Volta’sannouncementofhispilewasscarcelytwomonthsoldwhentwoEnglishmen，Messrs。NicholsonandCarlisle，madethediscoverythatthecurrentfromthegalvanicbatteryhadadecidedeffectuponcertainchemicals，amongotherthingsdecomposingwaterintoitselements，hydrogenandoxygen。OnMay7，1800，theseinvestigatorsarrangedtheendsoftwobrasswiresconnectedwiththepolesofavoltaicpile，composedofalternatesilverandzincplates，sothatthecurrentcomingfromthepilewasdischargedthroughasmallquantityof"NewRiverwater。""A
　　finestreamofminutebubblesimmediatelybegantoflowfromthepointofthelowerwireinthetubewhichcommunicatedwiththesilver，"wroteNicholson，"andtheoppositepointoftheupperwirebecametarnished，firstdeeporangeandthenblack……"Theproductofgasduringtwohoursandahalfwastwo-
　　thirtiethsofacubicinch。"Itwasthenmixedwithanequalquantityofcommonair，"continuesNicholson，"andexplodedbytheapplicationofalightedwaxenthread。"
　　Thisdemonstrationwasthebeginningoftheveryimportantscienceofelectro-chemistry。
　　TheimportanceofthisdiscoverywasatoncerecognizedbySirHumphryDavy，whobeganexperimentingimmediatelyinthisnewfield。Heconstructedaseriesofbatteriesinvariouscombinations，withwhichheattackedthe"fixedalkalies，"thecompositionofwhichwasthenunknown。Veryshortlyhewasabletodecomposepotashintobrightmetallicglobules，resemblingquicksilver。Thisnewsubstancehenamed"potassium。"Theninrapidsuccessiontheelementarysubstancessodium，calcium，strontium，andmagnesiumwereisolated。
　　Itwassoondiscovered，also，thatthenewelectricity，liketheold，possessedheatingpowerundercertainconditions，eventothefusingofpiecesofwire。ThisobservationwasprobablyfirstmadebyFrommsdorff，butitwaselaboratedbyDavy，whoconstructedabatteryoftwothousandcellswithwhichheproducedabrightlightfrompointsofcarbon——theprototypeofthemodernarclamp。HemadethisdemonstrationbeforethemembersoftheRoyalInstitutionin1810。
　　Butthepracticalutilityofsuchalightforilluminatingpurposeswasstillathingofthefuture。Theexpenseofconstructingandmaintainingsuchanelaboratebattery，andtherapidinternaldestructionofitsplates，togetherwiththeconstantpolarization，rendereditsuseinpracticalilluminationoutofthequestion。ItwasnotuntilanothermethodofgeneratingelectricitywasdiscoveredthatDavy’sdemonstrationcouldbeturnedtopracticalaccount。
　　InDavy’sownaccountofhisexperimenthesays:
　　"Whenpiecesofcharcoalaboutaninchlongandone-sixthofaninchindiameterwerebroughtneareachotherwithinthethirtiethorfortiethofaninch，abrightsparkwasproduced，andmorethanhalfthevolumeofthecharcoalbecameignitedtowhiteness；
　　and，bywithdrawingthepointsfromeachother，aconstantdischargetookplacethroughtheheatedair，inaspaceequaltoatleastfourinches，producingamostbrilliantascendingarchoflight，broadandconicalinforminthemiddle。Whenanysubstancewasintroducedintothisarch，itinstantlybecameignited；
　　platinameltedasreadilyinitaswaxinacommoncandle；
　　quartz，thesapphire，magnesia，lime，allenteredintofusion；fragmentsofdiamondandpointsofcharcoalandplumbagoseemedtoevaporateinit，evenwhentheconnectionwasmadeinthereceiverofanair-pump；buttherewasnoevidenceoftheirhavingpreviouslyundergonefusion。Whenthecommunicationbetweenthepointspositivelyandnegativelyelectrifiedwasmadeintheairrarefiedinthereceiveroftheair-pump，thedistanceatwhichthedischargetookplaceincreasedastheexhaustionwasmade；andwhentheatmosphereinthevesselsupportedonlyone-
　　fourthofaninchofmercuryinthebarometricalgauge，thesparkspassedthroughaspaceofnearlyhalfaninch；and，bywithdrawingthepointsfromeachother，thedischargewasmadethroughsixorseveninches，producingamostbrilliantcoruscationofpurplelight；
　　thecharcoalbecameintenselyignited，andsomeplatinawireattachedtoitfusedwithbrilliantscintillationsandfellinlargeglobulesupontheplateofthepump。
　　Allthephenomenaofchemicaldecompositionwereproducedwithintenserapiditybythiscombination。"[1]
　　Butthisexperimentdemonstratedanotherthingbesidesthepossibilityofproducingelectriclightandchemicaldecomposition，thisbeingtheheatingpowercapableofbeingproducedbytheelectriccurrent。
　　ThusDavy’sexperimentoffusingsubstanceslaidthefoundationofthemodernelectricfurnaces，whichareofparamountimportanceinseveralgreatcommercialindustries。
　　WhilesomeoftheresultsobtainedwithDavy’sbatterieswerepracticallyassatisfactoryascouldbeobtainedwithmoderncellbatteries，thebatteriesthemselveswereanythingbutsatisfactory。Theywereexpensive，requiredconstantcareandattention，and，whatwasmoreimportantfromanexperimentalstandpointatleast，werenotconstantintheiractionexceptforaverylimitedperiodoftime，thecurrentsoon"runningdown。"Numerousexperimenters，therefore，setaboutdevisingasatisfactorybattery，andwhen，in1836，JohnFrederickDaniellproducedthecellthatbearshisname，hisinventionwasepoch-
　　makinginthehistoryofelectricalprogress。TheRoyalSocietyconsidereditofsufficientimportancetobestowtheCopleymedalupontheinventor，whosedeviceisthedirectparentofallmoderngalvaniccells。
　　FromthetimeoftheadventoftheDaniellcellexperimentsinelectricitywererenderedcomparativelyeasy。Inthemeanwhile，however，anothergreatdiscoverywasmade。
　　ELECTRICITYANDMAGNETISM
　　Formanyyearstherehadbeenagrowingsuspicion，amountinginmanyinstancestobeliefinthecloserelationshipexistingbetweenelectricityandmagnetism。
　　Beforethewinterof1815，however，itwasabeliefthatwassurmisedbutnotdemonstrated。ButinthatyearitoccurredtoJeanChristianOersted，ofDenmark，topassacurrentofelectricitythroughawireheldparallelwith，butnotquitetouching，asuspendedmagneticneedle。Theneedlewasinstantlydeflectedandswungoutofitsposition。
　　"ThefirstexperimentsinconnectionwiththesubjectwhichIamundertakingtoexplain，"wroteOersted，"weremadeduringthecourseoflectureswhichIheldlastwinteronelectricityandmagnetism。Fromthoseexperimentsitappearedthatthemagneticneedlecouldbemovedfromitspositionbymeansofagalvanicbattery——onewithaclosedgalvaniccircuit。
　　Since，however，thoseexperimentsweremadewithanapparatusofsmallpower，Iundertooktorepeatandincreasethemwithalargegalvanicbattery。
　　"Letussupposethatthetwooppositeendsofthegalvanicapparatusarejoinedbyametalwire。ThisIshallalwayscalltheconductorforthesakeofbrevity。
　　Placearectilinearpieceofthisconductorinahorizontalpositionoveranordinarymagneticneedlesothatitisparalleltoit。Themagneticneedlewillbesetinmotionandwilldeviatetowardsthewestunderthatpartoftheconductorwhichcomesfromthenegativepoleofthegalvanicbattery。Ifthewireisnotmorethanfour-fifthsofaninchdistantfromthemiddleofthisneedle，thisdeviationwillbeaboutforty-fivedegrees。
　　Atagreaterdistancetheangleofdeviationbecomesless。Moreover，thedeviationvariesaccordingtothestrengthofthebattery。Theconductorcanbemovedtowardstheeastorwest，solongasitremainsparalleltotheneedle，withoutproducinganyotherresultthantomakethedeviationsmaller。
　　"Theconductorcanconsistofseveralcombinedwiresormetalcoils。Thenatureofthemetaldoesnotaltertheresultexcept，perhaps，tomakeitgreaterorless。Wehaveusedwiresofplatinum，gold，silver，brass，andiron，andcoilsoflead，tin，andquicksilverwiththesameresult。Iftheconductorisinterruptedbywater，alleffectisnotcutoff，unlessthestretchofwaterisseveralincheslong。
　　"Theconductorworksonthemagneticneedlethroughglass，metals，wood，water，andresin，throughclayvesselsandthroughstone，forwhenweplacedaglassplate，ametalplate，oraboardbetweentheconductorandtheneedletheeffectwasnotcutoff；eventhethreetogetherseemedhardlytoweakentheeffect，andthesamewasthecasewithanearthenvessel，evenwhenitwasfullofwater。Ourexperimentsalsodemonstratedthatthesaideffectswerenotalteredwhenweusedamagneticneedlewhichwasinabrasscasefullofwater。
　　"Whentheconductorisplacedinahorizontalplaneunderthemagneticneedlealltheeffectswehavedescribedtakeplaceinpreciselythesameway，butintheoppositedirectiontowhattookplacewhentheconductorwasinahorizontalplaneabovetheneedle。
　　"Iftheconductorismovedinahorizontalplanesothatitgraduallymakesever-increasingangleswiththemagneticmeridian，thedeviationofthemagneticneedlefromthemagneticmeridianisincreasedwhenthewireisturnedtowardstheplaceoftheneedle；itdecreases，ontheotherhand，whenitisturnedawayfromthatplace。
　　"Aneedleofbrasswhichishunginthesamewayasthemagneticneedleisnotsetinmotionbytheinfluenceoftheconductor。Aneedleofglassorrubberlikewiseremainsstaticundersimilarexperiments。Hencetheelectricalconductoraffectsonlythemagneticpartsofasubstance。Thattheelectricalcurrentisnotconfinedtotheconductingwire，butiscomparativelywidelydiffusedinthesurroundingspace，issufficientlydemonstratedfromtheforegoingobservations。"[2]
　　TheeffectofOersted’sdemonstrationisalmostincomprehensible。Byitwasshownthecloserelationshipbetweenmagnetismandelectricity。Itshowedthewaytotheestablishmentofthescienceofelectrodynamics；
　　althoughitwasbytheFrenchsavantAndreMarieAmpere1775-1836thatthesciencewasactuallycreated，andthiswithinthespaceofoneweekafterhearingofOersted’sexperimentindeflectingtheneedle。AmperefirstreceivedthenewsofOersted’sexperimentonSeptember11，1820，andonthe18thofthesamemonthheannouncedtotheAcademythefundamentalprinciplesofthescienceofelectro-dynamics——
　　sevendaysofrapidprogressperhapsunequalledinthehistoryofscience。
　　Ampere’sdistinguishedcountryman，Arago，afewmonthslater，gavethefinishingtouchestoOersted’sandAmpere’sdiscoveries，bydemonstratingconclusivelythatelectricitynotonlyinfluencedamagnet，butactuallyproducedmagnetismunderpropercircumstances——acomplementalfactmostessentialinpracticalmechanicsSomefouryearsafterArago’sdiscovery，Sturgeonmadethefirst"electro-magnet"bywindingasoftironcorewithwirethroughwhichacurrentofelectricitywaspassed。Thisstudyofelectro-magnetswastakenupbyProfessorJosephHenry，ofAlbany，NewYork，whosucceededinmakingmagnetsofenormousliftingpowerbywindingtheironcorewithseveralcoilsofwire。Oneofthesemagnets，excitedbyasinglegalvaniccelloflessthanhalfasquarefootofsurface，andcontainingonlyhalfapintofdiluteacids，sustainedaweightofsixhundredandfiftypounds。
　　ThusbyOersted’sgreatdiscoveryoftheintimaterelationshipofmagnetismandelectricity，withfurtherelaborationsanddiscoveriesbyAmpere，Volta，andHenry，andwiththeinventionofDaniell’scell，thewaywaslaidforputtingelectricitytopracticaluse。
　　Soonfollowedtheinventionandperfectionoftheelectro-magnetictelegraphandahostofotherbutlittlelessimportantdevices。
　　FARADAYANDELECTRO-MAGNETICINDUCTION
　　Withthesegreatdiscoveriesandinventionsathand，electricitybecamenolongeratoyora"playthingforphilosophers，"butofenormousandgrowingimportancecommercially。Still，electricitygeneratedbychemicalaction，eveninaveryperfectcell，wasbothfeebleandexpensive，and，withal，onlyapplicableinacomparativelylimitedfield。Anotherimportantscientificdiscoverywasnecessarybeforesuchthingsaselectrictractionandelectriclightingonalargescaleweretobecomepossible；butthatdiscoverywassoonmadebySirMichaelFaraday。
　　Faraday，thesonofablacksmithandabookbinderbytrade，hadinterestedSirHumphryDavybyhisadmirablenotesonfourofDavy’slectures，whichhehadbeenabletoattend。Althoughadvisedbythegreatscientistto"sticktohisbookbinding"ratherthanenterthefieldofscience，Faradaybecame，attwenty-twoyearsofage，Davy’sassistantintheRoyalInstitution。There，forseveralyears，hedevotedallhissparehourstoscientificinvestigationsandexperiments，perfectinghimselfinscientifictechnique。
　　Afewyearslaterhebecameinterested，likeallthescientistsofthetime，inArago’sexperimentofrotatingacopperdiskunderneathasuspendedcompass-
　　needle。Whenthisdiskwasrotatedrapidly，theneedlewasdeflected，orevenrotatedaboutitsaxis，inamannerquiteinexplicable。Faradayatonceconceivedtheideathatthecauseofthisrotationwasduetoelectricity，inducedintherevolvingdisk——notonlyconceivedit，butputhisbeliefinwriting。Forseveralyears，however，hewasunabletodemonstratethetruthofhisassumption，althoughhemaderepeatedexperimentstoproveit。Butin1831hebeganaseriesofexperimentsthatestablishedforeverthefactofelectro-magneticinduction。
　　Inhisfamouspaper，readbeforetheRoyalSocietyin1831，Faradaydescribesthemethodbywhichhefirstdemonstratedelectro-magneticinduction，andthenexplainedthephenomenonofArago’srevolvingdisk。
　　"Abouttwenty-sixfeetofcopperwire，one-twentiethofaninchindiameter，werewoundroundacylinderofwoodasahelix，"hesaid，"thedifferentspiresofwhichwerepreventedfromtouchingbyathininterposedtwine。Thishelixwascoveredwithcalico，andthenasecondwireappliedinthesamemanner。Inthiswaytwelveheliceswere"superposed，eachcontaininganaveragelengthofwireoftwenty-sevenfeet，andallinthesamedirection。Thefirst，third，fifth，seventh，ninth，andeleventhoftheseheliceswereconnectedattheirextremitiesendtoendsoastoformonehelix；
　　theotherswereconnectedinasimilarmanner；andthustwoprincipalheliceswereproduced，closelyinterposed，havingthesamedirection，nottouchinganywhere，andeachcontainingonehundredandfifty-fivefeetinlengthofwire。
　　Oneoftheseheliceswasconnectedwithagalvanometer，theotherwithavoltaicbatteryoftenpairsofplatesfourinchessquare，withdoublecoppersandwellcharged；yetnottheslightestsensibledeflectionofthegalvanometerneedlecouldbeobserved。
　　"Asimilarcompoundhelix，consistingofsixlengthsofcopperandsixofsoftironwire，wasconstructed。
　　Theresultingironhelixcontainedtwohundredandeightfeet；butwhetherthecurrentfromthetroughwaspassedthroughthecopperortheironhelix，noeffectupontheothercouldbeperceivedatthegalvanometer。
　　"Intheseandmanysimilarexperimentsnodifferenceinactionofanykindappearedbetweenironandothermetals。
　　"Twohundredandthreefeetofcopperwireinonelengthwerepassedroundalargeblockofwood；othertwohundredandthreefeetofsimilarwirewereinterposedasaspiralbetweentheturnsofthefirst，andmetalliccontacteverywherepreventedbytwine。Oneoftheseheliceswasconnectedwithagalvanometerandtheotherwithabatteryofahundredpairsofplatesfourinchessquare，withdoublecoppersandwellcharged。Whenthecontactwasmade，therewasasuddenandveryslighteffectatthegalvanometer，andtherewasalsoasimilarslighteffectwhenthecontactwiththebatterywasbroken。Butwhilstthevoltaiccurrentwascontinuingtopassthroughtheonehelix，nogalvanometricalappearancesofanyeffectlikeinductionupontheotherhelixcouldbeperceived，althoughtheactivepowerofthebatterywasprovedtobegreatbyitsheatingthewholeofitsownhelix，andbythebrilliancyofthedischargewhenmadethroughcharcoal。
　　"Repetitionoftheexperimentswithabatteryofonehundredandtwentypairsofplatesproducednoothereffects；butitwasascertained，bothatthisandattheformertime，thattheslightdeflectionoftheneedleoccurringatthemomentofcompletingtheconnectionwasalwaysinonedirection，andthattheequallyslightdeflectionproducedwhenthecontactwasbrokenwasintheotherdirection；and，also，thattheseeffectsoccurredwhenthefirstheliceswereused。
　　"TheresultswhichIhadbythistimeobtainedwithmagnetsledmetobelievethatthebatterycurrentthroughonewiredid，inreality，induceasimilarcurrentthroughtheotherwire，butthatitcontinuedforaninstantonly，andpartookmoreofthenatureoftheelectricalwavepassedthroughfromtheshockofacommonLeydenjarthanofthatfromavoltaicbattery，and，therefore，mightmagnetizeasteelneedlealthoughitscarcelyaffectedthegalvanometer。
　　"Thisexpectationwasconfirmed；foronsubstitutingasmallhollowhelix，formedroundaglasstube，forthegalvanometer，introducingasteelneedle，makingcontactasbeforebetweenthebatteryandtheinducingwire，andthenremovingtheneedlebeforethebatterycontactwasbroken，itwasfoundmagnetized。
　　"Whenthebatterycontactwasfirstmade，thenanunmagnetizedneedleintroduced，andlastlythebatterycontactbroken，theneedlewasfoundmagnetizedtoanequaldegreeapparentlywiththefirst；butthepoleswereofthecontrarykinds。"[3]
　　ToFaradaytheseexperimentsexplainedthephenomenonofArago’srotatingdisk，thediskinducingthecurrentfromthemagnet，and，inreacting，deflectingtheneedle。Toprovethis，heconstructedadiskthatrevolvedbetweenthepolesofanelectro-magnet，connectingtheaxisandtheedgeofthediskwithagalvanometer。
　　"……Adiskofcopper，twelveinchesindiameter，fixeduponabrassaxis，"hesays，"wasmountedinframessoastoberevolvedeitherverticallyorhorizontally，itsedgebeingatthesametimeintroducedmoreorlessbetweenthemagneticpoles。Theedgeoftheplatewaswellamalgamatedforthepurposeofobtaininggoodbutmovablecontact；apartroundtheaxiswasalsopreparedinasimilarmanner。
　　"Conductorsorcollectorsofcopperandleadwereconstructedsoastocomeincontactwiththeedgeofthecopperdisk，orwithotherformsofplateshereaftertobedescribed。Theseconductorswe’reaboutfourincheslong，one-thirdofaninchwide，andone-fifthofaninchthick；oneendofeachwasslightlygrooved，toallowofmoreexactadaptationtothesomewhatconvexedgeoftheplates，andthenamalgamated。Copperwires，one-sixteenthofaninchinthickness，attachedintheordinarymannerbyconvolutionstotheotherendsoftheseconductors，passedawaytothegalvanometer。
　　"Allthesearrangementsbeingmade，thecopperdiskwasadjusted，thesmallmagneticpolesbeingaboutone-halfaninchapart，andtheedgeoftheplateinsertedabouthalftheirwidthbetweenthem。Oneofthegalvanometerwireswaspassedtwiceorthricelooselyroundthebrassaxisoftheplate，andtheotherattachedtoaconductor，whichitselfwasretainedbythehandincontactwiththeamalgamatededgeofthediskatthepartimmediatelybetweenthemagneticpoles。Underthesecircumstancesallwasquiescent，andthegalvanometerexhibitednoeffect。Buttheinstanttheplatemovedthegalvanometerwasinfluenced，andbyrevolvingtheplatequicklytheneedlecouldbedeflectedninetydegreesormore。"[4]
　　Thisrotatingdiskwasreallyadynamoelectricmachineinminiature，thefirsteverconstructed，butwhosedirectdescendantsaretheordinarydynamos。
　　Moderndynamosrangeinpowerfromlittlemachinesoperatingmachineryrequiringonlyfractionsofahorsepowertogreatdynamosoperatingstreet-carlinesandlightingcities；butallarebuiltonthesameprincipleasFaraday’srotatingdisk。Bythisdiscoverytheuseofelectricityasapracticalandeconomicalmotivepowerbecamepossible。
　　STORAGEBATTERIES
　　WhenthediscoveriesofFaradayofelectro-magneticinductionhadmadepossiblethemeansofeasilygeneratingelectricity，thenextnaturalstepwastofindameansofstoringitoraccumulatingit。This，however，provednoeasymatter，andasyetapracticalstorageorsecondarybatterythatisneithertoocumbersome，toofragile，nortooweakinitsactionhasnotbeeninvented。Ifasatisfactorystoragebatterycouldbemade，itisobviousthatitsrevolutionaryeffectscouldscarcelybeoverestimated。Inthesinglefieldofaeronautics，itwouldprobablysolvethequestionofaerialnavigation。Littlewonder，then，thatinventorshavesoughtsoeagerlyfortheinventionofsatisfactorystoragebatteries。Asearlyas1803Ritterhadattemptedtomakesuchasecondarybattery。In1843Grovealsoattemptedit。Butitwasnotuntil1859，whenGastonPlancheproducedhisinvention，thatanythinglikeareasonablysatisfactorystoragebatterywasmade。Planchediscoveredthatsheetsofleadimmersedindilutesulphuricacidwereverysatisfactoryfortheproductionofpolarizationeffects。Heconstructedabatteryofsheetsofleadimmersedinsulphuricacid，and，afterchargingtheseforseveralhoursfromthecellsofanordinaryBunsenbattery，wasabletogetcurrentsofgreatstrengthandconsiderableduration。Thisbattery，however，fromitsconstructionoflead，wasnecessarilyheavyandcumbersome。
　　Faureimproveditsomewhatbycoatingtheleadplateswithred-lead，thusincreasingthecapacityofthecell。Faure’sinventiongaveafreshimpetustoinventors，andshortlyafterthemarketwasfilledwithstoragebatteriesofvariouskinds，mostofthemmodificationsofPlanche’sorFaure’s。Theardorofenthusiasticinventorssoonflagged，however，forallthesestoragebatteriesprovedoflittlepracticalaccountintheend，ascomparedwithotherknownmethodsofgeneratingpower。
　　Threemethodsofgeneratingelectricityareingeneraluse:staticorfrictionalelectricityisgeneratedby"plate"or"static"machines；galvanic，generatedbybatteriesbasedonVolta’sdiscovery；andinduced，orfaradic，generatedeitherbychemicalormechanicalaction。Thereisstillanotherkind，thermo-electricity，thatmaybegeneratedinamostsimplemanner。In1821Seebecle，ofBerlin，discoveredthatwhenacircuitwasformedoftwowiresofdifferentmetals，iftherebeadifferenceintemperatureatthejunctureofthesetwometalsanelectricalcurrentwillbeestablished。
　　Inthiswayheatmaybetransmitteddirectlyintotheenergyofthecurrentwithouttheinterpositionofthesteam-engine。Batteriesconstructedinthiswayareoflowresistance，however，althoughbyarrangingseveralofthemin"series，"currentsofconsiderablestrengthcanbegenerated。Asyet，however，theyareoflittlepracticalimportance。
　　AboutthemiddleofthecenturyClerk-Maxwelladvancedtheideathatlightwaveswerereallyelectro-
　　magneticwaves。Ifthisweretrueandlightprovedtobesimplyoneformofelectricalenergy，thenthesamewouldbetrueofradiantheat。Maxwelladvancedthistheory，butfailedtosubstantiateitbyexperimentalconfirmation。ButDr。HeinrichHertz，afewyearslater，byaseriesofexperiments，demonstratedthecorrectnessofMaxwell’ssurmises。Whatarenowcalled"Hertzianwaves"arewavesapparentlyidenticalwithlightwaves，butofmuchlowerpitchorperiod。InhisexperimentsHertzshowedthat，underproperconditions，electricsparksbetweenpolishedballswereattendedbyetherwavesofthesamenatureasthoseoflight，butofapitchofseveralmillionsofvibrationspersecond。Thesewavescouldbedealtwithasiftheywerelightwaves——reflected，refracted，andpolarized。
　　Thesearethewavesthatareutilizedinwirelesstelegraphy。
　　ROENTGENRAYS，ORX-RAYS
　　InDecemberof1895wordcameoutofGermanyofascientificdiscoverythatstartledtheworld。Itcamefirstasarumor，littlecredited；thenasapronouncedreport；atlastasademonstration。Ittoldofanewmanifestationofenergy，invirtueofwhichtheinteriorofopaqueobjectsismadevisibletohumaneyes。Onehadonlytolookintoatubecontainingascreenofacertaincomposition，anddirectedtowardsapeculiarelectricalapparatus，toacquireclairvoyantvisionmorewonderfulthanthediscreditedsecond-sightofthemedium。Coinswithinapurse，nailsdrivenintowood，spectacleswithinaleathercase，becameclearlyvisiblewhensubjectedtotheinfluenceofthismagictube；andwhenahumanhandwasheldbeforethetube，itsbonesstoodrevealedinweirdsimplicity，asiftheliving，palpitatingfleshaboutthemwerebuttheshadowysubstanceofaghost。
　　Notonlycouldthehumaneyeseetheseastoundingrevelations，buttheimpartialevidenceofinanimatechemicalscouldbebroughtforwardtoprovethatthemindharborednoillusion。Thephotographicfilmrecordedthethingsthattheeyemightsee，andghostlypicturesgaloresoongaveaquietustothedoubtsofthemostsceptical。WithinamonthoftheannouncementofProfessorRoentgen’sexperimentscommentuponthe"X-ray"andthe"newphotography"hadbecomeapartofthecurrentgossipofallChristendom。
　　Itishardlynecessarytosaythatsucharevolutionarythingasthediscoveryofaprocesswherebyopaqueobjectsbecametransparent，ortranslucent，wasnotachievedatasingleboundwithnointermediatediscoveries。
　　In1859theGermanphysicistJuliusPlucker1801-1868noticedthatwhentherewasanelectricaldischargethroughanexhaustedtubeatalowpressure，onthesurroundingwallsofthetubenearthenegativepole，orcathode，appearedagreenishphosphorescence。
　　Thisdiscoverywassoonbeinginvestigatedbyanumberofotherscientists，amongothersHittorf，Goldstein，andProfessornowSirWilliamCrookes。TheexplanationsgivenofthisphenomenonbyProfessorCrookesconcernusheremoreparticularly，inasmuchashisviewsdidnotaccordexactlywiththoseheldbytheothertwoscientists，andashisresearchesweremoredirectlyconcernedinthediscoveryoftheRoentgenrays。Heheldthattheheatandphosphorescenceproducedinalow-pressuretubewerecausedbystreamsofparticles，projectedfromthecathodewithgreatvelocity，strikingthesidesoftheglasstube。Thecompositionoftheglassseemedtoenterintothisphosphorescencealso，forwhileleadglassproducedbluephosphorescence，sodaglassproducedayellowishgreen。Thecompositionoftheglassseemedtobechangedbyalong-continuedpeltingoftheseparticles，thephosphorescenceafteratimelosingitsinitialbrilliancy，causedbytheglassbecoming"tired，"asProfessorCrookessaid。Thuswhensomeopaquesubstance，suchasiron，isplacedbetweenthecathodeandthesidesoftheglasstubesothatitcastsashadowinacertainspotontheglassforsomelittletime，itisfoundonremovingtheopaquesubstanceorchangingitspositionthattheareaofglassatfirstcoveredbytheshadownowrespondedtotheraysinadifferentmannerfromthesurroundingglass。
　　Thepeculiarray’s，nowknownasthecathoderays，notonlycastashadow，butaredeflectedbyamagnet，sothatthepositionofthephosphorescenceonthesidesofthetubemaybealteredbytheproximityofapowerfulmagnet。Fromthisitwouldseemthattheraysarecomposedofparticleschargedwithnegativeelectricity，andProfessorJ。J。ThomsonhasmodifiedtheexperimentofPerrintoshowthatnegativeelectricityisactuallyassociatedwiththerays。Thereisreasonforbelieving，therefore，thatthecathoderaysarerapidlymovingchargesofnegativeelectricity。Itispossible，also，todeterminethevelocityatwhichtheseparticlesaremovingbymeasuringthedeflectionproducedbythemagneticfield。
　　Fromthefactthatopaquesubstancescastashadowintheseraysitwasthoughtatfirstthatallsolidswereabsolutelyopaquetothem。Hertz，however，discoveredthatasmallamountofphosphorescenceoccurredontheglassevenwhensuchopaquesubstancesasgold-leaforaluminiumfoilwereinterposedbetweenthecathodeandthesidesofthetube。ShortlyafterwardsLenarddiscoveredthatthecathoderayscanbemadetopassfromtheinsideofadischargetubetotheoutsideair。Forconveniencetheseraysoutsidethetubehavesincebeenknownas"Lenardrays。"
　　IntheclosingdaysofDecember，1895，ProfessorWilhelmKonradRoentgen，ofWurzburg，announcedthathehadmadethediscoveryoftheremarkableeffectarisingfromthecathoderaystowhichreferencewasmadeabove。Hefoundthatifaplatecoveredwithaphosphorescentsubstanceisplacednearadischargetubeexhaustedsohighlythatthecathoderaysproducedagreenphosphorescence，thisplateismadetoglowinapeculiarmanner。Theraysproducingthisglowwerenotthecathoderays，althoughapparentlyarisingfromthem，andarewhathavesincebeencalledtheRoentgenrays，orX-rays。
　　Roentgenfoundthatashadowisthrownuponthescreenbysubstancesheldbetweenitandtheexhaustedtube，thecharacteroftheshadowdependinguponthedensityofthesubstance。Thusmetalsarealmostcompletelyopaquetotherays；suchsubstancesasbonemuchlessso，andordinaryfleshhardlysoatall。
　　Ifacoinwereheldinthehandthathadbeeninterposedbetweenthetubeandthescreenthepictureformedshowedthecoinasablackshadow；andthebonesofthehand，whilecastingadistinctshadow，showeddistinctlylighter；whilethesofttissuesproducedscarcelyanyshadowatall。Thevalueofsuchadiscoverywasobviousfromthefirst；andwasstillfurtherenhancedbythediscoverymadeshortlythat，photographicplatesareaffectedbytherays，thusmakingitpossibletomakepermanentphotographicrecordsofpicturesthroughwhatweknowasopaquesubstances。
　　WhataddsmateriallytothepracticalvalueofRoentgen’sdiscoveryisthefactthattheapparatusforproducingtheX-raysisnowsosimpleandrelativelyinexpensivethatitiswithinthereachevenofamateurscientists。Itconsistsessentiallyofaninductioncoilattachedeithertocellsorastreet-currentplugforgeneratingtheelectricity，afocustube，andaphosphorescencescreen。Thesefocustubesaremadeinvariousshapes，butperhapsthemostpopularareintheformofaglassglobe，notunlikeanordinarysmall-sizedwater-bottle，thistubebeingclosedandexhausted，andhavingthetwopolesanodeandcathodesealedintotheglasswalls，butprotrudingateitherendforattachmenttotheconductingwiresfromtheinductioncoil。Thistubemaybemountedonastandataheightconvenientformanipulation。Thephosphorescencescreenisusuallyaplatecoveredwithsomeplatino-cyanideandmountedintheendofaboxofconvenientsize，theoppositeendofwhichissoshapedthatitfitsthecontouroftheface，shuttingoutthelightandallowingtheeyesoftheobservertofocalizeonthescreenattheend。Formakingobservationstheoperatorhassimplytoturnonthecurrentofelectricityandapplythescreentohiseyes，pointingittowardstheglowingtube，whentheshadowofanysubstanceinterposedbetweenthetubeandthescreenwillappearuponthephosphorescenceplate。
　　Thewonderfulshadowpicturesproducedonthephosphorescencescreen，orthephotographicplate，wouldseemtocomefromsomepeculiarformoflight，buttheexactnatureoftheseraysisstillanopenquestion。
　　WhethertheRoentgenraysarereallyaformoflight——thatis，aformof"electro-magneticdisturbancepropagatedthroughether，"isnotfullydetermined。
　　Numerousexperimentshavebeenundertakentodeterminethis，butasyetnoproofhasbeenfoundthattheraysareaformoflight，althoughthereappearstobenothingintheirpropertiesinconsistentwiththeirbeingso。ForthemomentmostinvestigatorsarecontenttoadmitthatthetermX-rayvirtuallybegsthequestionastotheintimatenatureoftheformofenergyinvolved。
　　VIII。THECONSERVATIONOFENERGY
　　Aswehaveseen，itwasin1831thatFaradayopenedupthefieldofmagneto-electricity。Reversingtheexperimentsofhispredecessors，whohadfoundthatelectriccurrentsmaygeneratemagnetism，heshowedthatmagnetshavepowerundercertaincircumstancestogenerateelectricity；heproved，indeed，theinterconvertibilityofelectricityandmagnetism。
　　Thenheshowedthatallbodiesaremoreorlesssubjecttotheinfluenceofmagnetism，andthatevenlightmaybeaffectedbymagnetismastoitsphenomenaofpolarization。Hesatisfiedhimselfcompletelyofthetrueidentityofallthevariousformsofelectricity，andoftheconvertibilityofelectricityandchemicalaction。
　　Thushelinkedtogetherlight，chemicalaffinity，magnetism，andelectricity。And，moreover，heknewfullwellthatnooneofthesecanbeproducedinindefinitesupplyfromanother。"Nowhere，"hesays，"isthereapurecreationorproductionofpowerwithoutacorrespondingexhaustionofsomethingtosupplyit。"
　　WhenFaradaywrotethosewordsin1840hewastreadingontheveryheelsofagreatergeneralizationthananywhichheactuallyformulated；nay，hehaditfairlywithinhisreach。Hesawagreattruthwithoutfullyrealizingitsimport；itwasleftforothers，approachingthesametruthalonganotherpath，topointoutitsfullsignificance。
　　ThegreatgeneralizationwhichFaradaysonarrowlymissedisthetruthwhichsincethenhasbecomefamiliarasthedoctrineoftheconservationofenergy——thelawthatintransformingenergyfromoneconditiontoanotherwecanneversecuremorethananequivalentquantity；that，inshort，"tocreateorannihilateenergyisasimpossibleastocreateorannihilatematter；
　　andthatallthephenomenaofthematerialuniverseconsistintransformationsofenergyalone。"Somephilosophersthinkthisthegreatestgeneralizationeverconceivedbythemindofman。Bethatasitmay，itissurelyoneofthegreatintellectuallandmarksofthenineteenthcentury。Itstandsapart，sostupendousandsofar-reachinginitsimplicationsthatthegenerationwhichfirstsawthelawdevelopedcouldlittleappreciateit；onlynow，throughthevistaofhalfacentury，dowebegintoseeitinitstrueproportions。
　　Avastgeneralizationsuchasthisisneveramushroomgrowth，nordoesitusuallyspringfullgrownfromthemindofanysingleman。Alwaysanumberofmindsareverynearatruthbeforeanyonemindfullygraspsit。Pre-eminentlytrueisthisofthedoctrineoftheconservationofenergy。NotFaradayalone，buthalfadozendifferentmenhadaninklingofitbeforeitgainedfullexpression；indeed，everymanwhoadvocatedtheundulatorytheoryoflightandheatwasvergingtowardsthegoal。ThedoctrineofYoungandFresnelwasasahighwayleadingsurelyontothewideplainofconservation。Thephenomenaofelectro-
　　magnetismfurnishedanothersuchhighway。Buttherewasyetanotherroadwhichledjustassurelyandevenmorereadilytothesamegoal。Thiswastheroadfurnishedbythephenomenaofheat，andthemenwhotravelleditweredestinedtooutstriptheirfellow-workers；though，aswehaveseen，wayfarersonotherroadswerewithinhailingdistancewhentheleaderspassedthemark。
　　Inordertodoevenapproximatejusticetothemenwhoenteredintothegreatachievement，wemustrecallthatjustatthecloseoftheeighteenthcenturyCountRumfordandHumphryDavyindependentlyshowedthatlabormaybetransformedintoheat；andcorrectlyinterpretedthisfactasmeaningthetransformationofmolarintomolecularmotion。Wecanhardlydoubtthateachofthesemenofgeniusrealized——vaguely，atanyrate——thattheremustbeaclosecorrespondencebetweentheamountofthemolarandthemolecularmotions；hencethateachofthemwasinsightofthelawofthemechanicalequivalentofheat。Butneitherofthemquitegraspedorexplicitlystatedwhateachmustvaguelyhaveseen；andforjustaquarterofacenturynooneelseevencameabreasttheirlineofthought，letalonepassingit。
　　Butthen，in1824，aFrenchphilosopher，SadiCarnot，caughtstepwiththegreatEnglishmen，andtookalongleapaheadbyexplicitlystatinghisbeliefthatadefinitequantityofworkcouldbetransformedintoadefinitequantityofheat，nomore，noless。Carnotdidnot，indeed，reachtheclearviewofhispredecessorsastothenatureofheat，forhestillthoughtitaformof"imponderable"fluid；buthereasonednonethelessclearlyastoitsmutualconvertibilitywithmechanicalwork。Butimportantashisconclusionsseemnowthatwelookbackuponthemwithclearervision，theymadenoimpressionwhateveruponhiscontemporaries。
　　Carnot’sworkinthislinewasanisolatedphenomenonofhistoricalinterest，butitdidnotenterintotheschemeofthecompletednarrativeinanysuchwayasdidtheworkofRumfordandDavy。
　　ThemanwhoreallytookupthebrokenthreadwhereRumfordandDavyhaddroppedit，andwoveitintoacompletedtexture，cameuponthescenein1840。
　　HishomewasinManchester，England；hisoccupationthatofamanufacturer。HewasafriendandpupilofthegreatDr。Dalton。HisnamewasJamesPrescottJoule。Whenposterityhasdoneitsfinaljugglingwiththenamesofthenineteenthcentury，itisnotunlikelythatthenameofthisManchesterphilosopherwillbeahouseholdword，likethenamesofAristotle，Copernicus，andNewton。
　　ForJoule’sworkitwas，doneinthefifthdecadeofthecentury，whichdemonstratedbeyondallcavilthatthereisapreciseandabsoluteequivalencebetweenmechanicalworkandheat；thatwhatevertheformofmanifestationofmolarmotion，itcangenerateadefiniteandmeasurableamountofheat，andnomore。
　　Joulefound，forexample，thatatthesea-levelinManchesterapoundweightfallingthroughsevenhundredandseventy-twofeetcouldgenerateenoughheattoraisethetemperatureofapoundofwateronedegreeFahrenheit。Therewasnothinghaphazard，nothingaccidental，aboutthis；itborethestampofunalterablelaw。AndJoulehimselfsaw，whatothersintimeweremadetosee，thatthistruthismerelyaparticularcasewithinamoregenerallaw。Ifheatcannotbeinanysensecreated，butonlymademanifestasatransformationofanotherkindofmotion，thenmustnotthesamethingbetrueofallthoseotherformsof"force"——light，electricity，magnetism——whichhadbeenshowntobesocloselyassociated，somutuallyconvertible，withheat？Allanalogyseemedtourgethetruthofthisinference；allexperimenttendedtoconfirmit。Thelawofthemechanicalequivalentofheatthenbecamethemaincorner-stoneofthegreaterlawoftheconservationofenergy。
　　Butwhilethiscitationisfreshinmind，wemustturnourattentionwithallhastetoacountryacrosstheChannel——toDenmark，inshort——andlearnthatevenasJouleexperimentedwiththetransformationofheat，aphilosopherofCopenhagen，Coldingbyname，hadhituponthesameidea，andcarrieditfartowardsademonstration。Andthen，withoutpausing，wemustshiftyetagain，thistimetoGermany，andconsidertheworkofthreeothermen，whoindependentlywereonthetrackofthesametruth，andtwoofwhom，itmustbeadmitted，reacheditearlierthaneitherJouleorColding，ifneitherbroughtittoquitesoclearademonstration。ThenamesofthesethreeGermansareMohr，Mayer，andHelmholtz。Theirshareinestablishingthegreatdoctrineofconservationmustnowclaimourattention。
　　AstoKarlFriedrichMohr，itmaybesaidthathisstatementofthedoctrineprecededthatofanyofhisfellows，yetthatotherwiseitwasperhapsleastimportant。
　　In1837thisthoughtfulGermanhadgraspedthemaintruth，andgivenitexpressioninanarticlepublishedintheZeitschriftfurPhysik，etc。Butthearticleattractednoattentionwhatever，evenfromMohr’sowncountrymen。Still，Mohr’stitletorankasonewhoindependentlyconceivedthegreattruth，andperhapsconceiveditbeforeanyothermanintheworldsawitasclearly，eventhoughhedidnotdemonstrateitsvalidity，isnottobedisputed。
　　Itwasjustfiveyearslater，in1842，thatDr。JuliusRobertMayer，practisingphysicianinthelittleGermantownofHeilbronn，publishedapaperinLiebig’sAnnalenon"TheForcesofInorganicNature，"inwhichnotmerelythemechanicaltheoryofheat，buttheentiredoctrineoftheconservationofenergy，isexplicitlyifbrieflystated。TwoyearsearlierDr。Mayer，whilesurgeontoaDutchIndiavesselcruisinginthetropics，hadobservedthatthevenousbloodofapatientseemedredderthanvenousbloodusuallyisobservedtobeintemperateclimates。Heponderedoverthisseeminglyinsignificantfact，andatlastreachedtheconclusionthatthecausemustbethelesseramountofoxidationrequiredtokeepupthebodytemperatureinthetropics。Ledbythisreflectiontoconsiderthebodyasamachinedependentonoutsideforcesforitscapacitytoact，hepassedonintoanovelrealmofthought，whichbroughthimatlasttoindependentdiscoveryofthemechanicaltheoryofheat，andtothefirstfullandcomprehensiveappreciationofthegreatlawofconservation。Blood-letting，themodernphysicianholds，wasapracticeofverydoubtfulbenefit，asarule，tothesubject；butonce，atleast，itledtomarvellousresults。Nostrawisgosmallthatitmaynotpointthereceptivemindofgeniustonewandwonderfultruths。
　　MAYER’SPAPEROF1842
　　ThepaperinwhichMayerfirstgaveexpressiontohisrevolutionaryideasborethetitleof"TheForcesofInorganicNature，"andwaspublishedin1842。Itisoneofthegemsofscientificliterature，andfortunatelyitisnottoolongtobequotedinitsentirety。
　　Seldomifeverwasagreatrevolutionarydoctrineexpoundedinbriefercompass:
　　"Whatarewetounderstandby’forces’？andhowaredifferentforcesrelatedtoeachother？Thetermforceconveysforthemostparttheideaofsomethingunknown，unsearchable，andhypothetical；whilethetermmatter，ontheotherhand，impliesthepossession，bytheobjectinquestion，ofsuchdefinitepropertiesasweightandextension。Anattempt，therefore，torendertheideaofforceequallyexactwiththatofmatterisonewhichshouldbewelcomedbyallthosewhodesiretohavetheirviewsofnatureclearandunencumberedbyhypothesis。
　　"Forcesarecauses；andaccordinglywemaymakefullapplicationinrelationtothemoftheprinciplecausaaequateffectum。Ifthecausechastheeffecte，thenc=e；if，initsturn，eisthecauseofasecondeffectoff，wehavee=f，andsoon:c=e=f……=c。
　　Inaseriesofcausesandeffects，atermorapartofatermcannever，asisapparentfromthenatureofanequation，becomeequaltonothing。Thisfirstpropertyofallcauseswecalltheirindestructibility。
　　"Ifthegivencausechasproducedaneffecteequaltoitself，ithasinthatveryactceasedtobe——chasbecomee。If，aftertheproductionofe，cstillremainedinthewholeorinpart，theremustbestillfurthereffectscorrespondingtothisremainingcause:thetotaleffectofcwouldthusbe>e，whichwouldbecontrarytothesuppositionc=e。Accordingly，sincecbecomese，andebecomesf，etc。，wemustregardthesevariousmagnitudesasdifferentformsunderwhichoneandthesameobjectmakesitsappearance。Thiscapabilityofassumingvariousformsisthesecondessentialpropertyofallcauses。Takingbothpropertiestogether，wemaysay，causesanINDESTRUCTIBLE
　　quantitatively，andquantitativelyCONVERTIBLEobjects。
　　"Thereoccurinnaturetwocauseswhichapparentlyneverpassoneintotheother，"saidMayer。"Thefirstclassconsistsofsuchcausesaspossessthepropertiesofweightandimpenetrability。Thesearekindsofmatter。Theotherclassiscomposedofcauseswhicharewantinginthepropertiesjustmentioned——
　　namely，forces，calledalsoimponderables，fromthenegativepropertythathasbeenindicated。ForcesarethereforeINDESTRUCTIBLE，CONVERTIBLE，IMPONDERABLEOBJECTS。
　　"Asanexampleofcausesandeffects，takematter:
　　explosivegas，HO，andwater，HO，arerelatedtoeachotherascauseandeffect；thereforeHO=
　　HO。ButifHObecomesHO，heat，cal。，makesitsappearanceaswellaswater；thisheatmustlikewisehaveacause，x，andwehavethereforeHOX=
　　HOcal。Itmightbeasked，however，whetherHO
　　isreally=HO，andx=cal。，andnotperhapsHO=
　　cal。，andx=HO，whencetheaboveequationcouldequallybededuced；andsoinmanyothercases。Thephlogisticchemistsrecognizedtheequationbetweencal。andx，orphlogistonastheycalledit，andinsodoingmadeagreatstepinadvance；buttheyinvolvedthemselvesagaininasystemofmistakesbyputtingxinplaceofO。InthiswaytheyobtainedH=
　　HOx。
　　"Chemistryteachesusthatmatter，asacause，hasmatterforitseffect；butwemaysaywithequaljustificationthattoforceasacausecorrespondsforceaseffect。Sincec=e，ande=c，itisnaturaltocallonetermofanequationaforce，andtheotheraneffectofforce，orphenomenon，andtoattachdifferentnotionstotheexpressionforceandphenomenon。Inbrief，then，ifthecauseismatter，theeffectismatter；ifthecauseisaforce，theeffectisalsoaforce。
　　"Thecausethatbringsabouttheraisingofaweightisaforce。Theeffectoftheraisedweightis，therefore，alsoaforce；or，expressedinamoregeneralform，SEPARATIONINSPACEOFPONDERABLEOBJECTSISA
　　FORCE；andsincethisforcecausesthefallofbodies，wecallitFALLINGFORCE。Fallingforceandfall，or，stillmoregenerally，fallingforceandmotion，areforcesrelatedtoeachotherascauseandeffect——forcesconvertibleintoeachother——twodifferentformsofoneandthesameobject。Forexample，aweightrestingonthegroundisnotaforce:itisneitherthecauseofmotionnoroftheliftingofanotherweight。Itbecomesso，however，inproportionasitisraisedabovetheground。
　　Thecause——thatis，thedistancebetweenaweightandtheearth，andtheeffect，orthequantityofmotionproduced，beartoeachother，asshownbymechanics，aconstantrelation。
　　’Gravitybeingregardedasthecauseofthefallingofbodies，agravitatingforceisspokenof；andthustheideasofPROPERTYandofFORCEareconfoundedwitheachother。Preciselythatwhichistheessentialattributeofeveryforce——thatis，theUNIONofindestructibilitywithconvertibility——iswantingineveryproperty:
　　betweenapropertyandaforce，betweengravityandmotion，itisthereforeimpossibletoestablishtheequationrequiredforarightlyconceivedcausalrelation。
　　Ifgravitybecalledaforce，acauseissupposedwhichproduceseffectswithoutitselfdiminishing，andincorrectconceptionsofthecausalconnectionsofthingsaretherebyfostered。Inorderthatabodymayfall，itisjustasnecessarythatitbeliftedupasthatitshouldbeheavyorpossessgravity。Thefallofbodies，therefore，oughtnottobeascribedtotheirgravityalone。Theproblemofmechanicsistodeveloptheequationswhichsubsistbetweenfallingforceandmotion，motionandfallingforce，andbetweendifferentmotions。Hereisacaseinpoint:Themagnitudeofthefallingforcevisdirectlyproportionaltheearth’sradiusbeingassumed——ootothemagnitudeofthemassm，andtheheightd，towhichitisraised——thatis，v=md。Iftheheightd=l，towhichthemassmisraised，istransformedintothefinalvelocityc=lofthismass，wehavealsov=mc；
　　butfromtheknownrelationsexistingbetweendandc，itresultsthat，forothervaluesofdorofc，themeasureoftheforcevismcsquared；accordinglyv=md=mcsquared。Thelawoftheconservationofvisvivaisthusfoundtobebasedonthegenerallawoftheindestructibilityofcauses。
　　"Inmanycasesweseemotionceasewithouthavingcausedanothermotionortheliftingofaweight。Butaforceonceinexistencecannotbeannihilated——itcanonlychangeitsform。Andthequestionthereforearises，whatotherformsisforce，whichwehavebecomeacquaintedwithasfallingforceandmotion，capableofassuming？Experiencealonecanleadustoaconclusiononthispoint。Thatwemayexperimenttoadvantage，wemustselectimplementswhich，besidescausingarealcessationofmotion，areaslittleaspossiblealteredbytheobjectstobeexamined。Forexample，ifwerubtogethertwometalplates，weseemotiondisappear，andheat，ontheotherhand，makeitsappearance，andthereremainstobedeterminedonlywhetherMOTIONisthecauseofheat。Inordertoreachadecisiononthispoint，wemustdiscussthequestionwhether，inthenumberlesscasesinwhichtheexpenditureofmotionisaccompaniedbytheappearanceofheat，themotionhasnotsomeothereffectthantheproductionofheat，andtheheatsomeothercausethanthemotion。
　　"Aseriousattempttoascertaintheeffectsofceasingmotionhasneverbeenmade。Withoutwishingtoexcludeapriorithehypothesiswhichitmaybepossibletoestablish，therefore，weobserveonlythat，asarule，thiseffectcannotbesupposedtobeanalterationinthestateofaggregationofthemovedthatis，rubbing，etc。bodies。Ifweassumethatacertainquantityofmotionvisexpendedintheconversionofarubbingsubstanceminton，wemustthenhavemv-n，andn=mv；andwhennisreconvertedintom，vmustappearagaininsomeformorother。
　　Bythefrictionoftwometallicplatescontinuedforaverylongtime，wecangraduallycausethecessationofanimmensequantityofmovement；butwoulditeveroccurtoustolookforeventhesmallesttraceoftheforcewhichhasdisappearedinthemetallicdustthatwecouldcollect，andtotrytoregainitthence？
　　Werepeat，themotioncannothavebeenannihilated；
　　andcontrary，orpositiveandnegative，motionscannotberegardedas=oanymorethancontrarymotionscancomeoutofnothing，oraweightcanraiseitself。
　　"Withouttherecognitionofacausalrelationbetweenmotionandheat，itisjustasdifficulttoexplaintheproductionofheatasitistogiveanyaccountofthemotionthatdisappears。Theheatcannotbederivedfromthediminutionofthevolumeoftherubbingsubstances。Itiswellknownthattwopiecesoficemaybemeltedbyrubbingthemtogetherinvacuo；butletanyonetrytoconverticeintowaterbypressure，howeverenormous。Theauthorhasfoundthatwaterundergoesariseoftemperaturewhenshakenviolently。
　　Thewatersoheatedfromtwelvetothirteendegreescentigradehasagreaterbulkafterbeingshakenthanithadbefore。Whencenowcomesthisquantityofheat，whichbyrepeatedshakingmaybecalledintoexistenceinthesameapparatusasoftenasweplease？
　　Thevibratoryhypothesisofheatisanapproachtowardsthedoctrineofheatbeingtheeffectofmotion，butitdoesnotfavortheadmissionofthiscausalrelationinitsfullgenerality。Itratherlaysthechiefstressonrestlessoscillations。
　　"Ifitbeconsideredasnowestablishedthatinmanycasesnoothereffectofmotioncanbetracedexceptheat，andthatnoothercausethanmotioncanbefoundfortheheatthatisproduced，weprefertheassumptionthatheatproceedsfrommotiontotheassumptionofacausewithouteffectandofaneffectwithoutacause。Justasthechemist，insteadofallowingoxygenandhydrogentodisappearwithoutfurtherinvestigation，andwatertobeproducedinsomeinexplicablemanner，establishesaconnectionbetweenoxygenandhydrogenontheonehand，andwaterontheother。
　　"Wemayconceivethenaturalconnectionexistingbetweenfallingforce，motion，andheatasfollows:
　　Weknowthatheatmakesitsappearancewhentheseparateparticlesofabodyapproachnearertoeachother；condensationproducesheat。Andwhatappliestothesmallestparticlesofmatter，andthesmallestintervalsbetweenthem，mustalsoapplytolargemassesandtomeasurabledistances。Thefallingofaweightisadiminutionofthebulkoftheearth，andmustthereforewithoutdoubtberelatedtothequantityofheattherebydeveloped；thisquantityofheatmustbeproportionaltothegreatnessoftheweightanditsdistancefromtheground。Fromthispointofviewweareeasilyledtotheequationsbetweenfallingforce，motion，andheatthathavealreadybeendiscussed。
　　"Butjustaslittleastheconnectionbetweenfallingforceandmotionauthorizestheconclusionthattheessenceoffallingforceismotion，cansuchaconclusionbeadoptedinthecaseofheat。Weare，onthecontrary，ratherinclinedtoinferthat，beforeitcanbecomeheat，motionmustceasetoexistasmotion，whethersimple，orvibratory，asinthecaseoflightandradiantheat，etc。
　　"Iffallingforceandmotionareequivalenttoheat，heatmustalsonaturallybeequivalenttomotionandfallingforce。JustasheatappearsasanEFFECTofthediminutionofbulkandofthecessationofmotion，soalsodoesheatdisappearasaCAUSEwhenitseffectsareproducedintheshapeofmotion，expansion，orraisingofweight。
　　"Inwater-millsthecontinualdiminutioninbulkwhichtheearthundergoes，owingtothefallofthewater，givesrisetomotion，whichafterwardsdisappearsagain，callingforthunceasinglyagreatquantityofheat；and，inversely，thesteam-engineservestodecomposeheatagainintomotionortheraisingofweights。Alocomotivewithitstrainmaybecomparedtoadistillingapparatus；theheatappliedundertheboilerpassesoffasmotion，andthisisdepositedagainasheatattheaxlesofthewheels。"
　　Mayerthencloseshispaperwiththefollowingdeduction:
　　"Thesolutionoftheequationssubsistingbetweenfallingforceandmotionrequiresthatthespacefallenthroughinagiventime——e。g。，thefirstsecond——
　　shouldbeexperimentallydetermined。Inlikemanner，thesolutionoftheequationssubsistingbetweenfallingforceandmotionontheonehandandheatontheotherrequiresananswertothequestion，Howgreatisthequantityofheatwhichcorrespondstoagivenquantityofmotionorfallingforce？Forinstance，wemustascertainhowhighagivenweightrequirestoberaisedabovethegroundinorderthatitsfallingforcemaybeequivalenttotheraisingofthetemperatureofanequalweightofwaterfrom0degreesto1degreescentigrade。Theattempttoshowthatsuchanequationistheexpressionofaphysicaltruthmayberegardedasthesubstanceoftheforegoingremarks。
　　"Byapplyingtheprinciplesthathavebeensetforthtotherelationssubsistingbetweenthetemperatureandthevolumeofgases，wefindthatthesinkingofamercurycolumnbywhichagasiscompressedisequivalenttothequantityofheatsetfreebythecompression；
　　andhenceitfollows，theratiobetweenthecapacityforheatofairunderconstantpressureanditscapacityunderconstantvolumebeingtakenas=1。421，thatthewarmingofagivenweightofwaterfrom0degreesto1degreescentigradecorrespondstothefallofanequalweightfromtheheightofaboutthreehundredandsixty-fivemetres。Ifwecomparewiththisresulttheworkingofourbeststeam-engines，weseehowsmallapartonlyoftheheatappliedundertheboilerisreallytransformedintomotionortheraisingofweights；andthismayserveasjustificationfortheattemptsattheprofitableproductionofmotionbysomeothermethodthantheexpenditureofthechemicaldifferencebetweencarbonandoxygen——moreparticularlybythetransformationintomotionofelectricityobtainedbychemicalmeans。"[1]
　　MAYERANDHELMHOLTZ
　　Here，then，wasthisobscureGermanphysician，leadingthehumdrumlifeofavillagepractitioner，yetseeingsuchvisionsasnohumanbeingintheworldhadeverseenbefore。
　　Thegreatprinciplehehaddiscoveredbecamethedominatingthoughtofhislife，andfilledallhisleisurehours。Heapplieditfarandwide，amidallthephenomenaoftheinorganicandorganicworlds。Ittaughthimthatbothvegetablesandanimalsaremachines，boundbythesamelawsthatholdswayoverinorganicmatter，transformingenergy，butcreatingnothing。
　　Thenhismindreachedoutintospaceandmetauniversemadeupofquestions。Eachstarthatblinkeddownathimasherodeinanswertoanight-callseemedaninterrogation-pointasking，HowdoIexist？WhyhaveInotlongsinceburnedoutifyourtheoryofconservationbetrue？Noonehadhithertoeventriedtoanswerthatquestion；fewhadsomuchasrealizedthatitdemandedananswer。ButtheHeilbronnphysicianunderstoodthequestionandfoundananswer。
　　Hismeteorichypothesis，publishedin1848，gaveforthefirsttimeatenableexplanationofthepersistentlightandheatofoursunandthemyriadothersuns——anexplanationtowhichweshallrecurinanotherconnection。
　　Allthistimeourisolatedphilosopher，hisbrainaflamewiththeglowofcreativethought，wasquiteunawarethatanyoneelseintheworldwasworkingalongthesamelines。AndtheoutsideworldwasequallyheedlessoftheworkoftheHeilbronnphysician。Therewasnofriendtoinspireenthusiasmandgivecourage，nokindredspirittoreactonthismasterfulbutlonelymind。Andthisisthemoreremarkablebecausetherearefewothercaseswhereamaster-originatorinsciencehascomeuponthesceneexceptasthepupilorfriendofsomeothermaster-originator。Ofthemenwehavenoticedinthepresentconnection，YoungwasthefriendandconfrereofDavy；Davy，theprotegeofRumford；
　　Faraday，thepupilofDavy；Fresnel，theco-workerwithArago；Colding，theconfrereofOersted；Joule，thepupilofDalton。ButMayerisanisolatedphenomenon——oneofthelonemountain-peakintellectsofthecentury。ThatestimatemaybeexaggeratedwhichhascalledhimtheGalileoofthenineteenthcentury，butsurelynolukewarmpraisecandohimjustice。
　　Yetforalongtimehisworkattractednoattentionwhatever。In1847，whenanotherGermanphysician，HermannvonHelmholtz，oneofthemostmassiveandtoweringintellectsofanyage，hadbeenindependentlyledtocomprehensionofthedoctrineoftheconservationofenergyandpublishedhistreatiseonthesubject，hehadhardlyheardofhiscountrymanMayer。Whenhedidhearofhim，however，hehastenedtorenounceallclaimtothedoctrineofconservation，thoughtheworldatlargegiveshimcreditofindependenteventhoughsubsequentdiscovery。
　　JOULE’SPAPEROF1843
　　Meantime，inEngland，Joulewasgoingonfromoneexperimentaldemonstrationtoanother，obliviousofhisGermancompetitorsandalmostaslittlenoticedbyhisowncountrymen。HereadhisfirstpaperbeforethechemicalsectionoftheBritishAssociationfortheAdvancementofSciencein1843，andnooneheededitintheleast。Itiswellworthourwhile，however，toconsideritatlength。Itbearsthetitle，"OntheCalorificEffectsofMagneto-Electricity，andtheMechanicalValueofHeat。"Thefulltext，aspublishedintheReportoftheBritishAssociation，isasfollows:
　　"Althoughithasbeenlongknownthatfineplatinumwirecanbeignitedbymagneto-electricity，itstillremainedamatterofdoubtwhetherheatwasevolvedbytheCOILSinwhichthemagneto-electricitywasgenerated；anditseemedindeednotunreasonabletosupposethatCOLDwasproducedthereinordertomakeupfortheheatevolvedbytheotherpartofthecircuit。Theauthorthereforehasendeavoredtoclearupthisuncertaintybyexperiment。Hisapparatusconsistedofasmallcompoundelectro-magnet，immersedinwater，revolvingbetweenthepolesofapowerfulstationarymagnet。Themagneto-electricitydevelopedinthecoilsoftherevolvingelectro-magnetwasmeasuredbyanaccurategalvanometer；andthetemperatureofthewaterwastakenbeforeandaftereachexperimentbyaverydelicatethermometer。
　　Theinfluenceofthetemperatureofthesurroundingatmosphericairwasguardedagainstbycoveringtherevolvingtubewithflannel，etc。，andbytheadoptionofasystemofinterpolation。Byanextensiveseriesofexperimentswiththeaboveapparatustheauthorsucceededinprovingthatheatisevolvedbythecoilsofthemagneto-electricalmachine，aswellasbyanyotherpartofthecircuit，inproportiontotheresistancetoconductionofthewireandthesquareofthecurrent；themagnetohaving，undercomparablecircumstances，thesamecalorificpowerasthevoltaicelectricity。
　　"ProfessorJacobi，ofSt。Petersburg，badshownthatthemotionofanelectro-magneticmachinegeneratesmagneto-electricityinoppositiontothevoltaiccurrentofthebattery。Theauthorhadobservedthesamephenomenononarranginghisapparatusasanelectro-magneticmachine；buthadfoundthatnoadditionalheatwasevolvedonaccountoftheconflictofforcesinthecoiloftheelectro-magnet，andthattheheatevolvedbythecoilremained，asbefore，proportionaltothesquareofthecurrent。Again，byturningthemachinecontrarytothedirectionoftheattractiveforces，soastoincreasetheintensityofthevoltaiccurrentbytheassistanceofthemagneto-electricity，hefoundthattheevolutionofheatwasstillproportionaltothesquareofthecurrent。Theauthordiscovered，therefore，thattheheatevolvedbythevoltaiccurrentisinvariablyproportionaltothesquareofthecurrent，howevertheintensityofthecurrentmaybevariedbymagneticinduction。ButDr。Faradayhasshownthatthechemicaleffectsofthecurrentaresimplyasitsquantity。Thereforeheconcludedthatintheelectro-
　　magneticengineapartoftheheatduetothechemicalactionsofthebatteryislostbythecircuit，andconvertedintomechanicalpower；andthatwhentheelectro-magneticengineisturnedCONTRARYtothedirectionoftheattractiveforces，agreaterquantityofheatisevolvedbythecircuitthanisduetothechemicalreactionsofthebattery，theover-plusquantitybeingproducedbytheconversionofthemechanicalforceexertedinturningthemachine。Byadynamometricalapparatusattachedtohismachine，theauthorhasascertainedthat，inalltheabovecases，aquantityofheat，capableofincreasingthetemperatureofapoundofwaterbyonedegreeofFahrenheit’sscale，isequaltothemechanicalforcecapableofraisingaweightofabouteighthundredandthirtypoundstotheheightofonefoot。"[2]
　　JOULEORMAYER？
　　TwoyearslaterJoulewishedtoreadanotherpaper，butthechairmanhintedthattimewaslimited，andaskedhimtoconfinehimselftoabriefverbalsynopsisoftheresultsofhisexperiments。Hadthechairmanbutknownit，hewascurtailingapapervastlymoreimportantthanalltheotherpapersofthemeetingputtogether。However，thesynopsiswasgiven，andonemanwastheretohearitwhohadthegeniustoappreciateitsimportance。ThiswasWilliamThomson，thepresentLordKelvin，nowknowntoalltheworldasamongthegreatestofnaturalphilosophers，butthenonlyanovitiateinscience。HecametoJoule’said，startedrollingtheballofcontroversy，andsubsequentlyassociatedhimselfwiththeManchesterexperimenterinpursuinghisinvestigations。
　　ButmeantimetheacknowledgedleadersofBritishscienceviewedthenewdoctrineaskance。Faraday，Brewster，Herschel——thosewerethegreatnamesinphysicsatthatday，andnooneofthemcouldquiteacceptthenewviewsregardingenergy。Forseveralyearsnoolderphysicist，speakingwithrecognizedauthority，cameforwardinsupportofthedoctrineofconservation。Thisculminatingthoughtofthefirsthalfofthenineteenthcenturycamesilentlyintotheworld，unheraldedandunopposed。Thefifthdecadeofthecenturyhadseenitelaboratedandsubstantiallydemonstratedinatleastthreedifferentcountries，yeteventheleadersofthoughtdidnotsomuchasknowofitsexistence。In1853Whewell，thehistorianoftheinductivesciences，publishedasecondeditionofhishistory，and，asHuxleyhaspointedout，hedidnotsomuchasrefertotherevolutionizingthoughtwhicheventhenwasafulldecadeold。
　　Bythistime，however，thebattlewasbrewing。Therisinggenerationsawtheimportanceofalawwhichtheirelderscouldnotappreciate，andsoonitwasnoisedabroadthatthereweremorethanoneclaimanttothehonorofdiscovery。ChieflythroughtheeffortsofProfessorTyndall，theworkofMayerbecameknowntotheBritishpublic，andamostregrettablecontroversyensuedbetweenthepartisansofMayerandthoseofJoule——abittercontroversy，inwhichDavy’scontentionthatscienceknowsnocountrywasnotalwaysregarded，andwhichleftitsscarsupontheheartsandmindsofthegreatmenwhosepersonalinterestswereinvolved。
　　Andsotothisdaythequestionwhoisthechiefdiscovererofthelawoftheconservationofenergyisnotsusceptibleofacategoricalanswerthatwouldsatisfyallphilosophers。ItisgenerallyheldthatthefirstchoiceliesbetweenJouleandMayer。ProfessorTyndallhasexpressedthebeliefthatinfutureeachofthesemenwillbeequallyrememberedinconnectionwiththiswork。Buthistorygivesusnowarrantforsuchahope。
　　Posterityinthelongrundemandsalwaysthatitsheroesshallstandalone。WhoremembersnowthatRobertHookecontestedwithNewtonthediscoveryofthedoctrineofuniversalgravitation？Thejudgmentofposterityisunjust，butitisinexorable。Andsowecanlittledoubtthatacenturyfromnowonenamewillbementionedasthatoftheoriginatorofthegreatdoctrineoftheconservationofenergy。ThemanwhosenameisthusrememberedwillperhapsbespokenofastheGalileo，theNewton，ofthenineteenthcentury；
　　butwhetherthenamethusdignifiedbythefinalverdictofhistorywillbethatofColding，Mohr，Mayer，Helmholtz，orJoule，isnotas，yetdecided。