GLASS of the operation is judged of by the workmen dipping iron rods from time to time into the mixture and examining the appearance of the drops withdrawn. A nearly homogeneous pro- duct, which becomes transparent on cooling, indicates that the most refractory ingredients have been all dissolved. Their mixture has been facilitated by the continual disengage- ment of carbonic acid gas, which in its escape caused the whole to be thrown into ebullition. Some of the gas remains in the mass, render- ing it spongy and full of vesicles. Unless in the manufacture of the finer qualities of glass, for which the purest materials are employed, there is also a scum, called glass gall or san- diver, floating upon the surface, consisting of the insoluble matters, and the sulphates of soda and lime not taken up by the mixture. This is removed by ladling, and the "metal" is next fined, which is done by increasing the heat to the highest degree, and keeping the contents of the pots in a state of perfect fluidity from 10 to 30 hours ; in this time the bubbles disappear and the insoluble matters settle to the bottom. The furnace is then allowed to cool until the metal has become viscid, so that it may be taken out and worked; and it is afterward kept at sufficiently high temperature to maintain the glass in this condition, that it may be used as required. The arrangements of the great circular glass furnaces, with their central fire surrounded with eight to twelve pots, each reached by its own arch under the general dome, admit of enough material being melted at once to employ all hands the first four working days of the week, the men work- ing day and night in six-hour shifts. The ma- terials of the furnaces and pots, in order that they may withstand the excessive heat and the action of the various melted ingredients, must be carefully selected from the most refractory substances, and the work must be most skil- fully executed. The construction of the great melting pots is an object of special solicitude, and the placing of a new one in the furnace while this is in operation is a task of no little apparent difficulty and danger. In England Fw. 8. Melting Pots. they are made of the best Stourbridge fire clay, mixed with about one fifth part of ground pot- sherds. The work is done entirely by hand, no machinery having yet been invented for that purpose. An average-sized pot is about 4 ft. high, 4 ft. in diameter at top, and somewhat smaller at the bottom, and will contain about 25 cwt. of melted glass. The average duration of a pot in the furnace is about eight weeks. In the case of window and ordinary bottle glass, the pot is a plain round vessel open at the top ; but in melting flint glass, it being necessary to protect the metal from all external impurities, the top of the pot is made in the form of an arch or hood, with a small opening on one side near the top, which corresponds with the nose hole of the furnace, and from which the workman withdraws the melted glass. Ordinarily two kinds of furnaces are used in addition to the annealing oven, one for melting the glass, and the other for reheat- ing it at different stages during the process of manufacture. One of the most important im- provements in the manufacture of glass has been the adoption of the Siemens regenerating gas furnace. (See FURNACE.) The novelty of this system consists in taking up the waste heat from the furnace in large chambers, and using it for raising to a higher temperature the ele- ments of combustion. The whole of the fuel, except the inorganic portions, is converted into gas, not in the furnace itself, but in adjacent " producers." The gas and air passing through separate chambers, and having each been heated to a high degree in the waste-heat chambers, meet on entering the furnace, and there ignite, producing a heat of wonderful intensity. The advantages of this system are a greater inten- sity of heat produced from less fuel, and, what is very important in the manufacture of glass, a degree of cleanliness which cannot be at- tained by the older methods of melting. .The intensity of the heat produced is indicated by the fact that in a sheet-glass furnace contain- ing 1,800 cubic feet, materials for about 16 tons of glass in eight large pots are melted and refined into a liquid mass in 25 hours. Such is a mere outline of the means employed to bring the materials of glass into their desired combination. The production of each kind of glass is a separate branch of manufacture, in- volving many curious details and processes, too numerous even to be named in this account. The tools employed are few and simple, and differ but little from those described in the work of Blancourt " On the Art of Glass," published in London in 1699. The first in im- portance is the pipe or blowing tube, made of wrought iron, 4 or 5 ft. long, with a bore from ^ to 1 in. in diameter, a little larger at the mouth end than at the other. It is a long hand, partly covered with wood, with which, the end being heated red hot, the workman reaches into the pot of melted matter and gathers up the quantity he requires, and which afterward holds- the article in the manipulations to which he subjects it; and it is at the same time the air tube through which the breath is forced to expand the vessel, or through which water is sometimes blown to produce the same effect by the steam it generates. A solid rod of iron, called a punty or pontil, serves to receive the
