TEMPERED GLASS. 561 



Such, then, is De la Bastie's toughened glass, which possesses 

 enormous cohesive power, and offers great resistance to the force of 

 impact. There is, however, one peculiarity which, for the present, 

 tells against it in a slight degree it cannot be cut through with a 

 diamond. Scratched its surface can be, but there the action of the 

 diamond ceases. This drawback only applies in the case of window- 

 glass in odd-sized frames ; for the practice of the present day, with 

 builders, is to make window-sashes of certain fixed dimensions, and 

 glass-manufacturers work to these dimensions. It is not at all improb- 

 able, however, that ere long a means will be divised for cutting tough- 

 ened glass to any size or shape ; experiments are, in fact, now being 

 conducted with this view, and so far as they have gone they give 

 promise of success. But if toughened glass cannot be cut by the 

 diamond, it can be readily cut and polished by the wheel, as for lustres 

 and the like, so that wine-glasses and articles of cut glass-ware can 

 be toughened directly they are made, and cut and polished subse- 

 quently. 



Superficial observers have afiected to detect in the toughening 

 process a similar condition of matter to that which obtains in Prince 

 Rupert's drops. The error of such a conclusion, however, becomes 

 evident upon a little consideration. Prince Rupert's drops are made 

 by allowing melted glass to fall into cold water ; the result of which 

 is a small peai--shaped drop, which will stand smart blows upon the 

 thick end without injury; but the moment the thin end, or tail, is 

 broken, the drop flies into fragments. Now, glass and water, and a's 

 far as present knowledge goes no other substances besides, expand 

 while passing from the fluid into the solid condition. The theory of 

 the Rupert drops is, that the glass being cooled suddenly, by being 

 dropped into cold water, expansion is checked by reason of a hard 

 skin being formed on the outer surface. This exterior coating pre- 

 vents the interior atoms from expanding and arranging themselves in 

 such a way as to give the glass a fibrous nature, as they would if the 

 glass was allowed to cool very gradually. An examination of the 

 Rupert's drop shows the inner substance to be fissured and divided 

 into a number of small particles. They exist, in fact, in a state of 

 compression, with but little mutual cohesion, and are only held to- 

 gether by the external skin. So long as the skin remains intact the 

 tendency of the inner particles to expand and fill their proper space is 

 checked and resisted by the superior compressive strain of the skin. 

 Nor is the balance of the opposing forces disturbed by blows on the 

 thick end of the drop, which vibrates as a whole, the vibrations not 

 being transmitted from the exterior to the interior. But by breaking 

 off the tail of the drop a vibratory movement is communicated along 

 the crystalline surface, admitting of internal expansion, by which the 

 cohesion of the particles composing the external skin is overcome, and 

 the glass is at once reduced to fragments. As the skin of toughened 



VOL. TII. 36 



