the unseen world and other essays-第3章

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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　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