206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



our so-called atoms are really divisible, we can not 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 phenomena of radio- 

 activity. 



To-night we have to deal with a substance directly concerned with 

 the iconoclastic radioactive changes — with the very phenomena which 

 cause us to stop and think about our definitions of atoms and ele- 

 ments. For the lead obtained from radioactive minerals appears to 

 have resulted, together with helium, from the radioactive decompo- 

 sition of elements of higher atomic weight. Skeptical at first, the 

 whole chemical world has now come to acknowledge that the well- 

 defined element, helium (discovered by Sir William Ramsay 23 years 

 ago), is one of the decomposition products of radium. Radium 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 member 

 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,G70 years are demanded for the decomposi- 

 tion of half of any given portion of radium, according to the exact 

 measurements of Professors Boltwood and Gleditsch. 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 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 in T 

 variable presence of lead in radium minerals. Thus we must accept 

 a kind of limited transmutation of the elements, although not of the 

 immediately profitable tj^pe sought by the ancient alchemists. 



Interesting and significant as all of this is, nevertheless 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 conjunction with the uranium, 

 which even after careful purification soon becomes radioactive and 

 gives every indication of suffering slow disintegration. 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 disintegration 



