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XXXII. The Ionization produced h(j Hot Platinum in xlir. 



Trinity College, Cambridge. 

 July 4, 1903. 



To the Editors of the Philosophical Magazine. 



Gentlemen, — j 



IX his very interesting paper on " The Positive Ionization 

 produced by Hot Platinum in Air at Low Pressures/' in 

 the July number of the Philosophical Magazine, Mr. 

 Richardson comes to the conclusion that the positive leak 

 from a hot platinum wire in air at low pressures is not a 

 definite function o£ the temperature of the wire, but that it 

 gradually dies away with time.' He states further that " the 

 phenomenon is exactly analogous to a monomolecular che- 

 mical reaction. The quantity which corresponds to the 

 velocity^ of the reaction is probably a function of the pressure 

 of the gas and of the temperature of the wire. 



^' In air at atmospheric pressure the falhng off of the con- 

 ductivity with time is much slower than at low pressures. 

 Thus Mr. H. A. Wilson * found the following numbers . . .■" 



While I entirely agree with most of his conclusions, I do not 

 think that the results which I have described in the paper from 

 which Mr. Richardson quotes justify the conclusion that the 

 rate of decay of the positive leak is less at high pressures 

 than at low. I think also that only a part of the positive leak 

 can be correctly described as analogous to a monomolecular 

 reaction. 



I have show^n {loc. cit.) that on first heating platinum in air 

 at atmospheric pressure a large leak is obtained, the greater 

 part of which dies away in a few minutes. The temperatures 

 at which I worked were higher than Mr. Richardson's, and 

 the rate of decay of the initial temporary leak in my expe- 

 riments was greater than the rates of decay which Mr. 

 Richardson's experiments at much lower pressures indicate. 

 The numbers w^hich Mr. Richardson quotes as showing a 

 slow rate of decay at atmospheric pressure are values of the 

 more permanent part of the positive leak obtained after the 

 temporary part of the leak had entirely disappeared. 



It was shown in my paper [loc. cit.) that the comparatively 

 permanent part of the leak w^hich remains slowly diminishes 

 with long continued heating of the platinum. In air, how- 

 ever, it does not fall to a zero value, but a permanent leak 

 ultimately remains which does not decay with time. This 



* Phil. Trans. A. vol. cxcvii. p. 415 (1901). 



•^in 



