a history of science-4-第10节

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al　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　simplest　mathematics　show；　in　the　light　of　Avogadro's　law；　not　only　that　each　molecule　of　water　must　contain　two　hydrogen　atoms　（a　point　previously　in　dispute）；　but　that　the　original　molecules　of　hydrogen　and　oxygen　must　have　been　composed　in　each　case　of　two　atoms…else　how　could　one　volume　of　oxygen　supply　an　atom　for　every　molecule　of　two　volumes　of　water？

What；　then；　does　this　imply？　　Why；　that　the　elementary　atom　has　an　avidity　for　other　atoms；　a　longing　for　companionship；　an　〃affinity〃call　it　what　you　willwhich　is　bound　to　be　satisfied　if　other　atoms　are　in　the　neighborhood。　　Placed　solely　among　atoms　of　its　own　kind；　the　oxygen　atom　seizes　on　a　fellow　oxygen　atom；　and　in　all　their　mad　dancings　these　two　mates　cling　togetherpossibly　revolving　about　each　other　in　miniature　planetary　orbits。　Precisely　the　same　thing　occurs　among　the　hydrogen　atoms。　But　now　suppose　the　various　pairs　of　oxygen　atoms　come　near　other　pairs　of　hydrogen　atoms　（under　proper　conditions　which　need　not　detain　us　here）；　then　each　oxygen　atom　loses　its　attachment　for　its　fellow；　and　flings　itself　madly　into　the　circuit　of　one　of　the　hydrogen　couplets；　andpresto！there　are　only　two　molecules　for　every　three　there　were　before；　and　free　oxygen　and　hydrogen　have　become　water。　The　whole　process；　stated　in　chemical　phraseology；　is　summed　up　in　the　statement　that　under　the　given　conditions　the　oxygen　atoms　had　a　greater　affinity　for　the　hydrogen　atoms　than　for　one　another。

As　chemists　studied　the　actions　of　various　kinds　of　atoms；　in　regard　to　their　unions　with　one　another　to　form　molecules；　it　gradually　dawned　upon　them　that　not　all　elements　are　satisfied　with　the　same　number　of　companions。　Some　elements　ask　only　one；　and　refuse　to　take　more；　while　others　link　themselves；　when　occasion　offers；　with　two；　three；　four；　or　more。　Thus　we　saw　that　oxygen　forsook　a　single　atom　of　its　own　kind　and　linked　itself　with　two　atoms　of　hydrogen。　　Clearly；　then；　the　oxygen　atom；　like　a　creature　with　two　hands；　is　able　to　clutch　two　other　atoms。　　But　we　have　no　proof　that　under　any　circumstances　it　could　hold　more　than　two。　Its　affinitie