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。