darwin and modern science-第175节

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　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　alpha…rays。　　If　the　electroscope　be　screened　from　the　alpha…rays；　it　is　found　that　the　activity　of　the　rod　in　beta…　an　gamma…rays　increases　for　some　35　minutes　and　then　diminishes　（Fig。　3。）。

These　complicated　relations　have　been　explained　satisfactorily　and　completely　by　Rutherford　on　the　hypothesis　of　successive　changes　of　the　radio…active　matter　into　one　new　body　after　another。　　（Rutherford；　〃Radio…　activity〃　（2nd　edition）；　Cambridge；　1905；　page　379。）　　The　experimental　curve　represents　the　resultant　activity　of　all　the　matter　present　at　a　given　moment；　and　the　process　of　disentangling　the　component　effects　consists　in　finding　a　number　of　curves；　which　express　the　rise　and　fall　of　activity　of　each　kind　of　matter　as　it　is　produced　and　decays；　and；　fitted　together；　give　the　curve　of　the　experiments。

Other　methods　of　investigation　also　are　open。　　They　have　enabled　Rutherford　to　complete　the　life…history　of　radium　and　its　products；　and　to　clear　up　doubtful　points　left　by　the　analysis　of　the　curves。　　By　the　removal　of　the　emanation；　the　activity　of　radium　itself　has　been　shown　to　consist　solely　of　alpha…rays。　　This　removal　can　be　effected　by　passing　air　through　the　solution　of　a　radium　salt。　　The　emanation　comes　away；　and　the　activity　of　the　deposit　which　it　leaves　behind　decays　rapidly　to　a　small　fraction　of　its　initial　value。　　Again；　some　of　the　active　deposits　of　the　emanation　are　more　volatile　than　others；　and　can　be　separated　from　them　by　the　agency　of　heat。

From　such　evidence　Rutherford　has　traced　a　long　series　of　disintegration　products　of　radium；　all　but　the　first　of　which　exist　in　much　too　minute　quantities　to　be　detected　otherwise　than　by　their　radio…activities。　　Moreover；　two　of　these　products　are　not　themselves　appreciably　radio…　active；　though　they　are　born　from　radio…active　parents；　and　give　rise　to　a　series　of　radio…active　descendants。　　Their　presence　is　inferred　from　such　evidence　as　the　rise　of　beta　and　gamma　radio…activity　in　the　solid　newly　deposited　by　the　emanation；　this　rise　measuring　t