Thegradualpermeationofthefieldbythegreatdoctrineofconservationsimplyrepeatedthehistoryoftheintroductionofeverynovelandrevolutionarythought。Necessarilytheeldergeneration，towhomallformsofenergywereimponderablefluids，mustpassawaybeforethenewconceptioncouldclaimthefield。
Eventhewordenergy，thoughYounghadintroduceditin1807，didnotcomeintogeneralusetillsometimeafterthemiddleofthecentury。Tothegeneralityofphilosophersthewordphysicistwasevenlessinfavoratthistimethevariousformsofenergywerestillsubtilefluids，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，ofsuchworkersasThomsonLordKelvin，Rankine，andTyndallinGreatBritain，ofHelmholtzandClausiusinGermany，andofRegnaultinFrance，dealingwithvariousmanifestationsofheat，placedtheevidencebeyondthereachofcriticism。
Outofthesestudies，justatthemiddleofthecentury，towhichtheexperimentsofMayerandJoulehadled，grewthenewscienceofthermo-dynamics。Outofthemalsogrewinthemindofoneoftheinvestigatorsanewgeneralization，onlysecondinimportancetothedoctrineofconservationitself。ProfessorWilliamThomsonLordKelvininhisstudiesinthermodynamicswasearlyimpressedwiththefactthatwhereasallthemolarmotiondevelopedthroughlabororgravitycouldbeconvertedintoheat，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，expressedhisbeliefthatelectricityandmagnetismarebutmanifestationsofvariousconditionsofstressandmotionintheetherealmediumelectricityadisplacementofstrain，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。Itisassumedonallsidesthattheetherisacontinuous，incompressiblebody，possessingrigidityandelasticity。LordKelvinhasevencalculatedtheprobabledensityofthisether，anditscoefficientofrigidity。Asmightbesupposed，itisallbutinfinitelytenuousascomparedwithanytangiblesolid，anditsrigidityisbutinfinitesimalascomparedwiththatofsteel。Inaword，itcombinespropertiesoftangiblematterinawaynotknowninanytangiblesubstance。Thereforewecannotpossiblyconceiveitstrueconditioncorrectly。Thenearestapproximation，accordingtoLordKelvin，isfurnishedbyamouldoftransparentjelly。Itisacrude，inaccurateanalogy，ofcourse，thedensityandresistanceofjellyinparticularbeingutterlydifferentfromthoseoftheether；butthequiversthatrunthroughthejellywhenitisshaken，andtheelastictensionunderwhichitisplacedwhenitsmassistwistedabout，furnishsomeanalogytothequiversandstrainsintheether，whichareheldtoconstituteradiantenergy，magnetism，andelectricity。
Thegreatphysicistsofthedaybeingatoneregardingtheexistenceofthisall-pervadingether，itwouldbeamanifestpresumptionforanyonestandingwithoutthepaletochallengesofirmlyrootedabelief。
And，indeed，inanyevent，thereseemslittlegroundonwhichtobasesuchachallenge。Yetitmaynotbealtogetheramisstoreflectthatthephysicistofto-dayisnomorecertainofhisetherthanwashispredecessoroftheeighteenthcenturyoftheexistenceofcertainallegedsubstanceswhichhecalledphlogiston，caloric，corpusclesoflight，andmagneticandelectricfluids。
Itwouldbebuttherepetitionofhistoryshoulditchancethatbeforethecloseofanothercenturytheethershouldhavetakenitsplacealongwiththesediscardedcreationsofthescientificimaginationofearliergenerations。Thephilosopherofto-dayfeelsverysurethatanetherexists；butwhenhesaysthereis\"nodoubt\"ofitsexistencehespeaksincautiously，andstepsbeyondtheboundsofdemonstration。HedoesnotKNOWthatactioncannottakeplaceatadistance；
hedoesnotKNOWthatemptyspaceitselfmaynotperformthefunctionswhichheascribestohisspace-fillingether。
Meantime，however，theether，beitsubstanceorbeitonlydream-stuff，isservinganadmirablepurposeinfurnishingafulcrumformodernphysics。Notalonetothestudentofenergyhasitprovedinvaluable，buttothestudentofmatteritselfaswell。Outofitshypotheticalmistinesshasbeenrearedthemosttenabletheoryoftheconstitutionofponderablematterwhichhasyetbeensuggested——or，atanyrate，theonethatwillstandasthedefinitivenineteenth-centuryguessatthis\"riddleoftheages。\"Imean，ofcourse，thevortextheoryofatoms——thatprofoundandfascinatingdoctrinewhichsuggeststhatmatter，inallitsmultiformphases，isneithermorenorlessthanetherinmotion。
TheauthorofthiswonderfulconceptionisLordKelvin。
Theideawasborninhismindofahappyunionofmathematicalcalculationswithconcreteexperiments。
ThemathematicalcalculationswerelargelytheworkofHermannvonHelmholtz，who，abouttheyear1858，hadundertakentosolvesomeuniqueproblemsinvortexmotions。Helmholtzfoundthatavortexwhirl，onceestablishedinafrictionlessmedium，mustgoon，theoretically，unchangedforever。InalimitedmediumsuchawhirlmaybeV-shaped，withitsendsatthesurfaceofthemedium。Wemayimitatesuchavortexbydrawingthebowlofaspoonquicklythroughacupofwater。Butinalimitlessmediumthevortexwhirlmustalwaysbeaclosedring，whichmaytakethesimpleformofahooporcircle，orwhichmaybeindefinitelycontorted，looped，or，sotospeak，knotted。Whethersimpleorcontorted，thisendlesschainofwhirlingmattertheparticlesrevolvingabouttheaxisoftheloopastheparticlesofastringrevolvewhenthestringisrolledbetweenthefingers
must，inafrictionlessmedium，retainitsformandwhirlonwithundiminishedspeedforever。
WhilethesetheoreticalcalculationsofHelmholtzwerefreshinhismind，LordKelvinthenSirWilliamThomsonwasshownbyProfessorP。G。Tait，ofEdinburgh，anapparatusconstructedforthepurposeofcreatingvortexringsinair。Theapparatus，whichanyonemayduplicate，consistedsimplyofaboxwithaholeboredinoneside，andapieceofcanvasstretchedacrosstheoppositesideinlieuofboards。Fumesofchlorideofammoniaaregeneratedwithinthebox，merelytorendertheairvisible。Bytappingwiththebandonthecanvassideofthebox，vortexringsofthecloudedairaredrivenout，preciselysimilarinappearancetothosesmoke-ringswhichsomeexperttobacco-
smokerscanproducebytappingontheircheeks，ortothoselargeroneswhichwesometimesseeblownoutfromthefunnelofalocomotive。
TheadvantageofProfessorTait’sapparatusisitsmanageablenessandthecertaintywithwhichthedesiredresultcanbeproduced。BeforeLordKelvin’sinterestedobservationitthrewoutringsofvarioussizes，whichmovedstraightacrosstheroomatvaryingratesofspeed，accordingtotheinitialimpulse，andwhichbehavedverystrangelywhencomingincontactwithoneanother。If，forexample，arapidlymovingringovertookanothermovinginthesamepath，theoneinadvanceseemedtopause，andtospreadoutitsperipherylikeanelasticband，whilethepursuerseemedtocontract，tillitactuallyslidthroughtheorificeoftheother，afterwhicheachringresumeditsoriginalsize，andcontinueditscourseasifnothinghadhappened。When，ontheotherhand，tworingsmovinginslightlydifferentdirectionscameneareachother，theyseemedtohaveanattractionforeachother；yetiftheyimpinged，theyboundedaway，quiveringlikeelasticsolids。Ifaneffortweremadetograsportocutoneoftheserings，thesubtlethingshrankfromthecontact，andslippedawayasifitwerealive。
Andallthewhilethebodywhichthusconducteditselfconsistedsimplyofawhirlintheair，madevisible，butnototherwiseinfluenced，bysmokyfumes。
Presentlythefrictionofthesurroundingairworetheringaway，anditfadedintothegeneralatmosphere——
often，however，notuntilithadpersistedformanyseconds，andpassedclearacrossalargeroom。Clearly，iftherewerenofriction，thering’sinertiamustmakeitapermanentstructure。OnlythefrictionlessmediumwaslackingtofulfilalltheconditionsofHelmholtz’sindestructiblevortices。AndatonceLordKelvinbethoughthimofthefrictionlessmediumwhichphysicistshadnowbeguntoaccept——theall-pervadingether。
Whatifvortexringswerestartedinthisether，musttheynothavethepropertieswhichthevortexringsinairhadexhibited——inertia，attraction，elasticity？
Andarenotthesethepropertiesofordinarytangiblematter？Isitnotprobable，then，thatwhatwecallmatterconsistsmerelyofaggregationsofinfinitesimalvortexringsintheether？
ThusthevortextheoryofatomstookforminLordKelvin’smind，anditsexpressiongavetheworldwhatmanyphilosophersofourtimeregardasthemostplausibleconceptionoftheconstitutionofmatterhithertoformulated。Itisonlyatheory，tobesure；
itsauthorwouldbethelastpersontoclaimfinalityforit。\"Itisonlyadream，\"LordKelvinsaidtome，inreferringtoitnotlongago。Butithasabasisinmathematicalcalculationandinanalogicalexperimentsuchasnoothertheoryofmattercanlayclaimto，andithasaunifyingormonistictendencythatmakesit，forthephilosophicalmind，littlelessthanfascinating。
Trueorfalse，itisthedefinitivetheoryofmatterofthetwentiethcentury。
Quiteasidefromthequestionoftheexactconstitutionoftheultimateparticlesofmatter，questionsastothedistributionofsuchparticles，theirmutualrelations，properties，andactions，cameinforafullshareofattentionduringthenineteenthcentury，thoughthefoundationsforthemodernspeculationswerefurnishedinapreviousepoch。Themostpopulareighteenth-
centuryspeculationastotheultimateconstitutionofmatterwasthatofthelearnedItalianpriest，RogerJosephBoscovich，publishedin1758，inhisTheoriaPhilosophiaeNaturalis。\"Inthistheory，\"accordingtoanearlycommentator，\"thewholemassofwhichthebodiesoftheuniversearecomposedissupposedtoconsistofanexceedinglygreatyetfinitenumberofsimple，indivisible，inextendedatoms。TheseatomsareenduedbytheCreatorwithREPULSIVEandATTRACTIVE
forces，whichvaryaccordingtothedistance。Atverysmalldistancestheparticlesofmatterrepeleachother；
andthisrepulsiveforceincreasesbeyondalllimitsasthedistancesarediminished，andwillconsequentlyforeverpreventactualcontact。Whentheparticlesofmatterareremovedtosensibledistances，therepulsiveisexchangedforanattractiveforce，whichdecreasesininverseratiowiththesquaresofthedistances，andextendsbeyondthespheresofthemostremotecomets。\"
Thisconceptionoftheatomasamerecentreofforcewashardlysuchascouldsatisfyanymindotherthanthemetaphysical。Noonemadeaconspicuousattempttoimproveupontheidea，however，tilljustatthecloseofthecentury，whenHumphryDavywasled，inthecourseofhisstudiesofheat，tospeculateastothechangesthatoccurintheintimatesubstanceofmatterunderalteredconditionsoftemperature。Davy，aswehaveseen，regardedheatasamanifestationofmotionamongtheparticlesofmatter。Asallbodieswithwhichwecomeincontacthavesometemperature，Davyinferredthattheintimateparticlesofeverysubstancemustbeperpetuallyinastateofvibration。
Suchvibrations，hebelieved，producedthe\"repulsiveforce\"whichincommonwithBoscovichheadmittedasholdingtheparticlesofmatteratadistancefromoneanother。Toheatasubstancemeansmerelytoincreasetherateofvibrationofitsparticles；thusalso，plainly，increasingtherepulsiveforcesandexpandingthebulkofthemassasawhole。Ifthedegreeofheatappliedbesufficient，therepulsiveforcemaybecomestrongenoughquitetoovercometheattractiveforce，andtheparticleswillseparateandtendtoflyawayfromoneanother，thesolidthenbecomingagas。
NotmuchattentionwaspaidtotheseverysuggestiveideasofDavy，becausetheywerefoundedontheideathatheatismerelyamotion，whichthescientificworldthenrepudiated；buthalfacenturylater，whenthenewtheoriesofenergyhadmadetheirway，therecamearevivalofpracticallythesameideasoftheparticlesofmattermoleculestheywerenowcalled
whichDavyhadadvocated。ThenitwasthatClausiusinGermanyandClerk-MaxwellinEnglandtookuptheinvestigationofwhatcametobeknownasthekinetictheoryofgases——thenowfamiliarconceptionthatallthephenomenaofgasesareduetothehelter-
skelterflightoftheshowersofwidelyseparatedmoleculesofwhichtheyarecomposed。Thespecificideathatthepressureor\"spring\"ofgasesisduetosuchmolecularimpactswasduetoDanielBournelli，whoadvanceditearlyintheeighteenthcentury。Theidea，thenlittlenoticed，hadbeenrevivedaboutacenturylaterbyWilliamHerapath，andagainwithsomesuccessbyJ。J。Waterston，ofBombay，about1846；butitgainednodistinctfootinguntiltakeninhandbyClausiusin1857andbyClerk-Maxwellin1859。
TheconsiderationsthatledClerk-Maxwelltotakeupthecomputationsmaybestatedinhisownwords，asformulatedinapaper\"OntheMotionsandCollisionsofPerfectlyElasticSpheres。\"
\"Somanyofthepropertiesofmatter，especiallywheninthegaseousform，\"hesays，\"canbededucedfromthehypothesisthattheirminutepartsareinrapidmotion，thevelocityincreasingwiththetemperature，thattheprecisenatureofthismotionbecomesasubjectofrationalcuriosity。DanielBournelli，Herapath，Joule，Kronig，Clausius，etc。，haveshownthattherelationsbetweenpressure，temperature，anddensityinaperfectgascanbeexplainedbysupposingtheparticlestomovewithuniformvelocitiesinstraightlines，strikingagainstthesidesofthecontainingvesselandthusproducingpressure。Itisnotnecessarytosupposeeachparticletotraveltoanygreatdistanceinthesamestraightline；fortheeffectinproducingpressurewillbethesameiftheparticlesstrikeagainsteachother；sothatthestraightlinedescribedmaybeveryshort。M。Clausiushasdeterminedthemeanlengthofpathintermsoftheaverageoftheparticles，andthedistancebetweenthecentresoftwoparticleswhenthecollisiontakesplace。Wehaveatpresentnomeansofascertainingeitherofthesedistances；butcertainphenomena，suchastheinternalfrictionofgases，theconductionofheatthroughagas，andthediffusionofonegasthroughanother，seemtoindicatethepossibilityofdeterminingaccuratelythemeanlengthofpathwhichaparticledescribesbetweentwosuccessivecollisions。Inordertolaythefoundationofsuchinvestigationsonstrictmechanicalprinciples，Ishalldemonstratethelawsofmotionofanindefinitenumberofsmall，hard，andperfectlyelasticspheresactingononeanotheronlyduringimpact。
Ifthepropertiesofsuchasystemofbodiesarefoundtocorrespondtothoseofgases，animportantphysicalanalogywillbeestablished，whichmayleadtomoreaccurateknowledgeofthepropertiesofmatter。
Ifexperimentsongasesareinconsistentwiththehypothesisofthesepropositions，thenourtheory，thoughconsistentwithitself，isprovedtobeincapableofexplainingthephenomenaofgases。Ineithercaseitisnecessarytofollowouttheseconsequencesofthehypothesis。
\"Insteadofsayingthattheparticlesarehard，spherical，andelastic，wemay，ifweplease，saytheparticlesarecentresofforce，ofwhichtheactionisinsensibleexceptatacertainverysmalldistance，whenitsuddenlyappearsasarepulsiveforceofverygreatintensity。Itisevidentthateitherassumptionwillleadtothesameresults。Forthesakeofavoidingtherepetitionofalongphraseabouttheserepulsivebodies，Ishallproceedupontheassumptionofperfectlyelasticsphericalbodies。Ifwesupposethoseaggregatemoleculeswhichmovetogethertohaveaboundingsurfacewhichisnotspherical，thentherotatorymotionofthesystemwillcloseupacertainproportionofthewholevisviva，ashasbeenshownbyClausius，andinthiswaywemayaccountforthevalueofthespecificheatbeinggreaterthanonthemoresimplehypothesis。\"[1]
TheelaborateinvestigationsofClerk-Maxwellservednotmerelytosubstantiatethedoctrine，butthrewafloodoflightupontheentiresubjectofmoleculardynamics。
Soonthephysicistscametofeelascertainoftheexistenceoftheseshowersofflyingmoleculesmakingupagasasiftheycouldactuallyseeandwatchtheirindividualactions。Throughstudyoftheviscosityofgases——thatistosay，ofthedegreeoffrictionaloppositiontheyshowtoanobjectmovingthroughthemortoanothercurrentofgas——anideawasgained，withtheaidofmathematics，oftherateofspeedatwhichtheparticlesofthegasaremoving，andthenumberofcollisionswhicheachparticlemustexperienceinagiventime，andofthelengthoftheaveragefreepathtraversedbythemoleculebetweencollisions，Thesemeasurementswereconfirmedbystudyoftherateofdiffusionatwhichdifferentgasesmixtogether，andalsobytherateofdiffusionofheatthroughagas，boththesephenomenabeingchieflyduetothehelter-skelterflightofthemolecules。
Itissufficientlyastonishingtobetoldthatsuchmeasurementsasthesehavebeenmadeatall，buttheastonishmentgrowswhenonehearstheresults。ItappearsfromClerk-Maxwell’scalculationsthatthemeanfreepath，ordistancetraversedbythemoleculesbetweencollisionsinordinaryair，isaboutone-half-millionthofaninch；whilethespeedofthemoleculesissuchthateachoneexperiencesabouteightbillionsofcollisionspersecond！Itwouldbehard，perhaps，tociteanillustrationshowingtherefinementsofmodernphysicsbetterthanthis；unless，indeed，oneotherresultthatfolloweddirectlyfromthesecalculationsbeconsideredsuch——thefeat，namely，ofmeasuringthesizeofthemoleculesthemselves。Clausiuswasthefirsttopointouthowthismightbedonefromaknowledgeofthelengthoffreepath；andthecalculationsweremadebyLoschmidtinGermanyandbyLordKelvininEngland，independently。
Theworkispurelymathematical，ofcourse，buttheresultsareregardedasunassailable；indeed，LordKelvinspeaksofthemasbeingabsolutelydemonstrativewithincertainlimitsofaccuracy。Thisdoesnotmean，however，thattheyshowtheexactdimensionsofthemolecule；itmeansanestimateofthelimitsofsizewithinwhichtheactualsizeofthemoleculemaylie。
Theselimits，LordKelvinestimates，areabouttheone-
ten-millionthofacentimetreforthemaximum，andtheone-one-hundred-millionthofacentimetrefortheminimum。Suchfiguresconveynoparticularmeaningtoourbluntsenses，butLordKelvinhasgivenatangibleillustrationthataidstheimaginationtoatleastavaguecomprehensionoftheunthinkablesmallnessofthemolecule。Heestimatesthatifaball，sayofwaterorglass，about\"aslargeasafootball，weretobemagnifieduptothesizeoftheearth，eachconstituentmoleculebeingmagnifiedinthesameproportion，themagnifiedstructurewouldbemorecoarse-grainedthanaheapofshot，butprobablylesscoarse-grainedthanaheapoffootballs。\"
Severalothermethodshavebeenemployedtoestimatethesizeofmolecules。Oneoftheseisbaseduponthephenomenaofcontactelectricity；anotheruponthewave-theoryoflight；andanotheruponcapillaryattraction，asshowninthetensefilmofasoap-bubble！
Nooneofthesemethodsgivesresultsmoredefinitethanthatduetothekinetictheoryofgases，justoutlined；
buttheimportantthingisthattheresultsobtainedbythesedifferentmethodsallofthemduetoLordKelvinagreewithoneanotherinfixingthedimensionsofthemoleculeatsomewhereaboutthelimitsalreadymentioned。Wemayfeelverysureindeed，therefore，thatthemoleculesofmatterarenottheunextended，formlesspointswhichBoscovichandhisfollowersoftheeighteenthcenturythoughtthem。Butallthis，itmustbeborneinmind，referstothemolecule，nottotheultimateparticleofmatter，aboutwhichweshallhavemoretosayinanotherconnection。Curiouslyenough，weshallfindthatthelatesttheoriesastothefinaltermoftheseriesarenotsoveryfarafieldfromthedreamingsoftheeighteenth-centuryphilosophers；
theelectronofJ。J。ThompsonshowsmanypointsofresemblancetotheformlesscentreofBoscovich。
Whatevertheexactformofthemolecule，itsoutlineissubjecttoincessantvariation；fornothinginmolecularscienceisregardedasmorefirmlyestablishedthanthatthemolecule，underallordinarycircumstances，isinastateofintensebutvariablevibration。Theentireenergyofamoleculeofgas，forexample，isnotmeasuredbyitsmomentum，butbythisplusitsenergyofvibrationandrotation，duetothecollisionsalreadyreferredto。Clausiushasevenestimatedtherelativeimportanceofthesetwoquantities，showingthatthetranslationalmotionofamoleculeofgasaccountsforonlythree-fifthsofitskineticenergy。Thetotalenergyofthemoleculewhichwecall\"heat\"
includesalsoanotherfactor——namely，potentialenergy，orenergyofposition，duetotheworkthathasbeendoneonexpanding，inovercomingexternalpressure，andinternalattractionbetweenthemoleculesthemselves。
Thispotentialenergywhichwillberecoveredwhenthegascontractsisthe\"latentheat\"ofBlack，whichsolongpuzzledthephilosophers。Itislatentinthesamesensethattheenergyofaballthrownintotheairislatentatthemomentwhentheballpoisesatitsgreatestheightbeforebeginningtofall。
Itthusappearsthatavarietyofmotions，realandpotential，enterintotheproductionoftheconditionwetermheat。Itis，however，chieflythetranslationalmotionwhichismeasurableastemperature；andthis，too，whichmostobviouslydeterminesthephysicalstateofthesubstancethatthemoleculescollectivelycompose——whether，thatistosay，itshallappeartoourbluntperceptionsasagas，aliquid，orasolid。Inthegaseousstate，aswehaveseen，thetranslationalmotionofthemoleculesisrelativelyenormous，themoleculesbeingwidelyseparated。Itdoesnotfollow，asweformerlysupposed，thatthisisevidenceofarepulsivepoweractingbetweenthemolecules。Thephysicistsofto-day，headedbyLordKelvin，declinetorecognizeanysuchpower。Theyholdthatthemoleculesofagasflyinstraightlinesbyvirtueoftheirinertia，quiteindependentlyofoneanother，exceptattimesofcollision，fromwhichtheyreboundbyvirtueoftheirelasticity；oronanapproachtocollision，inwhichlattercase，comingwithintherangeofmutualattraction，twomoleculesmaycircleabouteachother，asacometcirclesaboutthesun，thenrushapartagain，asthecometrushesfromthesun。
Itisobviousthatthelengthofthemeanfreepathofthemoleculesofagasmaybeincreasedindefinitelybydecreasingthenumberofthemoleculesthemselvesinacircumscribedspace。IthasbeenshownbyProfessorsTaitandDewarthatavacuummaybeproducedartificiallyofsuchadegreeofrarefactionthatthemeanfreepathoftheremainingmoleculesismeasurableininches。ThecalculationisbasedonexperimentsmadewiththeradiometerofProfessorCrookes，aninstrumentwhichinitselfisheldtodemonstratethetruthofthekinetictheoryofgases。SuchanattenuatedgasasthisisconsideredbyProfessorCrookesasconstitutingafourthstateofmatter，whichhetermsultra-
gaseous。
If，ontheotherhand，agasissubjectedtopressure，itsmoleculesarecrowdedclosertogether，andthelengthoftheirmeanfreepathisthuslessened。Ultimately，thepressurebeingsufficient，themoleculesarepracticallyincontinuouscontact。Meantimetheenormouslyincreasednumberofcollisionshassetthemoleculesmoreandmoreactivelyvibrating，andthetemperatureofthegashasincreased，as，indeed，necessarilyresultsinaccordancewiththelawoftheconservationofenergy。Noamountofpressure，therefore，cansufficebyitselftoreducethegastoaliquidstate。Itisbelievedthatevenatthecentreofthesun，wherethepressureisalmostinconceivablygreat，allmatteristoberegardedasreallygaseous，thoughthemoleculesmustbesopackedtogetherthattheconsistencyisprobablymorelikethatofasolid。
If，however，coincidentlywiththeapplicationofpressure，opportunitybegivenfortheexcessofheattobedissipatedtoacoldersurroundingmedium，themolecules，givingofftheirexcessofenergy，becomerelativelyquiescent，andatacertainstagethegasbecomesaliquid。Theexactpointatwhichthistransformationoccurs，however，differsenormouslyfordifferentsubstances。Inthecaseofwater，forexample，itisatemperaturemorethanfourhundreddegreesabovezero，centigrade；whileforatmosphericairitisonehundredandninety-fourdegreescentigradebelowzero，ormorethanahundredandfiftydegreesbelowthepointatwhichmercuryfreezes。
Beithighorlow，thetemperatureabovewhichanysubstanceisalwaysagas，regardlessofpressure，iscalledthecriticaltemperature，orabsoluteboiling-
point，ofthatsubstance。Itdoesnotfollow，however，thatbelowthispointthesubstanceisnecessarilyaliquid。Thisisamatterthatwillbedeterminedbyexternalconditionsofpressure。Evenfarbelowthecriticaltemperaturethemoleculeshaveanenormousdegreeofactivity，andtendtoflyasunder，maintainingwhatappearstobeagaseous，butwhattechnicallyiscalledavaporous，condition——thedistinctionbeingthatpressurealonesufficestoreducethevaportotheliquidstate。Thuswatermaychangefromthegaseoustotheliquidstateatfourhundreddegreesabovezero，butunderconditionsofordinaryatmosphericpressureitdoesnotdosountilthetemperatureisloweredthreehundreddegreesfurther。Belowfourhundreddegrees，however，itistechnicallyavapor，notagas；butthesoledifference，itwillbeunderstood，isinthedegreeofmolecularactivity。
Itthusappearedthattheprevalenceofwaterinavaporousandliquidratherthanina\"permanently\"
gaseousconditionhereontheglobeisamereincidentoftelluricevolution。Equallyincidentalisthefactthattheairwebreatheis\"permanently\"gaseousandnotliquidorsolid，asitmightbeweretheearth’ssurfacetemperaturetobeloweredtoadegreewhich，inthelargerview，mayberegardedastrifling。Betweentheatmospherictemperatureintropicalandinarcticregionsthereisoftenavariationofmorethanonehundreddegrees；werethetemperaturereducedanotherhundred，thepointwouldbereachedatwhichoxygengasbecomesavapor，andunderincreasedpressurewouldbealiquid。Thirty-sevendegreesmorewouldbringustothecriticaltemperatureofnitrogen。
Noristhisameretheoreticalassumption；itisadeterminationofexperimentalscience，quiteindependentoftheory。Thephysicistinthelaboratoryhasproducedartificialconditionsoftemperatureenablinghimtochangethestateofthemostpersistentgases。
Somefiftyyearssince，whenthekinetictheorywasinitsinfancy，Faradayliquefiedcarbonic-acidgas，amongothers，andtheexperimentsthusinauguratedhavebeenextendedbynumerousmorerecentinvestigators，notablybyCailletetinSwitzerland，byPictetinFrance，andbyDr。Thomas。AndrewsandProfessorJamesDewarinEngland。Inthecourseoftheseexperimentsnotonlyhasairbeenliquefied，buthydrogenalso，themostsubtleofgases；andithasbeenmademoreandmoreapparentthatgasandliquidare，asAndrewslongagoasserted，\"onlydistantstagesofalongseriesofcontinuousphysicalchanges。\"Ofcourse，ifthetemperaturebeloweredstillfurther，theliquidbecomesasolid；andthischangealsohasbeeneffectedinthecaseofsomeofthemost\"permanent\"gases，includingair。
Thedegreeofcold——thatis，ofabsenceofheat——
thusproducedisenormous，relativelytoanythingofwhichwehaveexperienceinnaturehereattheearthnow，yetthemoleculesofsolidifiedair，forexample，arenotabsolutelyquiescent。Inotherwords，theystillhaveatemperature，thoughsoverylow。Butitisclearlyconceivablethatastagemightbereachedatwhichthemoleculesbecameabsolutelyquiescent，asregardseithertranslationalorvibratorymotion。Suchaheatlessconditionhasbeenapproached，butasyetnotquiteattained，inlaboratoryexperiments。Itiscalledtheabsolutezerooftemperature，andisestimatedtobeequivalenttotwohundredandseventy-
threedegreesCentigradebelowthefreezing-pointofwater，orordinaryzero。
Atemperatureorabsenceoftemperaturecloselyapproximatingthisisbelievedtoobtainintheetherealoceanofinterplanetaryandinterstellarspace，whichtransmits，butisthoughtnottoabsorb，radiantenergy。
Wehereontheearth’ssurfaceareprotectedfromexposuretothiscold，whichwoulddepriveeveryorganicthingoflifealmostinstantaneously，solelybythethinblanketofatmospherewithwhichtheglobeiscoated。Itwouldseemasifthisatmosphere，exposedtosuchatemperatureatitssurface，musttherebeincessantlyliquefied，andthusfallbacklikeraintobedissolvedintogasagainwhileitstillismanymilesabovetheearth’ssurface。Thismaybethereasonwhyitsscurryingmoleculeshavenotlongagowanderedoffintospaceandlefttheworldwithoutprotection。
Butwhetherornotsuchliquefactionoftheairnowoccursinourouteratmosphere，therecanbenoquestionastowhatmustoccurinitsentiredepthwerewepermanentlyshutofffromtheheatinginfluenceofthesun，astheastronomersthreatenthatwemaybeinafutureage。Eachmolecule，notaloneoftheatmosphere，butoftheentireearth’ssubstance，iskeptaquiverbytheenergywhichitreceives，orhasreceived，directlyorindirectly，fromthesun。Lefttoitself，eachmoleculewouldwearoutitsenergyandfritteritoffintothespaceaboutit，ultimatelyrunningcompletelydown，assurelyasanyhuman-mademachinewhosepowerisnotfromtimetotimerestored。If，then，itshallcometopassinsomefutureagethatthesun’sraysfailus，thetemperatureoftheglobemustgraduallysinktowardstheabsolutezero。Thatistosay，themoleculesofgaswhichnowflyaboutatsuchinconceivablespeedmustdrophelplesstotheearth；
liquidsmustinturnbecomesolids；andsolidsthemselves，theirmolecularquiversutterlystilled，mayperhapstakeonpropertiesthenatureofwhichwecannotsurmise。
Yeteventhen，accordingtothecurrenthypothesis，theheatlessmoleculewillstillbeathinginstinctwithlife。Itsvortexwhirlwillstillgoon，uninfluencedbythedying-outofthosesubordinatequiversthatproducedthetransitoryeffectwhichwecalltemperature。
Forthosetransitorythrills，thoughdeterminingthephysicalstateofmatterasmeasuredbyourcrudeorgansofsense，werenomorethannon-essentialincidents；
butthevortexwhirlistheessenceofmatteritself。Someestimatesastotheexactcharacterofthisintramolecularmotion，togetherwithrecenttheoriesastotheactualstructureofthemolecule，willclaimourattentioninalatervolume。Weshallalsohaveoccasioninanotherconnectiontomakefullerinquiryastothephenomenaoflowtemperature。
MODERNDEVELOPMENTOFTHECHEMICALANDBIOLOGICALSCIENCES
ASregardschronology，theepochcoveredinthepresentvolumeisidenticalwiththatviewedintheprecedingone。Butnowasregardssubjectmatterwepassontothosediversephasesofthephysicalworldwhicharethefieldofthechemist，andtothoseyetmoreintricateprocesseswhichhavetodowithlivingorganisms。Soradicalarethechangesherethatweseemtobeenteringnewworlds；andyet，hereasbefore，thereareintimationsofthenewdiscoveriesawaybackintheGreekdays。
ThesolutionoftheproblemofrespirationwillremindusthatAnaxagorashalfguessedthesecret；andinthosediversifiedstudieswhichtellusoftheDaltonianatominitswonderfultransmutations，weshallberemindedagainoftheClazomenianphilosopherandhissuccessorDemocritus。
YetweshouldpresstheanalogymuchtoofarwerewetointimatethattheGreekoftheelderdayoranythinkerofamorerecentperiodhadpenetrated，eveninthevaguestway，allofthemysteriesthatthenineteenthcenturyhasrevealedinthefieldsofchemistryandbiology。AttheverymosttheinsightofthosegreatGreeksandofthewonderfulseventeenth-centuryphilosopherswhosooftenseemedonthevergeofourlaterdiscoveriesdidnomorethanvaguelyanticipatetheirsuccessorsofthislatercentury。Togainanaccurate，reallyspecificknowledgeofthepropertiesofelementarybodieswasreservedforthechemistsofarecentepoch。ThevagueGreekquestioningsastoorganicevolutionwereworld-widefromthepreciseinductionsofaDarwin。IfthemediaevalArabianendeavoredtodulltheknifeofthesurgeonwiththeuseofdrugs，hisresultshardlymerittobetermedevenananticipationofmodernanaesthesia。
Andwhenwespeakofpreventivemedicine——ofbacteriologyinallitsphases——wehavetodowithamarvellousfieldofwhichnopreviousgenerationofmenhadeventheslightestinkling。
Allinall，then，thosethatliebeforeusareperhapsthemostwonderfulandthemostfascinatingofallthefieldsofscience。
Asthechaptersoftheprecedingbookcarriedusoutintoamacrocosmofinconceivablemagnitude，ourpresentstudiesaretorevealamicrocosmofequallyinconceivablesmallness。Asthestudiesofthephysicistattemptedtorevealtheverynatureofmatterandofenergy，wehavenowtoseekthesolutionoftheyetmoreinscrutableproblemsoflifeandofmind。
I。THEPHLOGISTONTHEORYINCHEMISTRY
Thedevelopmentofthescienceofchemistryfromthe\"science\"ofalchemyisastrikingexampleofthecompleterevolutionintheattitudeofobserversinthefieldofscience。Ashasbeenpointedoutinaprecedingchapter，thealchemist，havingapreconceivedideaofhowthingsshouldbe，madeallhisexperimentstoprovehispreconceivedtheory；whilethechemistreversesthisattitudeofmindandbaseshisconceptionsontheresultsofhislaboratoryexperiments。Inshort，chemistryiswhatalchemynevercouldbe，aninductivescience。Butthistransitionfromonepointofviewtoanexactlyoppositeonewasnecessarilyaveryslowprocess。Ideasthathaveheldundisputedswayoverthemindsofsucceedinggenerationsforhundredsofyearscannotbeoverthrowninamoment，unlesstheagentofsuchanoverthrowbesoobviousthatitcannotbechallenged。Therudimentarychemistrythatoverthrewalchemyhadnothingsoobviousandpalpable。
Thegreatfirststepwasthesubstitutionoftheoneprinciple，phlogiston，forthethreeprinciples，salt，sulphur，andmercury。
Wehaveseenhowtheexperimentofburningorcalciningsuchametalaslead\"destroyed\"theleadassuch，leavinganentirelydifferentsubstanceinitsplace，andhowtheoriginalmetalcouldberestoredbytheadditionofwheattothecalcinedproduct。Tothealchemistthiswas\"mortification\"and\"revivification\"ofthemetal。For，aspointedoutbyParacelsus，\"anythingthatcouldbekilledbymancouldalsoberevivifiedbyhim，althoughthiswasnotpossibletothethingskilledbyGod。\"Theburningofsuchsubstancesaswood，wax，oil，etc。，wasalsolookeduponasthesame\"killing\"process，andthefactthatthealchemistwasunabletorevivifythemwasregardedassimplythelackofskillonhispart，andinnowiseaffectingthetheoryitself。
Buttheiconoclasticspirit，ifnottheacceptanceofalltheteachings，ofthegreatParacelsushadbeengraduallytakingrootamongthebetterclassofalchemists，andaboutthemiddleoftheseventeenthcenturyRobertBoyle1626-1691calledattentiontothepossibilityofmakingawrongdeductionfromthephenomenonofthecalcinationofthemetals，becauseofaveryimportantfactor，theactionoftheair，whichwasgenerallyoverlooked。
Andheurgedhiscolleaguesofthelaboratoriestogivegreaterheedtocertainotherphenomenathatmightpassunnoticedintheordinarycalcinatingprocess。Inhiswork，TheScepticalChemist，heshowedthereasonsfordoubtingthethreefoldconstitutionofmatter；andinhisGeneralHistoryoftheAiradvancedsomenovelandcarefullystudiedtheoriesastothecompositionoftheatmosphere。Thiswasanimportantstep，andalthoughBoyleisnotdirectlyresponsibleforthephlogistontheory，itisprobablethathisexperimentsontheatmosphereinfluencedconsiderablytherealfounders，BeckerandStahl。
Boylegaveverydefinitelyhisideaofhowhethoughtairmightbecomposed。\"Iconjecturethattheatmosphericalairconsistsofthreedifferentkindsofcorpuscles，\"hesays；\"thefirst，thosenumberlessparticleswhich，intheformofvaporsordryexhalations，ascendfromtheearth，water，minerals，vegetables，animals，etc。；inaword，whateversubstancesareelevatedbythecelestialorsubterranealheat，andthencediffusedintotheatmosphere。Thesecondmaybeyetmoresubtle，andconsistofthoseexceedinglyminuteatoms，themagneticaleffluviaoftheearth，withotherinnumerableparticlessentoutfromthebodiesofthecelestialluminaries，andcausing，bytheirinfluence，theideaoflightinus。Thethirdsortisitscharacteristicandessentialproperty，Imeanpermanentlyelasticparts。Varioushypothesesmaybeframedrelatingtothestructureoftheselaterparticlesoftheair。Theymightberesembledtothespringsofwatches，coiledupandendeavoringtorestorethemselves；towool，which，beingcompressed，hasanelasticforce；toslenderwiresofdifferentsubstances，consistencies，lengths，andthickness；ingreatercurlsorless，nearto，orremotefromeachother，etc。，yetallcontinuingspringy，expansible，andcompressible。Lastly，theymayalsobecomparedtothethinshavingsofdifferentkindsofwood，variousintheirlengths，breadth，andthickness。Andthis，perhaps，willseemthemosteligiblehypothesis，becauseit，insomemeasure，illustratestheproductionoftheelasticparticlesweareconsidering。Fornoartorcuriousinstrumentsarerequiredtomaketheseshavingswhosecurlsareinnowiseuniform，butseeminglycasual；andwhatismoreremarkable，bodiesthatbeforeseemedunelastic，asbeamsandblocks，willaffordthem。\"[1]
Althoughthisexplanationofthecompositionoftheairismostcrude，ithadtheeffectofdirectingattentiontothefactthattheatmosphereisnot\"merenothingness，\"buta\"something\"withadefinitecomposition，andthisservedasagoodfoundationforfutureinvestigations。Tobesure，Boylewasneitherthefirstnortheonlychemistwhohadsuspectedthattheairwasamixtureofgases，andnotasimpleone，andthatonlycertainofthesegasestakepartintheprocessofcalcination。JeanRey，aFrenchphysician，andJohnMayow，anEnglishman，hadpreformedexperimentswhichshowedconclusivelythattheairwasnotasimplesubstance；butBoyle’sworkwasbetterknown，andinitseffectprobablymoreimportant。ButwithallBoyle’sexplanationsofthecompositionofair，hestillbelievedthattherewasaninexplicablesomething，a\"vitalsubstance，\"whichhewasunabletofathom，andwhichlaterbecamethebasisofStahl’sphlogistontheory。Commentingonthismysterioussubstance，Boylesays:
\"The，difficultywefindinkeepingflameandfirealive，thoughbutforalittletime，withoutair，rendersitsuspiciousthattherebedispersedthroughtherestoftheatmospheresomeoddsubstance，eitherofasolar，astral，orotherforeignnature；onaccountofwhichtheairissonecessarytothesubstanceofflame！\"ItwasthisideathatattractedtheattentionofGeorgeErnstStahl1660-1734，aprofessorofmedicineintheUniversityofHalle，wholaterfoundedhisnewtheoryuponit。
Stahl’stheorywasadevelopmentofanearlierchemist，JohannJoachimBecker1635-1682，inwhosefootstepshefollowedandwhoseexperimentshecarriedfurther。
InmanyexperimentsStahlhadbeenstruckwiththefactthatcertainsubstances，whiledifferingwidely，fromoneanotherinmanyrespects，werealikeincombustibility。Fromthishearguedthatallcombustiblesubstancesmustcontainacommonprinciple，andthisprinciplehenamedphlogiston。Thisphlogistonhebelievedtobeintimatelyassociatedincombinationwithothersubstancesinnature，andinthatconditionnotperceivablebythesenses；butitwassupposedtoescapeasasubstanceburned，andbecomeapparenttothesensesasfireorflame。Inotherwords，phlogistonwassomethingimprisonedinacombustiblestructureitselfformingpartofthestructure，andonlyliberatedwhenthisstructurewasdestroyed。Fire，orflame，wasFREEphlogiston，whiletheimprisonedphlogistonwascalledCOMBINEDPHLOGISTON，orcombinedfire。Thepeculiarqualityofthisstrangesubstancewasthatitdislikedfreedomandwasalwaysstrivingtoconcealitselfinsomecombustiblesubstance。
Boyle’stentativesuggestionthatheatwassimplymotionwasapparentlynotacceptedbyStahl，orperhapsitwasunknowntohim。
Accordingtothephlogistictheory，thepartremainingafterasubstancewasburnedwassimplytheoriginalsubstancedeprivedofphlogiston。Torestoretheoriginalcombustiblesubstance，itwasnecessarytoheattheresidueofthecombustionwithsomethingthatburnedeasily，sothatthefreedphlogistonmightagaincombinewiththeashes。Thiswasexplainedbythesuppositionthatthemorecombustibleasubstancewasthemorephlogistonitcontained，andsincefreephlogistonsoughtalwaystocombinewithsomesuitablesubstance，itwasonlynecessarytomixthephlogisticatingagents，suchascharcoal，phosphorus，oils，fats，etc。，withtheashesoftheoriginalsubstance，andheatthemixture，thephlogistonthusfreedunitingatoncewiththeashes。Thistheoryfittedverynicelyasappliedtothecalcinedleadrevivifiedbythegrainsofwheat，althoughwithsomeotherproductsofcalcinationitdidnotseemtoapplyatall。
Itwillbeseenfromthisthatthephlogistictheorywasasteptowardschemistryandawayfromalchemy。Itledawayfromtheideaofa\"spirit\"inmetalsthatcouldnotbeseen，felt，orappreciatedbyanyofthesenses，andsubstitutedforitaprinciplewhich，althoughafalselyconceivedone，wasstillmuchmoretangiblethanthe\"spirit，\"sinceitcouldbeseenandfeltasfreephlogistonandweighedandmeasuredascombinedphlogiston。Thedefinitenessofthestatementthatametal，forexample，wascomposedofphlogistonandanelementwasmuchlessenigmatic，evenifwrong，thanthestatementofthealchemistthat\"metalsareproducedbythespiritualactionofthethreeprinciples，salt，mercury，sulphur\"——particularlywhenitisexplainedthatsalt，mercury，andsulphurwerereallynotwhattheirnamesimplied，andthattherewasnouniversallyacceptedbeliefastowhattheyreallywere。
Themetals，whicharenowregardedaselementarybodies，wereconsideredcompoundsbythephlogistians，andtheybelievedthatthecalciningofametalwasaprocessofsimplification。Theynoted，however，thattheremainsofcalcinationweighedmorethantheoriginalproduct，andthenaturalinferencefromthiswouldbethatthemetalmusthavetakeninsomesubstanceratherthanhavegivenoffanything。Butthephlogistianshadnotlearnedtheall-importantsignificanceofweights，andtheirexplanationofvariationinweightwaseitherthatsuchgainorlosswasanunimportant\"accident\"atbest，orthatphlogiston，beinglight，tendedtolightenanysubstancecontainingit，sothatdrivingitoutofthemetalbycalcinationnaturallylefttheresidueheavier。
Atfirstthephlogistontheoryseemedtoexplaininanindisputablewayalltheknownchemicalphenomena。Gradually，however，asexperimentsmultiplied，itbecameevidentthattheplaintheoryasstatedbyStahlandhisfollowersfailedtoexplainsatisfactorilycertainlaboratoryreactions。Tomeetthesenewconditions，certainmodificationswereintroducedfromtimetotime，givingthetheoryaflexibilitythatwouldallowittocoverallcases。Butasthenumberofinexplicableexperimentscontinuedtoincrease，andnewmodificationstothetheorybecamenecessary，itwasfoundthatsomeofthesemodificationsweredirectlycontradictorytoothers，andthusthesimpletheorybecametoocumbersomefromthenumberofitsmodifications。Itssupportersdisagreedamongthemselves，firstastotheexplanationofcertainphenomenathatdidnotseemtoaccordwiththephlogistictheory，andalittlelaterastothetheoryitself。Butasyettherewasnosatisfactorysubstituteforthistheory，which，evenifunsatisfactory，seemedbetterthananythingthathadgonebeforeorcouldbesuggested。
Butthegoodeffectsoftheeraofexperimentalresearch，towhichthetheoryofStahlhadgivensuchanimpetus，wereshowingintheattitudeoftheexperimenters。Theworksofsomeoftheolderwriters，suchasBoyleandHooke，wereagainsoughtoutintheirdustycornersandconsulted，andtheirsurmisesastothepossiblemixtureofvariousgasesintheairweremorecarefullyconsidered。Stillthephlogistontheorywasfirmlygroundedinthemindsofthephilosophers，whocanhardlybecensuredforadheringtoit，atleastuntilsomesatisfactorysubstitutewasoffered。Thefoundationforsuchatheorywasfinallylaid，asweshallseepresently，bytheworkofBlack，Priestley，Cavendish，andLavoisier，intheeighteenthcentury，butthephlogistontheorycannotbesaidtohavefinallysuccumbeduntiltheopeningyearsofthenineteenthcentury。