GLASS. 



bing, and Jilverhig ; w liich fee rcfpeftivcly. See alfo 

 jOOKing-Glass. 



Glass, Annealing or Ncaling of. The operation of an- 

 nealing of glafs is piTformcd in a peculiar furnace called the 

 Ucr, which confills of two parts, the tower and /«»•. The 

 veilels, as foon as made, are placed by the workmen on the 

 floor of the former to anneal : which done, they are drawn 

 flowly in a fort of pan, called yrnc/'c/, by an operator called 

 the farok-tiwn, all along the latter, the fpace of five or fix 

 yards,. to give them time to cool gradually ; fo that when 

 they reach the mouth of it, they are found quite cold. 

 Werrct, Not. to Neri, p. 243, fcq. 



This annealing is generally performed in a hot chamber, 

 built for the purpofc, at the top of the glafs-houfe, above 

 the crucibles, and a little below the chimney. Without this 

 precaution, the glal's would be liable to fly and break, by 

 the leafl change of heat and cold, by the fmallefl feratch, 

 and fometimes without any apparent external caufe. The 

 hard glafl'es, and lliofe efpecially that are made with alkali 

 and earths, require much more annealing than the foftcr and 

 more fufible glaflcs, containing in their compofition much 

 btharge. 



The particles of glafs by annealing are fuppofed to lofe 

 part of their fpringinefs, and their brittlcnefs at the fame 

 time. A gradual heating or cooling of glafs, according to 

 Dr. Hook, anneals or reduces its parts to a te.xture more 

 loofe, and eafy to be broke ; but withal more flexible than 

 before. And hence in fomemenfure the phenomena of glafs- 

 drops. 



Some of the phenomena depending on tiie fragihty of un- 

 annealed glafs deferve the attention of the curious. Thofe 

 ef the lachrymx, or glafs-drops, were among the firft taken 

 ROtice of ; and it has alfo been obferved, that hollow bells 

 made of unannealed glafs, with-a fmall hole in them, will fly 

 to pieces by the heat of the hand only, if the hole by 

 which the internal and external air communicate be flopped 

 with a finger. Phil. Tranl". N° 477. § 3. See Rii-i.ht'j 

 Drops. 



But lately feme vefll-ls made of fuch unannealed glafs 

 iiave been difcovered, which have the remarkable property 

 of refifting very hard flrokes given from witho'.it, though 

 they fliiver to pieces by the fliocks received from the fall of 

 very light and minute bodies dropped into their cavities. 

 Of this kind is the " Bologna phial." Thefe glafl'es maybe 

 made of any fliape ; all that needs be obferved in making 

 them, is to take care that their bottoms may be thicker than 

 their fides. The thicker the bottom is, the eafier do the 

 glafl'es break. One wlicfe bottom is three fingers breadth 

 ill thicknefsjflieswith as much eafeat leafl; as the thinneft glafs. 

 Some of thefe vefl"els have been tried with ftrokes of a mal- 

 let fufiicient to drive a nail into wood tolerably hard, and 

 have refifted frafture. T!:ey alfo refift the fliock of feveral 

 heavv bodies let fall into their cavities, from tlie height of 

 two or three feet. For inllance, muflcet-balls, pieces of 

 iron, or other metal ; pyrites, jafpcr, wood, bone, &c. but 

 this is not furprifiug, as other glafl'e* of the fame Cr/.i: do the 

 liinie. But the wonder is, that taking a fliiver of flint of the 

 fize of a fmall pea, and letting it fall into the glafs only from 

 the height of three inches, in about two feconds the glafs 

 flies, and fometimes in tlie very moment of the fliock ; nay 

 a bit of flint, no larger than a grain, dropt into feveral glafl'es 

 fucceflively, tho.igh it did not immediately break them, yet 

 they all flew, being fet by, in lefs than three quarters of an 

 hour. Phil. Franf. ibid, p 509. 



Some otber bodies produce a like effetl with flint : for in- 

 ftV'Ce, fappbire, porcelain, diamonds, hard tempered lleel, 



as alfo marbles, fuch as boys play with ; to which add pearl«> 

 from the animal kingdom. 



The experiment fiiccceded alfo when the glaflcs were held 

 in the hand, rcfted on a pillow, put in water, or filled with 

 water. It is alfo remarkable, that the glafl'es broke upon 

 their Vottoms being flightly rubbed with the linger, 

 though fome of them did not fly till half an hour after the 

 rubbing. 



If the glafl'es be every where extremely thin, they do not 

 break in thefe cireumftances. 



Some have pretended to account for thefe phenomena, 

 by faying, that the bodies dropped into thefe veffels caufe a 

 concuflion, that is ftronger than the cohefion of the parts of 

 the glafs, and that confequently a rupture of the fame mud 

 enfue. But why does not a ball of gold, filvcr, iron, cop- 

 per, or feveral other bodies, even a thouland times heavier 

 than a fliiver of flint, equally caufe this concuffion, and break 

 the glafl'es ? 



Mr. Eulcr has endeavoured to account for thefe appear- 

 ances from his Principles of Percuffion. He thinks this 

 experiment entirely overthrows the opinion of thofe who 

 meafure the force of percuflion by the i.'x I'iva ; and 

 he thinks the principles he has eitabhfhed give a clear folu- 

 tion of this phenomenon. According to thefe principles, 

 the extreme hardnefs of the flint, aiid alfo its angular figure-, 

 which makes the fpace of contad: with the glafs veflfel ex- 

 tremely fmall, ought to caui'e an impreflion on the glafs vaft- 

 ly greater than lead or any other metal ; and this may ac- 

 count for the flint's breaking the vcfl'el, though the bullet, 

 even falling from a coniiderable height, does no damage. 

 Mem. Acad. Berlin, 1745, p. 47' 



Hollow cups, made of the green bottle-glafs, fome of 

 them three inches thick at bottom, were inflantly broken, 

 by a fliiver of flint weighing about two grains, though they 

 had refifted the fliock of a mufl<et ball from the height of 

 three feet. Phil. Trauf ibid. p. 515. 



" The precife miechanieal caufe of this difpofition to 

 crack in unannealed glafs, is very difficult to explain," fay* 

 Aikin (Diet.', " but generally fpeaking, it is fuppofed to 

 be'tlie forcible contraftion of the outer part by fuddert cool- 

 ing, wliilft the inner portion is ilill foft and half-fluid, fo 

 that the whole fixes with a permanent ilrain or inequality of 

 prefl"ure of one part upon the other ; and as glafs is extreme- 

 ly elaflic, though brittle, any force which tears afunder a 

 portion, however fmall, of the tenfe part, communicates a 

 ilrong and fudden impulfe over the v.hole mafs.'' " Tliis 

 moll lingular phenomenon," (of the Rupert's drop) fays 

 the fame ingenious chemift, in confcquence of his own ex- 

 periments, " isobvioufly owing to lome permanent and very 

 Ilrong inequality of prefl'ure, for when they are heated fo 

 red, as to be foft and merely let to cool ef themfelves, 

 this property of burlling is entirely loft, and, at the 

 fame time, the fpecilic gravity of the drop is in. 

 creafed. The peculiar brittlenefs of the Bologna phial 

 is alfo removed by again heating and cooling flowly.** 

 The common window-glafs, when badly annealed, is cut 

 by the diamond with difficulty, and the cut often flies in a 

 direciion diflerent from what was intended, or the glafs en- 

 tirely breaks'. 

 , Among other more common defefls of glafs, we may 

 mention its liability to be afted upon by corrofive liquors, as 

 is the cafe when too much falineflux has been ufed. As impe- 

 netrable as glafs is to the common mcnftruums, we find it 

 eaten by the air in length of time, when expofcd in old win- 

 dows ; but the effefts of its being kept in a fubterraneous 

 place are much more flrange. Borrichiits tells us, that at 



il»e 



