Force of certain Tin Cells. 19 



rise of E.M.F. up to about *607 volt, but dilution with 1 per 

 cent, hydrochloric acid causes a slight diminution of E.M.F. 

 This and other facts render it probable that with this cell, 

 and to a less extent with the zinc-tin sulphate cell, increase of 

 solution-strength without increase in the percentage of con- 

 tained acid is associated with a rise of E.M.F. ; but that with 

 a corresponding increase of acidity, with a fall of E.M.F. 

 Thus it is that dilution with distilled water raises the E.M.F. 

 not because the metallic salt is diluted, but because the free 

 acid is more attenuated. 



Mutatis mutandis the same remarks apply to tin-copper or 

 other cells in which tin is the metal attacked (i. e. with con- 

 stant acidity and increasing strength of solution the E.M.F. 

 falls, but with increase of both the E.M.F. rises, or does not 

 fall). 



The heat of formation of zinc chloride and of stannous 

 chloride being known, the theoretical E.M.F. is directly 

 calculable. 



[Zn, Cl 2 , aq.] = 112840. [Sn, CI*, aq.] = 81140. 



112840-81140 = 31700; half of which multiplied into 

 4*32 x 10~ 5 gives *684 volt as the E.M.F. of zinc displacing 

 tin from solution of its chloride, and the difference between 

 the observed and calculated value is *684 — *549 = *135 volt. Or 

 to employ the terminology of Messrs. Wright and Thompson, 

 — *135 is the " Thermovoltaic constant " for tin in solution of 

 its chloride for certain strengths of solution (near *5 MC1 2 , 

 100 H 2 0). 



II. Cadmium- Tin Chloride Cell, 



A cell set up with electro-cadmium opposed to tin in 

 solutions of their chlorides, *5 M^Clg, 100 H 2 0, was found to 

 give an E.M.F. of *247 to *262 volt and an average value of 

 •249 volt. The solutions used contained the same amount of 

 free hydrochloric acid, namely about 1 per cent. 



The heats of formation being [Cd, Cl 2 , aq.] = 96250 and 

 [Sn, Cl 2 , aq.] =81140 and their difference 15110 or 7555 per 

 equivalent, the theoretical E.M.F. of cadmium displacing tin 

 from dilute solutions of its chloride is '326 volt, and the dif- 

 ference between this and the average observed value is '077. 



The E.M.F. of a zinc-cadmium chloride cell was also deter- 

 mined and found to be between *291 and *308 volt, giving an 

 average of -301 volt. This is below the value given by Messrs. 

 Wright and Thompson, owing to the solutions used by me 

 containing 1 per cent, free HOI, thev giving for this cell '329 



2 ' ' 



