912 
MR. W. CROOKES ON RADIANT MATTER SPECTROSCOPY: 
when the crystals were ignited, and the resulting earths re-treated with nitric acid 
and oxalic acid. After repeated fractionations I finally obtained in this manner a per¬ 
fectly white yttria and a terbia containing a small quantity of yttria. This terbia was 
added to the crude terbia from previous operations, and purified as already described 
(57). 
These operations gave me two earths,—yttria and terbia,—which, from the constancy 
of their H equivalents, were taken to be pure. The earths giving absorption spectra 
and having H equivalents other than 29'7 and 49'5, include erbia, holmia, and thulia. 
This portion was not further examined for the purposes of this investigation. 
Ytt erbia. 
66. Before considering it finally proved that the substance forming the citron-band 
spectrum was yttria, it was necessary to prepare ytterbia and ascertain its behaviour 
in the radiant matter tube, this earth and yttria being the only remaining earths to 
which the citron spectrum could possibly belong. 
The two metals have hydrogen equivalents—ytterbium 57‘9 and yttrium 29'7. 
The chemical reactions are also sufficiently different to render their separation a matter 
of no very great difficulty. 
67. Gadolinite is said by Nilson to contain most ytterbia, so this mineral was chosen 
in preference to samarskite. The crude earths were first purified from all the earths 
whose sulphates are difficultly soluble in potassic sulphate (22, 25, 31 to 36), then by the 
formic acid process from terbia (56, 57), and lastly by fractional precipitation with 
oxalic acid from the erbia earths (65). There remained an almost white yttria, which 
gave the citron-band spectrum very brilliantly. Now, gadolinite contains only about 
OT per cent, of ytterbia, and about 35 per cent, of yttria; therefore the ytterbia to 
yttria in this mixture was somewhat in the proportion of 1 to 300, and it gave the 
citron-band spectrum as brillantly as I had ever seen it. The probability was that the 
earth forming nearly the whole was the one giving the spectrum. 
68. Ytterbic nitrate decomposes on fusion almost as easily as erbic nitrate (60), 
whilst yttric nitrate resists decomposition much more energetically.* Fusion of the 
nitrates is also the best process for throwing out the erbia, holmia, and thulia, and is 
therefore the best for purifying gadolinite yttria, as this mineral is rich in the erbia 
earths and contains little terbia. 
The gadolinite yttria was converted into nitrate, fused for a short time, and extracted 
with water. The portions soluble and insoluble in water were again separately 
submitted to this treatment, until at last a colourless earth was obtained, the nitrate 
of which decomposed easily on fusion, and another whose nitrate resisted decomposition 
when exposed for a long time to nearly a red heat (70). 
The earth from the easily decomposed nitrate gave at first a faint citron-band 
* Marignac, ‘ Comptes Rendiis,’ vol. 90, p. 902. 
