That is actually the case in the hot springs alluded to, which give out an extremely fetid gas, composed of hydrogen impregnated with sulphur. CAROLINE. If I recollect right, steel and plumbago, which you mentioned in the last lesson, are both carburets of iron? MRS. B. Yes; and they are the only carburets of much consequence. A curious combination of metals has lately very much attracted the attention of the scientific world: I mean the meteoric stones that fall from the atmosphere. They consist principally of native or pure iron, which is never found in that state in the bowels of the earth; and contain also a small quantity of nickel and chrome, a combination likewise new in the mineral kingdom. These circumstances have led many scientific persons to believe that those substances have fallen from the moon, or some other planet, while others are of opinion either that they are formed in the atmosphere, or are projected into it by some unknown volcano on the surface of our globe. CAROLINE. I have heard much of these stones, but I believe many people are of opinion that they are formed on the surface of the earth, and laugh at their pretended celestial origin. MRS. B. The fact of their falling is so well ascertained, that I think no person who has at all investigated the subject, can now entertain any doubt of it. Specimens of these stones have been discovered in all parts of the world, and to each of them some tradition or story of its fall has been found connected. And as the analysis of all those specimens affords precisely the same results, there is strong reason to conjecture that they all proceed from the same source. It is to Mr. Howard that philosophers are indebted for having first analysed these stones, and directed their attention to this interesting subject. CAROLINE. But pray, Mrs. B., how can solid masses of iron and nickel be formed from the atmosphere, which consists of the two airs, nitrogen and oxygen? MRS. B. I really do not see how they could, and think it much more probable that they fall from the moon. --But we must not suffer this digression to take up too much of our time. The combinations of metals with each other are called alloys; thus brass is an alloy of copper and zinc; bronze, of copper and tin, &c. EMILY. And is not pewter also a combination of metal? MRS. B. It is. The pewter made in this country is mostly composed of tin, with a very small proportion of zinc and lead. CAROLINE. Block-tin is a kind of pewter, I believe? MRS. B. Properly speaking, block-tin means tin in blocks, or square massive ingots; but in the sense in which it is used by ignorant workmen, it is iron plated with tin, which renders it more durable, as tin will not so easily rust. Tin alone, however, would be too soft a metal to be worked for common use, and all tin-vessels and utensils are in fact made of plates of iron, thinly coated with tin, which prevents the iron from rusting. CAROLINE.