518 



REPORT — 1884. 



Here two alternative formula? are given ; it is a matter of opinion 

 which shows the least divergencies from the column of observed values. 



The first is the one most naturally suggested by the theoretical con- 

 siderations of section 20, the "8 standing for Zn/O — Ca/O, or what is 

 commonly called Zn/Cu. 



Electromotive force of a zinc-platinum, dilate sulphuric acid, cell; of re- 

 sistance -!- ohm, through which the specified currents are driven by two or 

 three Groves. Each plate exposing S3 square centimetres of surface on 

 either side. 1E.M.F. reckoned positive when it helps the current on. 



Here also are two alternative formula? given, of which the first agrees 

 best with the experimental results. But 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 see 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 copper when taken out. Theoretical 

 considerations would suggest something more like the second formula as 

 the probable E.M.F. ; the 1"2 being what is ordinarily called Zn/Pt. 



22. I can now continue the quotation of the remainder of the pre- 

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

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

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



III. — Statements believed bt the whiter to be true thocc.h 



NOT ENTIRELY ORTHODOX. 



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

 chemically will (unless it is actually attacked) be at a different potential 

 to the medium in contact with it, positive if the active element in the 

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

 negative. 



xiii. The above difference of potential can be calculated approximately 

 from the potential energy of combination between the substance and the 

 medium, the energy being measured by compelling the combination to 



