48 Mr. William CrooJces [Feb. 18, 



escence of GS — the citron band— and to enhance the iDhosphoreccence 

 of G^, the double green band, and the imitation of the Ya spectrum 

 is complete. 



I must here call attention to the experiments of Prof. A. E. 

 Nordenskiold, in the Comptes Rendus of the French Academy of 

 Sciences for November 2nd, 1886. This eminent savant is working in 

 the same direction as myself, with results which decidedly corroborate 

 my experiments. He has taken the crude mixture of yttria, erbia, 

 ytterbia, &c., just as it is precipitated from the minerals containing 

 these rare earths. This mixture, for brevity's sake, he calls gado- 

 linia, and he finds that this gadolinia, though palpably a compound 

 body, has always a constant atomic weight, whatever the mineral 

 from which it has been extracted. Or, to use Prof. Nordenskiold's 

 own words, " Oxide of gadolinium, thoiigli it is not the oxide of a simple 

 body, hut a mixture of three isomorphous oxides (even when it is derived 

 from totally different minerals found in localities far apart from each 

 other) possesses a constant atomic weight." Therefore, as he signi- 

 ficantly observes, " We are in presence of a fact altogether neiv in 

 chemistry. For the first time we are conf routed with the fact that 

 three isomorphous substances, of a kind that chemists are still com- 

 pelled to regard as elements, occur in nature not only always together, 

 but in the same proportions. It seems that chemists here find them- 

 selves face to face with a problem analogous to that presented to 

 astronomers in the origin of the minor planets." 



These facts throw a new light upon certain important chemical 

 questions. For the old yttrium passed muster as an element. It 

 had a definite atomic weight, it entered into combination with other 

 elements, and could be again separated from them as a whole. But 

 now we find that excessive and systematic fractionation has acted the 

 part of a chemical " sorting Demon," distributing the atoms of 

 yttrium into groups, with certainly different phosphorescent spectra, 

 and presumably different atomic weights, though, from the usual 

 chemical point of view, all these groups behave alike. Here, then, is 

 a so-called element whose spectrum does not emanate equally from all 

 its atoms ; but some atoms furnish some, other atoms others, of the 

 lines and bands of the compound spectrum of the element. Hence 

 the atoms of this element differ probably in weight, and certainly in 

 the internal motions they undergo. 



This is unlikely to be an isolated case. We may assume that the 

 principle is of general application to all the elements. In some, 

 possibly in all elements, the whole spectrum does not emanate from 

 all their atoms, but different spectral rays may come from different 

 atoms, and in the spectrum as we see it all these partial spectra are 

 present together. This may be interpreted to mean that there are 

 definite differences in the internal motions of the several groups of 

 which the atoms of a chemical element consist. For example, we 

 must now be prepared for some such events as that the seven series 

 of bands in the absorption-spcctruui of iodine may prove not all to 



