glass, and an interior substance, still soft, plastic, and constantly strained by a tendency to contract, to occupy smaller boundaries; but those boundaries cannot be moved without breaking. It is a struggle of forces. If the thickness of the glass be considerable, the constantly-increasing strain of contraction pulls so hard upon the shell, that the force of cohesion is unable to withstand it, and the shell, yielding with a shock, shivers the whole substance into fragments.
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.
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.
In this view the action of cooling glass is simple and easily understood, surely more simple than to imagine a tendency toward a fibrous constitution of substance, or the imperious "cohesive polarity" of the "Britannica" article.
Test this theory upon the Rupert-drop phenomenon, and its explanation will answer as well.
A small amount of fluid glass, when dropped into water, will immediately, by the action of its heat, envelop itself in a garment of steam, which protects its surface from contact with the water, until that surface is so cooled that such contact fails to crack it. To test this assumption, try the experiment with partially cooled or soft glass, and the result will be that all the drops will break in the water, on account of cracked surfaces. With fluid glass, many drops will be lost, not from the same cause, if the drops be not too large, but from excessive contraction; perhaps, out of a dozen only one or two will be saved.
The steam chills the surface of the glass much more rapidly than the air does, consequently the inner and fluid glass in the Rupert drop is inclosed in larger boundaries than if the drop had cooled in the air. Hence contractive force is very strongly exerted to draw in such excessive boundaries, but the curved form of the drop presents arches of strength to aid the power of cohesion and resist destruction.
One drop bursts in the water, another does the same, but perhaps the third is drawn forth entire, though curled and twisted, as if in the agony of its strain. Two of Nature's forces struggle fiercely for the mastery in this little drop, that gives no indication of the contest as it lies quietly before us. But break off the thread, and down goes the first of the little arches, that are holding up the surface against con-
