Conductivity of Flames containing Salt Vapours. 121 



cool the current was much smaller, but if the upper gauze was kept 

 hot by passing a current through it, then the current with a large 

 E.M.F. was independent of the distance between the electrodes, even 

 when the upper electrode was above the point of the flame. 



If both of the electrodes were hot, then the current, as the E.M.F. 

 was increased, attained a nearly constant value. Cooling the positive 

 electrode by raising it in the flame caused the current to increase 

 towards this saturation value much more slowly than before, while 

 cooling the negative electrode, the positive one being hot, caused the 

 current to show no sign of arriving at a maximum value. The current 

 was much greater when the negative electrode was hot, and the positive 

 electrode cool, than when the negative electrode was cool, and the 

 positive one hot. 



The fall of potential in the flame between the gauzes was examined 

 by putting in an insulated platinum wire, and finding the potential it 

 took up. When both the electrodes were hot, the fall of potential 

 closely resembled that observed in gases at low pressures- That is to say, 

 near each electrode there was a comparatively sudden fall of potential 

 much greater near the negative electrode than near the positive, with 

 a small and nearly uniform gradient in between. If either of the 

 electrodes was cooled, then the fall of potential near that electrode 

 became much greater, and often was nearly equal to the total drop of 

 potential between the electrodes. This effect was usually much more 

 marked in the case of cooling the negative electrode than with the 

 positive electrode. 



If the positive electrode was uppermost and somewhat cool, then 

 with small E.M.F.'s practically all the potential fall occurred near to 

 the positive electrode; but if the E.M.F. was sufficiently increased, 

 then a drop of potential appeared at the negative electrode, and with 

 a still greater E.M.F. this became greater than that at the positive 

 electrode, as it is in gases at low pressures. 



Some of the results obtained pointed to the conclusion that nearly 

 all the ionisation of the salt vapour takes place at the surfaces of the 

 glowing electrodes, and not throughout the volume of the flame. A 

 variety of experiments were tried to test this view, all of which con- 

 firmed its correctness. Thus, with two platinum foil electrodes opposite 

 one another in the flame, no increase in the current between them 

 occurred when a bead of salt was put between them, so that the salt 

 vapour from it passed between them without touching either electrode. 

 If the vapour came in contact with the negative electrode, there was 

 a great increase in the current, and a considerable but smaller increase 

 when it came in contact with the positive electrode. 



The relative velocities of the ions of alkali metal salts in the flame 

 were estimated by finding the potential gradient necessary to make the 

 ions travel down the flame against the upward current of gases.. 



