126 MESSRS. A. SMITHELLs. 11. M. DAWSON, AND H. A. WILSON: ELECTRICAL 



the case of salt vapours, the high temperatures at which alone the conductivity can 

 be examined, the correspondingly greater mobility of the molecules or ions, as well as 

 the enormous reduction in the density and viscosity of the medium, and in the 

 concentration of the salt, give ample ground for expecting characteristics in the 

 phenomena of conduction very different from those which occur with liquid electro- 

 lytes at ordinary temperatures, although in both cases the conduction may be of a 

 truly electrolytic character. 



To take first of all the relation between current strength and electromotive force, 

 we have in the case of liquid electrolytes, provided polarisation of the electrodes is 

 avoided, a strict applicability of Ohm's law. In the case of salt vapours this law 

 only applies for low electromotive forces. This was found both by AKRHENIUS and 

 ourselves. ABRHENIUS, from theoretical considerations, believes that Ohm's law 

 should hold also for higher electromotive forces, and he concludes that the divergence 

 from it must be regarded as only apparent. This, however, leaves the divergence 

 entirely unexplained. 



For an expression capable of representing the relationship between current strength 

 and electromotive force we were led, as already stated, to a formula derived by 

 Professor J. J. THOMSON and Mr. RUTHERFOBD from their study of the conductivity 

 of gases subjected to Rontgen rays. In a gas exposed to Rontgen rays a steady 

 supply of ions is supposed to be generated and the resulting concentration of ions is 

 then determined by the fact that the rate at which they combine is proportional to 

 the square of the number present, assuming equal numbers of positive and negative 

 ions to be distributed throughout the gas. In a flame containing salt vapour it may 

 be supposed that a steady supply of ionising salt is carried up between the electrodes, 

 so that the conditions would be, to this extent, analogous in the two cases. 



In applying the formula to our results we have had to recognise a feature dis- 

 tinguishing the behaviour of salt vapours from that of Rontgenised gases, namely, the 

 fact that the current strength continues to rise slowly even at high E.M.F.s. The 

 explanation of this difference does not seem to be difficult.* It would be accounted 

 for by the increased electrostatic field either bringing in ions from the neighbourhood 

 of the electrodes or increasing the rate of ionisation between them. 



We have now to consider our results in reference to the question of the state in 

 which the salts exist in the flame, and give rise to the conductivity. It could hardly 

 be expected, prima facie, that the salts would all be vaporised without change, for 

 even those among them that are most stable under ordinary conditions are likely to 

 undergo some change of composition at the very high temperature reigning in the 

 flame, and in the presence of various flame gases. The liability to change is indeed so 

 great, and, at the same time, the precise character and extent of the change so little 



* A kind of convective conduction, proportional to the E.M.F. which ARRHENIUS recognised in his 

 experiments on alkaline earth metals, cannot here be in question. We have referred to this subject in 

 another connection on p. 98. 



