Ofcoursetheatomitselfwasinnosensedisplaced，butAvogadro’slawsoonmadeitplainthattheatomhadoftenusurpedterritorythatdidnotreallybelongtoit。Inmanycasesthechemistshadsupposedthemselvesdealingwithatomsasunitswherethetrueunitwasthemolecule。Inthecaseofelementarygases，suchashydrogenandoxygen，forexample，thelawofequalnumbersofmoleculesinequalspacesmadeitclearthattheatomsdonotexistisolated，ashadbeensupposed。Sincetwovolumesofhydrogenunitewithonevolumeofoxygentoformtwovolumesofwatervapor，thesimplestmathematicsshow，inthelightofAvogadro’slaw，notonlythateachmoleculeofwatermustcontaintwohydrogenatomsapointpreviouslyindispute，butthattheoriginalmoleculesofhydrogenandoxygenmusthavebeencomposedineachcaseoftwoatoms——elsehowcouldonevolumeofoxygensupplyanatomforeverymoleculeoftwovolumesofwater？
What，then，doesthisimply？Why，thattheelementaryatomhasanavidityforotheratoms，alongingforcompanionship，an\"affinity\"——callitwhatyouwill——whichisboundtobesatisfiedifotheratomsareintheneighborhood。Placedsolelyamongatomsofitsownkind，theoxygenatomseizesonafellowoxygenatom，andinalltheirmaddancingsthesetwomatesclingtogether——possiblyrevolvingabouteachotherinminiatureplanetaryorbits。Preciselythesamethingoccursamongthehydrogenatoms。Butnowsupposethevariouspairsofoxygenatomscomenearotherpairsofhydrogenatomsunderproperconditionswhichneednotdetainushere，theneachoxygenatomlosesitsattachmentforitsfellow，andflingsitselfmadlyintothecircuitofoneofthehydrogencouplets，and——presto！——thereareonlytwomoleculesforeverythreetherewerebefore，andfreeoxygenandhydrogenhavebecomewater。Thewholeprocess，statedinchemicalphraseology，issummedupinthestatementthatunderthegivenconditionstheoxygenatomshadagreateraffinityforthehydrogenatomsthanforoneanother。
Aschemistsstudiedtheactionsofvariouskindsofatoms，inregardtotheirunionswithoneanothertoformmolecules，itgraduallydawneduponthemthatnotallelementsaresatisfiedwiththesamenumberofcompanions。Someelementsaskonlyone，andrefusetotakemore；whileotherslinkthemselves，whenoccasionoffers，withtwo，three，four，ormore。Thuswesawthatoxygenforsookasingleatomofitsownkindandlinkeditselfwithtwoatomsofhydrogen。Clearly，then，theoxygenatom，likeacreaturewithtwohands，isabletoclutchtwootheratoms。
Butwehavenoproofthatunderanycircumstancesitcouldholdmorethantwo。Itsaffinitiesseemsatisfiedwhenithastwobonds。But，ontheotherhand，theatomofnitrogenisabletoholdthreeatomsofhydrogen，anddoessointhemoleculeofammoniumNH3；whilethecarbonatomcanholdfouratomsofhydrogenortwoatomsofoxygen。
Evidently，then，oneatomisnotalwaysequivalenttoanotheratomofadifferentkindincombiningpowers。ArecognitionofthisfactbyFranklandabout1852，anditsfurtherinvestigationbyothersnotablyA。KekuleandA。S。Couper，ledtotheintroductionofthewordequivalentintochemicalterminologyinanewsense，andinparticulartoanunderstandingoftheaffinitiesor\"valency\"ofdifferentelements，whichprovedofthemostfundamentalimportance。Thusitwasshownthat，ofthefourelementsthatentermostprominentlyintoorganiccompounds，hydrogencanlinkitselfwithonlyasinglebondtoanyotherelement——ithas，sotospeak，butasinglehandwithwhichtograsp——whileoxygenhascapacityfortwobonds，nitrogenforthreepossiblyforfive，andcarbonforfour。Thewordsmonovalent，divalent，trivalent，tretrava-lent，etc。，werecoinedtoexpressthismostimportantfact，andthevariouselementscametobeknownasmonads，diads，triads，etc。Justwhydifferentelementsshoulddifferthusinvalencynooneasyetknows；itisanempiricalfactthattheydo。Andoncethenatureofanyelementhasbeendeterminedasregardsitsvalency，amostimportantinsightintothepossiblebehaviorofthatelementhasbeensecured。Thusaconsiderationofthefactthathydrogenismonovalent，whileoxygenisdivalent，makesitplainthatwemustexpecttofindnomorethanthreecompoundsofthesetwoelements——namely，H——O——writtenHObythechemist，andcalledhydroxyl；H——O——HH2O，orwater，andH——O——O——HH2O2，orhydrogenperoxide。Itwillbeobservedthatinthefirstofthesecompoundstheatomofoxygenstands，sotospeak，withoneofitshandsfree，eagerlyreachingout，therefore，foranothercompanion，andhence，inthelanguageofchemistry，forminganunstablecompound。Again，inthethirdcompound，thoughallhandsareclasped，yetonepairlinksoxygenwithoxygen；andthisalsomustbeanunstableunion，sincetheavidityofanatomforitsownkindisrelativelyweak。Thusthewell-knownpropertiesofhydrogenperoxideareexplained，itseasydecomposition，andtheeagernesswithwhichitseizesupontheelementsofothercompounds。
Butthemoleculeofwater，ontheotherhand，hasitsatomsarrangedinastateofstableequilibrium，alltheiraffinitiesbeingsatisfied。Eachhydrogenatomhassatisfieditsownaffinitybyclutchingtheoxygenatom；andtheoxygenatomhasbothitsbondssatisfiedbyclutchingbackatthetwohydrogenatoms。Thereforethetrio，linkedinthisclosebond，havenotendencytoreachoutforanyothercompanion，nor，indeed，anypowertoholdanothershoulditthrustitselfuponthem。Theyforma\"stable\"compound，whichunderallordinarycircumstanceswillretainitsidentityasamoleculeofwater，eventhoughthephysicalmassofwhichitisapartchangesitsconditionfromasolidtoagasfromicetovapor。
Butaconsiderationofthisconditionofstableequilibriuminthemoleculeatoncesuggestsanewquestion:Howcananaggregationofatoms，havingalltheiraffinitiessatisfied，takeanyfurtherpartinchemicalreactions？Seeminglysuchamolecule，whateveritsphysicalproperties，mustbechemicallyinert，incapableofanyatomicreadjustments。Andsoinpointoffactitis，solongasitscomponentatomsclingtooneanotherunremittingly。Butthis，itappears，ispreciselywhattheatomsarelittlepronetodo。Itseemsthattheyarefickletothelastdegreeintheirindividualattachments，andareaspronetobreakawayfrombondageastheyaretoenterintoit。Thustheoxygenatomwhichhasjustflungitselfintothecircuitoftwohydrogenatoms，thenextmomentflingsitselffreeagainandseeksnewcompanions。Itisforalltheworldliketheincessantchangeofpartnersinarollickingdance。ThisincessantdissolutionandreformationofmoleculesinasubstancewhichasawholeremainsapparentlyunchangedwasfirstfullyappreciatedbySte-ClaireDeville，andbyhimnameddissociation。Itisaprocesswhichgoesonmuchmoreactivelyinsomecompoundsthaninothers，andverymuchmoreactivelyundersomephysicalconditionssuchasincreaseoftemperaturethanunderothers。Butapparentlynosubstancesatordinarytemperatures，andnotemperatureabovetheabsolutezero，areabsolutelyfreefromitsdisturbinginfluence。
Henceitisthatmoleculeshavingallthevalencyoftheiratomsfullysatisfieddonotlosetheirchemicalactivity——sinceeachatomismomentarilyfreeintheexchangeofpartners，andmayseizeupondifferentatomsfromitsformerpartners，ifthoseitprefersareathand。
While，however，anappreciationofthisceaselessactivityoftheatomisessentialtoaproperunderstandingofitschemicalefficiency，yetfromanotherpointofviewthe\"saturated\"
molecule——thatis，themoleculewhoseatomshavetheirvalencyallsatisfied——maybethoughtofasarelativelyfixedorstableorganism。Eventhoughitmaypresentlybetorndown，itisforthetimebeingacompletedstructure；andaconsiderationofthevalencyofitsatomsgivesthebestclewthathashithertobeenobtainableastothecharacterofitsarchitecture。Howimportantthismatterofarchitectureofthemolecule——ofspacerelationsoftheatoms——maybewasdemonstratedaslongagoas1823，whenLiebigandWohlerproved，totheutterbewildermentofthechemicalworld，thattwosubstancesmayhavepreciselythesamechemicalconstitution——thesamenumberandkindofatoms——andyetdifferutterlyinphysicalproperties。ThewordisomerismwascoinedbyBerzeliustoexpressthisanomalousconditionofthings，whichseemedtonegativethemostfundamentaltruthsofchemistry。Namingtheconditionbynomeansexplainedit，butthefactwasmadeclearthatsomethingbesidesthemerenumberandkindofatomsisimportantinthearchitectureofamolecule。Itbecamecertainthatatomsarenotthrowntogetherhaphazardtobuildamolecule，anymorethanbricksarethrowntogetheratrandomtoformahouse。
Howdelicatemaybethegradationsofarchitecturaldesigninbuildingamoleculewaswellillustratedabout1850，whenPasteurdiscoveredthatsomecarboncompounds——ascertainsugars——canonlybedistinguishedfromoneanother，wheninsolution，bythefactoftheirtwistingorpolarizingarayoflighttotheleftortotheright，respectively。Butnoinklingofanexplanationofthesestrangevariationsofmolecularstructurecameuntilthediscoveryofthelawofvalency。Thenmuchofthemysterywasclearedaway；foritwasplainthatsinceeachatominamoleculecanholdtoitselfonlyafixednumberofotheratoms，complexmoleculesmusthavetheiratomslinkedindefinitechainsorgroups。Anditisequallyplainthatwheretheatomsarenumerous，theexactplanofgroupingmaysometimesbesusceptibleofchangewithoutdoingviolencetothelawofvalency。Itisinsuchcasesthatisomerismisobservedtooccur。
Bypayingconstantheedtothismatteroftheaffinities，chemistsareabletomakediagrammaticpicturesoftheplanofarchitectureofanymoleculewhosecompositionisknown。InthesimplemoleculeofwaterH2O，forexample，thetwohydrogenatomsmusthavereleasedeachotherbeforetheycouldjointheoxygen，andthemanneroflinkingmustapparentlybethatrepresentedinthegraphicformulaH——O——H。Withmoleculescomposedofalargenumberofatoms，suchgraphicrepresentationoftheschemeoflinkingisofcourseincreasinglydifficult，yet，withtheaffinitiesforaguide，itisalwayspossible。Ofcoursenoonesupposesthatsuchaformula，writteninasingleplane，canpossiblyrepresentthetruearchitectureofthemolecule:itisatbestsuggestiveordiagrammaticratherthanpictorial。Nevertheless，itaffordshintsastothestructureofthemoleculesuchasthefathersofchemistrywouldnothavethoughtitpossibleevertoattain。
PERIODICITYOFATOMICWEIGHTS
Theseutterlynovelstudiesofmoleculararchitecturemayseematfirstsighttotakefromtheatommuchofitsformerprestigeastheall-importantpersonageofthechemicalworld。Sincesomuchdependsuponthemerepositionoftheatoms，itmayappearthatcomparativelylittledependsuponthenatureoftheatomsthemselves。Butsuchaviewisincorrect，foroncloserconsiderationitwillappearthatatnotimehastheatombeenseentorenounceitspeculiarpersonality。Withincertainlimitsthecharacterofamoleculemaybealteredbychangingthepositionsofitsatomsjustasdifferentbuildingsmaybeconstructedofthesamebricks，buttheselimitsaresharplydefined，anditwouldbeasimpossibletoexceedthemasitwouldbetobuildastonebuildingwithbricks。Fromfirsttolastthebrickremainsabrick，whateverthestyleofarchitectureithelpstoconstruct；itneverbecomesastone。Andjustascloselydoeseachatomretainitsownpeculiarproperties，regardlessofitssurroundings。
Thus，forexample，thecarbonatommaytakepartintheformationatonetimeofadiamond，againofapieceofcoal，andyetagainofaparticleofsugar，ofwoodfibre，ofanimaltissue，orofagasintheatmosphere；butfromfirsttolast——fromglass-cuttinggemtointangiblegas——thereisnodemonstrablechangewhateverinanysinglepropertyoftheatomitself。Sofarasweknow，itssize，itsweight，itscapacityforvibrationorrotation，anditsinherentaffinities，remainabsolutelyunchangedthroughoutallthesevaryingfortunesofpositionandassociation。Andthesamethingistrueofeveryatomofalloftheseventy-oddelementarysubstanceswithwhichthemodernchemistisacquainted。Everyoneappearsalwaystomaintainitsuniqueintegrity，gainingnothingandlosingnothing。
Allthisbeingtrue，itwouldseemasifthepositionoftheDaltonianatomasaprimordialbitofmatter，indestructibleandnon-transmutable，hadbeenputtothetestbythechemistryofourcentury，andnotfoundwanting。SincethoseearlydaysofthecenturywhentheelectricbatteryperformeditsmiraclesandseeminglyreacheditslimitationsinthehandsofDavy，manynewelementarysubstanceshavebeendiscovered，butnosingleelementhasbeendisplacedfromitspositionasanundecomposablebody。
RatherhavetheanalysesofthechemistseemedtomakeitmoreandmorecertainthatallelementaryatomsareintruthwhatJohnHerschelcalledthem，\"manufacturedarticles\"——primordial，changeless，indestructible。
Andyet，oddlyenough，ithaschancedthathandinhandwiththeexperimentsleadingtosuchagoalhavegoneotherexperimentsaridspeculationsofexactlytheoppositetenor。Ineachgenerationtherehavebeenchemistsamongtheleadersoftheirsciencewhohaverefusedtoadmitthattheso-calledelementsarereallyelementsatallinanyfinalsense，andwhohavesoughteagerlyforproofwhichmightwarranttheirscepticism。ThefirstbitofevidencetendingtosupportthisviewwasfurnishedbyanEnglishphysician，Dr。WilliamProut，whoin1815calledattentiontoacuriousrelationtobeobservedbetweentheatomicweightofthevariouselements。AcceptingthefiguresgivenbytheauthoritiesofthetimenotablyThomsonandBerzelius，itappearedthatastrikinglylargeproportionoftheatomicweightswereexactmultiplesoftheweightofhydrogen，andthatothersdifferedsoslightlythaterrorsofobservationmightexplainthediscrepancy。Proutfeltthatitcouldnotbeaccidental，andhecouldthinkofnotenableexplanation，unlessitbethattheatomsofthevariousallegedelementsaremadeupofdifferentfixednumbersofhydrogenatoms。Coulditbethattheonetrueelement——theoneprimalmatter——ishydrogen，andthatallotherformsofmatterarebutcompoundsofthisoriginalsubstance？
Proutadvancedthisstartlingideaatfirsttentatively，inananonymouspublication；butafterwardsheespouseditopenlyandurgeditstenability。ComingjustafterDavy’sdissociationofsomesupposedelements，theideaprovedalluring，andforatimegainedsuchpopularitythatchemistsweredisposedtoroundouttheobservedatomicweightsofallelementsintowholenumbers。
Butpresentlyreneweddeterminationsoftheatomicweightsseemedtodiscountenancethispractice，andProut’sallegedlawfellintodisrepute。Itwasrevived，however，about1840，byDumas，whosegreatauthoritysecureditarespectfulhearing，andwhosecarefulredeterminationoftheweightofcarbon，makingitexactlytwelvetimesthatofhydrogen，aidedthecause。
SubsequentlyStas，thepupilofDumas，undertookalongseriesofdeterminationsofatomicweights，withtheexpectationofconfirmingtheProutianhypothesis。Buthisresultsseemedtodisprovethehypothesis，fortheatomicweightsofmanyelementsdifferedfromwholenumbersbymore，itwasthought，thanthelimitsoferroroftheexperiments。Itwasnoteworthy，however，thattheconfidenceofDumaswasnotshaken，thoughhewasledtomodifythehypothesis，and，inaccordancewithprevioussuggestionsofClarkandofMarignac，torecognizeastheprimordialelement，nothydrogenitself，butanatomhalftheweight，orevenone-fourththeweight，ofthatofhydrogen，ofwhichprimordialatomthehydrogenatomitselfiscompounded。Buteveninthismodifiedformthehypothesisfoundgreatoppositionfromexperimentalobservers。
In1864，however，anovelrelationbetweentheweightsoftheelementsandtheirothercharacteristicswascalledtotheattentionofchemistsbyProfessorJohnA。R。Newlands，ofLondon，whohadnoticedthatiftheelementsarearrangedseriallyinthenumericalorderoftheiratomicweights，thereisacuriousrecurrenceofsimilarpropertiesatintervalsofeightelementsThisso-called\"lawofoctaves\"attractedlittleimmediateattention，butthefactsitconnotessooncameundertheobservationofotherchemists，notablyofProfessorsGustavHinrichsinAmerica，DmitriMendeleeffinRussia，andLotharMeyerinGermany。Mendeleeffgavethediscoveryfullestexpression，explicatingitin1869，underthetitleof\"theperiodiclaw。\"
Thoughthisearlyexpositionofwhathassincebeenadmittedtobeamostimportantdiscoverywasveryfullyoutlined，thegeneralityofchemistsgaveitlittleheedtilladecadeorsolater，whenthreenewelements，gallium，scandium，andgermanium，werediscovered，which，onbeinganalyzed，werequiteunexpectedlyfoundtofitintothreegapswhichMendeleeffhadleftinhisperiodicscale。IneffecttheperiodiclawhadenabledMendeleefftopredicatetheexistenceofthenewelementsyearsbeforetheywerediscovered。Surelyasystemthatleadstosuchresultsisnomerevagary。Soverysoontheperiodiclawtookitsplaceasoneofthemostimportantgeneralizationsofchemicalscience。
Thislawofperiodicitywasputforwardasanexpressionofobservedrelationsindependentofhypothesis；butofcoursethetheoreticalbearingsofthesefactscouldnotbeoverlooked。AsProfessorJ。H。Gladstonehassaid，itforcesuponus\"theconvictionthattheelementsarenotseparatebodiescreatedwithoutreferencetooneanother，butthattheyhavebeenoriginallyfashioned，orhavebeenbuiltup，fromoneanother，accordingtosomegeneralplan。\"ItisbutashortstepfromthatpropositiontotheProutianhypothesis。
NEWWEAPONS——SPECTROSCOPEANDCAMERA
Buttheatomicweightsarenotaloneinsuggestingthecompoundnatureoftheallegedelements。Evidenceofatotallydifferentkindhascontributedtothesameend，fromasourcethatcouldhardlyhavebeenimaginedwhentheProutianhypothesis，wasformulated，throughthetraditionofanovelweapontothearmamentariumofthechemist——thespectroscope。Theperfectionofthisinstrument，inthehandsoftwoGermanscientists，GustavRobertKirchhoffandRobertWilhelmBunsen，cameaboutthroughtheinvestigation，towardsthemiddleofthecentury，ofthemeaningofthedarklineswhichhadbeenobservedinthesolarspectrumbyFraunhoferasearlyas1815，andbyWollastonadecadeearlier。ItwassuspectedbyStokesandbyFoxTalbotinEngland，butfirstbroughttodemonstrationbyKirchhoffandBunsen，thattheselines，whichwereknowntooccupydefinitepositionsinthespectrum，arereallyindicativeofparticularelementarysubstances。Bymeansofthespectroscope，whichisessentiallyamagnifyinglensattachedtoaprismofglass，itispossibletolocatethelineswithgreataccuracy，anditwassoonshownthatherewasanewmeansofchemicalanalysisofthemostexquisitedelicacy。Itwasfound，forexample，thatthespectroscopecoulddetectthepresenceofaquantityofsodiumsoinfinitesimalastheonetwo-hundred-thousandthofagrain。Butwhatwasevenmoreimportant，thespectroscopeputnolimituponthedistanceoflocationofthesubstanceittested，providedonlythatsufficientlightcamefromit。Theexperimentsitrecordedmightbeperformedinthesun，orinthemostdistantstarsornebulae；indeed，oneoftheearliestfeatsoftheinstrumentwastowrenchfromthesunthesecretofhischemicalconstitution。
Torendertheutilityofthespectroscopecomplete，however，itwasnecessarytolinkwithitanothernewchemicalagency——namely，photography。Thisnowfamiliarprocessisbasedonthepropertyoflighttodecomposecertainunstablecompoundsofsilver，andthusaltertheirchemicalcomposition。DavyandWedgwoodbarelyescapedthediscoveryofthevalueofthephotographicmethodearlyinthenineteenthcentury。Theirsuccessorsquiteoverlookedituntilabout1826，whenLouisJ。M。
Daguerre，theFrenchchemist，tookthematterinhand，andaftermanyyearsofexperimentationbroughtittorelativeperfectionin1839，inwhichyearthefamousdaguerreotypefirstbroughtthemattertopopularattention。InthesameyearMr。FoxTalbotreadapaperonthesubjectbeforetheRoyalSociety，andsoonafterwardstheeffortsofHerschelandnumerousothernaturalphilosopherscontributedtotheadvancementofthenewmethod。
In1843Dr。JohnW。Draper，thefamousEnglish-Americanchemistandphysiologist，showedthatbyphotographytheFraunhoferlinesinthesolarspectrummightbemappedwithabsoluteaccuracy；
alsoprovingthatthesilveredfilmrevealedmanylinesinvisibletotheunaidedeye。Thevalueofthismethodofobservationwasrecognizedatonce，and，assoonasthespectroscopewasperfected，thephotographicmethod，inconjunctionwithitsuse，becameinvaluabletothechemist。Bythismeanscomparisonsofspectramaybemadewithadegreeofaccuracynototherwiseobtainable；and，incaseofthestars，wholeclustersofspectramaybeplacedonrecordatasingleobservation。
Astheexaminationofthesunandstarsproceeded，chemistswereamazedordelighted，accordingtotheirvariouspreconceptions，towitnesstheproofthatmanyfamiliarterrestrialelementsaretobefoundinthecelestialbodies。Butwhatperhapssurprisedthemmostwastoobservetheenormouspreponderanceinthesiderealbodiesoftheelementhydrogen。Notonlyaretherevastquantitiesofthiselementinthesun’satmosphere，butsomeothersunsappearedtoshowhydrogenlinesalmostexclusivelyintheirspectra。Presentlyitappearedthatthestarsofwhichthisistruearethosewhitestars，suchasSirius，whichhadbeenconjecturedtobethehottest；whereasstarsthatareonlyred-hot，likeoursun，showalsothevaporsofmanyotherelements，includingironandothermetals。
In1878ProfessorJ。NormanLockyer，inapaperbeforetheRoyalSociety，calledattentiontothepossiblesignificanceofthisseriesofobservations。Heurgedthatthefactofthesunshowingfewerelementsthanareobservedhereonthecoolearth，whilestarsmuchhotterthanthesunshowchieflyoneelement，andthatonehydrogen，thelightestofknownelements，seemedtogivecolortothepossibilitythatourallegedelementsarereallycompounds，whichatthetemperatureofthehotteststarsmaybedecomposedintohydrogen，thelatter\"element\"itselfbeingalsodoubtlessacompound，whichmightberesolvedunderyetmoretryingconditions。
Here，then，waswhatmightbetermeddirectexperimentalevidenceforthehypothesisofProut。Unfortunately，however，itisevidenceofakindwhichonlyafewexpertsarecompetenttodiscuss——soverydelicateamatteristhespectralanalysisofthestars。Whatisstillmoreunfortunate，theexpertsdonotagreeamongthemselvesastothevalidityofProfessorLockyer’sconclusions。Some，likeProfessorCrookes，haveacceptedthemwithacclaim，hailingLockyeras\"theDarwinoftheinorganicworld，\"whileothershavesoughtadifferentexplanationofthefactshebringsforward。Asyetitcannotbesaidthatthecontroversyhasbeenbroughttofinalsettlement。Still，itishardlytobedoubtedthatnow，sincetheperiodiclawhasseemedtojoinhandswiththespectroscope，abeliefinthecompoundnatureoftheso-calledelementsisrapidlygaininggroundamongchemists。MoreandmoregeneralbecomesthebeliefthattheDaltonianatomisreallyacompoundradical，andthatbackoftheseemingdiversityoftheallegedelementsisasingleformofprimordialmatter。Indeed，inveryrecentmonths，directexperimentalevidenceforthisviewhasatlastcometohand，throughthestudyofradio-activesubstances。Inalaterchapterweshallhaveoccasiontoinquirehowthiscameabout。
IV。ANATOMYANDPHYSIOLOGYINTHEEIGHTEENTHCENTURY
ALBRECHTVONHALLER
AnepochinphysiologywasmadeintheeighteenthcenturybythegeniusandeffortsofAlbrechtvonHaller1708-1777，ofBerne，whoisperhapsasworthyofthetitle\"TheGreat\"asanyphilosopherwhohasbeensochristenedbyhiscontemporariessincethetimeofHippocrates。Celebratedasaphysician，hewasproficientinvariousfields，beingequallyfamedinhisowntimeaspoet，botanist，andstatesman，anddividinghisattentionbetweenartandscience。
AsachildHallerwassosicklythathewasunabletoamusehimselfwiththesportsandgamescommontoboysofhisage，andsopassedmostofhistimeporingoverbooks。WhentenyearsofagehebeganwritingpoemsinLatinandGerman，andatfifteenenteredtheUniversityofTubingen。Atseventeenhewrotelearnedarticlesinoppositiontocertainaccepteddoctrines，andatnineteenhereceivedhisdegreeofdoctor。SoonafterthishevisitedEngland，wherehiszealindissectingbroughthimundersuspicionofgrave-robbery，whichsuspicionmadeitexpedientforhimtoreturntotheContinent。AfterstudyingbotanyinBaselforsometimehemadeanextendedbotanicaljourneythroughSwitzerland，finallysettlinginhisnativecity，Berne，asapractisingphysician。Duringthistimehedidnotneglecteitherpoetryorbotany，publishinganonymouslyacollectionofpoems。
In1736hewascalledtoGottingenasprofessorofanatomy，surgery，chemistry，andbotany。Duringhislaborsintheuniversityheneverneglectedhisliterarywork，sometimeslivingandsleepingfordaysandnightstogetherinhislibrary，eatinghismealswhiledelvinginhisbooks，andsleepingonlywhenactuallycompelledtodosobyfatigue。Duringallthistimehewasincorrespondencewithsavantsfromallovertheworld，anditissaidofhimthatheneverleftaletterofanykindunanswered。
Haller’sgreatestcontributiontomedicalsciencewashisfamousdoctrineofirritability，whichhasgivenhimthenameof\"fatherofmodernnervousphysiology，\"justasHarveyiscalled\"thefatherofthemodernphysiologyoftheblood。\"Ithasbeensaidofthisfamousdoctrineofirritabilitythat\"itmovedallthemindsofthecentury——andnotinthedepartmentsofmedicinealone——inawayofwhichweofthepresentdayhavenosatisfactoryconception，unlesswecompareitwithourmodernDarwinism。\"[1]
TheprincipleofgeneralirritabilityhadbeenlaiddownbyFrancisGlisson1597-1677fromdeductivestudies，butHallerprovedbyexperimentsalongthelineofinductivemethodsthatthisirritabilitywasnotcommontoall\"fibreaswellastothefluidsofthebody，\"butsomethingentirelyspecial，andpeculiaronlytomuscularsubstance。Hedistinguishedbetweenirritabilityofmusclesandsensibilityofnerves。In1747hegaveasthethreeforcesthatproducemuscularmovements:elasticity，or\"deadnervousforce\"；irritability，or\"innatenervousforce\"；
andnervousforceinitself。Andin1752hedescribedonehundredandninetyexperimentsfordeterminingwhatpartsofthebodypossess\"irritability\"——thatis，thepropertyofcontractingwhenstimulated。Hisconclusionthatthisirritabilityexistsinmuscularsubstancealoneandisquiteindependentofthenervesproceedingtoitarousedacontroversythatwasneverdefinitelysettleduntillateinthenineteenthcentury，whenHaller’stheorywasfoundtobeentirelycorrect。
ItwasinpursuitofexperimentstoestablishhistheoryofirritabilitythatHallermadehischiefdiscoveriesinembryologyanddevelopment。Heprovedthatintheprocessofincubationoftheeggthefirsttraceoftheheartofthechickshowsitselfinthethirty-eighthhour，andthatthefirsttraceofredbloodshowedintheforty-firsthour。Byhisinvestigationsupontheloweranimalsheattemptedtoconfirmthetheorythatsincethecreationofgenuseveryindividualisderivedfromaprecedingindividual——theexistingtheoryofpreformation，inwhichhebelieved，andwhichtaughtthat\"everyindividualisfullyandcompletelypreformedinthegerm，simplygrowingfrommicroscopictovisibleproportions，withoutdevelopinganynewparts。\"
Inphysiology，besideshisstudiesofthenervoussystem，Hallerstudiedthemechanismofrespiration，refutingtheteachingsofHamberger1697-1755，whomaintainedthatthelungscontractindependently。Haller，however，incommonwithhiscontemporaries，failedutterlytounderstandthetruefunctionofthelungs。Thegreatphysiologist’sinfluenceuponpracticalmedicine，whilemostprofound，waslargelyindirect。Hewasatheoreticalratherthanapracticalphysician，yetheiscreditedwithbeingthefirstphysiciantousethewatchincountingthepulse。
BATTISTAMORGAGNIANDMORBIDANATOMY
AgreatcontemporaryofHallerwasGiovanniBattistaMorgagni1682-1771，whopursuedwhatSydenhamhadneglected，theinvestigationinanatomy，thussupplyinganecessarycounterparttothegreatEnglishman’swork。Morgagni’sinvestigationsweredirectedchieflytothestudyofmorbidanatomy——thestudyofthestructureofdiseasedtissue，bothduringlifeandpostmortem，incontrasttothenormalanatomicalstructures。Thisworkcannotbesaidtohaveoriginatedwithhim；forasearlyas1679Bonnethadmadesimilar，althoughlessextensive，studies；andlatermanyinvestigators，suchasLancisiandHaller，hadmadepost-mortemstudies。ButMorgagni’sDesedibusetcausismorborumperanatomenindagatiswasthelargest，mostaccurate，andbest-illustratedcollectionofcasesthathadeverbeenbroughttogether，andmarksanepochinmedicalscience。FromthetimeofthepublicationofMorgagni’sresearches，morbidanatomybecamearecognizedbranchofthemedicalscience，andtheeffectoftheimpetusthusgivenithasbeensteadilyincreasingsincethattime。
WILLIAMHUNTER
WilliamHunter1718-1783mustalwaysberememberedasoneofthegreatestphysiciansandanatomistsoftheeighteenthcentury，andparticularlyasthefirstgreatteacherofanatomyinEngland；buthisfamehasbeensomewhatovershadowedbythatofhisyoungerbrotherJohn。
HunterhadbeenintendedandeducatedfortheChurch，butontheadviceofthesurgeonWilliamCullenheturnedhisattentiontothestudyofmedicine。Hisfirstattemptatteachingwasin1746，whenhedeliveredaseriesoflecturesonsurgeryfortheSocietyofNavalPractitioners。Theselecturesprovedsointerestingandinstructivethathewasatonceinvitedtogiveothers，andhisreputationasalecturerwassoonestablished。Hewasanaturaloratorandstory-teller，andhecombinedwiththeseattractivequalitiesthatofthoroughnessandclearnessindemonstrations，andalthoughhislecturesweretwohourslonghemadethemsofullofinterestthathispupilsseldomtiredoflistening。Hebelievedthathecoulddogreatergoodtotheworldby\"publiclyteachinghisartthanbypractisingit，\"andevenduringthelastfewdaysofhislife，whenhewassoweakthathisfriendsremonstratedagainstit，hecontinuedhisteaching，faintingfromexhaustionattheendofhislastlecture，whichprecededhisdeathbyonlyafewdays。
FormanyyearsitwasHunter’sambitiontoestablishamuseumwherethestudyofanatomy，surgery，andmedicinemightbeadvanced，andin1765heaskedforagrantofaplotofgroundforthispurpose，offeringtospendseventhousandpoundsonits，erectionbesidesendowingitwithaprofessorshipofanatomy。Notbeingabletoobtainthisgrant，however，hebuiltahouse，inwhichwerelectureanddissectingrooms，andhismuseum。Inthismuseumwereanatomicalpreparations，coins，minerals，andnatural-historyspecimens。
Hunter’sweaknesswashisloveofcontroversyandhisresentmentofcontradiction。Thisbroughthimintostrainedrelationswithmanyoftheleadingphysiciansofhistime，notablyhisownbrotherJohn，whohimselfwasprobablynotentirelyfreefromblameinthematter。Hunterissaidtohaveexcusedhisownirritabilityonthegroundsthatbeingananatomist，andaccustomedto\"thepassivesubmissionofdeadbodies，\"
contradictionsbecamethemoreunbearable。Manyofthephysiologicalresearchesbegunbyhimwerecarriedonandperfectedbyhismorefamousbrother，particularlyhisinvestigationsofthecapillaries，butheaddedmuchtotheanatomicalknowledgeofseveralstructuresofthebody，notablyastothestructureofcartilagesandjoints。
JOHNHUNTER
InAbbotIslip’schapelinWestminsterAbbey，closetotheresting-placeofBenJonson，resttheremainsofJohnHunter1728-1793，famousintheannalsofmedicineasamongthegreatestphysiologistsandsurgeonsthattheworldhaseverproduced:amanwhosediscoveriesandinventionsarecountedbyscores，andwhosefieldofresearchwasonlylimitedbytheoutermostboundariesofeighteenth-centuryscience，althoughhiseffortsweredirectedchieflyalongthelinesofhisprofession。
UntilabouttwentyyearsofageyoungHunterhadshownlittleaptitudeforstudy，beingunusuallyfondofout-doorsportsandamusements；butaboutthattime，realizingthatsomeoccupationmustbeselected，heaskedpermissionofhisbrotherWilliamtoattemptsomedissectionsinhisanatomicalschoolinLondon。Tothesurpriseofhisbrotherhemadethisdissectionunusuallywell；andbeinggivenasecond，heacquittedhimselfwithsuchskillthathisbrotheratoncepredictedthathewouldbecomeagreatanatomist。Uptothistimehehadhadnotrainingofanykindtopreparehimforhisprofessionalcareer，andknewlittleofGreekorLatin——languagesentirelyunnecessaryforhim，asheprovedinallofhislifework。Ottleytellsthestorythat，whentwittedwiththislackofknowledgeofthe\"deadlanguages\"
inafterlife，hesaidofhisopponent，\"Icouldteachhimthatonthedeadbodywhichheneverknewinanylanguage，deadorliving。\"
Byhissecondyearindissectionhehadbecomesoskilfulthathewasgivenchargeofsomeoftheclassesinhisbrother’sschool；
in1754hebecameasurgeon’spupilinSt。George’sHospital，andtwoyearslaterhouse-surgeon。Havingbyoverworkbroughtonsymptomsthatseemedtothreatenconsumption，heacceptedthepositionofstaff-surgeontoanexpeditiontoBelleislein1760，andtwoyearslaterwasservingwiththeEnglisharmyatPortugal。Duringallthistimehewasconstantlyengagedinscientificresearches，manyofwhich，suchashisobservationsofgun-shotwounds，heputtoexcellentuseinlaterlife。OnreturningtoEnglandmuchimprovedinhealthin1763，heenteredatonceuponhiscareerasaLondonsurgeon，andfromthattimeforwardhisprogresswasapracticallyuninterruptedseriesofsuccessesinhisprofession。
Hunter’sworkonthestudyofthelymphaticswasofgreatservicetothemedicalprofession。Thisimportantnet-workofminutevesselsdistributedthroughoutthebodyhadrecentlybeenmadetheobjectofmuchstudy，andvariousstudents，includingHaller，hadmadeextensiveinvestigationssincetheirdiscoverybyAsellius。ButHunter，in1758，wasthefirsttodiscoverthelymphaticsintheneckofbirds，althoughitwashisbrotherWilliamwhoadvancedthetheorythatthefunctionofthesevesselswasthatofabsorbents。OneofJohnHunter’spupils，WilliamHewson1739-1774，firstgaveanaccount，in1768，ofthelymphaticsinreptilesandfishes，andaddedtohisteacher’sinvestigationsofthelymphaticsinbirds。Thesestudiesofthelymphaticshavebeenregarded，perhapswithjustice，asHunter’smostvaluablecontributionstopracticalmedicine。
In1767hemetwithanaccidentbywhichhesufferedaruptureofthetendoAchillis——thelargetendonthatformstheattachmentofthemusclesofthecalftotheheel。Fromobservationsofthisaccident，andsubsequentexperimentsupondogs，helaidthefoundationforthenowsimpleandeffectiveoperationforthecureofclubfeetandotherdeformitiesinvolvingthetendons。
In1772hemovedintohisresidenceatEarlscourt，Brompton，wherehegatheredabouthimagreatmenagerieofanimals，birds，reptiles，insects，andfishes，whichheusedinhisphysiologicalandsurgicalexperiments。Hereheperformedacountlessnumberofexperiments——more，probably，than\"anymanengagedinprofessionalpracticehaseverconducted。\"Theseexperimentsvariedinnaturefromobservationsofthehabitsofbeesandwaspstomajorsurgicaloperationsperformeduponhedgehogs，dogs，leopards，etc。Itissaidthatforfifteenyearshekeptaflockofgeeseforthesolepurposeofstudyingtheprocessofdevelopmentineggs。
Hunterbeganhisfirstcourseoflecturesin1772，beingforcedtodothisbecausehehadbeensorepeatedlymisquoted，andbecausehefeltthathecouldbettergaugehisownknowledgeinthisway。Lecturingwasasoretrialtohim，ashewasextremelydiffident，andwithoutwritingouthislecturesinadvancehewasscarcelyabletospeakatall。InthishepresentedamarkedcontrasttohisbrotherWilliam，whowasafluentandbrilliantspeaker。Hunter’slectureswereatbestsimplereadingsofthefactsashehadwrittenthem，thediffidentteacherseldomraisinghiseyesfromhismanuscriptandrarelystoppinguntilhiscompletelecturehadbeenreadthrough。Hislectureswere，therefore，instructiveratherthaninteresting，asheusedinfinitecareinpreparingthem；butappearingbeforehisclasseswassodreadedbyhimthatheissaidtohavebeeninthehabitoftakingahalf-drachmoflaudanumbeforeeachlecturetonervehimfortheordeal。OneisledtowonderbywhatnameheshalldesignatethatqualityofmindthatrendersaboldandfearlesssurgeonlikeHunter，whoisundauntedinthefaceofhazardousanddangerousoperations，astumbling，halting，and\"frightened\"
speakerbeforealittlebandof，atmost，thirtyyoungmedicalstudents。Andyetthissamethingisnotunfrequentlyseenamongtheboldestsurgeons。
Hunter’sOperationfortheCureofAneurismsItshouldbeanobject-lessontothosewho，ignorantlyorotherwise，preachagainstthepainlessvivisectionaspractisedto-day，thatbythesacrificeofasingledeerinthecauseofscienceHunterdiscoveredafactinphysiologythathasbeenthemeansofsavingthousandsofhumanlivesandthousandsofhumanbodiesfromneedlessmutilation。Werefertothediscoveryofthe\"collateralcirculation\"oftheblood，whichled，amongotherthings，toHunter’ssuccessfuloperationuponaneurisms。
Simplystated，everyorganormuscleofthebodyissuppliedbyonelargeartery，whosemaintrunkdistributesthebloodintoitslesserbranches，andthencethroughthecapillaries。Cuttingoffthismainartery，itwouldseem，shouldcutoffentirelytheblood-supplytotheparticularorganwhichissuppliedbythisvessel；anduntilthetimeofHunter’sdemonstrationthisbeliefwasheldbymostphysiologists。Butnaturehasmadeaprovisionforthispossiblestoppageofblood-supplyfromasinglesource，andhassoarrangedthatsomeofthesmallarterialbranchescomingfromthemainsupply-trunkareconnectedwithotherarterialbranchescomingfromsomeothersupply-trunk。Undernormalconditionsthemainarterialtrunkssupplytheirrespectiveorgans，thelittleconnectingarteriolesplayinganinsignificantpart。Butletthemainsupply-trunkbecutofforstoppedforwhateverreason，andaremarkablethingtakesplace。
Thelittleconnectingbranchesbeginatoncetoenlargeanddrawbloodfromtheneighboringuninjuredsupply-trunk，Thisenlargementcontinuesuntilatlastanewrouteforthecirculationhasbeenestablished，theorgannolongerdependingonthenowdefunctoriginalarterialtrunk，butgettingonaswellasbeforebythis\"collateral\"circulationthathasbeenestablished。
Thethoroughunderstandingofthiscollateralcirculationisoneofthemostimportantstepsinsurgery，foruntilitwasdiscoveredamputationswerethoughtnecessaryinsuchcasesasthoseinvolvingthearterysupplyingalegorarm，sinceitwassupposedthat，thearterybeingstopped，deathofthelimbandthesubsequentnecessityforamputationweresuretofollow。
Huntersolvedthisproblembyasingleoperationuponadeer，andhispracticalityasasurgeonledhimsoonaftertoapplythisknowledgetoacertainclassofsurgicalcasesinamostrevolutionaryandsatisfactorymanner。
WhatledtoHunter’sfar-reachingdiscoverywashisinvestigationastothecauseofthegrowthoftheantlersofthedeer。Wishingtoascertainjustwhatparttheblood-supplyontheoppositesidesoftheneckplayedintheprocessofdevelopment，or，perhapsmorecorrectly，toseewhateffectcuttingoffthemainblood-supplywouldhave，HunterhadoneofthedeerofRichmondParkcaughtandtied，whileheplacedaligaturearoundoneofthecarotidarteries——oneofthetwoprincipalarteriesthatsupplytheheadwithblood。Heobservedthatshortlyafterthistheantlerwhichwasonlyhalfgrownandconsequentlyveryvascularonthesideoftheobliteratedarterybecamecoldtothetouch——fromthelackofwarmth-givingblood。Therewasnothingunexpectedinthis，andHunterthoughtnothingofituntilafewdayslater，whenhefound，tohissurprise，thattheantlerhadbecomeaswarmasitsfellow，andwasapparentlyincreasinginsize。Puzzledastohowthiscouldbe，andsuspectingthatinsomewayhisligaturearoundthearteryhadnotbeeneffective，heorderedthedeerkilled，andonexaminationwasastonishedtofindthatwhilehisligaturehadcompletelyshutofftheblood-supplyfromthesourceofthatcarotidartery，thesmallerarterieshadbecomeenlargedsoastosupplytheantlerwithbloodaswellasever，onlybyadifferentroute。
Huntersoonhadachancetomakeapracticalapplicationoftheknowledgethusacquired。Thiswasacaseofpoplitealaneurism，operationsforwhichhadheretoforeprovedprettyuniformlyfatal。Ananeurism，asisgenerallyunderstood，isanenlargementofacertainpartofanartery，thisenlargementsometimesbecomingofenormoussize，fullofpalpitatingblood，andlikelytorupturewithfatalresultsatanytime。Ifbyanymeansthebloodcanbeallowedtoremainquietforevenafewhoursinthisaneurismitwillformaclot，contract，andfinallybeabsorbedanddisappearwithoutanyevilresults。Theproblemofkeepingthebloodquiet，withtheheartcontinuallydrivingitthroughthevessel，isnotasimpleone，andinHunter’stimewasconsideredsoinsurmountablethatsomesurgeonsadvocatedamputationofanymemberhavingananeurism，whileotherscutdownuponthetumoritselfandattemptedtotieoffthearteryaboveandbelow。Thefirstoftheseoperationsmaimedthepatientforlife，whilethesecondwaslikelytoprovefatal。
Inponderingoverwhathehadlearnedaboutcollateralcirculationandthetimerequiredforittobecomefullyestablished，Hunterconceivedtheideathatiftheblood-supplywascutofffromabovetheaneurism，thustemporarilypreventingtheceaselesspulsationsfromtheheart，thisbloodwouldcoagulateandformaclotbeforethecollateralcirculationcouldbecomeestablishedorcouldaffectit。Thepatientuponwhomheperformedhisnowcelebratedoperationwasafflictedwithapoplitealaneurism——thatis，theaneurismwaslocatedonthelargepoplitealarteryjustbehindtheknee-joint。Hunter，therefore，tiedoffthefemoral，ormainsupplyingarteryinthethigh，alittledistanceabovetheaneurism。Theoperationwasentirelysuccessful，andinsixweeks’timethepatientwasabletoleavethehospital，andwithtwosoundlimbs。NaturallythesimplicityandsuccessofthisoperationarousedtheattentionofEurope，and，alone，wouldhavemadethenameofHunterimmortalintheannalsofsurgery。Theoperationhaseversincebeencalledthe\"Hunterian\"operationforaneurism，butthereisreasontobelievethatDominiqueAnelbornabout1679performedasomewhatsimilaroperationseveralyearsearlier。Itisprobable，however，thatHunterhadneverheardofthisworkofAnel，andthathisoperationwastheoutcomeofhisownindependentreasoningfromthefactshehadlearnedaboutcollateralcirculation。Furthermore，Hunter’smodeofoperationwasamuchbetteronethanAnel’s，and，whileAnel’smustclaimpriority，thecreditofmakingitwidelyknownwillalwaysbeHunter’s。
ThegreatservicesofHunterwererecognizedbothathomeandabroad，andhonorsandpositionsofhonorandresponsibilityweregivenhim。In1776hewasappointedsurgeon-extraordinarytotheking；in1783hewaselectedamemberoftheRoyalSocietyofMedicineandoftheRoyalAcademyofSurgeryatParis；in1786hebecamedeputysurgeon-generalofthearmy；andin1790hewasappointedsurgeon-generalandinspector-generalofhospitals。Allthesepositionshefilledwithcredit，andhewasactivelyengagedinhistirelesspursuitofknowledgeandindischarginghismanydutieswheninOctober，1793，hewasstrickenwhileaddressingsomecolleagues，andfelldeadinthearmsofafellow-physician。
LAZZAROSPALLANZANI
Hunter’sgreatrivalamongcontemporaryphysiologistswastheItalianLazzaroSpallanzani1729-1799，oneofthemostpicturesquefiguresinthehistoryofscience。Hewasnoteducatedeitherasascientistorphysician，devoting，himselfatfirsttophilosophyandthelanguages，afterwardsstudyinglaw，andlatertakingorders。Buthewasakeenobserverofnatureandofaquestioningandinvestigatingmind，sothatheisrememberednowchieflyforhisdiscoveriesandinvestigationsinthebiologicalsciences。Oneimportantdemonstrationwashiscontroversionofthetheoryofabiogenesis，or\"spontaneousgeneration，\"aspropoundedbyNeedhamandBuffon。AtthetimeofNeedham’sexperimentsithadlongbeenobservedthatwhenanimalorvegetablematterhadlaininwaterforalittletime——longenoughforittobegintoundergodecomposition——thewaterbecamefilledwithmicroscopiccreatures，the\"infusoriaanimalculis。\"
Thiswouldtendtoshow，eitherthatthewaterortheanimalorvegetablesubstancecontainedthe\"germs\"oftheseminuteorganisms，orelsethattheyweregeneratedspontaneously。Itwasknownthatboilingkilledtheseanimalcules，andNeedhamagreed，therefore，thatifhefirstheatedthemeatorvegetables，andalsothewatercontainingthem，andthenplacedtheminhermeticallyscaledjars——ifhedidthis，andstilltheanimalculesmadetheirappearance，itwouldbeproof-positivethattheyhadbeengeneratedspontaneously。Accordinglybemadenumerousexperiments，alwayswiththesameresults——thatafterafewdaysthewaterwasfoundtoswarmwiththemicroscopiccreatures。Thethingseemedprovenbeyondquestion——providing，ofcourse，thattherehadbeennoslipsintheexperiments。
ButAbbeSpallanzanithoughtthathedetectedsuchslipsinNeedham’sexperiment。Thepossibilityofsuchslipsmightcomeinseveralways:thecontentsofthejarmightnothavebeenboiledforasufficientlengthoftimetokillallthegerms，ortheairmightnothavebeenexcludedcompletelybythesealingprocess。Tocoverboththesecontingencies，Spallanzanifirsthermeticallysealedtheglassvesselsandthenboiledthemforthree-quartersofanhour。Underthesecircumstancesnoanimalculesevermadetheirappearance——aconclusivedemonstrationthatrenderedNeedham’sgroundsforhistheoryatonceuntenable。[2]
AlliedtothesestudiesofspontaneousgenerationwereSpallanzani’sexperimentsandobservationsonthephysiologicalprocessesofgenerationamonghigheranimals。Heexperimentedwithfrogs，tortoises，anddogs；andsettledbeyondquestionthefunctionoftheovumandspermatozoon。Unfortunatelyhemisinterpretedthepartplayedbythespermatozoainbelievingthattheirsurroundingfluidwasequallyactiveinthefertilizingprocess，anditwasnotuntilsomefortyyearslater1824thatDumascorrectedthiserror。
THECHEMICALTHEORYOFDIGESTION
AmongthemostinterestingresearchesofSpallanzaniwerehisexperimentstoprovethatdigestion，ascarriedoninthestomach，isachemicalprocess。Inthishedemonstrated，asReneReaumurhadattemptedtodemonstrate，thatdigestioncouldbecarriedonoutsidethewallsofthestomachasanordinarychemicalreaction，usingthegastricjuiceasthereagentforperformingtheexperiment。Thequestionastowhetherthestomachactedasagrindingortrituratingorgan，ratherthanasareceptacleforchemicalaction，hadbeensettledbyReaumurandwasnolongeraquestionofgeneraldispute。Reaumurhaddemonstratedconclusivelythatdigestionwouldtakeplaceinthestomachinthesamemannerandthesametimeifthesubstancetobedigestedwasprotectedfromtheperistalicmovementsofthestomachandsubjectedtotheactionofthegastricjuiceonly。Hedidthisbyintroducingthesubstancestobedigestedintothestomachintubes，andthusprotectedsothatwhilethejuicesofthestomachcouldactuponthemfreelytheywouldnotbeaffectedbyanymovementsoftheorgan。
Followinguptheseexperiments，heattemptedtoshowthatdigestioncouldtakeplaceoutsidethebodyaswellasinit，asitcertainlyshouldifitwereapurelychemicalprocess。Hecollectedquantitiesofgastricjuice，andplacingitinsuitablevesselscontainingcrushedgrainorflesh，keptthemixtureataboutthetemperatureofthebodyforseveralhours。Afterrepeatedexperimentsofthiskind，apparentlyconductedwithgreatcare，Reaumurreachedtheconclusionthat\"thegastricjuicehasnomoreeffectoutofthelivingbodyindissolvingordigestingthefoodthanwater，mucilage，milk，oranyotherblandfluid。\"[3]Justwhyalloftheseexperimentsfailedtodemonstrateafactsosimpledoesnotappear；buttoSpallanzani，atleast，theywerebynomeansconclusive，andheproceededtoelaborateupontheexperimentsofReaumur。Hemadehisexperimentsinscaledtubesexposedtoacertaindegreeofheat，andshowedconclusivelythatthechemicalprocessdoesgoon，evenwhenthefoodandgastricjuiceareremovedfromtheirnaturalenvironmentinthestomach。Inthishewasopposedbymanyphysiologists，amongthemJohnHunter，butthetruthofhisdemonstrationscouldnotbeshaken，andinlateryearswefindHunterhimselfcompletingSpallanzani’sexperimentsbyhisstudiesofthepost-mortemactionofthegastricjuiceuponthestomachwalls。
ThatSpallanzani’sandHunter’stheoriesoftheactionofthegastricjuicewerenotatonceuniversallyacceptedisshownbyanessaywrittenbyalearnedphysicianin1834。InspeakingofsomeofSpallanzani’sdemonstrations，hewrites:\"Insomeoftheexperiments，inordertogivethefleshorgrainssteepedinthegastricjuicethesametemperaturewiththebody，thephialswereintroducedunderthearmpits。Butthisisnotafairmodeofascertainingtheeffectsofthegastricjuiceoutofthebody；
fortheinfluencewhichlifemaybesupposedtohaveonthesolutionofthefoodwouldbesecuredinthiscase。Theaffinitiesconnectedwithlifewouldextendtosubstancesincontactwithanypartofthesystem:substancesplacedunderthearmpitsarenotplacedatleastinthesamecircumstanceswiththoseunconnectedwithalivinganimal。\"Butjusthowthiswriterreachestheconclusionthat\"theexperimentsofReaumurandSpallanzanigivenoevidencethatthegastricjuicehasanypeculiarinfluencemorethanwateroranyotherblandfluidindigestingthefood\"[4]isdifficulttounderstand。
TheconcludingtouchesweregiventothenewtheoryofdigestionbyJohnHunter，who，aswehaveseen，atfirstopposedSpallanzani，butwhofinallybecameanardentchampionofthechemicaltheory。HunternowcarriedSpallanzani’sexperimentsfurtherandprovedtheactionofthedigestivefluidsafterdeath。Formanyyearsanatomistshadbeenpuzzledbypathologicallesionofthestomach，foundpostmortem，whennosymptomsofanydisorderofthestomachhadbeenevincedduringlife。Hunterrightlyconceivedthattheselesionswerecausedbytheactionofthegastricjuice，which，whileunabletoactuponthelivingtissue，continueditsactionchemicallyafterdeath，thusdigestingthewallsofthestomachinwhichithadbeenformed。
And，asusualwithhisobservations，beturnedthisdiscoverytopracticaluseinaccountingforcertainphenomenaofdigestion。
ThefollowingaccountofthestomachbeingdigestedafterdeathwaswrittenbyHunteratthedesireofSirJohnPringle，whenhewaspresidentoftheRoyalSociety，andthecircumstancewhichledtothisisasfollows:\"Iwasopening，inhispresence，thebodyofapatientofhisown，wherethestomachwasinpartdissolved，whichappearedtohimveryunaccountable，astherehadbeennoprevioussymptomthatcouldhaveledhimtosuspectanydiseaseinthestomach。Itookthatopportunityofgivinghimmyideasrespectingit，andtoldhimthatIhadlongbeenmakingexperimentsondigestion，andconsideredthisasoneofthefactswhichprovedaconvertingpowerinthegastricjuice……Thereareagreatmanypowersinnaturewhichthelivingprincipledoesnotenabletheanimalmatter，withwhichitiscombined，toresist——viz。，themechanicalandmostofthestrongestchemicalsolvents。Itrendersit，however，capableofresistingthepowersoffermentation，digestion，andperhapsseveralothers，whicharewellknowntoactonthesamematterwhendeprivedofthelivingprincipleandentirelytodecomposeit。\"
Hunterconcludeshispaperwiththefollowingparagraph:\"Theseappearancesthrowconsiderablelightontheprincipleofdigestion，andshowthatitisneitheramechanicalpower，norcontractionsofthestomach，norheat，butsomethingsecretedinthecoatsofthestomach，andthrownintoitscavity，whichthereanimalizesthefoodorassimilatesittothenatureoftheblood。
Thepowerofthisjuiceisconfinedorlimitedtocertainsubstances，especiallyofthevegetableandanimalkingdoms；andalthoughthismenstruumiscapableofactingindependentlyofthestomach，yetitisindebtedtothatviscusforitscontinuance。[5]
THEFUNCTIONOFRESPIRATION
Itisacuriouscommentaryonthecrudenotionsofmechanicsofpreviousgenerationsthatitshouldhavebeennecessarytoprovebyexperimentthatthethin，almostmembranousstomachofamammalhasnotthepowertopulverize，bymereattrition，thefoodsthataretakenintoit。However，theproofwasnowforthefirsttimeforthcoming，andthequestionofthegeneralcharacterofthefunctionofdigestionwasforeversetatrest。Almostsimultaneouslywiththisgreatadvance，correspondingprogresswasmadeinanalliedfield:themysteriesofrespirationwereatlastclearedup，thankstothenewknowledgeofchemistry。Thesolutionoftheproblemfollowedalmostasamatterofcourseupontheadvancesofthatscienceinthelatterpartofthecentury。HithertonoonesinceMayow，ofthepreviouscentury，whoseflashofinsighthadbeenstrangelyoverlookedandforgotten，hadevenvaguelysurmisedthetruefunctionofthelungs。ThegreatBoerhaavehadsupposedthatrespirationischieflyimportantasanaidtothecirculationoftheblood；hisgreatpupil，Haller，hadbelievedtothedayofhisdeathin1777
thatthemainpurposeofthefunctionistoformthevoice。Nogeniuscouldhopetofathomthemysteryofthelungssolongasairwassupposedtobeasimpleelement，servingameremechanicalpurposeintheeconomyoftheearth。
Butthediscoveryofoxygengavetheclew，andverysoonallthechemistsweretestingtheairthatcamefromthelungs——Dr。
Priestley，asusual，beinginthevan。Hisinitialexperimentsweremadein1777，andfromtheoutsettheproblemwasasgoodassolved。Otherexperimentersconfirmedhisresultsinalltheiressentials——notablyScheeleandLavoisierandSpallanzaniandDavy。Itwasclearlyestablishedthatthereischemicalactioninthecontactoftheairwiththetissueofthelungs；thatsomeoftheoxygenoftheairdisappears，andthatcarbonic-acidgasisaddedtotheinspiredair。Itwasshown，too，thattheblood，havingcomeincontactwiththeair，ischangedfromblacktoredincolor。Theseessentialswerenotindisputefromthefirst。
Butastojustwhatchemicalchangescausedtheseresultswasthesubjectofcontroversy。Whether，forexample，oxygenisactuallyabsorbedintotheblood，orwhetheritmerelyuniteswithcarbongivenofffromtheblood，waslongindispute。
Eachofthemaindisputantswasbiasedbyhisownparticularviewsastothemootpointsofchemistry。Lavoisier，forexample，believedoxygengastobecomposedofametaloxygencombinedwiththeallegedelementheat；Dr。Priestleythoughtitacompoundofpositiveelectricityandphlogiston；andHumphryDavy，whenheenteredthelistsalittlelater，supposedittobeacompoundofoxygenandlight。Suchmistakennotionsnaturallycomplicatedmattersanddelayedacompleteunderstandingofthechemicalprocessesofrespiration。Itwassometime，too，beforetheideagainedacceptancethatthemostimportantchemicalchangesdonotoccurinthelungsthemselves，butintheultimatetissues。Indeed，thematterwasnotclearlysettledatthecloseofthecentury。Nevertheless，theproblemofrespirationhadbeensolvedinitsessentials。Moreover，thevastlyimportantfacthadbeenestablishedthataprocessessentiallyidenticalwithrespirationisnecessarytotheexistencenotonlyofallcreaturessuppliedwithlungs，buttofishes，insects，andevenvegetables——inshort，toeverykindoflivingorganism。
ERASMUSDARWINANDVEGETABLEPHYSIOLOGY
SomeinterestingexperimentsregardingvegetablerespirationweremadejustatthecloseofthecenturybyErasmusDarwin，andrecordedinhisBotanicGardenasafoot-notetotheverse:
\"Whilespreadinairtheleavesrespiringplay。\"
Thesenotesareworthquotingatsomelength，astheygiveaclearideaofthephysiologicaldoctrinesofthetime1799，whiletakingadvancegroundastothespecificmatterinquestion:
\"Therehavebeenvariousopinions，\"Darwinsays，\"concerningtheuseoftheleavesofplantsinthevegetableeconomy。Somehavecontendedthattheyareperspiratoryorgans。ThisdoesnotseemprobablefromanexperimentofDr。Hales，VegetableStatics，p。
30。He，found，bycuttingoffbranchesoftreeswithapplesonthemandtakingofftheleaves，thatanappleexhaledaboutasmuchastwoleavesthesurfacesofwhichwerenearlyequaltotheapple；whenceitwouldappearthatappleshaveasgoodaclaimtobetermedperspiratoryorgansasleaves。Othershavebelievedthemexcretoryorgansofexcrementitiousjuices，butasthevaporexhaledfromvegetableshasnotaste，thisideaisnomoreprobablethantheother；addtothisthatinmostweatherstheydonotappeartoperspireorexhaleatall。
\"Theinternalsurfaceofthelungsorair-vesselsinmenissaidtobeequaltotheexternalsurfaceofthewholebody，oralmostfifteensquarefeet；onthissurfacethebloodisexposedtotheinfluenceoftherespiredairthroughthemedium，however，ofathinpellicle；bythisexposuretotheairithasitscolorchangedfromdeepredtobrightscarlet，andacquiressomethingsonecessarytotheexistenceoflifethatwecanlivescarcelyaminutewithoutthiswonderfulprocess。
\"Theanalogybetweentheleavesofplantsandthelungsorgillsofanimalsseemstoembracesomanycircumstancesthatwecanscarcelywithholdourconsenttotheirperformingsimilaroffices。
\"1。Thegreatsurfaceofleavescomparedtothatofthetrunkandbranchesoftreesissuchthatitwouldseemtobeanorganwelladaptedforthepurposeofexposingthevegetablejuicestotheinfluenceoftheair；this，however，weshallseeafterwardsisprobablyperformedonlybytheiruppersurfaces，yeteveninthiscasethesurfaceoftheleavesingeneralbearagreaterproportiontothesurfaceofthetreethanthelungsofanimalstotheirexternalsurfaces。
\"2。Inthelungofanimalstheblood，afterhavingbeenexposedtotheairintheextremitiesofthepulmonaryartery，ischangedincolorfromdeepredtobrightscarlet，andcertainlyinsomeofitsessentialpropertiesitisthencollectedbythepulmonaryveinandreturnedtotheheart。Toshowasimilarityofcircumstancesintheleavesofplants，thefollowingexperimentwasmade，June24，1781。Astalkwithleavesandseed-vesselsoflargespurgeEuphorbiahelioscopiahadbeenseveraldaysplacedinadecoctionofmadderRubiatinctorumsothatthelowerpartofthestemandtwooftheundermostleaveswereimmersedinit。
AfterhavingwashedtheimmersedleavesinclearwaterIcouldreadilydiscoverthecolorofthemadderpassingalongthemiddleribofeachleaf。Theredarterywasbeautifullyvisibleontheunderandontheuppersurfaceoftheleaf；butontheuppersidemanyredbrancheswereseengoingfromittotheextremitiesoftheleaf，whichontheothersidewerenotvisibleexceptbylookingthroughitagainstthelight。Onthisundersideasystemofbranchingvesselscarryingapalemilkyfluidwereseencomingfromtheextremitiesoftheleaf，andcoveringthewholeundersideofit，andjoiningtwolargeveins，oneoneachsideoftheredarteryinthemiddleriboftheleaf，andalongwithitdescendingtothefoot-stalkorpetiole。Onslittingoneoftheseleaveswithscissors，andhavingamagnifying-glassready，themilkybloodwasseenoozingoutofthereturningveinsoneachsideoftheredarteryinthemiddlerib，butnoneoftheredfluidfromtheartery。
\"AlltheseappearancesweremoreeasilyseeninaleafofPicristreatedinthesamemanner；forinthismilkyplantthestemsandmiddleriboftheleavesaresometimesnaturallycoloredreddish，andhencethecolorofthemadderseemedtopassfartherintotheramificationsoftheirleaf-arteries，andwastherebeautifullyvisiblewiththereturningbranchesofmilkyveinsoneachside。\"
Darwinnowgoesontodrawanincorrectinferencefromhisobservations:
\"3。Fromtheseexperiments，\"hesays，\"theuppersurfaceoftheleafappearedtobetheimmediateorganofrespiration，becausethecoloredfluidwascarriedtotheextremitiesoftheleafbyvesselsmostconspicuousontheuppersurface，andtherechangedintoamilkyfluid，whichisthebloodoftheplant，andthenreturnedbyconcomitantveinsontheundersurface，whichwereseentooozewhendividedwithscissors，andwhich，inPicris，particularly，rendertheundersurfaceoftheleavesgreatlywhiterthantheupperone。\"
Butinpointoffact，asstudiesofalatergenerationweretoshow，itistheundersurfaceoftheleafthatismostabundantlyprovidedwithstomata，or\"breathing-pores。\"Fromthestand-pointofthislaterknowledge，itisofinteresttofollowourauthoralittlefarther，toillustrateyetmorefullythepossibilityofcombiningcorrectobservationswithafaultyinference。
\"4。Astheuppersurfaceofleavesconstitutestheorganofrespiration，onwhichthesapisexposedintheterminationofarteriesbeneathathinpellicletotheactionoftheatmosphere，thesesurfacesinmanyplantsstronglyrepelmoisture，ascabbageleaves，whencetheparticlesofrainlyingovertheirsurfaceswithouttouchingthem，asobservedbyMr。MelvilleEssaysLiteraryandPhilosophical:Edinburgh，havetheappearanceofglobulesofquicksilver。Andhenceleaveswiththeuppersurfacesonwaterwitherassoonasinthedryair，butcontinuegreenformanydaysifplacedwiththeundersurfaceonwater，asappearsintheexperimentsofMonsieurBonnetUsagedesFeuilles。Hencesomeaquaticplants，asthewater-lilyNymphoea，havethelowersidesfloatingonthewater，whiletheuppersurfacesremaindryintheair。
\"5。Asthoseinsectswhichhavemanyspiracula，orbreathingapertures，aswaspsandflies，areimmediatelysuffocatedbypouringoiluponthem，Icarefullycoveredwithoilthesurfacesofseveralleavesofphlomis，ofPortugallaurel，andbalsams，andthoughitwouldnotregularlyadhere，Ifoundthemalldieinadayortwo。
\"Itmustbeaddedthatmanyleavesarefurnishedwithmusclesabouttheirfoot-stalks，toturntheirsurfacestotheairorlight，asmimosaorHedysarumgyrans。FromalltheseanalogiesI
thinktherecanbenodoubtbutthatleavesoftreesaretheirlungs，givingoutaphlogisticmaterialtotheatmosphere，andabsorbingoxygen，orvitalair。
\"6。Thegreatuseoflighttovegetationwouldappearfromthistheorytobebydisengagingvitalairfromthewaterwhichtheyperspire，andthencetofacilitateitsunionwiththeirbloodexposedbeneaththethinsurfaceoftheirleaves；sincewhenpureairisthusapplieditisprobablethatitcanbemorereadilyabsorbed。Hence，inthecuriousexperimentsofDr。PriestleyandMr。Ingenhouz，someplantspurifiedlessairthanothers——thatis，theyperspiredlessinthesunshine；andMr。Scheelefoundthatbyputtingpeasintowaterwhichabouthalfcoveredthemtheyconvertedthevitalairintofixedair，orcarbonic-acidgas，inthesamemannerasinanimalrespiration。
\"7。Thecirculationinthelungsorleavesofplantsisverysimilartothatoffish。Infishtheblood，afterhavingpassedthroughtheirgills，doesnotreturntotheheartasfromthelungsofair-breathinganimals，butthepulmonaryveintakingthestructureofanarteryafterhavingreceivedthebloodfromthegills，whichtheregainsamorefloridcolor，distributesittotheotherpartsoftheirbodies。Thesamestructureoccursintheliversoffish，whenceweseeinthoseanimalstwocirculationsindependentofthepoweroftheheart——viz。，thatbeginningattheterminationoftheveinsofthegillsandbranchingthroughthemuscles，andthatwhichpassesthroughtheliver；bothwhicharecarriedonbytheactionofthoserespectivearteriesandveins。\"[6]
Darwinishereatriflefancifulinforcingtheanalogybetweenplantsandanimals。Thecirculatorysystemofplantsisreallynotquitesoelaboratelycomparabletothatoffishesashesupposed。Buttheall-importantideaoftheuniformityunderlyingtheseemingdiversityofNatureishereexemplified，aselsewhereinthewritingsofErasmusDarwin；and，morespecifically，acleargraspoftheessentialsofthefunctionofrespirationisfullydemonstrated。