351. After an examination of the experiments of Walsh[A] Ingenhousz[B],
Cavendish[C], Sir H. Davy[D], and Dr. Davy[E], no doubt remains on my mind
as to the identity of the electricity of the torpedo with common and
voltaic electricity; and I presume that so little will remain on the minds
of others as to justify my refraining from entering at length into the
philosophical proofs of that identity. The doubts raised by Sir H. Davy
have been removed by his brother Dr. Davy; the results of the latter being
the reverse of those of the former. At present the sum of evidence is as
follows:--
[A] Philosophical Transactions, 1773, p. 461.
[B] Ibid. 1775, p. 1.
[C] Ibid. 1776, p. 196.
[D] Ibid. 1829, p. 15.
[E] Ibid. 1832, p. 259.
352. _Tension._--No sensible attractions or repulsions due to tension have
been observed.
353. _In motion_: i. Evolution of Heat; not yet observed; I have little or
no doubt that Harris's electrometer would show it (287. 359.).
354. ii. _Magnetism._--Perfectly distinct. According to Dr. Davy[A], the
current deflected the needle and made magnets under the same law, as to
direction, which governs currents of ordinary and voltaic electricity.
[A] Philosophical Transactions, 1832, p. 260.
355. iii. _Chemical decomposition._--Also distinct; and though Dr. Davy
used an apparatus of similar construction with that of Dr. Wollaston
(327.), still no error in the present case is involved, for the
decompositions were polar, and in their nature truly electro-chemical. By
the direction of the magnet it was found that the under surface of the fish
was negative, and the upper positive; and in the chemical decompositions,
silver and lead were precipitated on the wire connected with the under
surface, and not on the other; and when these wires were either steel or
silver, in solution of common salt, gas (hydrogen?) rose from the negative
wire, but none from the positive.
356. Another reason for the decomposition being electrochemical is, that a
Wollaston's apparatus constructed with _wires_, coated by sealing-wax,
would most probably not have decomposed water, even in its own peculiar
way, unless the electricity had risen high enough in intensity to produce
sparks in some part of the circuit; whereas the torpedo was not able to
produce sensible sparks. A third reason is, that the purer the water in
Wollaston's apparatus, the more abundant is the decomposition; and I have
found that a machine and wire points which succeeded perfectly well with
distilled water, failed altogether when the water was rendered a good
conductor by sulphate of soda, common salt, or other saline bodies. But in
Dr. Davy's experiments with the torpedo, _strong_ solutions of salt,
nitrate of silver, and superacetate of lead were used successfully, and
there is no doubt with more success than weaker ones.
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 Encyclopædia
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.
[A] The term _quantity_ in electricity is perhaps sufficiently definite
as to sense; the term _intensity_ is more difficult to define strictly.
I am using the terms in their ordinary and accepted meaning.
Table of the experimental Effects common to the Electricities derived from
different Sources[A].