a history of science-4-第6章

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profession；　as　we　know；　being　the　ministry。　There　is　hardly　another　case　in　history　of　a　man　not　a　specialist　in　science　accomplishing　so　much　in　original　research　as　did　this　chemist；　physiologist；　electrician；　the　mathematician；　logician；　and　moralist；　the　theologian；　mental　philosopher；　and　political　economist。　He　took　all　knowledge　for　his　field；　but　how　he　found　time　for　his　numberless　researches　and　multifarious　writings；　along　with　his　every…day　duties；　must　ever　remain　a　mystery　to　ordinary　mortals。

That　this　marvellously　receptive；　flexible　mind　should　have　refused　acceptance　to　the　clearly　logical　doctrines　of　the　new　chemistry　seems　equally　inexplicable。　　But　so　it　was。　　To　the　very　last；　after　all　his　friends　had　capitulated；　Priestley　kept　up　the　fight。　From　America　he　sent　out　his　last　defy　to　the　enemy；　in　1800；　in　a　brochure　entitled　〃The　Doctrine　of　Phlogiston　Upheld；〃　etc。　　In　the　mind　of　its　author　it　was　little　less　than　a　paean　of　victory；　but　all　the　world　beside　knew　that　it　was　the　swan…song　of　the　doctrine　of　phlogiston。　Despite　the　defiance　of　this　single　warrior　the　battle　was　really　lost　and　won；　and　as　the　century　closed　〃antiphlogistic〃　chemistry　had　practical　possession　of　the　field。



III。　CHEMISTRY　SINCE　THE　TIME　OF　DALTON

JOHN　DALTON　AND　THE　ATOMIC　THEORY

Small　beginnings　as　have　great　endingssometimes。　　As　a　case　in　point；　note　what　came　of　the　small；　original　effort　of　a　self…trained　back…country　Quaker　youth　named　John　Dalton；　who　along　towards　the　close　of　the　eighteenth　century　became　interested　in　the　weather；　and　was　led　to　construct　and　use　a　crude　water…gauge　to　test　the　amount　of　the　rainfall。　The　simple　experiments　thus　inaugurated　led　to　no　fewer　than　two　hundred　thousand　recorded　observations　regarding　the　weather；　which　formed　the　basis　for　some　of　the　most　epochal　discoveries　in　meteorology；　as　we　have　seen。　　But　this　was　only　a　beginning。　The　simple　rain…gauge　pointed　the　way　to　the　most　important　generalization　of　the　nineteenth　century　in　a　field　of　science　with　which；　to　the　casual　observer；　it　might　seem　to　have　no　alliance　whatever。　　The　wonderful　theory　of　atoms；　on　which　the　whole　gigantic　structure　of　modern　chemistry　is　founded；　was　the　logical　outgrowth；　in　the　mind　of　John　Dalton；　of　those　early　studies　in　meteorology。

The　way　it　happened　was　this：　　From　studying　the　rainfall；　Dalton　turned　naturally　to　the　complementary　process　of　evaporation。　He　was　soon　led　to　believe　that　vapor　exists；　in　the　atmosphere　as　an　independent　gas。　　But　since　two　bodies　cannot　occupy　the　same　space　at　the　same　time；　this　implies　that　the　various　atmospheric　gases　are　really　composed　of　discrete　particles。　These　ultimate　particles　are　so　small　that　we　cannot　see　themcannot；　indeed；　more　than　vaguely　imagine　themyet　each　particle　of　vapor；　for　example；　is　just　as　much　a　portion　of　water　as　if　it　were　a　drop　out　of　the　ocean；　or；　for　that　matter；　the　ocean　itself。　　But；　again；　water　is　a　compound　substance；　for　it　may　be　separated；　as　Cavendish　has　shown；　into　the　two　elementary　substances　hydrogen　and　oxygen。　　Hence　the　atom　of　water　must　be　composed　of　two　lesser　atoms　joined　together。　Imagine　an　atom　of　hydrogen　and　one　of　oxygen。　　Unite　them；　and　we　have　an　atom　of　water；　sever　them；　and　the　water　no　longer　exists；　but　whether　united　or　separate　the　atoms　of　hydrogen　and　of　oxygen　remain　hydrogen　and　oxygen　and　nothing　else。　　Differently　mixed　together　or　united；　atoms　produce　different　gross　substances；　but　the　elementary　atoms　never　change　their　chemical　naturetheir　distinct　personality。

It　was　about　the　year　1803　that　Dalton　first　gained　a　full　grasp　of　the　conception　of　the　chemical　atom。　　At　once　he　saw　that　the　hypothesis；　if　true；　furnished　a　marvellous　key　to　secrets　of　matter　hitherto　insolublequestions　relating　to　the　relative　proportions　of　the　atoms　themselves。　It　is　known；　for　example；　that　a　certain　bulk　of　hydrogen　gas　unites　with　a　certain　bulk　of　oxygen　gas　to　form　water。　If　it　be　true　that　this　combination　consists　essentially　of　the　union　of　atoms　one　with　another　（each　single　atom　of　hydrogen　united　to　a　single　atom　of　oxygen）；　then　the　relative　weights　of　the　original　masses　of　hydrogen　and　of　oxygen　must　be　also　the　relative　weights　of　each　of　their　respective　atoms。　If　one　pound　of　hydrogen　unites　with　five　and　one…half　pounds　of　oxygen　（as；　according　to　Dalton's　experiments；　it　did）；　then　the　weight　of　the　oxygen　atom　must　be　five　and　one…half　times　that　of　the　hydrogen　atom。　Other　compounds　may　plainly　be　tested　in　the　same　way。　Dalton　made　numerous　tests　before　he　published　his　theory。　He　found　that　hydrogen　enters　into　compounds　in　smaller　proportions　than　any　other　element　known　to　him；　and　so；　for　convenience；　determined　to　take　the　weight　of　the　hydrogen　atom　as　unity。　　The　atomic　weight　of　oxygen　then　becomes　（as　given　in　Dalton's　first　table　of　1803）　5。5；　that　of　water　（hydrogen　plus　oxygen）　being　of　course　6。5。　The　atomic　weights　of　about　a　score　of　substances　are　given　in　Dalton's　first　paper；　which　was　read　before　the　Literary　and　Philosophical　Society　of　Manchester；　October　21；　1803。　　I　wonder　if　Dalton　himself；　great　and　acute　intellect　though　he　had；　suspected；　when　he　read　that　paper；　that　he　was　inaugurating　one　of　the　most　fertile　movements　ever　entered　on　in　the　whole　history　of　science？

Be　that　as　it　may；　it　is　certain　enough　that　Dalton's　contemporaries　were　at　first　little　impressed　with　the　novel　atomic　theory。　Just　at　this　time；　as　it　chanced；　a　dispute　was　waging　in　the　field　of　chemistry　regarding　a　matter　of　empirical　fact　which　must　necessarily　be　settled　before　such　a　theory　as　that　of　Dalton　could　even　hope　for　a　bearing。　　This　was　the　question　whether　or　not　chemical　elements　unite　with　one　another　always　in　definite　proportions。　Berthollet；　the　great　co…worker　with　Lavoisier；　and　now　the　most　authoritative　of　living　chemists；　contended　that　substances　combine　in　almost　indefinitely　graded　proportions　between　fixed　extremes。　He　held　that　solution　is　really　a　form　of　chemical　combinationa　position　which；　if　accepted；　left　no　room　for　argument。

But　this　contention　of　the　master　was　most　actively　disputed；　in　particular　by　Louis　Joseph　Proust；　and　all　chemists　of　repute　were　obliged　to　take　sides　with　one　or　the　other。　For　a　time　the　authority　of　Berthollet　held　out　against　the　facts；　but　at　last　accumulated　evidence　told　for　Proust　and　his　followers；　and　towards　the　close　of　the　first　decade　of　our　century　it　came　to　be　generally　conceded　that　chemical　elements　combine　with　one　another　in　fixed　and　definite　proportions。

More　than　that。　　As　the　analysts　were　led　to　weigh　carefully　the　quantities　of　combining　elements；　it　was　observed　that　the　proportions　are　not　only　definite；　but　that　they　bear　a　very　curious　relation　to　one　another。　If　element　A　combines　with　two　different　proportions　of　element　B　to　form　two　compounds；　it　appears　that　the　weight　of　the　larger　quantity　of　B　is　an　exact　multiple　of　that　of　the　smaller　quantity。　This　curious　relation　was　noticed　by　Dr。　Wollaston；　one　of　the　most　accurate　of　observers；　and　a　little　later　it　was　confirmed　by　Johan　Jakob　Berzelius；　the　great　Swedish　chemist；　who　was　to　be　a　dominating　influence　in　the　chemical　world　for　a　generation　to　come。　　But　this　combination　of　elements　in　numerical　proportions　was　exactly　what　Dalton　had　noticed　as　early　as　1802；　and　what　bad　led　him　directly　to　the　atomic　weights。　So　the　confirmation　of　this　essential　point　by　chemists　of　such　authority　gave　the　strongest　confirmation　to　the　atomic　theory。

During　these　same　years　the　rising　authority　of　the　French　chemical　world；　Joseph　Louis　Gay…Lussac；　was　conducting　experiments　with　gases；　which　he　had　undertaken　at　first　in　conjunction　with　Humboldt；　but　which　later　on　were　conducted　independently。　In　1809；　the　next　year　after　the　publication　of　the　first　volume　of　Dalton's　New　System　of　Chemical　Philosophy；　Gay…Lussac　published　the　results　of　his　observations；　and　among　other　things　brought　out　the　remarkable　fact　that　gases；　under　the　same　conditions　as　to　temperature　and　pressure；　combine　always　in　definite　numerical　proportions　as　to　volume。　Exactly　two　volumes　of　hydrogen；　for　example；　combine　with　one　volume　of　oxygen　to　form　water。　　Moreover；　the　resulting　compound　gas　always　bears　a　simple　relation　to　the　combining　volumes。　In　the　case　just　cited；　the　union　of　two　volumes　of　hydrogen　and　one　of　oxygen　results　in　precisely　two　volumes　of　water　vapor。

Naturally　enough；　the　champions　of　the　atomic　theory　seized　upon　these　observations　of　Gay…Lussac　as　lending　strong　support　to　their　hypothesisall　of　them；　that　is；　but　the　curiously　self…reliant　and　self…sufficient　author　of　the　atomic　theory　himself；　who　declined　to　accept　the　observations　of　the　French　chemist　as　valid。　Yet　the　observations　of　Gay…Lussac　were　correct；　as　countless　chemists　since　then　have　demonstrated　anew；　and　his　theory　of　combination　by　volumes　became　one　of　the　foundation…stones　of　the　atomic　theory；　despite　the　opposition　of　the　author　of　that　theory。

The　true　explanation　of　Gay…Lussac's　law　of　combination　by　volumes　was　thought　out　almost　immediately　by　an　Italian　savant；　Amadeo；　Avogadro；　and　expressed　in　terms　of　the　atomic　theory。　The　fact　must　be；　said　Avogadro；　that　under　similar　physical　conditions　every　form　of　gas　contains　exactly　the　same　number　of　ultimate　particles　in　a　given　volume。　　Each　of　these　ultimate　physical　particles　may　be　composed　of　two　or　more　atoms　（as　in　the　case　of　water　vapor）；　but　such　a　compound　atom　conducts　itself　as　if　it　were　a　simple　and　indivisible　atom；　as　regards　the　amount　of　space　that　separates　it　from　its　fellows　under　given　conditions　of　pressure　and　temperature。　The　compound　atom；　composed　of　two　or　more　elementary　atoms；　Avogadro　proposed　to　distinguish；　for　purposes　of　convenience；　by　the　name　molecule。　　It　is　to　the　molecule；　considered　as　the　unit　of　physical　structure；　that　Avogadro's　law　applies。

This　vastly　important　distinction　between　atoms　and　molecules；　implied　in　the　law　just　expressed；　was　published　in　1811。　Four　years　later；　the　famous　French　physicist　Ampere　outlined　a　similar　theory；　and　utilized　the　law　in　his　mathematical　calculations。　And　with　that　the　law　of　Avogadro　dropped　out　of　sight　for　a　full　generation。　　Little　suspecting　that　it　was　the　very　key　to　the　inner　mysteries　of　the　atoms　for　which　they　were　seeking；　the　chemists　of　the　time　cast　it　aside；　and　let　it　fade　from　the　memory　of　their　science。

This；　however；　was　not　strange；　for　of　course　the　law　of　Avogadro　is　based　on　the　atomic　theory；　and　in　1811　the　atomic　theory　was　itself　still　being　weighed　in　the　balance。　The　law　of　multiple　proportions　found　general　acceptance　as　an　empirical　fact；　but　many　of　the　leading　lights　of　chemistry　still　looked　askance　at　Dalton's　explanation　of　this　law。　Thus　Wollaston；　though　from　the　first　he　inclined　to　acceptance　of　the　Daltonian　view；　cautiously　suggested　that　it　would　be　well　to　use　the　non…committal　word　〃equivalent〃　instead　of　〃atom〃；　and　Davy；　for　a　similar　reason；　in　his　book　of　1812；　speaks　only　of　〃proportions；〃　binding　himself　to　no　theory　as　to　what　might　be　the　nature　of　these　proportions。

At　least　two　great　chemists　of　the　time