traction. One arch, falling, brings down another, and, once started, they go in such rapid succession that the ear detects but one sound, one explosive burst, in which the imp of contraction exults in the ruin he has wrought.
The peculiarities of the Rupert drops are toughness, elasticity, and the property of breaking into small fragments when any fracture, however slight, is made; their strength to resist such fracture is, however, greater than that of annealed or unannealed glass.
When we consider that these same peculiarities are the characteristics of the so-called "toughened glass" of M. de la Bastie, and that the method of treating his "toughened" glass, in the cooling process, is at least analogous to that of the Rupert drops, we are forced to believe in a certain relationship between them.
The Rupert drop falls into a water-bath; M. de la Bastie's glass into an oleaginous bath, the exact composition of which has not been made public.
M. de la Bastie's glass is not malleable, is not unbreakable, but simply tougher, harder to break than the ordinary annealed glass; so also is the Rupert drop.
As the characteristic distinction between annealed glass and the Rupert drop is the excessive strain upon the molecules of the latter—contraction versus cohesion—it is fair to infer that the superior strength, toughness, and elasticity, of the drop are due to such strain. As it is harder to displace the key-stone of a loaded arch than of an unloaded one, the simile may hold good in this case, and the strain of contraction upon the molecules of the glass of a Rupert's drop may help resist any outside force tending to disturb cohesion. If an outside force could be so exerted as to act exactly in the same direction as the power of contraction acts, undoubtedly such force would be aided by contraction to destroy cohesion; but, acting in any other direction, contraction would aid cohesion to resist it. As the molecules of glass are exceedingly small, and as, in the cooling process, they one after another individually become rigid, the lines of their contractive strain become so complicated that it is very unlikely any outside force can be exerted in such direction as to unite its impulse with theirs against cohesion.
As the toughened glass of M. de la Bastie flies into many pieces when fracture is effected, in a manner analogous to the breaking of the Rupert drop, it is probable, at least, considering the process of the oil-bath, that such flying into fragments is due to a strain of contraction exerted by the molecules of its substance. And if such a strain exists, as the flying seems to prove, it is also reasonable to suppose that, exactly as in the case of the Rupert drop, this strain of contraction among the molecules of its mass produces the superior toughness, strength, and elasticity, which are claimed for this newly-invented glass.
