darwin and modern science-第143章

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　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　their　parents　uranium　and　thorium；　and　show　all　the　signs　of　distinct　chemical　individuality。　　They　are　strongly　radio…active；　while；　after　the　separation；　the　parents　uranium　and　thorium　are　found　to　have　lost　some　of　their　radio…activity。　　If　the　X…substances　be　kept；　their　radio…activity　decays；　while　that　of　the　uranium　or　thorium　from　which　they　were　obtained　gradually　rises　to　the　initial　value　it　had　before　the　separation。　　At　any　moment；　the　sum　of　the　radio…activity　is　constant；　the　activity　lost　by　the　product　being　equal　to　that　gained　by　the　parent　substance。　　These　phenomena　are　explained　if　we　suppose　that　the　X…product　is　slowly　produced　in　the　substance　of　the　parent；　and　decays　at　a　constant　rate。　　Uranium；　as　usually　seen；　contains　a　certain　amount　of　uranium…X；　and　its　radio…activity　consists　of　two　partsthat　of　the　uranium　itself；　and　that　of　the　X　product。　　When　the　latter　is　separated　by　means　of　its　chemical　reactions；　its　radio…activity　is　separated　also；　and　the　rates　of　decay　and　recovery　may　be　examined。

Radium　and　thorium；　but　not　uranium；　give　rise　to　radio…active　gases　which　have　been　called　emanations。　　Rutherford　has　shown　that　their　radio…　activity；　like　that　of　the　X　products；　suffers　decay；　while　the　walls　of　the　vessel　in　which　the　emanation　is　confined；　become　themselves　radio…　active。　　If　washed　with　certain　acids；　however；　the　walls　lose　their　activity；　which　is　transferred　to　the　acid；　and　can　be　deposited　by　evaporation　from　it　on　to　a　solid　surface。　　Here　again　it　is　clear　that　the　emanation　gives　rise　to　a　radio…active　substance　which　clings　to　the　walls　of　the　vessel；　and　is　soluble　in　certain　liquids；　but　not　in　others。

We　shall　return　to　this　point；　and　trace　farther　the　history　of　the　radio…　active　matter。　　At　present　we　wish　to　emphasise　the　fact　that；　as　in　other　cases；　the　radio…activity　of　the　emanation　is　accompanied　by　the　appearance　of　a　new　kind　of　substance　with　distinct　chemical　properties。

We　are　now　in　a　position　to　consider　as　a　whole　the　evidence　on　the　question　of　the　source　of　radio…active　energy。

（1）　　Radio…activity　is　accompanied　by　the　appearance　of　new　chemical　substances。　　The　energy　liberated　is　therefore　probably　due　to　the　associated　chemical　change。　　（2）　　The　activity　of　a　series　of　compounds　is　found　to　accompany　the　presence　of　a　radio…active　element；　the　activity　of　each　compound　depends　only　on　the　contents　of　the　element；　and　is　independent　of　the　nature　of　its　combination。　　Thus　radio…activity　is　a　property　of　the　element；　and　is　not　affected　by　its　state　of　isolation　or　chemical　combination。　（3）　　The　radio…activity　of　a　simple　transient　product　decays　in　a　geometrical　progression；　the　loss　per　second　being　proportional　to　the　mass　of　substance　still　left　at　the　moment；　and　independent　of　its　state　of　concentration　or　dilution。　　This　type　of　reaction　is　well　known　in　chemistry　to　mark　a　mono…molecular　change；　where　each　molecule　is　dissociated　or　altered　in　structure　independently。　　If　two　or　more　molecules　were　concerned　simultaneously；　the　rate　of　reaction　would　depend　on　the　nearness　of　the　molecules　to　each　other；　that　is；　to　the　concentration　of　the　material。　　（4）　　The　amount　of　energy　liberated　by　the　change　of　a　given　mass　of　material　far　transcends　the　amount　set　free　by　any　known　ordinary　chemical　action。　　The　activity　of　radium　decays　so　slowly　that　it　would　not　sink　to　half　its　initial　value　in　less　than　some　two　thousand　years；　and　yet　one　gramme　of　radium　emits　about　100　calories　of　heat　during　each　hour　of　its　existence。

The　energy　of　radio…activity　is　due　to　chemical　change；　but　clearly　to　no　chemical　change　hitherto　familiar　to　science。　　It　is　an　atomic　property；　characteristic　of　a　given　element；　and　the　atoms　undergo　the　change　individually；　not　by　means　of　interaction　among　each　other。　　The　conclusion　is　irresistible　that　we　are　dealing　with　a　fundamental　change　in　the　structure　of　the　individual　atoms；　which；　one　by　one；　are　dissociating　into　simpler　parts。　　We　are　watching　the　disintegration　of　the　〃atoms〃　of　the　chemist；　hitherto　believed　indestructible　and　eternal；　and　measuring　the　liberation　of　some　of　the　long…suspected　store　of　internal　atomic　energy。　　We　have　stumbled　on　the　transmutation　dreamed　by　the　alchemist；　and　discovered　the　process　of　a　veritable　evolution　of　matter。

The　transmutation　theory　of　radio…activity　was　formulated　by　Rutherford　（Rutherford；　〃Radio…activity〃　（2nd　edition）；　Cambridge；　1905；　page　307。）　and　Soddy　in　1903。　　By　its　light；　all　recent　work　on　the　subject　has　been　guided；　it　has　stood　the　supreme　test　of　a　hypothesis；　and　shown　power　to　suggest　new　investigations　and　to　co…ordinate　and　explain　them；　when　carried　out。　　We　have　summarised　the　evidence　which　led　to　the　conception　of　the　theory；　we　have　now　to　consider　the　progress　which　has　been　made　in　tracing　the　successive　disintegration　of　radio…active　atoms。

Soon　after　the　statement　of　the　transmutation　theory；　a　striking　verification　of　one　of　its　consequences　appeared。　　The　measurement　of　the　magnetic　and　electric　deflection　of　the　alpha…rays　suggested　to　Rutherford　the　idea　that　the　stream　of　projectiles　of　which　they　consisted　was　a　flight　of　helium　atoms。　　Ramsay　and　Soddy；　confining　a　minute　bubble　of　radium　emanation　in　a　fine　glass　tube；　were　able　to　watch　the　development　of　the　helium　spectrum　as；　day　by　day；　the　emanation　decayed。　　By　isolating　a　very　narrow　pencil　of　alpha…rays；　and　watching　through　a　microscope　their　impact　on　a　fluorescent　screen；　Rutherford　has　lately　counted　the　individual　alpha…projectiles；　and　confirmed　his　original　conclusion　that　their　mass　corresponded　to　that　of　helium　atoms　and　their　charge　to　double　that　on　a　univalent　atom。　　（〃Proc。　Roy。　Soc。〃　A；　page　141；　1908。）　　Still　more　recently；　he　has　collected　the　alpha…particles　shot　through　an　extremely　thin　wall　of　glass；　and　demonstrated　by　direct　spectroscopic　evidence　the　presence　of　helium。　　（〃Phil。　Mag。〃　February　1909。）

But　the　most　thorough　investigation　of　a　radio…active　pedigree　is　found　in　Rutherford's　classical　researches　on　the　successive　disintegration　products　of　radium；　in　order　to　follow　the　evidence　on　which　his　results　are　founded；　we　must　describe　more　fully　the　process　of　decay　of　the　activity　of　a　simple　radio…active　substance。　　The　decay　of　activity　of　the　body　known　as　uranium…X　is　shown　in　a　falling　curve　（Fig。　1。）。　　It　will　be　seen　that；　in　each　successive　22　days；　the　activity　falls　to　half　the　value　it　possessed　at　the　beginning。

This　change　in　a　geometrical　progression　is　characteristic　of　simple　radio…　active　processes；　and　can　be　expressed　mathematically　by　a　simple　exponential　formula。

As　we　have　said　above；　solid　bodies　exposed　to　the　emanations　of　radium　or　thorium　become　coated　with　a　radio…active　deposit。　　The　rate　of　decay　of　this　activity　depends　on　the　time　of　exposure　to　the　emanation；　and　does　not　always　show　the　usual　simple　type　of　curve。　　Thus　the　activity　of　a　rod　exposed　to　radium　emanation　for　1　minute　decays　in　accordance　with　a　curve　（Fig。　2）　which　represents　the　activity　as　measured　by　the