Contact Electricity of Metals* 117 



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 connexion with a new physical principle, for 

 exhibiting contact electricity by means of copper and zinc 

 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 disks for the contact electricity, 

 after the method of Volta, and my own quadrant electrometer 

 substituted for the gold-leaf electroscope by which Volta 

 himself obtained his electric indications. 



I was on the point of transmitting to the Royal 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 Poggendorfs Annalen for January, 1862, in 

 which results altogether in accordance 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 some- 

 thing to the available information regarding the properties of 

 matter to be found in Hankel's paper. I have made many 

 experiments from time to time since 1861 by the same method, 

 but have obtained results merely confirmatory of what had 

 been published by Pfaff in 1820 or 1821, showing the 

 phenomena of contact electricity to be independent of the 

 surrounding gas, and agreeing in the main with the numerical 

 values of the contact differences of different metals which 

 Hankel had published ; and I have therefore hitherto pub- 

 lished nothing except the slight statements regarding contact 

 electricity which appear in my l Electrostatics and Mag- 

 netism/ As interest has been recently re saved in the subject 



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 ilo of this amount in each of these four gases at pressures of two 

 or three millimetres. There seems every reason to believe that it would 

 be non-existent in high vacuum, such as that reached by JBottomley in his 

 Volta-contact experiments (§ 14 above). 



* First published in the British Association, Swansea meeting, August 

 1880, and ' Nature," April 4, 1881. 



