TEMPERED GLASS. 559 



It has, hovv^ever, exchanged its distinguishing characteristic of extreme 

 brittleness for a degree of toughness and elasticity Avhich 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 re- 

 suits of which fully corroborate this fact. From these it will suffice 

 to select a few by way of illustration. "Watch-glasses, which per- 

 fectly 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 lan- 

 tern-glasses, and the like, have been similarly thrown about by the 

 handful, stood upon, and otherwise maltreated, but without the slio-ht- 

 est injury accruing to them, except, perhaps, when a solitary specimen 

 which had been imperfectly tempered got in with the rest. Experi- 

 ments 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 

 Aveight-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 dimen- 

 sions, similai'ly 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 be- 

 fore 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 ob- 

 served that the test applied was that of long-sustained and gradually- 

 increasing pressure, which could rarely occur to glass articles in every- 

 day 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 ordi- 

 nary 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 Avas dropped on it from a height of one 



