702 
MR, W. CROOKES ON RADIANT MATTER SPECTROSCOPY. 
The contradictory behaviour of the sulphate to water (99) was now easily explained. 
The insoluble crystals, which from the brilliancy of their phosphorescent spectrum I 
had at first mistaken for the nearly pure sulphate of x, were merely calcic or strontic 
sulphate contaminated with perhaps not more than the one ten-thousandth part of x 
sulphate which it had carried down with them on crystallising. 
“ X ” in Cerite. 
116. In the corresponding yttrium research I was aided materially by the fact that 
the sought-for earth did not give an absorption spectrum (42). This enabled me to 
throw out a large number of obscurely known elements, and I therefore early 
endeavoured to ascertain whether the supposed new earth, x, did or did not give an 
absorption spectrum. At first I could not decide one way or the other. I frequently 
obtained a good orange-band spectrum when the solutions gave no trace of absorption 
spectrum, whilst on other occasions the solution showed good didymium and other 
bands. Gradually, however, it was noticed that whenever the didymium absorption 
bands were strong the orange-band spectrum was also particularly brilliant. More¬ 
over, amongst the earths enumerated in par. 113 as mixed with lime in the quest 
for x, I have mentioned that some of them gave the orange-band spectrum with 
increased intensity ; the earths of the cerium group were the most noteworthy, and 
these considerations made it probable that here would be found the location of x. 
117. On a former occasion, when searching for the citron-band-yielding earth, and 
examining cerite (22 to 25), I made use of the potassic-sulphate method of separating 
the two great sub-groups, viz., the cerium and the yttrium earths ; the former giving 
insoluble, and the latter soluble, double sulphates. I said (23) : — 
“ The precipitated double sulphates were dissolved in hydrochloric acid, and the 
earths precipitated as oxalates. After ignition and treatment with sulphuric acid, 
the mixed ceria, lanthana, and didymia were tested in the radiant matter tube, but 
the merest trace only of citron band was visible.” 
A repetition of the above experiment produced similar results. The contents of 
the tube were now removed, mixed with lime and excess of sulphuric acid, ignited, 
and again tested in the tube. This time the orange-band spectrum came out very 
brilliantly, showing in a striking manner the necessity of supplementing x with some 
other earth to bring out its phosphorescing properties. 
The cerium group, to which x was now almost certainly traced, consists of cerium, 
lanthanum, didymium, samarium, and perhaps yttrium-a (136). The other metals, 
enumerated in par. 101 as being precipitated by potassic sulphate, were found not to 
phosphoresce with a discontinuous spectrum, either alone or when mixed Avith lime. 
