902 
MR. W. CROOKES ON RADIANT MATTER SPECTROSCOPY: 
yttrium group, and zirconium. The only remaining elements among which this 
elusive body would probably be found are those members of the yttrium family which 
are not precipitated by potassic sulphate. 
37. On the other hand, the body giving the citron band spectrum did not behave 
like one of the known earths. A rich residue was fused with sodic carbonate, and 
the mass extracted with water. The insoluble residue, on testing in the usual way, 
was rich in citron band, but subsequent treatment of the aqueous solution gave rue an 
earth which also gave the citron band strongly. 
An acid solution of the citron body was precipitated by ammonia and ammonic 
chloride. The earth was not completely precipitated, but after a long boiling some 
remained in solution. I have since ascertained that the detection of the citron-band 
body in solution under these circumstances is only owing to the marvellous delicacy 
of the test, which carries our powers of recognition far beyond the resources of 
ordinary chemistry. 
38. Besides obtaining indirect evidence that the citron band was not due to certain 
elements, I tried special experiments with each substance, brought to the highest 
possible state of purity. In many cases I detected more or less traces of citron band ; 
but I had come to the conclusion, abundantly warranted by facts, that this citron 
band was an extraordinarily sensitive test of the presence of the element causing it; 
and the minute chemistry of many of these earthy metals being insufficiently known, 
it was not surprising that traces of one of them should adhere to another in spite of 
repeated attempts to purify it out. With each successive fractional precipitation the 
citron band became fainter, showing that with perseverance the last trace would 
probably disappear. The time this process would have occupied, in my opinion, 
seemed not worth the little additional evidence it would have afforded. 
39. Taking into consideration the extremely small quantity of phosphorescent 
material which had so far been obtained, all these experiments justified me in 
assuming that the body sought for not only belonged to the group of earths, but 
also most probably to the sub-group not precipitated by potassic sulphate to which 
yttria belongs. As, however, the number of these metals has increased so much 
within the last few years, and as the quantity of material which I had up to the 
present at my disposal was too small to admit of a satisfactory chemical examination 
being made of it, search was commenced among other, sources known to be rich in 
these metals. Besides, not only did the majority of the substances I had up till 
now obtained in anything like quantity indicate the citron band earth to belong to 
the yttria group (33, 34, 36), but also that either the earth itself showed an 
absorption band in the spectroscope, or was invariably accompanied by one which 
did. On the other hand, I had a certain amount of evidence that the earth sought 
for did not show a band in the spectroscope (24) ; but remembering the extremely 
small quantity of very impure substance experimented with, the evidence on this 
point was not at all conclusive. 
