34 ELECTRICITY IN MOTION. 



dipped into water and into an acid, connected together by a small 

 tube, the acid, becoming positive, sends its superfluous fluid 

 through the charcoal into the water; and if a wire of copper be 

 dipped into water and a solution of alkaline sulphuret, connected 

 with each other, the sulphuret, becoming negative, will draw the 

 fluid from the copper on which it acts : and in all these cases the 

 direction of the current is truly determined, as it may be shewn 

 by composing a battery of a number of alterations of this kind, 

 and either examining the state of its different parts by electrical 

 tests, or connecting wires with its extremities, which, when im- 

 mersed into a portion of water, will exhibit the production of 

 oxygen gas where they emit the electric fluid, and of hydrogen 

 where they receive it. These processes of oxidation and of sul- 

 phuration may be opposed to each other, or they may be com. 

 bined in various ways, the sum or difference of the separate actions 

 being obtained by their union ; thus it usually happens, that both 

 the metals employed are oxidable in some degree, and the oxida. 

 tion, which takes place at the surface of the better conductor, 

 tends to impede the whole effect, perhaps by impeding the passage 

 of the fluid through the surface. The most oxidable of the metals, 

 and probably the wont conductor, is zinc ; the next is iron ; then. 

 come tin, lead copper, silver, gold, and platina. 



In the same manner as a wire charged with positive electricity 

 causes an extrication of oxygen gas, so the supply of electricity 

 through the more conducting metal promotes the oxidation of the 

 zinc of a galvanic battery ; and the effect of this circulation may be 

 readily exhibited, by fixing a wire of zinc and another of silver 

 or platina, in an acid, while one end of each is loose, and may 

 be brought together or separated at pleasure : for at the moment 

 that the contact takes place, a stream of bubbles rising from the 

 platina, and a white cloud of oxid falling from the zinc, indicate 

 both the circulation of the fluid and the increase of the chemical 

 action. But when, on the other hand, a plate of zinc is mad* 

 negative by the action of an acid on the greater part of its surface, 

 a detached drop of water has less effect on it, than in the natural 

 state: while a plate of iron, which touches the zinc, and forms a 

 part of the circle with it, is very readily oxidated at a distant 

 point: such a plate must therefore be considered, with regard to 

 this effect, as being made positive by the electricity which it re- 

 ceives from the acid or the water ; unless something like a com* 



