270 



On Radiant Matter Spectroscopy. 



[May 31, 



Pure yttria was also prepared from yttro-tantalite, euxenite, tyrite, 

 and also from plaster of Paris and common limestone. In no case 

 could I detect any difference in the position or intensity of the lines 

 shown by their phosphorescent spectra. 



The Phosphorescent Spectrum of Yttria. 



The spectrum shown by pure ignited yttric sulphate in a radiant 

 matter tube is one of the most beautiful objects in the whole range of 

 spectroscopy. The spectrum is best seen under low dispersion and 

 not too narrow a slit. It consists essentially of a broad red band, an 

 intensely brilliant citron band, and two almost equally brilliant green 

 bands. Other fainter lines are also seen, but they are not characteristic. 

 Coloured drawings and maps of the spectrum to scale accompany the 

 paper. This description applies to the spectrum shown either by pure 

 yttric sulphate or by an earth tolerably rich in yttria. When traces 

 are present the citron band only is seen. A little more yttria brings 

 out the first and then the second green band, and finally, as the pro- 

 portion of yttria increases, the red and blue bands appear. 



The paper next gives a description of experiments made with pure 

 yttria, and with various compounds of it, to see which would give the 

 most characteristic spectrum. The sulphate heated to redness was 

 found to give the best results. Pure yttria precipitated by ammonia 

 did not phosphoresce in the slightest degree, and, necessarily, no 

 citron band spectrum was to be* seen. The yttria was removed from 

 the tube, converted into sulphate, heated to redness, and again tested. 

 This time it gave the citron band magnificently. This shows what 

 apparently trivial circumstances will alter the whole course of an 

 investigation. In 1881, when searching for discontinuous phos- 

 phorescent spectra, I tried a similar experiment with pure precipitated 

 yttria, and entirely missed its citron band spectrum. Had I first 

 treated the yttria with sulphuric acid instead of testing the earth 

 itself in the radiant matter tube the results would have been very 

 different, and this research would probably have never been under- 

 taken. 



Yttria was now prepared by igniting' the precipitated oxalate at a 

 red heat. On testing it in the radiant matter tube it phosphoresced 

 with feeble intensity, the light being about one-twentieth of that 

 given by the ignited sulphate under similar conditions. The citron 

 band was almost as sharp as the sodium line, and was shifted one 

 division towards the blue end. The two green bands were visible, 

 but very hazy and indistinct, and only to be resolved into bands with 

 difficulty. 



It is an old and probably a true saying that every element could be 

 detected everywhere had we sufficiently delicate tests for it. Early 

 observations had prepared me for the wide distribution of the element 



