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owing to fornc permanent and very ftrong in-equality of preffure, for when they are heatedfo red as to be foft and merely let to cool ofthemfelves, this property of burfting is entirelyloft, and at the fame time the fpecific gravity ofthe drop is encreafed. s

The peculiar brittlenefs of the Bologna phialis alfo removed by again heating and coolingflowly.

A defeat in the annealing of common win-dow glafs is alfo fliewn when cut by the dia-mond. When the glafs is well annealed thediamond cuts it with moderate eafe, making anuniform fmooth furrow, at firft dark, but whichgradually opens and then appears as a brightfilver thread ; but when the glafs is badly an-nealed, the diamond works with much moredifficulty, the cut opens very flowly, and oftenflies into a different direction, or the glafs en-tirely breaks.

The other more common defeats of glafs area liability to be acted on by corrofive liquors(which takes place when too much faline fluxhas been ufed) and alfo a number of vifible im-perfections, fome of which materially injurethe foundnefs as well as beauty of the manu-factured articles. The chief of thefe vifibledefects are Jlria h , threads , tears , and knots. TheStria are undulating waves in the glafs, per-fectly tranfparent and vitrified, but which pro-duce much ftrange diftortion when ufed forwindows or for optical purpofes. This defeftarifes from the imperfedt mixture of the ma-terials, and the great difference in their fpecificgravities. For the gravity of glafs made Amplywith alkali and fand is about 2.3 or 2.4, thatof alkali and crucible clay, about 2.5 ; that ofalkali and chalk 2.7 or 2.8 ; whilft the vitrifiedoxyd of manganefe alone weighs 3.2 and theglafs of lead 7.2 nearly. Therefore when thefeare all together melted in the glais-pot, if theyare not thoroughly mixed, they are in the cafeof liquors of unequal denfity in contact witheach other and flightly agitated fo as to fhakedie different materials into ltreaks or waves.

The defedt from this caufe is feen veryftrikingly, in ordinary prifms, or pieces of folidglafs of a certain^thicknefs, which are feldomquite uniform in denfity throughout. For verynice purpofes it is often of ufe when fmallmoveable crucibles are ufed, to invert themwhen the glafs is melted and empty the contents,whereby the heavier parts become mixed withthe lighter as they fall through them.

Threads in glafs-making are thofe ftreaky

* Own Expcr.

filaments which arife from the vitrification ofthe clay. They are generally green and oftenrender the glafs more liable to crack at thefeparts.

Another and one of the wo'rft defects isTears, or drops of vitrified clay falling downfrom the furnace into the pots and entangledwith the glafs. Articles made of glafs withthis defeCt are always very brittle, and gene-rally break of themfelves by flight changes ofheat and cold. This is the more likely to hap-pen in proportion as the tear is nearer the fur-face.

Glafs when not fufficiently refined by con-tinuance of the melting heat is always full offmall bubbles. This fault may alfo happenfrom a deficiency of flux which renders theglafs lefs fufible, and therefore ftiffer duringthe ordinary time and degree of heating fo thatthe bubbles cannot eafily difengage themfelves.Hence the foft fufible glaffes with much leadare much lefs liable to this fault than the hardgreen bottle glafs which is made only of alkaliand earth.

Another defeCt is knots which arife eitherfrom a portion of fand that has efcaped vitri-fication and remains entangled in the glafs, orfrom a remaining quantity of glafs-gall; or frombits of the crucible which may be accidentallyknocked off by the iron inftruments ufed in theworking.

Of the different kinds of Glafs.

Though an infinite variety is found in thequality and compofition of different glaffes,there are fome principal kinds made for totallydiftindt purpofes which may be fliortly noticed.

The fiint glafs of London and other parts ofEngland (called by foreigners crffal) is thatbeautiful, foft, brilliant, fine glafs of which boththe common and fineft articles of white glafsin domeftic or ornamental ufe are manufac-tured. Many optical inftruments are alfo madeof the fame material. It is particularly diftin-guifhed for the quantity of litharge which en-ters into its compofition, on which account itis by far the heavieft, the mod brilliant, thefofteft, and the eafieft to work, and alfo themoll expenfive.

The precife proportions of ingredients arenot ufually known, but the following is faidto make an article, of the bell quality, namely;120 parts of fine clean white fand, 40 of pearl-afti well purified, 35 of litharge or elfe minium,13 of nitre, and a fmall quantity of black oxydof manganefe. The diftinft ufe of thefe in-

h Loyfel.