1888.] and the Minimum-point, tyc, of a Voltaic Couple. 301 



with a magnesium-platinum couple, and extremely different with, 

 unlike substances. 



In the present paper my object is to describe a few similar experi- 

 ments, made to examine the influence of liquids of different chemical 

 composition, upon this phenomenon and upon the degrees of electro- 

 motive force produced by further additions of the substances. All 

 the solutions were made with distilled water,, and the substances 

 employed were of considerable degree of purity. The voltaic cell 

 consisted in each case of zinc and platinum in distilled water, and its 

 electromotive force was balanced by that of a suitable thermo-electric 

 pile (see ' Proceedings of the Birmingham Philosophical Society,' 

 vol. 4, p. 130), and the measurements made under that condition. 



The electromotive force of a ziDC-platinum couple in ordinary 

 distilled water at 16° C. is about 1*088 volt ; provided the zinc is free 

 from oxide, and the platinum contains no absorbed hydrogen. The 

 presence of hydrogen (not removable by rubbing hut removable by 

 heating to redness) may reduce the electromotive force to 0*91 volt, 

 and a film of oxide upon the zinc may reduce it 1 or 2 per cent., 

 whilst carbonic acid absorbed by the water from the air, &c, may 

 increase it about 2 per cent. In all cases, therefore, where very 

 exact measurements of electromotive force are necessary, these cir- 

 cumstances have to be considered. In the present case the measure- 

 ments are sufficiently accurate for the purposes intended. 



A series of measurements were made with a zinc-platinum couple 

 in water, adding uniform quantities of hydrochloric acid up to 

 0"15 grain per 465 grains of water, and heating the platinum to 

 redness previous to each measurement. The variations of electro- 

 motive force obtained were nearly the same as when the platinum was 

 not heated, the only material difference being that the electromotive 

 force throughout was about 0*10 volt higher. 



The following are the results of the experiments made upon the 

 influence of the chemical energy of the liquid. The numbers are 

 corrected for the influence of hydrogen absorbed by the platinum. 



Table I.— KI0 3 in 465 grains of Water at 15° C. 



Grains. 



Volts. 



Grains. 



Yolts. 



Grains. 



Volts. 



37-05 



1-40586 



22-05 



1 -26 



7-05 



1 -1456 



34 -05 



1 -36296 



19-05 



1 • 2428 



4-05 



11313 



31-05 



1-3172 



16-05 



1 -2085 



105 



1-1370 



28 -05 



1 2829 



13 -05 



1 • 2028 



94 



1-0884 



25-05 



1 2743 



10-05 



1-14 



water 





The strongest solution employed was a saturated one. Four 

 different solutions, each weaker than 0*94 grain, gave the same 



2 a 2 



