Andthesamekindofinferencemustapplytoallthematuredstarswhichweseeintheheavens。
Whenwedulytakeallthesethingsintotheaccount，thecaseofoursolarsystemwillappearasonlyoneofathousandcasesofevolutionanddissolutionwithwhichtheheavensfurnishus。
Otherstars，likeoursun，haveundoubtedlystartedasvaporousmasses，andhavethrownoffplanetsincontracting。Theinferencemayseemaboldone，butitafterallinvolvesnootherassumptionthanthatofthecontinuityofnaturalphenomena。Itisnotlikely，therefore，thatthesolarsystemwillforeverbelefttoitself。Starswhichstronglygravitatetowardeachother，whilemovingthroughaperenniallyresistingmedium，mustintimebedrawntogether。ThecollisionofourextinctsunwithoneofthePleiades，afterthismanner，wouldverylikelysufficetogenerateevenagrandernebulathantheonewithwhichwestarted。Possiblytheentiregalacticsystemmay，inaninconceivablyremotefuture，remodelitselfinthisway；andpossiblythenebulafromwhichourowngroupofplanetshasbeenformedmayhaveoweditsorigintothedisintegrationofsystemswhichhadaccomplishedtheircareerinthedepthsofthebygoneeternity。
Whentheproblemisextendedtothesehugedimensions，theprospectofanultimatecessationofcosmicalworkisindefinitelypostponed，butatthesametimeitbecomesimpossibleforustodealverysecurelywiththequestionswehaveraised。Themagnitudesandperiodswehaveintroducedaresonearlyinfiniteastobafflespeculationitself:Onepoint，however，weseemdimlytodiscern。Supposingthestellaruniversenottobeabsolutelyinfiniteinextent，wemayholdthatthedayofdoom，sooftenpostponed，mustcomeatlast。Theconcentrationofmatteranddissipationofenergy，sooftenchecked，mustintheendprevail，sothat，asthefinaloutcomeofthings，theentireuniversewillbereducedtoasingleenormousball，deadandfrozen，solidandblack，itspotentialenergyofmotionhavingbeenalltransformedintoheatandradiatedaway。SuchaconclusionhasbeensuggestedbySirWilliamThomson，anditisquiteforciblystatedbytheauthorsof“TheUnseenUniverse。“
Theyremindusthat“iftherebeanyoneformofenergylessreadilyorlesscompletelytransformablethantheothers，andiftransformationsconstantlygoon，moreandmoreofthewholeenergyoftheuniversewillinevitablysinkintothislowergradeastimeadvances。“Nowradiantheat，aswehaveseen，issuchalowergradeofenergy。“Ateachtransformationofheat-energyintowork，alargeportionisdegraded，whileonlyasmallportionistransformedintowork。Sothatwhileitisveryeasytochangeallofourmechanicalorusefulenergyintoheat，itisonlypossibletotransformaportionofthisheat-energybackagainintowork。Aftereachchange，too，theheatbecomesmoreandmoredissipatedordegraded，andlessandlessavailableforanyfuturetransformation。Inotherwords，“ourauthorscontinue，“thetendencyofheatistowardsequalization；heatisparexcellencethecommunistofouruniverse，anditwillnodoubtultimatelybringthesystemtoanend……Itisabsolutelycertainthatlife，sofarasitisphysical，dependsessentiallyupontransformationsofenergy；itisalsoabsolutelycertainthatageafteragethepossibilityofsuchtransformationsisbecominglessandless；and，sofarasweyetknow，thefinalstateofthepresentuniversemustbeanaggregationintoonemassofallthematteritcontains，i。e。thepotentialenergygone，andapracticallyuselessstateofkineticenergy，i。e。
uniformtemperaturethroughoutthatmass。“Thusourauthorsconcludethatthevisibleuniversebeganintimeandwillintimecometoanend；andtheyaddthatunderthephysicalconditionsofsuchauniverse“immortalityisimpossible。“
Concerningthelatterinferenceweshallbyandbyhavesomethingtosay。Meanwhilethiswholespeculationastothefinalcessationofcosmicalworkseemstome——asitdoestomyfriend，ProfessorClifford[3]——bynomeanstrustworthy。Theconditionsoftheproblemsofartranscendourgraspthatanysuchspeculationmustremainanunverifiableguess。IdonotgowithProfessorCliffordindoubtingwhetherthelawsofmechanicsareabsolutelythesamethroughouteternity；Icannotquitereconcilesuchadoubtwithfaithintheprincipleofcontinuity。Butitdoesseemtomeneedful，beforeweconcludethatradiatedenergyisabsolutelyandforeverwasted，thatweshouldfindoutwhatbecomesofit。Whatwecallradiantheatissimplytransversewave-motion，propagatedwithenormousvelocitythroughanoceanofsubtleetherealmatterwhichbathestheatomsofallvisibleorpalpablebodiesandfillsthewholeofspace，extendingbeyondtheremoteststarwhichthetelescopecanreach。Whetherthereareanyboundsatalltothisetherealocean，orwhetheritisasinfiniteasspaceitself，wecannotsurmise。Ifitbelimited，thepossibledispersionofradiantenergyislimitedbyitsextent。Heatandlightcannottravelthroughemptiness。Iftheetherisboundedbysurroundingemptiness，thenarayofheat，onarrivingatthislimitingemptiness，wouldbereflectedbackassurelyasaballissentbackwhenthrownagainstasolidwall。
Ifthisbethecase，itwillnotaffectourconclusionsconcerningsuchatinyregionofspaceasisoccupiedbythesolarsystem，butitwillseriouslymodifySirWilliamThomson’ssuggestionastothefateoftheuniverseasawhole。Theradiancethrownawaybythesunisindeedlostsofarasthefutureofoursystemisconcerned，butnotasingleunitofitislostfromtheuniverse。Soonerorlater，reflectedbackinalldirections，itmustdoworkinonequarteroranother，sothatultimatestagnationbecomesimpossible。Itistruethatnosuchreturnofradiantenergyhasbeendetectedinourcorneroftheworld；butwehavenotyetsofardisentangledalltheforce-relationsoftheuniversethatweareentitledtoregardsuchareturnasimpossible。Thisisonewayofescapefromtheconsummationofthingsdepictedbyourauthors。Anotherwayofescapeisequallyavailable，ifwesupposethatwhiletheetheriswithoutboundsthestellaruniversealsoextendstoinfinity。
Forinthiscasethereproductionofnebulousmassesfitforgeneratingnewsystemsofworldsmustgoonthroughspacethatisendless，andconsequentlytheprocesscannevercometoanendandcanneverhavehadabeginning。Wehave，therefore，threealternatives:eitherthevisibleuniverseisfinite，whiletheetherisinfinite；orbotharefinite；orbothareinfinite。Onlyonthefirstsupposition，Ithink，dowegetauniversewhichbeganintimeandmustendintime。Betweensuchstupendousalternativeswehavenogroundsforchoosing。Butitwouldseemthatthethird，whetherstrictlytrueornot，bestrepresentsthestateofthecaserelativelytoourfeeblecapacityofcomprehension。Whetherabsolutelyinfiniteornot，thedimensionsoftheuniversemustbetakenaspracticallyinfinite，sofarashumanthoughtisconcerned。Theyimmeasurablytranscendthecapabilitiesofanygaugewecanbringtobearonthem。
Accordinglyallthatwearereallyentitledtohold，astheoutcomeofsoundspeculation，istheconceptionofinnumerablesystemsofworldsconcentratingoutofnebulousmasses，andthenrushingtogetheranddissolvingintosimilarmasses，asbubblesuniteandbreakup——nowhere，nowthere——intheirplayonthesurfaceofapool，andtothistremendousseriesofeventswecanassignneitherabeginningnoranend。
[3]FortnightlyReview，April，1875。
Wemustnowmakesomemoreexplicitmentionoftheetherwhichcarriesthroughspacetheraysofheatandlight。Inclosestconnectionwiththevisiblestellaruniverse，thevicissitudesofwhichwehavebrieflytraced，theall-pervadingetherconstitutesasortofunseenworldremarkableenoughfromanypointofview，buttowhichthetheoryofourauthorsascribescapacitieshithertounsuspectedbyscience。Theveryexistenceofanoceanofetherenvelopingthemoleculesofmaterialbodieshasbeendoubtedordeniedbymanyeminentphysicists，thoughofcoursenonehavecalledinquestionthenecessityforsomeinterstellarmediumforthetransmissionofthermalandluminousvibrations。
Thisscepticismhasbeen，Ithink，partiallyjustifiedbythemanydifficultiesencompassingtheconception，intowhich，however，weneednothereenter。Thatlightandheatcannotbeconveyedbyanyoftheordinarysensibleformsofmatterisunquestionable。Noneoftheformsofsensiblemattercanbeimaginedsufficientlyelastictopropagatewave-motionattherateofonehundredandeighty-eightthousandmilespersecond。
Yetarayoflightisaseriesofwaves，andimpliessomesubstanceinwhichthewavesoccur。Thesubstancerequiredisonewhichseemstopossessstrangelycontradictoryproperties。Itiscommonlyregardedasan“ether“orinfinitelyraresubstance；
but，asProfessorJevonsobserves，wemightaswellregarditasaninfinitelysolid“adamant。““SirJohnHerschelhascalculatedtheamountofforcewhichmaybesupposed，accordingtotheundulatorytheoryoflight，tobeexertedateachpointinspace，andfindsittobe1，148，000，000，000timestheelasticforceofordinaryairattheearth’ssurface，sothatthepressureoftheetheruponasquareinchofsurfacemustbeabout17，000，000，000，000，orseventeenbillionsofpounds。“[4]Yetatthesametimetheresistanceofferedbytheethertotheplanetarymotionsistoominutetobeappreciable。“Allourordinarynotions，“saysProfessorJevons，“mustbelaidasideincontemplatingsuchanhypothesis；yet[itis]nomorethantheobservedphenomenaoflightandheatforceustoaccept。WecannotdenyeventhestrangesuggestionofDr。Young，thattheremaybeindependentworlds，somepossiblyexistingindifferentpartsofspace，butothersperhapspervadingeachother，unseenandunknown，inthesamespace。Forifweareboundtoadmittheconceptionofthisadamantinefirmament，itisequallyeasytoadmitapluralityofsuch。“
[4]Jevons’sPrinciplesofScience，Vol。II。p。145。Thefigures，whichintheEnglishsystemofnumerationreadasseventeenbillions，wouldintheAmericansystemreadasseventeentrillions。
Theether，therefore，isunlikeanyoftheformsofmatterwhichwecanweighandmeasure。Insomerespectsitresemblesafluid，insomerespectsasolid。Itisbothhardandelastictoanalmostinconceivabledegree。Itfillsallmaterialbodieslikeaseainwhichtheatomsofthematerialbodiesareasislands，anditoccupiesthewholeofwhatwecallemptyspace。Itissosensitivethatadisturbanceinanypartofitcausesa“tremourwhichisfeltonthesurfaceofcountlessworlds。“Ouroldexperiencesofmattergiveusnoaccountofanysubstancelikethis；yettheundulatorytheoryoflightobligesustoadmitsuchasubstance，andthattheoryisaswellestablishedasthetheoryofgravitation。Obviouslywehavehereanenlargementofourexperienceofmatter。Theanalysisofthephenomenaoflightandradiantheathasbroughtusintomentalrelationswithmatterinadifferentstatefromanyinwhichwepreviouslyknewit。Forthesuppositionthattheethermaybesomethingessentiallydifferentfrommatteriscontradictedbyallthetermswehaveusedindescribingit。Strangeandcontradictoryasitspropertiesmayseem，aretheyanymorestrangethanthepropertiesofagaswouldseemifwewereforthefirsttimetodiscoveragasafterheretoforeknowingnothingbutsolidsandliquids？Ithinknot；andtheconclusionimpliedbyourauthorsseemstomeeminentlyprobable，thatintheso-calledetherwehavesimplyastateofmattermoreprimitivethanwhatweknowasthegaseousstate。Indeed，theconceptionsofmatternowcurrent，andinheritedfrombarbarousages，arelikelyenoughtobecrudeintheextreme。Itisnotstrangethatthestudyofsuchsubtleagenciesasheatandlightshouldobligeustomodifythem；anditwillnotbestrangeifthestudyofelectricityshouldentailstillfurtherrevisionofourideas。