of Electrical and Chemical Changes. 95 



tion of a new combination, which is negative with respect to 

 the metal ; for after the formation of the sulphuret of copper, 

 the plate of copper that has been first plunged into the solu- 

 tion exhibits the same negative state with respect to polished 

 copper, whether introduced into saline solutions, or alkaline 

 or acid menstrua. The electrical effect therefore does not de- 

 pend on so simple a condition as would at first appear, and it 

 may be in fact referred to the combinations containing two 

 metallic substances and one fluid. 



The gray sulphuret of copper is negative, in solutions of 

 hydro-sulphuret, to clean copper, and the superficial coatuig 

 has apparendy similar electrical powers to this substance. 



Copper, in the state of protoxide, is negative, not only with 

 respect to metallic copper, but likewise with respect to the 

 sulphuret; a circumstance which explains many singular and 

 apparently anomalous circumstances with respect to the action 

 of hydro-sulphuret on copper. I have often found the order 

 which I have mentioned, of metallic copper being positive 

 with respect to copper that had been a few seconds in solution 

 of hydro-sulphuret, reversed in a singular and capricious 

 way ; but on investigating the cause, I found that the copper 

 was tarnished ; and on heating any kind of polished copper 

 strongly, so as to produce a thin coating of oxide any where 

 on its surface, it became strongly negative to copper plunged 

 in solution of hydro-sulphuret : the same effect was produced 

 by the action of acids. 



There are some singular circumstances connected with the 

 violent and intense chemical action of copper on solutions 

 of hydro-sulphurets, which are worthy of being described. 

 When a piece of copper connected with the multiplier has 

 been for a minute in strong solution of hydro-sulphuret of 

 potassa, on introducing a piece of polished copper connected 

 with the other wire, there is often a violent and momentary 

 negative charge communicated to it, which sends the needle 

 through a whole revolution: it then oscillates, and almost 

 immediately returns, and takes the direction which indicates 

 that the piece first plunged in is negative. This effect con- 

 tinues for some minutes, then becomes weaker ; at last the two 

 sides are in equilibrium, and the piece which was first plunged 

 in now becomes positive with respect to the other. The first 

 described of these effects seems to depend upon the discharge, 

 by the clean copper, of the negative electricity accumulated 

 by the contact of the plate first plunged in, before the rela- 

 tive states produced by the metallic contact and the regular 

 currents occur ; and the second, to the detaching or peeling 

 off of the coat of sulphuret, which has the effect of exposing 



a clean 



