darwin and modern science-第174节

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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　about　225。　　As　thus　prepared；　radium　is　a　well…marked　chemical　element；　forming　a　series　of　compounds　analogous　to　those　of　barium　and　showing　a　characteristic　line　spectrum。　　But；　unlike　most　other　chemical　elements；　it　is　intensely　radio…active；　and　produces　effects　some　two　million　times　greater　than　those　of　uranium。

In　1899　E。　Rutherford；　then　of　Montreal；　discovered　that　the　radiation　from　uranium；　thorium　and　radium　was　complex。　　（Rutherford；　〃Radio…activity〃　（2nd　edition）；　Cambridge；　1905。）　　Three　types　of　rays　were　soon　distinguished。　　The　first；　named　by　Rutherford　alpha…rays；　are　absorbed　by　thin　metal　foil　or　a　few　centimetres　of　air。　　When　examined　by　measurements　of　the　deflections　caused　by　magnetic　and　electric　fields；　the　alpha…rays　are　found　to　behave　as　would　positively　electrified　particles　of　the　magnitude　of　helium　atoms　possessing　a　double　ionic　charge　and　travelling　with　a　velocity　about　one…tenth　that　of　light。　　The　second　or　beta　type　of　radiation　is　much　more　penetrating。　　It　will　pass　through　a　considerable　thickness　of　metallic　foil；　or　many　centimetres　of　air；　and　still　affect　photographic　plates　or　discharge　electroscopes。　　Magnetic　and　electric　forces　deflect　beta…rays　much　more　than　alpha…rays；　indicating　that；　although　the　speed　is　greater；　approaching　in　some　cases　within　five　per　cent。　that　of　light；　the　mass　is　very　much　less。　　The　beta…rays　must　be　streams　of　particles；　identical　with　those　of　cathode　rays；　possessing　the　minute　mass　of　J。J。　Thomson's　corpuscle；　some　eight…hundredth　part　of　that　of　a　hydrogen　atom。　　A　third　or　gamma　type　of　radiation　was　also　detected。　　More　penetrating　even　than　beta…rays；　the　gamma…rays　have　never　been　deflected　by　any　magnetic　or　electric　force　yet　applied。　　Like　Rontgen　rays；　it　is　probable　that　gamma…rays　are　wave…pulses　in　the　luminiferous　aether；　though　the　possibility　of　explaining　them　as　flights　of　non…　electrified　particles　is　before　the　minds　of　some　physicists。

Still　another　kind　of　radiation　has　been　discovered　more　recently　by　Thomson；　who　has　found　that　in　high　vacua；　rays　become　apparent　which　are　absorbed　at　once　by　air　at　any　ordinary　pressure。

The　emission　of　all　these　different　types　of　radiation　involves　a　continual　drain　of　energy　from　the　radio…active　body。　　When　M。　and　Mme　Curie　had　prepared　as　much　as　a　gramme　of　radium　chloride；　the　energy　of　the　radiation　became　apparent　as　an　evolution　of　heat。　　The　radium　salt　itself；　and　the　case　containing　it；　absorbed　the　major　part　of　the　radiation；　and　were　thus　maintained　at　a　temperature　measurably　higher　than　that　of　the　surroundings。　　The　rate　of　thermal　evolution　was　such　that　it　appeared　that　one　gramme　of　pure　radium　must　emit　about　100　gramme…calories　of　heat　in　an　hour。　　This　observation；　naturally　as　it　follows　from　the　phenomena　previously　discovered；　first　called　attention　to　the　question　of　the　source　of　the　energy　which　maintains　indefinitely　and　without　apparent　diminution　the　wonderful　stream　of　radiation　proceeding　from　a　radio…active　substance。　　In　the　solution　of　this　problem　lies　the　point　of　the　present　essay。

In　order　to　appreciate　the　evidence　which　bears　on　the　question　we　must　now　describe　two　other　series　of　phenomena。

It　is　a　remarkable　fact　that　the　intensity　of　the　radiation　from　a　radio…　active　body　is　independent　of　the　external　conditions　of　temperature；　pressure；　etc。　which　modify　so　profoundly　almost　all　other　physical　and　chemical　processes。　　Exposure　to　the　extreme　cold　of　liquid　air；　or　to　the　great　heat　of　a　furnace；　leaves　the　radio…activity　of　a　substance　unchanged；　apparent　exceptions　to　this　statement　having　been　traced　to　secondary　causes。

Then；　it　is　found　that　radio…activity　is　always　accompanied　by　some　chemical　change；　a　new　substance　always　appears　as　the　parent　substance　emits　these　radiations。　　Thus　by　chemical　reactions　it　is　possible　to　separate　from　uranium　and　thorium　minute　quantities　of　radio…active　materials　to　which　the　names　of　uranium…X　and　thorium…X　have　been　given。　　These　bodies　behave　differently　from　t