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covered with a grate of very ftrong iron barson which the fuel is thrown, and the flamedraws very ftrong and fierce round the pots,and pafles out together with the fmoke, in onebody through the top of the dome which islengthened into a chimney for the fpace of afew feet. The precife conftrudion of the glafs-houfe can only be underftood by figures, whichcannot be given in this place. At the topof the dome between the pots and the chimney,is a kind of broad covered fhelf which is heatedby the flame in its paffage round it, but to amuch lefs degree of intenfity than the pots, andferves as a receptacle for the glafs as foon aswrought, in which it may cool llowly and gra-dually. This is the annealing oven.

Very great care is required in building aglafs-houfe to form the bricks of an earth whichcombines in the higheft degree the qualities ofdenfity and infufibility, fo as to enable them towithftand the unceafing action of very ftrongheat for a great length of time, for the fires arefeldom fuffered to go out from the time thefurnace begins to be in adtion, till the inevitablewear and fufion of the walls renders a repairneceflary, which may be in about two or threeyears. But ftill greater attention is requiredin feledting proper materials for the glafs-pots,as tliefe have to withftand for a given time notonly the conftant adtion of a very fierce fire,but alfo the folvent power of the glafs itfelfwith the variety of powerful fluxes in full vitri-fication. The pots therefore, being made ofearth, mull neceffarily be themfelves alwaysgradually diflfolving in their contents, and hence,befides the property of difficult fufibility, anearth of confiderable purity is required for thefe,fo that a fmall admixture with the glafs may notinjure its quality.

The chief material both of the walls of thefurnace and of the pots is clay, as it is of al-moft every veflel and fubftance deftined to beara long and violent heat. This is mixed withfand, in different proportions according to itsovality, for the fire-bricks and other parts of thefurnace. On an average a fine ftiff clay willrequire about two-thirds as much of fand tobring it to fuch a confidence that it will workeafily, dry into a very compadt mafs, and refillthe impreffion of fire for a length of time. Stillhowever, this addition of fand renders it infome degree fufible, fo that when the fire is inudlivity, drops of vitrified earth are conftantlyand llowly falling from the walls, fome ofwhich cannot be prevented from dropping intothe glafs and mixing with it. The crucibles

are made of a ftill more refradlory mixture,which confifts Amply of raw and of burnt clay,the latter is called technically cement, and isprocured from the remains of the former fur-naces when pulled down for reparation. Asclay lofes its plafticity by baking, it then an-fwers all the purpofes of fand in diminiffiingthe tenacity of the mafs, and efpecially in lef-fening the fhrinkage whilft drying, and beingthe fame kind of earth as the unbaked, it addsnothing to the vitrefcibility of the material,for as has been already (hewn, earthy com-pounds are, ceteris paribus, fufible in proportionto the number of ingredients of which they arecompofed.

The particular manipulations employed inconftrucling a glafs-furnace and pots are fo-reign to the prefent purpofe: very great pre-caution is ufed in drying them thoroughly andvery gradually. This is peculiarly neceflary inmaking the pots, for as they are intended tohold a confiderable weight of glafs, and to lallmany months, they muft be made very thickand ftrong, and therefore would readily crackwithout much care. "When finiffied they arefirft kept in a warm fheltered room for manyweeks to diffipate much of their moifture, andthe fmall fiffures formed by the unequal fhrink-ing of the clay are clofed up by beating gentlywith a mallet. They are afterwards heatedextremely gradually in a fmall oven made forthis purpofe, and are llowly brought to a redheat and kept there the requifite time, afterwhich they are removed, whilft ftill hot, tothe furnace and foldered down to their place byfire clay. A ftill further fhrinkage takes placewhen in the furnace, for which reafon they arelet to ftand empty for a day or two before theyare fitted to receive the materials for glafs.Thefe pots laft on an average about a year, andhence they muft be changed once or twiceduring the continuance of the furnace itfelf.

The fuel ufed in this country is conftantlycoal, and the beft is of the kind that gives aftrong fteady blaze. As the glafs-pots openonly outward none of the fuel or flame comesin contact with their contents, except throughany accidental crack in the clay foldering.

Confiderable variety prevails refpedting theexadt form and conftrudtion of the furnace andcrucibles, in different countries, and alfo ac-cording to the kind of manufacture, and in par-ticular the plate-glafs furnace requires a differentarrangement.

Of the Fufton and ’wording of Glafs.

The fand, alkali, and other materials for the