BURNING
, the action of fire on some pabulum or fuel, by which its minute parts are put into a violent motion, and some of them, assuming the nature of fire, fly off in orbem, while the rest are dissipated in vapour or reduced to ashes..
Burning-Glass, or Burning-Mirror, a machine by which the sun's rays are collected into a point; and by that means their force and effect are extremely heightened, so as to burn objects placed in it.
Burning glasses are of two kinds, convex and concave. The convex ones are lenses, which acting according to the laws of refraction, incline the rays of light towards the axis, and unite them in a point or focus. The concave ones are mirrors or reflectors, whether made of polished metal or silvered glass, and which acting by the laws of reflection, throw the rays back into a point or focus before the glass.
The use of burning glasses it appears is very ancient, many of the old authors relating some effects of them. Diodorus Siculus, Lucian, Dion, Zonaras, Galen, Anthemius, Eustatius, Tzetzes, and others, relate that by means of them Archimedes set fire to the Roman fleet at the siege of Syracuse. Tzetzes is so particular in his account of this matter, that his description suggested to Kircher the method by which it was probably accomplished. That author says that “Archimedes set fire to Marcellus's navy by means of a burning glass composed of small square mirrors, moving every way upon hinges; which when placed in the sun's rays, directed them upon the Roman fleet, so as to reduce it to ashes at the distance of a bowshot.” And the burning power of reflectors is mentioned in Euclid's Optics, theor. 31. Again, Aristophanes, in his comedy of The Clouds, introduces Socrates as examining Strepsiades about a method he had discovered of getting clear of his debts. He replies, that “he thought of making use of a burning-glass which he had hitherto used in kindling his fire; for should they bring a writ against me, I'll immediately place my glass in the sun at some little distance from it, and set it on fire.” Pliny and Lactantius have also spoken of glasses that burn by refraction. The former calls them balls or globes of crystal or glass, which being exposed to the sun, transmit a heat sufficient to set sire to cloth, or corrode the dead flesh of those patients who stand in need of caustics; and the latter, after Clemens Alexandrinus, observes that fire may be kinkled by interposing glasses filled with water between the sun and the object, so as to transmit the rays to it.
Among the ancients the most celebrated burning mirrors were those of Archimedes and Proclus; by the former was burnt the fleet of Marcellus, as above mentioned; and by the latter, the navy of Vitellius, besieging Byzantium, according to Zonaras was burnt to ashes.
Among the moderns, the most remarkable burningglasses, are those of Magine of 20 inches diameter: of Sepatala of Milan, near 42 inches diameter, and which burnt at the distance of 15 feet; of Settala of Villette, of Tschirnhausen, of Buffon, of Trudaine, and of Parker.
Villette, a French artist at Lyons, made a large mirror, which was bought by Tavernier, and presented to the king of Prussia; a second, bought by the king of Denmark; a third, presented to the Royal Academy by the king of France; and a 4th came to England, and was publicly shewn. This mirror is 47 inches wide, being a segment of a sphere of 76 inches radius; so that its focus is about 38 inches from the vertex; and its substance is a composition of tin, copper, and tin-glass. Some of its effects were as follow: |
| sec. | |
| A silver sixpence melted in | 7 1/2 |
| A George the 1st's halfpenny in | 16 |
| and runs with a hole in | 34 |
| Tin melts in | 3 |
| Cast iron in | 16 |
| Slate in | 3 |
| A fossil shell calcines in | 7 |
| Piece of Pompey's pillar vitrifies, the black part in | 50 |
| the white part in | 54 |
| Copper ore in | 8 |
| Bone calcines in 4, and vitrifies in | 33 |
Tschirnhausen's reflecting mirrors produced equally surprizing effects; as they may be seen described in the Acta Erudit. for 1687, pa. 52. And other persons have made very good ones of wood, straw, paper, ice, and other substances capable of taking a proper form and polish.
Every lens, whether convex, plano-convex, or convexo-convex, collects the sun's rays, dispersed over its convexity, into a point by refraction; and it is therefore a burning-glass. The most considerable of this kind is that made by Tschirnhausen, and described in the same Acta Erudit. The diameters of his lenses are from 3 to 4 feet, having the focus at the distance of 12 feet, and its diameter an inch and a half. To make the focus more vivid, the rays are collected a second time, by a second lens parallel to the first, and placed at such a distance that the diameter of the cone of rays formed by the first lens is equal to the diameter of the second; so that it receives them all; and the focus is reduced from an inch and a half to half the quantity, and consequently its force is quadrupled. This glass vitrifies tiles, slates, pumice-stones &c. in a moment. It melts sulphur, pitch, and all rosins, under water; the ashes of vegetables, woods and other matters, are transmuted into glass; and every thing applied to its focus is either melted, changed into a calx, or into fumes. The author observes that it succeeds best when the matter applied is laid on a hard charcoal well burnt. —But though the force of the solar rays be thus found so surprizing, yet the rays of the full moon, collected by the same burning-glass, do not shew the least increase of heat.
Sir Isaac Newton presented a burning-glass to the Royal Society, consisting of 7 concave glasses, so placed that all their foci join in one physical point. Each glass is about 11 1/2 inches diameter: six of them are placed contiguous to, and round the seventh, forming a kind of spherical segment, whose subtense is about 34 1/2 inches: the common focus is about 22 1/2 inches distant, and about an inch in diameter. This glass vitrifies brick or tile in 1 second, and melts gold in 30 seconds.
M. Buffon also made a variety of very powerfulburningglasses, both as mirrors and as lenses; but at length concluded with one which is probably of the same nature with that of Archimedes, and consisted of 400 mirrors reflecting their rays all to one point, and with which he could melt lead and tin at the distance of 140 feet; and with others he consumed substances at the distance of 210 feet. See Philos. Trans. vol. 44; or Buffon's Histoire Naturelle, Suppl. vol. 1; or Montucla's Histoire des Math. vol. i. pa. 246.
It would seem there is no substance capable of resisting the efficacy of modern burning-glasses; though water &c. are not affected by them at all. Thus, Messrs Macquer and Baumé have succeeded in melting small portions of platina by means of a concave glass, 22 inches diameter, and 28 inches focus; though this metal is not fusible by the strongest fires that can be excited in furnaces, or sustainèd by any chemical apparatus. Yet it was long since observed, by the Academicians del Cimento, that spirit of. wine could not be sired by any burning-glass which they used; and notwithstanding the great improvements these instruments have since received, M. Nollet has not been able, by the most powerful burning mirrors, to set fire to any inflammable liquors whatever.
However, a large burning lens, for fusing and vitrifying such substances as resist the fires of furnaces, and especially for the application of heat in vacuo, and in certain other circumstances in which heat cannot be applied by other means, has long been a desideratum with persons concerned in philosophical experiments: and this it appears is now in a great measure accomplished by Mr. Parker, an ingenious glass manufacturer in Fleet-street, London. His lens is made of flint glass, and is 3 feet in diameter, but when fixed in its frame exposes a surface of 32 inches in the clear; the length of the focus is 6 feet 8 inches, and its diameter one inch. The rays from this large lens are received and transmitted through a smaller, of 13 inches diameter in the clear within the frame, its focal length 29 inches, and diameter of its focus 3-8ths of an inch: so that this second lens increases the power of the former more than 7 times, or as the square of 8 to the square of 3.
From a great number of experiments made with this lens, the following are selected to serve as specimens of its powers:
| Substances fused; with their weight, and time of fusion. | Time in sec. | Weight in grs. |
| Scoria of wrought iron | 2 | 12 |
| Common slate | 2 | 10 |
| Silver, pure | 3 | 20 |
| Platina, pure | 3 | 10 |
| Nickell | 3 | 16 |
| Cast Iron, a cube | 3 | 10 |
| Kearsh | 3 | 10 |
| Gold, pure | 4 | 20 |
| Crystal pebble | 6 | 7 |
| Cauk, or terra ponderosa | 7 | 10 |
| Lava | 7 | 10 |
| Asbestos | 10 | 10 |
| Bar Iron, a cube | 12 | 10 |
| Steel, a cube | 12 | 10 |
| Garnet | 17 | 10 |
| Copper, pure | 20 | 33 |
| Onyx | 20 | 10 |
| SUBSTANCES FUSED, &c. | Time in sec. | Wgt. in grs. |
| Zeolites | 23 | 10 |
| Pumice Stone | 24 | 10 |
| Oriental Emerald | 25 | 2 |
| Jasper | 25 | 10 |
| White Agate | 30 | 10 |
| Flint, oriental | 30 | 10 |
| Topaz, or chrysolite | 45 | 3 |
| Common Limestone | 55 | 10 |
| White <*>homboidal Spar | 60 | 10 |
| Volcanic Clay | 60 | 10 |
| Cornish Moorstone | 60 | 10 |
| Rough Cornelian | 75 | 10 |
| Rotten Stone | 80 | 10 |
Burning Zone, or Torrid Zone, the space within 23 1/2 degrees of the equator, both north and south.
