developed during the Combination of Zinc and Iodine. 291 



reactions taking place, assumed that this correspondence 

 existed. Recent experiments have shown, however, that this 

 is not a true assumption in many cases, and Helmholtz has 

 recently treated the subject from a new point of view*. 



The following is, as I understand it, the outcome of his 

 work on chemical combination, in so far as it applies to 

 finding the connection between the heats of combination and 

 the electromotive force of the cell. 



In general, when an infinitely small current de passes 

 through a cell at a given absolute temperature 6, a certain 

 amount of heat, dQ, must be supplied to or removed from 

 the cell to keep its temperature constant, apart from the heat 

 developed by the electric resistance of the liquid. 



If the electromotive force of the cell corresponds exactly 

 to the heat given out by the reaction taking place in the cell, 

 then no heat need be added or removed. 



Now the measurement of this cooling or heating of the 

 cell would be difficult in practice, but, applying the second 

 law of thermodynamics, Helmholtz obtains the following 

 formula, showing that a measure of dQ can be obtained from 

 the variations in electromotive force due to temperature — 



e^de=JdQ, 



where dp is the variation in the electromotive force due to 

 temperature. 



It is necessary, then, in practice to measure the electromo- 

 tive force of the cell experimented upon at different tempera- 

 tures and to find the variation, if any, of the electromotive force. 



In a cell of one volt electromotive force, at 300° ab. temp., 

 a variation of *0001 volt per deg. Cent, would mean a correc- 

 tion of *03 volt or 3 per cent, to be applied before determining 

 the heat of combination from the electromotive force. 



The heat of combination can seldom be obtained with much 

 accuracy from books on thermal chemistry, the conditions 

 under which the reactions have been performed in the calori- 

 meter differing considerably from the conditions in the cell. 



For example, three numbers are given by Naumann as 

 representing the heat of combination of zinc and iodine in 

 presence of water — 



1st. The heat of combination of zinc and iodine in water, 

 60540 calories. 



2nd. The heat of solution of zinc iodide in water, 11310 

 calories (Znl 2 , 400H 2 O). 



* Die Thermodynamic, Chemischer Vorganger. Wissenschaftliche 

 Abhandlungen, von Hermann Helmholtz : Zweiter Band. 



Y2 



