165 RUPERT'S DROP$. 



The cylinder thus open at one end is returned to the mouth of the 

 furnace, win-re it is cut by the aid of cold water, and ripped up 

 through its whole length by a pair of iron shears ; after which it is 

 gradually heated on an earthern table, in order to unfold its length, 

 while the workman with another iron tool alternately raises and 

 depresses the two halves of the cylinder: by which process, the 

 one half accommodates itself to the same flat form as the other. 



Plate-glass is the last and most valuable kind, and is thus called 

 from its being cast in plates or large sheets: it is almost exclusively 

 employed for mirrors or looking-glasses, and for the windows of 

 carriages. 



Plate.glass was formerly blown ; but that, method having been 

 found very inconvenient, casting was invented ; namely, the liquid 

 metal is conveyed from the furnace to a large table, on which it is 

 poured, and all excrescences, or bubble?, are immediately re- 

 moved by a roller that is swiftly passed over it. It is then an. 

 nealed iu the manner already referred to. 



SECTION IV. 

 Rupert's Drops. liatarian Tears. Bolognian Phial. 



THESE are peculiar modifications of glass, for the purpose of de.. 

 ception or amusement. 



Rupert's Drops, an elegant glass toy, are simply formed by pouring 

 a small solid lump of green bottle glass, when red-hot, into water, 

 by which means the rounded lump assumes gradually a lengthened 

 form, terminating with a fine and nearly capillary tail, at the e.x- 

 tn inity. This solid lump will bear very considerable violence on 

 the massy end without injury, and is altogether extremely tou^h ; 

 but whenever the smallest portion of the thinner end is broken olf, 

 the whole bursts with a smart snap, and instantly crumbles into 

 innumerable fragments as small as fine sand ; which, from their 

 yery oionteDett, and ,?' imperfection of their crystallization, do 

 no other injury to the hand that holds the drop, than that of pro. 

 during a slight sting from the suddt n concussion. 



This curious and i xtraordinary fragility is obviously owing to 

 gome p< rmanent and very strong inequality of pressure ; for when 

 the Huprrt's drops are heated so red as to be soft, and are let to 

 cool gradually of themselves, and, consequently, to become better 

 annealed, this | ru| eriy of bursting is entirely lost, and, at the same 

 time, the specific gravity of the drop is increased. 



