SODDY (1917) Complexity Elements

December 27, 2017 | Author: laiagabe | Category: Ion, Atoms, Chemistry, Electron, Matter
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The Complexity of the Chemical Elements Author(s): Frederick Soddy Source: The Scientific Monthly, Vol. 5, No. 5 (Nov., 1917), pp. 451-462 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/22557 . Accessed: 25/03/2014 13:32 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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COMPLEXITY

OF CHEMICAJ4 ELEMENTS

THE COMPLEXITY

451

OF THE CHEMICAL

ELEMENTS1

By Professor FREDERICK

THE

SODDY, M.A., F.R.S.

elementsof thechemistare nowknownto be complex

in three differentsenses. 'In the firstsense the complexityis one that concernsthe general nature of matter,and thereforeof all the elementsin commonto greater or less degree. It followsfromthe relationsbetweenmatterand electricitywhichhave developedgraduallyduringthe past century as the resultof experimentsmade and theoriesborn withinthe four walls of this institution. Associated initially with the names of Davy and Faraday, theyhave onlyin these days come to full fruition'as the resultof the verybrilliantelucidationof the real nature of electricityby your distinguishedprofessor of physics,Sir JosephThomson. Such an advance, developing with long intervalsof apparent stagnation, slowlyand fitfully, needs to be reviewed from generation to generation,disentangled fromthe undergrowththat obscures it, and its clear conclusionsdrivenhome. This complexityof the chemicalelementsis a consequenceof the conditionthat neitherfree electricitynor free matter can be studied alone, except in very special phenomena. Our experimentalknowledgeof matterin quantityis necessarilyconfinedto the complexof matterand electricitywhich constitutesthe material world. This applies even to the "free" elementsof the chemist,which in reality are no more freethen than theyare in their compounds. The is merelythat,whereasin the latterthe elementsare difference combinedwith otherelements,in the so-called free state they are combinedwith electricity. I shall touchbut brieflyon this first;aspect, as in principleit is now fairly well understood. But its consistentand detailedapplicationto the studyof chemical characteris still lacking. The second sense in whichthe elements,or some of themat least, are knownnow to be complexhas, in sharp contrastto the first,developedsuddenlyand startlinglyfrom the recognitionin radioactivechanges,of different radio-elements, nonseparable by chemicalmeans,now called isotopes. The natural corollaryof this is that the chemicalelementrepresentsrather I Lecture before the Royal Institutionof Great Britain.

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a type of element,the membersof the type being only chemically alike. Alike theyare in most of those propertieswhich were studiedpriorto the last decade of last centuryand which are due, as we now think,to the outer shells of the atom, so alike that all the criteria,hithertorelied upon by the chemist as being the most infallibleand searching,would declare them to be identical. The apparent identitygoes even deeper into the region reached by X-ray spectrumanalysis which fails to is foundonlyin that distinguishbetweenthem. The difference innermostregionof all,'thenucleusof the atom,of whichradioactive phenomenafirstmade us aware. But, thoughthese phenomenapointedthe way, and easily showed to be differentwhat the chemist and 'spectroscopist would have decidedto be identical,it did more. It showedthat althoughthe finerand newercriteria,relied upon by the chemist in his analysis of matter,must of necessityfail in these cases, being ultimatelyelectricalin character,yetthe difference should be obvious in that most studied and distinctivecharacteristicof all-the criterionby whichDalton firstdistinguished kinds of atoms-the atomic weight. Those who the different have devoted themselvesto the exact determinationof these weights have now confirmedthe differencein two separate cases, which,in absence of what perhaps theymightregard as "preconceivednotions,"theywere unable to discoverforthemselves. This is the experimentaldevelopmentto which I wish more especiallyto directyour attention. It indicatesthat the chemical analysis of matter is, even within its own province, superficialrather thanultimate,and that thereare indefinitely more distinctelementsthan the ninety-twopossible types of elementaccommodatedby the presentperiodic system. The thirdsense in whichthe elementsare knownto be complex is that which,in the form of philosophicalspeculations, has come down to us from the ancients, which inspired the labors of the alchemistsof the Middle Ages, and which in the formof Prout's hypothesishas reappeared in scientificchemistry. It is the sense that deniesto naturethe rightto be complex, and fromthe'earliesttimes,faithoutstrippingknowledge, has underlainthe beliefthat all the elementsmust be built up of the same primordialstuff. The facts of radioactive pheare indeedmade nomenahave shownthat all the radio-elements up of lead and helium, and this has definitelyremovedthe question fromthe region'of pure speculation. We know that helium is certainlya material'constituentof the elementsin the Proutian sense,and it would,be harmless,if probablyfruit-

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less, to anticipatethe day of fullerknowledgeby atom building and unbuildingon paper. Apart altogetherfrom this, however, the existence of isotopes, the generalizationconcerning the Periodic Law that has arisen fromthe studyof radioactive changeon the one hand and the spectraof X-rayson the other, and experimentson the scatteringof a-particlesby matter,do give us for the firsttime a definiteconceptionas to what con-' betweenone elementand another. We stitutesthe difference can say how gold would result from lead or mercury,even though the control of the processes necessary to effectthe change still eludes us. The nuclear atom proposed by Sir Ernest Rutherford, even though,admittedly, it is onlya general and incompletebeginningto a completetheoryof atomicstructure, enormouslysimplifiesthe correlationof a large number of diversefacts. This and what survivesof the old electronic theory of matter,in so far as it attemptedto explain the periodiclaw,'will thereforebe brieflyreferredto in conclusion. THE FREE ELEMENT A COMPOUND OF MATTER AND ELECTRICITY

AlthoughDavy and Faraday were the contemporariesof Dalton, it mustbe rememberedthat it tookchemistsfiftyyears to put the atomictheoryon a definiteand unassailable basis, so thatneitherof theseinvestigatorshad the benefitof the very clear view we hold to-day. Davy was the originatorof the first electrochemical theoryof chemicalcombination,and Faraday's and electricityare one dictum.," the forcesof chemicalaffinity and thesame," it is safe to say, inspiresall the modernattempts to reduce chemicalcharacterto a science in the sense of someas well as expressed thingthat can be measuredqua:ntitatively, qualitatively. Faraday's work on the laws of electrolysisand the discoverythat followedfrom it, when the atomic theory came to be fullydeveloped,that all monovalentatomsor radicles carry the same charge, that divalent atoms carry twice this chargeand'so on,can be regardedto-dayas a simpleextension of the law of multipleproportionsfrom compoundsbetween matter and matter to compounds.between matter.and electricity. Long beforethe electriccharge had been isolated,.o.r the propertiesof electricitydivorcedfrommatter discovered, the same law of multipleproportionswhich led, withoutany possibilityof escape, to an atomic theoryof matter,led, as Helmholtzpointed out in his well-knownFaraday lecture to the ChemicalSocietyin thistheaterin 1881,to an atomictheory of electricity. The work of Hittorfon the migrationof ions, the bold and

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upsettingconclusionof Arrheniusthat in solutionmanyof the compoundshithertoregarded as most stable exist dissociated into ions, the realizationthat most of the reactionsthat take of and are utilizedfor the identification place instantaneously, elementsin chemicalanalysis,are reactionsof ions ratherthan of the elementin question,made very familiarto chemiststhe betweenthe propertiesof the elementsin enormousdifference the chargedand in the electricallyneutralstate. More slowly appreciated,and not yet perhaps sufficiently emphasized,was the unparalleledintensityof these charges in comparison with anythingthat electrical science can show, whichcan be expressedtritelyby the statementthat the charge on a milligramof hydrogenions would raise the potentialof the world 100,000 volts. Or, if we consider another aspect, and calculatehow manyfreehydrogenions you could forceinto a bottlewithoutburstingit, provided,of course,that you could do so withoutdischargingthe ions, you would findthat,were the bottle of the strongeststeel, the breech of a gun, for example,it would burst,by reason of the mutualrepulsionof the charges,beforeas much was put in as would, in the formof hydrogengas, show the spectrumof the elementin a vacuum tube. Then came the fundamentaladvances in our knowledgeof the nature of electricity,its isolation as the electron,or atom the great extensionof the conceptionof of negativeelectricity, throughgases, the ions to explain the conductionof-electricity theoreticalreasoning,due in part to Heaviside,that the electron must possess inertia inverselyproportionalto the diameterof the sphereon whichit is concentratedby reason of the electromagneticprinciplesdiscoveredby Faraday, leading to the allembracingmonismthat all mass may be of electro-magnetic origin,

This put the coping-stoneto the conclusionthat the elements as we apprehendthem in ordinarymatter are always compounds. In the "free"' state they are compoundsof the elementin multipleatomicproportionswith the electron. The ions,whichare thereal chemicallyuncombinedatomsof matter, can no more exist freein quantitythan can the electrons. The compoundmay be individualas betweenthe atom and the electron,or it may be statistical,affectingthe total number merelyof the opposite charges, and the elementpresumably will be an insulator or conductorof electricityaccordingly. Analogously,with compounds,the former condition applies to unionizedcompoundssuch as are met with in the domainof

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organic chemistry,or ionized, as in the importantclasses of inorganiccompounds,the acids, bases and salts. Just as the chemisthas long regardedthe union of hydrogenand chlorine as precededby the decompositionof the hydrogenand chlorine molecule,so he should now furtherregard the union itself as a decompositionof the hydrogenatom intothe positiveion and the negativeelectron,and a combinationof the latterwith the chlorineatom. One of the barriersto the properunderstandingand quantitativedevelopmentof chemicalcharacterfromthis basis is, perhaps,the conventionalidea derivedfromelectrostatics,that opposite electric charges neutralize one another. In atomic electricityor chemistry,though the equality of the opposite chargesis a necessaryconditionfor existence,thereis' no such thingas neutralization,or the electricallyneutralstate. Every atom being the seat of distinct opposite charges, intensely localized, the state of electricneutralitycan apply only to a remotepoint outside it, remote in comparisonwith its own diameter. We are gettingback to the conceptionof Berzelius, with some possibilityof understandingit, that the atom of hydrogen,for example, may be stronglyelectro-positive, and with regard to one that of chlorinestronglyelectro-negative, another,and yet each may be electricallyneutral in the molar sense. Some day it may be possible to map the electricfield surroundingeach of the ninety-two possibletypesof atom,over distances comparable with the atomic diameter. Then the studyof chemicalcharacterwould becomea sciencein Kelvin's sense, of somethingthat could be reduced to a number. But the mathematicalconceptionsand methodsof attack used in electrostaticsfor macroscopicdistances are ill-suitedfor the purposes of chemistry,which will have to develop methodsof its own. We have to face an apparent paradox that the greaterthe affinitythat binds togetherthe material and electrical constituentsof the atom, the less is its combiningpower in the is in in. chemicalsense. In otherwords,the chemicalaffinity of matterfor electrons. The helium verse ratio to the affinity atomsoffera verysimpleand instructivecase. Helium is nonvalent and in the zero family,possessing absolutelyno power of chemical combinationthat can be detected. Yet we know the atom possesses two electrons,for in radioactivechange it is expelled withoutthem as the a-particle. The discharge of electricitythroughit and positive-rayanalysis show that the electrons,or certainlyone of them,are detachableby electric

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agencies, althoughnot by chemical agencies. One would expect helium to act as a diad, forminghelides analogous to

oxides.

Professor Armstrongfor long advocated the view that these inertgases reallyare endowedwith such strongchemical that theyare compoundsthat have neverbeen decomaffinities for electrons posed. They certainlyhave such strongaffinities that the atom,the complexof the + ion and electrons,can not be decomposedchemically. Yet, in this 'case,wherethe affinity of the matterfor the electronis at a maximum,the chemical combiningpower is absent. These gases seem to furnishthe nearest standardwe have to electricneutralityin the atomicsense. The negativecharge of the electronsexactly'satisfies the positive charge of the matter, and the atomic complex is chemically,because electrically,neutral. In the case of the electro-positiveelements, hydrogenand the alkali-metals,one electronmorethan satisfies the positive charge on the ion, and' so long as the equality of oppositecharges is not altered,the electrontries to get away. In the case of the electro-negative elements,such as the halogens, the negative charge, though equal presumablyto the to neutralizethe atom. Hence these positive,is not sufficient one having more and the groups show strongmutual affinity, other less negative electricitythan would make the system atomicallyneutrallike helium. The electronexplains well the merelynumericalaspect of valency. But chemicalcombining power itselfseems to require the idea that equal and opposite charges in the atomic sense are only exactlyequivalentin the case of the inertgases. None of these ideas are now new, but theirconsistentapplicationto the studyof chemicalcompounds seems curiouslyto hang fire,as though somethingwere still lacking. for the chemistconsistentlyto realize that It is so difficult is due to a dissociatingas well as to a comchemicalaffinity effect. There is only one biningtendencyand is a differential affinity, probably,and it is the same as that betweenoppositely chargedspheres. But, atomiccharges being enormousand the distancesover whichtheyoperatein chemicalphenomenabeing is colossal,even in comparisonwith chemminute,this affinity is due ical standards. What the chemistrecognizesas affinity to relativelyslight differencesbetween the magnitudeof the universal tendencyof the electronto combinewith matterin atoms. Over all is the necessaryconthe case of the different ditionthat the oppositecharges should be equivalent,but this

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being satisfied,the individualatoms display the tendenciesinherentin theirstructure,some to lose, othersto;gain electrons, in order,as we believe from Sir Joseph Thomson's teaching, to accommodatethe numberof electronsin the outermostring to some definitenumber. Chemical affinityneeds that some shall lose as well as othersgain. Chemicalunionis always preceded by a dissociation. The tendency'to combine,only, is specificto any particular-atom,'but the energy and driving power of combinationis-the universal attractionof the + for the- change,of matterforthe electron. THE ELECTRICAL THEORY OF MATTER

Anotherbarrierthatundoubtedlyexiststo the betterappreciation of the modernpoint of view, even among those most willingto learn,is the confusionthat exists betweenthe earlier and the presentattemptto explain the relationbetweenmatter and electricity. We know negative 'electricityapart from matter as the electron. We know positive electricityapart from the electron,the' hydrogenion and the radiant helium atom or a-particleof radioactivechange for example,and it is matter in the'free or electricallyuncombinedcondition. Indeed, if you want to find matter free and uncombined,the simpleelementaryparticleof matterin the sense of complexity being discussed,you will go, paradoxically,to what the chemist termsa compoundratherthan to that whichhe termsthe free element. If this compoundis ionized completelyit constitutes the nearestapproachto matterin the freestate. Thus all acids owe their commonacidic quality to really free hydrogen,the hydrogenion, a particle more differentfrom the hydrogen atom than the atom is fromthe hydrogenmolecule. Positive electricityis thus emphaticallynot the mere absence of electricity,and any electricaltheoryof matter purportingto explain matterin termsof electricitydoes so by the palpable sophistryof callingtwo fundamentally different things name. the same The by dualismremainswhetheryou speak of matterand electricity,or of positive and negative electricity, and the chemistwould do well to stick to his conceptionof matter,until the physicisthas got a new name for positive electricitywhichwill not confuseit with the onlykind of electricitythat can exist apart frommatter. On the other hand, the theory of the electro-magnetic originof mass or inertiais a true monism. It tries to explain consistentlytwo things-the inertia of the electron and the inertia of matter-by the same cause. The inertia of the

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formerbeing accountedforby the well-knownelectro-magnetic principlesof Faraday, by the assumptionthat the charge on the electronis concentratedintoa sphereof appropriateradius; the 2,000-foldgreaterinertiaof the hydrogenion, for example, can be accountedfor by shrinkingthe sphereto one two-thousandth of the electronicradius. But the electricaldualism remainscompletelyunexplained. Call the electronE and the hydrogenion H. The facts are that two E's repel one anotherwith the same force and according to the same law as two H's repel each other,or as an H attracts an E. These very remarkablepropertiesof H and E are not explained by the explanation of the inertia. Are E and H made up of the same stuffor of two different stuffs? We do not know, and certainlyhave no good reason to assume, that matterminus its electronsis made of the same thing as the electron. We have still to reckonwithtwo different things. THE CHEMICALELEMENTS NOT NECESSARILY HOMOGENEOUS I pass now to the second and mostnovel sense in whichthe elements,or some of themat least, are complex. In their discoveryof new radioactiveelements,M. and Mme. Curie used radioactivityas a method of chemical analysis precisely as Bunsen and Kirchoff, and later Sir WilliamCrookes,used spectrumanalysis to discovercaesium and rubidium,and thallium. The new methodyieldedat once,fromuraniumminerals,three new radio-elements, radium,poloniumand actinium. According to the theoryof Sir Ernest Rutherfordand myself,these elements are intermediatemembers in a long sequence of changes of the parent element uranium. In a mineral the various membersof the series mustcoexistin equilibrium,provided none succeed in escaping fromthe mineral,in quantities inverselyproportionalto their respectiverates of change, or to theirperiods of average life. Radium directlyproportional. changes sufficiently slowly to accumulate in small but ponderable quantityin a uranium mineral,and so it was shown to be a new memberof the alkaline-earthfamilyof elements, with atomic weight 226.0, occupyinga vacant place in the periodic table. Polonium changes 4,500 times more rapidly, and can onlyexist to the extentof a few hundredthsof a milligram in a ton of uraniummineral. Actiniumalso, thoughits life period is still unknown,and verypossiblyis quite long,is scarce for anotherreason,that it is not in the main line of disintegration,but in a branchseries whichclaims onlya few per cent. of the uranium atoms disintegrating. In spite of this,

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poloniumand actiniumhave .just as much right to be considered new elements,probably,as radium has. Polonium has great resemblancein chemicalcharacterboth to bismuthand tellurium,but was separated fromthe firstby Mme. Curie and fromthe secondby Marckwald. In the positionit occupies as the last memberof the sulphurgroup,bismuthand tellurium are its neighborsin the periodictable. Actiniumresemblesthe rare-earthelements,and most closely lanthanum,but an enrichmentof the proportionof actiniumfrom lanthanumhas been effectedby Giesel. The smallnessof the quantitiesalone preventstheir completeseparation in the formof pure compounds as was done for radium. The three gaseous members,the emanations of radium, actiniumand thorium,were put in their proper place in the periodictable almost as soon as radium was, for,being chemically inertgases, theircharacterizationwas simple. They are the last membersof the argon family,and the fact that there are three of about the same atomic weight was probablythe first indication,although'not clearly appreciated,that more than one chemicalelementcould occupythe same place in the periodictable. The extensionof the three disintegrationseries proceeded apace; new members were being continuallyadded, but no other new radio-elements-new,that is, in possessing a new chemical character-were discovered. The four longest-lived to be added,radio-leador radium-D,as it is now moreprecisely termed,and ioniumin the uraniumseries, and mesothorium-I and radiothoriumin the thoriumseries,could not be separated from other constituents always present in the minerals, radium-D fromlead, ionium and radiothoriumfromthorium, and mesothorium-Ifromradium. An appreciable proportion of the radioactivityof a uraniummineral is due to radium-D and its products,and its separationwouldhave been a valuable technical achievement,but, thoughmany attemptshave been made, this has never been accomplished,and, we know now, probablyneverwill be. Seven years ago it was the generalopinionin the thencomparativelyundevelopedknowledgeofthe chemistry of theradioelements,that therewas nothingespeciallyremarkablein this. The chemistis familiarwithmanypairs or groupsof elements, the separationof whichis laboriousand difficult, and the radiochemisthad notthenfullyappreciatedthe power of radioactive analysis in detectinga veryslightchange in the proportionsof two elements,one or bothof whichwere radioactive. The case

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is not at all like that of the rare-earthgroup of elements,for example,in whichthe equivalentor atomicweightis used as a guide to the progress of the separation. Here the total differencein the equivalentof the completelyseparated elements is only a very small percentage of the equivalent, and the separation must already have proceededa long way before it can be ascertained. Human natureplays its part in scientificadvances,and the chemistis human like the rest. My own views on the matter developedwith some speed when,in 1910, I came across a new case of this phenomenon. Tryingto findout the chemicalcharwhich had been kept secret for techacter of mesothorium-I, nical reasons, I found it to have preciselythe same chemical characteras radium,a discoverywhich was made in the same year by Marckwald,and actuallyfirstpublishedby him. I delayed my publicationsome monthsto completea very careful fractionalcrystallizationof the barium-radium-mesothorium-I chlorideseparated fromthorianite. Althougha great number of fractionations and the radiumwas enriched, were performed, with regardto the barium,several hundredtimes,the ratio between the radium and mesothorium-Iwas, within the very small marginof errorpossible in carefulradioactivemeasurements,not affectedby the process. I feltjustifiedin concluding fromthis case, and its analogy with the several othersimilar cases then known,that radium and mesothorium-Iwere nonseparable by chemicalprocesses,and had a chemicalcharacter not merelylike, but identical. It followedthat some of the commonelementsmight similarlybe mixturesof chemically identicalelements. In the cases cited,the non-separablepairs differin atomic weightfrom2 to 4 units. Hence the lack of any regularnumericalrelationshipsbetweenthe atomicweights would on this view follownaturally.2 This idea was elaborated in the Chemical Society's Annual Report on Radioactivityfor 1910, in the concludingsectionsummingup the positionat that time. This was, I think,the beginningof the conceptionof differentelementsidentical chemically,which later came to be termed "isotopes," though it is sometimesattributedto K. Fajans, whose valuable contributionsto radioactivityhad not at that date commenced,and whose firstcontributionto this subject did not appear till 1913. In the six or seven years thathave elapsed the view has received complete vindication. Really, three distinct lines of advance convergedto a commonconclusion,and, so far as is 2 Trans. Chem. Soc., 1911, XCIX., 72.

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possible,these may be disentangled. First, therehas been the exact chemicalcharacterizationfromthe new point of view of every one of the membersof the three disintegrationseries, with lives over one minute. Secondly,came the sweepinggeneralizationsin the interpretationof the periodic law. Lastly, therehas been the firstbeginningsof our experimentalknowledge of atomicstructure,which got beyondthe electronicconstituentsand at the materialatom itself. In pursuance of the first,Alexander Fleck, at my request, commenceda careful systematicstudy of the chemical character of all the radio-elementsknownof which our knowledge was lackingor imperfect, to see whichwere and whichwere not separable fromknownchemicalelements. Seldom can the results of so muchlong and laboriouschemicalworkbe expressed in so few words.- Every one, that it was possible to examine, was foundto be chemicallyidenticaleitherwith some common elementor with another of the new radio-elements. Of the more importantcharacterizations,mesothorium-Ilwas found to be non-separablefromactinium,radium-Afrompolonium, the three B-members and radium-D from lead, the three C-members. and radium-E from bismuth, actinium-D and thorium-Dfromthallium. These results naturallytook some time to complete,and became known fairly widely to others workingin the subject beforetheywere published,throughA. S. Russell, an.old student,who was then carryingon his inin Manchester. Their interprevestigationsin :radioactivity: tation constitutesthe second line of advance. Before that is considered,it may firstbe said that every case of chemical non-separabilityput forwardhas stood the test of time,and all the many skilledworkerswho have pitted their chemicalskill against Nature in this quest have merely confirmed it. The evidenceat the presentday is too numerous and detailedto recount. It comesfromsources,such as in the technicalextractionof mesothoriumfrommonazite,where one process is repeated a nearly endless number of times; from trials of a very great varietyof methods,as, for example, in the investigationson radium-D and lead by Paneth and von Hevesy; it is drawn fromtotallynew methods,as in the beautiful.proofby the same authorsof the electro-chemical identity of thesetwo isotopes; it is at the basis of the use of radioactive elementsas indicatorsfor studyingthe propertiesof a common element,isotopic with it, at concentrationstoo feeble to be otherwisedealt with, and fromlarge numbersof isolated observations,as well as prolongedsystematicresearches. One

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of the finestexamplesof the latterkind of work,the Austrian researcheson ionium,will be dealt withlater. The mostrecent, whichappearedlast month,is by T. W. Richardsand N. F. Hall, who subjectedlead fromAustraliancarnotite,containingtherefore radium-D,to over a thousand fractionalcrystallizations in the formof chloride,withoutappreciablyalteringthe atomic weightor the ,8-activity.So that it may be-safely stated that no one who has ever reallytestedthis conclusionnow doubtsit, and afterall theyalone have a rightto an opinion. by chemicalmethods This statementof the non-separability of pairs or groups of elementssuffersperhaps frombeing in a negativeform. It looks too much like a mere negativeresult, a failure,but in realityit is one of the most sweepingpositive generalizationsthat could be made. Ionium wo say is nonseparable fromthorium,but every chemistknows thoriumis readily separated from every other known element. Hence, one now knows every detail of the chemistryof the vast majority of these new radio-elementsby proxy,even when their life is to be measuredin minutesor seconds,as completelyas if they were obtainable,like thoriumis, by the ton. The differenceit makes can only be appreciatedby those who have lived throughearlier days, when, in some cases, dealing with the separation of radio-constituentsfrom complex minerals, after every chemical separation one took the separated parts to the electroscopeto findout wherethe desiredconstituentwas. As the evidenceaccumulatedthat we had to deal here with somethingnew and fundamental,the questionnaturallyarose whetherthe spectrumof isotopes would be the same. The spectrumis known,like the chemicalcharacter,to be an elecand it was to be expected tronicratherthan mass phenomenon, that the identityshould extendto the spectrum. The question has been testedverythoroughlyby A. S. Russell and R. Rossi in this country,and by the Austrian workersat the Radium Institut of Vienna, for ionium and thorium,and by various workersforthe various isotopesof lead. No certaindifference has been found,and it may be concludedthat the spectra of isotopes are identical. This identityprobablyextends to the X-ray spectra,Rutherfordand Andrada havingshownthat the spectrumof the y-raysof radium-Bis identicalwiththe X-ray spectrumof its isotope,lead. (To be continued)

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