376 



Dr. M. Wiklerman. On Galvanic Cells [Nov. 8, 



(5) Equation (YI) allows us to calculate the heat of reaction in galvanic combi- 

 nations created by light, from the observed E.M.F. and its temperature coefficient. 

 The much greater deflections obtained here induced the author to make vigorous 

 efforts to determine the temperature coefficient of the E.M.F. observed, but the 

 enormous experimental difficulties entailed prevented him from getting more than 

 the general rt suit that it can only be very small (experiments with ZBr solutions 

 and LiBr solutions). For the same reason the heat of reaction can be determined 

 only approximately. 



(6) As to the solution tension in light and in dark, there are differences between 

 cells created by light and ordinary galvanic cells; the first are dependent upon the 

 capacity of absorption of light, the effect of previous illumination, physical changes 

 in the dark, the intensity of light, etc. ; all this makes the solution tension of an 

 electrode in light sui generis, distinct from the solution tension of ordinary galvanic 

 cells. 



(7) The E.M.F. of constant cells reversible in respect of the anion is found also 

 to be directly proportional to its intensity (2E = c . I (YII)). Since the light 

 energv falling upon the plate is also directly proportional to the intensity of light, 



i.e., L = K.I, we have 2E = L, i.e., the E.M.F. or the work done by the given 

 K 



system forms always the same fraction of the total light falling upon it, and this 

 must hold good for monochromatic light as well. Since the solution pressure of a 

 substance in the dark is a constant, it follows from (VII) that 



-log, Vi = c.I : 0-860 T.10- 4 -log e Prf and log^P/ = K".I + K'" (VIII) 



This equation gives the variation of the solution pressure P^ of the electrode in 

 light in dependence upon its intensity ; if P^ is known from other sources the 

 absolute value of P/ can be calculated for each intensity of light from (VIII). 



(6) The theory of thermogalvanic cells is the following. An analysis 

 of the chemical reactions going on in such systems (two equal 

 pktes immersed in a solution, one plate being kept at a higher 

 temperature than the other) shows that, e.g., Ag plates in AgN0 3 

 solution must form a constant cell reversible in respect of the cation, 

 that Ag-BrAg plates in NaBr solution must form constant cells 

 reversible in respect of the anion, and Ag plates in NaCl solution must 

 give an inconstant irreversible combination, etc. 



The E.M.F. of a thermogalvanic combination evidently consists of : 

 (1) The potential difference between one of the plates and the solution 

 at T /}i (2) the potential difference of the same plate and the same 

 solution at T y/ and (3) the potential difference between two solutions of 

 the different temperatures, i.e., 2 E = EI - E 2 + E 3 . 



Now we have for the single electrical potential differences E] = p + 



T, at T /} E 2 = p + T /y at T /y where p is the heat of ionisation of 



+ 



one gramme-atom of ions (Ag in the above mentioned constant cell 

 reversible in respect of the cation), while E 3 can be put = E' (T, - T,,), 

 i.e., the force driving the ions from one part of the solution at one 

 temperature to the other part of the solution at another temperature 

 must be directly proportional to the difference of the two temperatures, 



