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。