a history of science-4-第8章

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〃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　essentially　composed　of　such　binary　combinations。　For　many　years　this　view　enjoyed　almost　undisputed　sway。　It　received　what　seemed　strong　confirmation　when　Faraday　showed　the　definite　connection　between　the　amount　of　electricity　employed　and　the　amount　of　decomposition　produced　in　the　so…called　electrolyte。　But　its　claims　were　really　much　too　comprehensive；　as　subsequent　discoveries　proved。


ORGANIC　CHEMISTRY　AND　THE　IDEA　OF　THE　MOLECULE

When　Berzelius　first　promulgated　his　binary　theory　he　was　careful　to　restrict　its　unmodified　application　to　the　compounds　of　the　inorganic　world。　　At　that　time；　and　for　a　long　time　thereafter；　it　was　supposed　that　substances　of　organic　nature　had　some　properties　that　kept　them　aloof　from　the　domain　of　inorganic　chemistry。　It　was　little　doubted　that　a　so…called　〃vital　force〃　operated　here；　replacing　or　modifying　the　action　of　ordinary　〃chemical　affinity。〃　It　was；　indeed；　admitted　that　organic　compounds　are　composed　of　familiar　elementschiefly　carbon；　oxygen；　hydrogen；　and　nitrogen；　but　these　elements　were　supposed　to　be　united　in　ways　that　could　not　be　imitated　in　the　domain　of　the　non…living。　It　was　regarded　almost　as　an　axiom　of　chemistry　that　no　organic　compound　whatever　could　be　put　together　from　its　elementssynthesizedin　the　laboratory。　To　effect　the　synthesis　of　even　the　simplest　organic　compound；　it　was　thought　that　the　〃vital　force〃　must　be　in　operation。

Therefore　a　veritable　sensation　was　created　in　the　chemical　world　when；　in　the　year　1828；　it　was　announced　that　the　young　German　chemist；　Friedrich　Wohler；　formerly　pupil　of　Berzelius；　and　already　known　as　a　coming　master；　had　actually　synthesized　the　well…known　organic　product　urea　in　his　laboratory　at　Sacrow。　　The　〃exception　which　proves　the　rule〃　is　something　never　heard　of　in　the　domain　of　logical　science。　　Natural　law　knows　no　exceptions。　　So　the　synthesis　of　a　single　organic　compound　sufficed　at　a　blow　to　break　down　the　chemical　barrier　which　the　imagination　of　the　fathers　of　the　science　had　erected　between　animate　and　inanimate　nature。　Thenceforth　the　philosophical　chemist　would　regard　the　plant　and　animal　organisms　as　chemical　laboratories　in　which　conditions　are　peculiarly　favorable　for　building　up　complex　compounds　of　a　few　familiar　elements；　under　the　operation　of　universal　chemical　laws。　　The　chimera　〃vital　force〃　could　no　longer　gain　recognition　in　the　domain　of　chemistry。

Now　a　wave　of　interest　in　organic　chemistry　swept　over　the　chemical　world；　and　soon　the　study　of　carbon　compounds　became　as　much　the　fashion　as　electrochemistry　had　been　in　the；　preceding　generation。

Foremost　among　the　workers　who　rendered　this　epoch　of　organic　chemistry　memorable　were　Justus　Liebig　in　Germany　and　Jean　Baptiste　Andre　Dumas　in　France；　and　their　respective　pupils；　Charles　Frederic　Gerhardt　and　Augustus　Laurent。　　Wohler；　too；　must　be　named　in　the　same　breath；　as　also　must　Louis　Pasteur；　who；　though　somewhat　younger　than　the　others；　came　upon　the　scene　in　time　to　take　chief　part　in　the　most　important　of　the　controversies　that　grew　out　of　their　labors。

Several　years　earlier　than　this　the　way　had　been　paved　for　the　study　of　organic　substances　by　Gay…Lussac's　discovery；　made　in　1815；　that　a　certain　compound　of　carbon　and　nitrogen；　which　he　named　cyanogen；　has　a　peculiar　degree　of　stability　which　enables　it　to　retain　its　identity　and　enter　into　chemical　relations　after　the　manner　of　a　simple　body。　A　year　later　Ampere　discovered　that　nitrogen　and　hydrogen；　when　combined　in　certain　proportions　to　form　what　he　called　ammonium；　have　the　same　property。　Berzelius　had　seized　upon　this　discovery　of　the　compound　radical；　as　it　was　called；　because　it　seemed　to　lend　aid　to　his　dualistic　theory。　He　conceived　the　idea　that　all　organic　compounds　are　binary　unions　of　various　compound　radicals　with　an　atom　of　oxygen；　announcing　this　theory　in　1818。　Ten　years　later；　Liebig　and　Wohler　undertook　a　joint　investigation　which　resulted　in　proving　that　compound　radicals　are　indeed　very　abundant　among　organic　substances。　　Thus　the　theory　of　Berzelius　seemed　to　be　substantiated；　and　organic　chemistry　came　to　be　defined　as　the　chemistry　of　compound　radicals。

But　even　in　the　day　of　its　seeming　triumph　the　dualistic　theory　was　destined　to　receive　a　rude　shock。　　This　came　about　through　the　investigations　of　Dumas；　who　proved　that　in　a　certain　organic　substance　an　atom　of　hydrogen　may　be　removed　and　an　atom　of　chlorine　substituted　in　its　place　without　destroying　the　integrity　of　the　original　compoundmuch　as　a　child　might　substitute　one　block　for　another　in　its　play…house。　Such　a　substitution　would　be　quite　consistent　with　the　dualistic　theory；　were　it　not　for　the　very　essential　fact　that　hydrogen　is　a　powerfully　electro…positive　element；　while　chlorine　is　as　strongly　electro…negative。　Hence　the　compound　radical　which　united　successively　with　these　two　elements　must　itself　be　at　one　time　electro…positive；　at　another　electro…negativea　seeming　inconsistency　which　threw　the　entire　Berzelian　theory　into　disfavor。

In　its　place　there　was　elaborated；　chiefly　through　the　efforts　of　Laurent　and　Gerhardt；　a　conception　of　the　molecule　as　a　unitary　structure；　built　up　through　the　aggregation　of　various　atoms；　in　accordance　with　〃elective　affinities〃　whose　nature　is　not　yet　understood　A　doctrine　of　〃nuclei〃　and　a　doctrine　of　〃types〃　of　molecular　structure　were　much　exploited；　and；　like　the　doctrine　of　compound　radicals；　became　useful　as　aids　to　memory　and　guides　for　the　analyst；　indicating　some　of　the　plans　of　molecular　construction；　though　by　no　means　penetrating　the　mysteries　of　chemical　affinity。　They　are　classifications　rather　than　explanations　of　chemical　unions。　　But　at　least　they　served　an　important　purpose　in　giving　definiteness　to　the　idea　of　a　molecular　structure　built　of　atoms　as　the　basis　of　all　substances。　Now　at　last　the　word　molecule　came　to　have　a　distinct　meaning；　as　distinct　from　〃atom；〃　in　the　minds　of　the　generality　of　chemists；　as　it　had　had　for　Avogadro　a　third　of　a　century　before。　Avogadro's　hypothesis　that　there　are　equal　numbers　of　these　molecules　in　equal　volumes　of　gases；　under　fixed　conditions；　was　revived　by　Gerhardt；　and　a　little　later；　under　the　championship　of　Cannizzaro；　was　exalted　to　the　plane　of　a　fixed　law。　Thenceforth　the　conception　of　the　molecule　was　to　be　as　dominant　a　thought　in　chemistry　as　the　idea　of　the　atom　had　become　in　a　previous　epoch。


CHEMICAL　AFFINITY

Of　course　the　atom　itself　was　in　no　sense　displaced；　but　Avogadro's　law　soon　made　it　plain　that　the　atom　had　often　usurped　territory　that　did　not　really　belong　to　it。　In　many　cases　the　chemists　had　supposed　themselves　dealing　with　atoms　as　units　where　the　true　unit　was　the　molecule。　In　the　case　of　elementary　gases；　such　as　hydrogen　and　oxygen；　for　example；　the　law　of　equal　numbers　of　molecules　in　equal　spaces　made　it　clear　that　the　atoms　do　not　exist　isolated；　as　had　been　supposed。　　Since　two　volumes　of　hydrogen　unite　with　one　volume　of　oxygen　to　form　two　volumes　of　water　vapor；　the　sim