the unseen world and other essays-第2章

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the　suggestions　presently　to　be　set　forth　concerning　the　Unseen　World；　and　we　shall　find　it　most　convenient　to　begin；　like　our　authors；　with　a　brief　statement　of　what　the　principle　of　continuity　teaches　as　to　the　proximate　beginning　and　end　of　the　visible　universe。　I　shall　in　the　main　set　down　only　results；　having　elsewhere＇2＇　given　a　simple　exposition　of　the　arguments　upon　which　these　results　are　founded。

＇1＇　The　Unseen　Universe；　or；　Physical　Speculations　on　a　Future　State。　＇Attributed　to　Professors　TAIT　and　BALFOUR　STEWART。＇　New　York：　Macmillan　&　Co。　1875。　8vo。　pp。　212。

＇2＇　Outlines　of　Cosmic　Philosophy；　based　on　the　Doctrine　of　Evolution。　Boston：　J。　R。　Osgood　&　Co。　1875。　2　vols。　8vo。


The　first　great　cosmological　speculation　which　has　been　raised　quite　above　the　plane　of　guesswork　by　making　no　other　assumption　than　that　of　the　uniformity　of　nature；　is　the　well…known　Nebular　Hypothesis。　Every　astronomer　knows　that　the　earth；　like　all　other　cosmical　bodies　which　are　flattened　at　the　poles；　was　formerly　a　mass　of　fluid；　and　consequently　filled　a　much　larger　space　than　at　present。　It　is　further　agreed；　on　all　hands；　that　the　sun　is　a　contracting　body；　since　there　is　no　other　possible　way　of　accounting　for　the　enormous　quantity　of　heat　which　he　generates。　The　so…called　primeval　nebula　follows　as　a　necessary　inference　from　these　facts。　There　was　once　a　time　when　the　earth　was　distended　on　all　sides　away　out　to　the　moon　and　beyond　it；　so　that　the　matter　now　contained　in　the　moon　was　then　a　part　of　our　equatorial　zone。　And　at　a　still　remoter　date　in　the　past；　the　mass　of　the　sun　was　diffused　in　every　direction　beyond　the　orbit　of　Neptune；　and　no　planet　had　an　individual　existence；　for　all　were　indistinguishable　parts　of　the　solar　mass。　When　the　great　mass　of　the　sun；　increased　by　the　relatively　small　mass　of　all　the　planets　put　together；　was　spread　out　in　this　way；　it　was　a　rare　vapour　or　gas。　At　the　period　where　the　question　is　taken　up　in　Laplace's　treatment　of　the　nebular　theory；　the　shape　of　this　mass　is　regarded　as　spheroidal；　but　at　an　earlier　period　its　shape　may　well　have　been　as　irregular　as　that　of　any　of　the　nebulae　which　we　now　see　in　distant　parts　of　the　heavens；　for；　whatever　its　primitive　shape；　the　equalization　of　its　rotation　would　in　time　make　it　spheroidal。　That　the　QUANTITY　of　rotation　was　the　same　then　as　now　is　unquestionable；　for　no　system　of　particles；　great　or　small；　can　acquire　or　lose　rotation　by　any　action　going　on　within　itself；　any　more　than　a　man　could　pick　himself　up　by　his　waistband　and　lift　himself　over　a　stone　wale　So　that　the　primitive　rotating　spheroidal　solar　nebula　is　not　a　matter　of　assumption；　but　is　just　what　must　once　have　existed；　provided　there　has　been　no　breach　of　continuity　in　nature's　operations。　Now　proceeding　to　reason　back　from　the　past　to　the　present；　it　has　been　shown　that　the　abandonment　of　successive　equatorial　belts　by　the　contracting　solar　mass　must　have　ensued　in　accordance　with　known　mechanical　laws；　and　in　similar　wise；　under　ordinary　circumstances。　each　belt　must　have　parted　into　fragments；　and　the　fragments　chasing　each　other　around　the　same　orbit；　must　have　at　last　coalesced　into　a　spheroidal　planet。　Not　only　this；　but　it　has　also　been　shown　that　as　the　result　of　such　a　process　the　relative　sizes　of　the　planets　would　be　likely　to　take　the　order　which　they　now　follow；　that　the　ring　immediately　succeeding　that　of　Jupiter　would　be　likely　to　abort　and　produce　a　great　number　of　tiny　planets　instead　of　one　good…sized　one；　that　the　outer　planets　would　be　likely　to　have　many　moons；　and　that　Saturn；　besides　having　the　greatest　number　of　moons；　would　be　likely　to　retain　some　of　his　inner　rings　unbroken；　that　the　earth　would　be　likely　to　have　a　long　day　and　Jupiter　a　short　one；　that　the　extreme　outer　planets　would　be　not　unlikely　to　rotate　in　a　retrograde　direction；　and　so　on；　through　a　long　list　of　interesting　and　striking　details。　Not　only；　therefore；　are　we　driven　to　the　inference　that　our　solar　system　was　once　a　vaporous　nebula；　but　we　find　that　the　mere　contraction　of　such　a　nebula；　under　the　influence　of　the　enormous　mutual　gravitation　of　its　particles；　carries　with　it　the　explanation　of　both　the　more　general　and　the　more　particular　features　of　the　present　system。　So　that　we　may　fairly　regard　this　stupendous　process　as　veritable　matter　of　history；　while　we　proceed　to　study　it　under　some　further　aspects　and　to　consider　what　consequences　are　likely　to　follow。

Our　attention　should　first　be　directed　to　the　enormous　waste　of　energy　which　has　accompanied　this　contraction　of　the　solar　nebula。　The　first　result　of　such　a　contraction　is　the　generation　of　a　great　quantity　of　heat；　and　when　the　heat　thus　generated　has　been　lost　by　radiation　into　surrounding　space　it　becomes　possible　for　the　contraction　to　continue。　Thus；　as　concentration　goes　on；　heat　is　incessantly　generated　and　incessantly　dissipated。　How　long　this　process　is　to　endure　depends　chiefly　on　the　size　of　the　contracting　mass；　as　small　bodies　radiate　heat　much　faster　than

large　ones。　The　moon　seems　to　be　already　thoroughly　refrigerated；　while　Jupiter　and　Saturn　are　very　much　hotter　than　the　earth；　as　is　shown　by　the　tremendous　atmospheric　phenomena　which　occur　on　their　surfaces。　The　sun；　again；　generates　heat　so　rapidly；　owing　to　his　great　energy　of　contraction；　and　loses　it　so　slowly；　owing　to　his　great　size；　that　his　surface　is　always　kept　in　a　state　of　incandescence。　His　surface…temperature　is　estimated　at　some　three　million　degrees　of　Fahrenheit；　and　a　diminution　of　his　diameter　far　too　small　to　be　detected　by　the　finest　existing　instruments　would　suffice　to　maintain　the　present　supply　of　heat　for　more　than　fifty　centuries。　These　facts　point　to　a　very　long　future　during　which　the　sun　will　continue　to　warm　the　earth　and　its　companion　planets；　but　at　the　same　time　they　carry　on　their　face　the　story　of　inevitable　ultimate　doom。　If　things　continue　to　go　on　as　they　have　all　along　gone　on；　the　sun　must　by　and　by　grow　black　and　cold；　and　all　life　whatever　throughout　the　solar　system　must　come　to　an　end。　Long　before　this　consummation；　however；　life　will　probably　have　become　extinct　through　the　refrigeration　of　each　of　the　planets　into　a　state　like　the　present　state　of　the　moon；　in　which　the　atmosphere　and　oceans　have　disappeared　from　the　surface。　No　doubt　the　sun　will　continue　to　give　out　heat　a　long　time　after　heat　has　ceased　to　be　needed　for　the　support　of　living　organisms。　For　the　final　refrigeration　of　the　sun　will　long　be　postponed　by　the　fate　of　the　planets　themselves。　The　separation　of　the　planets　from　their　parent　solar　mass　seems　to　be　after　all　but　a　temporary　separation。　So　nicely　balanced　are　they　now　in　their　orbits　that　they　may　well　seem　capable　of　rolling　on　in　their　present　courses　forever。　But　this　is　not　the　case。　Two　sets　of　circumstances　are　all　the　while　striving；　the　one　to　drive　the　planets　farther　away　from　the　sun；　the　other　to　draw　them　all　into　it。　On　the　one　hand；　every　body　in　our　system　which　contains　fluid　matter　has　tides　raised　upon　its　surface　by　the　attraction　of　neighbouring　bodies。　All　the　planets　raise　tides　upon　the　surface　of　the　sun　and　the　periodicity　of　sun…spots　（or　solar　cyclones）　depends　upon　this　fact。　These　tidal　waves　act　as　a　drag　or　brake　upon　the　rotation　of　the　sun；　somewhat　diminishing　its　rapidity。　But；　in　conformity　with　a　principle　of　mechanics　well　known　to　astronomers；　though　not　familiar　to　the　general　reader；　all　the　motion　of　rotation　thus　lost　by　the　sun　is　added　to　the　planets　in　the　shape　of　annual　motion　of　revolution；　and　thus　their　orbits　all　tend　to　enlarge；they　all　tend　to　recede　somewhat　from　the　sun。　But　this　state　of　things；　though　long…enduring　enough；　is　after　all　only　temporary；　and　will　at　any　rate　come　to　an　end　when　the　sun　and　planets　have　become　solid。　Meanwhile　another　set　of　circumstances　is　all　the　time　tending　to　bring　the　planets　nearer　to　the　sun；　and　in　the　long　run　must　gain　the　mastery。　The　space　through　which　the　planets　move　is　filled　with　a　kind　of　matter　which　serves　as　a　medium　for　the　transmission　of　heat　and　light；　and　this　kind　of　matter；　though　different　in　some　respects　from　ordinary　ponderable　matter；　is　yet　like　it　in　exerting　friction。　This　friction　is　almost　infinitely　little；　yet　it　has　a　wellnigh　infinite　length　of　time　to　work　in；　and　during　all　this　wellnigh　infinite　length　of　time　it　is　slowly　eating　up　the　momentum　of　the　planets　and　diminishing　their　ability　to　maintain　their　distances　from　the　sun。　Hence　in　course　of　time　the　planets　will　all　fall　into　the　sun；　one　after　another；　so　that　the　solar　system　will　end；　as　it　began；　by　consisting　of　a　single　mass　of　matter。

But　this　is　by　no　means　the　end　of　the　story。　When　two　bodies　rush　together；　each　parts　with　some　of　its　energy　of　motion；　and　this　lost　energy　of　motion　reappears　as　heat。　In　the　concussion　of　two　cosmical　bodies；　like　the　sun　and　the　earth；　an　enormous　quantity　of　motion　is　thus　converted　into　heat。　Now　heat；　when　not　allowed　to　radiate；　or　when　generated　faster　than　it　can　be　radiated；　is　transformed　into　motion　of　expansion。　Hence　the　shock　of　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　cas