550 Lord KeMn [May 21, 



induces electric conductivity in the air all round it. If I were to 

 show you an experiment proving this, you 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 

 conductance in air is only one of many marvels of the " 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 transparent for them. 



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

 zontal copper plate, fixed to the insulated electrode of the electrometer. 

 I fix a zinc plate, supported by a metal stem which is in metallic 

 connection with the sheath of the electrometer, 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 plptes, connected as now, 

 copper to insulated quadrants and zinc t,o tl^e 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 can see. I leave it to itself now, 

 resting on its paraffin supports and not touching the zinc, and the 

 spot of light goes back to where it was ; showing about three-quarters 

 of a volt positive. 



§ 42. I now take this copper wire, which is metallically connected 

 with the zinc plate and the sheath of the electrometer, and bring it to 

 touch the under side of the copper shelf on which the uranium is sup- 

 ported by its paraffin insulators. Instantly the spot of light moves 

 towards the metallic zero, and after a few vibrations settles there. I 

 break the contact ; instantly the spot of light begins to return to its 

 previous position, where it settles again in less than half a minute. 

 You see, therefore, that if I re-make and keep made the metallic 

 contact between the zinc and copper plates, a current is continuously 

 maintained through the connecting wire, by which heat is generated 

 and radiated away, or carried away by the air ; as long as the con- 

 tact is kept made. What is the source of the energy thus produced ? 

 If we take away the uranium, and send cool fumes from a spirit- 

 lamp, or shed Rontgen rays or ultra-violet light, between the zinc 

 and copper, the results of breaking and making contact would be just 

 what you see with uranium. So would they be — you have already, 

 in fact, seen them (§ 5) — without either Eontgen rays or ultra-violet 

 light, but with the copper and zinc a little closer together and with 

 a drop of water between them : and so would they be with dry ice, 

 or with hot glass, between and touched by the zinc and copper. In 

 each of these six cases we have a source of energy ; the well-known 

 eluctro-chemical energy given by the oxidation of zinc in the last 



