142 Production of Light by the Me combination of Ions. 



of 28,000 cm. per sec. (the velocity of the stream of vapour), 

 and that one per cent, of the current was carried by an excess 

 of positive ions over negative ions, there would be a drop in 

 potential between b and a point 1 cm. from it of 14*5 volts*. 

 In a repetition of the experiment described in my previous 

 article I found that with a current of *06 milliampere the 

 difference in potential between b and a point 1 cm. toward 

 the cathode was somewhat less than 1 volt. In this experi- 

 ment the total potential difference between the electrodes 

 was 70 volts, and the distance was 1*1 cm. 



We are therefore justified in saying that the number of 

 negative ions per unit volume is very approximately the 

 same as that of the positive ions. Therefore, the number of 

 both positive and negative ions in the vicinity of b is inde- 

 pendent of the current. Therefore, the rate of recombina- 

 tion in this region is independent of the presence of a field, 

 and the assumption that the light is due to the recombination 

 is entirely reasonable. 



This reasoning does not hold in the immediate vicinity of 

 the cathode, for there the change in the electric force is 

 rapid. However, in that region the gas is not luminous 

 when a field is applied between b and c. Consequently the 

 conditions in that region are also quite in harmony with the 

 idea that the light is due to recombination. 



It should be noted that the preceding discussion does not 

 claim that all light is produced by recombination. If, as 

 has been found by Strutt, the luminous vapours rising from 

 arsenic and antimony arcs are not affected by the electric 

 field, there is no reason to think that such light is produced 

 by the recombination of ions, but such an experiment would 

 prove equally well that this light does not come from positive 

 ions, for we could not have the negative ions drawn out and 

 the positive ones left without the very great electrostatic 

 effects which have been discussed in the preceding paragraphs. 

 If this experiment with arsenic is an argument against the 

 idea that the light in the mercury vapour is produced by 

 recombination, as Strutt believes it is, it is equally an 

 argument against his idea that the light in the mercury 

 vapour is due to the vibration of positive ions. 



It is, however, quite unnecessary to draw such a conclusion 

 from this result. If the light from arsenic and antimony 

 vapours does not behave in an electric field in the same way 

 as that from mercury and sodium vapours, it seems mora 

 reasonable to assume that the light is produced by some 

 different mechanism. 



* Pliys. Rev. xii. p. 79(1901). 



