MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 
151 
reflected light, and are only large enough to partially cover the uranium plate. The 
plate and the two films are supported on a light aluminium frame. 
535. When the tube is exhausted to within a few millionths of an atmosphere, and 
the pole d is made negative, the uranium film b casts a strong shadow on the uranium 
plate a. No phosphorescence can be detected on the part of the plate which is 
beneath the films, but where the plate is uncovered it shines with its characteristic 
greenish light. 
536. On making contact with the coil, the film b flashes out suddenly, all over its 
surface, with a yellowish-green phosphorescence, which instantly disappears except at 
the part where it is stuck to the aluminium support at its back. The plate does not 
seem to become phosphorescent quite suddenly, but the phosphorescence remains 
permanent as long as the coil is kept at work. 
537. When the contact hammer of the coil is screwed so as to make the spark 
excessively faint, the thin film remains more luminous than the plate ; but if the screw 
is turned so as to get an intense spark, the luminosity of the film sinks, and that of 
the uncovered part of the plate increases; the plate now being three or four times 
brighter than the film. 
538. If an intense spark be used and the contact made and broken suddenly, so as 
to send one spark at a time through the tube, only the film becomes visible, flashing 
out with a brilliant yellowisli-green light, the plate, even in its uncovered part, 
remaining dark. 
539. Exactly similar phenomena take place if the pole e be made negative, so as to 
experiment with the film of German glass, c. The only difference is in the colour of 
the phosphorescent light, which is much yellower with German than with uranium glass. 
In other tubes, English glass, which phosphoresces with a bluish light, has been found 
to behave in the same way. A very thin film of quartz casts a perfectly opaque 
shadow, but it does not become phosphorescent. The same thing occurs with mica. 
540. These experiments are conclusive against the phosphorescence being caused by 
the radiation of phosphorogenic ultra-violet light from a thin layer of arrested mole¬ 
cules at the surface of the glass, for were this the cause the film could under no 
circumstances be superior to the plate. 
The momentary phosphorescence of the film, and its rapid fading out, prove more than 
this. The molecular bombardment is too much for the thin film. It responds to it at 
first, but immediately gets heated by the impacts and then ceases to be luminous, 
except where it is attached to the aluminium support, which conducts the heat away. 
The plate, however, stands the heating without getting hot enough to lose its power of 
phosphorescing. The experiment demonstrates for the first time the transference to a 
finite distance, from molecule to molecule, of even that short period vibration on which 
phosphorescence depends, and it further exhibits experimentally the decadence of 
phosphorescence by transformation into a disturbance of longer period, which is 
quickly propagated from molecule to molecule, and carried away as heat. 
