514 



iiEroirr — 1884. 



by platinum, wliicli barely pulls at all until it is coated and alloyed with 

 bydrotren. 



This answci" cannot be considered as complete, and in order to complete 

 it consider a more precise experiment. 



ArranfjfC a series of common dilute acid voltameters witb their plates 

 respectively, zinc zinc, zinc-iron, zinc-copper, and zinc-platinum. Pass 

 one curieufc through the series from zinc to the other metal, and measure 

 the diO'erences of potential between the plates in eti,ch cell. Now the same 

 chemical action is going on in each. In each, zinc is dissolved at one side 

 and hydrogen evolved at the other — the only dilference being that it is 

 liberated from surfaces of znic, iron, copper, platinum, in the four cases. 

 What is to prevent the E.M.F, between the terminals of each volta- 

 meter from being the same P But it is not the same (2}a.c6 Prof. Exner) ; 

 the zinc-zinc cell shows the greatest difference of potential between its 

 terminals, the zinc-iron less ; and the zinc-platinum may easily show 

 a reverse difference because it helps the current on instead of hinder- 

 ing it. It will bo understood that the fvcclso behaviour of the cells is 

 determined \v the intensity of the current (i.e. current per area) — if it is 

 weak, even rhe zinc-iron cell may help it on, but the zinc-platinum will 

 help it on most ; if it is very strong, even the zinc-platinum will retard it, 

 but the others will retard it more, and the zinc-zinc most. 



Now why is all this ? Take the difl'crenco between the heats of 

 formation of Zn, SO.) and of H^, SO4, at the comma, and you will li;ne 

 the total energy assimilated by tlie current in each cell. This energy is 

 the same in all the cells, but not in all does it take the same form. In 

 the zinc-platinum cell it mainly results in driving the current forward. 

 In the zinc-zinc cell it wholly results in a Peltier (or Bouty) generation 

 of heat. In going out of the cell by a cathode zinc plate, it has to move 

 hydrogen towards it, and (//«(> fnctu) oxygen away from it, in opposition 

 to the strong chemical attraction ; thus it will do work and liljorato 

 energy, which, since tiiere is nothing better to do, must exhibit itself as 

 heat. At an iron surface less heat is generated, and at a copper less still; 

 but, at any cathode which attracts oxygen, some heat must be gcnei-ated 

 by a current made to do work in opposition to this attraction. 



In the zinc-zinc cells there is no propulsion of electricity at all by the 

 cell ; on one side, where the current entei'S, zinc is dissolved and the 

 current helped forward with the full energy (or nearly the full energy) of 

 the combination, so that no (or nearly no) waste enei'gy or heat is there 

 produced ; but on the other side, where the curi-ent leaves, tlio same 

 combination is (not exactly undone but) opposed and the current hiudere'l 

 •with (probably something less than) the full energy of the combiuation. 

 and there the heat of combination is generated. 



Thus, regarding the passage of hydrogen to the cathode as a virtual 

 separation of (or S0,|) from it, Ave may say in general that in any one 

 of the above cells, used as a voltameter, the energy available for helpini;' 

 the current on is that represented by the^difference between the combina- 

 tion energies of the substances respectively attacked and liberated; i.(., 

 Zn, SO, — Ho, S0.| ; but tliat besides this the combination M, SOji i» 

 virtually undone, and since its energy appears as a generation of heat at 

 the cathode, it is so much to be- subtraeted from the propelling force 

 available for the current, only the balance being left for this purpose, 

 viz. — 



Zn, SO4-U.,, SO^-M, SO,. 



