660 
MR. W. CROOKES ON MOLECULAR PHYSICS IN HIOH VACUA. 
used now in Paris for coating clock faces, which remain luminous after dark, is invalu¬ 
able in these researches for the preparation of phosphorescent screens whereon to trace 
the paths and trajectories of the molecules. It shines with a bright blue-violet light, 
and, when on a surface of several square niches, is sufficient to light up a room. Modifi¬ 
cations of these phosphorescent sulphides shine with a yellow, orange, and green light. 
630. The only body I have yet met with which surpasses the luminous sulphides 
both in brilliancy and variety of colour is the diamond. Most of these gems, whether 
cut or in the rough, when coming from the South African fields, phosphoresce of a 
brilliant light blue colour. Diamonds from other localities shine with different colours, 
such as bright blue, pale blue, apricot, red, yellowish-green, orange, and bright green. 
One beautiful green diamond in my collection when phosphorescing in a good vacuum 
gives almost as much light as a candle : the light is pale green—almost white. A 
beautiful collection of diamond crystals kindly lent me by Professor Maskelyne 
phosphoresce with nearly all the colours of the rainbow, the different faces glowing 
with different shades of colour. 
631. 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 colour the earth 
or stone possesses naturally, the colour 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” colour, glowing alike in the vacuum, thus 
corroborating E. Becquerel’s results on the action of fight on alumina and its com¬ 
pounds in the phosphoroscope (‘ Annales de Chimie et de Physique,’ser. 3, vol. lvii.). 
Nothing can be more beautiful than the effect presented by a mass of rough rubies 
when the molecular discharge plays on them in a high vacuum. They glow as if they 
were red hot, and the illuminating effect is almost equal to that of the diamond under 
similar circumstances. 
632. By the kindness of M. Ch. FEiL,who has placed large masses of his artificial ruby 
crystals at my service, I have been enabled to compare the behaviour of the artificially 
formed crystals with that of the natural ruby. In the vacuum there is no difference 
whatever; the colour of the phosphorescence emitted by M. Feil’s crystals is of just 
as intense a colour, and quite as pure in character, as that given by the natural stone. 
This affords another proof, if one were needed, that Messrs. Fremy and Feil have 
actually succeeded in the artificial formation of the veritable ruby, and have not simply 
obtained crystals which imitate it in hardness and colour. 
633. The appearance of the alumina glow in the spectroscope is remarkable. There 
is a faint continuous spectrum ending in the red somewhere near the line B; then a 
black space, and next an intensely brilliant and sharp red line to which nearly the 
whole of the intensity of the coloured glow is due. The wave-length of this red fine, 
which appears characteristic of this form of alumina, is 689A m.m.in., as near as 
I can measure in my spectroscope; the maximum probable error being about ff; - 3. 
