The phenomenon of thermo-electricity--the discovery of Seebeck, is another proof of the very close connection of the physical forces. We witness their being resolved as it were into each other, electricity producing heat, and heat again electricity; and it is from these curious results that the arguments in favour of their intimate relations and actual identity have been drawn. It will, however, be found to be the best philosophy to regard these forces as dissimilar, until we are enabled to prove them to be only modified forms of one principle or power. At the same time it must not be forgotten that in natural operations we invariably find the combined action of several forces producing a single phenomenon. The important fact to be particularly regarded is, that we have evidence that every substance which is unequally heated becomes the source of this very remarkable form of electricity.[148] There exist a few fishes gifted with the very extraordinary power of producing electrical phenomena by an effort of muscular or nervous energy. The _Gymnotus electricus_, or electrical eel, and the _Raia torpedo_, a species of ray, are the most remarkable. This power is, it would appear, given to these curious creatures for purposes of defence, and also for enabling them to secure their prey. The _Gymnotus_ of the South America rivers, will, it is said, when in full vigour, send forth a discharge of electricity sufficiently powerful to knock down a man, or to stun a horse; while it can destroy fishes, through a considerable space, by exerting its strange artillery.[149] Faraday’s description of a _Gymnotus_, paralyzing and seizing its prey, is too graphic and important to be omitted.