Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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The
Sensation Is The Same When The Fish Is Placed Between Two Metallic
Plates, The Edges Of Which Do Not Touch, And The Person Applies Both
Hands At Once To These Plates.
The interposition of one metallic plate
prevents the communication if that plate be touched with one hand
only, while the interposition of two metallic plates does not prevent
the shock when both hands are applied.
In the latter case it cannot be
doubted that the circulation of the fluid is established by the two
arms.
If, in this situation of the fish between two plates, there exist any
immediate communication between the edges of these two plates, no
shock takes place. The chain between the two surfaces of the electric
organ is then formed by the plates, and the new communication,
established by the contact of the two hands with the two plates,
remains without effect. We carried the torpedo with impunity between
two plates of metal, and felt the strokes it gave only at the instant
when they ceased to touch each other at the edges.
Nothing in the torpedo or in the gymnotus indicates that the animal
modifies the electrical state of the bodies by which it is surrounded.
The most delicate electrometer is no way affected in whatever manner
it is employed, whether bringing it near the organs or insulating the
fish, covering it with a metallic plate, and causing the plate to
communicate by a conducting wire with the condenser of Volta. We were
at great pains to vary the experiments by which we sought to render
the electrical tension of the torpedo sensible; but they were
constantly without effect, and perfectly confirmed what M. Bonpland
and myself had observed respecting the gymnoti, during our abode in
South America.
Electrical fishes, when very vigorous, act with equal energy under
water and in the air. This observation led us to examine the
conducting property of water; and we found that, when several persons
form the chain between the superior and inferior surface of the organs
of the torpedo, the shock is felt only when these persons join hands.
The action is not intercepted if two persons, who support the torpedo
with their right hands, instead of taking one another by the left
hand, plunge each a metallic point into a drop of water placed on an
insulating substance. On substituting flame for the drop of water, the
communication is interrupted, and is only re-established, as in the
gymnotus, when the two points immediately touch each other in the
interior of the flame.
We are, doubtless, very far from having discovered all the secrets of
the electrical action of fishes which is modified by the influence of
the brain and the nerves; but the experiments we have just described
are sufficient to prove that these fishes act by a concealed
electricity, and by electromotive organs of a peculiar construction,
which are recharged with extreme rapidity. Volta admits that the
discharges of the opposite electricities in the torpedos and the
gymnoti are made by their own skin, and that when we touch them with
one hand only, or by means of a metallic point, we feel the effect of
a lateral shock, the electrical current not being directed solely the
shortest way. When a Leyden jar is placed on a wet woollen cloth
(which is a bad conductor), and the jar is discharged in such a manner
that the cloth makes part of the chain, prepared frogs, placed at
different distances, indicate by their contractions that the current
spreads itself over the whole cloth in a thousand different ways.
According to this analogy, the most violent shock given by the
gymnotus at a distance would be but a feeble part of the stroke which
re-establishes the equilibrium in the interior of the fish.* (* The
heterogeneous poles of the double electrical organs must exist in each
organ. Mr. Todd has recently proved, by experiments made on torpedos
at the Cape of Good Hope, that the animal continues to give violent
shocks when one of these organs is extirpated. On the contrary, all
electrical action is stopped (and this point, as elucidated by
Galvani, is of the greatest importance) if injury be inflicted on the
brain, or if the nerves which supply the plates of the electrical
organs be divided. In the latter case, the nerves being cut, and the
brain left untouched, the torpedo continues to live, and perform every
muscular movement. A fish, exhausted by too numerous electrical
discharges, suffered much more than another fish deprived, by dividing
the nerves, of any communication between the brain and the
electromotive apparatus. Philosophical Transactions 1816.) As the
gymnotus directs its stroke wherever it pleases, it must also be
admitted that the discharge is not made by the whole skin at once, but
that the animal, excited perhaps by the motion of a fluid poured into
one part of the cellular membrane, establishes at will the
communication between its organs and some particular part of the skin.
It may be conceived that a lateral stroke, out of the direct current,
must become imperceptible under the two conditions of a very weak
discharge, or a very great obstacle presented by the nature and length
of the conductor. Notwithstanding these considerations, it appears to
me very surprising that shocks of the torpedo, strong in appearance,
are not propagated to the hand when a very thin plate of metal is
interposed between it and the fish.
Schilling declared that the gymnotus approached the magnet
involuntarily. We tried in a thousand ways this supposed influence of
the magnet on the electrical organs, without having ever observed any
sensible effect. The fish no more approached the magnet, than a bar of
iron not magnetic. Iron-filings, thrown on its back, remained
motionless.
The gymnoti, which are objects of curiosity and of the deepest
interest to the philosophers of Europe, are at once dreaded and
detested by the natives. They furnish, indeed, in their muscular
flesh, pretty good aliment; but the electric organ fills the greater
part of their body, and this organ is slimy, and disagreeable to the
taste; it is accordingly separated with care from the rest of the eel.
The presence of gymnoti is also considered as the principal cause of
the want of fish in the ponds and pools of the Llanos.
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