INDUSTKIAL PROGRESS DURING THE YEAR 1875. x lv 



and a half meters without fracture. Watch-glasses made 

 of it remained intact when thrown across the room. The 

 hardening process is not difficult nor costly, and it prom- 

 ises to become of great practical importance. Mr. Pock- 

 in o-ton states that he has examined by polarized light some 

 specimens of this hardened glass, prepared by himself ac- 

 cording to De la Bastie's method. Having prepared a small 

 cube in this manner, its sides w r ere ground plane and pol- 

 ished, and on examination by the polariscope it became 

 at once evident that the contraction of the exterior of the 

 mass must exert a powerful compressing force upon the in- 

 terior. The outer surface of the glass can be made, accord- 

 ing to his experiments, nearly twice as hard as ordinary 

 glass. On grinding away either surface it is evident that 

 the interior of the mass consists of ordinary glass, being lit- 

 tle, if at all, harder than before the application of De la Bas- 

 tie's process, and subject to fracture in the ordinary way. 

 There appears to be a limit beyond which the opposite sur- 

 faces can not be unequally removed without producing such 

 phenomena as, under the polariscope, show the existence of 

 unsymmetrical tensions ; but there is practically no limit be- 

 yond which both surfaces may not be simultaneously re- 

 moved, as is shown by dissolving away the softer portions 

 by means of hydrofluoric acid. De Luynes and Feil the 

 former well known from his researches on the Prince Ru- 

 pert's drop have also made some experiments on the hard- 

 ened glass of M. De la Bastie. They find that this glass 

 presents many points of analogy with the Prince Rupert's 

 drop, as well in the mode of production as of fracture. 

 Though it is not ordinarily possible to cut a piece of this 

 glass with a saw, a drill, or a file without its flying in pieces, 

 yet in some cases it may be done. A disk, for example, may 

 be drilled through its centre without fracture, though not 

 elsewhere. A square plate of St. Gobain glass thus hard- 

 ened showed in polarized light a black cross, the lines of 

 which were parallel to the sides. It is always possible to 

 saw such a plate along these lines without fracture, though 

 beyond them, either parallel or transverse to them, any at- 

 tempt to cut the plate fractures it. If the two fragments of 

 a plate thus cut be examined in polarized light, the arrange- 

 ment of the dark bands and colored fringes shows the molec- 



