106 



Mr. W. Crookes on 



[Dec. 5, 



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 

 preliminary note and a diagram* of the variation of viscosity of 

 air, hydrogen, and other gases at exhaustions between 240 M and 

 •1 M. From these and other unpublished results, the author finds 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 phos- 

 phorescence can be detected. When, however, the spectral and other 

 characteristics of the gas begin to disappear, the viscosity also com- 

 mences to decline, and at an exhaustion at which the green phos- 

 phorescence 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 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 discharge. 



When the exhaustion is sufficiently high for the length of path 

 between successive collisions to be greater than the distance between 

 the fly and the glass, the swiftly moving rebounding molecules spend 

 their energy, in part or in whole, on the sides of the vessel, and the 

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



