G L A

G L A

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ounces, and this is.nearly the extent of thepower of the common lamp-blowing. But thebulbs for thermometers, or other philofophicalpurpofes are much iefs. Two or three of theqoramoncft operations may be delcribed.

To feal a tube hermetically, if fmall, it isfufficient to hold it in the flame for a little timefiowly turning it round, when the end will melt,and falling in, will clofe the cavity with a neatbutton. This may be afliffed by pufhing theloftened ends in towards the common centerwith an iron needle. But if the tube be verylarge, this button would be too clumfy,and beingthick would be in danger of breaking on cooling.It is therefore neceflary to leflen the quantity ofglafs, which is done in the following way: foftenthe end of the tube in the flame, and apply to ita piece of another tube of nearly the fame fize(fragments of tubes being always abundant inthis bufinefs) which will flick firmly to it.Then foften the tube to be fealed a little higherup than the point of juncture, and pull the twofiowly in contrary directions till they feparate.The tube will then draw out at the heated partinto two fhort thin funnels, and a little turningand management of the flame will readily feal thatwhich is wanted, leaving the joined ends andabout half an inch of the lower part of thetube on the wafte piece.

To bend a tube, if of a narrow bore and theglafs is pretty thick, it is only neceflary to holdit in the weaker part of the flame, and foften itfor about an inch or two of its length, and bendit fiowly into the required ihape. In this waybarometer tubes are bent. But if the tube bewide, and tne glafs thin, this way of bendingentirely deftroys the cylindrical form of thebore at the bent part, making a double flattening.To avoid this, firft feal up one end of the tube,and then whillt bending it at the required part,blow fleadily and gently into the open end, andthe preffure of the breath will counteract thefalling in of the fides of the bending portion,and keep the bore cylindrical. The clofed endis then cut off by the file, to do which make adeep fcratch with one edge of a fine three-cornered file on the part intended to be cut, thenbreak the tube with a fmart pull in that directionin which the fcratched part will be outermofl,and it will feparate in general with great accu-racy at this point.

To join two tubes, heat them botli in theflame, and apply them together when white hot,turning them round to fintfh the confolidation,or elfe to avoid the thick ring of glafs whichthis produces, previoufly clofe one end of one

tube, and when the two are fully joined blewinto the open end of the other tube, and pullthem out a little at the point of juncture, till anequal cylinder is formed.

To form a bulb (of a thermometer for ex-ample) choofe a tube of a very equal bore, fealthe end in the ufual manner, and to collect agreater mafs of glafs at the end, prefs upwardson it while quite hot with any iron inflrument,fo as to confolidate and Ihorten it a little; letit remain in the hotted part of the flame till thelump of gl^fs is quite white hot, then remove it,put your lips to the open end without lofs oftime, holding it with the hot part lowed, andblow moderately and fleadily.. The lump ofhot glafs will immediately open into a bulb, thefize of which can be regulated at pleafure.

Glafs may be fpun out into threads of almoftindefinite minutenefs by means of the blow-pipe.When no thicker than fine hair, it is extremelyflexible and elaftic, and if ftill finer it may bewound almoft like common thread withoutbreaking. The way of doing it is very fimple,A piece of glafs tube is heated in the lamp,and the end drawn out into a thread by meansof another piece of glafs cemented to it. Whena fine thread is once drawn, the end is carriedround a reel or wheel two to three feet in dia-meter, and by turning the wheel and continuingto heat the tube, an endlefs thread is drawnout, winding round it as long as the artift pleafesor the glafs lafts. TBe quicker the wheel re-volves, and the hotter the glafs is kept, thefirmer is the thread, which may thus be madeas delicate as a fingle filk-worm’s thread, withextreme flexibility. Different coloured threadsare made in this way by ufing very deeplycoloured glafles inftead of common glafs.

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A Angular change occurs in the texture ofglafs, more particularly of green bottle glafs,made only of fand, lime, and faline allies, whenexpofed for fome time to a moderate red heat,or any higher temperature, but below its melt-ing point. This is peculiarly obfervable whenit has been in contact with fand, and hence itfrequently takes place in green glafs retortslong expofed to a high heat in fand-bath diftil-lations. Neuman appears the firft who noticedthis change, which was afterwards examinedmore at large by Reaumur, and from the por-cellanous texture which the glafs aflumes whenthus changed, it has been commonly calledReaumur’s porcelain. This ingenious phiiofopherhad the idea that much advantage might bemade of this, fact in rendering glafs much