RADIOACTIVE LEAD RICHARDS. 215 



experiments at Harvard and elsewhere. A * and B were from two dif- 

 ferent specimens of radioactive lead, C from ordinary lead, all very 

 carefully purified. The range covered is about from 3,000 to 2,000 

 wavelength — far in the ultra-violet. Very recently Prof. 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 wavelength — 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 considerable difference in the atomic 

 weight produces only a barely perceptible effect on the wavelengths 

 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 re- 

 fractive index. All the formula} 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 compounds of the two kinds of 

 lead, affect the refractive indices of the salts? Is the refractive 

 index of a given salt of radioactive 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 car- 

 ried 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 radioactive lead within one part ill nearly twenty thousand, a 

 result which shows that density is a less important factor in de- 

 termining refractive index than had been previously assumed. 



Both of 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 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 solubility of 

 each kind of lead is the same, and that the densities of the solutions 

 are different, the density of the radioactive solution being less to an 

 extent consistent with the smaller molecular weight. These results, 



1 References are to photographs not reproduced here. 



