a history of science-4-第11节

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atoms。　　But　we　have　no　proof　that　under　any　circumstances　it　could　hold　more　than　two。　Its　affinities　seem　satisfied　when　it　has　two　bonds。　　But；　on　the　other　hand；　the　atom　of　nitrogen　is　able　to　hold　three　atoms　of　hydrogen；　and　does　so　in　the　molecule　of　ammonium　（NH3）；　while　the　carbon　atom　can　hold　four　atoms　of　hydrogen　or　two　atoms　of　oxygen。

Evidently；　then；　one　atom　is　not　always　equivalent　to　another　atom　of　a　different　kind　in　combining　powers。　　A　recognition　of　this　fact　by　Frankland　about　1852；　and　its　further　investigation　by　others　（notably　A。　Kekule　and　A。　S。　Couper）；　led　to　the　introduction　of　the　word　equivalent　into　chemical　terminology　in　a　new　sense；　and　in　particular　to　an　understanding　of　the　affinities　or　〃valency〃　of　different　elements；　which　proved　of　the　most　fundamental　importance。　Thus　it　was　shown　that；　of　the　four　elements　that　enter　most　prominently　into　organic　compounds；　hydrogen　can　link　itself　with　only　a　single　bond　to　any　other　elementit　has；　so　to　speak；　but　a　single　hand　with　which　to　graspwhile　oxygen　has　capacity　for　two　bonds；　nitrogen　for　three　（possibly　for　five）；　and　carbon　for　four。　The　words　monovalent；　divalent；　trivalent；　tretrava…lent；　etc。；　were　coined　to　express　this　most　important　fact；　and　the　various　elements　came　to　be　known　as　monads；　diads；　triads；　etc。　　Just　why　different　elements　should　differ　thus　in　valency　no　one　as　yet　knows；　it　is　an　empirical　fact　that　they　do。　　And　once　the　nature　of　any　element　has　been　determined　as　regards　its　valency；　a　most　important　insight　into　the　possible　behavior　of　that　element　has　been　secured。　Thus　a　consideration　of　the　fact　that　hydrogen　is　monovalent；　while　oxygen　is　divalent；　makes　it　plain　that　we　must　expect　to　find　no　more　than　three　compounds　of　these　two　elementsnamely；　HO（written　HO　by　the　chemist；　and　called　hydroxyl）；　HOH　（H2O；　or　water）；　and　HOOH　（H2O2；　or　hydrogen　peroxide）。　It　will　be　observed　that　in　the　first　of　these　compounds　the　atom　of　oxygen　stands；　so　to　speak；　with　one　of　its　hands　free；　eagerly　reaching　out；　therefore；　for　another　companion；　and　hence；　in　the　language　of　chemistry；　forming　an　unstable　compound。　Again；　in　the　third　compound；　though　all　hands　are　clasped；　yet　one　pair　links　oxygen　with　oxygen；　and　this　also　must　be　an　unstable　union；　since　the　avidity　of　an　atom　for　its　own　kind　is　relatively　weak。　Thus　the　well…known　properties　of　hydrogen　peroxide　are　explained；　its　easy　decomposition；　and　the　eagerness　with　which　it　seizes　upon　the　elements　of　other　compounds。

But　the　molecule　of　water；　on　the　other　hand；　has　its　atoms　arranged　in　a　state　of　stable　equilibrium；　all　their　affinities　being　satisfied。　　Each　hydrogen　atom　has　satisfied　its　own　affinity　by　clutching　the　oxygen　atom；　and　the　oxygen　atom　has　both　its　bonds　satisfied　by　clutching　back　at　the　two　hydrogen　atoms。　Therefore　the　trio；　linked　in　this　close　bond；　have　no　tendency　to　reach　out　for　any　other　companion；　nor；　indeed；　any　power　to　hold　another　should　it　thrust　itself　upon　them。　They　form　a　〃stable〃　compound；　which　under　all　ordinary　circumstances　will　retain　its　identity　as　a　molecule　of　water；　even　though　the　physical　mass　of　which　it　is　a　part　changes　its　condition　from　a　solid　to　a　gas　from　ice　to　vapor。

But　a　consideration　of　this　condition　of　stable　equilibrium　in　the　molecule　at　once　suggests　a　new　question：　How　can　an　aggregation　of　atoms；　having　all　their　affinities　satisfied；　take　any　further　part　in　chemical　reactions？　Seemingly　such　a　molecule；　whatever　its　physical　properties；　must　be　chemically　inert；　incapable　of　any　atomic　readjustments。　And　so　in　point　of　fact　it　is；　so　long　as　its　component　atoms　cling　to　one　another　unremittingly。　　But　this；　it　appears；　is　precisely　what　the　atoms　are　little　prone　to　do。　It　seems　that　they　are　fickle　to　the　last　degree　in　their　individual　attachments；　and　are　as　prone　to　break　away　from　bondage　as　they　are　to　enter　into　it。　　Thus　the　oxygen　atom　which　has　just　flung　itself　into　the　circuit　of　two　hydrogen　atoms；　the　next　moment　flings　itself　free　again　and　seeks　new　companions。　It　is　for　all　the　world　like　the　incessant　change　of　partners　in　a　rollicking　dance。　　This　incessant　dissolution　and　reformation　of　molecules　in　a　substance　which　as　a　whole　remains　apparently　unchanged　was　first　fully　appreciated　by　Ste。…Claire　Deville；　and　by　him　named　dissociation。　　It　is　a　process　which　goes　on　much　more　actively　in　some　compounds　than　in　others；　and　very　much　more　actively　under　some　physical　conditions　（such　as　increase　of　temperature）　than　under　others。　　But　apparently　no　substances　at　ordinary　temperatures；　and　no　temperature　above　the　absolute　zero；　are　absolutely　free　from　its　disturbing　influence。　Hence　it　is　that　molecules　having　all　the　valency　of　their　atoms　fully　satisfied　do　not　lose　their　chemical　activitysince　each　atom　is　momentarily　free　in　the　exchange　of　partners；　and　may　seize　upon　different　atoms　from　its　former　partners；　if　those　it　prefers　are　at　hand。

While；　however；　an　appreciation　of　this　ceaseless　activity　of　the　atom　is　essential　to　a　proper　understanding　of　its　chemical　efficiency；　yet　from　another　point　of　view　the　〃saturated〃　moleculethat　is；　the　molecule　whose　atoms　have　their　valency　all　satisfiedmay　be　thought　of　as　a　relatively　fixed　or　stable　organism。　Even　though　it　may　presently　be　torn　down；　it　is　for　the　time　being　a　completed　structure；　and　a　consideration　of　the　valency　of　its　atoms　gives　the　best　clew　that　has　hitherto　been　obtainable　as　to　the　character　of　its　architecture。　　How　important　this　matter　of　architecture　of　the　moleculeof　space　relations　of　the　atomsmay　be　was　demonstrated　as　long　ago　as　1823；　when　Liebig　and　Wohler　proved；　to　the　utter　bewilderment　of　the　chemical　world；　that　two　substances　may　have　precisely　the　same　chemical　constitutionthe　same　number　and　kind　of　atomsand　yet　differ　utterly　in　physical　properties。　The　word　isomerism　was　coined　by　Berzelius　to　express　this　anomalous　condition　of　things；　which　seemed　to　negative　the　most　fundamental　truths　of　chemistry。　　Naming　the　condition　by　no　means　explained　it；　but　the　fact　was　made　clear　that　something　besides　the　mere　number　and　kind　of　atoms　is　important　in　the　architecture　of　a　molecule。　It　became　certain　that　atoms　are　not　thrown　together　haphazard　to　build　a　molecule；　any　more　than　bricks　are　thrown　together　at　random　to　form　a　house。

How　delicate　may　be　the　gradations　of　architectural　design　in　building　a　molecule　was　well　illustrated　about　1850；　when　Pasteur　discovered　that　some　carbon　compoundsas　certain　sugarscan　only　be　distinguished　from　one　another；　when　in　solution；　by　the　fact　of　their　twisting　or　polarizing　a　ray　of　light　to　the　left　or　to　the　right；　respectively。　But　no　inkling　of　an　explanation　of　these　strange　variations　of　molecular　structure　came　until　the　discovery　of　the　law　of　valency。　　Then　much　of　the　mystery　was　cleared　away；　for　it　was　plain　that　since　each　atom　in　a　molecule　can　hold　to　itself　only　a　fixed　number　of　other　atoms；　complex　molecules　must　have　their　atoms　linked　in　definite　chains　or　groups。　And　it　is　equally　plain　that　where　the　atoms　are　numerous；　the　exact　plan　of　grouping　may　sometimes　be　susceptible　of　change　without　doing　violence　to　the　law　of　valency。　It　is　in　such　cases　that　isomerism　is　observed　to　occur。

By　paying　constant　heed　to　this　matter　of　the　affinities；　chemists　are　able　to　make　diagrammatic　pictures　of　the　plan　of　architecture　of　any　molecule　whose　composition　is　known。　In　the　simple　molecule　of　water　（H2O）；　for　example；　the　two　hydrogen　atoms　must　have　released　each　other　before　they　could　join　the　oxygen；　and　the　manner　of　linking　must　apparently　be　that　represented　in　the　graphic　formula　HOH。　With　molecules　composed　of　a　large　number　of　atoms；　such　graphic　representation　of　the　scheme　of　linking　is　of　course　increasingly　difficult；　yet；　with　the　affinities　for　a　guide；　it　is　always　possible。　Of　course　no　one　supposes　that　such　a　formula；　written　in　a　single　plane；　can　possibly　represent　the　true　architecture　of　the　molecule：　it　is　at　best　suggestive　or　diagrammatic　rather　than　pictorial。　Nevertheless；　it　affords　hints　as　to　the　structure　of　the　molecule　such　as　the　fathers　of　chemistry　would　not　have　thought　it　possible　ever　to　attain。


PERIODICITY　OF　ATOMIC　WEIGHTS

These　utterly　novel　studies　of　molecular　architecture　may　seem　at　first　sight　to　take　from　the　atom　much　of　its　former　prestige　as　the　all…important　personage　of　the　chemical　world。　Since　so　much　depends　upon　the　mere　position　of　the　atoms；　it　may　appear　that　comparatively　little　depends　upon　the　nature　of　the　atoms　themselves。　　But　such　a　view　is　incorrect；　for　on　closer　consideration　it　will　appear　that　at　no　time　has　the　atom　been　seen　to　renounce　its　peculiar　personality。　Within　certain　limits　the　character　of　a　molecule　may　be　altered　by　changing　the　positions　of　its　atoms　（just　as　different　buildings　may　be　constructed　of　the　same　bricks）；　but　these　limits　are　sharply　defined；　and　it　would　be　as　impossible　to　exceed　them　as　it　would　be　to　build　a　stone　building　with　bricks。　From　first　to　last　the　brick　remains　a　brick；　whatever　the　style　of　architecture　it　helps　to　construct；　it　never　becomes　a　stone。　And　just　as　closely　d