ON ELECTRICITY IN MOTION. 521 



when heated or cooled, and it is found that the disposition, assumed by 

 the fluid, bears a certain relation to the direction in which the stone trans- 

 mits the light most readily ; some parts of the crystal being rendered 

 always positively and others negatively electrical, by an increase of 

 temperature. 



The most remarkable of the phenomena, attending the excitation of 

 electricity by chemical changes, are those which have lately received the 

 appellation of galvanic. Some of the effects which have been considered 

 as belonging to galvanism are probably derived from the electrical powers 

 of the animal body, and the rest have been referred by Mr. Volta, and 

 many other philosophers on the continent, to the mere mechanical actions 

 of bodies possessed of different properties with regard to electricity. 

 Thus, they have supposed that when a circulation of the electric fluid is 

 produced through a long series of substances in a certain direction, the 

 differences of their attractions and of their conducting powers, which 

 must remain the same throughout the process, keep up this perpetual 

 motion, in defiance of the general laws of mechanical forces. In this 

 country it has been generally maintained, that no explanation founded on 

 such principles could be admissible, even if it were in all other respects 

 sufficient and satisfactory, which the mechanical theory of galvanism cer- 

 tainly is not. 



The phenomena of galvanism appear to be principally derived from an 

 inequality in the distribution of the electric fluid, originating from 

 chemical changes, and maintained by means of the resistance opposed to 

 its motion, by a continued alternation of substances of different kinds, 

 which furnishes a much stronger obstacle to its transmission than any of 

 those substances alone would have done. The substances employed must 

 neither consist wholly of solids nor of fluids, and they must be of three 

 different kinds, possessed of different powers of conducting electricity; 

 but whether the difference of their conducting powers is of any other con- 

 sequence than as it accompanies different chemical properties, is hitherto 

 undetermined. Of these three substances, two must possess a power of 

 acting mutually on each other, while the other appears to serve principally 

 for making a separate connexion between them : and this action may be of 

 two kinds, or perhaps of more ; the one is oxidation, or the combination 

 of a metal or an inflammable substance with a portion of oxygen derived 

 from water or from an acid, the other sulfuration, or a combination with 

 the sulfur contained in a solution of an alkaline sulfuret. 



We may represent the effects of all galvanic combinations, by con- 

 sidering the oxidation as producing positive electricity in the acting liquid, 

 and the sulfuration as producing negative electricity, and by imagining 

 that this electricity is always communicated to the best conductor of the 

 other substances concerned, so as to produce a circulation in the direction 

 thus determined. For example, when two wires of zinc and silver, 

 touching each other, are separately immersed in an acid, the acid, becoming 

 positively electrical, imparts its electricity to the silver, and hence it flows 

 back into the zinc : when the ends of a piece of charcoal are dipped into 

 water and into an acid, connected together by a small tube, the acid, 



