208 ANNUAL, REPORT SMITHSONIAN" INSTITUTION, 1918. 



in the least invalidated by the new discoveries of radioactivity. On 

 the contrary, the atomic theory is entrenched to-day more firmly than 

 ever before in its history. 



Interesting speculations by Doctors Russell, Fleck, Soddy, and Fa- 

 jans and others have interpreted in extremely ingenious and plausible 

 fashion the several transitory steps of the changes, and indicate the 

 reasons why the end products of the decomposition both of uranium 

 and thorium should be very similar to lend, if not identical with it. 

 Therefore a careful study of the properties of lead of indubitably 

 radioactive origin became a matter of great interest, as a step toward 

 confirming these speculations, especially in comparison with the 

 properties of ordinary lead. Such investigations should throw light 

 on the nature of radium and uranium and the extraordinary changes 

 which those metals suffer. Moreover, by analogy, the resulting con- 

 clusions might be more or less applicable to the relations of other 

 elements to each other; and the comparison of this new kind of lead 

 with ordinary lead might afford important information as to the 

 essential attributes of elementary substances in general, in case any. 

 differences between the two kinds should be found. 



Before the subject had been taken up at Harvard University, chem- 

 ists had already recognized the fact that the so-called uranium-lead 

 is indeed qualitatively very like ordinary lead. It yields a black 

 sulphide, a yellow chromate, and a white sulphate, all very sparingly 

 soluble in water, just as ordinary lead does. Continued fractional 

 crystallization or precipitation had been shown by Professor Soddy 

 and others to separate no foreign substance. Hence great similarity 

 was proved ; but this does not signify identity. Identity is to be es- 

 tablished only by quantitative researches. Plato recognized long ago, 

 in an often-quoted epigram, that when weights and measures are left 

 out little remains of any art. Modern science echoes this dictum in 

 its insistence on quantitative data ; science becomes more scientific as 

 it becomes more exactly quantitative. 



One of the most striking and significant of the quantitative proper- 

 ties of an element is its atomic weight — a number computed from the 

 proportion by weight in which it combines with some other element, 

 taken as 1 a standard. There is no need, before this distinguished audi- 

 ence, of emphasizing the importance of the familiar table of atomic 

 weights ; but a few parenthetical words about their character is per- 

 haps not out of place. As has been more than once said, the atomic 

 weights of the relatively permanent elements, which constitute almost 

 all of the crust of the earth, seem to be concerned with the ultimate 

 nature of things, and must have been fixed at the very beginning of 

 the universe, if indeed the universe ever had any beginning. They 

 are silent, apparently unchanging witnesses of the transition from the 

 imagined chaos of old philosophy to the existing cosmos. The crystal 



