IHAVEselectedto-nighttheparticularsubjectofYeastfortworeasons——or，rather，Ishouldsayforthree。Inthefirstplace，becauseitisoneofthesimplestandthemostfamiliarobjectswithwhichweareacquainted。Inthesecondplace，becausethefactsandphenomenawhichIhavetodescribearesosimplethatitispossibletoputthembeforeyouwithoutthehelpofanyofthosepicturesordiagramswhichareneededwhenmattersaremorecomplicated，andwhich，ifIhadtorefertothemhere，wouldinvolvethenecessityofmyturningawayfromyounowandthen，andtherebyincreasingverylargelymydifficultyalreadysufficientlygreatinmakingmyselfheard。Andthirdly，IhavechosenthissubjectbecauseIknowofnofamiliarsubstanceformingpartofourevery-dayknowledgeandexperience，theexaminationofwhich，withalittlecare，tendstoopenupsuchveryconsiderableissuesasdoesthissubstance——yeast。
　　Inthefirstplace，Ishouldliketocallyourattentiontoafactwithwhichthewholeofyouare，tobeginwith，perfectlyacquainted，Imeanthefactthatanyliquidcontainingsugar，anyliquidwhichisformedbypressingoutthesucculentpartsofthefruitsofplants，oramixtureofhoneyandwater，iflefttoitselfforashorttime，beginstoundergoapeculiarchange。Nomatterhowclearitmightbeatstarting，yetafterafewhours，oratmostafewdays，ifthetemperatureishigh，thisliquidbeginstobeturbid，andby-and-bybubblesmaketheirappearanceinit，andasortofdirty-lookingyellowishfoamorscumcollectsatthesurface；whileatthesametime，bydegrees，asimilarkindofmatter，whichwecallthe"lees，"sinkstothebottom。
　　Thequantityofthisdirty-lookingstuff，thatwecallthescumandthelees，goesonincreasinguntilitreachesacertainamount，andthenitstops；andbythetimeitstops，youfindtheliquidinwhichthismatterhasbeenformedhasbecomealteredinitsquality。Tobeginwithitwasameresweetishsubstance，havingtheflavourofwhatevermightbetheplantfromwhichitwasexpressed，orhavingmerelythetasteandtheabsenceofsmellofasolutionofsugar；butbythetimethatthischangethatIhavebeenbrieflydescribingtoyouisaccomplishedtheliquidhasbecomecompletelyaltered，ithasacquiredapeculiarsmell，and，whatisstillmoreremarkable，ithasgainedthepropertyofintoxicatingthepersonwhodrinksit。Nothingcanbemoreinnocentthanasolutionofsugar；nothingcanbelessinnocent，iftakeninexcess，asyouallknow，thanthosefermentedmatterswhichareproducedfromsugar。
　　Well，again，ifyounoticethatbubbling，or，asitwere，seethingoftheliquid，whichhasaccompaniedthewholeofthisprocess，youwillfindthatitisproducedbytheevolutionoflittlebubblesofair-likesubstanceoutoftheliquid；andIdaresayyouallknowthisair-likesubstanceisnotlikecommonair；itisnotasubstancewhichamancanbreathewithimpunity。Youoftenhearofaccidentswhichtakeplaceinbrewers'vatswhenmengoincarelessly，andgetsuffocatedtherewithoutknowingthattherewasanythingevilawaitingthem。AndifyoutriedtheexperimentwiththisliquidIamtellingofwhileitwasfermenting，youwouldfindthatanysmallanimalletdownintothevesselwouldbesimilarlystifled；andyouwoulddiscoverthatalightlowereddownintoitwouldgoout。Well，then，lastly，ifafterthisliquidhasbeenthusalteredyouexposeittothatprocesswhichiscalleddistillation；thatistosay，ifyouputitintoastill，andcollectthematterswhicharesentover，youobtain，whenyoufirstheatit，acleartransparentliquid，which，however，issomethingtotallydifferentfromwater；itismuchlighter；
　　ithasastrongsmell，andithasanacridtaste；anditpossessesthesameintoxicatingpowerastheoriginalliquid，butinamuchmoreintensedegree。Ifyouputalighttoit，itburnswithabrightflame，anditisthatsubstancewhichweknowasspiritsofwine。
　　NowthesefactswhichIhavejustputbeforeyou——allbutthelast——havebeenknownfromextremelyremoteantiquity。Itis，Ihopeoneofthebestevidencesoftheantiquityofthehumanrace，thatamongtheearliestrecordsofallkindsofmen，youfindatimerecordedwhentheygotdrunk。Wemayhopethatthatmusthavebeenaverylateperiodintheirhistory。NotonlyhavewetherecordofwhathappenedtoNoah，butifweturntothetraditionsofadifferentpeople，thoseforefathersofourswholivedinthehighlandsofNorthernIndia，wefindthattheywerenotlessaddictedtointoxicatingliquids；andI
　　havenodoubtthattheknowledgeofthisprocessextendsfarbeyondthelimitsofhistoricallyrecordedtime。
　　Anditisaverycuriousthingtoobservethatallthenameswehaveofthisprocess，andallthatbelongstoit，arenamesthathavetheirrootsnotinourpresentlanguage，butinthoseolderlanguageswhichgobacktothetimesatwhichthiscountrywaspeopled。Thatword"fermentation"forexample，whichisthetitleweapplytothewholeprocess，isaLatinterm；andatermwhichisevidentlybaseduponthefactoftheeffervescenceoftheliquid。ThentheFrench，whoareveryfondofcallingthemselvesaLatinrace，haveaparticularwordforferment，whichis'levure'。And，inthesameway，wehavetheword"leaven，"thosetwowordshavingreferencetotheheavingup，ortotheraisingofthesubstancewhichisfermented。NowthosearewordswhichwegetfromwhatImaycalltheLatinsideofourparentage；butifweturntotheSaxonside，thereareanumberofnamesconnectedwiththisprocessoffermentation。
　　Forexample，theGermanscallfermentation——andtheoldGermansdidso——"gahren；"andtheycallanythingwhichisusedasafermentbysuchnames，suchas"gheist"and"geest，"andfinallyinlowGerman，"yest"；"andthatwordyouknowisthewordourSaxonforefathersused，andisalmostthesameasthewordwhichiscommonlyemployedinthiscountrytodenotethecommonfermentofwhichIhavebeenspeaking。Sotheyhaveanothername，theword"hefe，"whichisderivedfromtheirverb"heben，"whichsignifiestoraiseup；andtheyhaveyetathirdname，whichisalsoonecommoninthiscountryIdonotknowwhetheritiscommoninLancashire，butitiscertainlyverycommonintheMidlandcountries，theword"barm，"whichisderivedfromarootwhichsignifiestoraiseortobearup。Barmisasomethingborneup；andthusthereismuchmorerealrelationthaniscommonlysupposedbythosewhomakepuns，betweenthebeerwhichamantakesdownhisthroatandthebieruponwhichthatprocess，ifcarriedtoexcess，generallylandshim，fortheyarebothderivedfromtherootsignifyingbearingup；theonethingisborneuponmen'sshoulders，andtheotheristhefermentedliquidwhichwasborneupbythefermentationtakingplaceinitself。
　　Again，Ispokeoftheproduceoffermentationas"spiritofwine。"Nowwhataverycuriousphrasethatis，ifyoucometothinkofit。Theoldalchemiststalkedofthefinestessenceofanythingasifithadthesamesortofrelationtothethingitselfasaman'sspiritissupposedtohavetohisbody；andsotheyspokeofthisfineessenceofthefermentedliquidasbeingthespiritoftheliquid。Thuscameaboutthatextraordinaryambiguityoflanguage，invirtueofwhichyouapplypreciselythesamesubstantivenametothesoulofmanandtoaglassofgin!Andthenthereisstillyetoneothermostcuriouspieceofnomenclatureconnectedwiththismatter，andthatistheword"alcohol"itself，whichisnowsofamiliartoeverybody。Alcoholoriginallymeantaveryfinepowder。ThewomenoftheArabsandotherEasternpeopleareinthehabitoftingingtheireyelasheswithaveryfineblackpowderwhichismadeofantimony，andtheycallthat"kohol；"andthe"al"issimplythearticleputinfrontofit，soastosay"thekohol。"Anduptothe17thcenturyinthiscountrythewordalcoholwasemployedtosignifyanyveryfinepowder；youfinditinRobertBoyle'sworksthatheuses"alcohol"foraveryfinesubtlepowder。Butthenthisnameofanythingveryfineandverysubtlecametobespeciallyconnectedwiththefineandsubtlespiritobtainedfromthefermentationofsugar；andIbelievethatthefirstpersonwhofairlyfixeditasthepropernameofwhatwenowcommonlycallspiritsofwine，wasthegreatFrenchchemistLavoisier，socomparativelyrecentistheuseofthewordalcoholinthisspecialisedsense。
　　SomuchbywayofgeneralintroductiontothesubjectonwhichIhavetospeakto-night。WhatIhavehithertostatedissimplywhatwemaycallcommonknowledge，whicheverybodymayacquainthimselfwith。Andyouknowthatwhatwecallscientificknowledgeisnotanykindofconjuration，aspeoplesometimessuppose，butitissimplytheapplicationofthesameprinciplesofcommonsensethatweapplytocommonknowledge，carriedout，ifImaysospeak，toknowledgewhichisuncommon。
　　Andallthatweknownowofthissubstance，yeast，andalltheverystrangeissuestowhichthatknowledgehasledus，havesimplycomeoutoftheinveteratehabit，andaveryfortunatehabitforthehumanraceitis，whichscientificmenhaveofnotbeingcontentuntiltheyhaveroutedoutallthedifferentchainsandconnectionsofapparentlysimplephenomena，untiltheyhavetakenthemtopiecesandunderstoodtheconditionsuponwhichtheydepend。Iwilltrytopointouttoyounowwhathashappenedinconsequenceofendeavouringtoapplythisprocessof"analysis，"aswecallit，thisteazingoutofanapparentlysimplefactintoallthelittlefactsofwhichitismadeup，totheascertainedfactsrelatingtothebarmortheyeast；secondly，whathascomeoftheattempttoascertaindistinctlywhatisthenatureoftheproductswhichareproducedbyfermentation；thenwhathascomeoftheattempttounderstandtherelationbetweentheyeastandtheproducts；andlastly，whatverycurioussideissuesifImaysocallthem——havebranchedoutinthecourseofthisinquiry，whichhasnowoccupiedsomewhereabouttwocenturies。
　　Thefirstthingwastomakeoutpreciselyandclearlywhatwasthenatureofthissubstance，thisapparentlymerescumandmudthatwecallyeast。AndthatwasfirstcommencedseriouslybyawonderfuloldDutchmanofthenameofLeeuwenhoek，wholivedsometwohundredyearsago，andwhowasthefirstpersontoinventthoroughlytrustworthymicroscopesofhighpowers。Now，Leeuwenhoekwenttoworkuponthisyeastmud，andbyapplyingtoithighpowersofthemicroscope，hediscoveredthatitwasnomeremudsuchasyoumightatfirstsuppose，butthatitwasasubstancemadeupofanenormousmultitudeofminutegrains，eachofwhichhadjustasdefiniteaformasifitwereagrainofcorn，althoughitwasvastlysmaller，thelargestofthesenotbeingmorethanthetwo-thousandthofaninchindiameter；while，asyouknow，agrainofcornisalargething，andtheverysmallestoftheseparticleswerenotmorethantheseven-thousandthofaninchindiameter。
　　Leeuwenhoeksawthatthismuddystuffwasinrealityaliquid，inwhichtherewerefloatingthisimmensenumberofdefinitelyshapedparticles，allaggregatedinheapsandlumpsandsomeofthemseparate。Thatdiscoveryremained，sotospeak，dormantforfullyacentury，andthenthequestionwastakenupbyaFrenchdiscoverer，who，payinggreatattentionandhavingtheadvantageofbetterinstrumentsthanLeeuwenhoekhad，watchedthesethingsandmadetheastoundingdiscoverythattheywerebodieswhichwereconstantlybeingreproducedandgrowing；thanwhenoneoftheseroundedbodieswasonceformedandhadgrowntoitsfullsize，itimmediatelybegantogiveoffalittlebudfromoneside，andthenthatbudgrewoutuntilithadattainedthefullsizeofthefirst，andthat，inthisway，theyeastparticlewasundergoingaprocessofmultiplicationbybudding，justaseffectualandjustascompleteastheprocessofmultiplicationofaplantbybudding；andthusthisFrenchman，CagniarddelaTour，arrivedattheconclusion——verycreditabletohissagacity，andwhichhasbeenconfirmedbyeveryobservationandreasoningsince——thatthisapparentlymuddyrefusewasneithermorenorlessthanamassofplants，ofminutelivingplants，growingandmultiplyinginthesugaryfluidinwhichtheyeastisformed。Andfromthattimeforthwehaveknownthissubstancewhichformsthescumandtheleesastheyeastplant；andithasreceivedascientificname——whichImayusewithoutthinkingofit，andwhichIwillthereforegiveyou——namely，"Torula。"
　　Well，thiswasacapitaldiscovery。Thenextthingtodowastomakeouthowthistorulawasrelatedtotheotherplants。Iwon'twearyyouwiththewholecourseofinvestigation，butImaysumupitsresults，andtheyarethese——thatthetorulaisaparticularkindofafungus，aparticularstaterather，ofafungusormould。Therearemanymouldswhichundercertainconditionsgiverisetothistorulacondition，toasubstancewhichisnotdistinguishablefromyeast，andwhichhasthesamepropertiesasyeast——thatistosay，whichisabletodecomposesugarinthecuriouswaythatweshallconsiderby-and-by。SothattheyeastplantisaplantbelongingtoagroupoftheFungi，multiplyingandgrowingandlivinginthisveryremarkablemannerinthesugaryfluidwhichis，sotospeak，thenidusorhomeoftheyeast。
　　That，inafewwords，is，asfarasinvestigation——bythehelpofone'seyeandbythehelpofthemicroscope——hastakenus。Butnowthereisanobserverwhosemethodsofobservationaremorerefinedthanthoseofmenwhousetheireye，eventhoughitbeaidedbythemicroscope；amanwhoseesindirectlyfurtherthanwecanseedirectly——thatis，thechemist；andthechemisttookupthisquestion，andhisdiscoverywasnotlessremarkablethanthatofthemicroscopist。Thechemistdiscoveredthattheyeastplantbeingcomposedofasortofbag，likeabladder，insidewhichisapeculiarsoft，semifluidmaterial——thechemistfoundthatthisouterbladderhasthesamecompositionasthesubstanceofwood，thatmaterialwhichiscalled"cellulose，"andwhichconsistsoftheelementscarbonandhydrogenandoxygen，withoutanynitrogen。ButthenhealsofoundthefirstpersontodiscoveritwasanItalianchemist，namedFabroni，intheendofthelastcenturythatthisinnermatterwhichwascontainedinthebag，whichconstitutestheyeastplant，wasasubstancecontainingtheelementscarbonandhydrogenandoxygenandnitrogen；thatitwaswhatFabronicalledavegeto-animalsubstance，andthatithadthepeculiaritiesofwhatarecommonlycalled"animalproducts。"
　　Thisagainwasanexceedinglyremarkablediscovery。Itlayneglectedforatime，untilitwassubsequentlytakenupbythegreatchemistsofmoderntimes，andthey，withtheirdelicatemethodsofanalysis，havefinallydecidedthat，inallessentialrespects，thesubstancewhichformsthechiefpartofthecontentsoftheyeastplantisidenticalwiththematerialwhichformsthechiefpartofourownmuscles，whichformsthechiefpartofourownblood，whichformsthechiefpartofthewhiteoftheegg；that，infact，althoughthislittleorganismisaplant，andnothingbutaplant，yetthatitsactivelivingcontentscontainasubstancewhichiscalled"protein，"whichisofthesamenatureasthesubstancewhichformsthefoundationofeveryanimalorganismwhatever。
　　Nowwecomenexttothequestionoftheanalysisoftheproducts，ofthatwhichisproducedduringtheprocessoffermentation。Sofarbackasthebeginningofthe16thcentury，inthetimesoftransitionbetweentheoldalchemyandthemodernchemistry，therewasaremarkableman，VonHelmont，aDutchman，whosawthedifferencebetweentheairwhichcomesoutofavatwheresomethingisfermentingandcommonair。Hewasthemanwhoinventedtheterm"gas，"andhecalledthiskindofgas"gassilvestre"——sotospeakgasthatiswild，andlivesinoutofthewayplaces——havinginhismindtheidentityofthisparticularkindofairwiththatwhichisfoundinsomecavesandcellars。Then，thegradualprocessofinvestigationgoingon，itwasdiscoveredthatthissubstance，thencalled"fixedair，"wasapoisonousgas，anditwasfinallyidentifiedwiththatkindofgaswhichisobtainedbyburningcharcoalintheair，whichiscalled"carbonicacid。"Thenthesubstancealcoholwassubjectedtoexamination，anditwasfoundtobeacombinationofcarbon，andhydrogen，andoxygen。Thenthesugarwhichwascontainedinthefermentingliquidwasexaminedandthatwasfoundtocontainthethreeelementscarbon，hydrogen，andoxygen。Sothatitwascleartherewereinsugarthefundamentalelementswhicharecontainedinthecarbonicacid，andinthealcohol。
　　AndthencamethatgreatchemistLavoisier，andheexaminedintothesubjectcarefully，andpossessedwiththatbrilliantthoughtofhiswhichhappenstobepropoundedexactlyapropostothismatteroffermentation——thatnomatteriseverlost，butthatmatteronlychangesitsformandchangesitscombinations——heendeavouredtomakeoutwhatbecameofthesugarwhichwassubjectedtofermentation。Hethoughthediscoveredthatthewholeweightofthesugarwasrepresentedbythecarbonicacidproduced；thatinotherwords，supposingthistumblertorepresentthesugar，thattheactionoffermentationwasasitwerethesplittingofit，theonehalfgoingawayintheshapeofcarbonicacid，andtheotherhalfgoingawayintheshapeofalcohol。
　　Subsequentinquiry，carefulresearchwiththerefinementsofmodernchemistry，havebeenappliedtothisproblem，andtheyhaveshownthatLavoisierwasnotquitecorrect；thatwhathesaysisquitetrueforabout95percent。ofthesugar，butthattheother5percent。，ornearlyso，isconvertedintotwootherthings；oneofthem，matterwhichiscalledsuccinicacid，andtheothermatterwhichiscalledglycerine，whichyouallknownowasoneofthecommonestofhouseholdmatters。Itmaybethatwehavenotgottotheendofthisrefinedanalysisyet，butatanyrate，IsupposeImaysay——andIspeakwithsomelittlehesitationforfearmyfriendProfessorRoscoeheremaypickmeupfortrespassinguponhisprovince——butIbelieveImaysaythatnowwecanaccountfor99percent。atleastofthesugar，andthat99percent。issplitupintothesefourthings，carbonicacid，alcohol，succinicacid，andglycerine。Sothatitmaybethatnoneofthesugarwhateverdisappears，andthatonlyitsparts，sotospeak，arere-arranged，andifanyofitdisappears，certainlyitisaverysmallportion。
　　Nowthesearethefactsofthecase。Thereisthefactofthegrowthoftheyeastplant；andthereisthefactofthesplittingupofthesugar。Whatrelationhavethesetwofactstooneanother？
　　Foraverylongtimethatwasagreatmatterofdispute。TheearlyFrenchobservers，todothemjustice，discernedtherealstateofthecase，namely，thattherewasaverycloseconnectionbetweentheactuallifeoftheyeastplantandthisoperationofthesplittingupofthesugar；andthatonewasinsomewayorotherconnectedwiththeother。
　　Allinvestigationsubsequentlyhasconfirmedthisoriginalidea。Ithasbeenshownthatifyoutakeanymeasuresbywhichotherplantsoflikekindtothetorulawouldbekilled，andbywhichtheyeastplantiskilled，thentheyeastlosesitsefficiency。Butacapitalexperimentuponthissubjectwasmadebyaverydistinguishedman，Helmholz，whoperformedanexperimentofthiskind。
　　Hehadtwovessels——oneofthemwewillsupposefullofyeast，butoverthebottomofit，asthismightbe，wastiedathinfilmofbladder；consequently，throughthatthinfilmofbladderalltheliquidpartsoftheyeastwouldgo，butthesolidpartswouldbestoppedbehind；thetorulawouldbestopped，theliquidpartsoftheyeastwouldgo。Andthenhetookanothervesselcontainingafermentablesolutionofsugar，andheputoneinsidetheother；andinthiswayyouseethefluidpartsoftheyeastwereabletopassthroughwiththeutmosteaseintothesugar，butthesolidpartscouldnotgetthroughatall。Andhejudgedthus:ifthefluidpartsarethosewhichexcitefermentation，then，inasmuchasthesearestopped，thesugarwillnotferment；andthesugardidnotferment，showingquiteclearly，thatanimmediatecontactwiththesolid，livingtorulawasabsolutelynecessarytoexcitethisprocessofsplittingupofthesugar。Thisexperimentwasquiteconclusiveastothisparticularpoint，andhashadverygreatfruitsinotherdirections。
　　Well，then，theyeastplantbeingessentialtotheproductionoffermentation，wheredoestheyeastplantcomefrom？Here，again，wasanothergreatproblemopenedup，for，asIsaidatstarting，youhave，underordinarycircumstancesinwarmweather，merelytoexposesomefluidcontainingasolutionofsugar，oranyformofsyruporvegetablejuicetotheair，inorder，afteracomparativelyshorttime，toseeallthesephenomenaoffermentation。Ofcoursethefirstobvioussuggestionis，thatthetorulahasbeengeneratedwithinthefluid。Infact，itseemsatfirstquiteabsurdtoentertainanyotherconviction；
　　butthatbeliefwouldmostassuredlybeanerroneousone。
　　Towardsthebeginningofthiscentury，inthevigoroustimesoftheoldFrenchwars，therewasaMonsieurAppert，whohadhisattentiondirectedtothepreservationofthingsthatordinarilyperish，suchasmeatsandvegetables，andinfacthelaidthefoundationofourmodernmethodofpreservingmeats；andhefoundthatifheboiledanyofthesesubstancesandthentiedthemsoastoexcludetheair，thattheywouldbepreservedforanytime。Hetriedtheseexperiments，particularlywiththemustofwineandwiththewortofbeer；andhefoundthatifthewortofbeerhadbeencarefullyboiledandwasstoppedinsuchawaythattheaircouldnotgetatit，itwouldneverferment。
　　Whatwasthereasonofthis？That，again，becamethesubjectofalongstringofexperiments，withthisultimateresult，thatifyoutakeprecautionstopreventanysolidmattersfromgettingintothemustofwineorthewortofbeer，underthesecircumstances——thatistosay，ifthefluidhasbeenboiledandplacedinabottle，andifyoustufftheneckofthebottlefullofcottonwool，whichallowstheairtogothroughandstopsanythingofasolidcharacterhoweverfine，thenyoumayletitbefortenyearsanditwillnotferment。Butifyoutakethatplugoutandgivetheairfreeaccess，then，soonerorlaterfermentationwillsetup。Andthereisnodoubtwhateverthatfermentationisexcitedonlybythepresenceofsometorulaorother，andthatthattorulaproceedsinourpresentexperience，frompre-existingtorulae。
　　Theselittlebodiesareexcessivelylight。Youcaneasilyimaginewhatmustbetheweightoflittleparticles，butslightlyheavierthanwater，andnotmorethanthetwo-thousandthorperhapsseven-thousandthofaninchindiameter。Theyarecapableoffloatingaboutanddancinglikemotesinthesunbeam；theyarecarriedaboutbyallsortsofcurrentsofair；thegreatmajorityofthemperish；butoneortwo，whichmaychancetoenterintoasugarysolution，immediatelyenterintoactivelife，findtheretheconditionsoftheirnourishment，increaseandmultiply，andmaygiverisetoanyquantitywhateverofthissubstanceyeast。
　　And，whatevermaybetrueornotbetrueaboutthis"spontaneousgeneration，"asitiscalledinregardtoallotherkindsoflivingthings，itisperfectlycertain，asregardsyeast，thatitalwaysowesitsorigintothisprocessoftransportationorinoculation，ifyoulikesotocallit，fromsomeotherlivingyeastorganism；andsofarasyeastisconcerned，thedoctrineofspontaneousgenerationisabsolutelyoutofcourt。Andnotonlyso，buttheyeastmustbealiveinordertoexertthesepeculiarproperties。Ifitbecrushed，ifitbeheatedsofarthatitslifeisdestroyed，thatpeculiarpoweroffermentationisnotexcited。Thuswehavecometothisconclusion，astheresultofourinquiry，thatthefermentationofsugar，thesplittingofthesugarintoalcoholandcarbonicacid，glycerine，andsuccinicacid，istheresultofnothingbutthevitalactivityofthislittlefungus，thetorula。
　　Andnowcomesthefurtherexceedinglydifficultinquiry——howisitthatthisplant，thetorula，producesthissingularoperationofthesplittingupofthesugar？Fabroni，towhomIreferredsometimeago，imaginedthattheeffervescenceoffermentationwasproducedinjustthesamewayastheeffervescenceofasedlitzpowder，thattheyeastwasakindofacid，andthatthesugarwasacombinationofcarbonicacidandsomebasetoformthealcohol，andthattheyeastcombinedwiththissubstance，andsetfreethecarbonicacid；justaswhenyouaddcarbonateofsodatoacidyouturnoutthecarbonicacid。ButofcoursethediscoveryofLavoisierthatthecarbonicacidandthealcoholtakentogetherareverynearlyequalinweighttothesugar，completelyupsetthishypothesis。AnotherviewwasthereforetakenbytheFrenchchemist，Thenard，anditisstillheldbyaveryeminentchemist，M。
　　Pasteur，andtheirviewisthis，thattheyeast，sotospeak，eatsalittleofthesugar，turnsalittleofittoitsownpurposes，andbysodoinggivessuchashapetothesugarthattherestofitbreaksupintocarbonicacidandalcohol。
　　Well，then，thereisathirdhypothesis，whichismaintainedbyanotherverydistinguishedchemist，Liebig，whichdenieseitheroftheothertwo，andwhichdeclaresthattheparticlesofthesugarare，asitwere，shakenasunderbytheforcesatworkintheyeastplant。NowI
　　amnotgoingtotakeyouintotheserefinementsofchemicaltheory，I
　　cannotforamomentpretendtodoso，butImayputthecasebeforeyoubyananalogy。Supposeyoucomparethesugartoacardhouse，andsupposeyoucomparetheyeasttoachildcomingnearthecardhouse，thenFabroni'shypothesiswasthatthechildtookhalfthecardsaway；
　　Thenard'sandPasteur'shypothesisisthatthechildpullsoutthebottomcardandthusmakesittumbletopieces；andLiebig'shypothesisisthatthechildcomesbyandshakesthetableandtumblesthehousedown。IappealtomyfriendhereProfessorRoscoewhetherthatisnotafairstatementofthecase。
　　Havingthus，asfarasIcan，discussedthegeneralstateofthequestion，itremainsonlythatIshouldspeakofsomeofthosecollateralresultswhichhavecomeinaveryremarkablewayoutoftheinvestigationofyeast。Itoldyouthatitwasveryearlyobservedthattheyeastplantconsistedofabagmadeupofthesamematerialasthatwhichcomposeswood，andofaninteriorsemifluidmasswhichcontainsasubstance，identicalinitscomposition，inabroadsense，withthatwhichconstitutesthefleshofanimals。Subsequently，afterthestructureoftheyeastplanthadbeencarefullyobserved，itwasdiscoveredthatallplants，highandlow，aremadeupofseparatebagsor"cells，"astheyarecalled；thesebagsorcellshavingthecompositionofthepurematterofwood；having
　　thesamecomposition，broadlyspeaking，asthesacoftheyeastplant，andhavingintheirinterioramoreorlessfluidsubstancecontainingamatterofthesamenatureastheproteinsubstanceoftheyeastplant。Andthereforethisremarkableresultcameout——thathowevermuchaplantmaydifferfromananimal，yetthattheessentialconstituentofthecontentsofthesevariouscellsorsacsofwhichtheplantismadeup，thenitrogenousproteinmatter，isthesameintheanimalasintheplant。Andnotonlywasthisgraduallydiscovered，butitwasfoundthatthesesemifluidcontentsoftheplantcellhad，inmanycases，aremarkablepowerofcontractilityquitelikethatofthesubstanceofanimals。Andabout24
　　or25yearsago，namely，abouttheyear1846，tothebestofmyrecollection，averyeminentGermanbotanist，HugoVonMohl，conferreduponthissubstancewhichisfoundintheinterioroftheplantcell，andwhichisidenticalwiththematterfoundintheinsideoftheyeastcell，andwhichagaincontainsananimalsubstancesimilartothatofwhichweourselvesaremadeup——heconferreduponthisthattitleof"protoplasm，"whichhasbroughtotherpeopleagreatdealoftroublesince!Ibegparticularlytosaythat，becauseIfindmanypeoplesupposethatIwastheinventorofthatterm，
　　whereasithasbeeninexistenceforatleasttwenty-fiveyears。Andthenotherobservers，takingthequestionup，cametothisastonishingconclusionworkingfromthisbasisoftheyeast，thatthedifferencesbetweenanimalsandplantsarenotsomuchinthefundamentalsubstanceswhichcomposethem，notintheprotoplasm，butinthemannerinwhichthecellsofwhichtheirbodiesarebuiltuphavebecomemodified。Thereisasenseinwhichitistrue——andtheanalogywaspointedoutverymanyyearsagobysomeFrenchbotanistsandchemists——thereisasenseinwhichitistruethateveryplantissubstantiallyanenormousaggregationofbodiessimilartoyeastcells，eachhavingtoacertainextentitsownindependentlife。Andthereisasenseinwhichitisalsoperfectlytrue——althoughitwouldbeimpossibleformetogivethestatementtoyouwithproperqualificationsandlimitationsonanoccasionlikethis——butthereisalsoasenseinwhichitistruethateveryanimalbodyismadeupofanaggregationofminuteparticlesofprotoplasm，comparableeachofthemtotheindividualseparateyeastplant。Andthosewhoareacquaintedwiththehistoryofthewonderfulrevolutionwhichhasbeenworkedinourwholeconceptionofthesemattersinthelastthirtyyears，willbearmeoutinsayingthatthefirstgermofthem，toaverygreatextent，wasmadetogrowandfructifybythestudyoftheyeastplant，whichpresentsuswithlivingmatterinalmostitssimplestcondition。
　　Thenthereisyetonelastandmostimportantbearingofthisyeastquestion。Thereisonedirectionprobablyinwhichtheeffectsofthecarefulstudyofthenatureoffermentationwillyieldresultsmorepracticallyvaluabletomankindthananyother。LetmerecalltoyourmindsthefactwhichIstatedatthebeginningofthislecture。SupposethatIhadhereasolutionofpuresugarwithalittlemineralmatterinit；andsupposeitwerepossibleformetotakeuponthepointofaneedleonesingle，solitaryyeastcell，measuringnomoreperhapsthanthethree-thousandthofaninchindiameter——notbiggerthanoneofthoselittlecolouredspecksofmatterinmyownbloodatthismoment，theweightofwhichitwouldbedifficulttoexpressinthefractionofagrain——andputitintothissolution。Fromthatsingleone，ifthesolutionwerekeptatafairtemperatureinawarmsummer'sday，therewouldbegenerated，inthecourseofaweek，enoughtorulaetoformascumatthetopandtoformleesatthebottom，andtochangetheperfectlytastelessandentirelyharmlessfluid，syrup，intoasolutionimpregnatedwiththepoisonousgascarbonicacid，impregnatedwiththepoisonoussubstancealcohol；andthat，invirtueofthechangesworkeduponthesugarbythevitalactivityoftheseinfinitesimallysmallplants。Nowyouseethatthisisacaseofinfection。Andfromthetimethatthephenomenonoffermentationwerefirstcarefullystudied，ithasconstantlybeensuggestedtothemindsofthoughtfulphysiciansthattherewasasomethingastoundinglysimilarbetweenthisphenomenaofthepropagationoffermentationbyinfectionandcontagion，andthephenomenaofthepropagationofdiseasesbyinfectionandcontagion。
　　Outofthissuggestionhasgrownthatremarkabletheoryofmanydiseaseswhichhasbeencalledthe"germtheoryofdisease，"theidea，infact，thatweoweagreatmanydiseasestoparticleshavingacertainlifeoftheirown，andwhicharecapableofbeingtransmittedfromonelivingbeingtoanother，exactlyastheyeastplantiscapableofbeingtransmittedfromonetumblerofsaccharinesubstancetoanother。Andthatisaperfectlytenablehypothesis，onewhichinthepresentstateofmedicineoughttobeabsolutelyexhaustedandshownnottobetrue，untilwetaketootherswhichhavelessanalogyintheirfavour。Andtherearesomediseasesmostassuredlyinwhichitturnsouttobeperfectlycorrect。Therearesomeformsofwhatarecalledmalignantcarbunclewhichhavebeenshowntobeactuallyeffectedbyasortoffermentation
　　ifImayusethephrase，byasortofdisturbanceanddestructionofthefluidsoftheanimalbody，setupbyminuteorganismswhicharethecauseofthisdestructionandofthisdisturbance；andonlyrecentlythestudyofthephenomenawhichaccompanyvaccinationhasthrownanimmenselightinthisdirection，tendingtoshowbyexperimentsofthesamegeneralcharacterasthattowhichIreferredasperformedbyHelmholz，thatthereisamostastonishinganalogybetweenthecontagionofthathealingdiseaseandthecontagionofdestructivediseases。Forithasbeenmadeoutquiteclearly，byinvestigationscarriedoninFranceandinthiscountry，thattheonlypartofthevaccinematterwhichiscontagious，whichiscapableofcarryingonitsinfluenceintheorganismofthechildwhoisvaccinated，isthesolidparticlesandnotthefluid。Byexperimentsofthemostingeniouskind，thesolidpartshavebeenseparatedfromthefluidparts，andithasthenbeendiscoveredthatyoumayvaccinateachildasmuchasyoulikewiththefluidparts，butnoeffecttakesplace，thoughanexcessivelysmallportionofthesolidparticles，themostminutethatcanbeseparated，isamplysufficienttogiverisetoall
　　thephenomenaofthecowpock，byaprocesswhichwecancomparetonothingbutthetransmissionoffermentationfromonevesselintoanother，bythetransporttotheoneofthetorulaparticleswhichexistintheother。Andithasbeenshowntobetrueofsomeofthemostdestructivediseaseswhichinfectanimals，suchdiseasesasthesheeppox，suchdiseasesasthatmostterribleanddestructivedisorderofhorses，glanders，thatinthese，also，theactivepoweristhelivingsolidparticle，andthattheinertpartisthefluid。However，donotsupposethatIampushingtheanalogytoofar。Idonotmeantosaythattheactive，solidpartsinthesediseasedmattersareofthesamenatureaslivingyeastplants；but，sofarasitgoes，thereisamostsurprisinganalogybetweenthetwo；andthevalueoftheanalogyisthis，thatbyfollowingitoutwemaysometimeorothercometounderstandhowthesediseasesarepropagated，justasweunderstand，now，aboutfermentation；andthat，inthisway，someofthegreatestscourgeswhichafflictthehumanracemaybe，ifnotprevented，atleastlargelyalleviated。
　　ThisistheconclusionofthestatementswhichIwishedtoputbeforeyou。Youseewehavenotbeenabletohaveanyaccessories。Ifyouwillcomeinsuchnumberstohearalectureofthiskind，allIcansayis，thatdiagramscannotbemadebigenoughforyou，andthatitisnotpossibletoshowanyexperimentsillustrativeofalectureonsuchasubjectasIhavetodealwith。Ofcoursemyfriendsthechemistsandphysicistsareverymuchbetteroff，becausetheycannotonlyshowyouexperiments，butyoucansmellthemandhearthem!Butinmycasesuchaidsarenotattainable，andthereforeIhavetakenasimplesubjectandhavedealtwithitinsuchawaythatIhopeyouallunderstandit，atleastsofarasIhavebeenabletoputitbeforeyouinwords；andhavingonceapprehendedsuchoftheideasandsimplefactsofthecaseasitwaspossibletoputbeforeyou，youcanseeforyourselvesthegreatandwonderfulissuesofsuchanapparentlyhomelysubject。