REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 379 
follows from this that in Nl, 2, 3 and 4 the thermo E.M.F.’s are very small, so that 
E.M.F. is created by the total light or blue screen almost exclusively at a constant 
temperature. (The direction of the current will be discussed when inconstant cells 
will be dealt with.) 
Similar were the results obtained with the system Ag plate in light, Ag-BrAg 
plate in the dark and BrNa solution. The matter at once becomes different where 
an intimate contact is established between the tw r o parts which are to form the 
electrode, e.g., when an insoluble mercury salt is in contact with liquid mercury, as 
we shall see later on. 
XI. The Systems (Hg, Solid Hg 2 Cl 2 in OT normal NaCl and OT normal KC1 Solution ), 
(Hg, Solid Hg 2 Br 2 in OT normal KBr and OT normal NaBr Solution). Plates AT2 
and A r 22', Table X. 
The E.M.F. of the systems reversible in respect of the cation or anion is composed 
of the electrical potentials of the electrodes to the liquid in light and in the dark and 
of the electrical potential of the liquid in light to the liquid in the dark. The system 
Hg-Hg 2 Cl 2 in OT normal CINa solution in light and in dark was investigated on 
July 25 and 26, 1904, and the system Hg-Hg 2 Cl 2 in OT normal KC1 on July 27, 
1904, both with acetylene at a distance = 88 centims. from quartz vessel (the light of 
Hg in the quartz vessel was about 2 centims.). No deflection could be established, 
at any rate it did not reach a millimetre. Plate N22 gives the photographic curve 
for CINa. The system Hg-Hg 2 Br 2 in OT normal BrK and OT normal BrNa was 
investigated with the arc at the distances of 118 and 47 centims. from the quartz 
vessel. No deflection was obtained (Plate N22'). This proves at the same time that 
not only does the electrical potential of Hg-Hg 2 Cl 2 or Hg-Hg 2 Br 2 to the solutions 
change in light exceedingly little, but the electrical potential between the CINa or C1K 
solution , or BrK or BrNa solution in the light and in the dark can only be exceedingly 
small, if at all measurable. 
Thus the proof is given that in the systems dealt with before, which are reversible 
in respect of the anion or cation, the variation of the osmotic pressure of the solution 
in light and resulting from its electrical potential between the solution in light and 
in the dark can be only very small, and that the last, in comparison with the 
electrical potentials of the plates to the solution in light and in the dark, is a 
negligible quantity. 
XII. What is to be understood under the “ Electrode Reversible in respect of the 
Anion.” 
In the consideration of the reactions going on in systems reversible in respect of 
the anion, we assume (with Nernst) that both Ag and ClAg or BrAg of the 
electrodes take part in the reversible reaction, the result of which is that the 
anion Cl, Br, &c., &c., either passes from the electrode into the solution or separates 
3 c 2 
