PRINCE RUPERT'S DROPS. 317 



admitting of internal expansion, by which the coliesion of the particles 

 composing the external skin is overcome, and the glass is at once re- 

 duced to fragments." 



In the "American Cyclopaedia" (revised edition), under the word 

 "annealing," are found the following enplanations : "When this" 

 (glass) "is melted and shaped into articles which ai"e 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 con- 

 stantly 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 rami- 

 fications in difterent directions throuo-hout thewliole mass." 



In the " Encyclopoedia Britannica " (ninth edition), under the 

 word " annealing," is found as follows concerning the phenomena of 

 unannealed glass and Prince Rupert's droj^s: "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 propex'ly-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 droj)S it is plainly visible that the 

 interior substance is cracked in every direction, and ready to fly to 

 pieces." 



The practical glass-maker, desii'ous 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 founda- 

 tion of the theory of the Rupert drop, and of the process of annealing, 

 in the article of The Popujlar Science Monthly, and in the " Ameri- 

 can Cyclopaedia," 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,' bismuth,'' gray cast-iron,' and antimony,* 

 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. 



' Ganot's " Physics," edition of 1873, p. 261. = Miller's " Chemistry," vol. ii., p. 604. 



* Bauerman's " Metallurgy of IroD," p. 233. * Miller's " Chemistry," vol. ii., p. 595. 



