196 



Dr. C. R. A. Wright and Mr. C. Thompson. [May 3, 



corresponding with the difference in heat of formation of zinc and 

 silver sulphates (85700 gram-degrees = 1843 volts) it is only- 

 depressed by an amount short of this by some 5 or 6 decivolts. 

 Similarly in the ammoniacal cells where (as in the caustic soda cells) 

 the action consists in the oxidation of a metal and the solution of the 

 oxide formed in the ammoniacal liquor, Julius Thomsen gives the 

 heat values — 



Zinc Zn,0 = 85430 = 1*837 volt. 



Copper Cu 2 ,0 = 40810 = 0*877 „ 



Silver Ag 2 ,0 = 5900 = 0*127 „ 



Whence the E.M.F. corresponding with the total chemical change 

 must somewhat exceed these amounts by the quantity representing 

 the respective heats of solution in ammonia liquor of the metallic 

 oxides : the highest values observed with zinc and copper fall dis- 

 tinctly, short of these amounts, whilst the numbers obtained with 

 many kinds of aeration plates in weaker solutions exhibit a large 

 deficiency ; on the other hand, cells containing silver as oxidisable 

 metal show no large falling off, and in the case of the highest values 

 an actual excess of E.M.F. , again indicating a somewhat large negative 

 value for the thermo-voltaic constant applicable to silver in contact 

 with ammoniacal fluids. 



It is noticeable that the values of K 2 deduced above are not widely 

 different from those equivalent to the difference in heat of oxidation 

 of the various metals, silver excepted : thus with the caustic soda 

 cells — 



?K = SSI™ 6 ™ 06 - 3513 ° = -° 755 ™ lts - 

 Observed values from — 0'678 to —0*691. 



With the sulphuric acid cells the differences between the heat of 

 formation of copper sulphate, and that of zinc, cadmium, and silver 

 sulphates respectively, are +50130, +33540, and —35570, corre- 

 sponding with— 



Volts. 



Copper replaced by zinc = +1*078 

 Copper replaced by cadmium = +0*721 

 Copper replaced by silver = —0*765 



Observed values of K%. 

 + 0*970 to +1*054 

 + 0*720 to +0*725 

 About -0*020 



With the ammoniacal cells the differences between the heat of 

 formation of cuprous oxide and of zinc and silver oxides respectively 

 are +44620, and —34910, corresponding with — 



