52 Messrs. Ayrton and Perry on Prof. Exner's 



water immediately before being connected with the zinc plate; 

 and then it could only reach it if the whole chemical action 

 were electrically efficient, and it' there were no chemical resist- 

 ance due to the affinity of the platinized surface for oxygen. 



" It is also to be remarked, that the permanent working 

 electromotive force of a galvanic element consisting of zinc 

 and a less oxidizable metal immersed in sulphuric acid can 

 never exceed the number 2,056,200, derived from the full 

 thermal equivalent for a single cell of Smee's, since the che- 

 mical action is identical in all such cases, and the mechanical 

 value of the external effects can never exceed that of the che- 

 mical action. In a pair consisting of zinc and tin, the electro- 

 motive force has been found by Poggendorff to be only about 

 half that of a pair consisting of zinc and copper, and conse- 

 quently less than half that of a single cell of Smee's." 



The alteration in the electromotive force of galvanic cells 

 produced by gas dissolved in the liquid has been known for a 

 long time, indeed before the experiments of De Fonville, De 

 Lerain, Marie-Davy, referred to by Prof. Exner; and we may 

 draw attention to some experiments of our own, which show 

 that the polarization phenomena in voltameters are wonder- 

 fully influenced by the state of the platinum plates and the 

 liquid as regards absorbed gas*. 



Thi?*dly, Prof. Exner states that the contact theory requires 

 that the permanent electromotive force of a Smee's should 

 depend on the nature of the negative plate, while the chemical 

 theory requires that it should be independent of the negative 

 metal; further, that his experiments agree with the latter con- 

 clusion and disagree with the former. Now the first part of 

 this statement is wrong; for the contact theory does not require 

 per se that the electromotive force should, or should not, de- 

 pend on the negative metal. What the contact theory tells us 

 is that the electromotive force of the cell will depend on the 

 electromotive force of contact of every pair of dissimilar sub- 

 stances in the circuit, and therefore will depend, among other 

 things, on the contact difference of potentials of the negative 

 metal with possibly a gas, the gas with the liquid, the liquid 

 possibly with another gas, the gas with the positive metal, 

 and the positive metal with the negative. Further, the con- 

 clusion itself to which Prof. Exner has arrived, viz. that the 

 working electromotive force of a Smee's cell does not depend 

 on the negative plate, has, like his other conclusions, been 

 disputed. For Beetzt has recently published the results of 



* " A Preliminary Account of Reduction of Observations on Strained 

 Material," &c, "by John Perry and W. E. Ayrton, Proc. Roy. Soc. No. 30, 

 pp. 411-435 (1880). 



t " On the Nature of Galvanic Polarization," by W. Beetz, Annalen der 

 Physik und Chemie, B. x. H. 3, pp. 348-371 (1880). 



