574 - lECTORE X.IV. 



but a thin plate of a conducting substance, when insulated, may be excited 

 almost as easily as an electric, commonly so called. 



Vapours are generally in a negative state, but if they rise from metallic 

 substances, or even from some kinds of heated glass, the effect is uncertain, 

 probably on account of some chemical actions which interfere with it. Sul- 

 fur becomes electrical in cooling, and wax candles are said to be sometimes found 

 in a state so electrical, when they are taken out of their moulds, as to attract 

 the particles of dust which are floating near them. The tourmalin, and 

 several other crystallized stones, become electrical 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 transmits 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 elec- 

 tricity by chemical changes, are those which have lately received the appella- 

 tion of galvanic. Some of the effects which have been considered as belong- 

 ing 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 philo- 

 sophers on the continent, to the mere mechanical actions of bodies possessed 

 of diflferent 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 

 certainly is not. 



The phenomena of galvanism appear to be principally derived from an in- 

 equality 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 



