892 
MR. W. CROOKES OK RADIANT MATTER SPECTROSCOPY: 
specially directed for some years past, considerable interest attaching to a solid body 
whose molecules vibrate in a few directions only, giving rise to spectrum lines or 
bands on a dark background. 
The citron hand spectrum. 
2. For a long time past I have been haunted by a bright citron-coloured band or 
line appearing in these phosphorescent spectra, sometimes as a sharp line, at others as 
a broader nebulous band, but having always a characteristic appearance and occurring 
uniformly in the same spot. This band I first saw in the summer of 1879, and from 
that date down to a comparatively short time ago all my efforts to clear up the 
mystery proved vain. By chemical means it was not difficult to effect a partial 
separation of a certain mineral or earthy body into two parts, one giving little or no 
citron band, the other giving one stronger than the original band ; and by again 
treating this latter portion by appropriate chemical means, the citron band-forming 
body could frequently be still more concentrated; but further than this for a long 
time it seemed impossible to go. I soon came to the conclusion that the substance I 
was in search of was an earth, but on attempting to determine its chemical properties 
I was baffled. A more Proteus-like substance a chemist never had to deal with. In 
my preliminary note, above referred to, speaking of the possibility that some of these 
spectrum-forming bodies might be new chemical elements, I said—The chemist must 
be on his guard against certain pitfalls which catch the unwary. I allude to the 
profound modification which the presence of fluorine, phosphorus, boron, &c., causes in 
the chemical reactions of many elements, and to the interfering action of a large 
quantity of one body on the chemical properties of another which may be present in 
small quantities.” 
3. This warning was not unnecessary. No Will-o’-the-Wisp ever led the unwary 
traveller into so many pitfalls and sloughs of despond as the hunt for this phantom 
band has entrapped me. I have started with a large quantity of substance which, 
from preliminary observations, promised to be a rich mine of the desired body, and 
have worked it up chemically to a certain point, when the citron band vanished, and 
could not be again detected in either precipitate or filtrate. Half-a-dozen times in 
the last four years the research has been given up as hopeless, and only a feeling of 
humiliation at the thought of a chemist being beaten by any number of anomalies 
made me renew each time the attack. Likewise, the tedious character of the research 
made a long continuance of failures very disheartening. To perform a spectrum test, 
the body under examination must be put in a tube and exhausted to a very high 
point before the spectroscope can be brought to bear on it. Instead of a few 
minutes, many hours are occupied in each operation, and the tentative gropings m 
the dark, pre-eminently characteristic of this kind of research, have to be extended 
over a long period of time. 
4. I soon found that the best way to bring out the band was to treat the substance 
