402 H. V. Gill — Electric Discharge in Geissler Tubes. 



Toepler* investigated these air disturbances by the 'strobo- 

 scopic method ' of Plateau, and found that there are periodic 

 expansions and contractions of the air along the line of dis- 

 charge ; these results are illustrated in his paper, which it will 

 be useful to consult in connection with the experiments we are 

 about to describe. 



Antalikf and also Quet and Seguing found that powder, such 

 as finely divided carbon, placed on a plate between the ter- 

 minals arranged itself in ridges at regular intervals along the 

 line of discharge. Joly§ made some interesting experiments 

 of a somewhat similar nature. These experimentalists, how- 

 ever, seem to have attributed the results to purely electrical 

 causes. 



The different effects produced by sparks have been imitated 

 by means of hydraulic models, in which currents of water 

 were made to take the place of the electric currents. We 

 read for example : J "But the case in which the imitation of 

 stratified light is striking is when one produces currents of 

 liquids or air, interrupted at regular intervals by the vibrations 

 of a tongue at the mouth of the tube by which the jet of 

 water is projected on the layer of powder." 



We now pass to the consideration of the stratified discharge 

 in vacuum, in connection with which are three facts on which 

 our theory rests, which will be seen at once to be but logical 

 deductions from the experiments in air just cited. 



These facts may be stated as follows : 



1st. The discharge which gives rise to stratification is never 

 a continuous one, but is intermittent — and as we have seen is 

 probably oscillatory. 



2d. The gas at a stratum is not in the same physical condi- 

 tion as at an obscure place, but is of a quasi-resisting nature. 



3d. It follows from the first that the strata are not places of 

 continuous brightness, but are alternately bright and obscure. 



That the first exists is proved by the following facts. 



Spottiswoode^f examined the striae in a rapidly rotating mir- 

 ror, and noticed that the image of a stratum presented, not a 

 continuous streak of light, but a series of short bright lines sep- 

 arated by dark intervals. His remarks refer to the effect of a 

 single spark of his great coil. He states that sometimes the 

 strise have a general progressive movement, usually from the 

 positive pole. He showed that this apparent movement is 



* Pogg. Ann., cxxxiv, p. 194, 1868; see "Recent Researches," J. J. Thomson. 



f Id. cliv, p 14, 1815. 



\ Comptes Rendus, xlviii, p. 840, 1859. 



§ Proc. Roy. Soc , xlvii, p. 78, 1890. 



|| C. Decharme. Ann. Chim. Phys. (5) Ixxix, p. 416, etc., 1883. 



^[ Proc Roy. Soc, xxv, p. 73. 



