the unseen world and other essays-第3部分

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　sun　and　planet　would　at　once　result　in　the　vaporization　of　both　bodies；　and　there　can　be　no　doubt　that　by　the　time　the　sun　has　absorbed　the　outermost　of　his　attendant　planets；　he　will　have　resumed　something　like　his　original　nebulous　condition。　He　will　have　been　dilated　into　a　huge　mass　of　vapour；　and　will　have　become　fit　for　a　new　process　of　contraction　and　for　a　new　production　of　life…bearing　planets。

We　are　now；　however；　confronted　by　an　interesting　but　difficult　question。　Throughout　all　this　grand　past　and　future　career　of　the　solar　system　which　we　have　just　briefly　traced；　we　have　been　witnessing　a　most　prodigal　dissipation　of　energy　in　the　shape　of　radiant　heat。　At　the　outset　we　had　an　enormous　quantity　of　what　is　called　〃energy　of　position；〃　that　is；　the　outer　parts　of　our　primitive　nebula　had　a　very　long　distance　through　which　to　travel　towards　one　another　in　the　slow　process　of　concentration；　and　this　distance　was　the　measure　of　the　quantity　of　work　possible　to　our　system。　As　the　particles　of　our　nebula　drew　nearer　and　nearer　together；　the　energy　of　position　continually　lost　reappeared　continually　as　heat；　of　which　the　greater　part　was　radiated　off；　but　of　which　a　certain　amount　was　retained。　All　the　gigantic　amount　of　work　achieved　in　the　geologic　development　of　our　earth　and　its　companion　planets；　and　in　the　development　of　life　wherever　life　may　exist　in　our　system；　has　been　the　product　of　this　retained　heat。　At　the　present　day　the　same　wasteful　process　is　going　on。　Each　moment　the　sun's　particles　are　losing　energy　of　position　as　they　draw　closer　and　closer　together；　and　the　heat　into　which　this　lost　energy　is　metamorphosed　is　poured　out　most　prodigally　in　every　direction。　Let　us　consider　for　a　moment　how　little　of　it　gets　used　in　our　system。　The　earth's　orbit　is　a　nearly　circular　figure　more　than　five　hundred　million　miles　in　circumference；　while　only　eight　thousand　miles　of　this　path　are　at　any　one　time　occupied　by　the　earth's　mass。　Through　these　eight　thousand　miles　the　sun's　radiated　energy　is　doing　work；　but　through　the　remainder　of　the　five　hundred　million　it　is　idle　and　wasted。　But　the　case　is　far　more　striking　when　we　reflect　that　it　is　not　in　the　plane　of　the　earth's　orbit　only　that　the　sun's　radiance　is　being　poured　out。　It　is　not　an　affair　of　a　circle；　but　of　a　sphere。　In　order　to　utilize　all　the　solar　rays；　we　should　need　to　have　an　immense　number　of　earths　arranged　so　as　to　touch　each　other；　forming　a　hollow　sphere　around　the　sun；　with　the　present　radius　of　the　earth's　orbit。　We　may　well　believe　Professor　Tyndall；　therefore；　when　he　tells　us　that　all　the　solar　radiance　we　receive　is　less　than　a　two…billionth　part　of　what　is　sent　flying　through　the　desert　regions　of　space。　Some　of　the　immense　residue　of　course　hits　other　planets　stationed　in　the　way　of　it；　and　is　utilized　upon　their　surfaces；　but　the　planets；　all　put　together；　stop　so　little　of　the　total　quantity　that　our　startling　illustration　is　not　materially　altered　by　taking　them　into　the　account。　Now　this　two…billionth　part　of　the　solar　radiance　poured　out　from　moment　to　moment　suffices　to　blow　every　wind；　to　raise　every　cloud；　to　drive　every　engine；　to　build　up　the　tissue　of　every　plant；　to　sustain　the　activity　of　every　animal；　including　man；　upon　the　surface　of　our　vast　and　stately　globe。　Considering　the　wondrous　richness　and　variety　of　the　terrestrial　life　wrought　out　by　the　few　sunbeams　which　we　catch　in　our　career　through　space；　we　may　well　pause　overwhelmed　and　stupefied　at　the　thought　of　the　incalculable　possibilities　of　existence　which　are　thrown　away　with　the　potent　actinism　that　darts　unceasingly　into　the　unfathomed　abysms　of　immensity。　Where　it　goes　to　or　what　becomes　of　it；　no　one　of　us　can　surmise。

Now　when；　in　the　remote　future；　our　sun　is　reduced　to　vapour　by　the　impact　of　the　several　planets　upon　his　surface；　the　resulting　nebulous　mass　must　be　a　very　insignificant　affair　compared　with　the　nebulous　mass　with　which　we　started。　In　order　to　make　a　second　nebula　equal　in　size　and　potential　energy　to　the　first　one；　all　the　energy　of　position　at　first　existing　should　have　been　retained　in　some　form　or　other。　But　nearly　all　of　it　has　been　lost；　and　only　an　insignificant　fraction　remains　with　which　to　endow　a　new　system。　In　order　to　reproduce；　in　future　ages；　anything　like　that　cosmical　development　which　is　now　going　on　in　the　solar　system；　aid　must　be　sought　from　without。　We　must　endeavour　to　frame　some　valid　hypothesis　as　to　the　relation　of　our　solar　system　to　other　systems。

Thus　far　our　view　has　been　confined　to　the　career　of　a　single　star；our　sun；with　the　tiny；　easily…cooling　balls　which　it　has　cast　off　in　the　course　of　its　development。　Thus　far；　too；　our　inferences　have　been　very　secure；　for　we　have　been　dealing　with　a　circumscribed　group　of　phenomena；　the　beginning　and　end　of　which　have　been　brought　pretty　well　within　the　compass　of　our　imagination。　It　is　quite　another　thing　to　deal　with　the　actual　or　probable　career　of　the　stars　in　general；　inasmuch　as　we　do　not　even　know　how　many　stars　there　are；　which　form　parts　of　a　common　system；　or　what。　are　their　precise　dynamic　relations　to　one　another。　Nevertheless　we　have　knowledge　of　a　few　facts　which　may　support　some　cautious　inferences。　All　the　stars　which　we　can　see　are　undoubtedly　bound　together　by　relations　of　gravitation。　No　doubt　our　sun　attracts　all　the　other　stars　within　our　ken；　and　is　reciprocally　attracted　by　them。　The　stars；　too；　lie　mostly　in　or　around　one　great　plane；　as　is　the　case　with　the　members　of　the　solar　system。　Moreover；　the　stars　are　shown　by　the　spectroscope　to　consist　of　chemical　elements　identical　with　those　which　are　found　in　the　solar　system。　Such　facts　as　these　make　it　probable　that　the　career　of　other　stars；　when　adequately　inquired　into；　would　be　found　to　be　like　that　of　our　own　sun。　Observation　daily　enhances　this　probability；　for　our　study　of　the　sidereal　universe　is　continually　showing　us　stars　in　all　stages　of　development。　We　find　irregular　nebulae；　for　example；　we　find　spiral　and　spheroidal　nebulae；　we　find　stars　which　have　got　beyond　the　nebulous　stage；　but　are　still　at　a　whiter　heat　than　our　sun；　and　we　also　find　many　stars　which　yield　the　same　sort　of　spectrum　as　our　sun。　The　inference　seems　forced　upon　us　that　the　same　process　of　concentration　which　has　gone　on　in　the　case　of　our　solar　nebula　has　been　going　on　in　the　case　of　other　nebulae。　The　history　of　the　sun　is　but　a　type　of　the　history　of　stars　in　general。　And　when　we　consider　that　all　other　visible　stars　and　nebulae　are　cooling　and　contracting　bodies；　like　our　sun；　to　what　other　conclusion　could　we　very　well　come？　When　we　look　at　Sirius；　for　instance；　we　do　not　see　him　surrounded　by　planets；　for　at　such　a　distance　no　planet　could　be　visible；　even　Sirius　himself；　though　fourteen　times　larger　than　our　sun；　appearing　only　as　a　〃twinkling　little　star。〃　But　a　comparative　survey　of　the　heavens　assures　us　that　Sirius　can　hardly　have　arrived　at　his　present　stage　of　concentration　without　detaching；　planet…forming　rings；　for　there　is　no　reason　for　supposing　that　mechanical　laws　out　there　are　at　all　different　from　what　they　are　in　our　own　system。　And　the　same　kind　of　inference　must　apply　to　all　the　matured　stars　which　we　see　in　the　heavens。

When　we　duly　take　all　these　things　into　the　account；　the　case　of　our　solar　system　will　appear　as　only　one　of　a　thousand　cases　of　evolution　and　dissolution　with　which　the　heavens　furnish　us。　Other　stars；　like　our　sun；　have　undoubtedly　started　as　vaporous　masses；　and　have　thrown　off　planets　in　contracting。　The　inference　may　seem　a　bold　one；　but　it　after　all　involves　no　other　assumption　than　that　of　the　continuity　of　natural　phenomena。　It　is　not　likely；　therefore；　that　the　solar　system　will　forever　be　left　to　itself。　Stars　which　strongly　gravitate　toward　each　other；　while　moving　through　a　perennially　resisting　medium；　must　in　time　be　drawn　together。　The　collision　of　our　extinct　sun　with　one　of　the　Pleiades；　after　this　manner；　would　very　likely　suffice　to　generate　even　a　grander　nebula　than　the　one　with　which　we　started。　Possibly　the　entire　galactic　system　may；　in　an　inconceivably　remote　future；　remodel　itself　in　this　way；　and　possibly　the　nebula　from　which　our　own　group　of　planets　has　been　formed　may　have　owed　its　origin　to　the　disintegration　of　systems　which　had　accomplished　their　career　in　the　depths　of　the　bygone　eternity。

When　the　problem　is　extended　to　these　huge　dimensions；　the　prospect　of　an　ultimate　cessation　of　cosmical　work　is　indefinitely　postponed；　but　at　the　same　time　it　becomes　impossible　for　us　to　deal　very　securely　with　the　questions　we　have　raised。　The　magnitudes　and　periods　we　have　introduced　are　so　nearly　infinite　as　to　baffle　speculation　itself：　One　point；　however；　we　seem　dimly　to　discern。　Supposing　the　stellar　universe　not　to　be　absolutely　infinite　in　extent；　we　may　hold　that　the　day　of　doom；　so　often　postponed；　must　come　at　last。　The　concentration　of　matter　and　dissipation　of　energy；　so　often　checked；　must　in　the　end　prevail；　so　that；　as　the　final　outcome　of　things；　the　entire　universe　will　be　reduced　to　a　single　enormous　ball；　dead　and　frozen；　solid　and　black；　its　potential　energy　of　motion　having　been　all　transformed　into　heat　and　radiated　away。　Such　a　conclusion　has　been　suggested　by　Sir　William　Thomson；　and　it　is　quite　forcibly　stated　by　the　authors　of　〃The　Unseen　Universe。〃　They　remind　us　that　〃if　there　be　any　one　form　of　energy　less　readily　or　less　completely　transformable　than　the　others；　and　if　transformations　constantly　go　on；　more　and　more　of　the　whole　energy　of　the　universe　will　inevitably　sink　into　this　lower　grade　as　time　advances。〃　Now　radiant　heat；　as　we　have　seen；　is　such　a　lower　grade　of　energy。　〃At　each　transformation　of　heat…energy　into　work；　a　large　portion　is　degraded；　while　only　a　small　portion　is　transformed　into　work。　So　that　while　it　is　very　easy　to　change　all　of　our　mechanical　or　useful　energy　into　heat；　it　is　only　possible　to　transform　a　portion　of　this　heat…energy　back　again　into　work。　After　each　change；　too；　the　heat　becomes　more　and　more　dissipated　or　degraded；　and　less　and　less　available　for　any　future　transformation。　In　other　words；〃　our　authors　continue；　〃the　tendency　of　heat　is　towards　equalization；　heat　is　par　exc