Geo. Diamond, no doubt, is one of them.
Har. So it has commonly been reckoned, and the purest of all; but late experiments have shown, that though it is the hardest body in nature it may be totally dispersed into vapour by a strong fire, so that mineralogists will now hardly allow it to be a stone at all, but class it among inflammable substances. The precious stones abovementioned owe their colours chiefly to some metallic mixture. They are in general extremely hard, so as to cut glass, and one another; but diamonds will cut all the rest.
Geo. But are they not exceedingly rare?
Tut. Yes; and in this rarity consists the greatest part of their value. They are, indeed, beautiful objects; but the figure they make in proportion to their expense is so very small, that their high price may be reckoned one of the principal follies among mankind. What proportion can there possibly be between the real worth of a glittering stone as big as a hazelnut, and a magnificent house and gardens, or a large tract of country covered with noble woods and rich meadows and cornfields? And as to the mere glitter, a large lustre of cut glass has an infinitely greater effect on the eye than all the jewels of a foreign prince.
Geo. Will you please to tell us how glass is made?
Tut. Willingly. The base of it is, as I said before, some earth of the siliceous class. Those commonly used are flint and sand. Flint is first burnt or calcined, which makes it quite white, like enamel; and it is then powdered. This is the material sometimes used for some very white glasses; but sand is that commonly preferred, as being already in a powdery form. The white crystalline sands are used for fine glass; the brown or yellow for the common sort. As these earths will not melt of themselves, the addition in making glass is somewhat that promotes their fusion. Various things will do this; but what is generally used is an alkaline salt, obtained from the ashes of burnt vegetables. Of this there are several kinds, as potash, pearlash, barilla, and kelp. The salt is mixed with the sand in a certain proportion, and the mixture then exposed in earthen pots to a violent heat, till it is thoroughly melted. The mass is then cooled till it is nearly of the consistence of dough, and in this state it is fashioned by blowing and the use of shears and other instruments. You must see this done some time, for it is one of the most curious and pleasing of all manufactures; and it is not possible to form an idea of the ease and dexterity with which glass is wrought, without an actual view.
Har. I should like very much to see it, indeed.
Geo. Where is glass made in this country?
Tut. In many places. Some of the finest in London, but the coarser kinds generally where coals are cheap; as at Newcastle and its neighbourhood, in Lancashire, at Stourbridge, Bristol, and in South Wales. I should have told you, however, that in our finest and most brilliant glass, a quantity of the oxide of lead is put, which vitrifies with the other ingredients, and gives the glass more firmness and density. The blue, yellow, and red glasses are coloured with the oxides of other metals. As to the common green glass, it is made with an alkali that has a good deal of calcareous earth remaining with the ashes of the plant. But to understand all the different circumstances of glassmaking, one must have a thorough knowledge of chymistry.
Geo. I think making of glass is one of the finest inventions of human skill.