With regard to the hardness of glass, Merret mentions as the thirteenth property possessed by that substance, ‘that it only receives sculpture or cutting from a _Diamond_ or _Emery_ stone.’ But such a statement would be likely to give an exaggerated idea of the hardness of glass. If we take the scale of hardness used by the mineralogist, it will be found that there are few kinds of glass that do not fall between the fifth and sixth divisions of that scale. In other words, it would be difficult to find a specimen of glass on which a crystal of apatite (phosphate of lime) would make any impression, whereas all glass in ordinary use is readily scratched by felspar. It is possible, however, that some kinds of Bohemian glass may equal the latter mineral in hardness; it is indeed a common statement that certain Bohemian or German ‘combustion-tubes’ will strike fire with steel. On the other hand, the presence of lead tends to make a soft glass; our cut flint is perceptibly softer than common window-glass, and perhaps the most important defect of the paste used to imitate precious stones—such paste may contain as much as 50 per cent. of lead oxide—is to be found in its comparative softness. At the same time, the greater the amount of lead in a glass, the greater its dispersive power on the light that passes through it. Hence the brilliancy and fire of flint-glass, and still more of artificial gems. Apart from the varieties containing lead, samples of glass differ little in weight; the specific gravity may range between 2·4 and 2·8. That of flint-glass, on the other hand, varies from 3 to 3·8; indeed in some optical glasses containing a large percentage of lead, and again in the paste used for false jewellery, the specific gravity may be as high as 4·5 or even 5. The high melting-point, or more definitely the high softening-point, of certain kinds of Bohemian and German glass, makes them invaluable in the laboratory of the chemist. On the other hand, the ready fusibility of glass containing lead was, as we shall see, one of the causes that promoted the adoption of such a glass in our furnaces. Thus we find that the potash-lime glass of Bohemia, containing a high percentage of silica, excels in hardness and resistance to heat; on the other hand, the various kinds of glass containing lead are soft and easily fusible, and at the same time they combine a high specific gravity with a wide dispersive power. What we may call the maritime or soda-lime glass takes an intermediate place in all these respects. This is indeed an additional reason for regarding this great family of ‘Mediterranean’ glass as the normal type. The two essential elements, then, required by the glass-maker are, in the first place, silica, and secondly an alkali, in each case as pure as possible, and in a convenient form for mixing and fusing together. I do not propose here to do more than indicate the source of these materials.