darwin and modern science-第173部分

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　century　since　this　class　of　observation　has　claimed　the　close　attention　of　astronomers；　something　considerable　has　been　discovered　already　and　there　seems　scarcely　a　discernible　limit　to　what　will　be　known　in　this　field　a　century　from　now。　　Some　of　the　results　which　I　have　set　forth　may　then　be　shown　to　be　false；　but　it　seems　profoundly　improbable　that　we　are　being　led　astray　by　a　Will…of…the…Wisp。


XXIX。　　THE　EVOLUTION　OF　MATTER。

By　W。C。D。　WHETHAM；　M。A。；　F。R。S。　Trinity　College；　Cambridge。

The　idea　of　evolution　in　the　organic　world；　made　intelligible　by　the　work　of　Charles　Darwin；　has　little　in　common　with　the　recent　conception　of　change　in　certain　types　of　matter。　　The　discovery　that　a　process　of　disintegration　may　take　place　in　some　at　least　of　the　chemical　atoms；　previously　believed　to　be　indestructible　and　unalterable；　has　modified　our　view　of　the　physical　universe；　even　as　Darwin's　scheme　of　the　mode　of　evolution　changed　the　trend　of　thought　concerning　the　organic　world。　　Both　conceptions　have　in　common　the　idea　of　change　throughout　extended　realms　of　space　and　time；　and；　therefore；　it　is　perhaps　not　unfitting　that　some　account　of　the　most　recent　physical　discoveries　should　be　included　in　the　present　volume。

The　earliest　conception　of　the　evolution　of　matter　is　found　in　the　speculation　of　the　Greeks。　　Leucippus　and　Democritus　imagined　unchanging　eternal　atoms；　Heracleitus　held　that　all　things　were　in　a　continual　state　of　fluxPanta　rei。

But　no　one　in　the　Ancient　Worldno　one　till　quite　modern　timescould　appreciate　the　strength　of　the　position　which　the　theory　of　the　evolution　of　matter　must　carry　before　it　wins　the　day。　　Vague　speculation；　even　by　the　acute　minds　of　philosophers；　is　of　little　use　in　physical　science　before　experimental　facts　are　available。　　The　true　problems　at　issue　cannot　even　be　formulated；　much　less　solved；　till　the　humble　task　of　the　observer　and　experimenter　has　given　us　a　knowledge　of　the　phenomena　to　be　explained。

It　was　only　through　the　atomic　theory；　at　first　apparently　diametrically　opposed　to　it；　that　the　conception　of　evolution　in　the　physical　world　was　to　gain　an　established　place。　　For　a　century　the　atomic　theory；　when　put　into　a　modern　form　by　Dalton；　led　farther　and　farther　away　from　the　idea　of　change　in　matter。　　The　chemical　elements　seemed　quite　unalterable；　and　the　atoms；　of　which　each　element　in　modern　view　is　composed；　bore　to　Clerk　Maxwell；　writing　about　1870；　〃the　stamp　of　manufactured　articles〃　exactly　similar　in　kind；　unchanging；　eternal。

Nevertheless　throughout　these　years；　on　the　whole　so　unfavourable　to　its　existence；　there　persisted　the　idea　of　a　common　origin　of　the　distinct　kinds　of　matter　known　to　chemists。　　Indeed；　this　idea　of　unity　in　substance　in　nature　seems　to　accord　with　some　innate　desire　or　intimate　structure　of　the　human　mind。　　As　Mr　Arthur　Balfour　well　puts　it；　〃There　is　no　a　priori　reason　that　I　know　of　for　expecting　that　the　material　world　should　be　a　modification　of　a　single　medium；　rather　than　a　composite　structure　built　out　of　sixty　or　seventy　elementary　substances；　eternal　and　eternally　different。　　Why　then　should　we　feel　content　with　the　first　hypothesis　and　not　with　the　second？　　Yet　so　it　is。　　Men　of　science　have　always　been　restive　under　the　multiplication　of　entities。　　They　have　eagerly　watched　for　any　sign　that　the　different　chemical　elements　own　a　common　origin；　and　are　all　compounded　out　of　some　primordial　substance。　　Nor；　for　my　part；　do　I　think　that　such　instincts　should　be　ignored。。。that　they　exist　is　certain；　that　they　modify　the　indifferent　impartiality　of　pure　empiricism　can　hardly　be　denied。〃　　（〃Report　of　the　74th　Meeting　of　the　British　Association〃　（Presidential　Address；　Cambridge　1904）；　page　9；　London；　1905。）

When　Dalton's　atomic　theory　had　been　in　existence　some　half　century；　it　was　noted　that　certain　numerical　relations　held　good　between　the　atomic　weights　of　elements　chemically　similar　to　one　another。　　Thus　the　weight　（88）　of　an　atom　of　strontium　compared　with　that　of　hydrogen　as　unity；　is　about　the　mean　of　those　of　calcium　（40）　and　barium　（137）。　　Such　relations；　in　this　and　other　chemical　groups；　were　illustrated　by　Beguyer　de　Chancourtois　in　1862　by　the　construction　of　a　spiral　diagram　in　which　the　atomic　weights　are　placed　in　order　round　a　cylinder　and　elements　chemically　similar　are　found　to　fall　on　vertical　lines。

Newlands　seems　to　have　been　the　first　to　see　the　significance　of　such　a　diagram。　　In　his　〃law　of　octaves；〃　formulated　in　1864；　he　advanced　the　hypothesis　that；　if　arranged　in　order　of　rising　atomic　weight；　the　elements　fell　into　groups；　so　that　each　eighth　element　was　chemically　similar。　　Stated　thus；　the　law　was　too　definite；　no　room　was　left　for　newly…　discovered　elements；　and　some　dissimilar　elements　were　perforce　grouped　together。

But　in　1869　Mendeleeff　developed　Newland's　hypothesis　in　a　form　that　attracted　at　once　general　attention。　　Placing　the　elements　in　order　of　rising　atomic　weight；　but　leaving　a　gap　where　necessary　to　bring　similar　elements　into　vertical　columns；　he　obtained　a　periodic　table　with　natural　vacancies　to　be　filled　as　new　elements　were　discovered；　and　with　a　certain　amount　of　flexibility　at　the　ends　of　the　horizontal　lines。　　From　the　position　of　the　vacancies；　the　general　chemical　and　physical　properties　of　undiscovered　elements　could　be　predicted；　and　the　success　of　such　predictions　gave　a　striking　proof　of　the　usefulness　of　Mendeleeff's　generalisation。

When　the　chemical　and　physical　properties　of　the　elements　were　known　to　be　periodic　functions　of　their　atomic　weights；　the　idea　of　a　common　origin　and　common　substance　became　much　more　credible。　　Differences　in　atomic　weight　and　differences　in　properties　alike　might　reasonably　be　explained　by　the　differences　in　the　amount　of　the　primordial　substance　present　in　the　various　atoms；　an　atom　of　oxygen　being　supposed　to　be　composed　of　sixteen　times　as　much　stuff　as　the　atom　of　hydrogen；　but　to　be　made　of　the　same　ultimate　material。　　Speculations　about　the　mode　of　origin　of　the　elements　now　began　to　appear；　and　put　on　a　certain　air　of　reality。　　Of　these　speculations　perhaps　the　most　detailed　was　that　of　Crookes；　who　imagined　an　initial　chaos　of　a　primordial　medium　he　named　protyle；　and　a　process　of　periodic　change　in　which　the　chemical　elements　successively　were　precipitated。

From　another　side　too；　suggestions　were　put　forward　by　Sir　Norman　Lockyer　and　others　that　the　differences　in　spectra　observed　in　different　classes　of　stars；　and　produced　by　different　conditions　in　the　laboratory；　were　to　be　explained　by　changes　in　the　structure　of　the　vibrating　atoms。

The　next　step　in　advance　gave　a　theoretical　basis　for　the　idea　of　a　common　structure　of　matter；　and　was　taken　in　an　unexpected　direction。　　Clerk　Maxwell's　electromagnetic　theory　of　light；　accepted　in　England；　was　driven　home　to　continental　minds　by　the　confirmatory　experiments　of　Hertz；　who　in　1888　detected　and　measured　the　electromagnetic　waves　that　Maxwell　had　described　twenty　years　earlier。　　But；　if　light　be　an　electromagnetic　phenomenon；　the　light　waves　radiated　by　hot　bodies　must　take　their　origin　in　the　vibrations　of　electric　systems。　　Hence　within　the　atoms　must　exist　electric　charges　capable　of　vibration。　　On　these　lines　Lorentz　and　Larmor　have　developed　an　electronic　theory　of　matter；　which　is　imagined　in　its　essence　to　be　a　conglomerate　of　electric　charges；　with　electro…magnetic　inertia　to　explain　mechanical　inertia。　　（Larmor；　〃Aether　and　Matter〃；　Cambridge；　1900。）　　The　movement　of　electric　charges　would　be　affected　by　a　magnetic　field；　and　hence　the　discovery　by　Zeeman　that　the　spectral　lines　of　sodium　were　doubled　by　a　strong　magnetic　force　gave　confirmatory　evidence　to　the　theory　of　electrons。

Then　came　J。J。　Thomson's　great　discovery　of　minute　particles；　much　smaller　than　any　chemical　atom；　forming　a　common　constituent　of　many　different　kinds　of　matter。　　（Thomson；　〃Conduction　of　Electricity　through　Gases〃　（2nd　edition）；　Cambridge；　1906。）　　If　an　electric　discharge　be　passed　between　metallic　terminals　through　a　glass　vessel　containing　air　at　very　low　pressure；　it　is　found　that　rectilinear　rays；　known　as　cathode　rays；　proceed　from　the　surface　of　the　cathode　or　negative　terminal。　　Where　these　rays　strike　solid　objects；　they　give　rise　to　the　Rontgen　rays　now　so　well　known；　but　it　is　with　the　cathode　rays　themselves　that　we　are　concerned。　　When　they　strike　an　insulated　conductor；　they　impart　to　it　a　negative　charge；　and　Thomson　found　that　they　were　deflected　from　their　path　both　by　magnetic　and　electric　forces　in　the　direction　in　which　negatively　electrified　particles　would　be　deflected。　　Cathode　rays　then　were　accepted　as　flights　of　negatively　charged　particles；　moving　with　high　velocities。　　The　electric　and　magnetic　deflections　give　two　independent　measurements　which　may　be　made　on　a　cathode　ray；　and　both　the　deflections　involve　theoretically　three　unknown　quantities；　the　mass　of　the　particles；　their　electric　charge　and　their　velocity。　　There　is　strong　cumulative　evidence　that　all　such　particles　possess　the　same　charge；　which　is　identical　with　that　associated　with　a　univalent　atom　in　electrolytic　liquids。　　The　number　of　unknown　quantities　was　thus　reduced　to　twothe