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



stopper having a loop underneath from which is suspended by 



two silk fibres a needle of decarbonized iron The needle 



was now placed at right angles to the tube and the point made 

 positive. After a few trials at different exhausts a beautiful 

 tongue-shaped stratification was obtained and it was then pos- 

 sible by altering the amount of current to make the apex of a 

 stratum impinge on one or other end of the needle : on which- 

 ever end the stratum touched that end was pushed away by it, 

 showing clearly that the balance of forces which held together 

 the molecules composing a stratum are sufficient to render it 

 viscous, and unyielding to a small resistance." 



De la Rue and Miiller* also came to the same conclusion from 

 other experiments that there were vortices in the gas at a 

 stratum. 



In a more recent paperf on this subject we read : " Another 

 effect, to which I have before alluded, is the apparent resistance 

 which the sharply-defined edges of the strata offer to the pene- 

 tration of the wire. The edge acts as if it had an elastic skin, 

 or a sort of surface tension, bending in as the wire pushes 

 against it, and finally snapping back to its original position, 

 leaving the wire well within the luminous disk. Too little is 

 known about the strata to make any discussion of these facts 

 worth while, and I merely record them without comment.";); 



Having thus enumerated the chief facts on which our theory 

 rests, it will be seen at once that it is their natural conse- 

 quence. It may be stated in general terms as follows : 



* Nature, Aug., 1883, p. 383. 



f R. W. Wood, Physical Review, Nov.-Dec, p. 204, 1896. 



\ It may be well to recall some of the conclusions arrived at by de la Rue and 

 Miiller (Phil. Trans., vol. clxix, p. 230) 



" 1°. The discharge in a vacuum tube does not differ essentially from that in air 

 and other gases at ordinary atmospheric pressures : it cannot be considered as a 

 current in the ordinary acceptation of the term, but must be of the nature of a 

 disruptive discharge, the molecules of gas acting as carriers of electrification." 



'• 2°. As the exhaustion proceeds, the potential necessary to cause a current to 

 pass diminishes up to a certain point, whence it again increases, and the strata 

 thicken and diminish in number till a point is reached at which, notwithstanding 

 the high e. m. f. available, no discharge through the residual gas could be obtained." 



" 3°. All strata have their origin at the positive pole." . . . 



" 6°. If the discharge is irregular and the strata indistinct, any alteration in the 

 amount of current makes the strata distinct and steady." 



" 7°. The greatest heat is in the vicinity of the strata." . . . 



" 11°. At the same pressure and with the same current the diameter of the tube 

 affects the character and closeness of the strata." 



In a later memoir (Phil. Trans., pt. 1, vol. clxxi, p. 108), they continue: 



" 1°. For all gases there is a pressure which offers the least resistance to the 

 passage of an electric discharge. After the minimum has been reached the resist- 

 ance to a discharge rapidly increases as the pressure of the medium decreases." . . . 



" 3°. When the discharge takes place there is a sudden dilatation of the medium 

 in addition to and distinct from that caused by heat. This dilatation ceases instantly 

 when the discharge ceases." . . . 



" 6°. The electric arc and the stratified discharge are modifications of the same 

 phenomenon." 



