CHAP, in.] ELECTRICITY AND MAGNETISM. 133 



solvent ; it has been " killed," as workmen say, for it 

 has been turned into sulphate of zinc. The battery will 

 cease to act, therefore, either when the zinc has all dis- 

 solved away, or when the acid has become exhausted, 

 that is to say, when it is all turned into sulphate of zinc. 

 Stout zinc plates will last a long time, but the acids 

 require to be renewed frequently, the spent liquor being 

 emptied out. 



161. Local Action. When the circuit is not closed 

 the current cannot flow, and there should be no chemical 

 action so long as the battery is producing no current. 

 The impure zinc of commerce, however, does not re- 

 main quiescent in the acid, but is continually dissolving 

 and giving off hydrogen bubbles. This local action, 

 as it is termed, is explained in the following manner : 

 The impurities in the zinc consist of particles of iron, 

 arsenic, and other metals. Suppose a particle of iron to 

 be on the surface anywhere and in contact with the acid. 

 It will behave like the copper plate of a battery towards 

 the zinc particles in its neighbourhood, for a local differ- 

 ence of potential will be set up at the point where there 

 is metallic contact, causing a local current to run from 

 the particles of zinc through the acid to the particle of 

 iron, and so there will be a constant wasting of the zinc, 

 both when the battery circuit is closed and when it is open. 



162. Amalgamation of Zinc. We see now why a 

 piece of ordinary commercial zinc is attacked on being 

 placed in acid. There is local action set up all over its 

 surface in consequence of the metallic impurities in it. 

 To do away with this local action, and abolish the 

 wasting of the zinc while the battery is at rest, it is usual 

 to amalgamate the surface of the zinc plates with 

 mercury. The surface to be amalgamated should be 

 cleaned by dipping into acid, and then a few drops of 

 mercury should be poured over the surface and rubbed 

 into it with a bit of linen rag tied to a stick. The 

 mercury unites with the zinc at the surface, forming a 



