第23章

Ofcoursetheatomitselfwasinnosensedisplaced，butAvogadro’slawsoonmadeitplainthattheatomhadoftenusurpedterritorythatdidnotreallybelongtoit。Inmanycasesthechemistshadsupposedthemselvesdealingwithatomsasunitswherethetrueunitwasthemolecule。Inthecaseofelementarygases，suchashydrogenandoxygen，forexample，thelawofequalnumbersofmoleculesinequalspacesmadeitclearthattheatomsdonotexistisolated，ashadbeensupposed。Sincetwovolumesofhydrogenunitewithonevolumeofoxygentoformtwovolumesofwatervapor，thesimplestmathematicsshow，inthelightofAvogadro’slaw，notonlythateachmoleculeofwatermustcontaintwohydrogenatoms(apointpreviouslyindispute)，butthattheoriginalmoleculesofhydrogenandoxygenmusthavebeencomposedineachcaseoftwoatoms——elsehowcouldonevolumeofoxygensupplyanatomforeverymoleculeoftwovolumesofwater？

What，then，doesthisimply？Why，thattheelementaryatomhasanavidityforotheratoms，alongingforcompanionship，an"affinity"——callitwhatyouwill——whichisboundtobesatisfiedifotheratomsareintheneighborhood。Placedsolelyamongatomsofitsownkind，theoxygenatomseizesonafellowoxygenatom，andinalltheirmaddancingsthesetwomatesclingtogether——possiblyrevolvingabouteachotherinminiatureplanetaryorbits。Preciselythesamethingoccursamongthehydrogenatoms。Butnowsupposethevariouspairsofoxygenatomscomenearotherpairsofhydrogenatoms(underproperconditionswhichneednotdetainushere)，theneachoxygenatomlosesitsattachmentforitsfellow，andflingsitselfmadlyintothecircuitofoneofthehydrogencouplets，and——presto！——thereareonlytwomoleculesforeverythreetherewerebefore，andfreeoxygenandhydrogenhavebecomewater。Thewholeprocess，statedinchemicalphraseology，issummedupinthestatementthatunderthegivenconditionstheoxygenatomshadagreateraffinityforthehydrogenatomsthanforoneanother。

Aschemistsstudiedtheactionsofvariouskindsofatoms，inregardtotheirunionswithoneanothertoformmolecules，itgraduallydawneduponthemthatnotallelementsaresatisfiedwiththesamenumberofcompanions。Someelementsaskonlyone，andrefusetotakemore；whileotherslinkthemselves，whenoccasionoffers，withtwo，three，four，ormore。Thuswesawthatoxygenforsookasingleatomofitsownkindandlinkeditselfwithtwoatomsofhydrogen。Clearly，then，theoxygenatom，likeacreaturewithtwohands，isabletoclutchtwootheratoms。

Butwehavenoproofthatunderanycircumstancesitcouldholdmorethantwo。Itsaffinitiesseemsatisfiedwhenithastwobonds。But，ontheotherhand，theatomofnitrogenisabletoholdthreeatomsofhydrogen，anddoessointhemoleculeofammonium(NH3)；whilethecarbonatomcanholdfouratomsofhydrogenortwoatomsofoxygen。

Evidently，then，oneatomisnotalwaysequivalenttoanotheratomofadifferentkindincombiningpowers。ArecognitionofthisfactbyFranklandabout1852，anditsfurtherinvestigationbyothers(notablyA。KekuleandA。S。Couper)，ledtotheintroductionofthewordequivalentintochemicalterminologyinanewsense，andinparticulartoanunderstandingoftheaffinitiesor"valency"ofdifferentelements，whichprovedofthemostfundamentalimportance。Thusitwasshownthat，ofthefourelementsthatentermostprominentlyintoorganiccompounds，hydrogencanlinkitselfwithonlyasinglebondtoanyotherelement——ithas，sotospeak，butasinglehandwithwhichtograsp——whileoxygenhascapacityfortwobonds，nitrogenforthree(possiblyforfive)，andcarbonforfour。Thewordsmonovalent，divalent，trivalent，tretrava-lent，etc。，werecoinedtoexpressthismostimportantfact，andthevariouselementscametobeknownasmonads，diads，triads，etc。Justwhydifferentelementsshoulddifferthusinvalencynooneasyetknows；itisanempiricalfactthattheydo。Andoncethenatureofanyelementhasbeendeterminedasregardsitsvalency，amostimportantinsightintothepossiblebehaviorofthatelementhasbeensecured。Thusaconsiderationofthefactthathydrogenismonovalent，whileoxygenisdivalent，makesitplainthatwemustexpecttofindnomorethanthreecompoundsofthesetwoelements——namely，H——O——(writtenHObythechemist，andcalledhydroxyl)；H——O——H(H2O，orwater)，andH——O——O——H(H2O2，orhydrogenperoxide)。Itwillbeobservedthatinthefirstofthesecompoundstheatomofoxygenstands，sotospeak，withoneofitshandsfree，eagerlyreachingout，therefore，foranothercompanion，andhence，inthelanguageofchemistry，forminganunstablecompound。Again，inthethirdcompound，thoughallhandsareclasped，yetonepairlinksoxygenwithoxygen；andthisalsomustbeanunstableunion，sincetheavidityofanatomforitsownkindisrelativelyweak。Thusthewell-knownpropertiesofhydrogenperoxideareexplained，itseasydecomposition，andtheeagernesswithwhichitseizesupontheelementsofothercompounds。

Butthemoleculeofwater，ontheotherhand，hasitsatomsarrangedinastateofstableequilibrium，alltheiraffinitiesbeingsatisfied。Eachhydrogenatomhassatisfieditsownaffinitybyclutchingtheoxygenatom；andtheoxygenatomhasbothitsbondssatisfiedbyclutchingbackatthetwohydrogenatoms。Thereforethetrio，linkedinthisclosebond，havenotendencytoreachoutforanyothercompanion，nor，indeed，anypowertoholdanothershoulditthrustitselfuponthem。Theyforma"stable"compound，whichunderallordinarycircumstanceswillretainitsidentityasamoleculeofwater，eventhoughthephysicalmassofwhichitisapartchangesitsconditionfromasolidtoagasfromicetovapor。

Butaconsiderationofthisconditionofstableequilibriuminthemoleculeatoncesuggestsanewquestion:Howcananaggregationofatoms，havingalltheiraffinitiessatisfied，takeanyfurtherpartinchemicalreactions？Seeminglysuchamolecule，whateveritsphysicalproperties，mustbechemicallyinert，incapableofanyatomicreadjustments。Andsoinpointoffactitis，solongasitscomponentatomsclingtooneanotherunremittingly。Butthis，itappears，ispreciselywhattheatomsarelittlepronetodo。Itseemsthattheyarefickletothelastdegreeintheirindividualattachments，andareaspronetobreakawayfrombondageastheyaretoenterintoit。Thustheoxygenatomwhichhasjustflungitselfintothecircuitoftwohydrogenatoms，thenextmomentflingsitselffreeagainandseeksnewcompanions。Itisforalltheworldliketheincessantchangeofpartnersinarollickingdance。ThisincessantdissolutionandreformationofmoleculesinasubstancewhichasawholeremainsapparentlyunchangedwasfirstfullyappreciatedbySte-ClaireDeville，andbyhimnameddissociation。Itisaprocesswhichgoesonmuchmoreactivelyinsomecompoundsthaninothers，andverymuchmoreactivelyundersomephysicalconditions(suchasincreaseoftemperature)thanunderothers。Butapparentlynosubstancesatordinarytemperatures，andnotemperatureabovetheabsolutezero，areabsolutelyfreefromitsdisturbinginfluence。

Henceitisthatmoleculeshavingallthevalencyoftheiratomsfullysatisfieddonotlosetheirchemicalactivity——sinceeachatomismomentarilyfreeintheexchangeofpartners，andmayseizeupondifferentatomsfromitsformerpartners，ifthoseitprefersareathand。

While，however，anappreciationofthisceaselessactivityoftheatomisessentialtoaproperunderstandingofitschemicalefficiency，yetfromanotherpointofviewthe"saturated"

molecule——thatis，themoleculewhoseatomshavetheirvalencyallsatisfied——maybethoughtofasarelativelyfixedorstableorganism。Eventhoughitmaypresentlybetorndown，itisforthetimebeingacompletedstructure；andaconsiderationofthevalencyofitsatomsgivesthebestclewthathashithertobeenobtainableastothecharacterofitsarchitecture。Howimportantthismatterofarchitectureofthemolecule——ofspacerelationsoftheatoms——maybewasdemonstratedaslongagoas1823，whenLiebigandWohlerproved，totheutterbewildermentofthechemicalworld，thattwosubstancesmayhavepreciselythesamechemicalconstitution——thesamenumberandkindofatoms——andyetdifferutterlyinphysicalproperties。ThewordisomerismwascoinedbyBerzeliustoexpressthisanomalousconditionofthings，whichseemedtonegativethemostfundamentaltruthsofchemistry。Namingtheconditionbynomeansexplainedit，butthefactwasmadeclearthatsomethingbesidesthemerenumberandkindofatomsisimportantinthearchitectureofamolecule。Itbecamecertainthatatomsarenotthrowntogetherhaphazardtobuildamolecule，anymorethanbricksarethrowntogetheratrandomtoformahouse。

Howdelicatemaybethegradationsofarchitecturaldesigninbuildingamoleculewaswellillustratedabout1850，whenPasteurdiscoveredthatsomecarboncompounds——ascertainsugars——canonlybedistinguishedfromoneanother，wheninsolution，bythefactoftheirtwistingorpolarizingarayoflighttotheleftortotheright，respectively。Butnoinklingofanexplanationofthesestrangevariationsofmolecularstructurecameuntilthediscoveryofthelawofvalency。Thenmuchofthemysterywasclearedaway；foritwasplainthatsinceeachatominamoleculecanholdtoitselfonlyafixednumberofotheratoms，complexmoleculesmusthavetheiratomslinkedindefinitechainsorgroups。Anditisequallyplainthatwheretheatomsarenumerous，theexactplanofgroupingmaysometimesbesusceptibleofchangewithoutdoingviolencetothelawofvalency。Itisinsuchcasesthatisomerismisobservedtooccur。

Bypayingconstantheedtothismatteroftheaffinities，chemistsareabletomakediagrammaticpicturesoftheplanofarchitectureofanymoleculewhosecompositionisknown。Inthesimplemoleculeofwater(H2O)，forexample，thetwohydrogenatomsmusthavereleasedeachotherbeforetheycouldjointheoxygen，andthemanneroflinkingmustapparentlybethatrepresentedinthegraphicformulaH——O——H。Withmoleculescomposedofalargenumberofatoms，suchgraphicrepresentationoftheschemeoflinkingisofcourseincreasinglydifficult，yet，withtheaffinitiesforaguide，itisalwayspossible。Ofcoursenoonesupposesthatsuchaformula，writteninasingleplane，canpossiblyrepresentthetruearchitectureofthemolecule:itisatbestsuggestiveordiagrammaticratherthanpictorial。Nevertheless，itaffordshintsastothestructureofthemoleculesuchasthefathersofchemistrywouldnothavethoughtitpossibleevertoattain。

PERIODICITYOFATOMICWEIGHTS

Theseutterlynovelstudiesofmoleculararchitecturemayseematfirstsighttotakefromtheatommuchofitsformerprestigeastheall-importantpersonageofthechemicalworld。Sincesomuchdependsuponthemerepositionoftheatoms，itmayappearthatcomparativelylittledependsuponthenatureoftheatomsthemselves。Butsuchaviewisincorrect，foroncloserconsiderationitwillappearthatatnotimehastheatombeenseentorenounceitspeculiarpersonality。Withincertainlimitsthecharacterofamoleculemaybealteredbychangingthepositionsofitsatoms(justasdifferentbuildingsmaybeconstructedofthesamebricks)，buttheselimitsaresharplydefined，anditwouldbeasimpossibletoexceedthemasitwouldbetobuildastonebuildingwithbricks。Fromfirsttolastthebrickremainsabrick，whateverthestyleofarchitectureithelpstoconstruct；itneverbecomesastone。Andjustascloselydoeseachatomretainitsownpeculiarproperties，regardlessofitssurroundings。

Thus，forexample，thecarbonatommaytakepartintheformationatonetimeofadiamond，againofapieceofcoal，andyetagainofaparticleofsugar，ofwoodfibre，ofanimaltissue，orofagasintheatmosphere；butfromfirsttolast——fromglass-cuttinggemtointangiblegas——thereisnodemonstrablechangewhateverinanysinglepropertyoftheatomitself。Sofarasweknow，itssize，itsweight，itscapacityforvibrationorrotation，anditsinherentaffinities，remainabsolutelyunchangedthroughoutallthesevaryingfortunesofpositionandassociation。Andthesamethingistrueofeveryatomofalloftheseventy-oddelementarysubstanceswithwhichthemodernchemistisacquainted。Everyoneappearsalwaystomaintainitsuniqueintegrity，gainingnothingandlosingnothing。

Allthisbeingtrue，itwouldseemasifthepositionoftheDaltonianatomasaprimordialbitofmatter，indestructibleandnon-transmutable，hadbeenputtothetestbythechemistryofourcentury，andnotfoundwanting。SincethoseearlydaysofthecenturywhentheelectricbatteryperformeditsmiraclesandseeminglyreacheditslimitationsinthehandsofDavy，manynewelementarysubstanceshavebeendiscovered，butnosingleelementhasbeendisplacedfromitspositionasanundecomposablebody。

RatherhavetheanalysesofthechemistseemedtomakeitmoreandmorecertainthatallelementaryatomsareintruthwhatJohnHerschelcalledthem，"manufacturedarticles"——primordial，changeless，indestructible。

Andyet，oddlyenough，ithaschancedthathandinhandwiththeexperimentsleadingtosuchagoalhavegoneotherexperimentsaridspeculationsofexactlytheoppositetenor。Ineachgenerationtherehavebeenchemistsamongtheleadersoftheirsciencewhohaverefusedtoadmitthattheso-calledelementsarereallyelementsatallinanyfinalsense，andwhohavesoughteagerlyforproofwhichmightwarranttheirscepticism。ThefirstbitofevidencetendingtosupportthisviewwasfurnishedbyanEnglishphysician，Dr。WilliamProut，whoin1815calledattentiontoacuriousrelationtobeobservedbetweentheatomicweightofthevariouselements。Acceptingthefiguresgivenbytheauthoritiesofthetime(notablyThomsonandBerzelius)，itappearedthatastrikinglylargeproportionoftheatomicweightswereexactmultiplesoftheweightofhydrogen，andthatothersdifferedsoslightlythaterrorsofobservationmightexplainthediscrepancy。Proutfeltthatitcouldnotbeaccidental，andhecouldthinkofnotenableexplanation，unlessitbethattheatomsofthevariousallegedelementsaremadeupofdifferentfixednumbersofhydrogenatoms。Coulditbethattheonetrueelement——theoneprimalmatter——ishydrogen，andthatallotherformsofmatterarebutcompoundsofthisoriginalsubstance？

Proutadvancedthisstartlingideaatfirsttentatively，inananonymouspublication；butafterwardsheespouseditopenlyandurgeditstenability。ComingjustafterDavy’sdissociationofsomesupposedelements，theideaprovedalluring，andforatimegainedsuchpopularitythatchemistsweredisposedtoroundouttheobservedatomicweightsofallelementsintowholenumbers。

Butpresentlyreneweddeterminationsoftheatomicweightsseemedtodiscountenancethispractice，andProut’sallegedlawfellintodisrepute。Itwasrevived，however，about1840，byDumas，whosegreatauthoritysecureditarespectfulhearing，andwhosecarefulredeterminationoftheweightofcarbon，makingitexactlytwelvetimesthatofhydrogen，aidedthecause。

SubsequentlyStas，thepupilofDumas，undertookalongseriesofdeterminationsofatomicweights，withtheexpectationofconfirmingtheProutianhypothesis。Buthisresultsseemedtodisprovethehypothesis，fortheatomicweightsofmanyelementsdifferedfromwholenumbersbymore，itwasthought，thanthelimitsoferroroftheexperiments。Itwasnoteworthy，however，thattheconfidenceofDumaswasnotshaken，thoughhewasledtomodifythehypothesis，and，inaccordancewithprevioussuggestionsofClarkandofMarignac，torecognizeastheprimordialelement，nothydrogenitself，butanatomhalftheweight，orevenone-fourththeweight，ofthatofhydrogen，ofwhichprimordialatomthehydrogenatomitselfiscompounded。Buteveninthismodifiedformthehypothesisfoundgreatoppositionfromexperimentalobservers。

In1864，however，anovelrelationbetweentheweightsoftheelementsandtheirothercharacteristicswascalledtotheattentionofchemistsbyProfessorJohnA。R。Newlands，ofLondon，whohadnoticedthatiftheelementsarearrangedseriallyinthenumericalorderoftheiratomicweights，thereisacuriousrecurrenceofsimilarpropertiesatintervalsofeightelementsThisso-called"lawofoctaves"attractedlittleimmediateattention，butthefactsitconnotessooncameundertheobservationofotherchemists，notablyofProfessorsGustavHinrichsinAmerica，DmitriMendeleeffinRussia，andLotharMeyerinGermany。Mendeleeffgavethediscoveryfullestexpression，explicatingitin1869，underthetitleof"theperiodiclaw。"

Thoughthisearlyexpositionofwhathassincebeenadmittedtobeamostimportantdiscoverywasveryfullyoutlined，thegeneralityofchemistsgaveitlittleheedtilladecadeorsolater，whenthreenewelements，gallium，scandium，andgermanium，werediscovered，which，onbeinganalyzed，werequiteunexpectedlyfoundtofitintothreegapswhichMendeleeffhadleftinhisperiodicscale。IneffecttheperiodiclawhadenabledMendeleefftopredicatetheexistenceofthenewelementsyearsbeforetheywerediscovered。Surelyasystemthatleadstosuchresultsisnomerevagary。Soverysoontheperiodiclawtookitsplaceasoneofthemostimportantgeneralizationsofchemicalscience。

Thislawofperiodicitywasputforwardasanexpressionofobservedrelationsindependentofhypothesis；butofcoursethetheoreticalbearingsofthesefactscouldnotbeoverlooked。AsProfessorJ。H。Gladstonehassaid，itforcesuponus"theconvictionthattheelementsarenotseparatebodiescreatedwithoutreferencetooneanother，butthattheyhavebeenoriginallyfashioned，orhavebeenbuiltup，fromoneanother，accordingtosomegeneralplan。"ItisbutashortstepfromthatpropositiontotheProutianhypothesis。

NEWWEAPONS——SPECTROSCOPEANDCAMERA

Buttheatomicweightsarenotaloneinsuggestingthecompoundnatureoftheallegedelements。Evidenceofatotallydifferentkindhascontributedtothesameend，fromasourcethatcouldhardlyhavebeenimaginedwhentheProutianhypothesis，wasformulated，throughthetraditionofanovelweapontothearmamentariumofthechemist——thespectroscope。Theperfectionofthisinstrument，inthehandsoftwoGermanscientists，GustavRobertKirchhoffandRobertWilhelmBunsen，cameaboutthroughtheinvestigation，towardsthemiddleofthecentury，ofthemeaningofthedarklineswhichhadbeenobservedinthesolarspectrumbyFraunhoferasearlyas1815，andbyWollastonadecadeearlier。ItwassuspectedbyStokesandbyFoxTalbotinEngland，butfirstbroughttodemonstrationbyKirchhoffandBunsen，thattheselines，whichwereknowntooccupydefinitepositionsinthespectrum，arereallyindicativeofparticularelementarysubstances。Bymeansofthespectroscope，whichisessentiallyamagnifyinglensattachedtoaprismofglass，itispossibletolocatethelineswithgreataccuracy，anditwassoonshownthatherewasanewmeansofchemicalanalysisofthemostexquisitedelicacy。Itwasfound，forexample，thatthespectroscopecoulddetectthepresenceofaquantityofsodiumsoinfinitesimalastheonetwo-hundred-thousandthofagrain。Butwhatwasevenmoreimportant，thespectroscopeputnolimituponthedistanceoflocationofthesubstanceittested，providedonlythatsufficientlightcamefromit。Theexperimentsitrecordedmightbeperformedinthesun，orinthemostdistantstarsornebulae；indeed，oneoftheearliestfeatsoftheinstrumentwastowrenchfromthesunthesecretofhischemicalconstitution。

Torendertheutilityofthespectroscopecomplete，however，itwasnecessarytolinkwithitanothernewchemicalagency——namely，photography。Thisnowfamiliarprocessisbasedonthepropertyoflighttodecomposecertainunstablecompoundsofsilver，andthusaltertheirchemicalcomposition。DavyandWedgwoodbarelyescapedthediscoveryofthevalueofthephotographicmethodearlyinthenineteenthcentury。Theirsuccessorsquiteoverlookedituntilabout1826，whenLouisJ。M。

Daguerre，theFrenchchemist，tookthematterinhand，andaftermanyyearsofexperimentationbroughtittorelativeperfectionin1839，inwhichyearthefamousdaguerreotypefirstbroughtthemattertopopularattention。InthesameyearMr。FoxTalbotreadapaperonthesubjectbeforetheRoyalSociety，andsoonafterwardstheeffortsofHerschelandnumerousothernaturalphilosopherscontributedtotheadvancementofthenewmethod。

In1843Dr。JohnW。Draper，thefamousEnglish-Americanchemistandphysiologist，showedthatbyphotographytheFraunhoferlinesinthesolarspectrummightbemappedwithabsoluteaccuracy；

alsoprovingthatthesilveredfilmrevealedmanylinesinvisibletotheunaidedeye。Thevalueofthismethodofobservationwasrecognizedatonce，and，assoonasthespectroscopewasperfected，thephotographicmethod，inconjunctionwithitsuse，becameinvaluabletothechemist。Bythismeanscomparisonsofspectramaybemadewithadegreeofaccuracynototherwiseobtainable；and，incaseofthestars，wholeclustersofspectramaybeplacedonrecordatasingleobservation。

Astheexaminationofthesunandstarsproceeded，chemistswereamazedordelighted，accordingtotheirvariouspreconceptions，towitnesstheproofthatmanyfamiliarterrestrialelementsaretobefoundinthecelestialbodies。Butwhatperhapssurprisedthemmostwastoobservetheenormouspreponderanceinthesiderealbodiesoftheelementhydrogen。Notonlyaretherevastquantitiesofthiselementinthesun’satmosphere，butsomeothersunsappearedtoshowhydrogenlinesalmostexclusivelyintheirspectra。Presentlyitappearedthatthestarsofwhichthisistruearethosewhitestars，suchasSirius，whichhadbeenconjecturedtobet