BURNING-Glass, a convex glass commonly spherical, which being exposed directly to the sun, collects all the rays falling thereon into a very small space called the focus; where wood, or any other combustible matter being put, will be set on fire. The term burning-glass is also used to denote those concave mirrors, whether composed of glass quick-silvered, or of metalline matters, which burn by reflection, condensing the sun's rays into a focus similar to the former.
The use of burning-glasses appears to have been very ancient. Diodorus Siculus, Lucian, Dion, Zonaras, Galen, Anthemius, Eustathius, Tzetzes, and others, attest, 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 bow-shot." A very particular testimony we have also from Anthemius of Lydia, who takes pains to prove the possibility of setting fire to a fleet, or any other combustible body, at such a distance.
That the ancients were also acquainted with the use of catoptric or refracting burning-glasses, appears from a passage in Aristophanes's comedy of the clouds, which clearly treats of their effects. The author introduces Socrates as examining Strephiades about the 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, (says he), 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 burned by refraction. The former calls them balls or globes of glass or crystal, which, exposed to the sun, transmit a heat sufficient to set fire to cloth, or corrode away the dead flesh of those patients who stand in need of caustics; and the latter, after Clemens Alexandrinus, takes notice that fire may be kindled by interposing glasses filled with water between the sun and the object, so as to transmit the rays to it.
It seems difficult to conceive how they should know such glasses would burn without knowing they would magnify, which it is granted they did not, till towards the close of the 13th century, when spectacles were first thought on. For as to those passages in Plautus which seem to intimate the knowledge of spectacles, M. de la Hire observes, they do not prove any such thing; and he solves this, by observing, that their burning-glasses being spheres, either solid or full of water, their foci would be one fourth of their diameter distant from them. If then their diameter were supposed half a foot, which is the most we can allow, an object must be at an inch and a half distance to perceive it magnified; those at greater distances do not appear greater, but only more confused through the glass than out of it. It is no wonder, therefore, the magnifying property of convex glasses was unknown, and the burning one known. It is more wonderful there should be 300 years between the invention of spectacles and telescopes.
Among the ancients, the burning mirrors of Archimedes and Proclus are famous: the former we have already taken notice of; by the other, the navy of Vitellius besieging Byzantium, according to Zonaras, was burnt to ashes.
Among the moderns, the most remarkable burning mirrors are those of Settala, of Villette, of Tschirnhausen, and M. Buffon. Settala, canon of Padua, made a parabolic mirror, which, according to Schottus, burnt pieces of wood at the distance of 15 or 16 paces.
M. Tschirnhausen's mirror at least equals the former, both in bigness and effect. The following things are noted of it in the Acta Eruditorum. 1. Green wood takes fire instantaneously, so as a strong wind cannot extinguish it. 2. Water boils immediately; and eggs in it are presently edible. 3. A mixture of tin and lead, three inches thick, drops presently; and iron and steel-plate becomes red-hot presently, and a little after burns into holes. 4. Things not capable of melting, as stones, bricks, &c. become soon red-hot, like iron. 5. Slate becomes first white, then a black glass. 6. Tiles are converted into a yellow glass, and shells into a blackish yellow one. 7. A pumice stone, emitted from a volcano, melts in white glass; and, 8. A piece of crucible also vitrifies in eight minutes. 9. Bones are soon turned into an opaque glass, and earth into a black one. The breadth of this mirror is near three Leipsic ells, its focus two ells from it; it is made of copper, and its substance is not above double the thickness of the back of a knife.
Villette, a French artist of Lyons, made a large mirror, which was bought by Tavernier, and presented to the king of Persia; a second, bought by the king of Denmark; a third presented by the French king to the royal academy; a fourth has been in England, where it was publicly exposed. The effect hereof, as found by Dr Harris and Dr Desaguliers, are, that a silver sixpence is melted in 7" and , a king George'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"; a piece of Pompey's pillar at Alexandria vitrifies in the black part in 50", in the white in 54"; copper ore in 8"; bone calcines in 4", vitrifies in 33. An emerald melts into a substance like a turquoise stone; a diamond weighing four grains loses of its weight: the asbestos vitrifies; as all other bodies will do, if kept long enough in the focus; but when once vitrified, the mirror can go no farther with them. This mirror is 47 inches wide, and is ground to a sphere of 76 inches radius; so that its focus is about 38 inches from the vertex. Its substance is a composition of tin, copper, and tin-glass.
Every lens, whether convex, plano-convex, or convexo-convex, collects the sun's rays, dispersed over its convexity, into a point by refraction; and is therefore a burning-glass. The most considerable of this kind is that made by M. de Tschirnhausen: the diameter of his lens's are three and four feet, the focus at the distance of 12 feet, and its diameter an inch and a half. To make the focus the more vivid, it is collected a second time by a second lens parallel to the first, and placed in that point where 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, from an inch and a half, is contracted into the space of eight lines, and its force increased proportionably.
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, turned into a calx, or into smoke. Tschirnhausen observes, that it succeeds best when the matter applied is laid on a hard charcoal well burnt.
Sir Isaac Newton presented a burning-glass to the royal society, consisting of seven concave glasses, so placed, as that all their foci join in one physical point. Each glass is about 11 inches and a half in diameter: six of them are placed round the seventh, to which they are all contiguous; and they form a kind of segment of a sphere, whose subtense is about 34 inches and a half, and the central glass lies about an inch farther in than the rest. The common focus is about 22 inches and a half distant, and about an inch in diameter. This glass vitrifies brick or tile in 1", and melts gold in 30".
It would appear, however, that glass quicksilvered is a more proper material for burning-glasses than metals; for the effects of that speculum wherewith Mr Macquer melted the platina, seem to have been superior to those above mentioned, though the mirror itself was much smaller. The diameter of this glass was only 22 inches, and its focal distance 28. Black flint, when exposed to the focus, being powdered to prevent its crackling and flying about, and secured in a large piece of charcoal, bubbled up, and ran into transparent glass in less than half a minute. Hessian crucibles, and glass-house pots, vitrified completely in three or four seconds. Forged iron smoked, boiled, and changed into a vitreous scoria as soon as it was exposed to the focus. The gypsum of Montmartre, when the flat sides of the plates or leaves of which it is composed were presented to the glass, did not show the least disposition to melt; but, on presenting a transverse section of it, or the edges of the plates, it melted in an instant, with a hissing noise, into a brownish yellow matter. Calcareous stones did not completely melt: but there was detached from them, a circle more compact than the rest of the mass, and of the size of the focus; the separation of which seemed to be occasioned by the shrinking of the matter which had begun to enter into fusion. The white calx of antimony commonly called diaphoretic antimony, melted better than the calcareous stones, and changed into an opaque pretty glossy substance like white enamel. It was observed, that the whiteness of the calcareous stones and the antimonial calx was of great disadvantage to their fusion, by reason of their reflecting great part of the sun's rays; so that the subject could not undergo the full activity of the heat thrown upon it by the burning-glass. The case was the same with metallic bodies, which melted so much the more difficultly as they were more white and polished; and this difference was so remarkable, that in the focus of this mirror, so fusible a metal as silver, when its surface was polished, did not melt at all.
Plate LXVI. fig. 3. represents Mr Buffon's burning mirror, which he with great reason supposes to be of
of the same nature with that of Archimedes. It consists of a number of small mirrors of glass quicksilvered, all of which are held together by an iron frame. Each of these small mirrors is also moveable by a contrivance on the back part of the frame, that so their reflections may all coincide in one point. By this means they are capable of being accommodated to various heights of the sun, and to different distances. The adjusting them in this manner takes up a considerable time; but after they are so adjusted, the focus will continue unaltered for an hour or more.
Fig. 4. represents a contrivance of M. Buffon's for diminishing the thickness of very large refracting lenses. He observes, that in large lenses of this kind, and which are most convenient for many purposes, the thickness of the glass in the middle is so great, as very much to diminish their force. For this reason he proposes to form a burning-glass of concentric circular pieces of glass, each resting upon the other, as represented in the figure. His method is to divide the convex arch of the lens into three equal parts. Thus, suppose the diameter to be 26 inches, and the thickness in the middle to be three inches: By dividing the lens into three concentric circles, and laying the one over the other, the thickness of the middle piece needs be only one inch; at the same time that the lens will have the same convexity, and almost the same focal distance, as in the other case; while the effects of it must be much greater, on account of the greater thinness of the glass.