1879.] on Molecular Physics in High Vacua. 149 



tliey have been hammering and bombarding the glass till it is 

 appreciably warm, and at the same time they have been producing 

 another eJBfect on that glass — they have deadened its sensibility. The 

 glass has got tired, if I may use the expression, by the enforced 

 phosphorescence. Some change has been produced by this bombard- 



FlG. 9 A. 



ment which will prevent the glass from responding easily to additional 

 excitement ; but the part that the shadow has fallen on is not tired — 

 it has not been phosphorescing at all and is perfectly fresh ; there- 

 fore if I throw this star down, — I can easily do so by giving the 

 apparatus a slight jerk, for it has been most ingeniously constructed 

 with a hinge by Mr. Gimingham, — and so allow the rays from the 

 negative pole to fall uninterruptedly on to the end of the bulb, you 

 will suddenly see the black cross (c, d, Fig. 9 b) change to a luminous 



Fig. 9 b. 



one (e, /), because the background is only faintly phosphorescing, 

 whilst the part which had the black shadow on it retains its full 

 phosphorescent power. The luminous cross is now dying out. This 

 is a most delicate and venturous experiment, and I am fortunate in 

 having succeeded so well, for it is one that cannot bo rehearsed. 

 After resting for a time the glass seems to partly recover its power of 

 phosphorescing, but it is never so good as it was at first. 



We have, therefore, found an important fact connected with this 



