Inthemeantimethegreatimpetusgiventothestudyofradioactivityabroadledtoarapidsuccessionofnewdiscoveries.Manyscientistsengagedinthesearchforotherradioelements,usingthenewmethodofchemicalanalysis,withtheaidofradiation,thatwehadinaugurated.Thuswerefoundthemesothoriumnowusedbyphysiciansandmanufacturedindustrially,radio-thorium,ionium,protoactinium,radio-lead,andothersubstances.Atpresentweknow,inall,aboutthirtyradioelements(amongwhichthreearegases,oremanations),butamongthemallradiumstillplaysthemostimportantpart,becauseofthegreatintensityofitsradiation,whichdiminishesonlyextremelyslowlyduringthecourseofyears.
Theyearwasespeciallyimportantinthedevelopmentofthenewscience.Inthisyeartheinvestigationofradium,thenewchemicalelement,wasachieved,andPierreCuriedemonstratedtheastonishingdischargeofheatbythiselement,whichneverthelessremainedunalteredinappearance.InEngland,RamsayandSoddyannouncedagreatdiscovery.Theyprovedthatradiumcontinuallyproducesheliumgasandunderconditionsthatforceonetobelieveinanatomictransformation.If,indeed,radiumsaltheatedtoitsmeltingpointisconfinedforsometimeinasealedglasstube,entirelyemptiedofair,onecan,inreheatingit,makeitthrowoffasmallquantityofhelium,easytomeasureandtorecognizefromthecharacterofitsspectrum.Thisfundamentalexperimenthasreceivednumerousconfirmations.Itfurnishedusthefirstexampleofatransformationofatoms,independent,itistrue,ofourwill,butatthesametimeitreducestonothingthetheoryoftheabsolutefixityoftheatomicedifice.
Allthesefacts,alongwithothersformerlyknown,weremadetheobjectofasynthesisofthehighestvalue,inaworkbyE.RutherfordandF.Soddy,whoproposedatheoryofradioactivetransformations,to-dayuniversallyadopted.Accordingtothistheory,eachradioelement,evenwhenitappearsunchanged,isundergoingaspontaneoustransformation,andthemorerapidthetransformation,themoreintenseistheradiation.
Aradioactiveatomcantransformitselfintwoways:itcanexpelfromitselfanatomofhelium,which,thrownoffatanenormousspeedandwithapositivecharge,constitutesanAlpharay.Or,instead,itcandetachfromitsstructureamuchsmallerfragment,oneofthoseelectronstowhichwehavebecomeaccustomedinmodernphysics,andwhosemass,timessmallerthanthatofanatomofhydrogenwhenitsspeedismoderate,growsexcessivelywhenitsspeedapproachesthatoflight.Theseelectrons,whichcarryanegativecharge,formtheBetarays.Whateverthedetachedfragment,theresidualatomnolongerresemblestheprimitiveatom.Thuswhentheatomofradiumhasexpelledanatomofhelium,theresidueisanatomofgnseousemanation.Thisresiduechangesinitsturn,andtheprocessisnotarresteduntiltheattainmentofalastresiduewhichisstableanddoesnotgiveoffanyradiation.Thisstablematterisinactivematter.
ThustheAlphaandBetaraysresultfromthefragmentationofatoms.Gamma-raysarearadiationanalogoustolight,whichaccompaniesthecataclysmoftheatomictransformation.Theyareverypenetrating,andaretheonesmostusedinthetherapeuticmethodssofardeveloped.
Wecanseeinallthisthatradioelementsformfamilies,inwhicheachmemberderivesfromaprecedingmemberbydirectdescenttheprimaryelementsbeinguraniumandthorium.Wecaninparticularprovethatradiumisadescendantofuranium,andthatpoloniumisadescendantofradium.Sinceeachradioelement,atthesametimethatitisformedbythemothersubstance,destroysitself,itcannotaccumulateinthepresenceofthismothersubstancebeyondadeterminedproportion,whichexplainswhytherelationbetweenradiumanduraniumremainsconstantintheveryancientunalteredminerals.
Thespontaneousdestructionofradioelementstakesplaceaccordingtoafundamentallaw,calledtheexponentiallaw,accordingtowhichthequantityofeachradioelementdiminishesbyone-halfinatimealwaysthesame,calledaperiod,thistime-periodmakingitpossibletodeterminewithoutambiguitytheelementunderconsideration.Theseperiods,whichcanbemeasuredbydiversemethods,varygreatly.Theperiodofuraniumisseveralbillionsofyears;thatofradiumisaboutyears;thatofitsemanationalittlelessthanfourdays;andthereareamongthefollowingdescendantssomewhoseperiodisthesmallfractionofasecond.Theexponentiallawhasaprofoundphilosophicbearing;itindicatesthatthetransformationisproducedaccordingtothelawsofprobability.Thecausesthatdeterminethetransformationareamysterytous,andwedonotyetknowiftheyderivefromcausalconditionsoutsidetheatom,orfromconditionsofinternalinstability.Inmanycases,uptothepresent,noexterioractionhasshownitselfeffectiveininfluencingthetransformation.