Thefirstgreatcosmologicalspeculationwhichhasbeenraisedquiteabovetheplaneofguessworkbymakingnootherassumptionthanthatoftheuniformityofnature，isthewell-knownNebularHypothesis。Everyastronomerknowsthattheearth，likeallothercosmicalbodieswhichareflattenedatthepoles，wasformerlyamassoffluid，andconsequentlyfilledamuchlargerspacethanatpresent。Itisfurtheragreed，onallhands，thatthesunisacontractingbody，sincethereisnootherpossiblewayofaccountingfortheenormousquantityofheatwhichhegenerates。
Theso-calledprimevalnebulafollowsasanecessaryinferencefromthesefacts。Therewasonceatimewhentheearthwasdistendedonallsidesawayouttothemoonandbeyondit，sothatthematternowcontainedinthemoonwasthenapartofourequatorialzone。Andatastillremoterdateinthepast，themassofthesunwasdiffusedineverydirectionbeyondtheorbitofNeptune，andnoplanethadanindividualexistence，forallwereindistinguishablepartsofthesolarmass。Whenthegreatmassofthesun，increasedbytherelativelysmallmassofalltheplanetsputtogether，wasspreadoutinthisway，itwasararevapourorgas。AttheperiodwherethequestionistakenupinLaplace’streatmentofthenebulartheory，theshapeofthismassisregardedasspheroidal；butatanearlierperioditsshapemaywellhavebeenasirregularasthatofanyofthenebulaewhichwenowseeindistantpartsoftheheavens，for，whateveritsprimitiveshape，theequalizationofitsrotationwouldintimemakeitspheroidal。ThattheQUANTITYofrotationwasthesamethenasnowisunquestionable；fornosystemofparticles，greatorsmall，canacquireorloserotationbyanyactiongoingonwithinitself，anymorethanamancouldpickhimselfupbyhiswaistbandandlifthimselfoverastonewaleSothattheprimitiverotatingspheroidalsolarnebulaisnotamatterofassumption，butisjustwhatmustoncehaveexisted，providedtherehasbeennobreachofcontinuityinnature’soperations。Nowproceedingtoreasonbackfromthepasttothepresent，ithasbeenshownthattheabandonmentofsuccessiveequatorialbeltsbythecontractingsolarmassmusthaveensuedinaccordancewithknownmechanicallaws；andinsimilarwise，underordinarycircumstances。eachbeltmusthavepartedintofragments，andthefragmentschasingeachotheraroundthesameorbit，musthaveatlastcoalescedintoaspheroidalplanet。Notonlythis，butithasalsobeenshownthatastheresultofsuchaprocesstherelativesizesoftheplanetswouldbelikelytotaketheorderwhichtheynowfollow；thattheringimmediatelysucceedingthatofJupiterwouldbelikelytoabortandproduceagreatnumberoftinyplanetsinsteadofonegood-sizedone；thattheouterplanetswouldbelikelytohavemanymoons，andthatSaturn，besideshavingthegreatestnumberofmoons，wouldbelikelytoretainsomeofhisinnerringsunbroken；thattheearthwouldbelikelytohavealongdayandJupiterashortone；thattheextremeouterplanetswouldbenotunlikelytorotateinaretrogradedirection；andsoon，throughalonglistofinterestingandstrikingdetails。Notonly，therefore，arewedriventotheinferencethatoursolarsystemwasonceavaporousnebula，butwefindthatthemerecontractionofsuchanebula，undertheinfluenceoftheenormousmutualgravitationofitsparticles，carrieswithittheexplanationofboththemoregeneralandthemoreparticularfeaturesofthepresentsystem。
Sothatwemayfairlyregardthisstupendousprocessasveritablematterofhistory，whileweproceedtostudyitundersomefurtheraspectsandtoconsiderwhatconsequencesarelikelytofollow。
Ourattentionshouldfirstbedirectedtotheenormouswasteofenergywhichhasaccompaniedthiscontractionofthesolarnebula。Thefirstresultofsuchacontractionisthegenerationofagreatquantityofheat，andwhentheheatthusgeneratedhasbeenlostbyradiationintosurroundingspaceitbecomespossibleforthecontractiontocontinue。Thus，asconcentrationgoeson，heatisincessantlygeneratedandincessantlydissipated。Howlongthisprocessistoenduredependschieflyonthesizeofthecontractingmass，assmallbodiesradiateheatmuchfasterthanlargeones。Themoonseemstobealreadythoroughlyrefrigerated，whileJupiterandSaturnareverymuchhotterthantheearth，asisshownbythetremendousatmosphericphenomenawhichoccurontheirsurfaces。Thesun，again，generatesheatsorapidly，owingtohisgreatenergyofcontraction，andlosesitsoslowly，owingtohisgreatsize，thathissurfaceisalwayskeptinastateofincandescence。Hissurface-temperatureisestimatedatsomethreemilliondegreesofFahrenheit，andadiminutionofhisdiameterfartoosmalltobedetectedbythefinestexistinginstrumentswouldsufficetomaintainthepresentsupplyofheatformorethanfiftycenturies。Thesefactspointtoaverylongfutureduringwhichthesunwillcontinuetowarmtheearthanditscompanionplanets，butatthesametimetheycarryontheirfacethestoryofinevitableultimatedoom。Ifthingscontinuetogoonastheyhaveallalonggoneon，thesunmustbyandbygrowblackandcold，andalllifewhateverthroughoutthesolarsystemmustcometoanend。Longbeforethisconsummation，however，lifewillprobablyhavebecomeextinctthroughtherefrigerationofeachoftheplanetsintoastatelikethepresentstateofthemoon，inwhichtheatmosphereandoceanshavedisappearedfromthesurface。Nodoubtthesunwillcontinuetogiveoutheatalongtimeafterheathasceasedtobeneededforthesupportoflivingorganisms。Forthefinalrefrigerationofthesunwilllongbepostponedbythefateoftheplanetsthemselves。Theseparationoftheplanetsfromtheirparentsolarmassseemstobeafterallbutatemporaryseparation。Sonicelybalancedaretheynowintheirorbitsthattheymaywellseemcapableofrollingonintheirpresentcoursesforever。Butthisisnotthecase。Twosetsofcircumstancesareallthewhilestriving，theonetodrivetheplanetsfartherawayfromthesun，theothertodrawthemallintoit。Ontheonehand，everybodyinoursystemwhichcontainsfluidmatterhastidesraiseduponitssurfacebytheattractionofneighbouringbodies。Alltheplanetsraisetidesuponthesurfaceofthesunandtheperiodicityofsun-spotsorsolarcyclonesdependsuponthisfact。Thesetidalwavesactasadragorbrakeupontherotationofthesun，somewhatdiminishingitsrapidity。But，inconformitywithaprincipleofmechanicswellknowntoastronomers，thoughnotfamiliartothegeneralreader，allthemotionofrotationthuslostbythesunisaddedtotheplanetsintheshapeofannualmotionofrevolution，andthustheirorbitsalltendtoenlarge，——theyalltendtorecedesomewhatfromthesun。Butthisstateofthings，thoughlong-enduringenough，isafterallonlytemporary，andwillatanyratecometoanendwhenthesunandplanetshavebecomesolid。Meanwhileanothersetofcircumstancesisallthetimetendingtobringtheplanetsnearertothesun，andinthelongrunmustgainthemastery。Thespacethroughwhichtheplanetsmoveisfilledwithakindofmatterwhichservesasamediumforthetransmissionofheatandlight，andthiskindofmatter，thoughdifferentinsomerespectsfromordinaryponderablematter，isyetlikeitinexertingfriction。
Thisfrictionisalmostinfinitelylittle，yetithasawellnighinfinitelengthoftimetoworkin，andduringallthiswellnighinfinitelengthoftimeitisslowlyeatingupthemomentumoftheplanetsanddiminishingtheirabilitytomaintaintheirdistancesfromthesun。Henceincourseoftimetheplanetswillallfallintothesun，oneafteranother，sothatthesolarsystemwillend，asitbegan，byconsistingofasinglemassofmatter。