114 Lord Kelvin on 



Experiments made by Maclean and Goto in the Physical 

 Laboratory of the University of Glasgow in 1890*, proved 

 that polished zinc and polished copper, with fumes passing 

 np between them from the flame of a spirit-lamp 30 centi- 

 metres below, gave, when metallically connected to the 

 quadrants of an electrometer, deviations from the metallic 

 zero in the same direction, and of nearly the same amount, 

 as if cold water had been in place of the flame. This proved 

 that flame acted as an electrolytic conductor. They also 

 found that hot air from a large red-hot soldering bolt, put in 

 the place of the spirit-lamp, had no such effect; nor had 

 breathing upon the plates, nor the vapour of hot water, any 

 effect of the kind. In fact hot air, and either cloudy or clear 

 steam, act as very excellent insulators ; but there is some 

 wonderful agency in fumes from a flame, remaining even in 

 cooled fumes, in virtue of which the electric effect on zinc 

 and copper is nearly the same as if continuous water, instead 

 of fumes, were between the plates and in contact with bothf , 



A similar conclusion in respect to air traversed by ultra- 

 violet light was proved by Righi J, Hallwachs §, Elster and 

 Geitel ||, Branlylf. The same was proved for ordinary atmo- 

 spheric air, with Pontgen rays traversing it between plates of 

 zinc and copper, by Mr. Erskine Murray, in an experiment 

 suggested by Professor J. J. Thomson, and carried out in the 

 Cavendish Laboratory of the University of Cambridge **. 



§ -10. The substitution for ordinary air between zinc and 

 copper, of ice or hot glass, or of air or gas modified by flame 

 or by ultra-violet rays, or by Pontgen rays, or by uranium 

 (§§41, 42 below), gives us, no doubt, what would to some 

 degree fulfil Professor Lodge's idea of a " potentially-oxidiz- 

 ing " gas, and each one of the six fails wholly or partially to 

 " maintain electric force or voltaic difference of potential in 

 the space between them." In fact, Professor Lodge ; s brac- 

 keted sentence, so far as it can be understood, would be nearer 

 the truth if in it " cannot" were substituted for " can." I 

 hope no reader will consider this sentence too short or sharp. 

 I am quite sure that Professor Lodge will approve of its tone, 

 because in his letter to me of the 14th, he says, " In case of 

 divergence of view it is best to have both aspects stated as 



* Phil. Mag. Aug. 1890. 

 t Kelvin and Maclean, R.S.E., 1897. 

 X Rend. R. Ace. dei Lincei, 1888, 1889. 

 § Wiedemann's Annalen, xxxiv. (1888). 

 || Ibid, xxxviii., xli. (1888). 

 II Comptes Rendus, 1888, 1890. 

 ** Proc. R. S. March 1896. 



