Heat more or less expands all metals, which again contract in the cooling; and, with the expansion of cast-iron under peculiar circumstances, no danger of fracture is caused even by rapid cooling. Flint Glass, on the contrary, owing to its peculiar crystalline structure, especially when of unequal substance, is subject to fracture by sudden exposure to friction or the ordinary temperature of the atmosphere, and particularly so in frosty weather; it cannot, therefore, be too carefully cooled to allow a gradual contraction, and the crystalline particles to settle equably into their ultimate position. The process of annealing becomes to the workman and manufacturer of the greatest importance, and needs the most careful and scientific arrangements to prevent cold currents of air from coming in contact with the glass while gradually parting with its caloric. Metals being more or less crystallizable, under some circumstances, are subject to fracture by a sudden blow or friction; and Glass exceeds every other crystallizable sonorous body in this peculiarity. No contrivance can be practically too costly, which will effectually anneal so brittle a material as Glass; and it is false economy to save fuel in the process, when a greater body of caloric, at a greater cost of fuel, would give additional security. Fracture is supposed to arise from derangement of the atoms by unequal contraction or tension; thus, the interior of a hollow vessel taking more time to cool than the exterior, tends, by unequal contraction, to derangement and fracture, by crushing the particles; and this can only be remedied by the external heat of the annealing fire allowing the two surfaces to cool simultaneously. The more