192 Dr. C. R. A. Wright on the Determination of 



cadmium and 500 of pure mercury being employed for each 

 experiment. To insure solution it was found necessary to wash 

 the cadmium with dilute sulphuric acid just before use ; other- 

 wise portions remained un wetted and undissolved by the mer- 

 cury. The final result arrived at as the average of several 

 concordant observations was, that an evolution of heat to the 

 extent of 610 gramme-degrees per gramme-molecule (112 

 grammes) of cadmium took place during solution. Hence, 

 were cadmium sulphate formed from mercurial solution of 

 metal instead of crystalline precipitated metal, the heat of for- 

 mation expressed as Cd, 0, S0 3 aq. would be 89,500 — 610 = 

 88,890 (admitting that Thomsen's value 89,500 applies, with- 

 out correction, to the metal in the crystalline condition of that 

 experimented with). Consequently the heat of displacement of 

 copper by cadmium from the sulphate is 32,930 per gramme- 

 molecule, or 16,465 per gramme-equivalent, corresponding to 

 "726 volt. The observed values varied between '701 and *717, 

 averaging *707 with electro-copper and '706 with amalgamated 

 copper — again not differing from the value deduced from the 

 thermal data by an amount materially outside the limits of 

 experimental errors, especially those due to variation in the 

 heat of formation of salts according as the physical state of 

 the metal employed varies. 



It is, however, to be noticed that the above heat of solution of 

 crystalline cadmium in mercury only corresponds to an E.M.F. 

 of '0135 volt ; whilst the average difference in E.M.F. caused 

 by the substitution of fluid amalgamated cadmium for crystal- 

 line electro-metal was "7525 — '707 = '0455 volt, a considerably 

 greater amount ; so that amalgamating the crystalline metal 

 appears to produce a greater effect on the E.M.F. than corre- 

 sponds to the heat of solution. Just the same result is pro- 

 duced when cadmium and zinc are opposed (§ 121); on the 

 other hand, the effect on the E.M.F. of amalgamating silver 

 is sensibly the same as that corresponding to the heat of solu- 

 tion of silver in mercury (§ 129). Probably the difference 

 in the cases of silver and cadmium is due to the oxidizability 

 of the latter by dissolved air, thus rendering the outer surface 

 of the crystalline masses somewhat different from the interior. 



Cells containing Cadmium opposed to Zinc. 



121. The following table exhibits in brief the results of 

 upwards of thirty series of observations, mostly lasting over 

 several hours, during which period the E.M.F. developed by 

 anv given cell remained sensiblv steady: — 



