Electromotive Forces from Thermochemical Data. 229 



50,130 for solutions containing •25CUSO4IOOH2O, which 

 numbers I considered to be in sufficiently close agreement. 



But adopting 19,250 c. as the heat of replacement of 

 copper by tin we get a calculated E.M.F. of only "416 volt, 

 while experiment shows an E.M.F. of "55 volt as a mean 

 value ; thus showing that a tin-copper sulphate cell furnishes 

 an E.M.F. of about '144 volt in excess of its computed value. 

 Using this cell in a calorimeter, I found that with direct 

 currents of '1125 ampere passing through the cell there vvas 

 an absorption of heat, and when worked backwards against 

 the E.M.F. of the cell there was an evolution of heat apart 

 from that due to the square of the current and the resistance. 

 The internal resistance in one of these cells was reduced to 

 only '9 ohm, so that the heat due to it was very small. 



I conclude from these observations, and in consonance with 

 the theory already stated, that stannous sulphate has a negative 

 heat of dilution, which portion of its total energy, while 

 appearing as negative in the calorimeter, is unable to reduce 

 the electromotive force due to the "free energy" of the 

 undiluted salt, the negative heat of dilution being supplied 

 by fall of temperature. I have not been able to verify this 

 conclusion experimentally, and the subject is complicated by 

 the necessity of ha^ang free acid present in its solutions to 

 prevent the formation of basic salts. The effect both of 

 dilution and of the quantity of free acid present also greatly 

 influence the E.M.F. of stannous chloride cells, and some 

 points in regard to its action may be seen from the following 

 consideration. 



It is well known that if a strip of tin be placed vertically 

 in a vessel, containing at its lower part a strong solution of 

 stannous chloride (with free hydrochloric acid) and above it 

 water, at the line of junction of the liquids, where a dilute 

 solution of stannous chloride is formed by the intermixture, 

 metallic tin is deposited in brilliant crystals on the tin plate, 

 tin dissolving in the liquid below. This solution of a metal 

 in a concentrated solution of one of its salts and its deposition 

 from a dilute one is entirely contrary to what would be ex- 

 pected, were it not for the fact that in the relatively strong 

 hydrochloric acid in the lower liquid there is a store of 

 potential chemical energy which may either evolve heat in 

 diffusion and dilution, or, by attacking the tin plate in contact 

 with it, furnish a current which, flowing through the liquid 

 and metal, has the effect of depositing an equivalent amount 

 of tin from the solution above, in which the hydrochloric acid, 

 being more dilute, has less tendency to dissolve the tin, a 

 considerable portion of its energy having already been ex- 

 pended in the heat evolved during dilution. 



