GLASS. 



tliCTugli in rxtcrnal pppcaranccs.tlic difTcrtnccs were con fider- 

 able. All the pieces which liad hcon furroundod with char- 

 coal or with foot were externally of a deep blick colour, 

 which did not difappcar hy cxpof-ire to a flioiijj fue during 

 an hour, with free accefs of air. Coloured clays and fands 

 communicated diiT^rent fliades of a brown eolo'.ir ; and white 

 earths gave whitifli, gre\-ifli, or browiiifli tingr-.. White 

 fand, calcined flints, and gypfum, gave in general the 

 Cjreato.l whitcnefs, and tobacco-pipe cl;iy the grc:;tcll glt)lli- 

 neis and brightnefj. GlafffS compofed of earths witliotit al- 

 kaline fdlt, glafs of lead, flint-glafs, crown-glafs, looking. 

 glafs pliitos, a glafs prepared with calcined flints and a fixed 

 alkaline fait, and even green glafs which had been fiifed toge- 

 ther with a ninth part of alkaline fait, fiiffercd none of the 

 above alterations by cementation. Green bottle-glafs and 

 common window-glafs were moil fiifceptible of th-fe altera- 

 tions. 6. T!ie changes produced by cementation could not 

 proceed from any abforption of matter from the cementing 

 fubflance ; b?caufe no increafe of weight was given, and 

 becaiife the fame changes were produced ujion a piece of 

 glafs msrely by heat, without any cementing fubllancc. 

 See Com. Phil. Tcchn. p. 230^-255. 



Mr. Gregory Watt, in his valuable paper on bafalt (Phil. 

 Trauf. for 1804., cited by Aikin,) alleges this porcellanous 

 chau-re of glafs as an illuflration of his important principle, 

 I'l'a. " that bodies wliofe fibres have a natural tendency to a 

 cryftalline arrangement, or a polarity, when vitrified by a 

 fufficient heat and cooled hallily in the vitreous Hate, are able 

 fubfequently to return to their natural cryftailine arrange- 

 ment of fibre, when e.'^pofed to a heat merely fufficient to 

 foften the texture, though not enough for hifion. This, 

 in the inllance of bafalt, he fliev/s by the iingular cryllalli- 

 zations formed in the cells of fufed bafalt, long after it had 

 loll the liquidity of fufion. The circumllance of no mate- 

 rial change occurring in the weight of glafs by this eonver- 

 fion into the fibrous ftate, lliev.'s inconteftably that it cannot 

 be owing either to any thing gained during the procefs, nor 

 to any material lofs of the alkali ; and this is alfo rendered 

 manifeR by its return to the vitreous Hate and vitreous quali- 

 ties when again melted. This too may again be porcel- 

 lanized in the fame way, and again be melted into glafs, 

 and fo on alternately. " Sec Poucelaix. 



Gl.A-Hs-potj, the vefTels in the glafs manufafture vifed for 

 melting the glafs, Thofe for the white glafs works are made 

 of a tobacco-pipe clay, brought from tlie I fie of Wight, 

 which is firll well walhed, then calcined, and afterwards 

 ground to a i\ns powder in a mdl ; which being niixt with 

 water, is then trod with the bare feet till it is of a proper 

 conliflence, to mould ^N-ith the hands into the proper fliape 

 of the vefTels. ^Vhen thefe are thus made, they are after- 

 wards annealed over the furnace. Thofe for the green-glafs 

 work are made of the nonfuch, and another lort of clay from 

 Staffordfliire ; they make thefe fo large as to liold three or 

 four hundred weight of metal. And befides thefe, they have 

 a fmall fort called piling pots, which tiiey let upon the 

 larger, and which contain a finer and more nice metal fit for 

 the nicefl works, Neri. 



The clay tirat is ufed for this purpofe fliould be of the 

 purell and moft refractory kind, and well c'eanfed from all 

 fandy, ferruginous, and pyritous matters ; and to this it will 

 be proper to add ground crucililes, white fand, calcined 

 flints duly levigated, or a certain proportion of the fame 

 clay baked, and poimded not very linely. The quantity of 

 baked clay that ought to be mixed with the crude clay, to 

 prevent the pots from cracking when dried, or cxpofed to a 

 great heat, is nut abf^lutel) determined, but depends on 



the quahty of the crude clay, wliich ij more or leH fat. M . 

 D'Antic, in a memoir on tliii fubject, propofes the follov.-- 

 ing method of afcertaining it : the burnt and cnid<; clav, 

 bemg mixed in different proportions, fiiould be fonned into 

 cakes, one inch thick, and four inches long and wide. Lrt 

 thefe cakes be (lowly dried, and cxpofed to a \iolent heat, 

 till they become as hard and as much contracled a) ponTibl-, 

 and in this Rate be examined ; and the cake, he fay:, which 

 has fuffercd a diminution of its bulk equal only to an 

 eighteenth part, is made of the beft proportions. He ob- 

 ferves, in general, that moil clays require that the propor- 

 tion of the burnt fhould be to tiie frefh as four to five. 



It is of great importance that the material of which the 

 pots are made (liould be carefully feledled, as they are ex- 

 pofed to the adlion of a very fierce fire, and alfo to the fol- 

 vent aftivity of the glafs with its powerful fluxes. They 

 fhould alfo be made very thick and llrong, as they are in- 

 tended to lad for many months. Wficn finifhcd, they are 

 placed in a warm room in order to difcharge their moillure, 

 and any fmall fiffurts arifing from the unequal (hrinkior of 

 the clay are clofed by gently beating with a mallet. 1 hey 

 are afterwards gradually heated in a fmall oven, conftru6ted 

 for this purpole, and llowly brought to a red heat ; and 

 after rem.iiiiing for fome time in this Hate, they are removed 

 to the furnace, and fixed down in their places by fire-clay. 

 Here, on account of a further fhrinking, they remain for a 

 day or two bs-fore they are fit for receiving the glafs mate- 

 rials. Pots of this kind are faid to lail, at an average, about 

 a year. 



Gj..\.ss Tuhes^re of various lengths, diameters, and formj, 

 according to the purpofos to which they are applied. Thefe 

 are often formed with the lamp and blow-pipe, in the manner 

 defcribed under the article L.VMP-^/^-a'.-rj. In this way- 

 tubes are fealed hermetically (fee HEiiMKTir.\r, Seal), tubes 

 are bent, others arc joined, bulbs are annexed to tubes for 

 thermometers, &c., and glafs is drawn out into threads. 



It has been obferved, that glafs tubes laid before the 

 fire in an horizontal pofition, and with their extremities fup- 

 ported, have had a rotatory motion about their axes ; and 

 alfo a progrelTive motion towards the fire, even when their 

 fupport:! are inclined from the fire, fo that the tubes wilj 

 move a little upwards. See Phil. Tranf. N° 476. f T. 



When the progreffiTc motion of the tube is (lopped by 

 any obdacle, its motion about its axis will ilill continue. 

 When the tubes arc placed in a nearly upright pollure lea;'. 

 ing to the right hand, the motion will be from call to wefl ; 

 but if thoy lean to the left, the motion will be from w.ll t(». 

 fad ; when the nearer they are placed to the perfetlly up- 

 right pollure, the lefsthe motion will be cither way. 



If the tube be placed hori/onlally on a glafs plane, tho 

 fragment, for indancc, of coach window-glafs, indead ol" 

 moving towards the fire, will move from it, and about it* 

 axis, in a contrary- dircclion to what it had done before. 

 Nay, it v/ill recede from die fire and move a little up^vards, 

 when the plane inclines towards the fire. 



Ncitiicr tl'.c draught of air up the chimney, not attraftion 

 or repulfion, are the caufe of thcie phenomena. It feenii 

 rather owing to the fwcUing of the tube toward? the fir?» ; 

 fi)r allowing fuch fwelling, gravity mutf pull the tube dowi, 

 whe.T fupported near its extremities liori/ontaliy ; and a 

 freih part being expofcd to the fire and fwclhng out^ again, 

 muft fall douhi ag;iin, and fo on fuccelfively ; which will 

 produce a rotatory motion towards the fire. 



If the tube be fupported by two otlier tubes, and llielV be 



brought near to each other, and to the centre of the lup- 



ported lube, tlien its p.uts hanging over on each fide, beine 



hvrger than the part which lies between the fupporiers, will, 



Uu2 .ky 



