94 



NATURE 



[April 3, 19 19 



of lead, so that there could be no question as to lack 

 of identity of methods or precautions. 



Before leaving the subject of the relative atomic 

 weights of these two types of lead, it is not without 

 interest to note the exact absolute weights of the 

 atoms. If, as we have excellent reason for believing 

 on the basis of the brilliant work of Prof. R. A. 

 Millikan, a so-called gram-atom (the atomic weight in 

 grams) contains 6062 sextillion actual atoms, the 

 weights of the atoms of the two kinds of lead must be 

 respectively 342 and 340 septillionths of a gram. Their 

 extreme smallness as regards bulk may perhaps best 

 be inferred from the consideration that the smallest 

 object visible as a point in the common microscope 

 has a diameter probably about one thousand times as 

 great as an atom of lead." 



Evidently, on the basis of the quantitative results 

 just exhibited, we must admit that there is at least 

 one real difference between radio-active lead and the 

 common metal. Are there other differences? 



A question as to the density of each substance, and 

 therefore as to the bulk occupied by the respective 

 atoms, at once arises. Since the atom of uranium- 

 lead weighs less than the other, it must occupy less 

 space, supposing that it has the same density; or 

 else it must have less density, supposing tha"t it should 

 occupy the same space. The identity of the chemical 

 behaviour of the two types of lead suggests the 

 probability of the latter alternative, and this was, 

 therefore, assumed by Soddy; but experimental proof 

 was evidently desirable. Therefore an extended in- 

 vestigation of the density of the various kinds of lead 

 was carried out likewise in the Gibbs Memorial 

 Laboratory. As a matter of fact, the densities of 

 the several specimens were found to be very nearly 

 proportional to their atomic weights ; that is to say, 

 the bulk of the atom of radio-active lead is almost 

 exactly the same as the bulk of the atom of ordinary 

 lead, although the weights of these atoms are so 

 markedly different. 



Densities and Atomic Volufnes. 



Density 



Atomic 



Atomic 



weight ' volume 



Pure uranio-lead ... 20608 11-273 18-281 

 Australian mixture ... 206-34 11-289 18-278 

 Pure common lead ... 207-19 11337 18-277 



A distinctive property of elementary substances, 

 which has always been supposed to be concerned more 

 or less definitely with the atomic weight, is the 

 spectrum, depending upon the wave-lengths of light 

 emitted by the vapour. But, surprisingly enough, the 

 spectrum lines produced by these two sorts of lead, 

 when heated to the high temperature of the electric 

 arc, are so precisely alike, both as to their wave- 

 lengths and their intensities, that no ordinary 

 spectrum analysis shows any difference whatever. 

 This has been proved by careful experiments at 

 Harvard and elsewhere. A and B were from two 

 different specimens of radio-active lead, C from 

 ordinary lead, all very carefully purified. The range 

 covered is about from 3000 to 2000 wave-length — far 

 in the ultra-violet. Very recently Prof. W. D. 

 Harkins, of Chicago, and two assistants have de- 

 tected, with a very extended grating spectrum, an 

 exceedingly minute shift (0-000 1 per cent, of the wave- 

 length — an amount far too small to be shown by the 

 spectra exhibited) of one of the lines. The wonder is, 

 not that there should be a difference, but rather" that 

 they should be so very nearly identical. Evidently 

 the very considerable difference in the atomic weight 

 produces only a barely perceptible effect on the wave- 



'- If the smallest object visible in a microscope could be enlarged to the 

 width of this printed page, the atom; in it would appear about the size of the 

 dots on the letters i, or the periods, in the type above. 



NO. 2579, VOL. 103] 



lengths of light emitted by the several isotopic forms 

 of a given element, although a less difference in 

 atomic weight between two different elements (for 

 example, cobalt and nickel) is concomitant with utterly 

 divergent spectra. 



Another very interesting question, involving the 

 relations of substance both to light and to weight (or 

 rather density), is its refractive index. All the formulae 

 relating to molecular refraction involve the density of 

 the substance concerned. In the case under considera- 

 tion, do the differing weight's of the atoms, and there- 

 fore the differing densities of the same compounds 

 of the two kinds of lead, affect the refractive indices 

 of the salts? Is the refractive index of a given salt 

 of radio-lead identical with that of the same salt 

 of ordinary lead? Evidence on this point would 

 go far to decide whether density or atomic volume is 

 the more important thing in determining refractive 

 index. A very careful study carried out with the help 

 of Dr. W. C. Schumb at Harvard has, within the 

 past few months, shown that, as a matter of fact, the 

 refractive index of ordinary lead nitrate is identical 

 with that of the nitrate of uranium-lead within one 

 part in nearly twenty thousand— a result which shows 

 that density is a less important factor in determining 

 refractive index than had been previously assumed. 



Both these conclusions concerning light — that 

 drawn from the spectra and that drawn from the 

 refractive indices — have a yet more far-reaching 

 interest, for they give us a further clue as regards the 

 innermost nature of the atom. That part of the atom 

 which determines its weight seems to have, at least 

 in these cases, very little effect on that part of the 

 atom which determines its behaviour towards light. 



Immediately connected with the question of density 

 of the solid salts is the question as to the densities 

 of their saturated solutions, as well as to the extent 

 of saturation. Fajans and Eembert had recently 

 obtained results probably indicating that the molecular 

 solubility of each kind of lead is the same, and that 

 the densities of the solutions are different, the density 

 of the radio-lead solution being less to ^ an 

 extent consistent with its smaller molecular weight. 

 These results, however, left much to be desired in 

 the way of accuracy, and needed verification. There- 

 fore, a very careful investigation, begun at Harvard 

 with the assistance of Schumb, before the appear- 

 ance of Fajans's publication, furnished valuable know- 

 ledge on this point. 



Solubility of Two Kinds of Lead Nitrate.' 



Common lead Uranium-lead 



Per cent, salt in saturated 



solution (25-00°) 37-342 37-280 



Grams lead per 100 grams 



water 37-281 37-i3o 



Molecular solubility per 1000 



grams water 1-7993 1-7989 



Here, again, differences in weight alone are mani- 

 fest, and these are proportional to the differences in 

 the atomic weights ; the molecular behaviour is 

 essentially identical in the two sorts. Hence a differ- 

 ence in density between the two solutions must exist, 

 exactly consistent with the difference in the atomic 

 weights. 



The identity in solubility might also be inferred 

 from the impossibility of separating the two kinds of 

 lead from each other by fractional crystallisation. 

 This was predicted by Soddy, and tested by him and 

 by others. Various vain attempts have been made to 

 separate the different kinds of lead from one another, 



3 The uranium-lead uSed in these determinations was a speoimen from 

 Australia having the atomic weight 206-41, not quite like the earlier sample, 

 but not different in important degree. 



