138 



NATURE 



[Dec. 12, 1878 



■when positive rotation commences, becoming very rapid as the 

 dark space further iuBreases in size, and ultimately flattening 

 against the glass. 



Convergence of Molecular Rays to a Focus 



The subject next investigated is the convergence of the lines 

 of force to a focus, as ob-erved with the aluminium cup. As 

 this could not be accomplished during rapid rotation an instru- 

 ment was made having the cup-shaped negative pole fixed, 

 instead of movable. On exhaustion, the convergence of the 

 lines of force to a focu^ at the concave side was well observed. 

 When the dark space is very much larger than the cup it forms 

 an irregular ellipsoid drawn in towards the focal point. Inside 

 the luminous boundary a focus of dark violet light can be seen 

 converging, and, 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 re- 

 flected 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 espe- 

 cially if the cup is made positive, the whole bulb becomes 

 beautifully illuminated with greenish-yellow phosphorescent 

 light. 



This greenish -yellow phosphorescence, characteristic of high 

 exhaustions, is frequently spoken of in the paper. It must be 

 remembered, however, that the particular colour is due to the 

 special kind of soft German 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 4 M ^ no light other than this is seen in the 

 apparatus. At "9 M the phosphorescence is about at its 

 maximum. When the exhaustion reaches '15 M the spark has 

 a difficulty in passing, and the green light appears occasionally 

 in flashes only. At '06 M the vacuum is almost non-con- 

 ductive, 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 ob- 

 served. 



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 further from 

 the pole as the exhaustion increases. In the apparatus described, 

 at an exhaustion of I9'3 M, these two foci are seen simul- 

 taneously, 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 cf the positive pole in the tube makes 

 scarcely any diff'erence 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. 



c. The spectrum of the green light is a continuous one, most 

 of the red and the higher blue rays being absent ; while the 

 spectrum of the light observed in the tube at lower exhaustions 

 is characteristic of the residual gas. No difference can be 

 detected by spectrum examination in the green light, whether 

 the residual gas be nitrogen, hydrogen, or carbonic acid. 



d. The green phosphorescence commences at a different 

 exhaustion in different gases. 



e. The viscosity of a gas is almost as persistent a characteristic 

 of its individuality as its spectrum. The author refers to a pre- 

 liminary note and a diagram - of the variation of viscosity of air, 



' M signifies the millionth o£ an atmosphere. 



' Proc. Roy. Soc, Nov. 16, 1876, vol. xxv. p. 305. 



hydrogen, and other gases at exhaustions between 240 M and 

 "I M. From these and other unpublished results, the author 

 finds that the viscosity of a gas undergoes very little diminution 

 between atmospheric pressure and an exhaustion at which the 

 green phosphorescence can be detected. When, however, the 

 spectral and other characteristics of the gas begin to disappear, 

 the viscosity al>o commences to decline, and at an exhaustion at 

 which the green phosphorescence is most brilliant the viscosity- 

 has rapidly sunk to an insignificant amount. 



f The rays exciting green phosphorescence will not turn a 

 corner in the slightest degree, but nidiate from the negative pole 

 in straight lines, casting strong and sharply-defined shadows from 

 objects which happen to be in their path. On the other hand, 

 the ordinary luminescence of vacuum tubes will travel hither and 

 thither along any number of curves and angles. 



Projection of Molecular Shadows 



The author next examines the phenomena of shadows cast by 

 the green light. The best and sharpest shadows are cast by 

 flat disks and not by narrow pointed poles ; no green light what- 

 ever is seen in the shadow itself, no matter how thin or whatever 

 may be the substance from which it is thrown. 



From these and other experiments, fully described in the 

 paper, he ventures to advance the theory that the induction- 

 spark actually illuminates the lines of molecular pressure caused 

 by the electrical excitement of the negative pole. The thick- 

 ness of the dark space is the measure of the mean length of 

 the path between successive collisions of the molecules. The 

 extra velocity with which the molecules rebound from the ex- 

 cited negative pole keep back the more slowly-moving mole- 

 cules which are advancing towards that pole. The conflict 

 occm-s- at the boundary of the dark space, where the luminous 

 margin bears witness to the energy of the collisions. 



When the exhaustion is sufficiently high for the mean length 

 of path between successive collisions to be greater than the dis- 

 tance between the fly and the glass, the swiftly-moving, rebound- 

 ing molecules spend their force, in part or in whole, on the 

 sides of the vessel, and the production of light is the conse- 

 quence of this sudden arrest of velocity. The light actually 

 proceeds from the glass, and is caused by fluorescence or phos- 

 phorescence 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 direct impact of the molecules, ift 

 the same electrical state, on the surface of the glass. 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 TJiin 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 

 inclosed in a tube with terminals, and exhausted to a few mil- 

 lionths 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 

 suddenly 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 limiinous, 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 

 tlie 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.. 



