290 Mr. A. P. Laurie on the Electromotive Forces 



With these two cells the following experiments have been 

 made, w T ith a view to determining the changes in electromotive 

 force during the combination of zinc and iodine in presence 

 of water. 



These changes in E.M.F. no doubt correspond more or less 

 closely to the changes in the heat set free during the reaction ; 

 and therefore before describing the experiments it is necessary 

 to discuss the connection between these two phenomena from 

 the point of view of the recent work on the subject by Prof. 

 Helmholtz. 



When a current passes through a given zinc-iodine cell, 

 various changes take place requiring either the production or 

 absorption of heat. In the first place, there is the combination 

 of the zinc and iodine, producing heat ; in the second place, 

 there is the solution of the zinc iodide formed, producing 

 heat. On the other hand, the disintegration of the zinc, the 

 decomposition of the iodine molecule, and the removal of the 

 free iodine from its solution in zinc iodide, absorb heat. 



If now a series of cells is made up differing only in the 

 strength of the zinc-iodide solution, the heat of solution of 

 the zinc iodide formed in the cell during the passage of a 

 current, will vary with the strength of the iodide of zinc 

 solution, and that probably in a way familiar to chemists from 

 the behaviour of sulphuric acid when added in larger and 

 larger quantities to water. 



On the other hand, the heat of combination of the zinc and 

 iodine, and the heat absorbed in decomposing the iodine 

 molecule and disintegrating the zinc, will probably remain 

 the same. We shall, in fact, find that the small quantity 

 of zinc iodide dissolved on passing a current has a definite 

 heat of solution determined by the amount of zinc iodide 

 already dissolved in the water contained in the cell. 



Further, if a series of cells are made up differing only in 

 the amount of free iodine dissolved in a given iodide of zinc 

 solution, we shall find the heat absorbed in removing the 

 iodine from its solution in zinc iodide diminish as the amount 

 of free iodine present is increased. 



It is also probable that the heat of solution of a constant 

 quantity of iodine will vary with variations in the strength 

 of the iodide of zinc. 



Having thus stated what takes place in the cell, the 

 question next to be decided is, whether the heat given out by 

 these reactions corresponds exactly with the electromotive 

 force of the cell. 



Sir William Thomson, in publishing his formula connecting 

 the electromotive force of a cell with the heat set free by the 



