Lines of Molecular Pressure. 59 



point. Inside the luminous boundary a focus of dark violet light 

 can be seen con verging, arid, as the rays diverge on the other side 

 of the focus, spreading beyond the margin of the dark space — the 

 whole appearance being strikingly similar to the rays of the sun 

 reflected from a concave mirror through a foggy atmosphere. 



Green Phosphorescent Light of Molecular Impact. — At very high 

 exhaustions the dark space becomes so large that it fills the tube. 

 Careful scrutiny still shows the presence of the dark violet focus ; 

 and the part of the glass on which fall the rays diverging from this 

 focus shows a sharply defined spot of greenish-yellow light. On 

 still further exhaustion, and especially if the cup is made positive, 

 the whole bulb becomes beautifully illuminated with greenish-yellow 

 phosphorescent light. 



This greenish-yellow phosphorescence, characteristic of high ex- 

 haustions, is frequently spoken of in the paper. It must be remem- 

 bered, however, that the particular colour is due to the special kind 

 of soft G-erman glass used. Other kinds of glass phosphoresce 

 in a different colour. The phosphorescence takes place only under 

 the influence of the negative pole. At an exhaustion of 4M* no 

 light other than this is seen in the apparatus. At 0'9M the phos- 

 phorescence is about at its maximum. "When the exhaustion 

 reaches 0-15 M, the spark has a difficulty in passing, and the green 

 light appears occasionally in flashes only. At 0-06 M the vacuum 

 is almost non-conductive ; and a spark can be forced through only 

 by increasing the intensity of the coil and well insulating the tube 

 and wires leading to it. Beyond that exhaustion nothing has been 

 observed. 



Focus of Molecular Force. — In an apparatus specially constructed 

 for observing the position of the focus the author found that the 

 focal point of the green phosphorescent light was at the centre of 

 curvature, showing that the molecules by which it is produced are 

 projected in a direction normal to the surface of the pole. Before 

 reaching the best exhaustion for the green light, another focus of 

 blue-violet light is observed ; this varies in position, getting fur- 

 ther from the pole as the exhaustion increases. In the apparatus 

 described, at an exhaustion of 19'3M, these two foci are seen 

 simultaneously, the green being at the centre of curvature, while 

 the blue focus is at nearly twice the distance. 



Nature of the Green Phosphorescent Light. — The author adduces 

 the following characteristics of the green phosphorescent light, as 

 distinguishing it from the ordinary light observed in vacuum-tubes 

 at lower exhaustions : — 



a. The green focus cannot be seen in the space of the tube, but 

 where the projected beam strikes the glass only. 



b. The position of the positive pole in the tube makes scarcely 

 any difference to the direction and intensity of the lines of force 

 which produce the green light. The positive pole may be placed in 

 the tube either at the extremity opposite the negative pole, or below 

 it, or by its side. 



* M signifies the millionths of an atmosphere. 



