396 Sir Humphry Davy on the relations of 
The production of electrical currents by single metals and 
single fluids, though most distinct in the cases I have just 
named, yet occurs generally whenever new substances which 
can adhere to the metals are produced in chemical action. 
Thus in acid solutions of a certain strength pieces of the 
same zinc, tin, iron, and copper, exhibit similar phenomena ; 
the surface first plunged into the acid being tarnished, or re- 
taining a slight coat of oxide, is negative to the metal plunged 
in afterwards, and the relation is sustained in saline or alka- 
line solutions. The same effect is caused by producing a 
coat of oxide by heat on the surface, or even by applying it 
artificially. The oxidated surface is negative with respect to 
the other. 
Zinc, which dissolves in a strong solution of potassa, giving 
off hydrogen copiously, exhibits exactly the same phenomena 
in this solution ; the tarnished metal, or that first introduced, 
being negative with respect to the other. Tin likewise in 
solution of potassa, having been introduced long enough to 
have tarnished, is strongly negative with respect to polished 
tin. 
Even the noble metals obey the same law. Silver, that 
has been tarnished by the action of nitric acid, is negative to 
polished silver in diluted acid ; and gold and platinum, that 
have been acted on by aqua regia, are negative in that acid 
to the clean metals. 
The intimate connexion displayed in all these cases be- 
tween the chemical and electrical phenomena, becomes still 
more remarkable when the nature of the changes taking 
places in circles of this kind is considered. 
Oxygen, which may be considered as negative with respect 
