1877.] 



Contact of different Substances. 



307 



respectively in water and in a saturated aqueous solution of copper 

 sulphate, the two liquids being separated by a porous partition, if no 

 difference of potential arise between the liquids in contact, the copper 

 plate in water will be positive to the copper plate in the copper-sulphate 

 solutiou. The distribution of potential will be indicated by fig. 4. 



Fig. 4. 



Copper. Water. Sol. of Copper. 



cop. sulph. 



Experiment, however, shows that the copper plate in water is dis- 

 tinctly negative to the copper plate in the copper-sulphate solution. 

 Hence it follows that there is a considerable difference of potential be- 

 tween these two liquids in contact, the copper- sulphate solution being 

 positive to distilled water. 



The real distribution of potential in this case is therefore represented 

 in fig. 5 ; as before, these figures are not drawn to scale, but only repre- 

 sent qualitatively the distribution of potential. 



Fig. 5. 



Copper. Water. Sol. of Copper. 



cop. sulph. 



Similar experiments indicate (though, in the absence of perfectly reli- 

 able measures, perhaps not so conclusively) that a saturated solution of 

 copper sulphate is positive to dilute sulphuric acid and also to a solu- 

 tion of potassium cyanide when the first liquid is in contact with either 

 of the two latter. 



Indeed the experiments I have performed lead me to conclude that 

 the 11 to 13 per cent., by which the difference of potential exhibited by 

 the copper terminals of a Daniell's cell exceeds the difference of potential, 



