434 



NATURE 



[July 26, 19 17 



that of the parent, though its atomic weight is reduced 

 four units by the loss of the o particle, making the 

 passage across the table curiously alternating. Thus 

 the product of radium (Group \\.) by an o-ray change 

 is the emanation in the zero group, ot ionium 

 (Group IV.) radium, and so on, while in the thorium 

 series thorium (Group IV.) produces by an a-ray change 

 mesothorium-I (Group il.), which in subsequent 

 changes in which no o rays are expelled yields radio- 

 thorium, back in Group IV. again ("Chemistry of the 

 Radio-Elements," p. 29, first edition, 191 1). Nothing 

 at that time could be said about ^-ray changes. The 

 products were for the most part very short-lived and im- 

 perfectly characterised chemically, and several lacuna; 

 still existed in the series, masking the simplicity of the 

 process. But early in 1913 the whole scheme became 

 clear, and was pointed out first by A. S. Russell, in 

 a slightly imperfect form, independently by K. Fajans 

 from electrochemical evidence, and by myself in full 

 knowledge of Fleck's results, still for the most part 

 unpublished, all within the same month of February. 

 It was found that, making the assumption that 

 uranium-X was in reality two successive products 

 giving yS rays, a prediction Fajans and Gohring proved 

 to be correct within a month, and a slight alteration 

 in the order at the beginning of the uranium series, 

 every a-ray change produced a shift of place as de- 

 scribed, and every j8-ray change a shift of one_ place 

 in the opposite direction. Further, and most signifi- 

 cantly, when the successive members of the three dis- 

 integration series were put in the places in the table 

 dictated by these two rules, it was found that all the 

 elements occupying the same place were those which 

 had been found to be non-separable by chemical pro- 

 cesses from one another, and from the element already 

 occupying that place, if it was occupied, before the 

 discovery of radio-activity. For this reason the term 

 isotope was coined to express an element chemically 

 non-separable from the other, the term signifying "the 

 same place." 



So arranged, the three series extended from uranium 

 to thallium, and the ultimate product of each series 

 fell into the place occupied by the element lead. The 

 ultimate products of thorium should, because six a. 

 particles are expelled in the process, have an atomic 

 weight twenty-four units less than the parent, or about 

 208. The, main ultimate product of uranium, since 

 eight a particles are expelled in this case, should have 

 the atomic weight 206. Th.^ atomic weight of 

 ordinary lead is 2072, which made it appear very likely 

 that ordinary lead was a mixture of the two isotopes, 

 derived from uranium and thorium. The prediction 

 follows that lead, separated from a thorium mineral, 

 should have an atomic w-eight about a unit higher, 

 and that separated from uranium minerals about - a 

 unit lower, than the atomic weight of common lead, 

 and in each case this has now been satisfactorily estab- 

 lished. 



The Atomic Weight of Lead from Radio-active 

 Minerals. 

 It should be said that Boltwood and also Holmes 

 had, from geological evidence, both decided definitely 

 against its being possible that lead was a product of 

 thorium, because thorium minerals contain too little lead, 

 in proportion to the thorium, to accord with their geo- 

 logical ages ; whereas the conclusion that lead was 

 the ultimate product of the uranium series had been 

 thoroughly established by geological evidence, and has 

 been the means, in the hands of skilful investigators, 

 of ascertaining geological ages with a degree of pre- 

 cision not hitherto possible. Fortunately I was not de- 

 terred by the non possumus, for it looks as if both 

 conclusions are right ! An explanation of this para- 

 dox will be attempted later. In point of 



NO. 2491, VOL. 99] 



fact, there are exceedingly few thorium minerals 

 that do not contain uranium, and since the 

 rate of change of uranium . is about 26 times 

 that of thorium, onj part of uranium is equal as a lead 

 producer to 2-6 parts of thorium. Thus Ceylon 

 thorianite, one of the richest of thorium minerals, con- 

 taining 60 to 70 per cent, of ThOj, may contain 10 

 to 20, and even 30, per cent, of UjOg, and the lead from 

 it may be expected to consist of very similar quantities 

 of the two isotopes, to be, in fact, very similar to 

 ordinary lead. I know of only one mineral which is 

 suitable for this test. It was discovered at the same 

 time as thorianite, and from the same locality, Ceylon 

 thorite, a hydrated silicate containing some 57 per cent, 

 of thorium and i per cent, of uranium only. In the 

 original analysis no lead was recorded, but I found it 

 contained 0-4 per cent., which, if it were derived from 

 uranium only, would indicate a very hoary ancestry, 

 comparable, indeed, with the period of average life of 

 uranium itself. On the other hand, if all the lead (1) 

 is of radio-active origin, (2) is stable, and (3) is derived 

 from both constituents as the generalisation being dis- 

 cussed indicated, this 04 f>er cent, of lead should con- 

 sist 95-5 per cent, of the thorium isotope and 45 per 

 cent, of the uranium isotope. Thorite thus offered an 

 extremely favourable case for examination. 



In preliminary experiments in conjunction with H. 

 Hyman, in which only a gram or less of the lead was 

 available, the atomic weight was found relatively to 

 ordinary lead to be perceptibly higher, and the differ- 

 ence, rather less than ^ per cent., was of the expected 

 order. 



I was so fortunate as to secure a lot of 30 kilos of 

 this unique mineral, which was first carefully sorted 

 piece by piece from admixed thorianite and doubtful 

 specimens. From the 20 kilos of first-grade thorite 

 the lead was separated, purified, reduced to metal, and 

 cast in vacuo into a cylinder, and its density deter- 

 mined, together with that of a cylinder of common 

 lead similarly purified and prepared. Sir Ernest 

 Rutherford's theory of atomic structure, to be dealt 

 with in the latter part of this discourse, and the whole 

 of our knowledge as to what isotopes were, made it 

 appear probable that their atomic volumes, like their 

 chemical character and spectra, should be identical, 

 and therefore that their density should be proportional 

 to their atomic weight. The thorite lead proved to be 

 026 per cent, denser than the common lead. Taking 

 the figure 2072 for the atomic weight of common lead, 

 the calculated atomic weight of the specimen should be 

 20774. 



The two specimens of lead were fractionally distilled 

 in vacuo, and a comparison of the atomic weights of 

 the two middle fractions made by a development of one 

 of Stas's methods. The lead was converted into nitrate 

 in a quartz vessel, and then into chloride by a current; 

 of hydrogen chloride, in which it was heated at gradu- 

 ally increasing temperature to constant weight. Onlyj 

 single determinations have been done, and they gave] 

 the values 20720 for ordinary lead, and 207694 for' 

 the thorite lead, figures that are in the ratio ofj 

 100 to 100-24. This therefore favoured the conclusion! 

 that the atomic volume of isotopes is constant. 



At the request of Mr. Lawson, interned in Austria.! 

 and continuing his researches at the Radium Institute 

 under Prof. Stefan Meyer, the first fraction of the 

 distilled thorite lead was sent him, so that the work ,;• 

 could be checked. He reports that Prof. Honigschmid 

 has carried through an atomic weight determination 

 by the silver method, obtaining the value 2o777±o-oi4 

 as the mean of eight determinations. Hence the con- 

 clusion that the atomic weight of lead derived from 

 thorite is higher than that of common lead has been 

 put beyond reasonable doubt. 



Practicallv simultaneouslv with the. first announce- 



