_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
A: Physiological Effects B: Magnetic Deflection.
C: Magnets made.
D: Spark.
E: Heating Power.
F: True chemical Action.
G: Attraction and Repulsion.
H: Discharge by Hot Air.
_________________________________________________________ | | | | | | | | | | | | A | B | C | D | E | F | G | H | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | 1. Voltaic electricity | X | X | X | X | X | X | X | X | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | 2. Common electricity | X | X | X | X | X | X | X | X | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | 3. Magneto-Electricity | X | X | X | X | X | X | X | | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | 4. Thermo-Electricity | X | X | + | + | + | + | | | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | 5. Animal Electricity | X | X | X | + | + | X | | | |_________________________|___|___|___|___|___|___|___|___|
[A] Many of the s.p.a.ces in this table originally left blank may now be filled. Thus with _thermo-electricity_, Botto made magnets and obtained polar chemical decomposition: Antinori produced the spark; and if it has not been done before, Mr. Watkins has recently heated a wire in Harris"s thermo-electrometer. In respect to _animal electricity_, Matteucci and Linari have obtained the spark from the torpedo, and I have recently procured it from the gymnotus: Dr. Davy has observed the heating power of the current from the torpedo. I have therefore filled up these s.p.a.ces with crosses, in a different position to the others originally in the table. There remain but five s.p.a.ces unmarked, two under _attraction_ and _repulsion_, and three under _discharge by hot air_; and though these effects have not yet been obtained, it is a necessary conclusion that they must be possible, since the _spark_ corresponding to them has been procured. For when a discharge across cold air can occur, that intensity which is the only essential additional requisite for the other effects must be present.--_Dec. 13 1838._
-- 8. _Relation by Measure of common and voltaic Electricity._[A]
[A] In further ill.u.s.tration of this subject see 855-873 in Series VII.--_Dec. 1838._
361. Believing the point of ident.i.ty to be satisfactorily established, I next endeavoured to obtain a common measure, or a known relation as to quant.i.ty, of the electricity excited by a machine, and that from a voltaic pile; for the purpose not only of confirming their ident.i.ty (378.), but also of demonstrating certain general principles (366, 377, &c.), and creating an extension of the means of investigating and applying the chemical powers of this wonderful and subtile agent.
362. The first point to be determined was, whether the same absolute quant.i.ty of ordinary electricity, sent through a galvanometer, under different circ.u.mstances, would cause the same deflection of the needle. An arbitrary scale was therefore attached to the galvanometer, each division of which was equal to about 4, and the instrument arranged as in former experiments (296.). The machine (290.), battery (291.), and other parts of the apparatus were brought into good order, and retained for the time as nearly as possible in the same condition. The experiments were alternated so as to indicate any change in the condition of the apparatus and supply the necessary corrections.
363. Seven of the battery jars were removed, and eight retained for present use. It was found that about forty turns would fully charge the eight jars.
They were then charged by thirty turns of the machine, and discharged through the galvanometer, a thick wet string, about ten inches long, being included in the circuit. The needle was immediately deflected five divisions and a half, on the one side of the zero, and in vibrating pa.s.sed as nearly as possible through five divisions and a half on the other side.
364. The other seven jars were then added to the eight, and the whole fifteen charged by thirty turns of the machine. The Henley"s electrometer stood not quite half as high as before; but when the discharge was made through the galvanometer, previously at rest, the needle immediately vibrated, pa.s.sing _exactly_ to the same division as in the former instance.
These experiments with eight and with fifteen jars were repeated several times alternately with the same results.
365. Other experiments were then made, in which all the battery was used, and its charge (being fifty turns of the machine,) sent through the galvanometer: but it was modified by being pa.s.sed sometimes through a mere wet thread, sometimes through thirty-eight inches of thin string wetted by distilled water, and sometimes through a string of twelve times the thickness, only twelve inches in length, and soaked in dilute acid (298.).
With the thick string the charge pa.s.sed at once; with the thin string it occupied a sensible time, and with the thread it required two or three seconds before the electrometer fell entirely down. The current therefore must have varied extremely in intensity in these different cases, and yet the deflection of the needle was sensibly the same in all of them. If any difference occurred, it was that the thin string and thread caused greatest deflection; and if there is any lateral transmission, as M. Colladon says, through the silk in the galvanometer coil, it ought to have been so, because then the intensity is lower and the lateral transmission less.
366. Hence it would appear that _if the same absolute quant.i.ty of electricity pa.s.s through the galvanometer, whatever may be its intensity, the dejecting force upon the magnetic needle is the same._
367. The battery of fifteen jars was then charged by sixty revolutions of the machine, and discharged, as before, through the galvanometer. The deflection of the needle was now as nearly as possible to the eleventh division, but the graduation was not accurate enough for me to a.s.sert that the arc was exactly double the former arc; to the eye it appeared to be so.
The probability is, that _the deflecting force of an electric current is directly proportional to the absolute quant.i.ty of electricity pa.s.sed_, at whatever intensity that electricity may be[A].
[A] The great and general value of the galvanometer, as an actual measure of the electricity pa.s.sing through it, either continuously or interruptedly, must be evident from a consideration of these two conclusions. As constructed by Professor Ritchie with gla.s.s threads (see Philosophical Transactions, 1830, p. 218, and Quarterly Journal of Science, New Series, vol. i. p.29.), it apparently seems to leave nothing unsupplied in its own department.
368. Dr. Ritchie has shown that in a case where the intensity of the electricity remained the same, the deflection of the magnetic needle was directly as the quant.i.ty of electricity pa.s.sed through the galvanometer[A].
Mr. Harris has shown that the _heating_ power of common electricity on metallic wires is the same for the same quant.i.ty of electricity whatever its intensity might have previously been[B].
[A] Quarterly Journal of Science, New Series, vol. i. p. 33.
[B] Plymouth Transactions, page 22.