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 LXXXII. Theory of Ionization by Collision. 

 To the Editors of the Philosophical Magazine. 

 GrENTLEMBN, — 



IX a paper on the theory of ionization by collision, pub- 

 lished in the March number of the Philosophical 

 Magazine, Mr. Campbell criticises my work on that subject, 

 and explains what he imagines is the basis on which the 

 theory depends. I would like to point out a few facts in con- 

 nexion with that subject, and also to make some remarks on the 

 following statements which appear among his conclusions : — 



1. With regard to my assumption that the negative ion 

 does not undergo any permanent change in its properties as 

 a consequence of repeated collisions, " This assumption is 

 contradictory to the older view of ionization by collision, 

 according to which the negative ion when first formed is an 

 electron, but subsequently becomes a complex ion by gathering 

 round it neutral molecules." 



2. The mathematical argument is sketched, and it is pointed 

 out that it contains an error. 



With regard to the older views of the theory, as they 

 appeared in print, I think Mr. Campbell will find he is 

 mistaken. I have always held the same view with regard 

 to the electronic state of the negative ion when ionization by 

 collision is taking place. As far as I can find, the first paper 

 in which ionization by collision is mentioned as providing a 

 solution of any electrical problem is that which I published 

 in ' Nature,' August 9, 1900, describing the results obtained 

 with negative ions produced by Rontgen rays. A more 

 detailed examination was made of those experiments in the 

 paper in the Philosophical Magazine, Feb. 1901, in which it 

 was shown that if the negative ions generate others at a rate 

 a per centimetre under a force X in a gas at pressure p, the 

 three variables must be connected by an equation of the form 



- =/( — ), the hypothesis being that the negative ions act as 



ionizing agents throughout the whole time they are in the 

 gas, and that the positive ions are comparatively inactive. 

 The experiments are not so accurate as those made with 

 ultra-violet light, not because of the mathematical difficulty 

 of expressing the currents, but because the initial distribution 

 of the ions in the gas is unknown. They were quoted in my 

 book to which Mr. Campbell refers as showing that the 

 negative ions generated by Rontgen rays from the molecules 

 of any gas are all the same, being small compared with the 

 molecule of hydrogen, and identical with the ions set free 

 from a metal surface by ultra-violet light. This fact is of 



