1897.] on Contact Electricity of Metals. 661 



mentioned three cases ; and the energy drawn upon by the cooled 

 fumes, or by the Eontgen 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. 

 Mr. Becquerel has given irrefragable proof of the truth of his dis- 

 covery 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 seen, will be explained by the same 

 dynamical and chemical principles as those of the previously known 

 electrolytic action of cooled fumes from a spirit-lamp, and of air 

 traversed by Rontgen rays or ultra-violet light. 



Appendix. 



On a Method of Measuring Contact Electricity. \ 



In my reprint of papers on Electrostatics and Magnetism (§ 400, of 

 original date, January 1862) I described briefly this method, in con- 

 nection with a new physical principle, for exhibiting contact elec- 

 tricity by means of copper and ziiic quadrants substituted for the 

 uniform brass quadrants of my quadrant electrometer. In an extensive 

 series of experiments which I made in the years 1859-61, I had used 

 the same method, but with movable discs for the contact electricity, 

 after the method of Volta, and my own quadrant electrometer substi- 

 tuted for the gold-leaf electroscope by which Volta himself obtained 

 bis electric indications. 



I was on the point of transmitting to the Eoyal Society a paper 

 which I had written describing these experiments, and which I still 

 have in manuscript, when I found a paper by Hankel in Poggendorf 's 

 * Annalen ' for January, 1862, in which results altogether in accord^ 

 ance with my own were given, and I withheld my paper till I might 

 be able not merely to describe a new method, but if possible, add 

 something to the available information regarding the properties of 



* Experiments made in the Physical Laboratory of the University of Glasgow 

 [§33 of Kelvin, Beattie and Smolan, Proc. E.S.E. ; also 'Nature,' March°ll, 

 1897, and Phil. Mag. March 1898] show this electrolytic conductivity to be 

 produced by uranium to nearly the same amount in common air oxygen and 

 carbonic acid ; and to about one-third of the same amount in hydrogen, at 

 ordinary atmospheric pressure ; but only to about yi^ of this amount in each of 

 these four gases at pressures of 2 or 3 millimetres. There seems every reason 

 to believe that it would be non-existent in high vacuum, such as that reached by 

 Bottomley in his Volta-contact experiments (§14 above). 



t First published in the British Association, Swansea meeting, August 1880, 

 ond ' Nature,' April 4. 1881. 



2 o 2 



