ANNEALING is a process by which the extreme brittleness common to glass and some metals when cooled suddenly after melting, is avoided or removed. Such brittleness appears to be occasioned by some disturbance iu the regular arrangement of the constituent parti cles in rapid cooling, and is remedied by cool ing very slowly and regularly. A pretty ex ample of the effects of non-annealing is afforded by glass-tears, or Prince Rupert's Drops. These are made by letting drops of I melted glass fall into cold water, whereby they become suddenly solidified without annealing; 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 falls to pieces with a loud explosion.
Glass-houses are furnished with large an nealing-ovens, in which this object may be attained by the gradual removal of the glass articles from a hotter to a cooler part, or by allowing the heat of the oven to subside slowly. Analogous operations are employed
in the manufactures of cast-iron and other metals. Some malleable metals which crystal lize on cooling are brittle in their crystalline state, but are rendered tough by heating and rolling. Zinc, for example, though incapable of more than very slight extension under the hammer, without cracking, becomes almost as flexible and tough as copper after being rolled at a moderate heat.
Cast iron may he made malleable, without subjecting it to the process of puddling ; this is effected by a sort of annealing. The iron is kept for several hours at a temperature a little below its fusing point, and then allowed to cool slowly.