446 Prof. J. J. Thomson on the Passage 



reversing the commutator, the battery was thrown out of 

 circuit and the galvanometer in. In this case no deflexion of 

 the galvanometer was obtained though the current from 150 

 Daniell's cells had passed through the gas for several minutes. 

 As, however, it seemed possible that the polarization of the 

 electrodes, if it existed, might have disappeared in the short 

 time taken to reverse the commutator, another experiment was 

 tried. In this a very large resistance, formed by rubbing 

 graphite on a piece of ebonite, was put in parallel with the 

 hot-gas resistance. The graphite resistance was so large that 

 practically all the current went through the hot gas ; and the 

 galvanometer-needle was so still that any disturbance of it 

 could easily have been detected. On breaking the battery 

 circuit, however, no kick of the galvanometer could be 

 detected, though as before the current from a battery of 150 

 Daniell's cells had been flowing through the hot gas for 

 several minutes. 



Electrolytic polarization is connected with the work re- 

 quired to split up the electrolyte into its ions. In the case of the 

 hot gas on our supposition the molecules are already split up 

 into their ions, so that the absence of polarization in this 

 case is not inconsistent with its presence in electrolysis. 



At first sight it might seem that the fact that those gases 

 which dissociate conduct electricity on a scale altogether 

 different from those which do not, points to the theory that 

 the conductivity of electrolytes is due to the presence in the 

 liquid of free ions, the number of these being very large, in 

 seme cases so large that it corresponds to a dissociation of 

 more than 90 per cent, of the electrolyte. I think, however, 

 that a closer examination of the case will lead to the opposite 

 conclusion. Let us take the case of iodine for example, 

 which at high temperatures is known to dissociate largely. 

 The specific resistance of iodine vapour at a yellow heat is 

 comparable with -j^ of a megohm, or about the same order of 

 magnitude as that of glass at the temperature 300° C. This 

 resistance is enormously greater than that of a solution of 

 potassium iodide for example, containing the same number of 

 molecules per unit volume as the gas ; on the hypothesis of 

 free ions, with definite electrical charges, we should expect the 

 opposite result, as we cannot suppose that the resistance to the 

 motion of the free ions is as great in the gas as it is in the 

 liquid. The agreement between the conductivity of a salt 

 solution and the effect produced by the presence of the salt on 

 the freezing-point and vapour-pressure does not seem to be 

 at all a conclusive argument as to the existence of these 

 free ions ; for exactly the same result would follow on the hy- 

 pothesis that the conductivity was proportional to the chemical 





