Part 10 (1/2)

_Physiological effects._ n.o.bili has shown[A] that these currents are able to cause contractions in the limbs of a frog. v. _Spark._ The spark has not yet been seen.

[A] Bibliotheque Universelle, x.x.xvii. 15.

350. Only those effects are weak or deficient which depend upon a certain high degree of intensity; and if common electricity be reduced in that quality to a similar degree with the thermo-electricity, it can produce no effects beyond the latter.

V. _Animal Electricity._

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 ident.i.ty 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 ident.i.ty. 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 princ.i.p.ally 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 gla.s.s, 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 quant.i.ty 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 circ.u.mstances 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 circ.u.mstances of _quant.i.ty_ 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 _quant.i.ty_ 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].

Table headings