> " 



502 report— 1884. 



14. The oxidation energy of zinc per gramme-equivalent (i.e. 65 

 grammes of zinc or 10 of oxygen) is, according to the determinations 

 of Julius Thomsen, Andrews, and Favrc and Silbermann, S5430, 84*: 

 and 83915 respectively. 



The amount of electricity needed to deposit a gramme-equivalent of 

 zinc, or of any dyad element, is, according to the modern determination 

 of Lord Rayleigh, 1 10,320 units. 



Hence the value of , which is a ratio evidently independent of the 

 1 

 number of atoms dealt with, lies between *%%{{-§ and | ' probably. 



Let us say it is i-sHr-Hf or ^'^'- 



Now J in absolute measure is 42 x 10 6 ; so the value of v, according to 

 the above hypothesis, comes out 1'85 x 10 s , that is T85 volts. 



This, then, I say, is the step of potential between zinc and air. (To 

 avoid circumlocution I will speak as if the above hypothesis were ad- 

 mittedly true, and all I now say stands or falls with it.) 



All clean bright zinc is thus about 1'8 volts below the potential of the air 

 near it : tarnished or oxidised zinc will exhibit less difference, and it is per- 

 haps possible that perfectly oxidised ziDC need show no difference of poten- 

 tial at all between itself and the air. The step of potential by no means 

 therefore depends upon the occurrence of oxidation, it is the oxidation 

 tendency which causes it; but so far as oxidation actually takes place the 

 step diminishes. 



Proceed to consider a piece of copper similarly. Oxygen molecules 

 are straining at it also, but with less force. The combustion energy of 

 copper per gramme equivalent is given by the three authorities already 

 quoted as 37160, 38290, and 43770 respectively. These do not agree 

 well, and it is difficult to know which to take ; but Thomsen's results are, 

 I believe, generally relied on ; so, assuming his, the step of potential 



between copper and air will be — - — „ volts ; that is, about '8 volt. 



rr 19320 



This, then, is the amount by which clean bright copper differs from the air. 



Oxidised copper will differ less. Comparing this value for copper with 



that just obtained for zinc, we perceive that a piece of zinc and a piece 



of copper are, when separate, not at the same potential ; they differ by 



about a volt from each other. 



Now put the zinc and copper into direct metallic contact, and neglect 



for the present the third of a millivolt of E.M.F. developed at the junction, 



which acts so as to drive positive electricity from copper to zinc. A rush of 



electricity must take place from the copper to the zinc to equalise their 



potential ; it is impossible that they can remain at different potentials 



when directly united : all parts of a conductor must be at a uniform 



potential, and the rush has taken place because they were not so when 



put into contact. 



15. Picturing to ourselves the effectas produced by the straining oxygen 

 atoms we shall perceive that they could not get at either metal when sepa- 

 rate : first, because they surrounded it everywhere, and strained equally on 

 all sides ; and second, because being all charged with negative electricity 

 they could not move in on all sides at once without, so to speak, compressing 



1 ' 4*025 grammes of silver are deposited by an Ampere current in an hour.'— 

 Montreal Address. This gives the electrochemical equivalent of silver •01118, and 

 of hydrogen -00010352. 



