admitting of internal expansion, by which the cohesion of the particles composing the external skin is overcome, and the glass is at once reduced to fragments."
In the "American Cyclopædia" (revised edition), under the word "annealing," are found the following enplanations: "When this" (glass) "is melted and shaped into articles which are allowed to cool in the air, the glass becomes too brittle for any use. The exterior cools first and forms a contracted crust, which shelters the interior particles; so that these continue longer in a semi-fluid state, and are prevented from expanding, as glass does in cooling, and uniting with the rest to form an homogeneous mass. The inner parts are thus constantly tending to expand. If, on the contrary, the glass is placed in a hot oven, and this is allowed to cool very slowly, the particles of glass appear to assume a condition of perfect equilibrium of cohesive force without tension, so that the mass becomes tough and elastic." And, again, in the same article: "Dr. Ure explains this phenomenon" (the explosive breaking of Prince Rupert's drops) "by referring it to the tendency of a crack once formed in the glass to extend its ramifications in different directions throughout the whole mass."
In the "Encyclopædia Britannica" (ninth edition), under the word "annealing," is found as follows concerning the phenomena of unannealed glass and Prince Rupert's drops: "The particles of the glass have a cohesive polarity which dictates a certain regularity in their arrangement, but which requires some time for its development. When the vessels are suddenly cooled, the surface-molecules only can have had time to dispose themselves duly, while those within are kept by this properly-formed skin in a highly-constrained situation; and it is only so long as the surface-film keeps sound that this constraint can be resisted. In the Rupert's drops it is plainly visible that the interior substance is cracked in every direction, and ready to fly to pieces."
The practical glass-maker, desirous of thoroughly understanding the true theory of annealing glass, that from such a comprehension he may endeavor to accomplish more perfection in his process, refers to the authorities quoted above, and finds himself bewildered by the theories and explanations here given. He notices that the foundation of the theory of the Rupert drop, and of the process of annealing, in the article of The Popular Science Monthly, and in the "American Cyclopædia," is based upon the assertion that in passing from a fluid to a solid condition glass expands. Although well aware that certain substances, as water,[1] bismuth,[2] gray cast-iron,[3] and antimony,[4] expand while solidifying, yet he is constantly reminded, by phenomena occurring in the glass-house every moment under his eye, that the reverse of this takes place in the substance of glass.
