Obrazy na stronie

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

[ocr errors]

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 lime-stones. These dif. fer 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 oystershell lime.

T. They will ; and this is another source of calcareous earth. The shells of all animals, both land and sea, as oysters, muscles, 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 lime-stone beds; whence some have supposed that all calcareous earth is originally an animal production.

H. But where could animals enow 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 beside these purer calcareous earths, it is very frequently found mingled in different proportions with other earths. Thus marle, which is so much used in manuring land, and of which there are a great many kinds,

[ocr errors]

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 méthod 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.

T. Vinegar is not an acid strong enough to act upon a thing so solid

as a shell. But aqua fortis, or spirit of salt, will do it at once ;


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 heart-burn, 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,

« PoprzedniaDalej »