Page:Popular Science Monthly Volume 7.djvu/577

Rh It has, however, exchanged its distinguishing characteristic of extreme brittleness for a degree of toughness and elasticity which enables it to bear the impact of heavy falling weights and smart blows without the least injury. A great number of experiments have been made, the results of which fully corroborate this fact. From these it will suffice to select a few by way of illustration. Watch-glasses, which perfectly retain their transparency, have resisted every attempt to break them by crushing between the fingers, or by throwing them about indiscriminately on the bare floor. Glass plates, dishes, colored lantern-glasses, and the like, have been similarly thrown about by the handful, stood upon, and otherwise maltreated, but without the slightest injury accruing to them, except, perhaps, when a solitary specimen which had been imperfectly tempered got in with the rest. Experiments have also been carried out to ascertain the comparative strength of toughened and untoughened glass when submitted to bending stress. Here a number of pieces of glass, each measuring six inches in length, by five inches in breadth, and having a thickness of about one-fourth of an inch, were tried. Each sample in its turn was supported at the ends, and a stirrup-piece was hung upon the centre of the glass, a weight-rod hanging vertically from the under-side of the stirrup. With this arrangement applied to a piece of ordinary glass, the weight-rod was gradually loaded until a weight of 279 pounds was reached, when the glass broke. A piece of toughened glass of similar dimensions, similarly treated, did not give way until a strain of 1,348 pounds had been reached, and before it yielded a considerable deflection was produced in it, showing its elasticity. Had its strength been due to rigidity or inflexibility alone, it would not have assumed a curve before yielding to the pressure brought upon it.

Satisfactory as the above results may appear at the first glance, they will be seen upon reflection most inadequately to represent the relative strength of toughened and untoughened glass. It will be observed that the test applied was that of long-sustained and gradually-increasing pressure, which could rarely occur to glass articles in everyday use. Glass is subject to sudden, sharp blows, either from articles falling down on other substances or from extraneous bodies falling upon or being brought in contact with them. Hence it is clear that to obtain a true estimate of the new process, glass must be subjected to tests which fairly represent the conditions of the accidents to which it is ordinarily exposed. This estimate has been arrived at repeatedly by placing pieces of plate-glass in a frame and allowing weights to fall on them from given heights. One experiment from a number—and which was made publicly—will illustrate this test: A piece of ordinary glass, six inches long by five inches wide, and one-fourth of an inch thick, was placed in a small frame which supported the glass around its edges, and kept its under-side about half an inch from the floor. A four-ounce weight was dropped on it from a height of one