SEPARATION FROM THE BLOW-PIPE. 33

the flattened side of the glass bubble; there it sticks fast, so that the bubble is held between the tube on one side, and the rod on the other. A small piece of iron, wetted with cold water, is then drawn round that part of the glass which is connected with the tube, and the glass cracks in the circle traced by the cold iron. The workman gives a smart blow to his tube, the circular crack separates at once, and the glass is left attached to the solid iron rod on the flattened side, and having a round hole opposite to it on the other.

Now, then, the glass must be heated again, in order that this flattened globe may be converted into a plain surface, like the top of a round table. If I tell you how this is done, you will think it almost like a conjuring trick; and I believe it does appear unaccountable and surprising to every body who sees it for the first time.

The workman begins to twirl the iron rod in his hand, slowly at first, then faster and faster, as a woman twirls a mop. Every thing that is

34

PROCESS OF FLATTENING.

whirled round and round, has a tendency to fly

So it is with

off from the centre of motion. the particles of glass in the bubble; they obey the impulse given them; the glass becomes broader and broader, the round hole larger and larger, till at length it suddenly flies quite open, and the glass bubble is changed into a flat, round plate, measuring rather more than four feet across, and of equal thickness, except in the spot where it is attached to the iron rod ; there is seen the knot or lump of glass by which the rod was fixed to it, this is called the bull'seye; I have procured one from the glazier, and also a piece of the outer part of a table of glass. As they lie before you, you may easily imagine a wide circular plate of glass extending round this bull's-eye as a centre. The rod, like the blowing-pipe, is disengaged by touching the surrounding glass with a cold, wet iron, and the finished plate is put, resting on its edge, to cool gradually in an oven. Twelve such plates make

what glaziers call a crate of glass.*

*Lardner, chap. iv. and v.

BROAD-GLASS.-BOTTLE-GLASS.

35

When different manufactures are carried on in a country, the materials employed in one art, or the refuse of them, may be used in another, and by this means the article is made at a cheaper rate thus the soap-maker and the glass-maker both want soda, and the waste or refuse matter that is left after the soap is boiled, when mixed with sand and kelp, produces an inferior glass for windows, which is called broadglass.

The coarsest kind is the common green bottleglass. It is usually made of soap-boilers' waste and river sand, or of common sand and lime, with a little clay, and salt obtained by evaporating sea-water. When the glass is properly prepared, it is blown into the form of bottles, which are afterwards cooled very gradually in

an oven.

Besides these blown glasses, there is another kind which is used for looking-glasses, for the windows of carriages, and frequently for the windows of houses and shops. It is called plate-glass, and is made of sand, soda, nitre, and quicklime.

Lime is used as the flux, it promotes the melting of the sand and soda.*

It is said, that the idea of casting glass into plates, was suggested by an accident, which happened to a man employed in a glass-house. He was melting some of the material in a crucible, which is a kind of pot or vessel, made to bear the heat of a very strong fire; while engaged in this operation, he either overset the crucible, or spilled a great part of its contents: the melted mass falling on the large flag-stones, with which the place was paved, penetrated the joining of the pavement, and ran under one of the stones; this obliged the workman to take up the stone, in order to recover his glass: to his great surprise he found it lying beneath the stone, in the form of a plate or sheet of glass, such as could not have been produced by the usual method of blowing. He was very much impressed by this fact it set him to thinking so intently, that when night came he could not sleep. He saw at once that much better looking-glasses

Ure, ibid. and Lardner, p. 197.

ADVANTAGES OF CAST PLATES.

37

might be made by casting, if he could only contrive a good method of doing it. He resolved to try, and set about his experiments immediately, and with such good success, that before sunrise the next morning, he proved the possibility of making the great improvement which this fortunate accident had so unexpectedly suggested. This circumstance is said to have taken place towards the end of the fifteenth century.*

It was evident that great advantage would result from casting glass at once in the form of plates, especially when required to be very large a glass bubble can only be blown of a certain size, but there seems no limit to the dimensions of a cast plate, except the expense of the machinery required to produce it. The process is simple enough; something like that pursued in making sheet-lead.

When the glass is melted, it is poured out upon a table, which should be of metal, perfectly level, and furnished with iron ledges of

*Lardner, ibid. p. 138.