SCIENCE 



[N. S. Vol. XLIX. No. 1253 



tlieir wave-leng-ths and their intensities, that 

 no ordinary spectriun analysis shows any dif- 

 ference whatever. This has been proved by 

 careful experiments at Harvard and elsewhere, 

 and is made obvious by the photographs now 

 thrown on the screen. A and B were from 

 two different specimens of radioactive lead, C 

 from ordinary lead, all very carefully purified. 

 The range covered is about from 3,000 to 

 2,000 wave-length — ^far in the ultra-violet. 

 Very recently Professor W. D. Harkins, of 

 Chicago, and two assistants, have detected, 

 with a very extended grating spectrum, an 

 exceedingly minute shift (0.0001 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-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, involv- 

 ing the relations of substance both to light and 

 to weight (or rather density) is its refractive 

 index. All the formulas relating to molecular 

 refraction involve the density of the substance 

 concerned. In the case under consideration, 

 do the differing weights of the atoms, and 

 therefore the differing densities of the same 

 compomids of the two kinds of lead, affect 

 the refractive indices of the salts? It the re- 

 fractive index of a given salt of radio lead 

 identical with that of the same salt of or- 

 dinary lead? Evidence on this point would 

 go far to decide whether density or atomic 

 volume is the more important thing in deter- 

 mining 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 or ordinary lead nitrate is identical with 

 that of the nitrate of radiolead within one 

 part in nearly twenty thousand, a result which 

 shows that density is a less important factor 



in deterining refractive index than had been 

 previously assumed. 



Both of these conclusions concerning light — 

 that drawn from the spectra and that drawn 

 form the refractive indices — ^have a yet more 

 far reaching interest, for they give us a fur- 

 ther clue as regards the innermost nature of 

 the atom. That part of the atom which deter- 

 mines its weight seems to have, at least in 

 these cases, very little effect on that part of 

 the atom which determines its behavior toward 

 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 Lembert had recently obtained results 

 probably indicating that the molecular solu- 

 bility of each kind of lead is the same, and 

 that the densities of the solutions are differ- 

 ent, the density of the radiolead solution be- 

 ing less to an extent consistent with its smaller 

 molecular weight. These results, however, 

 left ranch to be desired in the way of accuracy, 

 and needed verification. Therefore a very 

 careful investigation, begun at Harvard with 

 the assistance of Schumb, before the appear- 

 ance of Fajan's publication, furnished valu- 

 able knowledge on this jwint. 



soLUBniirr of two kinds op lead nitrate* 



Common Uranium 



Lead Lead 



Per cent, salt in saturated so- 

 lution (25.00°) 37.342 37.280 



Grams lead per 100 grams 

 water 37.28 37.130 



Molecular solubility per 1,000 



grams water 1.7993 1.7989 



Here, again, differences in weight alone are 

 manifest, and these are proportional to the 

 differences in the atomic weights; the molec- 

 ular behavior is essentially identical in the 

 two sorts. 



The identity in solubility might also be in- 

 ferred from the imjiossihility of separating the 



* The uranium lead used in these determinations 

 was a specimen from Australia having the atomic 

 weight 206.41, not quite like the earlier sample, 

 but not different in important degree. 



