or FLAMES CONTAINING SALT VAPOURS. 
515 
were 5 centims. apart. When the upper electrode was negative, other conditions 
being the same, the current increased on introducing the bead, even with a potential 
difference of 1 or 2 volts. 
To prevent any ions reaching the lower electrode by passing down the sides of the 
flame where the velocity of the blast is small, a screen, PP (fig. 4), was placed above 
the lower electrode. The hole in the platinum gauze of the screen at D was 
2 centims. in diameter, and it was completely filled by the flame, which also passed 
through the gauze round the hole. The platinum grating on the lower electrode 
was bent up so that it was only 2 or 3 millims. below the gauze screen. Diagram 
No. 12 shows the results obtained with a bead of potassium carbonate. 
Diagi’am No. 12. 
It will be observed that the introduction of the bead produced no increase in the 
current when the upper electrode was positive until a definite E.M.F. was applied. 
It was possible to determine this E.M.F. within about 5 volts with certainty with 
salts of Cs, Rb, and K. With salts of sodium and lithium the amount of current 
obtained was small compared with the current through the flame without salt, which 
made it imjDossible to determine the necessary E.M.F. with any approach to accuracy. 
The following table gives the results obtained with carbonates of the alkali metals for 
the positive ions :—• 
Caesium. 104 volts. 
Rubidium. 100 ,, 
Potassium. 107 ,, 
Sodium.90 to 110 ,, 
Lithium.90 to 100 ,, 
The E.M.F. between the electrodes 5 centims. apart required to cause the positive 
ions of the carbonates of K, Rb, Cs, Na, and Li to move down against the blast of the 
flaune is about 100 volts, so that the positive ions of these salts must all have nearly 
the same velocity due to the slope of potential in the flame. With beads of the 
3 u 2 
