380 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
on the same, “ as if” we bad an electrode of “ Cl,” “Br,” &c., behaving in a CINa, BrNa 
solution as Zn or Cu in a ZnS0 4 , CuS0 4 solution. This “ as if” must be only taken 
as a useful illustration first introduced by N ernst for classification purposes. The 
electrode is a compound Ag-ClAg, Ag-BrAg and not a “Cl” or “Br” electrode, 
because otherwise the electrodes of all other combinations reversible in respect of the 
same anion ought to lead to the same values of the E.M.F. This however is not the 
case, as we see it from a comparison of the E.M.F.’s- obtained with Ag-BrAg and 
Hg-Hg 2 Br 2 in BrNa or BrK solution, obtained with Ag-ClAg and with Hg-Hg 2 Cl 2 in 
the same CINa solution. Thus we have in one case the compound Ag-ClAg electrode, 
in the other the compound Hg-Hg 2 Cl 2 electrode, which are different, have a different 
solution pressure, a different electrical potential with the same solution, a different 
heat of reaction, though the kind of reaction going on in the system under the 
action of the current (transportation of Cl, Br ions from one electrode to the other) 
is the same, and under the action of light the variation of the solution pressure and 
of the electrical potential of the different compound electrodes reversible in respect 
of the same anion to the same solution are different. 
XIII. The System Hg, Solid Hg 2 I 2 in IK Solution (Table X.). The System Hg, 
Solid SOJHga in Solutions o/S0 4 Na 2 , S0 4 K 2 , S0 4 (NH 4 ) 2 ( Plate JV23, Table XI.). 
The first system could not be investigated, since Hg 2 I 2 is decomposed by IK 
solution already in the dark ; the second system could, since the Hg 2 S0 4 is not 
decomposed by sulphates in the dark. The Hg 2 S0 4 being very much more affected 
by light than Hg 2 Cl 2 , Hg 2 Br 2 , it was of special interest. In all investigations of the 
mercury cells the method was the same. The quartz vessel was divided into two 
compartments by fixing tightly into it an ebonite wall, 1 millim. thick, 27 centims. 
wide, circa 2 centims. high, with Crookes cement and then asphaltum, each compartment 
serving as an electrode. Mercury was first brought into the two compartments, then 
the solid S0 4 Hg 2 (or Cl 2 Hg 2 , Br 2 -Hg 2 ) over both mercury surfaces several millims. 
high, and then the solution on the top a few millims. over the ebonite partition. The 
mercury, solid salt and solution were mixed at the quartz windows of the vessel 
and S0 4 Hg 2 entangled between the Hg and the quartz windows, so as to get vertical 
S0 4 Hg 2 electrodes (besides S0 4 Hg 2 being on the top). Two glass capillary tubes, 
sealed up at one end with long pieces of platinum wire leading from the closed ends 
and fixed in the ebonite top piece, were filled with mercury and the ebonite piece 
cautiously fixed in the quartz vessel. The heating of the HgPt junctions by light 
was thus as far as was practicable avoided. 
It was soon found that the E.M.F. considerably changes with the quantity of 
mercury. The greater the amount of mercury (the vertical surface of S0 4 Hg 2 ) the 
smaller the deflection—this is evidently due to the fact that the E.M.F. obtained is 
principally a thermo E.M.F. The E.M.F. obtained depends upon how the solid 
