Christopher Merret, our earliest English writer on glass, sets down the properties of the material under twenty-six heads, ‘by which we may easily differentiate it from all other bodies.’ From these I will select some four or five which will be sufficient for our purpose. Thus, of glass, he says: ‘’Tis a concrete of salt and sand or stones. ’Tis artificial. It melts in a strong fire. When melted ’tis tenacious and sticks together.... When melted it cleaves to iron, etc. ’Tis ductile whilst red-hot, and fashionable into any form, but not malleable, and it may be blown into a hollowness’ (_Art of Glass_, 1662). Here we have briefly expressed the real _differentiæ_ of glass. It is rather by these properties than by any virtue of transparency or of definite chemical composition that glass is to be distinguished from all other bodies; and it is only by duly taking advantage of these properties that the preparation of a vessel of glass is rendered possible. In passing from a liquid to a solid state there intervenes a viscous stage when the glass may be gathered at the end of an iron rod; the ductile, tenacious mass may now be drawn out into long threads, whose length and fineness are only limited by the difficulty of maintaining the requisite temperature. Again, if the rod upon which the mass is gathered is hollow, the glass may be blown out into a vesicle or bulb, the starting-point from which an endless variety of objects, bottles, cups, tubes, or even flat sheets of glass, may be subsequently formed. Until advantage was taken of this remarkable property of glass—its capability, I mean, of being blown out into a hollow vesicle when in a viscid condition—the art of the glass-maker was in a primitive stage. We may compare the glass prepared without the aid of the blowing-tube—that of the ancient Egyptians, for instance—to the pottery made by hand before the invention of the potter’s wheel. In dealing with the practical side of our subject—the materials from which glass is made, how these materials are first fritted and then fused together, and how the fused mass is subsequently dealt with—the best plan will be to approach the questions in each case from the point of view of the time and country. But as, on the one hand, for classical times, our sources of information for these practical details are but scanty, and as, on the other, I am not concerned with the industrial developments of the nineteenth century, it will be well to postpone any fuller treatment of such matters until I come to speak of the glass of late Mediæval and Renaissance times. I shall then be able to make use of contemporary accounts which will throw light on the processes of manufacture. A few preliminary notes on the chemical and physical properties of glass may, however, not be out of place.