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XIX. The Production of Light by the Recombination of Ions. 

 By CD. Child* 



IN a recent article by Strutt f a description is given of 

 experiments on the luminous vapour distilled from the 

 mercury arc. The conclusion is there reached that the light 

 given off by the vapour comes from the positive ions, while 

 the conclusion reached in a recent article by myself J was 

 the opposite of this. My reasoning led to the belief that 

 the light does not come from either the positive or the 

 negative ions alone, but from the two at the time of 

 recombining. I desire to present the following reasons for 

 continuing to hold the conclusions of my previous article. 



The experiment on which Strutt bases his conclusion can 

 be shown with the aid of fig. 1. The luminous vapour comes 



Fig. 1. 



f 

 1 

 1 



1 



c 



f 



I 



1 

 1 



b 



Os 



from an arc at the right, a is an exploring wire, b and c 

 are iron-wire nets connected to a source of current inde- 

 pendent of that producing the arc. When the current passes 

 /from b to c the region between these nets is luminous ; when 

 it passes in the opposite direction the region is non-luminous. 

 Strutt believes that this is due to the presence of positive 

 ions in the former case and to their absence in the latter 

 case. He states that in the former case one-half of the 

 negative ions may be drawn from the field without affecting 

 ihe luminosity to an appreciable extent, and therefore the 

 light cannot be due to the recombination of the ions. Strutt's 

 reasons for believing that many of the negative ions are 

 .taken out of this region when b is the anode is as follows. 



* Communicated by the Author, 

 t Proc. Roy. Soc. A. xci. p, 92 (1914). 

 X Phil. Mag. [6] xxyi. p. 906 (1913). 

 L2 



