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1、MetalsManfistbeganusingmetalsatleast5000yearsago.Hefoundthattheycouldbehammeredintotoolsandweaponsthatremainedsharp.Fromsmallbeginningstheuseofmetalshasbrowntosuchanextentthatcivilizationasweknowittodaycouldnotexistwiththem.Metalsmakeupalargepartoftheearth
2、souterlayer,orcrust.Butonlyafewofthemappearinthemetalformwearefamiliarwith.Mostmetals,suchasironandtin,arefoundcombinedwithotherchemicalelements.Mostofthesecompoundsdonotlookanythinglikemetals.Theyareoftenlumpsofrocks.Butmanyofthemcanbetreatedtoproducemeta
3、ls.Thesearecalledores.Notmanymetalsareusedwidelyintheirpurestate.Othermetalsareusuallyaddedtothemtoformmixturescalledalloys.Sometimeschemicalelementsotherthanmetalsareincludedinalloys.Themostimportantiscarbon.Inengineering,metalsandalloysareusuallycalledme
4、tals.Metalsaredividedintotowgroups,theferrous,whichcontainalargepercentageofiron,andthenonferrous,whichcontainnoiron.Steel,ourmostusefulmetalsisalloyofironandcarbon.Becauseofitsstrength,steelisusedinlargebuildingsandbridges.Itisalsousedtomakemotorcars,rail
5、waysandships.Mostofthemachinerythatmakespracticallyeverythingweuseismadeofironorsteel.Manyothermetalsareimportanttous,too.Thespoons,knivesandforksweeatwitharemadeofstainlesssteel.Wecookinaluminumboilers.Coppercablescarryelectricitytofactoriesandfamilies.Th
6、ecoinswecarryinourpocketscontaincopper,nickel,andzinc.Thelistoftheusesofmetalsisendless.Pureironisquiteaweakmetal,anditisnotveryhard,either.Butaddalittlecarbonaswellastracesofcertainothermetals,anditbecomesbothstrongandhard.Accordingtowhatisadded,theironca
7、nbegivenotherdesirablepropertiesaswell.Addingothersubstancestoametaltochangeitspropertiesiscalledalloying,andtheproductanalloy.Mostofthealloysofironarebetterknowntousassteels.Manyothermetalsformusefulalloys,too.Brassandbronzearecommonalloysofcopper.Solderi
8、sawell-knownleadalloy.Aluminumandzincalloysarealsoinwidespreaduse.However,itistheironalloysthatarebyfarthemostimportant.Thepigironthatismadeintheblastfurnacecanbeconsideredonekindofironalloy,butitisaverycrude
