3i6 THE POPULAR SCIENCE MONTHLY. 



An oven is so constructed that the heat of the glass is maintained 

 by a current of heated air in which articles to be annealed are placed, 

 and mechanism so contrived as very slowly to draw away the ware 

 into currents of lower temperature. Or the ware is annealed in kilns, 

 which are closed and scaled at a temperature a little less than that at 

 which glass becomes plastic, and heated air being thus confined the 

 kilns are many hours, often many days, in cooling. The more carefully 

 and slowly glass is annealed, the less liable it is to " ily." 



By cooling glass more rapidly than could occur in ordinary atmos- 

 pheric temperatures, that is, by a process the reverse of annealing, 

 Prince Rupert's drops are made. 



The ordinary way to make tliese scientific curiosities is to drop a 

 small quantity, usually less than half an ounce, of perfectly fluid glass 

 into water. In falling, the glass will assume the form of a tear, with 

 an elongated end extending into a thread. 



Rupert drops are clear, bright, and hard, and may be struck with 

 much violence upon the larger end without fracture, but if the thin, 

 though tough and very elastic thread of the other extremity be broken 

 otF, the whole drop will explode into numberless fragments, much finer 

 than the sand of which the glass was originally composed. 



Why does this happen? and why must glass-ware be annealed in 

 Older to be serviceable? There is evidently such similarity of phe- 

 nomena occurring in the drops and in unannealed glass that a satisfac- 

 tory theory for the one ought to lead to the explanation of the other. 



In an article on "Tempered Glass" contributed by Perry F. Nur- 

 sey, C. E., to the Popular Science Review^ and published in the Sep- 

 tember number of The Popular Sciexce Monthly, the following the- 

 ory of the Prince Rupert's drops is given : " Glass and water, and as 

 far as present knowledge goes no other substances besides, expand 

 while passing from the fluid into the solid condition. The theory of the 

 Rupert drops is, that the glass being cooled suddenly, by being dropped 

 into cold water, expansion is checked by reason of a hard skin being 

 formed on the outer surface. This exterior coating prevents the in- 

 terior atoms from expanding and arranging themselves in such a way 

 as to give the glass a fibrous nature, as they would if the glass were 

 allowed to cool very gradually. An examination of the Rupert's drop 

 shows the inner substance to be fissured and divided into a number of 

 small particles. They exist in fact in a state of compression, with but 

 little mutual cohesion, and are only held together by the external 

 skin. So long as the skin remains intact, the tendency of the inner 

 particles to expand and fill their proper space is checked and resisted 

 by the superior compressive strain of the skin. Nor is the balance of 

 the opposing forces disturbed by blows on the thick end of the drop, 

 which vibrates as a whole, the vibrations not being transmitted from 

 the exterior to the interior. But, by breaking off" the tail of the drop, 

 a vibratory movement is communicated along the crystalline surface, 



