第21章

Thegradualpermeationofthefieldbythegreatdoctrineofconservationsimplyrepeatedthehistoryoftheintroductionofeverynovelandrevolutionarythought。Necessarilytheeldergeneration，towhomallformsofenergywereimponderablefluids，mustpassawaybeforethenewconceptioncouldclaimthefield。

Eventhewordenergy，thoughYounghadintroduceditin1807，didnotcomeintogeneralusetillsometimeafterthemiddleofthecentury。Tothegeneralityofphilosophers(thewordphysicistwasevenlessinfavoratthistime)thevariousformsofenergywerestillsubtilefluids，andneverwasidearelinquishedwithgreaterunwillingnessthanthis。TheexperimentsofYoungandFresnelhadconvincedalargenumberofphilosophersthatlightisavibrationandnotasubstance；

butsogreatanauthorityasBiotclungtotheoldemissionideatotheendofhislife，in1862，andheldafollowing。

Meantime，however，thecompanyofbrilliantyoungmenwhohadjustservedtheirapprenticeshipwhenthedoctrineofconservationcameuponthescenehadgrownintoauthoritativepositions，andwerebattlingactivelyforthenewideas。Confirmatoryevidencethatenergyisamolecularmotionandnotan"imponderable"formofmatteraccumulateddaybyday。

TheexperimentsoftwoFrenchmen，HippolyteL。

FizeauandLeonFoucault，servedfinallytoconvincethelastlingeringscepticsthatlightisanundulation；

andbyimplicationbroughtheatintothesamecategory，sinceJamesDavidForbes，theScotchphysicist，hadshownin1837thatradiantheatconformstothesamelawsofpolarizationanddoublerefractionthatgovernlight。But，forthatmatter，theexperimentsthathadestablishedthemechanicalequivalentofheathardlyleftroomfordoubtastotheimmaterialityofthis"imponderable。"Doubtershadindeed，expressedscepticismastothevalidityofJoule’sexperiments，butthefurtherresearches，experimentalandmathematical，ofsuchworkersasThomson(LordKelvin)，Rankine，andTyndallinGreatBritain，ofHelmholtzandClausiusinGermany，andofRegnaultinFrance，dealingwithvariousmanifestationsofheat，placedtheevidencebeyondthereachofcriticism。

Outofthesestudies，justatthemiddleofthecentury，towhichtheexperimentsofMayerandJoulehadled，grewthenewscienceofthermo-dynamics。Outofthemalsogrewinthemindofoneoftheinvestigatorsanewgeneralization，onlysecondinimportancetothedoctrineofconservationitself。ProfessorWilliamThomson(LordKelvin)inhisstudiesinthermodynamicswasearlyimpressedwiththefactthatwhereasallthemolarmotiondevelopedthroughlabororgravitycouldbeconvertedintoheat，theprocessisnotfullyreversible。Heatcan，indeed，beconvertedintomolarmotionorwork，butintheprocessacertainamountoftheheatisradiatedintospaceandlost。Thesamethinghappenswheneveranyotherformofenergyisconvertedintomolarmotion。Indeed，everytransmutationofenergy，ofwhatevercharacter，seemscomplicatedbyatendencytodevelopheat，partofwhichislost。ThisobservationledProfessorThomsontohisdoctrineofthedissipationofenergy，whichheformulatedbeforetheRoyalSocietyofEdinburghin1852，andpublishedalsointhePhilosophicalMagazinethesameyear，thetitlebornebeing，"OnaUniversalTendencyinNaturetotheDissipationofMechanicalEnergy。"

FromtheprinciplehereexpressedProfessorThomsondrewthestartlingconclusionthat，"sinceanyrestorationofthismechanicalenergywithoutmorethananequivalentdissipationisimpossible，"theuniverse，asknowntous，mustbeintheconditionofamachinegraduallyrunningdown；andinparticularthattheworldweliveonhasbeenwithinafinitetimeunfitforhumanhabitation，andmustagainbecomesowithinafinitefuture。Thisthoughtseemssuchacommonplaceto-daythatitisdifficulttorealizehowstartlingitappearedhalfacenturyago。Agenerationtrained，asourshasbeen，inthedoctrinesoftheconservationanddissipationofenergyastheveryalphabetofphysicalsciencecanbutillappreciatethementalattitudeofagenerationwhichforthemostparthadnoteventhoughtitproblematicalwhetherthesuncouldcontinuetogiveoutheatandlightforever。ButthoseadvancethinkerswhohadgraspedtheimportofthedoctrineofconservationcouldatonceappreciatetheforceofThomson’sdoctrineofdissipation，andrealizethecomplementarycharacterofthetwoconceptions。

HereandthereathinkerlikeRankinedid，indeed，attempttofancyconditionsunderwhichtheenergylostthroughdissipationmightberestoredtoavailability，butnosuchefforthasmetwithsuccess，andintimeProfessorThomson’sgeneralizationandhisconclusionsastotheconsequencesofthelawinvolvedcametobeuniversallyaccepted。

Theintroductionofthenewviewsregardingthenatureofenergyfollowed，asIhavesaid，thecourseofeveryothergrowthofnewideas。Youngandimaginativemencouldacceptthenewpointofview；olderphilosophers，theirmindschannelledbypreconceptions，couldnotgetintothenewgroove。Sostrikinglytrueisthisintheparticularcasenowbeforeusthatitisworthwhiletonotetheagesatthetimeoftherevolutionaryexperimentsofthemenwhoseworkhasbeenmentionedasenteringintotheschemeofevolutionoftheideathatenergyismerelyamanifestationofmatterinmotion。Suchalistwilltellthestorybetterthanavolumeofcommentary。

Observe，then，thatDavymadehisepochalexperimentofmeltingicebyfrictionwhenhewasayouthoftwenty。YoungwasnoolderwhenhemadehisfirstcommunicationtotheRoyalSociety，andwasinhistwenty-seventhyearwhenhefirstactivelyespousedtheundulatorytheory。Fresnelwastwenty-sixwhenhemadehisfirstimportantdiscoveriesinthesamefield；andArago，whoatoncebecamehischampion，wasthenbuttwoyearshissenior，thoughforadecadehehadbeensofamousthatoneinvoluntarilythinksofhimasbelongingtoaneldergeneration。

Forbeswasunderthirtywhenhediscoveredthepolarizationofheat，whichpointedthewaytoMohr，thenthirty-one，tothemechanicalequivalent。Joulewastwenty-twoin1840，whenhisgreatworkwasbegun；

andMayer，whosediscoveriesdatefromthesameyear，wasthentwenty-six，whichwasalsotheageofHelmholtzwhenhepublishedhisindependentdiscoveryofthesamelaw。WilliamThomsonwasayouthjustpasthismajoritywhenhecametotheaidofJoulebeforetheBritishSociety，andbutsevenyearsolderwhenheformulatedhisowndoctrineofthedissipationofenergy。

AndClausiusandRankine，whoareusuallymentionedwithThomsonasthegreatdevelopersofthermo-dynamics，werebothfaradvancedwiththeirnovelstudiesbeforetheywerethirty。Withsuchalistinmind，wemaywellagreewiththefatherofinductivesciencethat"themanwhoisyounginyearsmaybeoldinhours。"

Yetwemustnotforgetthattheshieldhasareverseside。Forwasnotthegreatestofobservingastronomers，Herschel，pastthirty-fivebeforeheeversawatelescope，andpastfiftybeforehediscoveredtheheatraysofthespectrum？AndhadnotFaradayreachedmiddlelifebeforeheturnedhisattentionespeciallytoelectricity？Clearly，then，tomakethisphrasecomplete，Baconshouldhaveaddedthat"themanwhoisoldinyearsmaybeyounginimagination。"Here，however，evenmoreappropriatethanintheothercase——more’sthepity——wouldhavebeentheapplicationofhisqualifyingclause:"butthathappenethrarely。"

THEFINALUNIFICATION

Thereareonlyafewgreatgeneralizationsasyetthoughtoutinanysinglefieldofscience。Naturally，then，afteragreatgeneralizationhasfounddefinitiveexpression，thereisaperiodoflullbeforeanotherforwardmove。Inthecaseofthedoctrinesofenergy，thelullhaslastedhalfacentury。Throughoutthisperiod，itistrue，amultitudeofworkershavebeendelvinginthefield，andtothecasualobserveritmightseemasiftheiractivityhadbeenboundless，whilethepracticalapplicationsoftheirideas——asexemplified，forexample，inthetelephone，phonograph，electriclight，andsoon——havebeenlittlelessthanrevolutionary。Yetthemostcompetentoflivingauthorities，LordKelvin，couldassertin1895thatinfiftyyearshehadlearnednothingnewregardingthenatureofenergy。

This，however，mustnotbeinterpretedasmeaningthattheworldhasstoodstillduringthesetwogenerations。

Itmeansratherthattherankandfilehavebeenmovingforwardalongtheroadtheleadershadalreadytravelled。OnlyafewmenintheworldhadtherangeofthoughtregardingthenewdoctrineofenergythatLordKelvinhadatthemiddleofthecentury。Thefewleadersthensawclearlyenoughthatifoneformofenergyisinrealitymerelyanundulationorvibrationamongtheparticlesof"ponderable"matterorofether，allothermanifestationsofenergymustbeofthesamenature。Buttherankandfilewerenotevenwithinsightofthistruthforalongtimeaftertheyhadpartlygraspedthemeaningofthedoctrineofconservation。

When，lateinthefifties，thatmarvellousyoungScotchman，JamesClerk-Maxwell，formulatinginotherwordsanideaofFaraday’s，expressedhisbeliefthatelectricityandmagnetismarebutmanifestationsofvariousconditionsofstressandmotionintheetherealmedium(electricityadisplacementofstrain，magnetismawhirlintheether)，theideametwithnoimmediatepopularity。

Andevenlesscordialwasthereceptiongiventhesamethinker’stheory，putforwardin1863，thattheetherealundulationsproducingthephenomenonwecalllightdifferinnorespectexceptintheirwave-lengthfromthepulsationsofelectro-magnetism。

AtaboutthesametimeHelmholtzformulatedasomewhatsimilarelectro-magnetictheoryoflight；buteventheweightofthiscombinedauthoritycouldnotgivethedoctrinevogueuntilveryrecently，whentheexperimentsofHeinrichHertz，thepupilofHelmholtz，haveshownthataconditionofelectricalstrainmaybedevelopedintoawavesystembyrecurrentinterruptionsoftheelectricstateinthegenerator，andthatsuchwavestravelthroughtheetherwiththerapidityoflight。Sincethentheelectro-magnetictheoryoflighthasbeenenthusiasticallyreferredtoasthegreatestgeneralizationofthecentury；butthesoberthinkermustseethatitisreallyonlywhatHertzhimselfcalledit——onepierbeneaththegreatarchofconservation。

Itisaninterestingdetailofthearchitecture，butthepartcannotequalthesizeofthewhole。

Morethanthat，thisparticularpierisasyetbynomeansaveryfirmone。Ithas，indeed，beendemonstratedthatwavesofelectro-magnetismpassthroughspacewiththespeedoflight，butasyetnoonehasdevelopedelectricwavesevenremotelyapproximatingtheshortnessofthevisualrays。Themostthatcanpositivelybeasserted，therefore，isthatalltheknownformsofradiantenergy-heat，light，electro-magnetism——

travelthroughspaceatthesamerateofspeed，andconsistoftraversevibrations——"lateralquivers，"

asFresnelsaidoflight——knowntodifferinlength，andnotpositivelyknowntodifferotherwise。Ithas，indeed，beensuggestedthatthenewestformofradiantenergy，thefamousX-rayofProfessorRoentgen’sdiscovery，isalongitudinalvibration，butthisisameresurmise。Bethatasitmay，thereisnoonenowtoquestionthatallformsofradiantenergy，whatevertheirexactaffinities，consistessentiallyofundulatorymotionsofoneuniformmedium。

Afullcenturyofexperiment，calculation，andcontroversyhasthussufficedtocorrelatethe"imponderablefluids"ofourforebears，andreducethemalltomanifestationsofmotionamongparticlesofmatter。

Atfirstglimpsethatseemsanenormouschangeofview。Andyet，whencloselyconsidered，thatchangeinthoughtisnotsoradicalasthechangeinphrasemightseemtoimply。Forthenineteenth-centuryphysicist，indisplacingthe"imponderablefluids"ofmanykinds——oneeachforlight，heat，electricity，magnetism——hasbeenobligedtosubstituteforthemoneall-pervadingfluid，whosevariousquivers，waves，ripples，whirlsorstrainsproducethemanifestationswhichinpopularparlancearetermedformsofforce。

Thisall-pervadingfluidthephysicisttermstheether，andhethinksofitashavingnoweight。Ineffect，then，thephysicisthasdispossessedthemanyimponderablesinfavorofasingleimponderable——thoughthewordimponderablehasbeenbanishedfromhisvocabulary。

Inthisviewtheether——which，consideredasarecognizedscientificverity，isessentiallyanineteenth-

centurydiscovery——isaboutthemostinterestingthingintheuniverse。Somethingmoreastoitsproperties，realorassumed，weshallhaveoccasiontoexamineasweturntotheobversesideofphysics，whichdemandsourattentioninthenextchapter。

IX。THEETHERANDPONDERABLEMATTER

"Whateverdifficultieswemayhaveinformingaconsistentideaoftheconstitutionoftheether，therecanbenodoubtthattheinterplanetaryandinterstellarspacesarenotempty，butareoccupiedbyamaterialsubstanceorbodywhichiscertainlythelargestandprobablythemostuniformbodyofwhichwehaveanyknowledge。"

SuchwastheverdictpronouncedsomethirtyyearsagobyJamesClerk-Maxwell，oneoftheverygreatestofnineteenth-centuryphysicists，regardingtheexistenceofanall-pervadingplenumintheuniverse，inwhicheveryparticleoftangiblematterisimmersed。Andthisverdictmaybesaidtoexpresstheattitudeoftheentirephilosophicalworldofourday。Withoutexception，theauthoritativephysicistsofourtimeacceptthisplenumasaverity，andreasonaboutitwithsomethingofthesameconfidencetheymanifestinspeakingof"ponderable"matterorof，energy。Itistruetherearethoseamongthemwhoaredisposedtodenythatthisall-pervadingplenummeritsthenameofmatter。Butthatitisasomething，andavastlyimportantsomethingatthat，allareagreed。

Withoutit，theyallege，weshouldknownothingoflight，ofradiantheat，ofelectricityormagnetism；

withoutittherewouldprobablybenosuchthingasgravitation；nay，theyevenhintthatwithoutthisstrangesomething，ether，therewouldbenosuchthingasmatterintheuniverse。Ifthesecontentionsofthemodernphysicistarejustified，thenthisintangibleetherisincomparablythemostimportantaswellasthe"largestandmostuniformsubstanceorbody"intheuniverse。Itsdiscoverymaywellbelookeduponasoneofthemostimportantfeatsofthenineteenthcentury。

Foradiscoveryofthatcenturyitsurelyis，inthesensethatalltheknownevidencesofitsexistenceweregatheredinthatepoch。Truedreamersofallageshave，formetaphysicalreasons，imaginedtheexistenceofintangiblefluidsinspace——theyhad，indeed，peopledspaceseveraltimesoverwithdifferentkindsofethers，asMaxwellremarks——butsuchvaguedreamingsnomoreconstitutedthediscoveryofthemodernetherthanthedreamofsomepre-ColumbianvisionarythatlandmightliebeyondtheunknownwatersconstitutedthediscoveryofAmerica。InjusticeitmustbeadmittedthatHuyghens，theseventeenth-centuryoriginatoroftheundulatorytheoryoflight，caughtaglimpseofthetrueether；buthiscontemporariesandsomeeightgenerationsofhissuccessorswereutterlydeaftohisclaims；sohebearspracticallythesamerelationtothenineteenth-centurydiscoverersofetherthattheNorsemanbearstoColumbus。

ThetrueColumbusoftheetherwasThomasYoung。

Hisdiscoverywasconsummatedintheearlydaysofthenineteenthcentury，whenhebroughtforwardthefirst，conclusiveproofsoftheundulatorytheoryoflight。

Tosaythatlightconsistsofundulationsistopostulatesomethingthatundulates；andthissomethingcouldnotbeair，forairexistsonlyininfinitesimalquantity，ifatall，intheinterstellarspaces，throughwhichlightfreelypenetrates。Butifnotair，whatthen？Why，clearly，somethingmoreintangiblethanair；somethingsupersensible，evadingalldirecteffortstodetectit，yetexistingeverywhereinseeminglyvacantspace，andalsointerpenetratingthesubstanceofalltransparentliquidsandsolids，ifnot，indeed，ofalltangiblesubstances。

ThisintangiblesomethingYoungrechristenedtheLuminiferousEther。

IntheearlydaysofhisdiscoveryYoungthoughtoftheundulationswhichproducelightandradiantheatasbeinglongitudinal——aforwardandbackwardpulsation，correspondingtothepulsationsofsound——andassuchpulsationscanbetransmittedbyafluidmediumwiththepropertiesofordinaryfluids，hewasjustifiedinthinkingoftheetherasbeinglikeafluidinitsproperties，exceptforitsextremeintangibility。Butabout1818theexperimentsofFresnelandAragowithpolarizationoflightmadeitseemverydoubtfulwhetherthetheoryoflongitudinalvibrationsissufficient，anditwassuggestedbyYoung，andindependentlyconceivedanddemonstratedbyFresnel，thattheluminiferousundulationsarenotlongitudinal，buttransverse；andallthemorerecentexperimentshavetendedtoconfirmthisview。Butithappensthatordinaryfluids——

gasesandliquids——cannottransmitlateralvibrations；

onlyrigidbodiesarecapableofsuchavibration。Soitbecamenecessarytoassumethattheluminiferousetherisabodypossessingelasticrigidity——afamiliarpropertyoftangiblesolids，butonequiteunknownamongfluids。

Theideaoftransversevibrationscarriedwithitanotherpuzzle。Whydoesnottheether，whensetaquiverwiththevibrationwhichgivesusthesensationwecalllight，haveproducedinitssubstancesubordinatequivers，settingoutatrightanglesfromthepathoftheoriginalquiver？Suchperpendicularvibrationsseemnottoexist，elsewemightseearoundacorner；howexplaintheirabsence？Thephysicistcouldthinkofbutoneway:theymustassumethattheetherisincompressible。Itmustfillallspace——atanyrate，allspacewithwhichhumanknowledgedeals——perfectlyfull。

Thesepropertiesoftheether，incompressibilityandelasticrigidity，arequiteconceivablebythemselves；

butdifficultiesofthoughtappearwhenwereflectuponanotherqualitywhichtheetherclearlymustpossess——

namely，frictionlessness。Byhypothesisthisrigid，incompressiblebodypervadesallspace，imbeddingeveryparticleoftangiblematter；yetitseemsnottoretardthemovementsofthismatterintheslightestdegree。

Thisisundoubtedlythemostdifficulttocomprehendoftheallegedpropertiesoftheether。Thephysicistexplainsitasduetotheperfectelasticityoftheether，invirtueofwhichitclosesinbehindamovingparticlewithapushexactlycounterbalancingthestressrequiredtopenetrateitinfront。

Toapersonunaccustomedtothinkofseeminglysolidmatterasreallycomposedofparticlesrelativelywideapart，itishardtounderstandtheclaimthatetherpenetratesthesubstanceofsolids——ofglass，forexample——and，touseYoung’sexpression，whichwehavepreviouslyquoted，movesamongthemasfreelyasthewindmovesthroughagroveoftrees。Thisthought，however，presentsfewdifficultiestothemindaccustomedtophilosophicalspeculation。ButthequestionearlyaroseinthemindofFresnelwhethertheetherisnotconsiderablyaffectedbycontactwiththeparticlesofsolids。Someofhisexperimentsledhimtobelievethataportionoftheetherwhichpenetratesamongthemoleculesoftangiblematterisheldcaptive，sotospeak，andmadetomovealongwiththeseparticles。Hespokeofsuchportionsoftheetheras"bound"ether，incontradistinctiontothegreatmassof"free"ether。HalfacenturyafterFresnel’sdeath，whentheetherhypothesishadbecomeanacceptedtenetofscience，experimentswereundertakenbyFizeauinFrance，andbyClerk-MaxwellinEngland，toascertainwhetheranyportionofetherisreallythusboundtoparticlesofmatter；buttheresultsoftheexperimentswerenegative，andthequestionisstillundetermined。

Whiletheundulatorytheoryoflightwasstillfightingitsway，anotherkindofevidencefavoringtheexistenceofanetherwasputforwardbyMichaelFaraday，who，inthecourseofhisexperimentsinelectricalandmagneticinduction，wasledmoreandmoretoperceivedefinitelinesorchannelsofforceinthemediumsubjecttoelectro-magneticinfluence。Faraday’smind，likethatofNewtonandmanyotherphilosophers，rejectedtheideaofactionatadistance，andhefeltconvincedthatthephenomenaofmagnetismandofelectricinductiontoldstronglyfortheexistenceofaninvisibleplenumeverywhereinspace，whichmightveryprobablybethesameplenumthatcarriestheundulationsoflightandradiantheat。

Then，aboutthemiddleofthecentury，camethatfinalrevolutionofthoughtregardingthenatureofenergywhichwehavealreadyoutlinedintheprecedingchapter，andwiththatthecaseforetherwasconsideredtobefullyestablished。Theideathatenergyismerelya"modeofmotion"(toadoptTyndall’sfamiliarphrase)，combinedwiththeuniversalrejectionofthenotionofactionatadistance，madetheacceptanceofaplenumthroughoutspaceanecessityofthought——so，atanyrate，ithasseemedtomostphysicistsofrecentdecades。

Theproofthatallknownformsofradiantenergymovethroughspaceatthesamerateofspeedisregardedaspracticallyademonstrationthatbutoneplenum——oneether——isconcernedintheirtransmission。

Ithas，indeed，beententativelysuggested，byProfessorJ。OliverLodge，thattheremaybetwoethers，representingthetwooppositekindsofelectricity，buteventheauthorofthishypothesiswouldhardlyclaimforitahighdegreeofprobability。

ThemostrecentspeculationsregardingthepropertiesoftheetherhavedepartedbutlittlefromtheearlyideasofYoungandFresnel。Itisassumedonallsidesthattheetherisacontinuou