116 Lord Kelvin on 



can see. I leave it to itself now, resting on its paraffin 

 supports and not touching the zinc, and the spot of light goes 

 hack 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 electro- 

 meter, and bring it to touch the under side of the copper 

 shelf on which the uranium is supported by its paraffin insu- 

 lators. 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 contact is kept made. What 

 is the source of the energy thus produced ? If we took away 

 the uranium, and sent 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 

 Rontgen 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 

 electro-chemical energy given by the oxidation of zinc in the 

 last-mentioued three cases ; and the energy drawn upon by 

 the cooled fumes, or by the Rontgen rays or ultra-violet light, 

 acting in some hitherto unexplained manner, in the three 

 other cases. We may conjecture evaporations of metals ; we 

 have but little confidence in the probability of the idea. Or 

 does it depend on metallic carbides mixed among the metallic 

 uranium ? I venture on no hypothesis. M. Becquerel has 

 given irrefragable proof of the truth of his discovery of 

 radiation from uranium of something which we must admit to 

 be of the same species as light, and which may be compared 

 with phosphorescence. When the energy drawn upon by 

 this light is known, then, no doubt, the quasi electrolytic 

 phenomena, induced by uranium in air *, which you have 



* Experiments made in the Physical Laboratory of the University of 

 Glasgow [§ 33 of Kelvin, Beattie, and Smolan, Proc. R. S. E. ; also 

 « Nature,' March 11, 1897, and Phil. Mag. March 1898] show this electro- 

 lytic conductivity to be produced by uranium to nearly the same amount 



