Ions of Alkali Salt Vapours in Flames. 715 



It will be observed that in this equation the f's apply to 

 very small concentrations, while the g's are for a comparatively 

 large concentration. 



Sir J. J. Thomson has given the theory* of the relation 

 between the potential difference (V) between two electrodes 

 in a flame and the current (i). He obtains the equation 



When the electrodes are near together, as was the case in 

 the measurements of the currents with different salts, the 

 first term can be neglected, so that 



V: 



4iire z k^k\S'fl 



If % denotes the current obtained with another salt and the 

 same V, then 



©•-$)*(!)'• 



The experiments show that fk x =fk{. Hence, using (4), 



Here i and i' are the currents given by the same P.D. with 

 two different salt vapours of equal concentration. The ratio 

 i/i' ought therefore to be independent of the concentration. 

 "When the concentration of the salt is not too large 

 this is true, for then the current is proportional to a 

 power of the concentration, which is about one-half for 

 all salts. 



The following table t gives the currents observed when a 

 ^ normal solution of the chloride was sprayed into the flame 



* 'Conduction of Electricity through Gases,' 2nd ed. 



t " The Electrical Conductivity and Luminosity of Flames containing 

 Vaporized Salts," Smithells, Dawson, and Wilson : Phil. Trans. A. 

 ccxli. (1899). 



3 A 2 



