As to the mode in which heat is engendered, opinions are still very much divided. The chemical and mechanical explanations that have been given of the phenomenon are not universally applicable. All we can say at the present day is that the production of heat and the power of regulating their temperature pos sessed by organized beings is another of the hidden and singular laws or properties intro duced into the system of the universe with their creation.
Light.—Many unorganized bodies have the property of shining or giving out light for some time after they have been exposed to the bright rays of the sun, or have been heated in the fire, or when they are struck together or smartly compressed, and this certainly without any decomposition of their substance. The disen gagement of light, again, is a very uniform accompaniment of the decomposition and com position of inorganic substances, and it appears to be a very constant attendant upon electrical phenomena.
Various organic substances and products of organization have a similar property ; living vegetables, too, particularly the flowers, have been seen to give out light by authorities so respectable, that though the fact has been called in question by others of great name, there seems no sufficient reason for treating all that has been said on the subject as illusion : in the physical sciences negatives cannot be received as evidence of equal value with posi tives.
No one thinks of calling in question the luminousness of animals ; most of the innu merable inferior tribes that live in the sea, appear to possess and to manifest this pro perty at different seasons. The luminous ness of the ocean itself, so familiarly known, seems to depend On the presence of multitudes of infusory animals within its bosom. Many tribes of insects shine in the dark. The phe nomenon is not certainly known to be mani fested by any of the class of reptiles, birds, or mammalia. It appears to depend, in insects particularly, on the presence of a peculiar matter, secreted by their bodies and stored up in particular points, which, under the influence of a temperature elevated in a certain degree, and the contact of atmospheric air, enters into a kind of combustion during which light is emitted. Is the phenomenon dependent on
one common cause in both vegetables and animals, supposing that it does really occur among the former? Electrical phenomena are extensively ex hibited by the objects composing the unor ganized and the organized world. In fact, wherever there is composition and decompo sition going on, there are electrical phenomena manifested. The action of the immense mass of vegetables on the air, the evolution of oxygen in the sunshine, and the formation of carbonic acid during the • dark, has even been supposed by an ingenious natural philosopher of France (Pouillet) to be the principal source of the electricity of the atmosphere.
Galvanic electricity is excited by the contact of the different parts of which animal bodies consist, particularly of the nerves and muscular flesh ; the nerve of a frog's thigh exposed and isolated, touched with a piece of quivering flesh from the body of a bullock just slain, also isolated, causes the muscles to which the nerve is distributed to contract energetically (Hum boldt). The same phenomenon occurs when different other parts and fluids, particularly the blood, are used to form a chain. But the electrical phenomena manifested by animals at large, are weak when contrasted with those exhibited by certain fishes provided with spe cial voltaic piles or galvanic batteries by which they give at will, but not otherwise, electrical shocks of such violence as to stun larger ani mals and even to deprive smaller ones of life. This electricity of animals must be held as a vital phenomenon ; several of them have in deed a peculiar apparatus for the preparation of the shock, to speak of the phenomenon by its effects, in our ignorance of its essence or efficient cause, but this loses its power when the nerves that are abundantly distributed to it are divided.
Electrical phenomena are not so obviously displayed by any other tribe of animals as by fishes ; but it has been rendered .next to certain that muscular contractions are uniformly accom panied by a kind of electrical discharge from the nervous fibrils distributed to the special or gans of voluntary motion.