Intelligence and Miscellaneous Articles. 515 



a really workable and reliable electricity meter is already great and 

 daily increases. The author endeavours in this little hand-book to 

 give a clear idea of the principles and details of construction and 

 action of the various forms of meter for quantity, power, and 

 energy, which are or have been in practical use both here and 

 abroad. The figures are extremely well executed and help largely 

 to a clear understanding of the apparatus. At the end of the 

 book will be found a summary of the conditions under which 

 electrical energy is supplied in various cities, and the charges which 

 are made for it. Indeed no pains have been spared to make the 

 volume as handy and yet as complete as possible, and we would again 

 urge the desirability of its translation into English. J. L. H. 



LXVI. Intelligence and Miscellaneous Articles. 

 To the Editors of the Philosophical Magazine and Journal. 

 NOTE ON THE ELECTRICAL CONDUCTIVITY OF HOT GASES. 

 G-ENTLEUEX, Cambridge, May 14, 1891. 



IN reference to Dr. Arrhenius' remarks in the paper with the 

 above title in the last number of the 'Philosophical Magazine,' 

 1 have to observe : — 



1. With regard to the question as to the action of an excess of 

 hydrogen on the dissociation of HI, HBr, or HC1 : though Dr. 

 Arrhenius says that such an excess needwot diminish the dissociation, 

 the question is not really an open one, as Lemoine's experiments 

 (C. R. lxxxv. pp. 34-37) have proved that as a matter of fact an 

 excess of hvdrogen produces a large diminution in the dissociation 

 of HI. 



2. Dr. Arrhenius attributes the conductivities of the hot HC1, 

 HBr, HI, which I observed to the production of platinum dust 

 by the action of the halogens on the platinum of the vessel in 

 which the gases were contained, and also upon the electrodes. As, 

 however, the experiments were made in earthenware as well as 

 in platinum vessels, and with carbon, gold, and iron electrodes as 

 well as platinum ones, this explanation is quite inadmissible. 

 Again, not only the halogens, but also the vapours of most metals 

 conduct when hot, and this whether surrounded by nitrogen or 

 air. In my experiments the effect of surrounding the gas by 

 nitrogen instead of air was always tried. 



3. With reference to Crafts's experiments on the dissociation of 

 the halogens, I may observe that a trace of dissociation would 

 produce a much greater effect on the conductivity than on the 

 vapour-density, and that the temperature vaguely described as 

 yellow heat extends over a wide range, and two temperatures, each 

 of which might by different observers be described as yellow heat, 

 might easily differ by several hundred degrees Centigrade. 



I remain, Gentlemen, 



Your obedient servant, 



J. J. Thomson. 



