458 Prof. Williamson on the Dynamics 



forming chloride of zinc, and metallic copper will be deposited 

 from the chloride on the surface of the carbon. The zinc acts 

 just like the iron in my last experiment, only with a little more 

 power; and the carbon does not undergo any change whatever: 

 all the combination takes place on the surface of the zinc plate, 

 all the decomposition on the surface of the carbon plate, where the 

 copper is deposited. 



How is it, then, that we have not the full heat of combustion 

 evolved when the zinc combines, and the corresponding heat 

 absorbed or cold produced where the copper is deposited from 

 the chloride ? That nothing of the kind occurs is easily seen ; 

 but the answer to the question involves the theory of the battery, 

 in fact the understanding of our fairy's wand; for this little 

 wire has an electric current running through it as long as it 

 connects the combining and reducing plates. It has the same 

 properties (as you can see by the deflection of the magnetic 

 needle suspended beside it) which I mentioned as existing in 

 our wires on this table, and can produce the same kind of effect. 

 Our little instrument is, in fact, one cell of a galvanic battery. 

 If we want more power than this cell affords, we can select a 

 chemical process in which the heat evolved by the combination 

 is more in excess of the heat absorbed by the decomposition, so 

 that more heat will be actually available. Another way of get- 

 ting more power is by increasing the surface of the plates ; and 

 a third process is to increase the number of cells, taking care to 

 render the action in each cell dependent on that in the next by 

 connecting the zinc plate in the first with the carbon plate in 

 the second, the zinc in the second with the carbon in the third 

 cell, and so on in so-called polar order. 



Every cell of a galvanic battery consists necessarily of two 

 solid plates capable of conducting electricity, moistened either 

 with one liquid, or with two liquids kept from mixing by a 

 porous cell, in the pores of which they touch one another. On 

 the surface of one of these plates, which is called positive, a pro- 

 cess of combination of some kind goes on, whether oxidation or 

 chlorination, of which the effect is to evolve heat ; and at the 

 surface of the other plate, which is called negative, a process of 

 unburning goes on, by which heat is absorbed. It is not neces- 

 sary that the process of combination at the surface of the posi- 

 tive plate should take place, as in the instance before us, between 

 metallic particles from the very substance of the plate, and a 

 constituent of the liquid ; nor is it necessary that the decomposi- 

 tion on the surface of the negative plate should be always effected 

 by the deposition of a metallic body upon the surface of the plates. 

 I have here a little battery consisting of two cells, so constructed 

 that the chemical laws of its action are carried out in an entirely 



