Page:Popular Science Monthly Volume 7.djvu/578

560 foot, and the glass was broken. A piece of toughened glass of corresponding dimensions was then placed in the frame and the same weight dropped upon it several times from a height of ten feet, but without fracturing the glass. An eight-ounce weight was then substituted, and repeatedly dropped upon the glass from the same height as before, and with the same result, no impression whatever being made upon it. The eight-ounce weight was then thrown violently upon it several times, but without damaging it. Its destruction, however, was finally accomplished by means of a hammer. Perhaps the most crucial test to which toughened glass could be put would be to let it fall on iron. This has been done, and in public too. A thin glass plate was dropped from a height of four feet on to an iron grating, from which it rebounded about one foot, sustaining no injury whatever.

As singular as any other feature presented by toughened glass are the results of its destruction. Ordinary glass, upon being fractured, gives long, needle-shaped, and angular fragments. Not so toughened glass, which is instantaneously resolved into mere atoms. The whole mass is at once disintegrated into innumerable pieces, ranging in size from a pin's-point to an eighth of an inch in diameter. It sometimes occurs that pieces measuring half an inch or an inch across may remain whole, but these pieces are traversed in all directions by a net-work of fine lines of fracture, and with the fingers are easily reduced to fragments. Microscopical examination shows the fragments of toughened glass—large and small—to follow the same law as regards the form and character of the crystals, and on some of the larger crystals being broken up they have been found to separate into smaller ones of the same character. The edges of these fragments, too, are more or less smooth instead of being jagged and serrated as are those of fragments of ordinary glass. Hence a diminished tendency in the former to cause incised flesh-wounds when handled.

When glass has been imperfectly treated, as has sometimes happened in M. de la Bastie's experiments, it will not stand the same amount of rough usage as will perfectly-toughened specimens. The fact of the toughening process having been incomplete is made manifest upon the destruction of a sample in three different ways chiefly. Independently of its yielding at an early stage either to blows or pressure, it will show upon destruction either needle-fractures approaching in appearance those of ordinary glass, or pieces varying from the size of a sixpence to that of a half-crown will remain unbroken and untraversed by lines of fracture. Again, the mass may be wholly fractured, but on looking at the fragments edgewise a narrow, milky streak will be apparent midway between the upper and under sides of the glass, indicating that the influence of the bath has not extended through the glass. Where the process has been perfectly applied, no such phenomena are exhibited, the crystals being of uniform transparency throughout the whole mass.