OF FLAMES CONTAINING SALT VAPOUES. 
509 
Diagram No. 9. Diagram No. 10. Diagram No. 11. 
(4.) The Ionisation of the Salt Vapour. 
Arrhexius (‘Wied. Ann.,’ 42, p. 18, 1891) concluded from the results of bis 
experiments that the conductivity of salt vapours in flames is due to partial ionisation 
of the salt by the high temperature of the flame, and that the conductivity of a flame 
containing a salt vapour is very analogous to the conductivity of an aqueous solution 
of the salt. In our paper referred to above we have not seen any reason to doubt the 
general accuracy of Arrhexius’ conclusions. 
There are, however, a number of important facts which do not readily lend 
themselves to explanation by the hypothesis just mentioned. The phenomena of 
unipolar conduction are among these. Hittorf (see Wiedemaxn’s ‘ Elektricitat,’ 
vol. 4 B) showed that the current depends very greatly on the negative electrode, 
and that it is greater when a bead of salt in the flame is near the negative electrode 
than when it is near the positive electrode, and he concluded that nearly all the 
resistance to the passage of the current is at or near to the surface of the negative 
electrode, at any rate in the case of flames free from salt vapour. 
The experiments of Arrhexius and those described in our pajier referred to above, 
in which the current between two electrodes very near together in the flame was 
measured, were not adapted for the examination of unipolar conduction and allied 
phenomena, and it was sufficient, in considering the results obtained, to suppose the 
conductivity due to ionisation of the salt vapour, without making any further 
hypothesis as to exactly how and where the ionisation occurs. At the same time, it 
was more or less tacitly assumed that the salt vapour is ionised throughout the 
volume of the flame, just as a salt is ionised in an aqueous solution. 
I have concluded, from the results described in this paper, that the ionisation of the 
salt takes place entirely, or very nearly so, at the surface of the glowing platinum 
electrodes, and not throughout the volume of the flame. The experiments described 
above on the variation of the current with the distance between the electrodes, show 
that when the two electrodes are both kept hot, then the saturation current is 
independent of the distance between the electrodes, whereas if the salt vapour were 
