Theyrisebeforeme，thedeadandtheliving，inthemidstofthemostgratefulrecollections。Thefair，manlyfaceandstatelyfigureofmyfriend，Dr。SamuelParkman，himselffitforthehighestofficesofteaching，yetwillingtobemyfaithfulassistantinthetimeofneed，comebacktomewiththelongsighofregretforhisearlylosstoourearthlycompanionship。EveryyearIspeaktheeulogyofDr。
　　Ainsworth’spatienttoilasIshowhiselaboratepreparations：WhenI
　　takedownmy“AmericanCyclopaedia“andborrowinstructionfromthelearnedarticlesofDr。Kneeland，Iceasetoregretthathisindefatigableandintelligentindustrywasturnedintoabroaderchannel。AndwhatcanIsaytoocordialofmylongassociatedcompanionandfriend，Dr。Hodges，whoseadmirableskill，workingthroughtheswiftestandsurestfingersthateverheldascalpelamongus，hasdelightedclassafterclass，andfilledourMuseumwithmonumentswhichwillconveyhisnametounborngenerations？
　　Thisdaybelongs，however，nottomyselfandmyrecollections，buttoallofuswhoteachandallofyouwholisten，whetherexpertsinourspecialtiesoralienstotheirmysteries，ortimidneophytesjustenteringtheportalsofthehallofscience。Lookinwithme，then，whileIattempttothrowsomeraysintoitsinterior，whichshallilluminateafewofitspillarsandcornices，andshowatthesametimehowmanynichesandalcovesremainindarkness。
　　SCIENCEisthetopographyofignorance。Fromafewelevatedpointswetriangulatevastspaces，inclosinginfiniteunknowndetails。Wecastthelead，anddrawupalittlesandfromabysseswemayneverreachwithourdredges。
　　Thebestpartofourknowledgeisthatwhichteachesuswhereknowledgeleavesoffandignorancebegins。Nothingmoreclearlyseparatesavulgarfromasuperiormind，thantheconfusioninthefirstbetweenthelittlethatittrulyknows，ontheonehand，andwhatithalfknowsandwhatitthinksitknowsontheother。
　　Thatwhichistrueofeverysubjectisespeciallytrueofthebranchofknowledgewhichdealswithlivingbeings。Theirexistenceisaperpetualdeathandreanimation。Theiridentityisonlyanidea，forweputoffourbodiesmanytimesduringourlives，anddressinnewsuitsofbonesandmuscles。
　　“Thouartnotthyself；
　　Forthouexist’stonmanyathousandgrainsThatissueoutofdust。“
　　Ifitistruethatweunderstandourselvesbutimperfectlyinhealth，thistruthismoresignallymanifestedindisease，wherenaturalactionsimperfectlyunderstood，disturbedinanobscurewaybyhalf-
　　seencauses，arecreepingandwindingalonginthedarktowardtheirdestinedissue，sometimesusingourremediesassafestepping-stones，occasionally，itmaybe，stumblingoverthemasobstacles。
　　Iproposeinthislecturetoshowyousomepointsofcontactbetweenourignoranceandourknowledgeinseveralofthebranchesuponthestudyofwhichyouareentering。Imayteachyouaverylittledirectly，butIhopemuchmorefromthetrainsofthoughtIshallsuggest。Donotexpecttoomuchgroundtobecoveredinthisrapidsurvey。Ourtaskisonlythatofsendingoutafewpicketsunderthestarryflagofsciencetotheedgeofthatdarkdomainwheretheensignsoftheobstinaterebel，Ignorance，areflyingundisputed。Wearenotmakingareconnoissanceinforce，stilllessadvancingwiththemaincolumn。Buthereareafewroadsalongwhichwehavetomarchtogether，andwewishtoseeclearlyhowfarourlinesextend，andwheretheenemy’soutpostsbegin。
　　Beforetouchingthebranchesofknowledgethatdealwithorganizationandvitalfunctions，letusglanceatthatsciencewhichmeetsyouatthethresholdofyourstudy，andpreparesyouinsomemeasuretodealwiththemorecomplexproblemsofthelivinglaboratory。
　　CHEMISTRY。includestheartofseparatingandcombiningtheelementsofmatter，andthestudyofthechangesproducedbytheseoperations。
　　Wecanhardlysaytoomuchofwhatithascontributedtoourknowledgeoftheuniverseandourpowerofdealingwithitsmaterials。Ithasgivenusacatalogueraisonneofthesubstancesfounduponourplanet，andshownhoweverythinglivinganddeadisputtogetherfromthem。Itisaccomplishingwondersbeforeuseveryday，suchasArabianstory-tellersusedtostringtogetherintheirfables。Itspreadsthe，sensitivefilmontheartificialretinawhichlooksuponusthroughtheoptician’slensforafewseconds，andfixesanimagethatwilloutliveitsoriginal。Itquestionsthelightofthesun，anddetectsthevaporizedmetalsfloatingaroundthegreatluminary，——iron，sodium，lithium，andtherest，——asifthechemistofourremoteplanetcouldfillhisbell-glassesfromitsfieryatmosphere。Itlendsthepowerwhichflashesourmessagesinthrillsthatleavethelazychariotofdaybehindthem。Itsealsupafewdarkgrainsinironvases，andlo！atthetouchofasinglespark，risesinsmokeandflameamightyAfritwithavoicelikethunderandanarmthatshatterslikeanearthquake。ThedreamsofOrientalfancyhavebecomethesoberfactsofourevery-daylife，andthechemististhemagiciantowhomweowethem。
　　Toreturntothecolderscientificaspectofchemistry。Ithasshownushowbodiesstandaffectedtoeachotherthroughanalmostboundlessrangeofcombinations。Ithasgivenusamostingenioustheorytoaccountforcertainfixedrelationsinthesecombinations。
　　Ithassuccessfullyeliminatedagreatnumberofproximatecompounds，moreorlessstable，fromorganicstructures。Ithasinventedotherswhichformthebasisoflongseriesofwell-knowncompositesubstances。Infact，weareperhapsbecomingoverburdenedwithourlistofproximateprinciples，demonstratedandhypothetical。
　　HowmuchnearerhavewecometothesecretofforcethanLullyandGeberandthewholecrewofjugglingalchemists？Wehavelearnedagreatdealaboutthehow，whathavewelearnedaboutthewhy？
　　Whydoesironrust，whilegoldremainsuntarnished，andgoldamalgamate，whileironrefusestheallianceofmercury？
　　ThealchemistscalledgoldSol，thesun，andironMars，andpleasedthemselveswithfanciedrelationsbetweenthesesubstancesandtheheavenlybodies，bywhichtheypretendedtoexplainthefactstheyobserved。Someoftheirsuperstitionshavelingeredinpracticalmedicinetothepresentday，butchemistryhasgrownwiseenoughtoconfessthefactofabsoluteignorance。
　　Whatisitthatmakescommonsaltcrystallizeintheformofcubes，andsaltpetreintheshapeofsix-sidedprisms？Weseenoreasonwhyitshouldnothavebeenjusttheotherway，saltinprismsandsaltpetreincubes，orwhyeithershouldtakeanexactgeometricaloutline，anymorethancoagulatingalbumen。
　　Butalthoughwehadgivenupattemptingtoexplaintheessentialnatureofaffinitiesandofcrystallinetypes，wemighthavesupposedthatwehadatleastfixedtheidentityofthesubstanceswithwhichwedeal，anddeterminedthelawsoftheircombination。Allatoncewefindthatasimplesubstancechangesface，putsoffitscharacteristicqualitiesandresumesthematwill；——notmerelywhenweliquefyorvaporizeasolid，orreversetheprocess；butthatasolidisliterallytransformedintoanothersolidunderourowneyes。
　　Wethoughtweknewphosphorus。Wewarmaportionofitsealedinanemptytube，foraboutaweek。Ithasbecomeabrowninfusiblesubstance，whichdoesnotshineinthedarknoroxidateintheair。
　　Weheatitto500F。，anditbecomescommonphosphorusagain。Wetransmutesulphurinthesamesingularway。Nature，youknow，givesuscarbonintheshapeofcoalandinthatofthediamond。Itiseasytocallthesechangesbythenameallotropism，butnotthelessdotheyconfoundourhastygeneralizations。
　　Thesefactsofallotropismhavesomecorollariesconnectedwiththemratherstartlingtousofthenineteenthcentury。Theremaybeothertransmutationspossiblebesidesthoseofphosphorusandsulphur。
　　WhenDr。Prout，in1840，talkedaboutazoteandcarbonbeing“formed“
　　inthelivingsystem，itwaslookeduponasoneofthosefreaksoffancytowhichphilosophers，likeothermen，aresubject。ButwhenProfessorFaraday，in1851，says，atameetingoftheBritishAssociation，that“hishopesareinthedirectionofprovingthatbodiescalledsimplewerereallycompounds，andmaybeformedartificiallyassoonaswearemastersofthelawsinfluencingtheircombinations，“——whenhecomesforwardandsaysthathehastriedexperimentsattransmutation，andmeans，ifhislifeisspared，totrythemagain，——howcanwebesurprisedatthepopularstoryof1861，thatLouisNapoleonhasestablishedagold-factoryandisgluttingthemintsofEuropewithbullionofhisownmaking？
　　Andsowithreferencetothelawofcombinations。Theoldmaximwas，Corporanonaguntnisisoluta。Iftwosubstances，aandb，areinclosedinaglassvessel，c，wedonotexpecttheglasstochangethem，unlessaorborthecompoundabhasthepowerofdissolvingtheglass。ButifforaItakeoxygen，forbhydrogen，andforcapieceofspongyplatinum，Ifindthefirsttwocombinewiththecommonsignsofcombustionandformwater，thethirdinthemeantimeundergoingnoperceptiblechange。Ithasplayedthepartoftheunweddedpriest，whomarriesapairwithouttakingafeeorhavinganyfurtherrelationwiththeparties。Wecallthiscatalysis，catalyticaction，theactionofpresence，orbywhatlearnednamewechoose。Givewhatnametoitwewill，itisamanifestationofpowerwhichcrossesourestablishedlawsofcombinationataveryopenangleofintersection。Ithinkwemayfindananalogyforitinelectricalinduction，thedisturbanceoftheequilibriumoftheelectricityofabodybytheapproachofachargedbodytoit，withoutinterchangeofelectricalconditionsbetweenthetwobodies。
　　Butananalogyisnotanexplanation，andwhyafewdropsofyeastshouldchangeasaccharinemixturetocarbonicacidandalcohol，——alittleleavenleaveningthewholelump，——notbycombiningwithit，butbysettingamovementatwork，wenotonlycannotexplain，butthefactissuchanexceptiontotherecognizedlawsofcombinationthatLiebigisunwillingtoadmitthenewforceatalltowhichBerzeliushadgiventhenamesogenerallyaccepted。
　　Thephenomenaofisomerism，oridentityofcompositionandproportionsofconstituentswithdifferenceofqualities，andofisomorphism，oridentityofformincrystalswhichhaveoneelementsubstitutedforanother，wereequallysurprisestoscience；andalthoughthemechanismbywhichtheyarebroughtaboutcanbetoacertainextentexplainedbyareferencetothehypotheticalatomsofwhichtheelementsareconstituted，yetthisisonlyturningthedifficultyintoafractionwithaninfinitesimaldenominatorandaninfinitenumerator。
　　Sofarwehavestudiedtheworkingofforceanditsseeminganomaliesinpurelychemicalphenomena。Butwesoonfindthatchemicalforceisdevelopedbyvariousotherphysicalagencies，——byheat，bylight，byelectricity，bymagnetism，bymechanicalagencies；and，viceversa，thatchemicalactiondevelopsheat，light，electricity，magnetism，mechanicalforce，asweseeinourmatches，galvanicbatteries，andexplosivecompounds。Proceedingwithourexperiments，wefindthateverykindofforceiscapableofproducingallotherkinds，or，inMr。Faraday’slanguage，that“thevariousformsunderwhichtheforcesofmatteraremademanifesthaveacommonorigin，or，inotherwords，aresodirectlyrelatedandmutuallydependentthattheyareconvertibleoneintoanother。“
　　Outofthisdoctrinenaturallyspringsthatoftheconservationofforce，soablyillustratedbyMr。Grove，Dr。Carpenter，andMr。
　　Faraday。Thisideaisnonovelty，thoughitseemssoatfirstsight。
　　Itwasmaintainedanddisputedamongthegiantsofphilosophy。
　　DesCartesandLeibnitzdeniedthatanynewmotionoriginatedinnature，orthatanyeverceasedtoexist；allmotionbeinginacircle，passingfromonebodytoanother，onelosingwhattheothergained。Newton，ontheotherhand，believedthatnewmotionsweregeneratedandexistingonesdestroyed。Onthefirstsupposition，thereisafixedamountofforcealwayscirculatingintheuniverse。
　　Onthesecond，thetotalamountmaybeincreasingordiminishing。
　　Youwillfindinthe“AnnualofScientificDiscovery“for1858averyinterestinglecturebyProfessorHelmholtzofBonn，inwhichitismaintainedthatacertainportionofforceislostineverynaturalprocess，beingconvertedintounchangeableheat，sothattheuniversewillcometoastand-stillatlast，allforcepassingintoheat，andallheatintoastateofequilibrium。
　　Thedoctrinesoftheconvertibilityorspecificequivalenceofthevariousformsofforce，andofitsconservation，whichisitslogicalconsequence，areverygenerallyaccepted，asIbelieve，atthepresenttime，amongphysicists。Wearenaturallyledtothequestion，Whatisthenatureofforce？ThethreeillustriousphilosophersjustreferredtoagreeinattributingthegeneralmovementsoftheuniversetotheimmediateDivineaction。Thedoctrineof“preestablishedharmony“wasanespecialcontrivanceofLeibnitztoremovetheCreatorfromunworthyassociationwiththelessdivineactsoflivingbeings。Obsoleteasthisexpressionsoundstoourears，thephraselawsoftheuniverse，whichweusesoconstantlywithawiderapplication，appearstomeessentiallyidenticalwithit。