G94 
MR, W. CROOKES OK RADIAKT MATTER SPECTROSCOPY. 
keeps mercury vapour from passing through, since the iodide of sulphur holds its 
iodine very loosely, and fixes the mercury in the form of non-volatile red iodide. A 
glass tube containing finely divided copper must follow in order to keep sulphur out. 
With this blockade interposed between the pump and experimental tubes I have been 
unable to detect mercury vapour in any of the tubes, whether in the cold or on 
heating them. 
Mixed “ Citron ” and “ Orange ” Spectra. 
96. Since the date of my last paper I have devoted myself to the task of solving 
the problem presented by the double orange band first observed in 1881. With the 
yttrium experience as a guide it might be thought this would not be a difficult task, 
but in truth it helped me little beyond increasing my confidence that the new like 
the old spectrum was characteristic of an element. The extreme sensitiveness of the 
test is a drawback rather than a help. To the inexperienced eye one part of “ orange 
band ” substance in ten thousand gives as good an indication as one part in ten, and 
by far the greater part of the chemical work undertaken in the hunt for the spectrum¬ 
forming element has been performed upon material which later knowledge shows does 
not contain sufficient to respond to any known chemical test. It is as if the element 
sodium were to occur in ponderable quantity only in a few rare minerals seldom seen 
out of the collector’s cabinet. With only the yellow line to guide, and seeing the 
brilliancy with which an imponderable trace of sodium in a mineral declares its 
presence in the spectrum, I venture to think that a chemist would have about as stiff 
a hunt before he caught his yellow line as I had to bring my orange and citron bands 
to earth. 
Chemistry, except in few instances, as water-analysis and the detection of poisons, 
where necessity has stimulated minute research, takes little account of “ tracesand 
when an analysis adds up to 99'99, the odd O'Ol per cent, is conveniently put down to 
“impurities,” “ loss,” or “errors of analysis.” When, however, the 99 - 99 percent, 
constitutes the impurity and this exiguous 0‘01 is the precious material to be extracted, 
and wdien, moreover, its chemistry is absolutely unknown, the difficulties of the 
problem become enormously enhanced. Insolubility as ordinarily understood is a 
fiction, and separation by precipitants is nearly impossible. A new chemistry has to 
be slowdy built up, taking for data uncertain and deceptive indications, marred by the 
interfering power of mass in withdrawing soluble salts from a solution, and the 
solubility of nearly all precipitates in water or in ammoniacal salts when present in 
traces only. What is here meant by “ traces ” will be better understood if I give an 
instance. After six months’ work I obtained the earth didymia in a state which most 
chemists would call absolutely pure, for it contained not more than one part of 
*So recently as the year 1880 a high authority on blowpipe chemistry seriously argued that the yellow 
coloured flame “ which is ascribed to sodium ” is not caused by that metal, but is due to “ chemical 
water.” (‘ Chemical Hews,’ vol. sli., p. 159, April 2, 1880.) 
