UNBREAKABLE OB TOUGHENED GLASS. 
231 
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 manu- 
facturers work to these dimensions. It is not at all improbable, 
however, that ere long a means will be devised for cutting 
toughened 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 subsequently. 
Superficial observers have affected to detect in the toughen- 
ing process a similar condition of matter to that which obtains 
in Prince Rupert’s drops. The error of such a conclusion, how- 
ever, 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 pear-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 — as 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 prevents the interior atoms from expanding and arrang- 
ing 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 together 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 dis- 
turbed 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 sur- 
face, 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 
