final product of the transformation of radium.

This suggestion was first put forward by Boltwood (40), who has collected a large amount of evidence bearing on this subject. Since in old minerals the transformations have been in progress for periods of time, in some cases measured by hundreds of millions of years, it is obvious that the end product, if a stable element, should be an invariable companion of the radio element and be present in considerable quantity. Boltwood has shown that lead always occurs in radioactive minerals, and in many cases in amount about that to be expected from their uranium. content and age. It is difficult to settle definitely this very important problem until it can be experimentally shown that radium is transformed into lead, or, what should prove simpler in practice, that polonium changes into helium and lead. Unfortunately for a solution of this problem within a reasonable time, a very large quantity of polonium would be necessary. Mme. Curie and Debierne have obtained a very active preparation of polonium containing about 1/10th milligram of pure polonium. Rutherford and Boltwood and Curie and Debierne have both independently shown that polonium produces helium -a result to be expected, since it emits α particles.

Production of Helium.—In 1902 Rutherford and Soddy suggested that the helium which is invariably found in radioactive minerals was derived from the disintegration of radioactive matter. In 1903 Ramsay and Soddy definitely showed that helium was produced by radium and also by its emanation. From the observed mass of the α particle, it seemed probable from the first that the α particle was an atom of helium. This conclusion was confirmed by the work of Rutherford and Geiger (41), who showed that the α particle was an atom of helium carrying two unit charges of electricity. In order to prove definitely this relation, it was necessary to show that the a particles, quite independently of the active matter from which they were expelled, gave rise to helium. This was done by Rutherford and Royds (42), who allowed the α particles from a large quantity of emanation to be fired through the