NATURE OF. See Heat.

Greek Fire, a kind of artificial or fictitious fire, which is said to have been used with wonderful effect in the wars of the Christians and Saracens in the seventh century. Its ingredients are not known, the composition of it having been kept a profound secret; nor do the accounts that have come down to us admit of anything but conjecture as to its nature. From the descriptions of its effects, however, it seems probable that the principal ingredient was naphtha, with the addition of nitre, and perhaps sulphur. The use of naphtha, from its great inflammability, and the difficulty of quenching its flame, may account for the havoc and dismay which the Greek fire is said to have occasioned, and may have given rise to the stories of its burning with greater violence in water than in air, though a sufficient admixture of nitre, by the oxygen it gives out in combustion, might render such a composition for a time almost unextinguishable even by water. We are told too that it was employed with equal effect by sea and land, in battles or in sieges. "In two sieges," says Gibbon, "the deliverance of Constantinople may be chiefly ascribed to the novelty, the terrors, and the real efficacy of the Greek fire. The important Fire-Balls, secret of compounding and directing this artificial flame was imparted by Callinicus, a native of Heliopolis in Syria, who deserted from the service of the caliph to that of the emperor. It was either poured from the rampart in large boilers, or launched in red-hot balls of stone and iron, or darted with arrows and javelins, twisted round with flax and tow which had imbibed the inflammable oil; sometimes it was deposited in fire-ships, and was most commonly blown through long tubes of copper, which were planted on the prow of a galley, and fancifully shaped into the mouths of savage monsters that seemed to vomit a stream of liquid and consuming fire. This important art was preserved at Constantinople as the palladium of the state: the galleys and artillery might occasionally be lent to the allies of Rome, but the composition of the Greek fire was concealed with the most jealous scruple. The secret was confined above 400 years to the Romans of the East. It was at length either discovered or stolen by the Mohammedans; and in the holy wars of Syria and Egypt they retorted an invention, contrived against themselves, on the heads of the Christians. The few Gregois, as it is styled by the more early of the French writers, "cannons flying through the air," says Joinville, "like a winged long-tailed dragon, about the thickness of a hogshead, with the report of thunder and the velocity of lightning; and the darkness of the night was dispelled by this deadly illumination. The use of the Greek fire was continued to the middle of the fourteenth century, when the scientific or casual compound of nitre, sulphur, and charcoal, effected a new revolution in the art of war and the history of mankind."

It should be observed, that in all probability many different inventions have been described under the general name of Greek fire. Although the invention has usually been ascribed to Callinicus of Heliopolis, in the reign of Constantine Pogonatus, A.D. 658, it seems highly probable that naphtha (which abounds in many parts of the ancient Persian kingdom, in India, and on the shores of the Caspian) had been applied to the purposes of warfare at a much earlier period by the Oriental nations. This subject has been ably discussed by Dr Macculloch in the Quarterly Journal of Science, &c. vol. xiv.

Fire-Balls, called also Bolides and Fiery Meteors, globular masses of fire which appear at a considerable height above the earth, shining occasionally with very great splendour, and sometimes leaving a luminous track behind them. 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 earthly 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 sometimes 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 70 miles above the surface of the earth. The height of others has also been computed, and found to vary from 33 to 100 miles. The velocity at which they travel is estimated at from 5 to 33 miles in a second. One of 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 over an immense space, being observed in the Fire-Balls. southern provinces of France, and, according to one account, at Rome. During its course it frequently changed its shape; sometimes appearing in the form of one hall, 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 to objects a greenish cast.

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 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 with a pure bright light, then followed a tender yellow, mixed with azure, red, and green; which, with a coalition of bolder tints, and a reflexion 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 meteor, 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, 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 cause 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 broached 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. Another theory supposes that the Fire-Arms 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 earths 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. See Falling Stars, and Meteorites. Dispersed through the volumes of Von Humboldt's Cosmos the reader will find many curious and interesting observations on these phenomena.