212 Prof. B. F. Herroun on the Divergence of 



A further consideration of case 2 will be met with later in 

 this paper. T will merely mention that a considerable 

 difference in E.M.F. due to the superficial coating of a 

 copper plate with suboxide has been already noticed in the 

 use of standard Daniell cells by Dr. J. A. Fleming * and 

 myself t, and that what is rare and comparatively unimpor- 

 tant in the case of copper may become the rule and of greater 

 importance in some other combinations. 



In case 3 the nature of the effects of dissolved oxygen in 

 oxidizing a metallic surface or in removing traces of electro- 

 lytic hydrogen and its depolarizing effect in the latter case 

 are too obvious to need much comment ; only I would draw 

 attention to the fact that by its action the electromotive force 

 on open circuit, as measured by an electrometer, or with a 

 balance of difference of potential, as in Latimer Clark^s 

 method, or even when measured by a galvanometer of many 

 thousand ohms resistance, is liable to be in excess of that due 

 to the supposed chemical reaction, and therefore to give 

 false ideas as to the energy-producing power of the cell. 



Case 4 has been suggested as a very probable source of 

 divergence, since (1) in cells in which a very sparingly 

 soluble salt is employed, the solution, especially in the vicinity 

 of the electrode, may become saturated, and if any more salt 

 be formed it will necessarily be produced in the solid form, 

 Avhich may, or may not, be dissolved in the rest of the liquid ; 

 (2) it is difficult to see how the tendency of a salt to combine 

 molecularly with water to form a hydrate or to dissolve as a 

 solution, can directly affect the combination of the metal with 

 the acid radical to form that salt, and therefore we may be 

 justified in concluding that both the total heat evolved and 

 the E.M.F. of a cell are due to the algebraic sum of effects 

 of events which take place successively. But since the water 

 in different solutions is differently combined or related with 

 the anhydrous salt, it apj^ears possible that the energy which 

 in some cases is transformable into electric energy, or, to use 

 Helmholtz^s suggestive names, is " free,^' in other cases may 

 be simply " bound energy.^' 



The main interest of the question, however, centres in case 5, 

 since many physicists seem to have tacitly assumed that the 

 explanation of the divergence of actual E.M.F. s from the 

 computed values is simply that with cells giving a deficiency 

 of electromotive force a portion of the chemical energy 

 appears as heat in the cell, while cells giving an excessive 



* Plnl. Mag. ser. 5, vol. xx. p. 132 ; Proc. Pliys. Soc. vol. vii. p. 168. 

 t Phil. Mag. ser. 6, vol. xxi. p. 12 ; Proc. Phys. Soc. vol. vii. p. 276. 



