522 LECTURE LIV. 



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 sulfuret, connected with each other, the sulfuret, becoming nega- 

 tive, 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 shown by 

 composing a battery of a number of alternations of this kind, and either 

 examining the state of its different parts by electrical tests, or connecting 

 wires with its extremities, which, when immersed 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 sulfuration may be opposed to each other, or they may be combined 

 in various ways, the sum or difference of the separate actions being ob- 

 tained by their union ; thus it usually happens that both the metals 

 employed are oxidable in some degree, and the oxidation, 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 worst conductor, is zinc ; 

 the next is iron ; then come tin, lead, copper, silver, gold, and platina. 

 (Plate XL. Fig. 553.. .555.) 



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 made 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 receives from 

 the acid or the water ; unless something like a compensation be supposed 

 to take place, from the effects of induced electricity. Instead of the ex- 

 trication of hydrogen, the same causes will sometimes occasion a deposition 

 of a metal which has been dissolved, will prevent the solution of a metal 

 which would otherwise have been corroded, or produce some effects which 

 appear to indicate the presence of an alkali, either volatile or fixed. All 

 these operations may, however, be very much impeded by the interposition 

 of any considerable length of water, or of any other imperfect conductor. 

 (Plate XL. Fig. 556.) 



It is obvious, that since the current of electricity, produced by a galvanic 

 circle, facilitates those actions from which its powers are derived, the effect 

 of a double series must be more than twice as great as that of a single one : 

 and hence arises the activity of the pile of Volta, the discovery of which 

 forms the most important era in the history of this department of natural 



