MR. W. CROOKES ON MOLECULAR PHYSICS IN HIGH VACUA. 
659 
regia. Under the highest microscopical power it shows only as a stain, with no signs 
of granulation. 
626. A piece of glass from a tube which had cracked at incipient fusion was exa¬ 
mined under the microscope. The surface appeared curiously crumpled up and filled 
with minute bubbles, as if the glass had been boiling. 
Another piece of apparatus was constructed, in which a plate of German glass was 
held in the focus of the molecular bombardment. The vacuum was so good that 
no hydrogen or other lines could be seen in the spectrum of the emitted light. The 
focus was now allowed to play on the glass, when the glass soon became red hot. Gas 
appeared in the tube, and hydrogen lines now were visible in the spectrum. The gas 
was pumped out until hydrogen disappeared from the spectrum. It was now possible 
to heat the glass to dull redness without hydrogen coming in the tube ; but as soon as 
the heat approached the fusing point, the characteristic lines appeared. It was found 
that however highly I heated the glass, and then pumped the tube free from hydrogen, 
I had only to heat the glass to a still higher temperature to get a hydrogen spectrum 
in the tube. I consider the hydrogen comes from vapour of water, which is obstinately 
held in the superficial pores, and which is not entirely driven off by anything short of 
actual fusion of the glass. The bubbles noticed when the disintegrated and fused 
surface of the tube was examined under the microscope are probably caused by 
escaping vapour of water. 
627. When the negative discharge has been playing for some time on German glass, 
so as to render it strongly phosphorescent, the intensity of glow gradually diminishes 
(592). Some of this decline is due to the heating of the glass or to some other 
temporary action, for the glass partially recovers its property after rest; some is due 
to a superficial change of the surface of the glass; but part of the diminished sensitive¬ 
ness is due to the surface of the glass becoming coated with this brown stain. 
628. The luminous image of a hole in a plate of mica was projected from a platinum 
plate used as a negative pole, to the side of a glass bulb. The coil was kept playing 
for some time until the inside of the bulb was thoroughly darkened by projected 
platinum. Although a bundle of molecular rays could be seen all the time passing 
from the platinum, through the hole in the mica, to the glass, where it shone with 
a bright green light, I could detect no trace of extra darkening when the part of the 
glass formerly occupied by the green spot was carefully examined. Platinum is a 
metal which flies off in a remarkable manner when it forms the negative pole. It 
therefore appears from this experiment that the molecular stream does not consist of 
particles of the negative pole shot off from it. 
629. One of the most striking of the phenomena attending this research has been 
the remarkable power which the molecular rays in a high vacuum possess of causing 
phosphorescence in bodies on which they fall. Substances known to be phosphorescent 
under ordinary circumstances shine with great splendour when subjected to the nega¬ 
tive discharge in a high vacuum. Thus, a preparation of sulphide of calcium, much 
