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very important advantages, and the quantity ofalkali contained and that required to bring thewhole to a workable ftate is fo frnall as proba-bly to enable this glafs to refill all corrofiveliquors, at leaft as well as any other known kindof glafs.

Plate glafs is that moll beautiful and perfectglafs of which all the kinds of mirrors andlooking-glaffes are compofed, and of which thelarger articles are wrought, not by blowing asevery other glafs article is, but by cajling themelted glafs on a flat table.

The Venetians were long in the foie pof-feflion of the art of making mirror-glafs, butby them it was only manufactured by blowing,nearly in the way defcribed under crown orwindow glafs, and much of the common mirrorglafs is Hill prepared in this way.

In 1665 under the miniltry of the great Col-bert a company for bloiun-mirror glafs was firfteftabliflied near Cherbourg in Normandy , onthe plan of the Venetian manufacture, but thebeautiful art of calling glafs was invented inFrance about 1688, by one Abraham Thevart,and a company was foon ellablilhed for thisbranch of manufacture which was firft carriedon at Paris, and foon after removed to St. Gobin,where it ftill exifts in full activity, and undi-minilhed reputation.

Theplate-glafs cannot be made by blowing ofa larger fize than about fifty inches to be per-fect, but by the ingenious fubftitution of call-ing it may be made fo large and at the fametime fo perfeCt, that fcarcely any other limitscan be fet to the polfible fize of thefe plates thanthe heavy expence of the manufacture, in allits parts. As large plates as nine feet long,and wide in proportion, have been manufac-tured.

As this art of calling glafs requires morecare in the choice of materials and nicety inthe procefies of manufacture than moll otherbranches of glafs-making, fome more detailedaccount may be given of it as carried on inFrance , as far as it can be underftood withoutreference to plates.

The materials of the fineft plate glafs arewhite fand, foda, and lime, to which are addedmanganefe and zaffre, or any other oxyd ofcobalt for particular colouring purpofes, whichwill be prefently defcribed. The fand is ofthe fineft and whiteft kind, and is previoullypalled through a wire fieve of moderate clofe-nefs into water where it is well ftirred aboutand walhed till all dirt and impurity is got rid©f. The lharpeft grained fand is preferred,

and indeed it is found that the grains of mo-derate fize melt with the alkali fooner thaneither the very fine dull or the larger frag-ments, in the former cafe the fand clotting to-gether without mixing freely with the alkali, andin the latter the grains taking a longer time todiflolve on account of their bulk. The alkaliufed here is always foda, and there feems goodreafon to prefer this to potalh, as glaffes madewith foda are found to be fofter and to flowthinner when hot, and yet to be equally durablewhen cold, and in calling a large plate of whichthe perfection is to be without ftreak or bubbleit is obvious of how much importance it muft beto have it flow extremely liquid from the pot.Befides, the neutral falts with the balls of fodawhich conftitute the glafs-gall in this inftance,fuch as the muriat and fulphat of foda, appearto be diflipated more readily by the fire than thecorrefponding falts of potalh. The foda ufedhere is confiderably pure, or fuch as is feparatedfrom the rough allies of barilla, and other fodaplants by lixiviation.

Lime is of confiderable ufe here and addsmuch to the fufibility of the other materials,fupplying in this refpeCl the ufe of litharge inthe flint-glafs. Too much lime however im-pairs the colour and folidity of the glafs.About ■jV of the whole is as much lime as canbe ufed with propriety, and fome ufe as littleas ‘2'V*

The colouring or rather decolouring fub-ftances ufed are azure, or cobalt blue, andmanganefe. The latter is here in the ftate inwhich its eftefit is that of giving a flight redtinge, which mixes with the blue of the cobaltand the natural flight yellow of the other ma-terials, and altogether are found when properlyproportioned to neutralize each other fo thatfcarcely any definable tint remains.

Befides thefe ingredients there is always agreat quantity of fragments of glafs arifingfrom what is fpilt in the catling and the endscut off in fhaping the plates, which are madefriable by quenching in water when hot andufed in this ftate along with the frefli materials.

With regard to the proportions of the ingre-dients very confiderable latitude may be al-lowed. The quantity of foda is a good dealmore than neceffary merely to produce a goodglafs, as one of great fufibility is required. Thefollowing dofes are found to produce a veryfine glafs : 300 lbs. of fand ; 200 lbs. of foda;30 lbs. of lime; 32 ounces of manganefe;3 ounces of azure ; and 300 lbs. of fragments ofglafs.

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