MR. W. CROOKES OK MOLECULAR PHYSICS IK HIGH VACUA. 
645 
responds to the more energetic bombardment from the corrugated pole. This pheno¬ 
menon almost disappears at very high exhaustions, or if the tube is allowed to rest for 
some time. The tired glass then recovers its phosphorescent power to some extent, 
but not completely. 
Fig. 5. 
593. To obtain this action in a more striking manner, a tube was made having a 
metal cross on a hinge opposite the negative pole. The sharp image of the cross was 
projected on the phosphorescent end of the bulb, where it appeared black on a green 
ground. After the coil had been playing for some time a sudden blow caused the cross 
to fall down, when immediately there appeared on the glass a bright green cross on a 
darker background. The part of the glass formerly occupied by the shadow, having 
been protected from bombardment, now shone out with full intensity, whilst the 
adjacent parts of the glass had lost some of their sensitiveness, owing to previous 
bombardment. 
594. [This effect of deadening produced on glass by long-continued phosphorescence 
was shown in a very striking manner at a lecture delivered at the Royal Institution 
on April 4th, 1879, when the image of a cross was stencilled on the end of a large 
pear-shaped bulb. 
I subsequently experimented further with this bulb, and found that the image of 
the cross remained firmly stencilled on the glass. The bulb was then opened and the 
wide end heated in the blowpipe flame till it was quite soft and melted out of shape. 
It was then blown out again into its original shape, and re-exhausted; on connecting 
it with the induction coil, the metal cross being down out of the line of discharge, the 
original ghost of the cross was seen to be still there, showing that the deadening of 
the phosphorescing powers the glass produced by the first experiment at the Royal 
Institution had survived the melting-up and re-blowing out of the bulb.—August 12, 
1879.] 
595. When experimenting with this apparatus a shifting of the line of molecular 
discharge was noticed when the current was first turned on. The flat pole b (fig. 6) 
being negative and the line c d being normal to its surface, the spot of light falls 
accurately on d, when the exhaustion is sufficiently good to give a sharp oval image of 
the hole c. But at higher exhaustions, when the outline of the image of c becomes 
irregular and continually changing, the patch of light at the moment of making contact 
is sometimes seen at e, and then almost instantly travels from e to d, where it remains 
as long as the current passes. The passage of the spot from e to d is very rapid, and 
requires close attention to observe it. If the coil is now stopped for a longer or shorter 
