122 Electrical Conductivity of Flames containing Salt Vapours. 



This was done by putting a bead of salt between the two gauze 

 electrodes, and finding what E.M.F. was necessary to produce an 

 increase in the current between the electrodes when the bead was 

 put in. 



The potential gradient corresponding to this least E.M.F. was then 

 determined. In this way it was found that the positive ions of salts 

 of Li, Na, K, Eb, and Cs, all have nearly the same velocity in the 

 flame, whilst the negative ions of various salts of these metals also 

 have equal velocities which are about seventeen times as great as the 

 velocities of the positive ions. 



The velocity of the positive ions was estimated to be about 60 cm. 

 per second for one volt a cm., and that of the negative ions was about 

 1000 cm. per second. 



The relative velocities of the ions of various salts was also deter- 

 mined in a current of air at about 1000° C, which was obtained by 

 passing the air through a platinum tube 1*3 cm. in diameter, and 50 

 cm. long, heated in a gas-tube furnace. The method used was exactly 

 analogous to that used in the flame. The ions could be divided into 

 three classes, in each of which all the ions had equal velocities, viz. : — 



Telocity. 



1. Negative ions of salts of Li, Na, K, Kb, Cs, Ca, Sr, 



and Ba 26'0 cm.-sec. 



2. Positive ions of salts of Li, Na, K, Rb, and Cs 7*2 „ 



3. Positive ions of salts of Ca, Sr, and Ba 3 - 8 „ 



It thus appears that those ions which in solutions carry equal 

 charges have equal velocities in the gaseous state. This points to the 

 conclusion that the velocity of a gaseous ion in a given medium de- 

 pends only on its charge. The velocities are less than those calculated 

 for ions consisting of one atom, so that each ion appears to be a cluster 

 of atoms. If we regard this cluster as held together by the charge on 

 it, then it is reasonable to suppose that the size of the cluster will be 

 determined by the charge. Hence those ions having equal charges 

 will be of equal sizes, and consequently of equal masses, since the 

 atoms forming the cluster probably come from the medium rather 

 than from the small quantity of salt present. Consequently they all 

 have the same velocity under similar conditions. 



The two main results arrived at in this paper, viz., that the ionisa- 

 tion of the salt vapour in the flame takes place only at the surfaces of 

 the glowing electrodes, and that the velocity of the negative ions in 

 the flame is very much greater than the corresponding velocity of the 

 positive ions, enable the phenomena of unipolar conduction to be very 

 easily explained. For example, if one electrode is much hotter than 

 the other, then if the hot electrode is negative, it will give off negative 

 ions very freely, and there will be a large current; but if the hot 



