1878.] the Illumination of Lines of Molecular Pressure. 107 



production of light accompanies this sudden arrest of velocity. The 

 light proceeds from the glass, and is apparently caused by fluores- 

 cence or phosphorescence on its surface. No light is produced by a mica 

 or quartz screen, and the more fluorescent the material the better the 

 luminosity. Here the consideration arises that the greenish-yellow 

 light is an effect of the arrest of the negatively electrified molecules 

 by the surface of the glass ; but whether they actually strike the 

 glass, or whether at the boundary surface separating solid from gaseous 

 matter, there are intermediary layers of condensed gas which, taking 

 up the blow, pass it on to the layer beneath, are problems the solution 

 of which must be left to further research. The shadows are not 

 optical, but are molecular 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 observations 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 sud- 

 denly all over its surface with a yellowish phosphorescence, which, 

 however, instantly disappears. The uncovered part of the plate does 

 not become phosphorescent quite suddenly, but the phosphorescence is 

 permanent as long as the coil is kept at work. 



c. With an exceedingly faint spark the film remains more luminous 

 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 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 circumstances 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. 



