DUCTILITY

, a property of certain bodies, by which they are capable of being beaten, pressed, drawn, or stretched forth, without breaking; or by which they are capable of great alterations in their figure and dimensions, and of gaining in one way as they lose in another.

Such are metals, which, being urged by the hammer, gain in length and breadth what they lose in thickness; or, being drawn into wire through holes in iron, grow | longer as they become more slender. Such also are gums, glues, resins, and some other bodies; which, though not malleable, may yet be denominated ductile, in as much as, when softened by water, fire, or some other menstruum, they may be drawn into threads.

Some bodies are ductile both when they are hot and cold, and in all circumstances: such are metals, and especially gold and silver; other bodies are ductile only when they have a certain degree of heat; such as glass, and wax, and such like substances: others again are ductile only when cold, and brittle when hot; as some kinds of iron, viz, those called by workmen redshort, as also brass, and some metallic alloys.

The cause of ductility is very obscure; as depending much on hardness, a quality whose nature we know very little about. It is true, it is usual to account for hardness from the force of attraction between the particles of the hard body; and for ductility, from the particles of the ductile body being, as it were jointed, and entangled with each other. But without dwelling on any fanciful hypotheses about ductility, we may amuse ourselves with some truly amazing circumstances and phenomena of it, in the instances of gold, glass, and spider's-webs. Observing however that the ductility of metals decreases in the following order: gold, silver, copper, iron, tin, lead.

Ductility of Gold. One of the properties of gold is, to be the most ductile of all bodies; of which the gold beaters and gold wire-drawers, furnish us with abundant proof.

Fa. Mersenne, M. Rohault, Dr. Halley, &c, have made computations of it: but they trusted to the reports of the workmen. M. Reaumur, in the Memoires de l'Academie Royale des Sciences, an. 1713, took a surer way; he made the experiment himself. A single grain of gold, he found, even in the common gold leaf, used in most of our gildings, is extended into 36 and a half square inches; and an ounce of gold, which, in form of a cube, is not half an inch either high, broad, or long, is beat under the hammer into a surface of 146 and a half square feet; an extent almost double to what could be done in former times. In Fa. Mersenne's time, it was deemed prodigious, that an ounce of gold should form 1600 leaves; which, together, only made a surface of 105 square feet.

But the distension of gold under the hammer (how considerable soever) is nothing to that which it undergoes in the drawing-iron. There are gold leaves, in some parts scarce the 1/360000 part of an inch thick; but 1/360000 part of an inch is a considerable thickness, in comparison of that of the gold spun on silk in our gold thread.

To conceive this prodigious ductility, it is necessary to have some idea of the manner in which the wire drawers proceed. The wire, and thread we commonly call gold thread, &c, (which is only silver wire gilt, or covered over with gold), is drawn from a large ingot of silver, usually about thirty pounds weight. This they round into a cylinder, or roll, about an inch and a half in diameter, and twenty-two inches long, and cover it with the leaves prepared by the gold beater, laying one over another, till the cover is a good deal thicker than that in our ordinary gilding; and yet, even then, it is very thin; as will be easily conceived from the quantity of gold that goes to gild the thirty pounds of silver: two ounces ordinarily do the business; and, frequently, little more than one.

In effect, the full thickness of the gold on the ingot rarely exceeds 1/400 or 1/<*>00 part; and, sometimes not 1/1000 part of an inch.

But this thin coat of gold must be yet vastly thinner: the ingot is successively drawn through the holes of several irons, each smaller than the other, till it be as sine as, or siner than a hair. Every new hole lessens its diameter; but it gains in length what it loses in thickness; and, of consequence, increases in surface: yet the gold still covers it; it follows the silver in all its extension, and never leaves the minutest part bare, not even to the microscope. Yet, how inconceivably must it be attenuated while the ingot is drawn into a thread, whose diameter is 9000 times less than that of the ingot.

M. Reaumur, by exact weighing, and rigorous calculation, found, that one ounce of the thread was 3232 feet long; and the whole ingot 1163520 feet, Paris measure, or 96 French leagues; equal to 1264400 English feet, or 240 miles English; an extent which far surpasses what Fa. Mersenne, Furetiere, Dr. Halley, &c, ever dreamt of.

Mersenne says, that half an ounce of the thread is 100 toises, or fathoms long; on which footing, an ounce would only be 1200 feet: whereas, M. Reaumur finds it 3232. Dr. Halley makes 6 feet of the wire one grain in weight, and one grain of the gold 98 yards; and, consequently, the ten thousandth part of a grain, above one third of an inch. The diameter of the wire he found one-186th part of an inch; and the thickness of the gold one-154500th part of an inch. But this, too, comes short of M. Reaumur; for, on this principle, the ounce of wire would only be 2680 feet.

But the ingot is not yet extended to its full length. The greatest part of our gold thread is spun, or wound on silk; and, before it is spun, they flat it, by passing it between two rolls, or wheels of exceedingly well polished steel; which wheels, in flatting it, lengthen it by above one seventh. So that our 240 miles are now got to 274.

The breadth, now, of these laminæ, or plates, M. Reaumur finds, is only one-8th of a line, or one-96th of an inch; and their thickness one-3072d. The ounce of gold, then, is here extended to a surface of 1190 square feet; whereas, the utmost the gold beaters can do, we have observed, is to extend it to 146 square feet. But the gold, thus exceedingly extended, how thin must it be! From M. Reaumur's calculus, it is found to be one-175000th of a line, or one-2100000th of an inch; which is scarce one-13th of the thickness of Dr. Halley's gold.

But he adds, that this supposes the thickness of the gold every where equal, which is no ways probable; for in beating the gold leaves, whatever care they can bestow, it is impossible to extend them equally. This we easily find, by the greater opacity of some parts than others; for, where the leaf is thickest, it will gild the wire the thickest. |

M. Reaumur, computing what the thickness of the gold must be where thinnest, finds it only one-3150000th part of an inch. But what is the one-3150000th part of an inch? Yet this is not the utmost ductility of gold. for, instead of two ounces of gold to the ingot, which we have here computed upon, a single one might have been used; and, then, the thickness of the gold, in the thinnest places, would only be the 6300000th part of an inch.

And yet, as thin as the plates are, they might be made twice as thin, yet still be gilt; by only pressing them more between the flatter's wheels, they are extended to double the breadth and proportionably in length. So that their thickness, at last, will be reduced to one thirteen or fourteen millionth part of an inch.

Yet, with this amazing thinness of the gold, it is still a perfect cover for the silver: the best eye, or even the best microscope, cannot discover the least chasm, or discontinuity. There is not an aperture to admit alcohol of wine, the subtilest fluid in nature, or even light itself, unless it be owing to cracks occasioned by repeated strokes of the hammer. Add, that if a piece of this gold thread, or gold plate be laid to dissolve in aquafortis, the silver will be all excavated, or eat out, and the gold left entire, in little tubules.

It should be observed, that gold, when it has been struck for some time by a hammer, or violently compressed, as by gold wire drawers, becomes more hard, elastic and stiff, and less ductile, so that it is apt to be cracked or torn: the same thing happens to the other metals by percussion and compression. But ductility and tractability may be restored to metals in that state, by annealing them, or making them red hot. Gold seems to be more affected by percussion and annealing, than any other metals.

As to the Ductility of soft bodies, it is not yet carried to that pitch. The reader, however, must not be surprised that, among the ductile bodies of this class, we give the first place to the most brittle of all other, glass.

Ductility of Glass. We all know, that, when well penetrated with the heat of the fire, the workmen can figure and manage glass like sost wax; but what is most remarkable, it may be drawn, or spun out into threads exceedingly fine and long.

Our ordinary spinners do not form their threads of silk, flax, or the like, with half the ease, and expedition, as the glass spinners do threads of this brittle matter. We have some of them used in plumes for children's heads, and divers other works, much finer than any hair, and which bend and wave like it with every wind.

Nothing is more simple and easy than the method of making them: there are two workmen employed; the first holds one end of a piece of glass over the flame of a lamp; and, when the heat has softened it, a second operator applies a glass hook to the metal thus in fusion; and, withdrawing the hook again, it brings with it a thread of glass, which still adheres to the mass: then, fitting his hook on the circumference of a wheel about two feet and a half in diameter, he turns the wheel as fast as he pleases; which, drawing out the thread winds it on its rim; till, after a certain number of revolutions, it is covered with a skain of glass thread.

The mass in fusion over the lamp diminishes insensibly: being wound out, as it were, like a pelotoon, or clue of silk, upon the wheel; and the parts, as they recede from the flame, cooling, become more coherent to those next to them; and this by degrees: the parts nearest the fire are always the least coherent, and, of consequence, must give way to the effort the rest make to draw them towards the wheel.

The circumference of these threads is usually a flat oval, being three or four times as broad as thick: some of them seem scarce bigger than the thread of a silk worm, and are surprisingly flexible. If the two ends of such threads be knotted together, they may be drawn and bent, till the aperture, or space in the middle of the knot, doth not exceed one-4th of a line, or one-48th of an inch diameter.

Hence M. Reaumur advances, that the flexibility of glass increases in proportion to the fineness of the threads; and that, probably, had we but the art of drawing threads as fine as a spider's web, we might weave stuffs and cloths of them for wear. Accordingingly, he made some experiments this way: and found he could make threads fine enough, viz, as fine, in his judgment, as spider's thread, but he could never make them long enough to do any thing with them.

Ductility of Spider's-webs. See Web.