William Leighton Jr.

“ In the process of annealing, the heat of the oven keeps the surfaces of the glass articles from absolutely becoming rigid, so that they yield sufficiently to the strain of the contracting interior portions; and if the whole substance of each article cools exactly together, the exterior and interior all the time at the same temperature, there is no strain and the ware is perfectly annealed. ”

“ Rupert drops are clear, bright, and hard, and may be struck with much violence upon the larger end without fracture, but if the thin, though tough and very elastic thread of the other extremity be broken off, the whole drop will explode into numberless fragments, much finer than the sand of which the glass was originally composed. ”

“ The particles of the glass have a cohesive polarity which dictates a certain regularity in their arrangement, but which requires some time for its development. When the vessels are suddenly cooled, the surface-molecules only can have had time to dispose themselves duly, while those within are kept by this properly-formed skin in a highly-constrained situation; and it is only so long as the surface-film keeps sound that this constraint can be resisted. In the Rupert's drops it is plainly visible that the interior substance is cracked in every direction, and ready to fly to pieces. ”

“ As it is practically impossible to accomplish a perfect equality of temperature, a perfect equilibrium of the molecules cannot be obtained; but so near an approach to it is accomplished in a well-constructed annealing oven, that the cohesion of the glass is easily able to withstand the trifling strain. ”

“ 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 arranging themselves in such a way as to give the glass a fibrous nature, as they would if the glass were allowed to cool very gradually. ”

“ An oven is so constructed that the heat of the glass is maintained by a current of heated air in which articles to be annealed are placed, and mechanism so contrived as very slowly to draw away the ware into currents of lower temperature. ”

“ If the glass is so shaped as to be of unequal thickness in its different parts, it can seldom be cooled without fracture, and, if unbroken when cool, is liable to fracture with any subsequent change of temperature or by a sudden jar. ”

“ The ordinary way to make these scientific curiosities is to drop a small quantity, usually less than half an ounce, of perfectly fluid glass into water. In falling, the glass will assume the form of a tear, with an elongated end extending into a thread. ”

“ 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. ”

“ He finds the substance of glass always presenting the same vitreous, amorphous appearance, except in cases of devitrification, and, in the absence of any proof of such condition, cannot bring himself to believe in glass of a fibrous structure. ”

“ A piece of unannealed barometer-tube, forty inches long, measured when just drawn, will become about one-fourth of an inch shorter if annealed; whereas, if quickly cooled without annealing, it will only contract about one-eighth of an inch. ”