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Bedfordshire. A clayey stone called soap rock has exactly the feel and look of soap, and will even lather with water. The different kinds of slate, too, are stones of the argillaceous class; and very useful ones for covering houses, and other purposes.
H. Are writing slates like the slates used for covering houses ?
T. Yes; but their superior blackness and smoothness make them show better the marks of the pencil.
G. You have mentioned something of sand and flints, but you have not told us what sort of earths they are.
T. I reserved that till I spoke of the third great class of earths. This is the siliceous class, so named from silex, which is Latin for a flint-stone. They have also been called vitrifiable earths, because they are the principal ingredient in glass, named in Latin vitrum.
G. I have heard of flint glass.
T. Yes—but neither flint nor any other of the kind will make glass, even by the strongest heat, without some addition; but this we will speak of by and by. I shall now tell you the principal properties of these earths. They are all very hard, and will strike fire with steel, when in a mass large enough for the stroke. They mostly run into particular shapes, with sharp angles and points, and have a certain degree of transparency; which has made them also be called crystalline earths. They do not in the least soften with water, like clays; nor are they affected by acids, nor do they burn to lime, like the calcareous earths. As to the different kinds of them, flint has already been mentioned. It is a very common production in some parts, and is generally met with in pebbles or round lumps. What is called the shingle on the sea-shore chiefly consists of it; and the ploughed fields in some places are almost entirely covered with flint stones.
H. But do they not hinder the corn from growing ?
T. The corn, to be sure, cannot take root upon them; but I believe it has been found that the protection they afford to the young plants which grow under them, is more than equal to the harm they do by taking up room. Flints are also frequently found imbedded in chalk under the ground. Those used in the Staffordshire potteries chiefly come from the chalk-pits near Gravesend. So much for flints. You have seen white pebbles, which are semi-transparent, and when broken resemble white sugar-candy. They are common on the sea-shore, and in beds of rivers.
H. Oh, yes. We call them fire-stones. When they are rubbed together in the dark they send out great flashes of light, and have a particular smell.
T. True. The proper name of these is quartz. It is found in large quantities in the earth, and the ores of metals are often imbedded in it. Sometimes it is perfectly transparent, and then it is called crystal. Some of these crystals shoot into exact mathematical figures; and because many salts do the same, and are also transparent, they are called the crystals of such or such a salt.
G. Is not fine glass called crystal, too?
T. It is called so by way of simile: thus we say of a thing, “it is as clear as crystal.” But the only true crystal is an earth of the kind I have been describing. Well—now we come to sand ; for this is properly only quartz in a powdery state. If you examine the grains of sand singly, or look at them with a magnifying glass, you will find them all either entirely or partly transparent"; and in some of the white shining sands the grains are all little bright crystals.
H. But most sand is brown or yellowish.
T. That is owing to some mixture generally of the metallic kind. I believe I once told you that all sands were supposed to contain a small portion of gold. It is more certain that many of them contain iron.
G. But what could have brought this quartz and crystal into powder, so as to have produced all the sand in the world ?
T. That is not very easy to determine. On the sea-shore, however, the incessant rolling of the pebbles by the waves is enough in time to grind them to powder; and there is reason to believe that the greatest part of what is now dry land, was once sea, which may account for the vast beds of sand met with inland.
G. I have seen some stone so soft that one might crumble it between one's fingers, and then it seemed to turn to sand.
T. There are several of this kind, more or less solid, which are chiefly composed of sand conglutinated by some natural cement. Such are called sandstone, or freestone, and are used for various purposes, in building, making grindstones, and the like, according to their hardness.
H. Pray, what are the common pebbles that the streets are paved with ? I am sure they strike fire enough with the horses' shoes.
T. They are stones of the siliceous * kind, either pure or mixed with other earths. One of the hardest and best for this purpose is called granite, which is of various kinds and colours, but always consists of grains of different siliceous earths cemented together. The streets of London are paved with granite, brought from Scotland. In some other stones, these bits of different
Siliceous, that is, flinty.
earths dispersed through the cement are so large, as to look like plums in a pudding; whence they have obtained the name of pudding-stones.
G. I think there is a kind of stones that you have not yet mentioned-precious stones.
T. These, too, are all of the siliceous class ;—from the opaque, or half-transparent, as agate, jasper, cornelian, and the like, to the perfectly clear and brilliant ones, as ruby, emerald, topaz, sapphire, &c.
G. Diamond, no doubt, is one of them.
T. So it has commonly been reckoned, and the purest of all; but some late experiments have shown, that though it is the hardest body in nature, it may be totally dispersed into smoke and flame 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 above mentioned owe their different 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.
G. I suppose they must be very rare.
T. 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 worth of a glittering stone as big as a hazel-nut, and a magnificent house and gardens, or a large tract of country, covered with noble woods and rich meadows and corn-fields ? 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.
G. Will you please to tell us now how glass is made ?
T. 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 burned 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 something 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, pearl-ash, barilla, and kelp. The salt is mixed with the sand in a certain proportion, and the mixture then exposed in earthen pots to violent heat, till it is thoroughly melted. The mass is then taken white hot and fluid, in such quantities as are wanted, and fashioned by blowing and the use of sheers and other instruments. You must see this done some day, 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.
H. I should like very much to see it, indeed.
T. 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 calx 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 calxes 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 glass-making, one must have a thorough knowledge of chemistry.
G. I think the manufacture of glass is one of the finest inventions of human skill.
T. It is perhaps not of that capital importance that some other arts possess; but it has been a great addition to the comfort and pleasure of life in many ways. Nothing makes such clean and agreeable vessels as glass, which has the quality of not being corroded by any kind of liquor, as well as that of showing its contents by its transparency. Hence it is greatly preferable to the most precious metals for drinking out of; and for the same reasons it is preferred to every other material for chemical utensils, where the heat to be employed is not strong enough to melt it.
H. Then, glass windows!
T. Ay; that is a very material comfort in a climate like ours, where we so often wish to let in the light, and keep out the cold, wind, and rain. What could be more gloomy than to sit in the dark, or with no other light than came in through small holes covered with oiled paper or bladder, unable to see anything passing without doors ! Yet this must have been the case with the most sumptuous palaces before the invention of window-glass, a good deal later than that of bottles and drinking-glasses.
H. I think looking-glasses are very
beautiful. T. They are, indeed, very elegant pieces of furniture, and very costly too. The art of casting glass into large plates, big enough to reach almost from the bottom to the top of a room, is but lately introduced into this country from France. But the most splendid and brilliant manner of employing glass is in lustres and chandeliers, hung round with drops cut so as to reflect the light with all the colours of the rainbow. Some of the shops in London, filled with these articles, appear to realise all the wonders of an enchanted palace in the “ Arabian Nights' Entertainments.”
G. But are not spectacles and spying-glasses more useful than all these ?
T. I did not mean to pass them over, I assure you. By the curious invention of optical glasses of various kinds, not only the natural defects of the sight have been remedied, and old age has been in some measure lightened of one of its calamities, but the sense of seeing has been wonderfully extended. The telescope has brought distant objects within our view, while the microscope has given us a clear survey of small objects too minute for our unassisted eyes. By means of both, some of the brightest discoveries of the moderns have been made; so that glass has prc ved not less admirable in promoting science, than in contributing to splendour and convenience. Well,
I don't know that I have anything more at present to say relative to the class of earths. We have gone through the principal circumstances belonging to their three great divisions, the calcareous, argillaceous, and siliceous. You will remember, however, that most of the earths and stones offered by nature are not in any one of these kinds perfectly pure, but contain a mixture of one or both the others. There is not a pebble that you can pick up which would not exercise the skill of a mineralogist fully to ascertain its properties, and the materials of its composition. So inexhaustible is nature !- From ‘Evenings at Home.'
THERE are two species of the elephant—the Indian elephant, and the African elephant. The elephant of both kinds is a huge mass,