60 Mr. W. Crookes on the Illumination of 



e. 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 character- 

 istic of the residual gas. No difference can be detected by spec- 

 trum-examination in the green light, whether the residual gas be 

 nitrogen, hydrogen, or carbonic acid. 



d. The green phosphorescence commences at a different exhaus- 

 tion 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 preli- 

 minary note and a diagram* of the variation of viscosity of air, 

 hydrogen, and other gases at exhaustions between 240 M and 

 0-1 M. From these and other unpublished results, the author 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 

 also commences to decline ; and at an exhaustion at which the green 

 phosphorescence is most brilliant the viscosity has rapidly sunk to 

 an insignificant amount. 



/. The rays exciting green phosphorescence will not turn a corner 

 in the slightest degree, but radiate 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 lumi- 

 nescence of vacuum-tubes will travel hither and thither along any 

 number of curves and angles. 



Projection of Molecular Shadoivs. — 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 whatever 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 thickness of the dark space is 

 the measure of the mean length of the path between successive col- 

 lisions of the molecules. The extra velocity with which the mole- 

 cules rebound from the excited negative pole keep back the more 

 slowly moving molecules which are advancing towards that pole. 

 The conflict occurs 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 distance 

 between the fly and the glass, the swiftly moving rebounding mole- 

 cules spend their force, in part or in whole, on the sides of the 

 vessel, and the production of light is the consequence of this sudden 

 arrest of velocity. The light actually proceeds from the glass, and 



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



