96 Sir H. Davy oti the Relations 



a clean surface, and which effect is probably occasioned by the 

 oxidation of the positive side of the plate. 



There are few electrical actions more intense than those 

 produced by the operation of hydro-sulphurets on copper in 

 these different circumstances ; so much so, that I have con- 

 structed a Voltaic battery which decomposed water, by six 

 combinations, consisting merely of thin slips of copper, of 

 which one half had been exposed to the solution about a mi- 

 nute before the other half: of course, the oxidating surface 

 was on the side of the clean or latest exposed metal. 



With lead, and alloys of tin and lead and iron, there are 

 the same ph^enomena, but much feebler electrical action, the 

 metallic surface which is first introduced being the negative 

 surface ; and the principles of this kind of action are precisely 

 the same as those of copper and hydro-sulphurets. 



Zinc, platinum, and metals which have no chemical action 

 on solutions of hydro-sulphurets, produce no phaenomena of 

 this kind; silver and palladium, which act powerfully with 

 these menstrua, produce very decided effects ; but the com- 

 pounds they form in them being positive with respect to the 

 pure metals, the phajnomena are the reverse of those offered 

 by the more oxidable metals ; the surface plunged first into 

 the solution is the positive surface, and it retains this relation 

 in alkaline, acid, and saline solutions, presenting peculiarities 

 dependent upon the change of surface, which I shall refer to 

 again hereafter. 



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 phaenomena ; the 

 surface first plunged into the acid being tarnished, or retain- 

 ing a slight coat of oxide, is negative to the metal plunged in 

 afterwards, and the relation is sustained in saline or alkaline 

 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 phaenomena 

 in this solution ; the tarnished metal, or that first introduced, 

 being negative with respective to the other. Tin likewise in 

 solution of potassa, having been introduced long enough to 

 have tarnished, is strongly negative witli respect to polished 

 tin. 



Even the noble metals obey the same law. Silver, that has 



been 



