28 Messrs. Wright and Thompson on the Determination of 
therein being left out of consideration in valuing the molecular 
strength). From the heat of formation of aluminium-sulphate 
solution being greater than that of zinc-sulphate, it might 
naturally be expected that zinc would acquire the higher 
potential, as with magnesium-zince cells ; but this was not the 
case, aluminium-zinc-sulphate cells resembling cadmium-iron- 
sulphate cells (§ 175) in that the current actually generated 
flows in the direction opposite to that predicable from the heats 
of formation of the electrolytes : for this reason the cell-values 
are marked with the — sign. The following figures represent 
the mean readings during the first half hour after setting up: 
with aluminium-zine cells the readings slightly diminished in 
numerical value as time elapsed, and vice versa with the copper- 
aluminium cells :— 
Zinc-Aluminium. | Aluminium-Copper. 
Maximum mean reading ... — 548 +:588 
Minimum ,, a — 526 +566 
Average = 55 ties —°538 +:578 
iPropableverror ce esescaesees +:0034 +:0030 
From these figures the following valuations of the voltaic 
constant result; this constant is of + sign, since aluminium 
acquires the higher, and zinc the lower potential :— 
Varin = AALTNVINTUETA 3s « <'- ve sieiaicinieie baie euiseidaeelh sae eee +°538 
Zince-Copper— Aluminium-Copper { ee = +536 
Mean = +°537 
Julius Thomsen finds that Alz, O, SO3 aq.=150630; whence 
Ey=—‘982 volt. This value is probably not strictly appli- 
cable in the case of the cells now under discussion (which 
contained potash alum and not aluminium sulphate); but the 
error introduced by taking this value is but small as compared 
with the thermovyoltaic constant deduced, viz. 
537 —(—982) = +1519. 
VIII. Mercurous-Zine-sulphate Cells. 
179. A number of cells were examined consisting of amal- 
gamated-zine plates immersed in zinc-sulphate solution on the 
