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。