©N ELECTRICITT IN MOTION. ' 677 



appears to be occasioned by a long continuation of a slow motion of the fluid; 

 and this is perhaps best furnislied in galvanism by a surface of large extent^ 

 while some other effects may very naturally be expected to depend on the in- 

 tensity of the charge, independently of the quantity of charged surface. It 

 may easily be imagined, that the tension of the fluid must be nearly propor- 

 tional to the number of surfaces, imperfectly conducting, which are interposed 

 between the ends of a pile or battery, the density of the fluid becoming- 

 greater and greater by a limited quantity at each step; and it is easily un- 

 derstood, that any point of the pile may be rendered neutral, by a connexion 

 with the earth, while those parts, whichareaboveitorbelow it, will still preserve 

 their relations unaltered with respect to each other: the opposite extremi- 

 ties being, like the opposite surface of a charged jar, in contrary states, and a 

 partial discharge being produced, as often as they are connected by a con- 

 ducting substance. The various forms, in which the piles or troughs are con- 

 structed, are of little consequence to the theory of their operation : the most 

 convenient are the varnished troughs, in which plates of silvered zinc are ar- 

 ranged side by side, with intervening spaces for the reception of water, or of 

 an acid. (Plate XL. Fig. 55?.) 



It is unquestionable that the torpedo, the gymnotus electricus, and some 

 other fishes, have organs appropriated to the excitation of electricity, and that 

 they have a power of comnmnicating this electricity at pleasure to conduct- 

 ing substances in their neighbourhood. These organs somewhat resemble in 

 their appearance the plates of the galvanic pile, although we know nothing 

 of the immediate arrangement, from which their electrical properties are de- 

 rived; but the effect of the shock, which they produce, resembles in all re- 

 spects that of the weak charge of a very large battery. It has also been shown 

 by the experiments of Galvani, Volta, and Aldini, that the nerves and muscles 

 of the human body possess some electrical powers, although they are so much 

 less concerned in the phenomena which were at first attributed to them by 

 Galvani, than he originally supposed, that many philosophers have been in- 

 clined to consider the excitation of electricity as always occasioned by the 

 inanimate substances employed, and the spasmodic contractions of the muscles 

 as merely very delicate tests of the influence of foreign electricity on the nerves. 



Such is the general outline of the principal experiments and conclusions 



4 p 



