G r. A S s. 



the time when he was nt Rome, tiierc was Jug up a \v!;ole 

 houfe from under the kitchcH garden of a citi/.cn. The 

 hoiife had htfo-.i buried there ten ages, and there were found in 

 it fcvera! gUil's urns, or lacrymatorics. The glafs of thefe 

 had no holes made in it, as our old glafs in cliamber-windows 

 has, but Hill retained its fmooth furface and tranfpan>nce ; 

 but it was fpiit into a vaft number of thin lamina;, which 

 were as pellucid and fmo as Mufcovy glafs ; and in fome 

 places were tinged with all the beautiful colours that art could 

 have givtn. We are not acquainted perfectly with the an- 

 cient way of working their glafs ; but it is not probable 

 there could be any thing particular in the formation of the 

 veflel, to determine it to fplit thus into flakes; but that glafs 

 of the fame kind, in any form, would have done the fame. 

 Borrich de Ortu Cliemijc. 



There are other vifible imperfcftions in glafs, materially 

 injuring its foundnefs and beauty, and enumerated by Aikin 

 (l)i<ft. axi.Gliifs; under the denominations o{ Jlriie, threads, 

 tears, and knots. The former are undulating waves in the 

 ' glafs, ariling from the imperfeft mixture pf the materials, 

 and their different fpecific gravities. Accordingly, we may 

 obferve, that the molt tranlparent glafs is fubject to bubbles 

 and veins, the methods of preventing which are yet little 

 known ; and this is an inconvenience by v.-Iiich Mr. Dol- 

 lond's excellent difcovery is afFefted ; for the flint-glafs which 

 he ufesis peculiarly fubjeft to fmall veins, that dilhirb the rays 

 in their pafiage, and render the vihon confufed. This effeft is 

 owing to the denfity of thefe veins being greater than that 

 of the reft of tlie glafs, as appears from tlieir image received 

 on v.'hite paper, when the glafs is held between the paper 

 and a candle, or other luminous ohjett : for this image of a 

 vein, thus received, is a line brighter than the reft of the 

 image of the glafs, and this bright line is defined by a dark 

 edge or; each fide. But the bright line evidently fliews a 

 convergency of rays, which can only be effefted by the 

 »eins being denfer than the medium in which they are placed. 

 The reafon why flint glafs is more fiibjeft to veins than 

 other glafs is, fays the tranflator of Macqucr"s Cliemical 

 Dictionary, bccaufe it is compofed of materials of more 

 different denfities. 



Mr. Macquer, with a view of improving the manufac- 

 ti>re of this glafs, propofes to facilitate the union of the calx 

 «f lead and fand, of which it is compofed, by depriv- 

 ing the calx of lead, as much as pofhble, of its phlogJlon, 

 which may be done by combining the vitriolic acid with mi- 

 nium, or red-lead, and cxpofing this compofition to the opera- 

 tion of fire, to difengagc it from the acid ; and alfo by giving 

 ■thefe two fubitances the greateft poflible degrees of fluidity 

 and mobility, which nr.iy be done by mixing with the com- 

 pofition of them a cor.Cderable quantity of folvents. Kilt. 

 Acad. Scienc. for 1773. 



Threads in glafs, are thofe ftreaky filaments which arife 

 from the vitrification of the clay ; being generrJly green, 

 and rendering the glafs more liable to crack at thefe parts. 

 But one of the worft defefts (fays Aikin, ul'i fupra), is 

 *' tears, or drops of vitrified clay falling down from the 

 furnace into the pots, and entangled with the glafs. Arti- 

 cles made of glafs with this defett, are always very brittle, 

 and generally break of tliemfelves by flight changes of heat 

 and cold." Small bubbles appear in glafs not fiifficiently re- 

 lined by a continuance of the melting heat ; and thefe may 

 be owing to a deficiency of flux, fo that the glafs is Ids 

 fufible, and the bubbles cannot eafily be difengaged. 

 " Hence," fays the author now cited, " the foft fulible 

 glaffcs with much lead are much lefs liable to this fault than 

 the hard, green bottle-glafs, wliich is made only of alkali 



from bits of the crucible which may be accidentally knocked 

 off by the iron inftruments ufed in the working." 



Gl,.\':i< of /]iitlmcir.y. See 0.i;;'^.f ij/" Anti.moxy aid Vl- 

 TRU.\r yhittmomi ccratum. 



Gi-A.ss, Aximgta of. Sec AAfN<:i\, !-*.\NDiVL'n, ai.d 

 Cii..\it^,fu^ra. 



Oi.A.ss of Bora-. Sec Borax. 



Gla.ss, Cd'Hirlng of, tainiilale "fins. See GcM % 



Gl-.'is.'s, Gohl-colourcii. This kind of j^lafo may be made 

 by taking ten pounds of either of the comi.ofitions for hard 

 glafs, omitting tlie falt-petre ; and for every pound additlg 

 an ounce of calcined borax ; or, if this quantity doth not 

 render the glaft fufficlcntly fufible, two ounces ; ten ounces 

 of red tartar, of the deeped colour, two ounces of rnanga- 

 nefe, and two drams of cliarcoal of fallow, or any other loft 

 kind. Precipitate of filvcr baked on ghifs will llain it yellow, 

 and likcwile give a yellow colour on being mixed with and 

 melted with forty or fifty times their weight of vitreouj 

 compofilions ; the precipitate from aqua-fortis by fixe.i al- 

 kali feems to anfwer belt. Yellow glafles may alfo be ob- 

 tained with certain preparations of iron, particularly with 

 PruflTian blue. But Dr. Lewis obferves, that the colour does 

 not conftantly fuccecd, nor approach to the high yellow of 

 gold, with filver or with iron. The neareft imitations of 

 gold winch he has been able to produce, have been efTcclcd 

 with antimony and lead. Equ;:l parts of the glafs of anti- 

 mony, of flint calcined and powdered, and of miniu.T^, form- 

 ed a glafs of a high yellow ; and with two parts of glafs of 

 antimony, two of minium., and three of powdered flint, tlic 

 colour approached ItiU more to that of gold. The lail com- 

 pofition exhibited a multitudt of fmall fparkles interfperfed 

 tlu-ough its wliole fubilante, which ga\e it a beauijful ap- 

 pearance in the mafs, but were really imperftdlioas, owing 

 to air-bubbles. 



Neri directs, for a gold-yellow colour, cne part of red 

 tartar, and the fame quantity of manganefc, to be mixed 

 v.ith a hundred parts of frit. But Kunckel ohferves, that 

 thefe portions are faulty ; that otie part, or one and a quar- 

 ter of mangancfe, is fufF.cient for a hundred of frit ; but 

 that ^x-s. parts of tartar are hardly enougii, uiilefs the tartar 

 is of a dark red celour, alraolt blackiih ; and that he found 

 it expedient to add to the tiirtar about a fourth of its weight 

 of powdered charcoal. He adds, that the glafs fwclls up 

 very much in melting, and that it mull be left unllirred and 

 worked as it (lands in fufion. Mr. Samuel More, in repeat. 

 ing and varying this procefs, in order to render the colour 

 moreperfett, found that the mangancfe is entirely eflential 

 to the gold colour, and that the tartar is no ilherwifc of ufc, 

 than in virtue of the coaly matter, to which it is in part re- 

 duced by the fire ; the plilogiilon or inflammable part of the 

 coal appearing in Icveral experiments tii be the dirert ting- 

 ing fubitance. Mr. Pott alio, in his Xcuc Wichtige Phy- 

 ficaliflich-Chymifche Matcrien, &c. printed in 1762, ob- 

 ferves, that common coals give a yellow colour to glafs j 

 that different coaly matters differ in their tinging power; 

 that caput mortuum of foot and lamp-black anlwer better 

 than common charcoal ; and that tlie iparkling coal which 

 remains in the retort after the reitification ot the thick cm- 

 pyreumatic animal oils, is one of the molt active of tliefe 

 preparations. This pivparalion, he lays, powdered, and 

 then burnt again a httle in a dole veflel, is excellent for ting- 

 ing glafs, and gives yellow, brown, reddifli, or blackiih co- 

 lour, according to its' quantity ; but the frit luuil not be vci-y 



hard 



