Prof. Challis on a Theory of Galvanic Force. 439 



of the transverse dimensions of the channel is a necessary con- 

 dition, the stream being throughout confined within limits. But 

 when the extremities of the electrodes are in vacuum, the con- 

 tinuity of the channel is broken ; and as from hydrodynatnical 

 principles it may be shown that the sether is incapable, by the 

 mutual action of its parts, of maintaining a spiral movement, 

 this movement ceases, first at the extremities of the electrodes, 

 then through their whole lengths, and the current is consequently 

 stopped. 



12. It appears from experiment that discharges of great 

 lengths may be passed from the extremity of one electrode to 

 that of the other, within glass tubes of small bore, wholly or 

 nearly exhausted. This may be explained on the principles of 

 the theory by saying that the form and non-conductive quality of 

 the tube are favourable to the maintenance of the spiral move- 

 ment. 



13. The current is maintained when the electrodes are con- 

 nected with the earth instead of being connected with each other, 

 simply because the earth is a conductor which may be considered 

 to be of unlimited extent. On this account the drain which 

 takes place at the extremity of one of the electrodes cannot pro- 

 duce an exhaustion of sether sufficient to cause a revulsion of the 

 stream, nor can the flow at the extremity of the other electrode 

 cause a repletion which would send back the stream. 



14. It is stated in Davy's ' Elements of Chemical Philosophy ' 

 (vol. i. p. 152), that with the same battery an electric discharge 

 took place in air of ordinary density between charcoal points 

 distant from each other about one-thirtieth of an inch, and in 

 rarefied air which supported one-fourth of an inch of mercury, 

 the sparks passed through nearly half an inch : also that in the 

 former case the constant or galvanic discharge took place in the 

 heated air through a space equal at least to four inches, and in 

 the latter through a space of six or seven inches. These facts 

 prove experimentally, not only that air resists the passage both 

 of electricity and of the galvanic current in greater degree as its 

 density is greater, but seem to show also that the galvanic dis- 

 charge may go on while the battery is in a state of electric ten- 

 sion. This inference is consistent with the hydrodynamical 

 theory. 



15. The theory distinctly separates the electric from the gal- 

 vanic discharges, the former being abrupt and violent streams 

 produced by the sudden return of superficial atoms in disturbed 

 positions to their natural relative state, and the latter being the 

 continuous effect of a disturbed state of the interior atoms. 

 These conditions of the superficial and the interior atoms are, 

 however, so intimately connected, that when the galvanic current 



