DUCTILITY, the quality of adaptedness of solid bodies, particularly metals, which ren der them capable of being extended by draw ing into wire; as malleability is for being beaten into leaves. The order of metals in these two respects is as follows: Ductility — platinum, silver, iron, copper, gold, zinc, tin, lead and nickel; malleability — gold, silver, cop per, tin, platinum, lead, zinc, tron and nickel. By the device of coating a platinum wire one one hundredth of an inch in diameter with sil ver, and then drawing the two metals together as fine as possible, and then dissolving away the silver coating with nitric acid, a platinum wire three-one-hundred-thousandths of an inch in diameter has been obtained — so fine that one mile of it would weigh only one-twenty fifth of a grain. This wire has entirely super seded the spider's thread formerly used as the across in telescopes. The less ductile soft metals, such as magnesium, which cannot be drawn, are converted into the form of wire by the process of pressing or squirting the heated metal from a nozzle. One effect of drawing metals into wire is to increase to a great degree their ultimate tensile strength. Thus a wire of Swedish iron one-thirty-sixth of an inch in diameter has a breaking strain of 90,000 pounds per square inch; while a wire of one-two-hundred-and-fiftieth of an inch will not break until the strain is 134,000 per square inch. When glass is subjected to a sufficient degree of heat it can be managed like soft wax and may be drawn out into threads ex ceedingly long and fine. Ordinary spinners do not form their threads of silk, flax or the like, with half the ease and expedition the glass spinners do threads of this ordinarily brittle matter. Some of them are made into plumes
of filaments much finer than hair, which bend and wave, like hair, with every wind. Two workmen are employed in making them. The first holds one end of a piece of glass over a flame and when the heat has softened it the second operator applies a glass hook and draws out a thread of glass, which still adheres to the mass; then fitting his hook on the circumfer ence of a wheel about two and one-half feet in diameter, he turns the wheel as fast as he pleases till it is covered with a skein of glass thread. The parts, as they recede from the flame, by gradually cooling become more cohesive, the parts nearest the fire are least cohesive and consequently must give way to the effort made to draw them toward the wheel. These threads are com monly of a flat, oval shape, being three or four times as broad as thick; some of them seem scarcely bigger than the thread of a silk-worm, and are surprisingly flexible. If the two ends of such threads are knotted together they may be drawn and bent till the aperture or space in the middle of the knot does not exceed one fourth of a line or one-forty-eighth of an inch in diameter. The flexibility of glass increases in proportion to the fineness of the threads; and, probably, had we the art of drawing threads as fine as those of a spider's web, we might weave stuffs and cloths of them. Fa miliar instances of ductility in non-metallic sub stances are the %airing° of boiling sugar syrup in candy-making; and the similar drawing out of °threads° of glue from the carpenter's glue pot.