第20章

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-sixthofaninchindiameterwerebroughtneareachother(withinthethirtiethorfortiethofaninch)，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；

althoughitwasbytheFrenchsavantAndreMarieAmpere(1775-1836)thatthesciencewasactuallycreated，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。

In1859theGermanphysicistJuliusPlucker(1801-1868)noticedthatwhentherewasanelectricaldischargethroughanexhaustedtubeatalowpressure，onthesurroundingwallsofthetubenearthenegativepole，orcathode，appearedagreenishphosphorescence。

Thisdiscoverywassoonbeinginvestigatedbyanumberofotherscientists，amongothersHittorf，Goldstein，andProfessor(nowSirWilliam)Crookes。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，andhavingthetwopoles(anodeandcathode)sealedintotheglasswalls，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，H+O，andwater，HO，arerelatedtoeachotherascauseandeffect；thereforeH+O=

HO。ButifH+ObecomesHO，heat，cal。，makesitsappearanceaswellaswater；thisheatmustlikewisehaveacause，x，andwehavethereforeH+O+X=

HO+cal。Itmightbeasked，however，whetherH+O

isreally=HO，andx=cal。，andnotperhapsH+O=

cal。，andx=HO，whencetheaboveequationcouldequallybededuced；andsoinmanyothercases。Thephlogisticchemistsrecognizedtheequationbetweencal。andx，orphlogistonastheycalledit，andinsodoingmadeagreatstepinadvance；buttheyinvolvedthemselvesagaininasystemofmistakesbyputtingxinplaceofO。InthiswaytheyobtainedH=

HO+x。

"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:Themagnitudeofthefallingforcevisdirectlyproportional(theearth’sradiusbeingassumed——oo)tothemagnitudeofthemassm，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，thiseffectcannotbesupposedtobeanalterationinthestateofaggregationofthemoved(thatis，rubbing，etc。)bodies。Ifweassumethatacertainquantityofmotionvisexpendedintheconversionofarubbingsubstanceminton，wemustthenhavem+v-n，andn=m+v；andwhennisreconvertedintom，vmustappearagaininsomeformorother。

Bythefrictionoftwometallicplatescontinuedforaverylongtime，wecangraduallycausethecessationofanimmensequantityofmovement；butwoulditeveroccurtoustolookforeventhesmallesttraceoftheforcewhichhasdisappearedinthemetallicdustthatwecouldcollect，andtotrytoregainitthence？

Werepeat，themotioncannothavebeenannihilated；

andcontrary，orpositiveandnegative，motionscannotberegardedas=oanymorethancontrarymotionscancomeoutofnothing，oraweightcanraiseitself。

"Withouttherecognitionofacausalrelationbetweenmotionandheat，itisjustasdifficulttoexplaintheproductionofheatasitistogiveanyaccountofthemotionthatdisappears。Theheatcannotbederivedfromthediminutionofthevolumeoftherubbingsubstances。Itiswellknownthattwopiecesoficemaybemeltedbyrubbingthemtogetherinvacuo；butletanyonetrytoconverticeintowaterbypressure，howeverenormous。Theauthorhasfoundthatwaterundergoesariseoftemperaturewhenshakenviolently。

Thewatersoheated(fromtwelvetothirteendegreescentigrade)hasagreaterbulkafterbeingshakenthanithadbefore。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。