44 



3Ir, William Croohes 



[Feb. 18, 



before you (Fig. 2). Broadly speaking, there is a deep red band, a 

 very luminous citron-coloured band, a pair of greenish-blue bands, 

 and a blue band. These bands, it is true, varied slightly in relative 

 intensities and in sharpness with almost every sample of yttria 



Fi:^.Z. Yttrca 



J^y J. 1^ (Gaddiniaj 



Fzq. 4. YUriaej 'Sfamaria 59. 



I examined ; yet the general character of the spectrum remained 

 unchanged, and I habitually looked upon this spectrum as charac- 

 teristic of yttrium ; all the bands being visible when the earth was 

 present in quantity, whilst only the strongest of all — the citron 

 band — was visible when traces, such as millionths, were present. 

 But that the whole system of bands spelled yttrium, and nothing but 

 yttrium, I was firmly convinced. 



The differences in the spectra of yttrium prepared from different 

 sources are most distinctly seen on comparing the spectrum of 

 yttrium from samarskite with that from gadolinite, hielmite, monazite, 

 xenotime, fluocerite, euxenite, cerite, arrhenite, &c. Still, in spite of 

 these slight differences, the several yttriums are practically all the 

 same thing, and, as I have said, every living chemist a year ago 

 would have regarded them as identical. Eut they have since yielded 

 to persistent chemical fractionation, and I now call them old yttrium. 



One property, above all others, is relied on by chemists as an 

 indisputable proof of the identity of any particular chemical element. 



