Tut. 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 lime-stone beds; whence some have supposed that all calcareous earth is originally an animal production.
Har. But where could animals enough ever have lived to make mountains of their shells?
Tut. 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, consists of calcareous earth, united with clay and sand; and the more of this earth it contains, the richer manure it generally makes.
Geo. Is there any way of discovering it when it is mixed in this manner with other things?
Tut. Yes—there is an easy and sure method of discovering the smallest portion of it. All the varieties of calcareous earth that I have mentioned have 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.
Geo. 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.
Tut. Vinegar is not an acid strong enough to act upon a thing so solid as a shell. But sulphuric and muriatic acids 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 neutralizing 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 neutralizing the acid.
Geo. 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?
Tut. It is; but though magnesia has some properties in common with calcareous earths, it possesses others that are peculiar to itself.
Geo. Pray, what are the other uses of these earths?