ELECTPJCAL CONDUCTIVITY OF AIR AND SALT VAPOURS. 
429 
Diagram No. 10 shows the relation between the logs got from diagram No. 8. 
This gives log C = 12’8 log 6 — 45'8. 
According to these formulEe some current shonld l)e obtained at any temperature, 
however low. 
The above results show that the conductivity does not begin suddenly at a definite 
temperature, but always increases regularly with rise of temperature, so that the 
lowest temperature at which conductivity can be detected depends entirely on the 
sensitiveness of the galvanometer. 
The calculation of the energy required to ionize the conducting gas from the 
variation of the conductivity with the temperature will now be considered. 
The ionization practically all takes place in a thin layer close to the surface of 
the platinum, and the number of ions in this layer will depend on the rate at which 
they are formed and on their rate of recombination. 
For the purjjose of this calculation the rate of j^roduction of ions will be regarded 
as a function of the temperature and concentration of the gas in this layer. At 
any constant temperatnre, if C is the concentration of the gas and C' that of the 
j^ositive or negative ions supposed equal, we have 
C = aC'\ 
where a is a constant and it is assumed that one molecule of the gas dissociates 
into two ions. The case is evidently on these supjjositions exactly analogous to, 
say, the dissociation of N20^ into 2NO„ and the equation commonly employed to 
represent the variation of the dissociation with temperature in such cases can be 
employed. It is 
where g is the heat developed when 1 gramme molecule of the gas is formed from 
the ions by recombination, and x is the fraction of the gas dissociated at the 
absolute temperature T. g is supposed expressed in calories and is taken as being 
constant between T^ and Tg. 
If X is small then approximately 
The current when the potential gradient is small is equal to A n {k^ + F,), where 
n is the number of positive or negative ions in the gas per unit volume of the layer, 
k-^ and k .2 the velocities of the positive and negative ions respectively, and A a 
constant. Now k^ and k^ vary with the temperature, probably being proportional 
* See ‘ Lectures on Physical Chemistry,’ Vax’t Hoff, vol 1. 
