MR. W. CROOKES ON RADIANT MATTER SPECTROSCO-PY. 
695 
impurity in five hundred thousand parts of didymia (131). But this one part in half 
a million profoundly altered the character of didymia from a radiant-matter-spectro¬ 
scopic point of view, and the persistence of this very minute quantity of interfering 
impurity entailed another six months’ extra labour to eliminate these final “ traces ” 
and to ascertain the real reaction of didymia pure and simple (131). 
For a long time the “ citron-band ” and the “ orange-band ” spectra were confounded. 
That they were due to two different states or kinds of matter was not easily decided, 
since in all the early experiments I was dealing with a mixture; consequently the 
spectra obtained were not only mixed but differed considerably in the relative 
intensities and faintness of the different lines (146). 
97. At last having separated yttria and obtained its spectrum pure (71), the 
characteristic lines in the other spectrum or spectra could be provisionally mapped out 
by difference, and a systematic hunt was instituted for the new “ orange band 
substance, which to avoid periphrasis was termed x. Naturally my thoughts turned 
to samarskite and the yttria earths. A wide, prolonged survey over every available 
substance had convinced me that the number of bodies giving a discontinuous phos¬ 
phorescent spectrum is extremely limited, and to be counted on the fingers of one hand ; 
and having satisfactorily mated one of these spectra to yttria it became in the highest 
degree probable that the twin spectrum should, belong to one of the nearest chemical 
associates of yttria. 
Chemistry of the “ Orange-Band ” forming Body. 
98. At first it was necessary to take stock, as it were, of all the facts regarding x 
which had turned up during the search for the orange band. In the first place x is 
almost as widely distributed as yttria, generally occurring with the latter earth. 
Sometimes, however, the orange band was strong where the citron band was almost or 
quite absent. It is almost certainly one of the earthy metals, as it occurs in the 
insoluble oxalates, in the insoluble double sulphates, and in the precipitate with 
ammonia. It is not precipitated by sodic thiosulphate, and, moreover, it must be 
present in very minute quantities, since the ammonia precipitate is always extremely 
small, and as a rule x is not found in the filtrate from this precipitate. 
99. At this stage of the enquiry the chemical reaction of x were much more puzzling 
than with yttria. At the outset an anomaly presented itself. The orange band was 
prone to vanish in a puzzling manner. Frequently an accumulation of precipitates 
tolerably rich in x was worked up for purposes of concentration, when the spectrum 
reaction suddenly disappeared, showing itself neither in precipitate nor filtrate (3, 101, 
108, 115); whilst on other occasions, when following apparently the same procedure, 
the orange band became intensified and concentrated with no apparent loss. The 
behaviour of the sulphate to water was also very contradictory ; on some occasions it 
appeared to be almost insoluble, whilst occasionally it dissolved in water readily (115). 
100. For some time I debated whether the orange-band spectrum might not be a 
4 u 2 
