Lines of Molecular Pressure. 61 



is caused by fluorescence or phosphorescence on its surface. No 

 light is produced by a mica or quartz screen ; and the more fluo- 

 rescent the material the better the luminosity. Here the consi- 

 deration arises that the greenish -yellow light is an effect of the 

 direct impact of the molecules in the same electrical state on the 

 surface of the glass. The shadows are not optical, but are mole- 

 cular shadows, revealed only by an ordinary illuminating effect ; 

 this is proved by the sharpness of the shadow when projected from 

 a wide pole. 



Phosphorescence of Thin Films. — An experiment is next described 

 in which a film of uranium glass, sufficiently thin to show colours 

 of thin plates, is placed in front of a thick plate of the same glass, 

 the whole being enclosed in a tube with terminals and exhausted to 

 a few millionths of an atmosphere. Of this the following observa- 

 tions are recorded : — 



a. The uranium film, being next to the negative pole, casts a 

 strong shadow on the plate. 



b. On making contact with the coil, the thin film flashes out 

 suddenly all over its surface with a yellowish phosphorescence, 

 which, however, iustantly disappears. The uncovered part of the 

 plate does not become phosphorescent quite suddenly, but the phos- 

 phorescence is permanent as long as the coil is kept at work. 



c. With an exceedingly faint spark the film remains more lumi- 

 nous than the plate ; but on intensifying the spark, the luminosity 

 of the film sinks, and that of the uncovered part of the plate 

 increases. 



d. If a single intense spark be suddenly sent through the tube, 

 the film becomes very luminous, while the plate remains dark. 



These experiments are conclusive against the phosphorescence 

 being an effect of the radiation of the phosphorogenic ultra-violet 

 light from a thin layer of arrested molecules at the surface of the 

 glass ; for were this the case, the film could under no circum- 

 stances be superior to the plate. 



The momentary phosphorescence and rapid fading of the film 

 prove more than this. The molecular bombardment is too much 

 for the thin film. It responds thereto at first, but immediately 

 gets heated by the impacts, and then ceases to be luminous. The 

 plate, however, being thick, bears the hammering without growing 

 hot enough to lose its power of phosphorescing. 



Mechanical Actifoi of Projected Molecules. — "When the coil was first 

 turned on, the thin film was driven back at the moment of becoming 

 phosphorescent, showing that an actual material blow had been 

 given by the molecules. Experiments are next described in which 

 this mechanical action is rendered more evident. A small rotating 

 fly, capable of being moved about in any part of an exhausted bulb, 

 is used as an indicator ; and by appropriate means the molecular 

 shadow of an aluminium plate is projected along the bulb. Whether 

 entirely in or entirely out of the shadow, the indicator scarcely 

 moves ; but when immersed so that one half is exposed to molecular 

 impact, the fly rotates with extreme velocity. 



