17th.—The more I learn from my uncle’s kind geological conversations, the more I see the necessity of acquiring some knowledge of mineralogy, in order to understand them. In the mean time, Caroline and I find even the general views he gives us so interesting, that we seldom miss an opportunity of leading him to the subject. This morning he told us, that the debris of the hills which accumulate in alluvial districts usually continue in the loose form of gravel or sand, or mud, or clay, in which they were deposited. “Their visible transformation,” he said, “into stone is of rare occurrence; in some circumstances, however, especially on the sea coast, we may perceive the consolidation of the sand and gravel into thin strata. If a stream, impregnated with oxide of iron, should empty itself on the beach, it acts as a cement, and the process goes on rapidly. The northern coast of Cornwall affords some examples of this sort of petrification at home; and abroad it may be seen on a much larger scale on the shores of Greece, Karamania, Sicily, and the West Indies. Abundance of sea shells and other organic remains are found in it; and at Guadaloupe a human skeleton was discovered in the beach, imbedded in a mass of that description.
“Some springs of water are so highly loaded with calcareous particles, that the sediment they deposit soon hardens into stone; and the stalactites which I shewed you are formed in a similar manner, in the caverns and fissures of all limestone countries. Those were very small specimens, but in some places, for instance in the celebrated grotto of Antiparos, one of the Greek islands, they are found of enormous magnitude, forming rows and clusters of columns, that reach from the top to the bottom of that great cavern. The water in slowly dripping through the rock becomes saturated with lime; as the drops exude from the crevices, or trickle down the stalactites already formed, they are exposed to the air; the watery part then evaporates, and the lime forms a hard stony crust; in some cases assuming the shape of small crystals.”
When we reached home, my uncle obligingly laid M. De Choiseul Gouffier’s voyage on the table for us; and we all read with astonishment his description of that wonderful cavern, which is a thousand feet long, and full of these curious productions. The stalagmites that grow upwards from the floor, are equally curious. My uncle explained to us, that when the quantity of water that trickles through the roof is more than can be evaporated from the surface of the stalactite, the remainder falls on the floor, where the same process occurs; and thus the upper and lower concretions proceed till they meet each other and form an entire column. In the middle of the widest part of the cavern there is a stalagmite of twenty feet in diameter and twenty-four in height; and on this superb natural altar, another French nobleman had mass celebrated by his chaplain to more than five hundred people who surrounded it. The cavern was lighted by a hundred large torches and four hundred lamps; and the splendour of this illumination, reflected by the concretions which hung from the roof, or which lined the sides, is described as producing a very magnificent effect.
18th.—It will not be my uncle’s fault if I do not pick up some information in this delightful house, for every day he tells us something new. He has just been describing the method of casting plate glass; and I hope some day to see the whole operation myself.
The furnace for melting the materials is about eighteen feet long, and it is surrounded by ovens for annealing the plates of glass when made, that is, for cooling them slowly. The pots in which the materials are melted, are made of a sort of tough clay that is found at Stourbridge, in Worcestershire, as it has the property of standing the most intense heat; and they contain about twenty hundred weight of melted glass, or metal, as it is called by the workmen. The cuvettes, or cisterns, which convey the liquid glass to the casting table, are made of the same clay.
When the metal is sufficiently fluid, refined, and settled, which happens in about thirty-six hours, it is put, by means of ladles, into the cisterns, which are left in the furnace about six hours longer, till the little bubbles formed by this disturbance of the glass have all disappeared. The door of the furnace is now opened, and by a chain the cistern is drawn out upon an iron carriage, and conducted to the casting table. Here it is raised, by means of a crane, against two iron bars, which are so contrived as to incline the cistern, and empty the fiery torrent on the table.
This table is covered with a thick copper plate made very smooth on the surface; and it is supported on wheels, so that it can be moved from one annealing furnace to another. To regulate the thickness of the glass, two iron rulers are placed along the table, and on these rest the extremes of a very heavy roller, or cylinder of copper, which, as it moves along, drives the superfluous matter before it, and renders the two faces of the glass parallel. The iron rulers being moveable, serve also to determine the width of the glass plate, and to prevent the matter from running over the sides; the waste metal falls into a trough of water at the end of the table, and is reserved for the next melting.
As soon as the glass has cooled to a proper consistence it is examined; and if any bubbles or flaws are found, it is broken up and returned to the melting pot: but if it has a sound appearance, the table is rolled to the mouth of the annealing furnace, and the plate is carefully deposited there. The heat of this furnace is at first very great, but it is diminished every day for a fortnight, by which time the glass is sufficiently annealed. This process renders the glass less brittle; for, if suddenly cooled, my uncle says, it would fly into pieces when touched.
19th.—Much as we were all interested by the manufacture of plate glass, my uncle steadily refused to carry us any further yesterday than the annealing furnace: this evening, therefore, as soon as we were comfortably collected round the fire, after dinner, we reminded him that he was to describe both the grinding and polishing operations; and the following is the substance of what he said.
The annealing furnace generally contains six plates of glass; when they are withdrawn, they are cut square by a large diamond, which moves in a wooden frame, and they are then carried to the grinding room. There each plate is laid on a table, covered with a large slate or flag; and to keep the glass steady it is bedded on the slate in wet plaster of Paris, which you know has the property of setting, or becoming hard, in a few minutes. A smaller plate of glass is then laid on the larger one, and being properly loaded and drawn forwards and backwards, with a constant supply of fine sharp sand and water, the two glasses grind each other to a smooth even surface. A ledge round the lower glass prevents the sand and water from running off; and the upper or moveable glass has a strong plank cemented to it on which the weights are laid. An upright pin is fixed to this plank, to which a handle, like a coach wheel, is attached for the workmen to give motion to the glass, and much skill is required to vary this motion in every possible direction; for if they were frequently to repeat the same stroke, the glasses would grind each other into furrows. But no matter what pains are taken to vary this motion, the two surfaces have always a tendency to become slightly spherical, one convex and the other concave; and to prevent this, the upper glasses of the different grinding tables are occasionally changed, so that two convex or two concave plates mutually correct each other.