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PHOSPHORESCENCE. 84 
are good instances of a practically silent discharge accompanied by 
fairly well-marked luminosity. 
Electric discharges in vacuo give rise to very beautiful appear- 
ances, but accompanied by a light much less intense per fixed area 
than the light from the Glow-worm or Firefly. (The intensity of 
the light is one of the remarkable features of these insects.) 
By passage of the electric current through gases and vapours 
the light is generally enhanced, and varies in tint according to the 
kind of gas enclosed in the experimental tube employed. These two 
conditions of a rarified medium and the presence of certain gases in 
relatively small percentages appear to furnish the key to that 
glorious phenomenon of the Arctic Circle—the Aurora Borealis. 
Very striking results, too, are obtained by passing an electric 
current in vacuo so as to excite fluorescence in sensitive substances. 
We will now show the effect on uranium glass. We can go a step 
further and get a beautiful display of colour in substances—calcined 
shells, spars, &c.—which not only phosphoresce or fluoresce whilst 
the current is passing, but continue to glow somewhat feebly but 
sustainedly after the current is cut off. This phenomenon is closely 
related to the phosphorescence of substances like luminous paint 
after “‘ Insolation.” 
The discovery and separation of radio-active substances has re- 
vealed, as it were, a fresh world of wonders. Amongst the many 
points connected with radium I should like to dwell for a moment 
on what, perhaps, may be regarded as the chief one, namely: that 
the luminosity of radium is caused or accompanied by an incessant 
discharge of atoms or molecules of one or more substances carrying 
a high electrical charge which may or may not leave the particle 
When it comes into collision with neighbouring solids. Madame and 
Mons. Curie (all honour to them for their share in a splendid piece of 
: research work) found that objects even of an inert character adjacent 
