Wearenowbroughttooneoftheprofoundestspeculationsofmoderntimes，thevortex-atomtheoryofHelmholtzandThomson，inwhichtheevolutionofordinarymatterfrometherisplainlyindicated。Thereaderfirstneedstoknowwhatvortex-motionis；
andthishasbeensobeautifullyexplainedbyProfessorClifford，thatIquotehisdescriptionentire:“Imaginearingofindia-rubber，madebyjoiningtogethertheendsofacylindricalpiecelikealead-pencilbeforeitiscut，tobeputuponaroundstickwhichitwilljustfitwithalittlestretching。Letthestickbenowpulledthroughtheringwhilethelatteriskeptinitsplacebybeingpulledtheotherwayontheoutside。Theindia-rubberhasthenwhatiscalledvortex-motion。Beforetheendswerejoinedtogether，whileitwasstraight，itmighthavebeenmadetoturnaroundwithoutchangingposition，byrollingitbetweenthehands。Justthesamemotionofrotationithasonthestick，onlythattheendsarenowjoinedtogether。Alltheinsidesurfaceoftheringisgoingoneway，namely，thewaythestickispulled；andalltheoutsideisgoingtheotherway。Suchavortex-ringismadebythesmokerwhopurseshislipsintoaroundholeandsendsoutapuffofsmoke。Theoutsideoftheringiskeptbackbythefrictionofhislipswhiletheinsideisgoingforwards；thusarotationissetupallroundthesmoke-ringasittravelsoutintotheair。“Inthesecases，andinothersaswecommonlyfindit，vortex-motionowesitsorigintofrictionandisafterawhilebroughttoanendbyfriction。
Butin1858theequationsofmotionofanincompressiblefrictionlessfluidwerefirstsuccessfullysolvedbyHelmholtz，andamongotherthingsheprovedthat，thoughvortex-motioncouldnotbeoriginatedinsuchafluid，yetsupposingitoncetoexist，itwouldexisttoalleternityandcouldnotbediminishedbyanymechanicalactionwhatever。Avortex-ring，forexample，insuchafluid，wouldforeverpreserveitsownrotation，andwouldthusforeverretainitspeculiarindividuality，being，asitwere，markedofffromitsneighbourvortex-rings。UponthismechanicaltruthSirWilliamThomsonbasedhiswonderfullysuggestivetheoryoftheconstitutionofmatter。Thatwhichispermanentorindestructibleinmatteristheultimatehomogeneousatom；andthisisprobablyallthatispermanent，sincechemistsnowalmostunanimouslyholdthatso-calledelementarymoleculesarenotreallysimple，butowetheirsensibledifferencestothevariousgroupingsofanultimateatomwhichisalikeforall。
Relativelytoourpowersofcomprehensiontheatomendureseternally；thatis，itretainsforeverunalterableitsdefinitemassanditsdefiniterateofvibration。Nowthisisjustwhatavortex-ringwoulddoinanincompressiblefrictionlessfluid。
Thusthestartlingquestionissuggested，Whymaynottheultimateatomsofmatterbevortex-ringsforeverexistinginsuchafrictionlessfluidfillingthewholeofspace？SuchahypothesisisnotlessbrilliantthanHuyghens’sconjecturalidentificationoflightwithundulatorymotion；anditismoreoveralegitimatehypothesis，sinceitcanbebroughttothetestofverification。SirWilliamThomsonhasshownthatitexplainsagreatmanyofthephysicalpropertiesofmatter:itremainstobeseenwhetheritcanexplainthemall。
OfcoursetheetherwhichconveysthermalandluminousundulationsisnotthefrictionlessfluidpostulatedbySirWilliamThomson。Themostconspicuouspropertyoftheetherisitsenormouselasticity，apropertywhichweshouldnotfindinafrictionlessfluid。“Toaccountforsuchelasticity，“saysProfessorCliffordwhoseexpositionofthesubjectisstillmorelucidthanthatofourauthors，“ithastobesupposedthatevenwheretherearenomaterialmoleculestheuniversalfluidisfullofvortex-motion，butthatthevorticesaresmallerandmorecloselypackedthanthoseof[ordinary]matter，formingaltogetheramorefinelygrainedstructure。Sothatthedifferencebetweenmatterandetherisreducedtoameredifferenceinthesizeandarrangementofthecomponentvortex-rings。Now，whatevermayturnouttobetheultimatenatureoftheetherandofmolecules，weknowthattosomeextentatleasttheyobeythesamedynamiclaws，andthattheyactupononeanotherinaccordancewiththeselaws。Until，therefore，itisabsolutelydisproved，itmustremainthesimplestandmostprobableassumptionthattheyarefinallymadeofthesamestuff，thatthematerialmoleculeissomekindofknotorcoagulationofether。“[5]
[5]FortnightlyReview，June，1875，p。784。
AnotherinterestingconsequenceofSirWilliamThomson’spregnanthypothesisisthattheabsolutehardnesswhichhasbeenattributedtomaterialatomsfromthetimeofLucretiusdownwardmaybedispensedwith。Somewhatinthesamewaythatalooselysuspendedchainbecomesrigidwithrapidrotation，thehardnessandelasticityofthevortex-atomareexplainedasduetotheswiftrotarymotionofasoftandyieldingfluid。Sothatthevortex-atomisreallyindivisible，notbyreasonofitshardnessorsolidity，butbyreasonoftheindestructiblenessofitsmotion。
Supposing，now，thatweadoptprovisionallythevortextheory，——thegreatpowerofwhichiswellshownbytheconsiderationjustmentioned，——wemustnotforgetthatitisabsolutelyessentialtotheindestructiblenessofthematerialatomthattheuniversalfluidinwhichithasanexistenceasavortex-ringshouldbeentirelydestituteoffriction。Onceadmiteventhemostinfinitesimalamountoffriction，whileretainingtheconceptionofvortex-motioninauniversalfluid，andthewholecaseissofaralteredthatthematerialatomcannolongerberegardedasabsolutelyindestructible，butonlyasindefinitelyenduring。Itmayhavebeengenerated，inbygoneeternity，byanaturalprocessofevolution，andinfutureeternitymaycometoanend。Relativelytoourpowersofcomprehensionthepracticaldifferenceisperhapsnotgreat。
Scientificallyspeaking，HelmholtzandThomsonareaswellentitledtoreasonupontheassumptionofaperfectlyfrictionlessfluidasgeometersingeneralareentitledtoassumeperfectlineswithoutbreadthandperfectsurfaceswithoutthickness。Perfectlinesandsurfacesdonotexistwithintheregionofourexperience；yettheconclusionsofgeometryarenonethelesstrueideally，thoughinanyparticularconcreteinstancetheyareonlyapproximatelyrealized。Justsowiththeconceptionofafrictionlessfluid。Sofarasexperiencegoes，suchathinghasnomorerealexistencethanalinewithoutbreadth；andhenceanatomictheorybaseduponsuchanassumptionmaybeastrueideallyasanyofthetheoremsofEuclid，butitcangiveonlyanapproximativelytrueaccountoftheactualuniverse。Theseconsiderationsdonotatallaffectthescientificvalueofthetheory；buttheywillmodifythetenourofsuchtranscendentalinferencesasmaybedrawnfromitregarding，theprobableoriginanddestinyoftheuniverse。
Theconclusionsreachedinthefirstpartofthispaper，whileweweredealingonlywithgrossvisiblematter，mayhaveseemedboldenough；buttheyarefarsurpassedbytheinferencewhichourauthorsdrawfromthevortextheoryastheyinterpretit。Ourauthorsexhibitvariousreasons，moreorlesssound，forattributingtotheprimordialfluidsomeslightamountoffriction；andinsupportofthisviewtheyadduceLeSage’sexplanationofgravitationasadifferentialresultofpressure，andStruve’stheoryofthepartialabsorptionoflight-raysbytheether，——questionswithwhichourpresentpurposedoesnotrequireustomeddle。ApartfromsuchquestionsitiseverywayprobablethattheprimaryassumptionofHelmholtzandThomsonisonlyanapproximationtothetruth。Butifweaccredittheprimordialfluidwithevenaninfinitesimalamountoffriction，thenwearerequiredtoconceiveofthevisibleuniverseasdevelopedfromtheinvisibleandasdestinedtoreturnintotheinvisible。Thevortex-atom，producedbyinfinitesimalfrictionoperatingthroughwellnighinfinitetime，istobeultimatelyabolishedbytheagencywhichproducedit。Inthewordsofourauthors，“Ifthevisibleuniversebedevelopedfromaninvisiblewhichisnotaperfectfluid，thentheargumentdeducedbySirWilliamThomsoninfavouroftheeternityofordinarymatterdisappears，sincethiseternitydependsupontheperfectfluidityoftheinvisible。Infine，ifwesupposethematerialuniversetobecomposedofaseriesofvortex-ringsdevelopedfromaninvisibleuniversewhichisnotaperfectfluid，itwillbeephemeral，justasthesmoke-ringwhichwedevelopfromair，orthatwhichwedevelopfromwater，isephemeral，theonlydifferencebeinginduration，theselastingonlyforafewseconds，andtheothersitmaybeforbillionsofyears。“Thus，asourauthorssupposethat“theavailableenergyofthevisibleuniversewillultimatelybeappropriatedbytheinvisible，“theygoontoimagine，“atleastasapossibility，thattheseparateexistenceofthevisibleuniversewillsharethesamefate，sothatweshallhavenohuge，useless，inertmassexistinginafteragestoremindthepasser-byofaformofenergyandaspeciesofmatterthatislongsinceoutofdateandfunctionallyeffete。
Whyshouldnottheuniverseburyitsdeadoutofsight？“
Inonerespectperhapsnomorestupendoussubjectofcontemplationthanthishaseverbeenofferedtothemindofman。
Incomparisonwiththelengthoftimethusrequiredtoeffacethetinyindividualatom，theentirecosmicalcareerofoursolarsystem，oreventhatofthewholestarrygalaxy，shrinksintoutternothingness。Whetherweshalladopttheconclusionsuggestedmustdependontheextentofourspeculativeaudacity。
Wehaveseenwhereinitsprobabilityconsists，butinreasoninguponsuchascalewemayfitlybecautiousandmodestinacceptinginferences，andourauthors，wemaybesure，wouldbethefirsttorecommendsuchmodestyandcaution。Evenatthedimensionstowhichourtheorizinghasheregrown，wemayforinstancediscernthepossiblealternativeofasimultaneousorrhythmicallysuccessivegenerationanddestructionofvortex-atomswhichwouldgofartomodifytheconclusionjustsuggested。Butherewemustpauseforamoment，reservingforasecondpapertheweightierthoughtsastofuturitywhichourauthorshavesoughttoenwrapinthesesublimephysicalspeculations。
UPtothispoint，howeverremotefromordinaryevery-daythoughtsmaybetheregionofspeculationwhichwehavebeencalledupontotraverse，wehavestillkeptwithinthelimitsoflegitimatescientifichypothesis。Thoughwehaveventuredforagoodlydistanceintotheunknown，wehavenotyetbeenrequiredtoabandonourbaseofoperationsintheknown。Oftheviewspresentedintheprecedingpaper，somearewellnighcertainlyestablished，someareprobable，somehaveasortofplausibility，others——towhichwehaverefrainedfromgivingassent——maypossiblybetrue；butnoneareirretrievablybeyondthejurisdictionofscientifictests。Nosuggestionhassofarbeenbroachedwhichaverylittlefurtherincreaseofourscientificknowledgemaynotshowtobeeithereminentlyprobableoreminentlyimprobable。Wehavekeptprettyclearofmeresubjectiveguesses，suchasmenmaywrangleaboutforeverwithoutcomingtoanyconclusion。Thetheoryofthenebularoriginofourplanetarysystemhascometocommandtheassentofallpersonsqualifiedtoappreciatetheevidenceonwhichitisbased；andthemoreimmediateconclusionswhichwehavedrawnfromthattheoryareonlysuchasarecommonlydrawnbyastronomersandphysicists。Thedoctrineofanintermolecularandinterstellaretheriswrappedupinthewell-establishedundulatorytheoryoflight。SuchisbynomeansthecasewithSirWilliamThomson’svortex-atomtheory，whichto-dayisinsomewhatthesameconditionastheundulatorytheoryofHuyghenstwocenturiesago。