MR. W. CROOKES ON MOLECULAR PHYSICS IN HIGH VACUA. 
661 
This line coincides with the one described by E. Becquerel as being the most 
brilliant of the lines in the spectrum of the light of alumina, in its various forms, when 
glowing in the phosplioroscope. 
This coincidence affords a good proof of the identity of the phosphorescent light, 
whether the phosphorescence be produced by radiation, as in Becquerel’s experiments, 
or by molecular impact in a high vacuum. 
634. I have been favoured by my friend Professor Maskelyne with the following 
notes of results obtained on submitting to the molecular discharge various crystals 
which he lent me for the purpose of these experiments :— 
“Diamond crystals. A very small crystal, exhibiting large cube faces with the edges 
and angles truncated, was of a rich apricot colour, the dodecahedral faces of a clear yellow, 
and the octahedral of another yellow tint. No polarisation of the light was detected. 
Some were opaque ; some gave a bluish hazy light. 
“Emerald. A small hexagonal prism gave out a fine crimson-red colour. The 
light was polarised, apparently completely, in a plane perpendicular to the axis; this 
would correspond therefore to extraordinary rays which in emerald, as a negative 
crystal, represent the quicker rays vibrating presumably parallel to the optic axis of 
the crystal. 
“ Other emeralds behaved in the same way, though the illumination in two others 
experimented with appeared confined more particularly to one end—the end opposite to 
that at which the crystals presented some (in one instance fine) terminal faces. 
“ Beryls exhibited no corresponding phenomena. 
“ Sapphires gave out a bluish-grey light, distinctly polarised in a plane perpendicular 
to the axis. In this case, again, the ray developed corresponds to the extraordinary or 
quicker ray. 
“ Ruby gives out a transcendently fine crimson colour, exhibiting no marked dis¬ 
tinction in the plane of its polarisation, though in one part of a stone the colour was 
extinguished by a Nicol prism with its long diagonal parallel to the axis of the crystal, 
Here, therefore, also the light was that of the extraordinary ray. 
“ It seemed desirable to determine the nature of the phenomena in the case of 
positive crystals, and accordingly crystals of quartz, plrenakite, tinstone, and hyacinth, 
(zircon) were placed in a tube and experimented on. 
“ The only crystals that gave definite results were tinstone and hyacinth. A small 
crystal of the former mineral glowed with a fine yellow light, which was extinguished 
almost entirely when the long diagonal of the Nicol was perpendicular to the axis of 
the crystal. 
“ Here, therefore, the plane of polarisation of the emitted light was parallel to the 
axis of the crystal, and here it is again the quicker, though in this case (of an optically 
positive crystal) it is the ordinary ray which corresponds to the light evoked by the 
electric stream. 
“ So far, then, the experiments accord with the quicker vibrations being called into 
