502 



HErofiT — 1S84. 



14. The oxidation energy of zinc per gramme-cquiv.'ilont {I.e. r..'. 

 grammes of zinc or IG of oxygen) is, according to the dotcrminati nis 

 of Julins Thomson, Andrews, and Favre and .Silbcrniaun, 8>io'J, .li-J^, 

 and 83915 respectively. 



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

 zinc, or of any dyad element, is, accoi'ding to the modern deternu'nation 

 of Lord Rayleigh,' 10,;520 units. 



Hence the value of '. which is a ratio evidently independent of tijo 

 '1 

 number of atoms dealt with, lies between iv;;*!!", and -iy"'-ll| lirobaMv. 

 Let us i-ay it is ^ f; ;; \\ ;; or 4--i. 



Jfow J in absolute measure is i'i x lO'' ; so the value of v, accordius {<> 

 the above hypothesis, comes out 1 So x lU**, that is 1*85 volts. 



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

 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 Vd volts below the jiotcntial oftheai: 

 near it: tarnished.oroxidi.sed zinc will exhibit less difference, and it is pei'- 

 haps possible that perfectly oxidised zinc need show no diflf'erence of poten- 

 tial at all between itself and the air. The step of potential by no mcaii?: 

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

 tendencij which causes it ; but so fiir as oxidation actually takes place the 

 step diminishes. 



Proceed to consider a })iece of copper similarly. Oxygen molecules 

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

 copper per gramme equivalent is given by the three authorities already 

 quoted as 37100, 38290, and 43770 respectively. These do not agree 

 well, and it is difficult to know whi'^h to take ; but Thomsen's results are. 

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



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

 ^^ 19320 



This, then, is the amount by whicli 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 ot 

 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 effect as 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, compres.siDg 



'• 4'025 grammes of silver are deposited by an Amp^re current in an Isour. -- 

 Montreal Address. This giv(!S the electrochemical equivalent of silver •Oll1''i''""' 

 of hydrogen -00010:352. 



,!■ 



