March 27, 19 19] 



NATURE 



The particular subject chosen, namely, the problem 

 of radio-active lead, is one of peculiar and^ ex rS 

 ordinary interest, because it involves a readjustment 

 and enlargement of many rather firmly fixid Sea 

 concerning the chemical elements and their mutual 

 relations, as well as the nature of atoms 



Within the last twenty years the definition of these 

 uva words, ''elements" and "atoms," has been ren- 

 dered somewhat uncertain, and bids fair to suffer even 

 further change. Both of them are ancient words 

 both even ■ • - 



75 



and 



century since had acquired meanings 

 different from those of long ago. Thales thought of 

 but one element, and Aristotle's elements— earth air 

 hre, water, and the quintessence, derived perhaps 

 from yet more ancient philosophy— were not plentiful 

 enough to account for all the manifold phenomena of 

 -Nature. Democritus's old idea of the atom was asso- 

 ciated with the philosophical conception of indivisibility 

 rather than with the idea of chemical combination 

 in definite proportions. To-dav many chemists and 

 physicists think that the chemical atoms of the last 

 eontury are no longer to be considered as indivisible. 

 In that case, the old Greek name "atom" is no longer 

 fitting, because it denotes indivisibility. Someone has 

 .ven facetiously suggested that the word " tom "-indi- 

 cating divisibility— would be more appropriate ! More- 

 over, if our so-called atoms are really divisible, . we 

 cannot but be somewhat doubtful as to our definition 

 of the ultimate elements of the universe. The reason 

 for this new turn of thought is due, as you all know, 

 to the discovery of the unexpected and startling phenol 

 inena of radio-activity. 



To-night we have to deal with a substance directly 

 concerned with the iconoclastic radio-active changes-^ 

 with the very phenomena which cause us to stop and 

 think about our definitions of atoms and elements. 

 For the lead obtained from radio-active minerals 

 appears to have resulted, together with helium, from 

 the radio-active decomposition of elements of higher 

 atomic weight. Sceptical at first, the whole chemical 

 world has now come to acknowledge that the well- 

 defined element helium (discovered by Sir William 

 Ramsay twenty-three years ago) is one of the decom- 

 position products of radium. Radium itself is a sub- 

 stance which, in many respects, acts as an element, 

 with 226 as its atomic weight, and must be considered 

 as the heaviest member of the well-known calcium 

 family ; but its atoms appear to be so big and so com- 

 plex as to disintegrate because of lack of stability. The 

 disintegration is slow, and not to be hastened or re- 

 tarded by any agency known to man ; 1670 years 

 are demanded for the decomposition of half of any 

 given portion of radium, according to the exact 

 measurements of Profs. Boltvvood and Ellen Gleditsch. 

 Moreover, we have reason to believe that this decom- 

 position proceeds in a series of stages, successive atoms 

 of helium (five in all) being evolved with different 

 degrees of ease by any given atom of radium. In 

 the end most — indeed, probably all — of the residual 

 part of the radium appears to have been converted 

 into the peculiar kind of metallic lead with which 

 we are concerned to-night. The nature of the end- 

 product was first suggested by Boltwood, who pointed 

 out the invariable presence of lead in radium minerals. 

 Thus we must accept a kind of limited transmutation 

 of the elements, although not of the immediately 

 profitable type sought by the ancient alchemists. 



Intere'sting and significant as all this is, never- 

 theless the whole story has not yet been told. Radium 

 itself appears to come from the exceedingly slow 

 decomposition of uranium, an inference drawn 

 from the fact that radium is found only in conjunc- 

 tion with the uranium, which, even after careful puri- 

 fication, soon becomes radio-active, and gives every 

 indication of suffering slow disintegration. Moreover, 

 NO. 2578, VOL. 103] 



uranium is not the only other heavy element which 

 appears to be capable of decomposing and yie ding 

 elements of lower atomic weight.' Another, thorium^ 

 has a like propensity, although the steps in this caTe 

 are perhaps not so fully interpreted, or s^ generaUv 

 accepted In the process of disintegration fu these 

 heavy atoms yield strange radiations, some o hem 



fh.Tnn'.°''fl'"'''f ^','^' ^-''''»>'^' ^^hich bear awa^ 

 that part of the colossal energy of disintegration not 

 made manifest as heat. Thes? facts have been proved 

 beyond doubt by the brilliant work of Mme. cCrie 

 Sir Ernest Rutherford, and others. 



The nature of the rays and of the highly interestin 

 evanescent transition products and their relation '. 

 one another are too complex for discussion now We 

 are concerned rather with the nature of the more 

 permanent of the substances concerned, especially with 

 the starting point, uranium (possessing the heaviest of 

 aU atoms), radium, and the lead which seems to 

 result from their disintegration. Omitting the less 

 stable transition products, the most essential outcomes 

 are roughly indicated by a sort of genealogical tree 

 herewith shown :— . 



Hypothesis Concerning the Disintegration of Uranium. 



Uranium 



^ ^ 3 Helium^ 

 Radium 



i ^ I Helium -8 Helium 

 Emanation 



^ X 4 Helium j 

 Lead (Isotopic) 



Thus each atom of uranium is supposed to be con- 

 verted into radium by losing three atoms of helium, 

 and each atom of radium is supposed to be converted 

 into a kind of lead by losing five more, as already 

 stated. 



If uranium can thus disintegrate, should we call it 

 an element? .\nd should we call its smallest particles 

 atoms? The answers depend upon our definition of 

 these two words. If the word "element" is supposed 

 to designate a substance incapable of disintegration, ' 

 apparently it should not be applied to uranium ; 

 neither should the word "atom" be applied to the 

 smallest conceivable particles of this substance. But 

 no one would now maintain that any element is realh 

 incapable of disintegration. A method of still retaining 

 the terms in this and analogous cases is to define an 

 element as " a substance which has not yet been 

 decomposed artificially " — that is to sav, by the hand 

 of man ; and an atom as " the smallest particle of 

 such a substance, inferred from physico-chemical be- 

 haviour." The atom, then, is not to be considered as 

 wholly indivisible, but only as indivisible (or, at least, 

 as not yet divided) by artificial means. For, as in 

 the case of radium, the disintegration of uranium 

 cannot be hastened or retarded by any known earthl\- 

 agency. So long as it stays intact, the atom of 

 uranium behaves quantitatively in the same fashion as 

 any other atom ; Dalton's laws of definite and multiple 

 combining proportions apply without exception to its 

 compounds. In this connection one should remember 

 that the atomic theory as a whole, including Dalton's 

 and .Avogadro's generalisations, is not in the least 

 invalidated by the new discoveries of radio-activity. 

 On the contrary, the atomic theory is entrenched 

 to-day more firmly than ever before in its history. 



Interesting speculations by Drs. Russell, Fleck, 

 Soddy, Fajans, and others have interpreted in ex- 

 tremely ingenious and plausible fashion the several 

 transitory steps of the changes, and indicate the 

 reasons why the end-products of the decomposition 

 of both uranium and thorium should be very similar 



