170 Prof. G. Wiedemann on the Laws of the 



closed in tubes, this original impulse, and the quantity of 

 electricity necessary for a discharge, are on this very account 

 independent of the length and width of the tube — in so far at 

 least as the mutual action of the electricities accumulated 

 upon the electrodes, and that of the free electricity distributed 

 along the tube upon the electrodes themselves, can be neg- 

 lected. 



In like manner the tension required for a discharge is in- 

 dependent of the heating of the gas in the capillary tube, so 

 long as the glass is not decomposed by the heat and trans- 

 formed into a metallic conductor, because such a transforma- 

 tion would be equivalent to interposing a new electrode between 

 the original electrodes of the tube. 



The tension required for the commencement of a discharge 

 in different gases may depend, under conditions otherwise 

 similar, upon the nature of the gas itself, upon the capacity 

 for a charge of its several molecules, &c. 



In a gas-discharge proper the molecules do not fly from 

 one electrode to another ; against such an idea there may be 

 cited both the kinetic theory of gases, and the fact that 

 sodium vapour rendered luminous, either at an electrode or in 

 any part of a capillary tube, by a discharge passing through 

 it, remains nearly in the same place, instead of spreading 

 itself at once in smaller or greater quantity through the 

 whole tube. When also at higher pressure and with greater 

 charges, metallic discharges appear beside the gas-discharges 

 between two electrodes, whether in a free gas space or in a 

 tube, there are formed at the electrodes small tufts of lumi- 

 nous metallic particles, visible by the spectroscope. The 

 higher the pressure, the further these extend towards the 

 opposite electrode ; but only at a comparatively high pressure 

 do they span the entire interval. 



According to this, the passage of electricity between the 

 gas particles in a free space ought to take place at first 

 exactly as in a row of elastic balls hanging side by side. 

 When the charged gas-molecules A escaping from the elec- 

 trodes meet the more distant B, they separate the electricities 

 of the latter, and neutralize a part of their own electricity 

 with that of the opposite name of B, so that both molecules 

 appear to be charged with electricity of the same name. The 

 recoil resulting from this drives A back and B forward. A 

 receives fresh electricity either from the newly advancing 

 molecules or from the electrode, and again moves forward ; 

 while B, driven on by the electric recoil and the movement 

 imparted to it by A, gives up a part of its electricity to a 

 third portion, C, and falling back, again joins A, and so on. 



