“The smith’s work will not make the copper rust cease to be a poison,” replied Uncle Paul, “but it will prevent the rust’s forming. Of the common metals tin rusts the least. Exposed to the air a long time, it scarcely tarnishes. And then the rust, which forms in small quantities, is innocuous, like iron rust. To prevent copper from covering itself with poisonous green spots, to preserve it from rust, it must be kept from contact with damp air and also with certain alimentary substances such as vinegar, oil, grease—substances that provoke the rapid formation of rust. For this reason the copper saucepan is coated over with tin inside. Under the thin bed of tin which covers it, the copper cannot rust, because it is no longer in contact with the air. The tin remains; but this metal changes with difficulty, and, besides, its rust, if it forms any, is harmless. So they plate copper, that is to say they cover it with a thin bed of tin, to prevent its rusting, and thus to prevent the formation of the dangerous poison that might, some day or other, be mixed with our food. “They also tin iron, not to prevent the formation of poison, for the rust of this metal is harmless, but simply to preserve it from changing and covering itself with ugly red spots. This tinned iron is called tin-plate. Lids, coffee-pots, dripping-pans, graters, lanterns, and innumerable other things, are of tin-plate; that is to say, thin sheets of iron covered on both sides with a coating of tin.” ------------------------------------------------------------------------ CHAPTER XIV GOLD AND IRON “SOME metals never rust; such a one is gold. Ancient gold pieces found in the earth after centuries are as bright as the day they were coined. No dross, no rust covers their effigy and inscription. Time, fire, humidity, air, cannot harm this admirable metal. Therefore gold, on account of its unchangeable luster and its rarity, is preëminently the material for ornaments and coins. “Furthermore, gold is the first metal that man became acquainted with, long before iron, lead, tin, and the others. The reason why man’s attention was called to gold, long centuries before iron, is not hard to understand. Gold never rusts; iron rusts with such grievous facility that in a short time, if we are not careful, it is converted into a red earth. I have just told you that gold objects, however old they may be, have come to us intact, even after having been in the dampest ground. As for objects of iron, not one has reached us that was not in an unrecognizable state. Corroded with rust, they have become a shapeless earthy crust. Now I will ask Jules if the iron ore that is extracted from the bowels of the earth can be real, pure iron, such as we use.” “It seems to me not, Uncle; for if iron at any given moment is pure, it must rust with time and change to earthy matter, as does the blade of a knife buried in the ground.”