a history of science-4-第9节

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！


tery　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　exactly　the　same　circumstances　and　for　the　same　time；　I　obtained　a　quantity　of　alkali　which　must　have　been　more　than　twenty　times　greater；　but　no　traces　of　muriatic　acid。　There　was　much　probability　that　the　agate　contained　some　minute　portion　of　saline　matter；　not　easily　detected　by　chemical　analysis；　either　in　combination　or　intimate　cohesion　in　its　pores。　To　determine　this；　I　repeated　this　a　second；　a　third；　and　a　fourth　time。　　In　the　second　experiment　turbidness　was　still　produced　by　a　solution　of　nitrate　of　silver　in　the　tube　containing　the　acid；　but　it　was　less　distinct；　in　the　third　process　it　was　barely　perceptible；　and　in　the　fourth　process　the　two　fluids　remained　perfectly　clear　after　the　mixture。　The　quantity　of　alkaline　matter　diminished　in　every　operation；　and　in　the　last　process；　though　the　battery　had　been　kept　in　great　activity　for　three　days；　the　fluid　possessed；　in　a　very　slight　degree；　only　the　power　of　acting　on　paper　tinged　with　turmeric；　but　its　alkaline　property　was　very　sensible　to　litmus　paper　slightly　reddened；　which　is　a　much　more　delicate　test；　and　after　evaporation　and　the　process　by　carbonate　of　ammonia；　a　barely　perceptible　quantity　of　fixed　alkali　was　still　left。　The　acid　matter　in　the　other　tube　was　abundant；　its　taste　was　sour；　it　smelled　like　water　over　which　large　quantities　of　nitrous　gas　have　been　long　kept；　it　did　not　effect　solution　of　muriate　of　barytes；　and　a　drop　of　it　placed　upon　a　polished　plate　of　silver　left；　after　evaporation；　a　black　stain；　precisely　similar　to　that　produced　by　extremely　diluted　nitrous　acid。

〃After　these　results　I　could　no　longer　doubt　that　some　saline　matter　existing　in　the　agate　tubes　had　been　the　source　of　the　acid　matter　capable　of　precipitating　nitrate　of　silver　and　much　of　the　alkali。　Four　additional　repetitions　of　the　process；　however；　convinced　me　that　there　was　likewise　some　other　cause　for　the　presence　of　this　last　substance；　for　it　continued　to　appear　to　the　last　in　quantities　sufficiently　distinguishable；　and　apparently　equal　in　every　case。　I　had　used　every　precaution；　I　had　included　the　tube　in　glass　vessels　out　of　the　reach　of　the　circulating　air；　all　the　acting　materials　had　been　repeatedly　washed　with　distilled　water；　and　no　part　of　them　in　contact　with　the　fluid　had　been　touched　by　the　fingers。

〃The　only　substance　that　I　could　now　conceive　as　furnishing　the　fixed　alkali　was　the　water　itself。　　This　water　appeared　pure　by　the　tests　of　nitrate　of　silver　and　muriate　of　barytes；　but　potash　of　soda；　as　is　well　known；　rises　in　small　quantities　in　rapid　distillation；　and　the　New　River　water　which　I　made　use　of　contains　animal　and　vegetable　impurities；　which　it　was　easy　to　conceive　might　furnish　neutral　salts　capable　of　being　carried　over　in　vivid　ebullition。〃＇1＇　Further　experiment　proved　the　correctness　of　this　inference；　and　the　last　doubt　as　to　the　origin　of　the　puzzling　chemical　was　dispelled。

Though　the　presence　of　the　alkalies　and　acids　in　the　water　was　explained；　however；　their　respective　migrations　to　the　negative　and　positive　poles　of　the　battery　remained　to　be　accounted　for。　Davy's　classical　explanation　assumed　that　different　elements　differ　among　themselves　as　to　their　electrical　properties；　some　being　positively；　others　negatively；　electrified。　　Electricity　and　〃chemical　affinity；〃　he　said；　apparently　are　manifestations　of　the　same　force；　acting　in　the　one　case　on　masses；　in　the　other　on　particles。　Electro…positive　particles　unite　with　electro…negative　particles　to　form　chemical　compounds；　in　virtue　of　the　familiar　principle　that　opposite　electricities　attract　one　another。　When　compounds　are　decomposed　by　the　battery；　this　mutual　attraction　is　overcome　by　the　stronger　attraction　of　the　poles　of　the　battery　itself。

This　theory　of　binary　composition　of　all　chemical　compounds；　through　the　union　of　electro…positive　and　electro…negative　atoms　or　molecules；　was　extended　by　Berzelius；　and　made　the　basis　of　his　famous　system　of　theoretical　chemistry。　　This　theory　held　that　all　inorganic　compounds；　however　complex　their　composition；　are