394 Sir Humphry Davy on the relations of 
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 conti- 
nues 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 
