Electromotive Forces from Thermochemical JData. 211 



not occur, so that the aluminium plate merely acts as an im 

 perfect substitute for an entirely non-attackable substance 

 such as platinum or carbon, and having hydrogen, not alumi- 

 nium, deposited on its surface. 



That this is the case, and that the reason is probably to be 

 found in the superficial coating of the aluminium plate by an 

 oxide or srboxide, has been already put forward and received 

 experimental verification by Dr. A. P. Laurie^, who has in- 

 vestigated the effect of amalgamating the aluminium, which 

 better enables it to come in actual contact with the liquid and 

 causes the E.M.F. of the cell to actually reverse in direction. 



The heat developed by the oxidation or conversion into 

 sulphate of the zinc with the liberation of hydrogen would 

 be more than sufficient to account for the observed electro- 

 motive force ; hence if it be desired to trace any connexion 

 between the absorption or evolution of heat and the anomalous 

 E.M.F. of any cell, it is the actual chemical change that does 

 occur which must be made the basis of the calculation. 

 Further, the term " Thermovoltaic Constant " (employed by 

 Messrs. Wright and Thompson to denote the extent of de- 

 parture of the E.M.F. of a cell from thermochemical data; 

 zinc being taken as the standard from which the voltage is 

 reckoned) in cases such as aluminium can merely express the 

 fact that a plate of aluminium in its compact form, and 

 probably covered by its protecting film of oxide, is less actively 

 attacked than it ought to be from theoretical consideration 

 of the heat of formation of its salts ; but it must not be 

 accepted as denoting any occult power of transforming elec- 

 tric energy into heat or vice versa. 



From purely a priori considerations one would expect that 

 metals like aluminium and magnesium, which in the amalga- 

 mated or finely divided condition readily decompose water, 

 would, in the compact form, become covered superficially 

 with oxide and behave to a greater or less extent as a 

 hydrogen plate. It is therefore largely to this cause that one 

 would look for ati explanation of the large + values assigned 

 to the "thermovoltaic constants^' of aluminium, magnesium, 

 and possibly iron. The observation by Wright and Thomp- 

 son, that the substitution of dilute sulphuric acid for ferrous 

 sulphate as the liquid surrounding the iron plate of an iron- 

 copper cell uniformly tends to lower the E.M.F., which effect 

 they refer to "local action," distinctly supports the view that 

 even with iron there is a tendency to decompose water and 

 coat itself with hydrogen, and thus acquire a higher relative 

 potential . 



* Phil. Mag. ser. 5, vol. xxii. p. 213. 

 P2 



