SCIENCE 



[N. S. Vol. XLIX. No. 125.3 



ciated rather -with the philosophical conception 

 of indivisibility than with the idea of chem- 

 ical combination in definite proportions. To- 

 day many chemists and physicists think that 

 the chemical atoms of the last century are no 

 longer to be considered as indivisible. In 

 that case, the old Greek name " atom '' is no 

 longer fitting, because it denotes indivisibility. 

 Some one has even facetiously suggested that 

 the word "torn" — indicating divisibility — 

 vcould be more appropriate! Moreover, if our 

 so-called atoms are really divisible, we can not 

 but be somewhat doubtful as to our defijiition 

 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 un- 

 •expected and startling phenomena 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 radioactive minerals ap- 

 pears, to have resulted, together with helium, 

 from the radioactive decomposition of ele- 

 ments of higher atomic weight. Skeptical at 

 first, the whole chemical world has now come 

 to acknowledge that the well-defined element, 

 heliimi (discovered by Sir William Eamsey 

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

 position products of radium. Eadium itself 

 is a substance which, in many respects, acts as 

 an element, with 226 as its atomic weight, 

 and must be considered as the heaviest mem- 

 ber of the well-known calcium family; but 

 its atoms appear to be so big and so complex 

 as to disintegrate because of lack of stability. 

 The disintegration is slow, and not to be 

 hastened or retarded by any agency known to 

 man; 1,670 years are demanded for the 

 decomposition of half of any given portion 

 of radium, according to the exact measure- 

 ments of Professors Boltwood and EUen Gled- 

 itsch. Moreover, we have reason to believe 

 that this decomposition proceeds in a series 

 of stages, successive atoms of helium (five in 

 all) being evolved with difPerent 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 sug- 

 gested by Boltwood, who pointed out the in- 

 variable presence of lead in radium, minerals. 

 Thus we must accept a kind of limited trans- 

 mutation of the elements, although not of the 

 immediately profitable type sought by the an- 

 cient alchemists. 



Interesting and significant as all of this is, 

 nevertheless the whole story has not yet been 

 told. Eadium itself appears to come from 

 the exceedingly slow decomposition of ura- 

 nium, an inference drawn from the fact that 

 radium is found only in conjunction with 

 the uranium, which even after careful purifi- 

 cation soon becomes radioactive and gives 

 every indication of suffering slow disintegra- 

 tion. Moreover, uranium is not the only 

 other heavy element which appears to be 

 capable of decomposing and yielding elements 

 of lower atomic weight. Another, thorium, 

 has a like propensity, although the steps in 

 this case are perhaps not so fully interpreted, 

 nor so generally accepted. In the process of 

 disintegration all these heavy atoms yield 

 strange radiations, some of them akin to, Or 

 identical with X-rays, which bear away that 

 part of the colossal energy of disintegTation 

 not made manifest as heat. These facts have 

 been proved beyond doubt by the brilliant 

 work of Madame Curie, Sir Ernest Euther- 

 ford, and others. 



The nature of the rays, and of the highly 

 interesting evanescent transition products and 

 their relation to one another is 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 heav- 

 iest of all atoms), radium, and the lead which 

 seems to result from their disintegration. 

 Omitting the less stable transition products, 

 the most essential outcomes are roughly in- 

 dicated by a sort of genealogical tree herewith 

 shown : 



