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. Or the ware is annealed in kilns, which are closed and scaled at a temperature a little less than that at which glass becomes plastic, and heated air being thus confined the kilns are many hours, often many days, in cooling. The more carefully and slowly glass is annealed, the less liable it is to "fly."
By cooling glass more rapidly than could occur in ordinary atmospheric temperatures, that is, by a process the reverse of annealing, Prince Rupert's drops are made.
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.
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.
Why does this happen? and why must glass-ware be annealed in older to be serviceable? There is evidently such similarity of phenomena occurring in the drops and in unannealed glass that a satisfactory theory for the one ought to lead to the explanation of the other.
In an article on "Tempered Glass" contributed by Perry F. Nursey, C. E., to the Popular Science Review, and published in the September number of The Popular Science Monthly, the following theory of the Prince Rupert's drops is given: "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 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 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,