darwin and modern science-第142章

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f　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　mass　and　the　velocity。　　The　measurement　of　the　magnetic　and　electric　deflections　gave　two　independent　relations　between　the　unknowns；　which　could　therefore　be　determined。　　The　velocities　of　the　cathode　ray　particles　were　found　to　vary　round　a　value　about　one…tenth　that　of　light；　but　the　mass　was　found　always　to　be　the　same　within　the　limits　of　error；　whatever　the　nature　of　the　terminals；　of　the　residual　gas　in　the　vessel；　and　of　the　conditions　of　the　experiment。　　The　mass　of　a　cathode　ray　particle；　or　corpuscle；　as　Thomson；　adopting　Newton's　name；　called　it；　is　about　the　eight…hundredth　part　of　the　mass　of　a　hydrogen　atom。

These　corpuscles；　found　in　so　many　different　kinds　of　substance；　are　inevitably　regarded　as　a　common　constituent　of　matter。　　They　are　associated　each　with　a　unit　of　negative　electricity。　　Now　electricity　in　motion　possesses　electromagnetic　energy；　and　produces　effects　like　those　of　mechanical　inertia。　　In　other　words；　an　electric　charge　possesses　mass；　and　there　is　evidence　to　show　that　the　effective　mass　of　a　corpuscle　increases　as　its　velocity　approaches　that　of　light　in　the　way　it　would　do　if　all　its　mass　were　electromagnetic。　　We　are　led　therefore　to　regard　the　corpuscle　from　one　aspect　as　a　disembodied　charge　of　electricity；　and　to　identify　it　with　the　electron　of　Lorentz　and　Larmor。

Thus；　on　this　theory；　matter　and　electricity　are　identified；　and　a　great　simplification　of　our　conception　of　the　physical　structure　of　Nature　is　reached。　　Moreover；　from　our　present　point　of　view；　a　common　basis　for　matter　suggests　or　implies　a　common　origin；　and　a　process　of　development　possibly　intelligible　to　our　minds。　　The　idea　of　the　evolution　of　matter　becomes　much　more　probable。

The　question　of　the　nature　and　physical　meaning　of　a　corpuscle　or　electron　remains　for　consideration。　　On　the　hypothesis　of　a　universal　luminiferous　aether；　Larmor　has　suggested　a　centre　of　aethereal　strain　〃a　place　where　the　continuity　of　the　medium　has　been　broken　and　cemented　together　again　（to　use　a　crude　but　effective　image）　without　accurately　fitting　the　parts；　so　that　there　is　a　residual　strain　all　round　the　place。〃　　（Larmor；　loc。　cit。）　　Thus　he　explains　in　quasi…mechanical　terms　the　properties　of　an　electron。　　But　whether　we　remain　content　for　the　time　with　our　identification　of　matter　and　electricity；　or　attempt　to　express　both　of　them　in　terms　of　hypothetical　aether；　we　have　made　a　great　step　in　advance　on　the　view　that　matter　is　made　up　of　chemical　atoms　fundamentally　distinct　and　eternally　isolated。

Such　was　the　position　when　the　phenomena　of　radio…activity　threw　a　new　light　on　the　problem；　and；　for　the　first　time　in　the　history　of　science；　gave　definite　experimental　evidence　of　the　transmutation　of　matter　from　one　chemical　element　to　another。

In　1896　H。　Becquerel　discovered　that　compounds　of　the　metal　uranium　continually　emitted　rays　capable　of　penetrating　opaque　screens　and　affecting　photographic　plates。　　Like　cathode　and　Rontgen　rays；　the　rays　from　uranium　make　the　air　through　which　they　pass　a　conductor　of　electricity；　and　this　property　gives　the　most　convenient　method　of　detecting　the　rays　and　of　measuring　their　intensity。　　An　electroscope　may　be　made　of　a　strip　of　gold…leaf　attached　to　an　insulated　brass　plate　and　confined　in　a　brass　vessel　with　glass　windows。　　When　the　gold…leaf　is　electrified；　it　is　repelled　from　the　similarly　electrified　brass　plate；　and　the　angle　at　which　it　stands　out　measures　the　electrification。　　Such　a　system；　if　well　insulated；　holds　its　charge　for　hours；　the　leakage　of　electricity　through　the　air　being　very　slow。　　But；　if　radio…active　radiation　reach　the　air　within；　the　gold…leaf　falls；　and　the　rate　of　its　fall；　as　watched　through　a　microscope　with　a　scale　in　the　eye…piece；　measures　the　intensity　of　the　radiation。　　With　some　form　of　this　simple　instrument；　or　with　the　more　complicated　quadrant　electrometer；　most　radio…　active　measurements　have　been　made。

It　was　soon　discovered　that　the　activity　of　uranium　compounds　was　proportional　to　the　amount　of　uranium　present　in　them。　　Thus　radio…activity　is　an　atomic　property　dependent　on　the　amount　of　an　element　and　independent　of　its　state　of　chemical　combination。

In　a　search　for　radio…activity　in　different　minerals；　M。　and　Mme　Curie　found　a　greater　effect　in　pitch…blende　than　its　contents　of　uranium　warranted；　and；　led　by　the　radio…active　property　alone；　they　succeeded；　by　a　long　series　of　chemical　separations；　in　isolating　compounds　of　a　new　and　intensely　radio…active　substance　which　they　named　radium。

Radium　resembles　barium　in　its　chemical　properties；　and　is　precipitated　with　barium　in　the　ordinary　course　of　chemical　analysis。　　It　is　separated　by　a　prolonged　course　of　successive　crystallisation；　the　chloride　of　radium　being　less　soluble　than　that　of　barium；　and　therefore　sooner　separated　from　an　evaporating　solution。　　When　isolated；　radium　chloride　has　a　composition；　which；　on　the　assumption　that　one　atom　of　metal　combines　with　two　of　chlorine　as　in　barium　chloride；　indicates　that　the　relative　weight　of　the　atom　of　radium　is