* The dread Gymnotus. l. 202. The Gymnotus electricus is a native of the river of Surinam in South America; those which were brought over to England about eight years ago were about three or four feet long, and gave an electric shock (as I experienced) by putting one finger on the back near its head, and another of the opposite hand into the water near its tail. In their native country they are said to exceed twenty feet in length, and kill any man who approaches them in an hostile manner. It is not only to escape its enemies that this surprizing power of the fish is used, but also to take its prey; which it does by benumbing them and then devouring them before they have time to recover, or by perfectly killing them; for the quantity of the power seemed to be determined by the will or anger of the animal; as it sometimes struck a fish twice before it was suf∣ficiently benumbed to be easily swallowed.
The organs productive of this wonderful accumulation of electric matter have been accurately dissected and described by Mr. J. Hunter. Philos. Trans. Vol. LXV. And are so divided by membranes as to compose a very extensive surface, and are supplied with many pairs of nerves larger than any other nerves of the body; but how so large a quantity is so quickly accumulated as to produce such amazing effects in a fluid ill adapted for the purpose is not yet satisfactorily explained. The Torpedo possesses a similar power in a less degree, as was shewn by Mr. Walch, and another fish lately described by Mr. Paterson. Philo. Trans. Vol. LXXVI.
In the construction of the Leyden-Phial, (as it is called) which is coated on both sides, it is known, that above one hundred times the quantity of positive electricity can be condensed on every square inch of the coating on one side, than could have been ac∣cumulated on the same surface if there had been no opposite coating communicating with the earth; because the negative electricity, or that part of it which caused its ex∣pansion, is now drawn off through the glass. It is also well known, that the thinner the glass is (which is thus coated on both sides so as to make a Leyden-phial, or plate) the more electricity can be condensed on one of its surfaces, till it becomes so thin as to break, and thence discharge itself.
Now it is possible, that the quantity of electricity condensible on one side of a coated phial may increase in some high ratio in respect to the thinness of the glass, since the power of attraction is known to decrease as the squares of the distances, to which this cir∣cumstance of electricity seems to bear some analogy. Hence if an animal membrane, as thin as the silk-worm spins its silk, could be so situated as to be charged like the Leyden bottle, without bursting, (as such thin glass would be liable to do,) it would be difficult to calculate the immense quantity of electric fluid, which might be accumulated on its surface. No land animals are yet discovered which possess this power, though the air would have been a much better medium for producing its effects; perhaps the size of the necessary apparatus would have been inconvenient to land animals.
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