Contact Electricity of Metals. 115 



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 

 wonderful 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 nine- 

 teenth century more memorable than any one of all the 

 twenty-three centuries of scientific history which preceded it, 

 excepting the seventeeth 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 induces electric conductivity in the 

 air all round it. Jf I were to show you an experiment 

 proving this, vou might say it is witchcraft. But here is the 

 witch. You see, when I open the box, a piece of uranium of 

 about the size of a watch. This production of electric con- 

 ductance in air is only one of many marvels of the ft uranium 

 rays " discovered a year ago by Henri Becquerel, of no other 

 of which can I now speak to you, except that the wood and 

 paper of this box, and my hand, are to some degree trans- 

 parent for them. 



I now take the uranium out of its box and lay it on this 

 horizontal copper plate, fixed to the insulated electrode of the 

 electrometer. I fix a zinc plate, supported by a metal stem 

 which is in metallic connexion with the sheath of the elec- 

 trometer, horizontally over the copper plate at a distance of 

 about one centimetre from the top of the uranium. Look at 

 the spot of light ; it has already settled to very nearly the 

 position which you remember it took when we had a water- 

 arc between the copper and zinc plates, connected as now, 

 copper to insulated quadrants and zinc to the sheath. I now 

 lift the uranium, insulating it from the copper plate by three 

 very small pieces of solid paraffin, so as to touch neither plate, 

 or, again, to touch the zinc but not the copper. This change 

 makes but little difference to the spot of light. I tilt the 

 uranium now to touch the zinc above and the copper below ; 

 the spot of light comes to the metallic zero as nearly as you 



