soon came to light. Soddy found that the two elements, radium and mesothorium, although quite dissimilar in radioactive properties, were chemically so identical that it was impossible by chemical methods to separate one from the other. Other cases of this kind had long been suspected, viz. thorium and radio- thorium, thorium and ionium, and radium D and lead. He named such inseparable elements isotopes, since they appeared to oc- cupy the same place in the periodic classification of the elements.
Following the chemical study of the radioelements by Soddy, Fleck and von Hevesy, an important generalization connecting the chemical properties of the radioelements was announced independently in 1913 by Russell, Fajans and Soddy. After the expulsion of an a particle from a radioactive substance, the resulting product shifts two places in the direction of diminishing mass when the elements are arranged in families according to the Mendeleef classification.
The expulsion of a j3 particle causes a shift of one place in the opposite direction. For example, by the loss of an a particle from ionium of group IV., the resulting product, radium, belongs to group II., while the loss of another particle gives rise to the emanation which occupies the group O, and so on. By this method the chemical properties of all the known radioelements can be predicted from a knowledge of the radiations emitted from the products. This generalization can be viewed from another important standpoint. From the work of Moseley, the properties of an element are defined by the atomic number which is believed to represent the resultant positive charge on the nucleus. The loss of an a particle of mass 4, carrying two positive charges, lowers the atomic number by two units, while the emission of a /3 particle raises it by one unit. On looking through the table of the radioelements given above, it will be seen that many of them can be grouped under the same atomic number. These represent the radioactive isotopes, of which some of the more important are given below, preceded by the atomic numbers:
81 Tellurium (204), thorium D (208), actinium D (206).
82 Lead (207), uranium-lead (206), thorium-lead (208), radium D
(210), thorium B (212), radium B (214), actinium B (210).
83 Bismuth (208), radium E (210).
84 Polonium (210), thorium A (216), radium A (218), actinium A
(214). 86 Radium emanation (222), thorium emanation (220), actinium
emanation (218). 88 Radium (226), thorium X (224), mesothorium (228), actinium
X (222). 90 Thorium (232), radiothorium (228), ionium (230), uranium Xi
(234), uranium Y (230), radioactinium (226).
It will be seen that many of the radioactive elements are isotopic with known chemical elements. These radioactive isotopes differ not only in atomic weight but also in radioactive properties. The isotopes of lead are of special interest as they include the end-products of the uranium, thorium and actinium series a question that will be discussed more fully later. It has been found that the X-ray spectrum of the y rays from radium B is identical with that given by ordinary lead exposed to cathode rays in a vacuum tube, a result to be anticipated from the identity of their atomic number. It is of interest to note that polonium is a new type of chemical element which has no counterpart among the ordinary inactive elements.
Transformation of Uranium. In 1900 the late Sir W. Crookes found that the /3-ray activity of ordinary uranium could be removed by a single chemical operation and concentrated in an active res- idue. This is due to the separation of the product uranium X, of period 24 days, which emits /3 and 7 rays. A complete analysis of the transformations of uranium has been a matter of much difficulty. Boltwood showed that the o-ray activity of uranium was about twice as great as that of a corresponding a-ray product in the urani- um-radium series, indicating that uranium contained two successive a-ray products: This was confirmed by Geiger, who showed that the a rays from uranium consisted of two groups with ranges 2-5 and 2-9 cms. respectively. These two a-ray substances, called uranium I. and uranium II., are isotopic, with atomic weights 238 and 234 respectively. The latter, whose period is estimated at about 2 million years, exists in relatively very small quantity com- pared with uranium I. Following the generalization connecting the radiations and chemical properties of the series of radioelements, Fajans predicted the presence of a new product with properties analogous to tantalum, and promptly succeeded in isolating it
experimentally. The new product uranium X 2 , sometimes called brevium, has a period of 1-15 minutes and emits swift rays. The series of changes is thus:
Ur. !.-> Ur. X ,- Ur. X r-> Ur. II.-> Ionium.
We have seen that Antonoff discovered another /3-ray substance called uranium Y, separated with uranium Xi, which has a period of 24-6 hours. This exists in too small quantity to be in the main line of succession, but is to be regarded as a branch product of ura- nium Xi and is believed to be the first element of the subsidiary actinium series.
Rutherford and Geiger found the number of a particles emitted per gram of uranium per second to be 2-37 X IO 4 . From this the period of uranium is calculated to be about 6,000 million years.
Thorium. The first product observed in thorium was the emana- tion of period 54 seconds, and this gives rise to the active deposit, which has been shown to consist of at least four successive products called thorium A, B, C, D. The emanation, after the emission of an a particle, changes into a product of very short life emitting a rays. Its period was found by Geiger and Moseley to be about i/io second. The succeeding product, thorium B, emits only weak j3 and 7 rays with a period of 10-6 hours, changing into thorium C of period one hour. We have seen that thorium C breaks up in a com- plex way, emitting three distinct groups of particles. Thorium D is readily separated from C by the method of recoil. It emits pene- trating /3 and 7 rays with a half-period of 3 minutes. The active deposit as a whole decays ultimately with the period of thorium B, viz. IO'6 hours.
A special interest attaches to the product thorium X, first sep- arated by Rutherford and Soddy, since experiments with it laid the foundation of the general theory of radioactive transformations.
A close analysis of thorium has led to the discovery by Harm of a number of other important products. When the thorium X is separated from a thorium mineral or old thorium preparation, there appears with it another product called mesothorium I, of period 6-7 years, which is transformed with the emission of weak /3 rays into mesothorium 2, of period 6 hours, which emits swift /3 particles and penetrating 7 rays. This changes into an a-ray product, radio- thorium, of period 2 years, which is transformed into thorium X.
Radiothorium is an isotope of thorium, while mesothorium I is an isotope of radium. The radiothorium can readily be separated from a solution of mesothorium and obtained in a concentrated form. Mesothorium when first separated would show a very weak activity, but in consequence of the growth of its subsequent product radiothorium, its activity would increase for several years. After reaching a maximum it would ultimately decay with the period of mesothorium, viz. 6'7 years.
Actinium. Actinium of period about 20 years is believed to emit weak /3 rays changing into radioactinium, an a-ray product of period 19 days, first separated by Hahn. This changes into actinium X, an a-ray product of period 1 1 days, discovered by Godlewski. Then follows the actinium emanation of period 3-9 seconds, which gives rise to four further products named actinium A, B, C, D. Actinium A has the shortest life of any product whose rate of trans- formation has been directly determined. Its period, as determined by Geiger and Moseley and Fajans, is -002 second. After emitting an a particle, A changes into B, a product of period 36 minutes emitting weak /3 and 7 rays, analogous to thorium B. Actinium C of period 2-16 minutes undergoes a complex transformation, giving rise to two distinct groups of a particles. The main branch gives rise to actinium D of period 4-8 minutes, which is readily isolated by the recoil method. Actinium D, which emits /3 and 7 rays, is analogous in all respects to thorium D.
In the discussion above on branch products it has been shown that the parent of actinium, called protoactinium, has been recently isolated by Hahn and Soddy. This substance emits a rays and has an estimated period of 10,000 years. We have seen that the actin- ium series is believed to have its origin in a dual transformation of uranium X. The first branch product, representing about 4 % of the total, is believed to be uranium Y, a /3-ray product of period one day. This is directly transformed into protoactinium.
While very active preparations of actinium have been made, it has not been found possible to separate it entirely from the rare earths with which it is mixed. Protoactinium exists in much larger amounts and should be ultimately obtained in a pure state.
End-products of the Transformations (re-stated). After the radioactive transformations have come to an end, each of the elements uranium, thorium and actinium should g ; ve rise to an end or final product, which may be a known element or an unknown element of very slow period of transformation. Since the expulsion of an a particle lowers the mass of the atom by four units, and there are eight a-ray products, the atomic weight of the end atom should be 2388X4 = 206. The atomic weight of radium by this rule should be 2383X4 = 226, a result in good accord with experiment. The atomic weight of the end-product of uranium is close to that of lead, viz. 207, and Boltwood early suggested that lead was the end-product of radium. Since in
