a history of science-4-第9章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！



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；　however；　adopted　the　atomic　view　with　less　reservation。　　One　of　these　was　Thomas　Thomson；　professor　at　Edinburgh；　who；　in　1807；　had　given　an　outline　of　Dalton's　theory　in　a　widely　circulated　book；　which　first　brought　the　theory　to　the　general　attention　of　the　chemical　world。　The　other　and　even　more　noted　advocate　of　the　atomic　theory　was　Johan　Jakob　Berzelius。　　This　great　Swedish　chemist　at　once　set　to　work　to　put　the　atomic　theory　to　such　tests　as　might　be　applied　in　the　laboratory。　　He　was　an　analyst　of　the　utmost　skill；　and　for　years　be　devoted　himself　to　the　determination　of　the　combining　weights；　〃equivalents〃　or　〃proportions；〃　of　the　different　elements。　These　determinations；　in　so　far　as　they　were　accurately　made；　were　simple　expressions　of　empirical　facts；　independent　of　any　theory；　but　gradually　it　became　more　and　more　plain　that　these　facts　all　harmonize　with　the　atomic　theory　of　Dalton。　　So　by　common　consent　the　proportionate　combining　weights　of　the　elements　came　to　be　known　as　atomic　weightsthe　name　Dalton　had　given　them　from　the　firstand　the　tangible　conception　of　the　chemical　atom　as　a　body　of　definite　constitution　and　weight　gained　steadily　in　favor。

From　the　outset　the　idea　had　had　the　utmost　tangibility　in　the　mind　of　Dalton。　　He　had　all　along　represented　the　different　atoms　by　geometrical　symbolsas　a　circle　for　oxygen；　a　circle　enclosing　a　dot　for　hydrogen；　and　the　likeand　had　represented　compounds　by　placing　these　symbols　of　the　elements　in　juxtaposition。　Berzelius　proposed　to　improve　upon　this　method　by　substituting　for　the　geometrical　symbol　the　initial　of　the　Latin　name　of　the　element　representedO　for　oxygen；　H　for　hydrogen；　and　so　ona　numerical　coefficient　to　follow　the　letter　as　an　indication　of　the　number　of　atoms　present　in　any　given　compound。　This　simple　system　soon　gained　general　acceptance；　and　with　slight　modifications　it　is　still　universally　employed。　Every　school…boy　now　is　aware　that　H2O　is　the　chemical　way　of　expressing　the　union　of　two　atoms　of　hydrogen　with　one　of　oxygen　to　form　a　molecule　of　water。　　But　such　a　formula　would　have　had　no　meaning　for　the　wisest　chemist　before　the　day　of　Berzelius。

The　universal　fame　of　the　great　Swedish　authority　served　to　give　general　currency　to　his　symbols　and　atomic　weights；　and　the　new　point　of　view　thus　developed　led　presently　to　two　important　discoveries　which　removed　the　last　lingering　doubts　as　to　the　validity　of　the　atomic　theory。　In　1819　two　French　physicists；　Dulong　and　Petit；　while　experimenting　with　heat；　discovered　that　the　specific　heats　of　solids　（that　is　to　say；　the　amount　of　heat　required　to　raise　the　temperature　of　a　given　mass　to　a　given　degree）　vary　inversely　as　their　atomic　weights。　In　the　same　year　Eilhard　Mitscherlich；　a　German　investigator；　observed　that　compounds　having　the　same　number　of　atoms　to　the　molecule　are　disposed　to　form　the　same　angles　of　crystallizationa　property　which　he　called　isomorphism。

Here；　then；　were　two　utterly　novel　and　independent　sets　of　empirical　facts　which　harmonize　strangely　with　the　supposition　that　substances　are　composed　of　chemical　atoms　of　a　determinate　weight。　This　surely　could　not　be　coincidenceit　tells　of　law。　And　so　as　soon　as　the　claims　of　Dulong　and　Petit　and　of　Mitscherlich　had　been　substantiated　by　other　observers；　the　laws　of　the　specific　heat　of　atoms；　and　of　isomorphism；　took　their　place　as　new　levers　of　chemical　science。　　With　the　aid　of　these　new　tools　an　impregnable　breastwork　of　facts　was　soon　piled　about　the　atomic　theory。　And　John　Dalton；　the　author　of　that　theory；　plain；　provincial　Quaker；　working　on　to　the　end　in　semi…retirement；　became　known　to　all　the　world　and　for　all　time　as　a　master　of　masters。


HUMPHRY　DAVY　AND　ELECTRO…CHEMISTRY

During　those　early　years　of　the　nineteenth　century；　when　Dalton　was　grinding　away　at　chemical　fact　and　theory　in　his　obscure　Manchester　laboratory；　another　Englishman　held　the　attention　of　the　chemical　world　with　a　series　of　the　most　brilliant　and　widely　heralded　researches。　　This　was　Humphry　Davy；　a　young　man　who　had　conic　to　London　in　1801；　at　the　instance　of　Count　Rumford；　to　assume　the　chair　of　chemical　philosophy　in　the　Royal　Institution；　which　the　famous　American　had　just　founded。

Here；　under　Davy's　direction；　the　largest　voltaic　battery　yet　constructed　had　been　put　in　operation；　and　with　its　aid　the　brilliant　young　experimenter　was　expected　almost　to　perform　miracles。　And　indeed　he　scarcely　disappointed　the　expectation；　for　with　the　aid　of　his　battery　he　transformed　so　familiar　a　substance　as　common　potash　into　a　metal　which　was　not　only　so　light　that　it　floated　on　water；　but　possessed　the　seemingly　miraculous　property　of　bursting　into　flames　as　soon　as　it　came　in　contact　with　that　fire…quenching　liquid。　If　this　were　not　a　miracle；　it　had　for　the　popular　eye　all　the　appearance　of　the　miraculous。

What　Davy　really　had　done　was　to　decompose　the　potash；　which　hitherto　had　been　supposed　to　be　elementary；　liberating　its　oxygen；　and　thus　isolating　its　metallic　base；　which　he　named　potassium。　The　same　thing　was　done　with　soda；　and　the　closely　similar　metal　sodium　was　discoveredmetals　of　a　unique　type；　possessed　of　a　strange　avidity　for　oxygen；　and　capable　of　seizing　on　it　even　when　it　is　bound　up　in　the　molecules　of　water。　Considered　as　mere　curiosities；　these　discoveries　were　interesting；　but　aside　from　that　they　were　of　great　theoretical　importance；　because　they　showed　the　compound　nature　of　some　familiar　chemicals　that　had　been　regarded　as　elements。　　Several　other　elementary　earths　met　the　same　fate　when　subjected　to　the　electrical　influence；　the　metals　barium；　calcium；　and　strontium　being　thus　discovered。　Thereafter　Davy　always　referred　to　the　supposed　elementary　substances　（including　oxygen；　hydrogen；　and　the　rest）　as　〃unde…compounded〃　bodies。　These　resist　all　present　efforts　to　decompose　them；　but　how　can　one　know　what　might　not　happen　were　they　subjected　to　an　influence；　perhaps　some　day　to　be　discovered；　which　exceeds　the　battery　in　power　as　the　battery　exceeds　the