DUCTILITY, in physics, a property of certain bodies, whereby they are capa ble of being expanded, or stretched forth, by means of a hammer, press, &c.
The great ductility of some bodies, especially gold, is very surprising : the gold-beaters and wire-drawers furnish us with abundant proofs of this proper ty ; they, every day, reduce gold into lamellae inconceivably thin, yet without the least aperture, or pore, discoverable, even by the microscope : a single grain of gold may be stretched under the ham mer into a leaf that will cover many square inches, and yet the leaf remain so compact as not to transmit the rays of light, nor even admit spirit of wine to transude. Dr. Halley took the following method to compute the ductility of gold: he learned from the wire drawers that an ounce of gold is sufficient to gild, that is, to cover or coat a silver cylinder of forty-eight ounces weight, which cy linder may be drawn out into a wire so very fine, that two yards thereof shall only weigh one grain ; and consequently ninety-eight yards of the same wire only forty-nine grains : so that a single grain of gold here gilds ninety-eight yards ; Ind, of course, the ten-thousandth part of a grain is here above one-third of an inch long. And since the third part of an inch is yet capable of being divided into ten lesser parts visible to the naked eye, it it evident that the hundred thousandth part of a grain of gold may be seen without the assistance of a micro scope. Proceeding in his calculus, he found, at length, that a cube of gold whose side is the hundredth part of an inch, contains 2, 433,000,000 visible parts; and yet, though the gold wherewith such wire is coated be stretched to such • a degree, so intimately does its parts co here, that there is not any appearance of -the colour of the silver underneath: • Mr. Boyle, examining some leaf-gold,
c found that a grain and a quarter's weight I took up an area of fifty square inches ; supposing, therefore, the leaf divided by parallel lines one-hundredth of an inch . apart, a grain of gold will be divided into five hundred thousand minute squares, all discernible by a good eye : and the same author shews, that an ounce of gold drawn out into wire, would reach 155 miles and a half.
But M. Reaumur has carried the duc tility of gold to a still greater length : a gold wire, every body knows, is only a silver one gilt. This cylinder of silver, co vered with leaf-gold, they draw through the hole of an iron, and the gilding still keeps pace with the wire, stretch it to what length they can. Now M.. Reau mur shews that, in the common way of drawing gold-wire, a cylinder of silver twenty-two inches long and fifteen lines in diameter is stretched to 1,163,520 feet, or is 634,692 lines longer than be fore, which amounts to about ninety seven leagues. To wind this thread on silk fiir use they first flatten it, in doing which, it stretches at least one-seventh farther: so that the twenty-two inchesare now 111 leagues: but in the flattening, instead of one-seventh, they could stretch it one-fourth, which would bring it to 120 leagues. This appears a prodigious ex tension, and yet it is nothing to what this gentleman has proved gold to be ca pable of.