224 Prof. E. F. Herroun on the Divergence of 



rounding bodies, and one would expect it to be " bound 

 energy/' and to be a reversible thermal effect independent of 

 the effects due to the " free energy " of the chemical change. 

 Since the heat of solution of silver nitrate is a large nega- 

 tive quantity, —5440 or —10,880 for Ag2(N03)2,Aq, the heat 

 of formation of anhydrous AgNOg would be greater by that 

 amount than that measured when using its solutions. This 

 would give the heat of formation of 



[ Ag„ 0, N3O5] = 16,770 + 10,880 = 27,650, 



and thus give a calculated E.M.F. about '235 volt lower, 

 still leaving a deficiency of about "105 volt to be accounted 

 for by the tendency of silver to form a film of subsalt ; unless 

 indeed the heat of formation of silver nitrate has been 

 under-estimated, as the numbers given above for the heat of 

 replacement of silver by copper might indicate. 



These numbers, after allowance for the negative heat of 

 solution of silver nitrate, would give a calculated electro- 

 motive force almost exactly in agreement with the experi- 

 mental value. 



Lead. 



Lead, like silver, is a metal having a large negative " ther- 

 movoltaic constant," which also varies with the salt of the 

 metal employed. Wright and Thompson give for the differ- 

 ence of E and E^ in the case of lead in saturated solutions 

 of its chloride and bromide "222 and '235 volt, while for the 

 iodide only "002 volt. This difference, however, is readily 

 explained on reference to the numbers selected by these authors 

 in calculating the theoretical E.M.F., as, in the case of the 

 chloride and bromide, they employ the heats of formation of 

 the dissolved salts, while they use the heat of formation of 

 solid lead iodide. Inasmuch as they used saturated solutions 

 of the chloride and bromide, it is difficult to see why they 

 selected the values corresponding to the dissolved salts, since 

 any more salt formed must necessarily be produced in the 

 solid state, and the electromotive force would, consequently, 

 be determined by the heat of formation of the solid salts. 



Further, since there is no chemical attraction between 

 these compounds and water, and there is merely an absorp- 

 tion of heat on solution, in accordance with the view stated 

 above (in regard to silver nitrate), the E.M.F. will depend 

 upon the heat of formation of the solid salt, even though the 

 solution be not saturated. 



The values calculated from the dissolved and anhydrous 

 salts and the experimental values are compared below ; — 



