G L A

G L A

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are occafionally drawn out of the pots withan iron rod to examine the ftate of the ma-terials, and gradually the glafs becomes moreand more flexible, denfe, and lefs brittle, andat laft the glafs-gall is entirely diflipated, andthe vapours which it occafioned are no longerto be perceived. This is the firft and verywell defined ftep in the procefs of glafs-melt-

ing-

The glafs is now full of minute fpecks orbubbles which the continuance of the heatcaufes to expand and burft at the top, till at laftit refines beautifully clear, tranfparent, and co-lourlefs, as is feen by the famples which arefrom time to time taken out and cooled. Thisfecond procefs, namely, from the ceflation ofthe vapour of the glafs-gall and its thoroughremoval, to the time when the glafs is perfectlyclear and free from bubbles, is called the re-fining. The glafs is now compleat, but is oftoo thin a confiftence to be wrought, it is there-fore cooled by flopping the draught of the fireround the individual pot, and in cooling itthickens to the working point. The clay withwhich the opening was luted is then pickedoff and the working begun. But if the glafs isto be caft into plates (as all the large mirrors are)a much fliorter cooling is required, as for thispurpofe it is required to flow very thin andhot.

The particular manner of managing thepots, and the alternations of filling and work-ing them off varies confiderably, and dependschiefly on the convenience of the manufac-turer. Sometimes half the number of pots(which are generally fix in common glafs-houfes) are kept in the working ftate, whilftthe materials are melting or refining in thethree others, and fometimes they are all filledtogether. On an average it takes about forty-eight hours for the fine flint glaffes, from thetime that the pots are firft filled till the glafs isready for working, but there is no occafion toufe it immediately, as it may be allowed to re-main a confiderable time at a low working heatwithout any injury.

When the glafs is cooled down to the work-ing heat, which is a very full red, it has a kindof confiftence and tenacity not exadlly to befound in any other fubftance in nature. It isjuft foft enough to yield with the greateft eafeto any outer impreffion, even to the force of thebreath when urged pretty ftrongly in the centerof the glowing mafs, and may be bent, pulledout and fhaped in every poffible way ; and itstenacity is i'uch that it extends uniformly with-

out producing any cracks or fiffures, but whenftretched to the utmoft it pulls out to a folidfiring, the diameter of which is conflantly de-creaiing till it feparates from the mafs in athin capillary thread. As it cools it ftiffens,and becomes perfe£llv brittle, which takes placewhen no longer red hot, and at this time alfoit becomes tranfparent. Melted glafs adheresvery feebly to polifhed metal, fo that it iswrought with bright iron tools with the utmofteafe.

Almoft every kind of glafs veffel, and utenfil,common window glafs, and in fhort, almoftevery manufactured glafs article except plate-glafs, is fhaped out of a hollow globe formed byblowing. The inftrument ufed for this pur-pofe is Amply a hollow iron rod about four orfive feet long, which the workman firft dipsin the glafs-pot and turns about till a fufficientmafs of the melted glafs adheres to it, he thenholds, it near the ground by which the mafspulls out and lengthens by its own weight, andthen blows ftrongly through the tube. Thebreath thus penetrates the center of the red-hot mafs, and immediately extends it into anuniform hollow globe of the required thick-nefs. This mud be immediately blown out aslarge as intended, and the force of the breathkept upon it for a few feconds till it ftiffens bycooling, otherwife it would again fink in bythe compreffion of the denfer external air. Thisoperation produces a hollow globe, adheringby a neck to the iron rod, and is the originalform out of which the others are fafhioned.

It would be impofiible to give in a fhortfpace an adequate idea of the eafe and fimpledexterity by which, with a few inftruments,this rnoft beautiful fubftance is ftretched out,enlarged, clofed, perforated, and formed by afew ingenious manoeuvres into all the commonutenfils. As a Angle and fhort example, themaking of a common tumbler may be given.A hollow globe with a fhort neck being firftblown on the iron rod as above mentioned, itis taken off in the following way : an affiftantdips the end of a fhort folid iron rod into theglafs-pot, and brings it out with a little' of themelted glafs adhering, this is immediately thruftagainft the bottom of the globe at the partdire£tly oppofite the neck, to which it firmlyunites, fo that the globe becomes cementedupon this fecond tck! by means of the meltedglafs. The workman then wets a fmall pieceof iron with his mouth and lays it on the neckof the globe which is Hill extremely hot, and thiscracks it quite round in a fecond or two, fo that