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some, but mostly of inferior kinds. What we use chiefly comes from Italy. The Greek islands yield some fine sorts. That of Paros is of ancient fame for whiteness and purity, and the finest antique statues have been made of Parian marble.
H. I suppose black marble will not burn into white lime ?
T. Yes, it will. A violent heat will expel most of the colouring matter of marbles, and make them white. Chalk is another kind of calcareous earth. This is of a much softer consistence than marble; being easily cut with a knife, and marking things on which it is rubbed. It is found in great beds in the earth; and in some parts of England whole hills are composed of it.
G. Are chalk and whiting the same?
T. Whiting is made of the finer and purer particles of chalk washed out from the rest, and then dried in lumps. This, you know, is quite soft and crumbly. There are, besides, a great variety of stones in the earth, harder than chalk, but softer than marble, which will burn to lime, and are therefore called limestones. These differ much in colour and other properties, and accordingly furnish lime of different qualities. In general, the harder the limestone is, the firmer the lime made from it. Whole ridges of mountains in various parts are composed of limestone, and it is found plentifully in most of the hilly counties of England, to the great advantage of the inhabitants.
G. Will not oyster-shells burn into lime? I think I have heard of oyster-shell lime.
T. They will; and this is another source of calcareous earth. The shells of all animals, both land and sea, as oysters, mussels, cockles, crabs, lobsters, snails, and the like, and also egg-shells of all kinds, consist of this earth; and so does coral, which is formed by insects under the sea, and is very abundant in some countries. Vast quantities of shells are often found deep in the earth, in the midst of chalk and limestone beds; whence some have supposed that all calcareous earth is originally an animal production.
H. But where could animals enough ever have lived to make mountains of their shells ?
T. That, indeed, I cannot answer. But there are sufficient proofs that our world must long have existed in a very different state from the present. Well—but, besides these purer calcareous earths, it is very frequently found mingled in different proportions with other earths. Thus marl, which is so much used in manuring land, and of which there are a great many kinds, all consists of calcareous earth, united with clay and sand; and the more of this earth it contains, the richer manure it generally makes.
G. Is there any way of discovering it when it is mixed in this manner with other things ?
T. Yes—there is an easy and sure method of discovering the smallest portion of it. All calcareous earth has the property of dissolving in acids, and effervescing with them; that is, they bubble and hiss when acids are poured upon them. You may readily try this at any time with a piece of chalk or an oyster-shell.
G. I will pour some vinegar upon an oyster-shell as soon as I get home. But now I think of it, I have often done so in eating oysters, and I never observed it to hiss or bubble.
7. Vinegar is not an acid strong enough to act upon a thing so solid as a shell. But aquafortis, or spirit of salt, will do it at once; and persons who examine the nature of fossils always travel with a bottle of one of these acids, by way of a test of calcareous earth. Your vinegar will answer with chalk or whiting. This property of dissolving in acids, and what is called neutralising them, or taking away their sourness, has caused many of the calcareous earths to be used in medicine. You know that sometimes our food turns very sour upon the stomach, and occasions the pain called heartburn, and other uneasy symptoms. In these cases, it is common to give chalk or powdered shells, or other things of this kind, which afford relief by destroying the acid.
G. I suppose, then, magnesia is something of this sort, for I have often seen it given to my little sister, when they said her stomach was out of order ?
T. It is; but it has some peculiar properties which distinguish it from other calcareous earths, and particularly, it will not burn to lime. Magnesia is an artificial production, got from one of the ingredients in sea-water, called the bitter purging salt.
G. Pray, what are the other uses of these earths ?
T. Such of them as are hard stone, as the marbles and many of the limestones, are used for the same purposes as other stones. But their great use is in the form of lime, which is a substance of many curious properties that I will now explain to you. When fresh burnt it is called quick-lime, on account of the heat and life, as it were, which it possesses. Have you ever seen a lump put into water?
G. Yes, I have.
T. Were you not much surprised to see it swell and crack to pieces, with a hissing noise and a great smoke and heat?
G. I was, indeed. But what is the cause of this ?-how can cold water occasion so much heat?
T. I will tell you. The strong heat to which calcareous earth is exposed in making it lime, expels all the water. it contained (for all earths, as well as almost everything else, naturally contain water), and also a quantity of air which was united with it. At the same time it imbibes a good deal of fire, which remains fixed in its substance, even after it has grown cool to the touch. If water be now added to thiş quicklime, it is drunk in again with such rapidity, as to crack and break the lime to pieces. At the same time, most of the fire it had imbibed is driven out again, and makes itself sensible by its effects, burning all the things that it touches, and turning the water to steam. This operation is called slacking of lime. The water in which lime is slacked dissolves a part of it, and acquires a very pungent harsh taste: this is used in medicine under the name of lime-water. If, instead of soaking quicklime in water, it is exposed for some time to the air, it attracts moisture slowly, and by degrees falls to powder, without much heat or disturbance. But whether lime be slacked in water or air, it does not at first return to the state in which it was before, since it still remains deprived of its air; and on that account is still pungent and caustic. At length, however, it recovers this also from the atmosphere, and is then calcareous earth as at first. Now it is upon some of these circumstances that the utility of lime depends. In the first place, its burning and corroding quality makes it useful to the tanner, in looseningall the hair from the hides, and destroying the flesh and fat that adhere to them. And so in various other trades it is used as a great cleanser and purifier,
H. When lime is laid upon the ground, I suppose its use must be to burn up the weeds.
T. True—that is part of its use.
T. Properly objected. But the case is, that the farmer does not sow his seeds till the lime is rendered mild by exposure to the air and weather, and is well mixed with the soil. And even then it is reckoned a hot and forcing manure, chiefly fit for cold and wet lands. The principal use of lime, however, is as an ingredient in mortar. This, you know, is the cement by which bricks and stones are held together in building. It is made of fresh slacked lime and a proportion of sand well mixed together; and generally some chopped hair is put into it. The lime binds with the other ingredients; and in length of time, the mortar, if well made, becomes as hard or harder than the stone itself.
G. I have heard of the mortar in very old buildings being harder and stronger than any made at present.
T. That is only on account of its age. Burning lime and making mortar are as well understood now as ever; but in order to have it excellent, the lime should be of good quality, and used very fresh. Some sorts of lime have the property of making mortar which will harden under water, whence it is much valued for bridges, locks, wharfs, and the like.
G. Pray is not plaster of Paris a kind of lime? I know it will become hard by only mixing water with it, for I have used it to make casts of.
T. The powder you call plaster of Paris is made of an earth named gypsum, of which there are several kinds. Alabaster is a sort of this stone, and hard enough to be used like marble. The gypseous earths are of the calcareous kind, but they have naturally a portion of acid united with them, whence they will not effervesce on having acid poured on them. But they are distinguished by the property, that after being calcined or burned in the fire, and reduced to powder, they will set into a solid body by the addition of water alone. This makes them very useful for ornamental plasters, that are to receive a form or impression, such as the stucco for the ceiling of rooms.
Well—we have said enough about calcareous earths; now to another class, the Argillaceous.
G. I think I know what those are. Argilla is Latin for clay.
T. True; and they are also called clayey earths. In general, these earths are of a soft texture and a sort of greasy feel; but they are peculiarly distinguished by the property of becoming sticky on being tempered with water, so that they may be drawn out, and worked into form like a paste. Have you ever, when you were a little boy, made a clay house?
G. Yes, I have.
T. Then you well know the manner in which clay is tempered, and worked for this purpose.
H. Yes—and I remember helping to make little pots and mugs of clay.
T. Then you imitated the potter's trade; for all utensils of earthenware are made of clays either pure or mixed. This is one of the oldest arts among mankind, and one of the most useful. They furnish materials for building, too; for bricks and tiles are made of these earths. But in order to be fit for these purposes, it is necessary that clay should not only be soft and ductile, while it is forming, but capable of being hardened afterwards. And this it is, by the assistance of fire. Pottery ware and bricks are burned with a strong heat in kilns, by which they acquire a hardness equal to that of the hardest stones.
G. I think I have read of bricks being baked by the sun's heat alone in very hot countries.
T. True; and they may serve for building in climates where rain scarcely ever falls; but heavy showers would wash them away. Fire seems to change the nature of clays; for after they have undergone its operation, they become incapable of returning again to a soft and ductile state. You might steep brick dust or pounded pots in water ever so long without making it hold together in the least.
G. I suppose there are many kinds of clays ?
T. There are. Argillaceous or claylike earths differ greatly from each other in colour, purity, and other qualities. Some are perfectly white, as that of which tobacco-pipes are made. Others are blue, brown, yellow, and, in short, of all hues, which they owe to mixtures of other earths or metals. Those which burn red contain a portion of iron. No clays are found perfectly pure; but they are mixed with more or less of other earths. The common brick clays contain a large proportion of sand, which often makes them crumbly and perishable. In general, the finest earthenware is made of the purest and whitest clays; but other matters are mixed, in order to harden and strengthen them. Thus porcelain, or china, is made with a clayey earth 'mixed with a stone of a vitrifiable nature, that is, which may be melted into glass; and the fine pottery called queen's ware is a mixture of tobacco-pipe clay and flints burned and powdered. Common stone-ware is a coarse mixture of this sort. Some species of pottery are made with mixtures of burned and unburned clay; the former, as I told you before, being incapable of becoming soft again with water like a natural clay.
H. Are clays of no other use than to make pottery of?
T. Yes—the richest soils are those which have a proportion of clay; and marl, which I have already mentioned as a manure, generally contains a good deal of it. Then clay has the property of absorbing oil or grease, whence some kinds of it are used like soap for cleaning clothes. The substance called Fuller's earth is a mixed earth of the argillaceous kind; and its use in taking out the oil which naturally adheres to wool is so great, that it has been one cause of the superiority of our woollen cloths.
H. Then I suppose it is found in England ?