56o THE POPULAR SCIENCE MONTHLY. 



foot, and the glass was broken. A piece of toughened glass of corre- 

 sponding 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 wliatever 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 bemg 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-poiut 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 frag- 

 ments. 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 hap- 

 pened 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 difierent ways chiefly. In- 

 dependently of its yielding at an early stage either to blows or press- 

 ure, it wilfshow 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 untrav- 

 ersed 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 phe- 

 nomena are exhibited, the crystals being of uniform transparency 

 throughout the whole mass. 



