76 



NATURE 



[March 27, 1919 



to lead, if not identical with it. Therefore, a careful 

 study of the properties of lead of indubitably radio- 

 active origin became a matter of great interest, as a 

 step towards 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 conclusions might be more or less ap- 

 plicable 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 chemists had already recognised 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 crystallisation or precipitation had 

 been shown by Prof. Soddy and others to separate 

 no foreign substance. Hence great similarity was 

 proved, but this does not signify identity. Identity 

 is to be established only by quantitative researches. 

 Plato recognised long ago, in an oft-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 properties 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 a standard. There is no need, before this 

 distinguished audience, of emphasising the importance 

 of the familiar table of atomic weights; but a few 

 parenthetical words about their character is, perhaps, 

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

 the atomic weights of the relatively permanent 

 elements, which constitute almost all 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 of quartz in a newly hewn piece 

 of granite seems, and probably is, as compact and 

 perfect as it was just after it was formed aeons ago. 

 We cannot imagine that any of its properties have 

 essentially changed during its protracted imprison- 

 ment; and, so far as we can guess, the silicon and 

 oxygen of which it was made may have existed for 

 previous aeons, first as gas, and then as liquid. The 

 relative weights in which these two elements combine 

 must date at least from the inconceivably distant time 

 when the earth was " without form and void." 



Although, apparently, these numbers were thus 

 determined at the birth of our universe, they are, 

 philosophically speaking, in a different class from the 

 purely mathematical constants such as the relation of 

 circumference to the diameter of a circle. 3-14159... 

 is a geometrical magnitude entirely independent of any 

 kind of material, and it therefore belongs to the more 

 general class of numbers, together with simple 

 numerical relations, logarithmic and trigonometric 

 quantities, and other mathematical functions. On the 

 other hand, the atomic weights of the primeval 

 elements, although less general than these, are much 

 more general and fundamental than the constants of 

 NO. 2578, VOL. 103] 



astronomy, such as the so-called constant of gravity, 

 the length of the day and year, the proper motion of 

 the sun, and all the other incommensurable magni- 

 tudes which have been more or less accidentally 

 ordained in the cosmic system. The physico-chemical 

 constants, such as the atomic weights, lie in a group 

 between the mathematical constants and the astro- 

 nomical "constants," and their values have a signi- 

 ficance only less important than the former. 



In the lead from uranium we have a comparatively 

 youthful elementary substance which seems to have 

 been formed since the rocks in which it occurs had 

 crystallised. Is the atomic weight of this youthful 

 lead identical with that of the far more andent 

 common lead, which seems to be more nearly con- 

 temporary as to its origin with the silicon and oxygen 

 of quartz? 



The idea that different specimens of a given element 

 might have different atomic weights is by no means 

 new; it far antedates the discovery of radio-activity. 



Ever since the discovery of the definite combining 

 proportions of the elements and the ascription of 

 these proportions to the relative weights of the atoms, 

 the complete constancy of the atomic weights has 

 occasionally been questioned. More than once in the 

 past investigators have found apparent differences in 

 the weights of atoms of a single kind, but until very 

 recently all these irregularities have been proved to be 

 due to inaccurate experimentation. Nevertheless, even 

 thirty years ago the question seemed to me not de- 

 finitively answered, and careful experiments were made 

 with copper, silver, and sodium, obtained from widely 

 different sources, in the hope of finding differences in 

 the atomic weights, according to the source of the 

 material. No such differences whatever were found. 

 More recently Prof. Baxter compared the atomic 

 weights of iron and nickel in meteorites (from an 

 unknown, perhaps inconceivably distant, source) and 

 the same terrestrial metals. In these cases also the 

 results were negative. Thus copper, silver, sodium, 

 iron, and nickel all appeared to be perfectly definite 

 in nature, and their atoms, each after its own kind, 

 all alike. 



The general question remained, nevertheless, one of 

 profound interest to the theoretical chemist, because 

 it involved the very nature of the elenients themselves ; 

 and in its relation to the possible discovery of a 

 difference between uranium-lead and ordinary lead it 

 became a very crucial question. 



Early in 19 13, when the hypothesis of radio-active 

 disintegration had assumed definite shape. Dr. 

 Fajans's assistant. Max Lembert, journeyed to Cam- 

 bridge, Mass., bringing a large quantity of lead from 

 Bohemian radio-active sources in order that its 

 atomic weight might be determined by Harvard 

 methods, with the precision attainable there. The 

 Carnegie Institution of Washington gave generous 

 pecuniary assistance towards providing the necessary 

 apparatus in this and later investigations. 



The most important precautions to be taken in 

 such work are worthy of brief notice, because the 

 value of the results inevitably depends upon them. 

 The operation consists in weighing specimens of a 

 salt of the element in question, and then precipitating 

 one of the constituents In each specimen, determining 

 the weight of the precipitate, and thus the composi- 

 tion of the salt. In the first place, each portion of 

 substance to be weighed must be free from the sus- 

 picion of containing unheeded impurities, otherwise 

 its weight will mean little. This is an end not easily 

 attained, for liquids often attack their containing 

 vessels and absorb gases, crystals include and occlude 

 solvents, precipitates carry down polluting impurities. 



