Chemical Affinity in terms of Electromotive Force. 255 



is introduced by the circumstance that these values are cal- 

 culated from determinations of the heat evolved by the direct 

 combustion of metal to oxide, and that produced by solution 

 of the metallic oxide in dilute sulphuric acid — these determi- 

 nations being made by different observers necessarily working 

 with different specimens of metallic oxides, and their results 

 being averaged. Ditte has shown, however ( Comptes Rendus, 

 Ixxii. pp. 762 & 878, and lxxiii. p. 108), that the heat deve- 

 loped by dissolving a metallic oxide in an acid varies with the 

 degree of heat to which the oxide has previously been exposed. 

 Thus, with zinc oxide and dilute sulphuric acid, the following 

 values were obtained per gramme-equivalent: — 



Gramme-degrees. 

 Zinc oxide previously heated to 350° .... 9,890 

 „ „ „ dark red heat . 11,016 



„ „ „ bright „ . 12,138 



The value of this "heat of solution" was given by Favre as 

 10,455, and by Julius Thomsen as 11,705, the temperature to 

 which the zinc oxide had been previously heated not being 

 stated. 



23. The experiments of Raoult above adverted to (§ 18) 

 were conducted as follows : — A current from a Daniell bat- 

 tery was passed through a galvanometer and a voltameter 

 placed in a calorimeter, a derived circuit containing a sine 

 galvanometer being also applied to the voltameter-plates; so 

 that the current in this derived circuit virtually measured the 

 difference of potential subsisting between the voltameter-plates. 

 Calling /the "intensity of the derived current," and d that 

 due to a Daniell cell working through the same resistance, 

 and taking 23,900 as the "galvanic heat" (§ 17) of each 

 Daniell cell used, the total work done in the voltameter per 



f 



gramme-equivalent will be represented by ^ x 23,900 gramme- 

 degrees : subtracting this from the heat actually developed as 

 such, a value in gramme-degrees is obtained representing the 

 work done as chemical decomposition. In this way Raoult ob- 

 tained the following values for the chemical decomposition of 

 water and copper sulphate: — 



Water. Copper sulphate. 



33,858 gramme-degrees. 30,407 gramme-degrees. 



34,229 „ „ 29,765 „ 



33,998 „ „ 29,683 „ „ 



Mean . 34,028 „ „ 29,951 „ „ 



These mean values correspond respectively to 34,028 %J= 



T2 



