Crookes has presented to the Royal Society a paper on the color
emitted by pure alumina when submitted to the electric discharge _in
vacuo_, in answer to the statements of De Boisbaudran. In 1879 he had
stated that "next to the diamond, alumina, in the form of ruby, is
perhaps the most strikingly phosphorescent stone I have examined. It
glows with a rich, full red; and a remarkable feature is that it is of
little consequence what degree of color the earth or stone possesses
naturally, the color of the phosphorescence is nearly the same in all
cases; chemically precipitated amorphous alumina, rubies of a pale
reddish yellow, and gems of the prized 'pigeon's blood' color glowing
alike in the vacuum." These results, as well as the spectra obtained,
he stated further, corroborated Becquerel's observations. In
consequence of the opposite results obtained by De Boisbaudran,
Crookes has now re-examined this question with a view to clear up the
mystery. On examining a specimen of alumina prepared from tolerably
pure aluminum sulphate, shown by the ordinary tests to be free from
chromium, the bright crimson line, to which the red phosphorescent
light is due, was brightly visible in its spectrum. The aluminum
sulphate was then, in separate portions, purified by various processes
especially adapted to separate from it any chromium that might be
present; the best of these being that given by Wohler, solution in
excess of potassium hydrate and precipitation of the alumina by a
current of chlorine. The alumina filtered off, ignited, and tested in
a radiant matter tube gave as good a crimson line spectrum as did that
from the original sulphate.
A repetition of this purifying process gave no change in the result.
Four possible explanations are offered of the phenomena observed: "(1)
The crimson line is due to alumina, but it is capable of being
suppressed by an accompanying earth which concentrates toward one end
of the fractionations; (2) the crimson line is not due to alumina, but
is due to the presence of an accompanying earth concentrating toward
the other end of the fractionations; (3) the crimson line belongs to
alumina, but its full development requires certain precautions to be
observed in the time and intensity of ignition, degree of exhaustion,
or its absolute freedom from alkaline and other bodies carried down by
precipitated alumina and difficult to remove by washing; experience
not having yet shown which of these precautions are essential to the
full development of the crimson line and which are unessential; and
(4) the earth alumina is a compound molecule, one of its constituent
molecules giving the crimson line. According to this hypothesis,
alumina would be analogous to yttria."--_Nature._
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