468 The Electrical Properties of Flame. [Feb. 12, 



ance is nearly uniformly distributed along the flame. If now salt 

 vapour is put in anywhere between the electrodes the current is 

 increased. If, for example, we fill half the length of the flame with 

 salt vapour, we nearly double the current. 



When salt is put on one electrode, the flame can be used as a 

 rectifier for an alternating current, for when the salted electrode is 

 negative the resistance of the flame is much smaller than when it is 

 positive. 



I have measured the conductivities of a number of alkali salt 

 vapours in a current of air flowing along a platinum tube heated in 

 a gas furnace. An electrode was fixed along the axis of the tube, 

 and the current from it through the salt vapour to the surrounding 

 tube was measured with a galvanometer. It was found that at tem- 

 peratures above 1400° C, and with electro-motive forces of about 

 1000 volts, the current was proportional to the amount of salt passing 

 through the tube, and for difl'erent salts in equal quantities inversely 

 proportional to the electrochemical equivalent of the salt. This shows 

 that the quantity of electricity per molecule of salt is the same for all 

 salts. It was also found that the quantity of electricity carried per 

 molecule was equal to that carried per molecule when a solution of salt 

 in water is electrolysed. It appears therefore that the law^s of electro- 

 lysis discovered by Faraday for liquids apply also to salts in the state 

 of vapour. 



When a molecule of salt like sodium chloride dissociates into two 

 ions in water, the sodium atom forms the positive ion and the chlorine 

 atom the negative ion, and when a current is passed through the 

 solution the sodium is attracted to the negative electrode and the 

 chlorine goes to the positive electrode. We might expect the same 

 thing to happen when a current is passed through the salt vapour in 

 a flame. If we put two wires in the flame, and put some sodium salt 

 on one and then connect them to an induction coil, and pass a dis- 

 charge from the salted one to the other, we find that the yellow sodium 

 vapour appears at it when it is the negative pole but not when it is 

 positive. This shows that in the flame the positive ions of the salt 

 vapour contain the metal just as they do in solutions. The negative 

 ions, however, do not appear to be the same in flames as in solutions. 

 In flames the very high velocity of the negative ions indicates that 

 they are the electrons whose properties have been investigated in 

 vacuum tubes by Sir William Crookes and Sir Joseph Thomson. 

 The positive ion, then, is an atom or molecule, while the negative ion 

 is an electron, the mass of which is several thousand times smaller. 

 This is the explanation of the fact that the negative ions move 100 

 times more quickly than the positive ions. 



[H.A.W.] 



