a history of science-4-第7章

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eory　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　blowpipe？

Another　and　even　more　important　theoretical　result　that　flowed　from　Davy's　experiments　during　this　first　decade　of　the　century　was　the　proof　that　no　elementary　substances　other　than　hydrogen　and　oxygen　are　produced　when　pure　water　is　decomposed　by　the　electric　current。　It　was　early　noticed　by　Davy　and　others　that　when　a　strong　current　is　passed　through　water；　alkalies　appear　at　one　pole　of　the　battery　and　acids　at　the　other；　and　this　though　the　water　used　were　absolutely　pure。　This　seemingly　told　of　the　creation　of　elementsa　transmutation　but　one　step　removed　from　the　creation　of　matter　itselfunder　the　influence　of　the　new　〃force。〃　　It　was　one　of　Davy's　greatest　triumphs　to　prove；　in　the　series　of　experiments　recorded　in　his　famous　Bakerian　lecture　of　1806；　that　the　alleged　creation　of　elements　did　not　take　place；　the　substances　found　at　the　poles　of　the　battery　having　been　dissolved　from　the　walls　of　the　vessels　in　which　the　water　experimented　upon　had　been　placed。　Thus　the　same　implement　which　had　served　to　give　a　certain　philosophical　warrant　to　the　fading　dreams　of　alchemy　banished　those　dreams　peremptorily　from　the　domain　of　present　science。

〃As　early　as　1800；〃　writes　Davy；　〃I　had　found　that　when　separate　portions　of　distilled　water；　filling　two　glass　tubes；　connected　by　moist　bladders；　or　any　moist　animal　or　vegetable　substances；　were　submitted　to　the　electrical　action　of　the　pile　of　Volta　by　means　of　gold　wires；　a　nitro…muriatic　solution　of　gold　appeared　in　the　tube　containing　the　positive　wire；　or　the　wire　transmitting　the　electricity；　and　a　solution　of　soda　in　the　opposite　tube；　but　I　soon　ascertained　that　the　muriatic　acid　owed　its　existence　to　the　animal　or　vegetable　matters　employed；　for　when　the　same　fibres　of　cotton　were　made　use　of　in　successive　experiments；　and　washed　after　every　process　in　a　weak　solution　of　nitric　acid；　the　water　in　the　apparatus　containing　them；　though　acted　on　for　a　great　length　of　time　with　a　very　strong　power；　at　last　produced　no　effects　upon　nitrate　of　silver。

〃In　cases　when　I　had　procured　much　soda；　the　glass　at　its　point　of　contact　with　the　wire　seemed　considerably　corroded；　and　I　was　confirmed　in　my　idea　of　referring　the　production　of　the　alkali　principally　to　this　source；　by　finding　that　no　fixed　saline　matter　could　be　obtained　by　electrifying　distilled　water　in　a　single　agate　cup　from　two　points　of　platina　with　the　Voltaic　battery。

〃Mr。　Sylvester；　however；　in　a　paper　published　in　Mr。　Nicholson's　journal　for　last　August；　states　that　though　no　fixed　alkali　or　muriatic　acid　appears　when　a　single　vessel　is　employed；　yet　that　they　are　both　formed　when　two　vessels　are　used。　And　to　do　away　with　all　objections　with　regard　to　vegetable　substances　or　glass；　he　conducted　his　process　in　a　vessel　made　of　baked　tobacco…pipe　clay　inserted　in　a　crucible　of　platina。　I　have　no　doubt　of　the　correctness　of　his　results；　but　the　conclusion　appears　objectionable。　　He　conceives；　that　he　obtained　fixed　alkali；　because　the　fluid　after　being　heated　and　evaporated　left　a　matter　that　tinged　turmeric　brown；　which　would　have　happened　had　it　been　lime；　a　substance　that　exists　in　considerable　quantities　in　all　pipe…clay；　and　even　allowing　the　presence　of　fixed　alkali；　the　materials　employed　for　the　manufacture　of　tobacco…pipes　are　not　at　all　such　as　to　exclude　the　combinations　of　this　substance。

〃I　resumed　the　inquiry；　I　procured　small　cylindrical　cups　of　agate　of　the　capacity　of　about　one…quarter　of　a　cubic　inch　each。　They　were　boiled　for　some　hours　in　distilled　water；　and　a　piece　of　very　white　and　transparent　amianthus　that　had　been　treated　in　the　same　way　was　made　then　to　connect　together；　they　were　filled　with　distilled　water　and　exposed　by　means　of　two　platina　wires　to　a　current　of　electricity；　from　one　hundred　and　fifty　pairs　of　plates　of　copper　and　zinc　four　inches　square；　made　active　by　means　of　solution　of　alum。　After　forty…eight　hours　the　process　was　examined：　Paper　tinged　with　litmus　plunged　into　the　tube　containing　the　transmitting　or　positive　wire　was　immediately　strongly　reddened。　Paper　colored　by　turmeric　introduced　into　the　other　tube　had　its　color　much　deepened；　the　acid　matter　gave　a　very　slight　degree　of　turgidness　to　solution　of　nitrate　of　soda。　The　fluid　that　affected　turmeric　retained　this　property　after　being　strongly　boiled；　and　it　appeared　more　vivid　as　the　quantity　became　reduced　by　evaporation；　carbonate　of　ammonia　was　mixed　with　it；　and　the　whole　dried　and　exposed　to　a　strong　heat；　a　minute　quantity　of　white　matter　remained；　which；　as　far　as　my　examinations　could　go；　had　the　properties　of　carbonate　of　soda。　I　compared　it　with　similar　minute　portions　of　the　pure　carbonates　of　potash；　and　similar　minute　portions　of　the　pure　carbonates　of　potash　and　soda。　　It　was　not　so　deliquescent　as　the　former　of　these　bodies；　and　it　formed　a　salt　with　nitric　acid；　which；　like　nitrate　of　soda；　soon　attracted　moisture　from　a　damp　atmosphere　and　became　fluid。

〃This　result　was　unexpected；　but　it　was　far　from　convincing　me　that　the　substances　which　were　obtained　were　generated。　In　a　similar　process　with　glass　tubes；　carried　on　under　exa