Nature of. See the article HEAT.

Wild Fire, or Greek-Fire, a kind of artificial or fictitious fire, which burns even under water, and that with greater violence than out of it. It is composed of sulphur, naphtha, pitch, gum, and bitumen; and is only extinguishable by vinegar mixed with sand and urine, or by means of raw hides.

The motion or tendency of this fire is said to be contrary to that of natural fire, and always follows the direction in which it is thrown, whether it be downwards, sideways, or otherwise. The French call it Greek fire, or feu Greguais, because it was first used by the Greeks, about the year 660; as is observed by Petavius, on the authority of Nicetas, Theophanes, Cedrenus, and others.

The inventor, according to the same authority, was an engineer of Heliopolis, in Syria, named Callinicus, who first applied it in the sea-fight in which Constantine Pontonates commanded against the Saracens, near Cyzicus, in the Hellespont; and with such effect, that he therewith burnt the whole fleet, in which were thirty thousand men. But others think it of much older date, and consider Marcus Græchas as the inventor; an opinion which is supported by several passages both in the Greek and Roman writers, showing it to have been anciently used by both these nations in their wars. Constantine's successors used it on different occasions with equal advantage; and, what is remarkable, they were fortunate enough to keep the secret of the composition to themselves, so that in the year 960 no other nation was acquainted with the composition.

Hugh, king of Burgundy, having demanded ships of the Emperor Leo, for the siege of Fresno, desired likewise that they should be provided with the Greek fire.

Father Daniel gives a good description of the Greek fire, in his account of the siege of Damietta undertaken by St Louis. Every body, says that author, was astonished at the Greek fire, which the Turks then prepared, and the secret of which is now lost. They threw it out of a kind of mortar; and sometimes shot it with an odd sort of cross-bow, which was strongly bent by means of a handle. That thrown with the mortar sometimes appeared in the air of the size of a tun, with a long tail, and a noise like that of thunder. The French by degrees discovered the secret of extinguishing it, in which they several times succeeded.

Balls of Fire, in Meteorology, are luminous bodies, which appear at a considerable height above the earth, shining occasionally with very great splendour, and sometimes leaving a luminous track behind them after they have disappeared. The larger ones in this hemisphere proceed generally from north to south with great velocity, frequently exploding into globes of smaller size, and sometimes vanishing with a report, sometimes without one. They are not unfrequently attended with a discharge of solid compounds of earthy and metallic matters, called meteoric stones, the descent of which, though stubbornly denied for a time, was finally established about the beginning of this century.

These luminous appearances no doubt constituted part of the ancient prodigies, blazing stars or comets, which last they sometimes resemble in being attended with a train; but they frequently appear with a round and well-defined disk. The first of these of which we have any accurate account, was observed by Dr Halley and some other philosophers at different places, in the year 1719. From the slight observations they could take of its course, the perpendicular height of this body was computed at about seventy miles above the surface of the earth. The height of others has also been computed, and found to vary from thirty-three to a hundred miles. The velocity at which they travel is estimated at from five to thirty-three miles in a second. The most remarkable of these on record appeared on the 18th of August 1783, about nine o'clock in the evening. It was seen to the northward of Zetland, and took a southerly direction for an immense space, being observed in the southern provinces of France, and, according to one account, in the ecclesiastical capital of Italy. During its course it frequently changed its shape; sometimes appearing in the form of one ball, sometimes of two or more, sometimes with a train, sometimes without one. It passed over Edinburgh nearly in its zenith, and had then the appearance of a well-defined round body, extremely luminous, and of a greenish colour, the light which it diffused on the ground giving likewise a greenish cast to objects. After passing the zenith it was attended by a train of considerable length, which continually augmented, and at last entirely obliterated the head, so that it looked like a wedge, flying with the obtuse end foremost. The motion was not apparently rapid, by reason of its great height; though in reality it must have moved with great velocity, on account of the immense space over which it travelled in a short time. In other places its appearance was very different. At Greenwich we are told that "two bright balls parallel to each other led the way, the diameter of which appeared to be about two feet; and were followed by an expulsion of eight others, not elliptical, seeming gradually to mutilate, for the last was small. Between each two balls a luminous serrated body extended, and at the last a blaze issued which terminated in a point. Minute particles dilated from the whole. The balls were tinted first by a pure bright light, then followed a tender yellow, mixed with azure, red, and green; which, with a coalition of bolder tints, and a reflection from the other balls, gave the most beautiful rotundity and variation of colours that the human eye could be charmed with. The sudden illumination of the atmosphere, and the form and singular transition of this bright luminary, tended much to make it awful; nevertheless, the amazing vivid appearance of the different balls, and other rich connective parts not very easy to delineate, Fire-Balls gave an effect equal to the rainbow in the full zenith of its glory."

Dr Blagden, in a paper on the subject inserted in the seventy-fourth volume of the Philosophical Transactions, not only gave a particular account of this and other meteors of the kind, but offered several conjectures as to the probable causes of them. The first thing which occurred to philosophers on this subject was, that the meteors in question were burning bodies rising from the surface of the earth, and flying along the atmosphere with great rapidity. But this hypothesis was soon abandoned, on considering that there was no power known by which such bodies could either be raised to a sufficient height, or projected with the velocity of these meteors. The next hypothesis was, that they did not consist of one single body, but of a train of inflammable vapours, extending a vast way through the atmosphere, which being kindled at one end, displayed the luminous appearances in question by the fire running from one end of the train to the other. To this hypothesis, which was invented by Dr Halley, Dr Blagden objected that no just explanation was given of the nature of the vapours themselves, the manner in which they were raised up, or their regular arrangement in straight lines of such vast extent; or how they could be supposed to burn in such rarefied air. "Indeed," says he, "it is very difficult to conceive how vapours could be prevented, in those regions where there is in a manner no pressure, from spreading out on all sides in consequence of their natural elasticity, and instantly losing that degree of density which seems necessary for inflammation. Besides, it is to be expected, that such trains would sometimes take fire in the middle, and thus present the phenomenon of two meteors at the same time, receding from one another in a direct line."

For these and other reasons the hypothesis of Dr Halley was abandoned, and another substituted in its place. This was, that the meteors in question are permanent solid bodies, not rising from the earth, but revolving round it in very eccentric orbits, and thus in their perigee moving with inconceivable rapidity. But this hypothesis is likewise objectionable. Another theory supposes that the meteors in question are a kind of bodies which take fire as soon as they come within the atmosphere of the earth. Sir Humphry Davy's discovery that the earths and alkalis are metallic oxides, affords some plausibility to this theory, especially in cases where the meteors are accompanied with the descent of solid bodies to the earth. It has been suggested that the bases of the earth may exist in the meteor in a metallic state, and that when the body arrives within the range of our atmosphere, the strong affinity which it is well known these metallic bases have for oxygen, causes them to unite and produce violent combustion. The origin of fire-balls has also been ascribed to the agency of electricity, and there seems little doubt that this is really the true way of accounting for them.

Falling or shooting stars are the same phenomena upon a smaller scale. They are to be seen at all seasons, but most frequently during the prevalence of the northern lights, and most commonly in the lower regions of the atmosphere. The cause of these phenomena, as of those of a more magnificent description, has been attributed to electricity. This theory, however, was objected to by Sir Humphry Davy; and his opinions on every subject are entitled to the highest deference, and to deliberate consideration. He conceived them to be attributable to falling stones. But the subject of meteoric stones, including fire-balls and other meteors, will be treated of under Meteorology. It may also be mentioned, that Chladni, in his work on Fiery Meteors (Vienna, 1819), considers them as solid bodies, formed above the region of our atmosphere,