Conductivity of Hot Gases. 417 



means of such fine particles that convective conduction takes 

 place. 



That Prof. Thomson has really had to do with convective 

 conduction (for the most part) in his experiments seems to me 

 apparent from several circumstances. Convective conduction 

 is favoured relatively to electrolytic by low temperatures and 

 high E.M.F.'s. Now Prof. Thomson worked at a temperature 

 100° lower, and with an E.M.F. 60 times greater, than I em- 

 ployed in my experiments; in some of which, even, convective 

 conduction played a considerable part. Again, conduction 

 by means of solid particles obeys Ohm's Law ; while electro- 

 lytic conduction (with E.M.F. above 1 Dan.) is characterized 

 by large deviations from this law. Prof. Thomson found in 

 his experiments (with high E.M.F.) that Ohm's Law was 

 obeyed ; consequently the conduction was probably con- 

 vective, as it was in my experiments with salts of magnesium, 

 calcium, and strontium. 



Prof. Thomson's results are thus explained very naturally 

 by the following two assumptions : — ■ 



1. That, as in the analogous researches of Becquerel, 

 Blondlot, and Wiedemann and Ebert, a preponderating non- 

 electrolytic conduction through dust-particles took place. 



2. That free halogens, like chlorine in the experiments of 

 Crafts, had a corrosive influence on the platinum vessel and 

 electrodes, whereby the formation of fine particles (and conse- 

 quently the convection) was increased. 



The experiments with mercury vapour (Hg) show in the 

 most decisive manner that Prof. Thomson's view of the activity 

 of atoms in gaseous conduction is untenable : for it is in the 

 highest degree improbable that the opinion, grounded on so 

 many circumstances (e. g. vapour-density, specific heat) , that 

 gaseous mercury-molecules consist of single atoms, will be 

 sacrificed in order to save Prof. Thomson's theory. 



Finally, Prof. Thomson refers to my query : lt Who can 

 prove that iodine vapour decomposes entirely into ions ? It 

 might dissociate into uncharged atoms." This question 

 Prof. Thomson answers by asking two others connected 

 with it ; and these, as it chances, are very easy to dispose 

 of. The first is : " If the atoms of a gas (I 2 ) can be without 

 charge, why cannot those of a salt (NaOl) in solution?" 

 If a measurable quantity of uncharged atoms (Na and CI) 

 were present in a solution of NaCl, then, as in the case 

 of ordinary dissociation {e.g. of K 2 Al 2 S 4 0i 6 into K 2 S0 4 and 

 A1 2 S 3 12 ), we should be able to separate these components 

 by diffusion (dialysis). Now it is one of the most firmly 

 established facts that, in spite of numberless experiments, this 



Phil. Mag. 8. 5. Vol. 31. No. 192. J% 1891. 2 H 



