38 Messrs. Wright and Thompson on the Determination of 
Zn + 2HC1 = ZnCl 2 + H 2 , 
Cd-+ 2HC1 = CdCl 2 + H 2 , 
Zn + H 2 S0 4 = ZnS0 4 + H 2 , 
Cd + H 2 S0 4 = CdS0 4 + H 2 , 
these heat-developments being, per gramme-molecule, as fol- 
lows* : — 
Zn, Cl 2 , aq. . . . = 112840 Cd, Cl 2 , aq. . . . =96250 
H 2 ,C1 2 , aq. . . . = 78640 -Ha, Cl 2 , aq. . . . =78640 
Difference . . 34200 1 Difference . . 17610 
Diff. per gramme-equi- ") 1 71 nn ~®^' P er g ramme_e( l u i- "1 S8Q5 
valent .... J valent .... J 
Corresponding with volt *754 Corresponding with volt '388 
Zn,0,S0 3 ,aq. . . =106090 Cd, O, S0 3 , aq. . . =89500 
H 2 , O = 68200 H 2 , O =68200 
Difference . . 37890 1 Difference . . 21300 
Diff. per gramme-equi- \ -. ™ . k , Diff. per gramme-equi- ) 1 ackq 
valent .... J valent .... J 
Corresponding with volt *835 Corresponding with volt "470 
141. In order to see whether the electromotive forces actu- 
ally developed by these four voltaic combinations are really 
above the calculated values in the first two cases and below in 
the second two instances, when the disturbing effects of dis- 
solved air are eliminated, cells were set up like those described 
in § 85, and caused to generate feeble currents by employing 
large external resistances. In all cases it was found that when 
the errors due to dissolved air were eliminated and the read- 
ings became constant, the E.M.F. actually developed invariably 
fell short of the value corresponding with the net chemical action 
by an amount which increased with the current-density until 
the reduction became a large fraction of the E.M.F. observed 
with the smallest possible densities. With hydrochloric-acid 
cells the deficiency was not so great in the first instance, and 
* These figures are deduced from Julius Tbomsen's thermochemical 
data aud the mean value for the heat of formation of water arrived at in 
§ 31. Thomsen's values relate to the degree of dilution MCI,, 400 H 2 0, 
and MS0 4 , 400 H 2 O. Some experiments made by us on the amounts of 
heat evolved on diluting stronger solutions of zinc and cadmium chlorides 
and sulphates indicate that these values require slight corrections for 
stronger solutions than those used by Thomsen ; but the alterations thus 
produced in the net heat-development and in the E.M.F. corresponding 
thereto is but small. 
