642 



REPORT — I'JOO. 



usually increased the mean electromotive force, but when attended by such uniora 

 usually decreased it.' 



Determinations of the amounts of apparent electromotive force yielded by- 

 various alloys of copper and zinc when used as positive plates in a voltaic cell' 

 have been made by A. P. Laurie,- and are given in the first three columns of the 

 following table ; and I have calculated from these numbers the mean electromotive' 

 forces and the percentage changes of such forces of the alloys, according to the 

 method just referred to. They are as follows : — 



= Zn-=Uu. 



According to these numbers, whilst the apparent amounts of electromotive 

 force, as shown in column 3, increased in nearly all cases, and with tolerable 

 regularity, with the increased proportion of zinc in the alloys, the mean amounts, 

 as shown in column 4, behaved very differently ; thus with all the alloys con- 

 taining more than 33-8 per cent, of copper (agreeing with the formula Zn-Cu)- 

 there was a loss of mean electromotive force, and with less than 35-9 per cent, 

 there was in all cases a gain (see column 5). 



We may reasonably infer from the results obtained by dilution and mixture- 

 of electrolytes, and of metals with mercury,' that not only these, but the 

 results obtained with solid alloys of copper and zinc, are largely, if not 

 entirely, dependent upon two influences, viz., dilution and chemical union, the 

 I'ormer tending to increase and the latter to decrease the mean electromotive 

 fcrce, and that had there been no cases of chemical union in the foregoing 

 table there would have been none of loss of mean electromotive force. The 

 effect of chemical union upon electromotive force appears to have been greater 

 than that of dilution in all the mixtures which contained more than 33'8 per cent» 

 of copper; and the numbers in column 4 indicate what the apparent amounts of 



' See Proc. Birm. Phil. Soc, vol. viii. pp. 63-138. 

 ^ Jovrn. Chem. Soc, vol. liii. p. 104. 



