302 



Prof. R, B. Clifton on the 



[June 14, 



sulphate assume in contact a difference of potential, copper being positive 

 to the solution of copper sulphate. 



The difference of potential in this case appears to be about y 3 ^ of the 

 difference of potential of copper and water in contact ; but I do not yet 

 feel in a position to make any very definite statements with respect to 

 the relative magnitudes of the differences of potential of metals and 

 liquids in contact. 



When the liquid employed was commercial sulphuric acid diluted with 

 twenty times its volume of distilled water, the difference of potential 

 observed was very small, so small, indeed, that copper and this dilute acid 

 in contact may be regarded as practically at the same potential ; a very 

 small difference of potential, however, exists in this case, and copper is 

 negative to this dilute acid. 



Copper and a strong aqueous solution of caustic potash assume in con- 

 tact a small difference of potential, larger, however, than that observed 

 in the last case, and copper is negative to the solution of caustic potash. 



There is a very marked difference of potential between copper and an 

 aqueous solution of potassium sulphide (liver of sulphur) in contact, 

 copper being negative to this solution. 



Between copper and a strong aqueous solution of potassium cyanide 

 in contact there is a very considerable difference of potential, copper 

 being negative to the solution. The difference of potential in this case is 

 comparable with the difference observed when zinc and copper are in 

 contact. 



Similar experiments were performed with the upper condenser-plate 

 of well-cleaned zinc, the liquid being connected with the lower plate by 

 a strip of zinc. When the liquid is distilled water there is a marked 

 difference of potential, zinc being positive to water ; and the magnitude of 

 this difference of potential appears to be, very approximately at least, the 

 same as that observed in the case of copper and water. 



With zinc and dilute sulphuric acid the difference of potential is 

 extremely small ; but zinc is slightly negative to the dilute acid (1 volume 

 of sulphuric acid in 20 volumes of distilled water). 



In experimenting with zinc the greatest care is necessary in cleaning 

 the surface of the zinc condenser-plate, as zinc oxide is strongly negative 

 to zinc ; when the plate is very slightly tarnished, the sign of the differ- 

 ence of potential exhibited by the condenser, in the case of water, is 

 reversed, the oxidized zinc being negative to distilled water. 



When the upper condenser-plate was of iron, and distilled water acted 

 as the lower plate, there was a small but distinct difference of potential, 

 iron being negative to water. 



The experiments above described seem to indicate that zinc and copper 

 are about equally positive to water, and that consequently zinc and copper 

 dipping in water are nearly at the same potential. 



This conclusion has hes al?o arrived at by Sir W. Thomson as tho 



