ANNEALING, or NEALING, a process by which glass is rendered less frangible ; and metals, which have become brittle, either in consequence of fusion or long-c.ontinued hammering, are again rendered malle able. When a glass vessel is allowed to cool immedi ately after being made, it will often sustain the shock of a pistol-bullet, or any other blunt body falling into it from a considerable height ; while a small splinter of flint, or an angular fragment of quartz, dropt gently into it, makes it entirely fly to pieces with great vio lence. This extreme fragility is prevented by anneal ing, or placing the vessels in an oven, where they take some days to cool. Similar phenomena are exhibited in a higher degree by glass-tears, or Prince Rupert's drops. They are procured by letting drops of melted glass fall into cold water. Their form resembles that of a pear, rounded at one extremity, and tapering to a very slender tail at the other. If a part of the tail be broken off, the whole drop flies to pieces with a loud explosion; and yet the tail of a drop.may be cut away by a glass cutter's wheel, or the thick end may be struck smartly with a hammer, without its sustaining any injury. When heated to redness, and permitted to cool gradually in the open air, they lose their peculiar properties, and do not differ sensibly from common glass.
The following explanation has been given of the phe nomena, presented by unannealed glass, and of the ef fects of annealing.
" Glass is one of those bodies, which increase in bulk. when passing from a fluid to a solid state. When it is allowed to crystallize regularly, the particles are so ar ranged, that it has a fibrous texture. It is elastic, and susceptible of long-continued vibrations ; but when a mass of melted glass is suddenly exposed to the cold, the surface crystallizes, and forms a solid shell round the interior fluid parts : This prevents them from ex panding when they become solid. They, therefore, have not the opportunity of a regular crystallization, but are compressed together with little mutual cohesion ; on the contrary, they press outward to occupy more space, but are prevented by the external crust. In con sequence of the effort of expansion in the internal parts. the greater number of glass-drops burst in cooling ; and those which remain entire, are not regularly crys tallized. A smart stroke upon them communicates a vibration to the whole mass, which is nearly synchro nous in every part ; and therefore the effort of expan sion has little more effect than if the body were at rest but the small tail and surface only are regularly crys tallized. If the tail be broken, this communicates a vi bration along the crystallized surface, without reaching the internal parts. By this they are allowed some ex
pansion ; and overcoming the cohesion of the thin outer shell, they burst it, and are dispersed in powder.
" In an unannealed glass-vessel, the same thing takes place. Sometimes the vibration may continue for a considerable time before the internal parts over come the resistance. If the vessel be very thin, the re gular crystallization extends through the whole thick ness, or at least the quantity of compressed matter in the middle is so inconsiderable, as to be incapable of bursting the external plate.
" By the process of annealing, the glass is kept for some time in a state approaching to fluidity ; the heat increases the bulk of the crystallized part, and renders it so soft that the internal parts have an opportunity of forming a regular crystallization." The preceding explanation is exposed to insuperable objections. For even granting that the surface of a plate of glass, when cooled, suddenly contracts itself more than if cooled slowly to the same degree, yet, since the only force which keeps together the two surfaces of the plate, is the cohesion of the glass interposed be tween them, it appears evident, that, according as the internal parts of the plate expand, its surfaces will re cede from each other, so as to afford sufficient room for these internal parts, and that consequently there can in this case be no tendency to rupture the surfaces. 2dly, In breaking the tail of a glass drop, we can see no cause why the vibration produced by the fracture should extend along the surface, instead of diffusing itself through the whole body of the drop, nor why a vibra tion, communicated to both the interior and exterior parts, should not have an equal tendency to rupture the glass, as a vibration communicated to the surface only ; and, according to this hypothesis, the moment that a fracture through the whole thickness of an unannealed vessel had taken place, the breaking should cease, for the vibration was then communicated both to the inte rior and exterior parts. The writer of this article con ceives that the properties of unannealed glass depend on a peculiar structure, extending uniformly through its whole substance, and that the bursting of a glass drop, by breaking off the or of an annealed glass by aopping a piece of flint into it, arises from a crack being thus begun, which afterwards extends its ramifications in different directions throughout the glass ;—but as he is at present engaged in a set of ex periments which seem to countenance this idea, he will take the opportunity of another article to communicate their results, and to prosecute this subject farther. See