357. iv. Physiological effects.—­These are so characteristic, that by them the peculiar powers of the torpedo and gymnotus are principally recognised.

358. v. Spark.—­The electric spark has not yet been obtained, or at least I think not; but perhaps I had better refer to the evidence on this point.  Humboldt, speaking of results obtained by M. Fahlberg, of Sweden, says, “This philosopher has seen an electric spark, as Walsh and Ingenhousz had done before him in London, by placing the gymnotus in the air, and interrupting the conducting chain by two gold leaves pasted upon glass, and a line distant from each other[A].”  I cannot, however, find any record of such an observation by either Walsh or Ingenhousz, and do not know where to refer to that by M. Fahlberg.  M. Humboldt could not himself perceive any luminous effect.

  [A] Edinburgh Phil.  Journal, ii. p. 249.

Again, Sir John Leslie, in his dissertation on the progress of mathematical and physical science, prefixed to the seventh edition of the Encyclopaedia Britannica, Edinb. 1830, p. 622, says, “From a healthy specimen” of the Silurus electricus, meaning rather the gymnotus, “exhibited in London, vivid sparks were drawn in a darkened room”; but he does not say he saw them himself, nor state who did see them; nor can I find any account of such a phenomenon; so that the statement is doubtful[A].

  [A] Mr. Brayley, who referred me to those statements, and has
  extensive knowledge of recorded facts, is unacquainted with any
  further account relating to them.

359.  In concluding this summary of the powers of torpedinal electricity, I cannot refrain from pointing out the enormous absolute quantity of electricity which the animal must put in circulation at each effort.  It is doubtful whether any common electrical machine has as yet been able to supply electricity sufficient in a reasonable time to cause true electro-chemical decomposition of water (330. 339.), yet the current from the torpedo has done it.  The same high proportion is shown by the magnetic effects (296. 371.).  These circumstances indicate that the torpedo has power (in the way probably that Cavendish describes,) to continue the evolution for a sensible time, so that its successive discharges rather resemble those of a voltaic arrangement, intermitting in its action, than those of a Leyden apparatus, charged and discharged many times in succession.  In reality, however, there is no philosophical difference between these two cases.

360.  The general conclusion which must, I think, be drawn from this collection of facts is, that electricity, whatever may be its source, is identical in its nature.  The phenomena in the five kinds or species quoted, differ, not in their character but only in degree; and in that respect vary in proportion to the variable circumstances of quantity and intensity[A] which can at pleasure be made to change in almost any one of the kinds of electricity, as much as it does between one kind and another.