1897.] on Contact Electricity of Metals. 527 



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 oxidised where dry, and polished only where wet by the 

 water connection, we find still the same electrolytic zero E ; but now 

 the spot of light moves to the left when we lift the zinc disc and 

 break the water connection. 



§ 8. The experiments of § 7, interpreted in connection 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 oxidised 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 for 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 electrolytic zero E, as it did when we had water connection be- 

 tween 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 electro- 

 meter 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 connection between them, vitreous and resinous elec- 

 tricities were induced upon the opposed surfaces of metallic copper 

 and oxidised copper respectively. This statement, which we know 

 from § 7 to be also true for zinc compared with oxidised zinc, is pro- 

 bably also true for every oxidisable 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. 



§ 9. Hitherto we have had no means of measuring the amount of 

 the Volta-contact electric force between dry metals, except observa- 

 tion of the degrees of deflection of the gold leaves of an electroscope, 

 or of the spot of light of the quadrant electrometer consequent upon 

 operations performed upon difierent pairs of metals, with dimensions 

 and distances of motion exactly the same, and comparison of these 

 deflections with the steady deflection from the metallic zero given by 

 polished zinc and copper connected conductively with one another by 

 water, and connected metallically with the two electrodes of an 



