Contact Electricity of Metals. 89 



trolytic zero E as before. But now, when we lift the zinc 

 disc and break the water connexion, the spot of light moves 

 to the right, instead of remaining steady as it does when both 

 the dry opposed surfaces are polished. If next we tarnish 

 the zinc disc by heat, as we did for the copper disc, and 

 repeat the experiment with wholly polished copper, and with 

 the zinc disc oxidized where dry, and polished only where 

 wet by the water connexion, we find still the same electro- 

 lytic zero E ; but now the spot of light moves to the left 

 when we lift the zinc disc and break the water connexion. 



§ 8. The experiments of § 7, interpreted in connexion with 

 those of § 5, prove that there are dry contact voltaic actions 

 between metallic copper and oxide of copper in contact with 

 it, and between metallic zinc and oxide of zinc in contact 

 with it ; according to which, dry oxide of copper is resinous 

 to copper in contact with it, and dry oxide of zinc is resinous 

 to zinc in contact with it, just as copper is resinous to zinc in 

 contact with it. We may verify this conclusion by another 

 interesting experiment. Taking, for instance, the oxidized 

 copper plate, with a little area polished for contacts, put a 

 little mound of copper, instead of the mound of water, on this 

 area for contact with the upper plate ; and fur the upper plate 

 take polished copper instead of polished zinc. If we operate 

 now as in § 7, the spot of light settles at the metallic zero 

 when the metallic contact is made, instead of at the electro- 

 lytic zero E, as it did when we had water connexion 

 between zinc and copper. But now, just as in § 7, the 

 spot of light moves to the right when the contact is broken 

 and the upper plate lifted, which proves that vitreous 

 electricity flows into the electrometer from the upper plate, 

 when its distance from the lower plate is increased after 

 breaking the metallic contact. We conclude that when the 

 two plates were parallel, and very near one another, and when 

 there was metallic connexion between them, vitreous and 

 resinous electricities were induced upon the opposed surfaces 

 of metallic copper and oxidized copper respectively. This 

 statement, which we know r from § 7 to be also true for zinc 

 compared with oxidized zinc, is probably also true for every 

 oxidizable metal compared with any one of its possible oxides. 

 It is true, as we shall see later (appended paper of 1880-81 ; 

 also Erskine-Murray's paper referred to in § 15), even for 

 platinum in its ordinary condition in our atmosphere of 21 

 per cent, oxygen and 79 per cent, nitrogen, voltaically tested 

 in comparison with platinum which has been recently kept 

 for several minutes or several hours in an atmosphere of pure 

 oxygen, or even in an atmosphere of 95 per cent, oxygen 

 and 5 per cent, nitrogen. 



