II; 



518 



REroRT — 1884. 



4i! M 



Mi;? i 



m 



a 



Here two ail-crnative formiilix) aro given ; it is a matter of opinion 

 wliioli sliows the least divorgoiicics from tho roluinn of observed values. 



The lirst is tho one most naturally suggestofl by the theoretieal oon- 

 sldcraiioMS of section 2U, tho "8 standing for Zti/0 — Ca/0, or what is 

 commonly called Zn/Cu. 



Electromotive force of a zixc-platixum, illhite .tiiljiliuric acid, cell; of re- 

 nixtance 1 ohm, throiirj/i, wliich the specified currents arc driven by tivo or 

 tlrrie Groves. Each jdalc expomn/ iJ'S Krpiare cevtimctres of surface oil 

 either side. I'J.Jlf.F. rcclconed positire whcti it helps the current on. 



Here also are two altci*native formula! given, of which the first agrees 

 best with tho experimental results, l^ut it is very strange that the 

 E.M.F. of this cell should be so low when the current is feeble ; it is 

 scarcely more than that of the copper cell. The only way I sec of 

 accounting for the error — if error it be — is, that the platinum was put 

 into the liquid after the copper plate, and it was sometimes found coated 

 with a very thin evanescent film of cop])er Avhen taken out. Theoretical 

 considerations would suggest something more lilce the second formula as 

 the ])Tobable E.M.F. ; the 1'2 being what is ordinarily called Zn/Pt. 



2-. I can now continue the quotation of tho remainder of the pre- 

 liminary notes with the certainty that they will be at any rate intel- 

 ligible : I begin with statements intended to bo true for substances of 

 every kind, and then specialise them for the case of metals. 



III. — St.\ii:mi:nts i'.ei.ikved by tiik wimtki; to r,v. 



NUT KXTII!i;i.V DiMlKlOiiX. 



n;UE THOCGH 



xil. A substance immersed in any medium tending to act upon it 

 ch.emically will (unless it is actually attacked) be at a dillei-ent potential 

 to tlic medium in contact with it, ])ositive if the active element in the 

 medium is electro-positive, negative if the active element is electro- 

 negative. 



xiii. Tho above difference of potential cnn be cileulated approximately 

 from the potentiiil energy of combination between the substance and the 

 medium, the energy being measured by compelling the combination to 



