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FLUXES. THE TERM EXPLAINED.

plant kali. The sand composed the body or substance of the glass-the use of the kali was to make the sand melt more readily.

Substances that are added to minerals to assist the power of fire in melting them, are called fluxes, and for a very plain reason. When any substance is melted, you know it becomes liquid, it runs, or flows. We have borrowed our word flux from the Latin word fluxus, which expresses the act of flowing. I dare say you have often observed, in ancient maps, the letters Flu. printed after the names of rivers; the Latin word for river is flumen: a river flows along between its banks. Our English word fluid has the same origin; we apply it to liquids, to substances that will flow, and spread themselves abroad, unless confined by the resistance of some solid body. The water in this tumbler would flow over the table, if it were not confined by the glass.

Several mineral substances that would be very difficult, perhaps impossible, to melt, if they were exposed alone to the action of fire, can be

melted with ease, when they are mixed in due proportion with something else. That something which causes an infusible mineral to melt, is called its flux; it enables heat to dissolve the dry, hard particles, and cause them to flow.

After this explanation, I think you will find no difficulty in understanding, that when we speak of the body, or substance, of any kind of glass, we mean the sand or stones employed in making it; and when we speak of the flux, we mean the material that assists in melting the sand, or whatever may be substituted for sand.

This tumbler is a specimen of flint-glass; it is called by that name, because the body of such glass was originally made of ground flints. The Venetians also employed a sort of pebble, resembling white marble, which they found in the river Tesino. Such stones make a beautiful, transparent glass, but the preparation of them for use is troublesome and expensive. Sand is now preferred for that purpose; it saves a great deal of trouble to the manufacturer, and is, in fact, rock reduced to powder by the gradual

30

FLUXES EMPLOYED.

operations of Nature. As the rocks of the ancient earth consisted of different kinds of stone, so the ruins of those rocks appear, at the present day, in various species of sand. For making the best glass, they choose sand which is fine, white, and shining; such sand, when examined through a microscope, appears like small fragments of rock-crystal. The English glass-houses are supplied with it from Lynn, in Norfolk, from Maidstone, in Kent, and from Alum Bay, in the Isle of Wight; it composes the body of our finest glass.

Now, for the flux to melt this sand, our manufacturers are not content with potash, (or salt obtained from the ashes of vegetables,) because glasses made of sand and salt only are too brittle, too easily broken; this defect, is remedied by adding to the flux a portion of red-lead : lead you know is a substance which is very heavy, melts easily, and bends easily. The consequence of this addition to the mixture is, that the lead imparts some of its own properties to the glass, which will be more easily melted,

heavier, less brittle, and not so white as if it had been made of sand and pearlash only. The purer the substances employed, the more transparent and beautiful the glass will be.

We come now to the glass used for windows. The best kind is called crown-glass; it is made of fine sand and kelp, or the salt obtained from the ashes of sea-weed.* No lead, or other metal, is used as a flux in making it. Crownglass is therefore much harder than flint-glass, and it would be more difficult to fashion into different shapes; but this is of no consequence, as it is designed especially for windows, which require only a plain surface.

In the first melting, transparent glass is not produced; but a greyish-white, tough mass, which is cut into brick-shaped pieces, and, when cold, piled up for future operations. After being melted a second time, and perhaps adding a portion of common salt to increase the transparency, the glass is fit for blowing.+ You may form some idea of this operation, if ever

you have seen soap bubbles blown by means of a tobacco-pipe. The glass-blower uses an iron tube, by repeatedly dipping it, he takes up from the mass of melted glass, as much as he supposes will be sufficient to form a sheet of glass of the usual size, which generally weighs ten or eleven pounds. He then rolls the lump sticking to the end of his tube, on an iron table, till it is of a roundish form; he now begins to blow, and the lump dilates into the form of a pear; it is then heated again, and a second blowing makes it swell to a still larger size, and rounder form; a third time it is heated and blown, which makes the bubble yet larger and thinner in substance. The side opposite the tube is now flattened by pressure against a table, or other smooth surface, and it is then ready to be disengaged from the tube used in blowing. I will endeavour to explain how this is done.

An assistant takes a solid iron rod, smaller and lighter than the tube used for blowing; he collects a small portion of melted glass on the end of this rod, and applies it to the centre of