_Annealing and Devitrification._--As the word "annealing" has been often used in this article, it will be well to explain what is its action. If a piece of molten glass be dropped into water, it will assume an oblong shape, the lower end of which will be round, while the other will taper off into a fine point. These drops, which have received the name of Prince Rupert's drops, look like pieces of ordinary glass, and if the small end of one of them be broken off, a sort of explosion takes place, and the whole mass flies into a thousand minute pieces, some of which will be found to have been driven to a considerable distance. Here then it appears, that when the skin, which is perfect and entire in the Rupert drop, is broken, the bond which held together the constituent particles is broken also, and so they are acted on by a repellent force, and fly away from one another. If hot water be poured into a thick common tumbler, it very generally cracks it: but if the tumbler be thin and of better manufacture, it will bear almost boiling water without cracking. In the first case it has been badly annealed; and besides this, glass being a bad conductor of heat, from its thickness, the heat imparted by the hot water expands the inner surface, while the outer coating, not being warmed, does not expand, and, retaining its original form, is burst. If, however, a tumbler be thick and properly annealed, there is not so much danger of its breaking, when a portion of it is exposed to a considerable rise of temperature. In the case of the Rupert drops, they are not annealed at all, and so there is no cohesive bond between the particles, such as there would be if they were properly annealed, that is, if, instead of being cooled suddenly from the molten state, they were allowed to cool in a heated chamber very slowly. After glass has been heated, the particles of which it is composed take a long time to rearrange themselves, so that in the manufacture of thermometers, it is necessary, after sealing up the bulb and tube which contain the mercury, to allow them to remain for a long time; otherwise the pressure of the air on the outside of the bulb, not being supported by any air on the inside, causes the particles of glass to become more compact, and thus renders the capacity of the thermometer bulb and tube smaller than it was, when the thermometer was first sealed. It seems that the process of annealing glass gives time for the particles to arrange themselves in such a way, that when the glass is cold, it will not be so liable to fracture from sudden changes of temperature.