the Electric Discharge in highly Rarefied Gaseous Media. 521 



completing the circuit, a luminous jet is formed which turns too 

 rapidly for its velocity of rotation to be measured, but which very 

 quickly spreads out so as to form at first for a few moments a 

 sector which continues to revolve, and soon afterwards a com- 

 plete circular sheet which no longer exhibits any perceptible 

 motion. 



It must not be thought that the action of magnetism ceases 

 when the gas is too far rarefied for there to be any longer a vi- 

 sible rotation. The action then shows itself under another form, 

 as is proved by experiments made at pressures of 3 to 2 millims. 

 Thus if the ring is employed as negative electrode, the violet 

 sheath which surrounds it is seen to be depressed at the 

 moment of magnetizing the soft iron, and to rise at the moment 

 of demagnetizing it. If, on the other hand, the ring is made 

 the positive electrode, the rose-coloured sheet, which occupies the 

 space between the ring and the central iron rod, is raised, as well 

 as the violet sheet which issues from the summit of the rod, at 

 the moment of magnetization, and depressed at the moment of 

 demagnetization. 



The following more complete experiment with dry nitrogen 

 proves that the rotation begins to show itself at higher pressures 

 when the ring is positive than when it is negative : — 



Pressure. 



Number of turns in one minute. 



Ring positive. 



Ring negative. 



millims. 

 35 

 29 

 21 

 16 

 12 



8 



6 



5 



12 

 27 

 45 

 67 

 99 

 115 



36 

 51 

 59 

 70 

 115 

 150 



At 4 millims. pressure the rotation is too rapid to be counted, 

 and at 3 millims. it appears to cease completely. The rose- 

 coloured halo is very bright when the top of the soft-iron rod is 

 positive. Moreover, when there is no longer any rotation, a 

 depression or elevation may be observed under the influence of 

 the magnetization, just as with atmospheric air. 



The pressure of vapour modifies the results obtained with dry 

 gases in some important respects. In the following experiment 

 with ordinary air reduced to a pressure of 2 millims., aqueous 

 vapour was introduced in successive quantities, so as to increase 

 the pressure solely by the effect of the presence of the vapour. 



