In the making of glass vessels by the glass blower (see GLASS) , they are of course quickly reduceil in temperature while the fused glass is being molded into the desired shape. The atoms of the glass thus rapidly compelled to assume a permanent position do not seem to be properly and firmly arranged together, and the vessel is very liable to be broken, either by a slight but smart blow, or a sudden increase or decrease in temperature. This brittleness is very ob servable in the lacrimw ritrew, or glass tears, known as Prince Rupert's drops, obtained by al lowing molten glass to fall into water, when the glass forms pear-shaped drops, which are so brit tle that if they be scratched with a file or the end be broken off the whole bursts asunder and falls down into a fine powder of glass. The same brittleness is exhibited in Bologna jars, or vials, which are small and very thick, and yet, if a minute angular fragment of any bard substance be dropped into the jar, the latter flies to pieces.
In the annealing of glass vessels, they are ar ranged in iron trays, and placed in a long oven, where they are gradually raised in temperature to near their fusing point by the trays being drawn along to the hottest part of the oven; and thereafter, the trays, with their contents, are very slowly drawn into a cooler and cooler part, till they become cold. The annealing operation
generally takes twelve hours for small articles, such as wine glasses; but days, and even a week or two, are required completely to anneal large vessels. Many articles of glass, such as tubes for steam-gauges. lamp-glasses, etc., are annealed by being immersed in cold water, which is very gradually raised to its boiling point, and there after cooled.
The theory of annealing is one of considerable technical intricacy, and scientists are not alto gether in agreement as regards many of its fea tures. For a full discussion of the theory and practice of the hardening, tempering, and anneal ing of steel, consult Howe, Metallurgy of Steel (New York, 1892).