and Ohmic Resistance of Gases. 365 



explained by conceiving of a polarized condition, in which 

 the molecules are in some way bound together by the electric 

 energy which is striving to force itself through them. As 

 the current increases the amount of the heat increases, until 

 it reaches a stage when some of the gas is freed from this 

 bondage — when the molecules not only separate from their 

 electric embrace, but split into their component atoms. Then, 

 if a large quantity of electricity is at hand to discharge itself, 

 the rate of discharge increases with enormous rapidity, 

 resulting in more dissociation, and the resistance is almost 

 entirely broken down. A good resume of the present state of 

 this hypothesis is given in ' Nature,' January 28th, 1897, 

 p. 310, and to this statement our determination of the re- 

 sistance makes an important addition. 



Hydrogen and oxygen cannot be dissociated to any appre- 

 ciable extent at ordinary temperatures and pressures, otherwise 

 water would form when they were mixed. We have no 

 vapour-densities of hydrogen or oxygen at temperatures 

 which show dissociation, bnt this is no reason for believing 

 that at temperatures of 3000 degrees or more dissociation 

 does not take place. Indeed, the burning of hydrogen and 

 oxygen gives us every reason for believing that the tendency 

 of both hydrogen and oxygen molecules to dissociate increases 

 with the temperature. Chlorine, bromine, and iodine are all 

 known to dissociate at high temperatures and to conduct 

 electricity well under those conditions. 



Another point in favour of ascribing the red glow of 

 hydrogen to dissociation is to be found in the fact that rarefied 

 aqueous vapour gives the pure " four-line " spectrum much 

 more easily than hydrogen itself. In order to give any 

 hydrogen spectrum at all, the vapour must be dissociated. 

 Of course the dissociation takes place only at the moment of 

 the discharge, the atoms combining again when cold. It is 

 caused by the heat of the discharge, and not by electrolysis, 

 although that too may take place at the electrodes. In short, 

 there is every reason to believe that at temperatures as high 

 as those with which we are dealing, the hydrogen is split 

 apart into hydrogen atoms, and that these atoms, or perhaps 

 the energy involved in the act of splitting them, are re- 

 sponsible for the " four-line " red spectrum. The fact cannot 

 be too strongly emphasized that this sort of dissociation is 

 very different indeed from the electrolytic dissociation of 

 solutions. 



In order to find if the structure which is dissociated by the 

 spark is the molecule or some more complex structure, 

 mercury vapour was subjected first to the continuous discharge 



Phil Mag. S. 5. Vol. 43. No. 264. May 1897. 2 F 



