Say rather _oxydating_, Mrs. B. --Rust is a word that should be exploded in chemistry. MRS. B. Take care, however, not to introduce the word oxydate, instead of rust, in general conversation; for you would probably not be understood, and you might be suspected of affectation. Metals differ very much in their affinity for each other; some will not unite at all, others readily combine together, and on this property of metals the art of _soldering_ depends. EMILY. What is soldering? MRS. B. It is joining two pieces of metal together, by a more fusible metal interposed between them. Thus tin is a solder for lead; brass, gold, or silver, are solder for iron, &c. CAROLINE. And is not _plating_ metals something of the same nature? MRS. B. In the operation of plating, two metals are united, one being covered with the other, but without the intervention of a third; iron or copper may thus be covered with gold or silver. EMILY. Mercury appears to me of a very different nature from the other metals. MRS. B. One of its greatest peculiarities is, that it retains a fluid state at the temperature of the atmosphere. All metals are fusible at different degrees of heat, and they have likewise each the property of freezing or becoming solid at a certain fixed temperature. Mercury congeals only at seventy-two degrees below the freezing point. EMILY. That is to say, that in order to freeze, it requires a temperature of seventy-two degrees colder than that at which water freezes. MRS. B. Exactly so. CAROLINE. But is the temperature of the atmosphere ever so low as that? MRS. B. Yes, often in Siberia; but happily never in this part of the globe. Here, however, mercury may be congealed by artificial cold; I mean such intense cold as can be produced by some chemical mixtures, or by the rapid evaporation of ether under the air-pump.* [Footnote *: By a process analogous to that described, page 155. of this volume.] CAROLINE. And can mercury be made to boil and evaporate? MRS. B. Yes, like any other liquid; only it requires a much greater degree of heat. At the temperature of six hundred degrees, it begins to boil and evaporate like water. Mercury combines with gold, silver, tin, and with several other metals; and, if mixed with any of them in a sufficient proportion, it penetrates the solid metal, softens it, loses its own fluidity, and forms an _amalgam_, which is the name given to the combination of any metal with mercury, forming a substance more or less solid, according as the mercury or the other metal predominates. EMILY. In the list of metals there are some whose names I have never before heard mentioned. MRS. B.