CONVERSATION XLI.

:

Of Animal Electricity of the Torpedo: of the Gymnotus Electricus, and of the Silurus Electricus.

Tutor. There are three kinds of fish which have been discovered that are possessed of the singular property of giving shocks very similar to those experienced by means of the Leyden jar.

Charles. I should like much to see them are they easily obtained?

Tutor. No, they are not: they are called the torpedo, the gymnotus electricus, and the silurus electricus.

James. Are they all of the same species ?

Tutor. No: the torpedo is a flat fish, seldom twenty inches long, and is common in various parts of the sea coast of Europe. The electric organs of this fish are placed on each side of the gills, where they fill up the whole thickness of the animal, from the lower to the upper surface, and are covered by the common skin of the body.

Charles. Can you lay hold of the fish by any other part of the body with impunity?

Tutor. Not altogether so: for if it be touched with one hand, it generally communicates a

very slight shock; but if it be touched with both hands at the same time, one being applied to the under, and the other to the upper surface of the body, a shock will be received similar to that which is occasioned by the Leyden jar.

James. Will not the shock be felt if both hands be put on one of the electrical organs at the same time?

Tutor. No: and this shows that the upper and lower surfaces of the electric organs are in opposite states of electricity, answering to the positive and negative sides of a Leyden phial.

Charles. Are the same substances conductors of the electric power of the torpedo, by which artificial electricity is conducted?

Tutor. Yes, they are: and if the fish, instead of being touched by the hands, be touched by conducting substances, as metals, the shock will be communicated through them. The circuit may also be formed by several persons joining hands, and the shock will be felt by them all at the same time. But the shock will not pass where there is the smallest interruption; it will not even be conducted through a chain.

James. Can you get sparks from it?

Tutor. No spark was ever obtained from the torpedo, nor could electric repulsion and attraction be produced by it.

Charles. Is it known how the power is accumulated?

Tutor. It seems to depend on the will of the VOL. III.-U

animal, for each effort is accompanied with a depression of its eyes, and it probably makes use of it as a means of self-defence.

James. Is this the case also with the other electrical fishes?

Tutor. The gymnotus possesses all the elec tric properties of the torpedo, but in a very superior degree. This fish has been called the electrical eel, on account of its resemblance to the common eel. It is found in the large rivers of South America.

Charles. Are these fishes able to injure others by this power?

Tutor. If small fishes are put into the water in which the gymnotus is kept, it will first stun, or perhaps kill them, and if the animal be hungry, it will then devour them. But fishes stunned by the gymnotus may be recovered, by being speedily removed into another vessel of

water.

The gymnotus is said to be possessed of a new kind of sense, by which it knows whether bodies, which are brought near him, are conductors or not.

Charles. Then it possesses the same knowledge by instinct which philosophers have gained by experiment.

Tutor. True: the following experiment, among others, is very decisive on this point.

Ex. The extremities of two wires were dip ped into the water of the vessel in which the

animal was kept; they were then bent, extended a great way, and terminated in two separate glasses full of water. These wires, being supported by non-conductors, at a considerable distance from each other, the circuit was incomplete but if a person put the fingers of both hands into the glasses in which the wires terminated, then the circuit was complete. While the circuit was incomplete, the fish never went near the extremities of the wires, as if desirous of giving the shock; but the moment the circuit was completed, either by a person, or any other conductor, the gymnotus immediately went towards the wires, and gave the shock, though the completion of the circuit was out of his sight.

James. How do they catch these kind of fish; the men would, probably, let them go on receiving the shock?

Tutor. In this way the property was, perhaps, first discovered. The gymnotus, as well as the others, may be touched, without any risk of the shock, with wax or with glass; but if it be touched with the naked finger, or with a metal, or a gold ring, the shock is felt up the arın. Charles. Does the silurus electricus produce the same effects as the others?

Tutor. This fish is found in some rivers in Africa, and it is known to possess the property of giving the shock, but no other particulars have been detailed respecting it.

With regard to the torpedo, its power of giving the benumbing sensation was known to the ancients, and from this it probably took its name.-In Fermin's Natural History of Surinam is some account of the trembling-eel, which Dr. Priestley conjectures to be different from the gymnotus: it lives in marshy places, from whence it cannot be taken, except when it is intoxicated. It cannot be touched with the hand, or with a stick, without feeling a terrible shock. If trod upon with shoes, the legs and thighs are affected in a similar manner.

CONVERSATION XLII.

General Summary of Electricity, with Experiments,

Tutor. You now understand what electricity is?

Charles. Yes, it is a fluid which seems to pervade all substances, and when undisturbed, it remains in a state of equilibrium.

James. And that certain portion which every body is supposed to contain, is called its natural share.