[F] Phil.  Trans. 1832, p. 259.  Dr. Davy, in making experiments on the torpedo, obtains effects the same as those produced by common and voltaic electricity, and says that in its magnetic and chemical power it does not seem to be essentially peculiar,—­p. 274; but he then says, p. 275, there are other points of difference; and after referring to them, adds, “How are these differences to be explained?  Do they admit of explanation similar to that advanced by Mr. Cavendish in his theory of the torpedo; or may we suppose, according to the analogy of the solar ray, that the electrical power, whether excited by the common machine, or by the voltaic battery, or by the torpedo, is not a simple power, but a combination of powers, which may occur variously associated, and produce all the varieties of electricity with which we are acquainted?”

At p. 279 of the same volume of Transactions is Dr. Ritchie’s paper, from which the following are extracts:  “Common electricity is diffused over the surface of the metal;—­voltaic electricity exists within the metal.  Free electricity is conducted over the surface of the thinnest gold leaf as effectually as over a mass of metal having the same surface;—­voltaic electricity requires thickness of metal for its conduction,” p. 280:  and again, “The supposed analogy between common and voltaic electricity, which was so eagerly traced after the invention of the pile, completely fails in this case, which was thought to afford the most striking resemblance.” p. 291.

266.  Notwithstanding, therefore, the general impression of the identity of electricities, it is evident that the proofs have not been sufficiently clear and distinct to obtain the assent of all those who were competent to consider the subject; and the question seemed to me very much in the condition of that which Sir H. Davy solved so beautifully,—­namely, whether voltaic electricity in all cases merely eliminated, or did not in some actually produce, the acid and alkali found after its action upon water.  The same necessity that urged him to decide the doubtful point, which interfered with the extension of his views, and destroyed the strictness of his reasoning, has obliged me to ascertain the identity or difference of common and voltaic electricity.  I have satisfied myself that they are identical, and I hope the experiments which I have to offer and the proofs flowing from them, will be found worthy the attention of the Royal Society.

267.  The various phenomena exhibited by electricity may, for the purposes of comparison, be arranged under two heads; namely, those connected with electricity of tension, and those belonging to electricity in motion.  This distinction is taken at present not as philosophical, but merely as convenient.  The effect of electricity of tension, at rest, is either attraction or repulsion at sensible distances.  The effects of electricity in motion or electrical currents may be considered as 1st, Evolution of heat; 2nd, Magnetism; 3rd, Chemical decomposition; 4th, Physiological phenomena; 5th, Spark.  It will be my object to compare electricities from different sources, and especially common and voltaic electricities, by their power of producing these effects.