1897.] on Contact Electricity of Metals. 549 



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 both.* 



A similar conclusion in respect to air traversed by ultra-violet 

 light was proved by Righi, f Hallwachs, J Elster and Geitel, § Branly. || 

 The same was proved for ordinary atmospheric air, with Eontgen 

 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 Cam- 

 bridge. H 



§ 40. 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 Eontgen rays, or by uranium (§§ 41, 42 below), gives us, 

 no doubt, what would to some degree fulfil Professor Lodge's idea of a 

 " potentially-oxidising " 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 bracketed 

 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 Pro- 

 fessor 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 crisply and distinctly as possible, so as to 

 emphasise the difference." I wish I could also feel sure that he will 

 agree with it, but I am afraid I cannot, because in the same letter he 

 says, " I am still unrepentant." 



Continuation of Lecture of May 21, 1897, 



§ 41. In conclusion, I bring before you one of the most won- 

 derful discoveries of the century now approaching its conclusion, 

 made by the third of three great men, Antoine Becquerel, Edmond 

 Becquerel, Henri Becquerel — father, son and grandson — who by their 

 inventive genius and persevering labour have worthily contributed to 

 the total of the scientific work of their time ; a total which has 

 rendered the nineteenth century more memorable than any one of all 

 the twenty-three centuries of scientific history which preceded it, 

 excepting the seventeenth century of the Christian era. 



You see this little box which I hold in my right hand, just as I 

 received it three months ago from my friend Professor Moissan, who 

 will be here this day week to show you his isolation of fluorine. It 



♦ Kelvin and Maclean, R.S.E. 1897. t Rend. R. Ace. dei Lincei, 1888, 1889. 



t Wiedemann's Annalen, 34, 1888. § Ibid. 38, 41, 1888. 



U Comptes Eendus, 1888, 1890. i Proc. R.S. March 1896. 

 Vol. XV. (No. 91.) 2 o 



