the laboratory notebooks of pierre and marie curie and the ... · pierre and marie curie, polonium...

Pierre and Marie Curie, polonium and radium

THE LABORATORY N O T E B O O K S OF P I E R R E AND MARIE CURIE AND THE DISCOVERY OF P O L O N I U M AND RADI UM

J. P. ADLOFF

63 Rue Saint-Urbain, 67100 Strasbourg, France

1 I n t r o d u c t i o n

It is an exceptional and moving privilege to remember at this historical place [Jfichymov] the centenary events which announced the rise of radiochemistry and conducted inexorably to the nuclear era of mankind. In the account of the early history of radioactivity Sankt Joachimsthal, now J~chymov, is intimately related to Pierre and Marie Curie who one hundred years ago discovered polonium and radium in a sample of Joachimsthal pitchblende.

In 1897 Maria Sklodowska, who had married Pierre Curie in 1895, concluded her studies at the Sorbonne in Paris and was thinking of a subject for a thesis. The X rays were still a topical question but had lost the charm of novelty. On the other hand, the uranic rays discovered in 1896 by Henri Becquerel raised a puzzling problem. The uranium salts appeared to maintain an undiminished ability to blacken a photographic plate over months. The law of energy conservation was solidly established since 50 years. What was the origin of this inexhaustible energy which apparently violated the Carnot principle, the first principle of thermodynamics, which states that energy can be transformed, but can never been created nor destroyed? Pierre Curie, a renowned physicist for his work on magnetism and crystal symmetry, had a presen- timent that the phenomenon was quite extraordinary and helped his wife in the decision. Marie Curie, in the biography of Pierre published in 1923, confirmed: we fel t the invest igat ion o f the phenomenon very attractive, so much the more the topic was quite new and required no bibliographical research [1].

A small laboratory was offered to the couple at the Parisian School for Physics and Chemistry. Pierre Curie was already involved in a work on crystal growth and had opened a laboratory diary on the 16 of September 1897. The writing of Marie Curie appears in the notebook on December 16, 1897. This day is the beginning of her work on uranic rays, first alone, later joined by Pierre Curie, which will lead within one year to the discoveries of two radioelements, prelude to two Nobel prizes.

In this lecture the work of Pierre and Marie Curie along the famous year 1898 will be reconstituted. Two sources of information are available: three laboratory notebooks kept at the French National Library and three publications in the Comptes Rendus de l 'Acad~mie des sciences, the weekly reports of the Academy of Sciences. The dates are not always indicated in the notebooks, however the progress of the work can be established by cross-checking with the publications. During the first months Marie Curie writes practically alone, comments by Pierre are limited to a few words. Subjec- tive remarks are exceptional; on February 6, Marie Curie registers with anger the temperature in the laboratory: 6.25~ followed by 10 exclamation marks. It is an evidence of the miserable working conditions.

The chronology and the connection of the documents are shown in Table 1. The three months covered in the first notebook are mostly used for setting up the equipment.

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The second d i a r y ends wi th the discovery of polonium. The las t notebook begins af te r an i n t e r rup t i on of 4 months ; six weeks l a t e r the th i rd publ ica t ion announces the discovery of r ad ium. The inscr ip t ions are pu r sued more or less r egu la r ly unt i l mid- 1900 wi th fu r the r i m p o r t a n t f indings such as the phenomenon of induced activity. Af te rwards the notes a re more sporadic. Mar ie Curie consecra ted most of her work to the i so la t ion of a macroscopic amoun t of pure r a d i u m chloride for the de t e rmina t ion of the a tomic weight of the rad ioe lement ; the correct va lue 225.9 appea r s in the l a s t page of the d iary .

Table 1. Chronology of the documents related to the work of Pierre and Marie Curie during 1898.

l N o t e b o o k P u b l i c a t i o n 16 December 1897 - 18 March 1898 18 March 1898 - 16 July 1898 12 April 1898 [2], 18 July 1898 [3]

November 1898 - 28 March 1902 26 December 1898 [4]

Wi th the sole notebooks i t would be difficult to recons t i tu te the content of the publ icat ions . P robab ly the au thors p repared draf t s which have d i sappeared ; it is l ikely tha t a t severa l occasions resu l t s have been r epor t ed on sheets r a t h e r t han in the diar ies . In fact, the publ ica t ions conta in informat ion which cannot be found in the l abo ra to ry notebooks and rec iprocal ly observat ions reg i s t e red in the d iar ies have not been inc luded in the publ ica t ions .

2 T h e s t r a t e g y

At the end of 1897 all knowledge on uranic rays was conta ined in the nine Becquerel publ ica t ions in the Comptes Rendus, most ly du r ing the first s emes t e r of 1896. After an in i t i a l exc i tement , the i n t e r e s t of sc ient is ts in the new rays faded rapidly . One reason was the pro l i fe ra t ion of false or doubtful observat ions of the emiss ion of r ad ia t ions s imi l a r to uran ic rays by a va r i e ty of subs tances inc luding glow worms. One can say t h a t the topic was mor ibund when Mar ie Curie en te red the scenery.

How u n d e r t a k e the subject chosen for Mar ie ' s thesis? One approach could be the search for subs tances sha r i ng the pecul ia r p rope r ty of u r a n i u m and its compounds. This seemed to be a logical process. I t is c lear ly expressed in the in t roduct ion of Marie Curie 's f i rs t publ ica t ion [2]: I have searched if substances other than uranium compounds render air conducting for electricity. This l a s t precis ion impl ic i t ly refers to the method used for the m e a s u r e m e n t of the rad ia t ion .

The pho tograph ic p la te by which rad ioac t iv i ty was discovered was a pr imi t ive and mere ly qua l i t a t ive r ad i a t i on detector . On the o ther hand, the ion iza t ion of a i r could be used for a quan t i t a t i ve de t e rmina t i on of the ac t ion of rays and thus the in tens i ty of the i r emission. However , a convenient m e a s u r e m e n t of very smal l in tens i t i es had st i l l to be imagined .

At th is poin t the genius of P ie r r e Curie was decisive. In 1880, P ie r re Curie toge ther wi th his b ro the r Jacques had discovered piezoelectr ici ty . This is the product ion of e lectr ic charges when a p re s su re is appl ied to h e m i h e d r a l c rys ta l s l ike quar tz . P ie r re Curie inven ted a device based on this phenomenon which is the key of the prodigious discoveries e f t h e Cur ies (Fig. 1).

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B " . . . . . ~'"""" "T ''~'~'

I,I,L, i " teme terre

~ePre

7C

Fig. 1. Scheme of the device used for the measurement of the ionization current. AB ionization chamber. CD connection to ground. E quadrant electrometer. Q piezoelectric quartz.

weight [5].

The apparatus consists of two metallic electrodes between which an electric field is applied. One plate is loaded with a powder of the material to be tested. The uranic rays produce electric charges in the air and a ionization current flows between the electrodes. The charges in the chamber are compensated by opposite charges obtained by applying a weight to a leaf of piezoelectric quartz. The compensation is controlled by a quadrant electrometer. The charges produced by the quartz are calculated from the weight applied and from the time during which the weight is applied. Eventually the ionization current can be calculated. For the first time the emission of uranic rays could be quantified on the basis of the current produced by the rays under controlled conditions.

The setting of the equipment was painful and fills 50 pages in the notebook. Humidity in the uncomfortable laboratory was the source of numerous difficulties. The handling of the device required considerable skill but Marie Curie now had an invaluable tool for routine measurements (Fig. 2).

3 Mar ie Curie 's f i r s t p u b l i c a t i o n

The first publication on the 12 of April with the title Radiat ions emitted by uran ium and thorium is signed Marie Sklodowska Curie [2]. Pierre Curie without whom the work would not have been possible is not associated as author. Two months before, Marie Curie had started the systematic search for compounds which may impart electric conductivity to air. She tested all samples at hand or borrowed from various collections. By courtesy of Henri Becquerel, she had access to the rich collection of minerals assembled by Professor Lacroix at the Museum of National History, the place where Becquerel had discovered radioactivity, There she found among other minerals pitchblende of various origins, in particular from Joachimsthal.

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Fig. 2. Marie Curie in the laboratory and a view of the equipment.

On February 18, she performed the first measurement of a pitchblende which was about twice as active as metallic uranium (Fig. 3). For each compound the weight in grams applied to the quartz and the compensation time in seconds are registered. For sake of comparison of activities the ratio weight over time (designated i in the notebook) was convenient. The ionization current i tself is calculated from the charac- teristics of the quartz and appears only in the publication. Metallic uranium prepared by Henri Moissan in 1896 by reduction of uranium oxide in the electric furnace which he had invented, was used as a reference for quantifying the relative strength of radioactive substances.

The intensi ty observed for pitchblende was quite unexpected since no uranium containing substance ought to be more active than the metal. I t was not commented in the notebook, but numerous tests of the equipment which followed immediately show that Marie Curie was extremely preoccupied by the finding. Fur the r significant results are shown in Table 2.

Table 2. A selection of substances measured by Marie Curie [2].

Sample metallic uranium (Moissan) thorium oxide potassium fluoxytantalate Joachimsthal pitchblende Johanngeorgenstadt pitchblende natural chalcolite

Intensity (pA) 24 53

2 83 67 52

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1,7. " '

3% .<.<_, t,-=, U--.

_ . /W) ~1

/o'o ' ,fJ ".} ~ ' . 3 ~ o,~o 5~ '.' ' ~ , </o

qS, ~'!'

Fig. 3. Marie Curie's first measurement of pitchblende on February 18, 1898. The compensation of charges produced in the chamber required a weight of 500 g applied during 24.7 s.

I t had been found ea r l i e r by Becquere l and conf i rmed by Mar ie Curie t h a t the emiss ion of r ays was specific p rope r ty of u r a n i u m atoms, i n d e p e n d e n t of the chemical combinat ion of the e lement . Accordingly the excess of ac t iv i ty of cer ta in mine ra l s h a d an unequivocal or igin which Mar ie Curie s t a t ed in following te rms: This fact is quite remarkable and suggests that these minerals may contain an element much more active than uran ium itself.

Evidence in favour of th is hypo thes i s was r ap id ly proven since Mar ie Curie knew how to p repa re ar t i f ic ia l chalcoli te: I have prepared chalcolite by the Debray method wi th pure products; this arti f icial chalcolite is not more active than other uran ium salts. Marie Curie then concluded t ha t the unknown e l emen t exis ts only in the u ran i fe rous mine ra l s which a re more act ive t han u ran ium.

At about the s ame t ime, on the 26 of Feb rua ry , Mar ie Curie d iscovered the r ad ioac t iv i ty of thor ium. This was a m a t t e r of chance since the compound a ppe a r s a t r andom in the mids t of o ther subs tances such as l a n t h a n u m n i t r a t e , t i t an ic acid, z i rconium oxide. (Fig. 4). One m a y notice t ha t Moissan u r a n i u m is m e a s u r e d r epea t ed - ly in o rder to check the correct work ing of the equ ipment . Thor ium oxide was found to be even more act ive t h a n u r a n i u m (Table 2). We know tha t u r a n i u m was in equ i l ib r ium wi th i ts f i rs t two descendan t s while t ho r ium was closer to complete equ i l ib r ium wi th al l daugh te r s .

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J. P. Adloff

. . . . j -

f C - f

J2-

5 ~ r _ _ 51"

mF.{ Is~ " - l i C E , ~n - - - : - - - ~ "~ ;:t.[

" ~m ~ - ~ : . - - ~ =

F~g. 4. Discovery of thorium radioactivity on February 26, 1898. Terre de la capsule is pure Th oxide. Ce exhibits a very feeble activity probably due to traces of Th. For other com-

pounds Marie Curie reports nothing certain.

However, the discovery of the radioactivity of thorium was not a premiere. Two months before Marie Curie's paper the German physicist Gerhardt Schmidt at Erlan- gen had published that thorium and its compounds blacken a photographic plate [6]. Marie Curie could not be aware of this work, but when Schmidt read Marie Curie's paper, he hastened to announce in the Comptes Rendus [7] that he was the discoverer of the radioactivity of thorium.

Interestingly enough, Marie Curie reported a feeble activity for two potassium salts and she was probably the first to record without knowing it the activity of 4~ She aIso proceeded to the first systematic investigation of the absorption and self-absorption of uranic and thoric rays.

A strange observation registered in the notebook is not mentioned in the publication. Marie Curie found that the activity of thorium oxide decreased when a stream of air was flown through the ionization chamber and afterwards it increased again during a few minutes. It is the same observation, but more salient, which led Rutherford to postulate a few months later the emission of a radioactive gas by thorium oxide. Thus one can think that Marie Curie was on the verge to make the same discovery. But she was probably much more intrigued by the activity of pitchblende. If she had concentrated her investigation on the erratic behaviour of thorium, perhaps the orientation of her work would have been fully changed.

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Fig. 5. Marie Curie proceeds to the measurement of a radioactive sample. With the right hand she lifts progressively the weight applied to the quartz; in the left hand she holds the

chronometer.

The sum of results acquired in two months is prodigious. The search of the element more active than uranium was now a mat ter of highest priori ty and urgency. Pierre Curie fascinated by Marie 's s findings abandoned his own research projects and joined his wife in the adventure (Fig. 5).

4 The discovery of po lonium

The second publication, this t ime signed by Pierre and Marie Curie, is based on experiments performed from April 9 to July 16. The title On a new substance, radioactive, contained in pitchblende [3] is eloquent for two reasons. I t announces tha t the search for the element much more active than uranium was successful and the word radioactive appears for the first time.

I t is noteworthy tha t research on uranic rays now turned from physics to chemistry. The obstacles were immense: separate and identify a substance whose chemical properties were completely unknown. The couple was helped by Gustave B~mont, an analyt ical chemist at the Paris ian School for Physics and Chemistry. The collaboration with B~mont lasted for 6 months.

On the other hand, the hypothetical element could be followed by its radioactivity. Marie Curie s tated in the biography of Pierre: The method we have used is a new one for chemical research based on radioactivity. It consists in separations performed with the ordinary procedures of analytical chemistry and in the measurement of the radioactivity of all compounds separated [1]. This sentence announces the beginning of radiochemistry. Marie Curie added tha t radioactivity acts like a specific analytical reagent with a high sensit ivity but she could not imagine tha t the limit of sensit ivity was a few atoms.

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J. P. Adloff

J q acid

I

evaporM a n ~

"azotate d ' u r a n i u m + X ? " alkaline fusion and a~d

H2S D,

Fig. 6. Simplified scheme of the first acidic t reatments of pitchblende.

P ie r r e Curie more incl ined to physics under took the ana lys i s of p i tchblende by subl imat ion . A l r e a d y in the f irst t r i a l i t was found tha t a ve ry smal l amount of subs tance about 10 t imes as act ive t h a n u r a n i u m could be ob ta ined and this method was pu r sued in pa ra l l e l wi th the chemical ana lys is .

The f irst t r e a t m e n t began on Apr i l 14, whereby 100 g of J o a c h i m s t h a l p i tchblende was ground and a t t acked by n i t r ic acid (Fig. 6). The acidic solut ion as well as the insoluble res idue were both active which could mean t ha t each f rac t ion conta ined a d i f ferent r ad ioac t ive subs tance or tha t a single one was d i s t r i bu t ed be tween the two phases . The au thors were excessively opt imis t ic in t h ink ing t h a t th is f i rs t s tep could a l r e ady be s ignif icant . P ie r re Curie measu red i m m e d i a t e l y the absorp t ion spec t rum of the solut ion and found a weak band non ex is ten t in u rany l n i t r a t e . He under l ined this r e su l t which in his mind was a f i rs t indica t ion of the presence of a new element . On he r side, Mar ie Curie evapora t ed the solution, measu red the ac t iv i ty and noted u r a n i u m azotate + X. She also though t tha t the mys te r ious e l emen t could be h idden in th is fract ion.

But i t was st i l l a long way to the success. F rom the notebooks i t a ppe a r s t ha t the au thor s followed s imu l t aneous ly severa l s t ra teg ies . The r e s idue was fused wi th po t a s s ium ca rbona te and a f t e rwards dissolved in acid. The t r e a t m e n t of the solut ion wi th H2S was a s ignif icant s tep since the p rec ip i t a te of sulf ides was active, as well as the r e s idua l so lu t ion which conta ined u ran ium.

The ac t iv i ty conta ined in the sulf ides was insoluble in a m m o n i u m sulf ide and thus could be s e p a r a t e d from As and Sb (Fig. 7). I t became clear t h a t X was associa ted with Pb, Cu and Bi. In the meanwhi l e P ie r r e Curie had s epa ra t ed by f rac t ional sub l imat ion a subs tance 100 t imes as active as u ran ium. Now a d i l emma had to be lifted: is the subs tance insoluble in a m m o n i u m sulf ide ident ica l wi th tha t ob ta ined by sub l imat ion? Does a known e l emen t sha re both proper t ies? Dur ing a whole week all possible e l emen t s and the i r compounds were tes ted: arsenic, iodine, sulfur , n a t u r a l sulf ides and m a n y others . None was active.

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Pb, CU, BI,As,Sb,X

~1 (NH4)2S

I I solution residue AS, Sb Pb, Cu,Bi,X

I q HNO 3

Qolution

JI H2SO 4

so~lon p~ec[p~ate PbSO,, X Cu, Bi, X

............ I I H ' ~ 4 D 4

solution X (part)

solution CU

NH 3

I precipitate

BI+X

Fig. 7. Principle of the t reatment of the precipitate of sulfides.

Thus the ac t iv i ty insoluble in a m m o n i u m sulfide was t h a t of a new e l emen t mixed wi th Cu, Pb and Bi. These th ree e lements could be eas i ly s e p a r a t e d and even tua l ly mos t of the ac t iv i ty r e m a i n e d wi th Bi. The au thors could not f ind an accura te we t me thod for the s epa ra t i on of the rad ioac t ive subs tance from Bi. A pa r t i a l concent ra t ion was ob ta ined by p rec ip i t a t ion wi th water : the f i rs t f ract ions are the more active.

Sudden ly on J u l y 13, wi thou t fu r the r comment , the symbol Po a ppe a r s in t he notebook (Fig. 8). P i e r r e Curie proceeds to the sub l ima t ion of about l g of a mix tu re of Bi, Pb and the supposed new e lemen t Po, 12 t imes more active t h a n u r a n i u m . In th is h i s tor ica l page he descr ibes the colour of f ract ions s e p a r a t e d whi le Mar i e Cur ie m e a s u r e s the act ivi t ies ; the h ighes t is t ha t of a fragment black, yellow orange, con-

siderably black 400 t imes more act ive t h a n U. This success was the r e su l t of the jo in t efforts of both sc ient i s t s since P ie r re Curie 's sub l ima t ion me thod was app l ied to Mar i e Curie 's sulf ides (Fig. 9).

The second notebook ends on J u l y 16 with a f inal t e s t for ac t iv i ty on a l a rge va r i e t y of compounds. This was a l as t p recau t ion before the discovery of a new e l emen t could be c la imed wi th confidence. Two days l a t e r the discovery of po lonium is announced, however caut iously: ... We believe that the substance we recovered from pitchblende

contains a heretofore unknown metal, similar to bismuth in its analytical properties.

I f the existence of this new metal is confirmed, we propose that it be named polonium

in honor o f the native land of one of us.

Pr io r to polonium, four e l emen t were given n a m e s re fe r r ing to countr ies : ge rma- nium, ru then ium, ga l l ium and scandium. But the des igna t ion of polonium had a pro- vocat ive s ignif icance since as a s t a t e Po land had d i s a p p e a r e d in 1795, be ing pa rce l l ed out be tween Pruss ia , Russ i a and the A u s t r i a n Empi re . F ranc ium, the l a s t d iscovered n a t u r a l r ad ioe lement , was found in 1939 by Margue r i t e Perey , a former l abo ra to ry a s s i s t a n t of Mar ie Curie .

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_ _ j~l

Fig. 8. The symbol Po appears in the notebook on July 13, 1898.

Fig. 9. Pierre and Marie Curie at the laboratory bench.

For the first t ime in the history of chemistry the existence of an invisible element

was claimed which could be identified solely on the basis of its emission ofuranic rays. In the 19th century no such claim was considered valid until a pure substance had

been isolated, the atomic weight of the element determined and its spectral lines

measured. Eugene Demarcay, the leading spectroscopist of the time, could not distin-

guish any new characterist ic line in the most active, sample of bismuth sulfide. The

authors admitted this fact does not favour the idea of the existence of a new metal.

However, Demarcay indicated tha t the absence of specific rays does not prove abso-

lutely tha t the sample contains only bismuth, since spectral analysis is not very

sensitive for heavy elements.

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The amount of Po in 100 g of pitchblende was at the most a few ng, well below the sensit ivity of the spectrometer. A few spectral lines of Po will be detected 12 years la ter on a sample of a tenth of mg extracted by Marie Curie and Andr~ Debierne from several tons of residues of Joachimsthal pitchblende.

One may wonder if the announcement of the discovery was premature. The excess of activity of pitchblende revealed in Marie Curie's first paper was a scoop which would undoubtedly incite other scientists to search for the origin of the phenomenon. Marie Curie had al ready missed the priori ty of the radioactivity of thorium and another mischance should be avoided.

More real is t ical ly the notebooks show tha t in their experimental conditions the authors had reached the extreme limit of concentration of Po in Bi. After each fractional precipitat ion the amount of mater ia l become smaller and in any case they could not have obtained a sample with an activity much higher than 400 t imes tha t of uranium and 130 t imes tha t of pitchblende. Hence there was no reason to delay the publication.

The isolation of Po from U had been accomplished although the authors were unaware of the relat ionship between the two elements. They considered the whole mater ia l as a mixture. They knew nothing at the time of radioactive decay. In this sense it was a mat te r of chance since the isolation was performed in a short t ime with respect to the 140 days half- life of Po. I t was only 4 years la ter tha t the Curies noticed with astonishment and great perplexity tha t Po was progressively disappearing.

Moreover they could not imagine tha t they were dealing with trace amounts of the new element. By simple reasoning the new substance must obviously be present in very small amount since it had not yet been found. However, it was not evident tha t this amount was below the l imit of any chemical means of detection.

The assignment of Po in the periodic table does not seem to have preoccupied the discoverers. Po was concentrated in the Bi fraction. This affinity led Marie Curie to consider tha t the two elements had similar properties. In fact, Po is an homologue of tel lur ium and the neighbour of Bi in the periodic table. The confusion is readi ly explained since Po was coprecipitated at trace concentrations with Bi sulfide and hydroxide.

The publication ended the short story of polonium for several years. The Curies were still preoccupied with the authent ici ty of the element and with honesty they did not hide their doubts. In 1899 they observed the phenomenon of induced radioactivi ty and Marie Curie raised the question: is Po which exhibits the spectral lines of Bi really

a new element or simply Bi activated by the radium contained in pitchblende? Still in 1902 the authors noted Po is a kind o f active Bi; it has not yet been proven that it contains a new element.

5 T h e d i s c o v e r y o f r a d i u m

Since the first chemical t rea tment the Curies suspected the presence of two new elements in pitchblende. A clue to this hypothesis was the distr ibution of the activity between several fractions (Fig. 6). The authors focused their at tention on the sulfides first because it seemed easier to search for the activity concentrated in a solid.

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~ P . A ~

The 4 months i n t e r rup t ion in the notebooks from J u l y to November has been the subject of severa l i n t e rp re t a t ions . Dur ing the s u m m e r the couple r e n t e d a fa rm in cen t ra l F rance . Eve Labouisse-Cur ie in the b iog raphy of he r mo the r points out everyday they discussed about the new metal, polonium and the other one which was still to be discovered. In September they returned to their humid workshop and resumed their investigations with a renewed enthusiasm [8]. On his side, Rober t Reid sugges ted t h a t both had to t ake a res t because of a s t range and in t ense t i r edness and r epea t ed fa in t ings which migh t have been the f i rs t signs of r ad i a t i on s ickness [9].

Su rp r i s ing ly in the l a s t d i a r y the wr i t ing of Mar ie Cur ie is absen t from November 17 to December 18, prec ise ly the t ime dur ing which the proofs for the exis tence of a second r a d i o e l e m e n t were accumula ted .

The t i t le of the t h i r d publ ica t ion which is co-authored by Gus tave B~mont is iden t ica l to t h a t of the second p a p e r wi th addi t ion of a s ingle word: On a new "strongly" radioactive substance contained in pitchblende [4]. In the f i rs t sentence the au thors r e m e m b e r t ha t t hey had ex t rac ted from pi tchblende a rad ioac t ive subs tance which m a y perhaps be considered as a new element . Aga in one notices the caution: the au thor s were s t i l l r e luc t an t to a s se r t officially the exis tence of polonium. But in the in t imacy of the l abo ra to ry the doubt is lifted: in the th i rd notebook the word polonium appea r s f requent ly .

The chemical behav iou r of the second radioact ive subs tance was s t r i k ing ly different from t h a t of polonium: i t did not p rec ip i t a te by hydrogen sulfide nor by ammonium sulfide: on the o ther h a n d i t coprec ip i ta ted wi th b a r i u m carbonate and sulfate. While Bi was the ca r r i e r of Po, Ba was the car r ie r of the new rad ioac t ive substance. The spec t ra of rad ioac t ive b a r i u m showed the charac te r i s t i c p a t t e r n of pure bar ium.

Once i t was sure t h a t the rad ioac t iv i ty was contained in b a r i u m it r ema ined to prove t ha t i t was not ba r ium, bu t a new element . This i m p o r t a n t demons t r a t i on was based on th ree tes ts . F i r s t , Mar ie Curie ver i f ied tha t n a t u r a l b a r i u m is not rad ioac t ive and conta ins no rad ioac t ive substance. She under took a f rac t ional c rys ta l l i sa t ion of 50 kg of commercia l b a r i u m chloride unt i l a f inal amount of 10 g was obtained. The res idue showed no ac t iv i ty wi th in the l imi t of sens i t iv i ty of the de tec tor which means t ha t the ac t iv i ty was less t h a n one h u n d r e d t h of t ha t of u ran ium.

As a very i m p o r t a n t point i t was possible to concent ra te the rad ioac t ive subs tance by f rac t ional c rys ta l l i sa t ion of b a r i u m chloride. This was based on the h igh solubi l i ty of the sa l t in w a t e r and i ts inso lubi l i ty in alcohol. The aqueous solut ion of radioact ive b a r i u m was t r e a t e d wi th alcohol and the f irst por t ions of the solid were much more act ive t h a n the r e m a i n i n g solution. The opera t ion was r epea t ed unt i l the act ivi ty of b a r i u m chloride was 900 t imes t h a t of u ran ium. At th is poin t the au thor s had to cease the concent ra t ion because the amount of ma te r i a l was too small . They ins i s t t ha t th is l imi t was provis ional : it is foreseeable that the activity would still have much increased if it would have been possible to pursue the concentration.

The th i rd and las t a r g u m e n t was decisive. Demarcay found in the spec t rum of the rad ioac t ive b a r i u m chloride severa l l ines which could not be ass igned to any known e l emen t [10]. The in t ens i ty of the most in tense new l ine increased wi th the radioac- t iv i ty of the subs tance , from very weak wi th the f i rs t s ample up to notable for the sample 900 t imes more act ive than u ran ium. Of course, the l ines of b a r i u m were the s t ronges t bu t the au thor s noted wi th g rea t sa t is fact ion t ha t the ma in l ine of the new

26 Czech. J. Phys. 49/Sl (1999)


. . . "~: ,~t c z . . . . . . A , - r -

,- <'=<-, ~ ,,7~ .--.;- &':~ .J'<,, i

<.%..,x- _ _ ~ . ~v,. 7

~,~ ._

�9 Ct= r )Y2= #, z~']v ~ = JJs', Z

" : ~ , , , , ~ _ 12. EC'i,'E +" . . . . . /-,::'~ ---- d , 3 Y Z,.2

/7 / i

&~dr

~ = 0, I/$5~ ;W:.:. O�9

lzz = / r o

Fig. 10�9 Marie Curie's first determination of the atomic weight of radium.

subs tance was more in t ense t h a n the weak l ines of ba r ium. F i n a l l y they wri te : we

t h i n k th is is a very serious reason to a t t r ibu te the new line to the radioact ive p a r t o f our

substance�9 The var ious reasons w h i c h we have e n u m e r a t e d lead us tot t h i n k tha t the

new radioact ive subs tance con ta ins a new e lemen t to wh ich we propose to give the n a m e

radium�9

Besides the spectroscopic ana lys i s , a second official proof for the exis tence of r a d i u m

was the m e a s u r e m e n t of the a tomic weight . Mar ie Curie proceeded to the de t e rmina - t ion us ing the class ical me thod based on the p rec ip i ta t ion of s i lver chlor ide from

rad ioac t ive b a r i u m chloride. The procedure was t e s ted wi th pu re BaC12 : the a tomic weight of b a r i u m was found be tween 137 and 139�9 On December 20 - th is da t e is t h a t

of the l a s t e n t r y in the notebook before the publ ica t ion - Mar ie Curie ob ta ined a f i rs t va lue of 142�9 for the a tomic weight of the rad ioac t ive me ta l on a 437 mg sample of rad i fe rous ba r ium, 227 t imes more act ive t han u r a n i u m (Fig. 10). This va lue was

s l ight ly h ighe r t h a n t h a t of b a r i u m bu t s t i l l w i th in l imi t s of er rors . Obviously the amoun t of r a d i u m was too smal l to change the a p p a r e n t a tomic weight of ba r ium.

When the Cur ies r a n out of m a t e r i a l t hey were aware t ha t vas t a moun t s of p i tchblende would be necessa ry in o rder to p repa re vis ible quan t i t i e s of r ad ium. They could not afford the pu rchase of th is expens ive ma te r i a l . Bu t they supposed correct ly

t ha t t he r e s idues of the ore af ter ex t rac t ion of u r a n i u m should conta in the new e lements Po and Ra. They also though t t h a t the res idues h a d no commerc ia l value�9 In a foot note of the pub l ica t ion the au thors t h a n k Professor Suess of the U n ive r s i t y of Vienna for his i n t e rven t ion wi th the A u s t r i a n gove rnmen t p ropr i e to r of the Joach im- s tha l mines�9 The Cur ies were offered free of charge 100 kg of r e s idues and the au thor s

acknowledged th i s wi l l g rea t l y fac i l i ta te our research .

C z e c h . J. Phys . 49 /S1 ( 1 9 9 9 ) 2 7

J. P. Adloff

With the foregoing discovery of Po, the authors had oddly enough begun with the most difficult part of the work. In its own right, radium had outstanding advantages; its concentration in the ore was about 5000 times greater than that of Po; it is a true chemical analogue of barium, from which it can be separated and it could be readily assigned to its correct position in the periodic table.

Probably in order to mark the difference with polonium the authors indicated in the title of the publication that the new element is strongly radioactive. Their radium was 900 times more active than uranium but polonium only 400 times. They commented: our preparation contains very likely a high proportion of barium; nevertheless the radio-activity is considerable; the radioactivity of pure radium must be enormous. Of course it could not be imagined that for an equal amount of substance polonium is 4000 times more active than radium.

At the end of this memorable year 1898 radiochemistry was born and 4 radioelements were known: U, Th, Po and Ra. However, despite of these prodigious discoveries, practically nothing was known about radioactivity itself.

I t was found that Po and Ra excite the fluorescence of a screen of barium platino- cyanide. This observation led the authors to conclude the publication of the discovery of radium in stating: a source of light which requires no energy can thus be obtained in contradiction, at least apparently, with the principle of Carnot. It is precisely this puzzle which had prompted Marie Curie to investigate the emission of uranic rays.

Acknowledgement

Madame H~lene Langevin is gratefully acknowledged for providing a copy of the notebooks of Pierre and Marie Curie.

References

[1] M. Curie: Pierre Curie, Editions Odile Jacob, Paris, 1996.

[2] M. Sklodowska Curie: Comptes Rendus Acad. Sciences 126 (1898) 1101.

[3] P. Curie and M. Curie: Comptes Rendus Acad. Sciences 127 (1898) 175.

[4] P. Curie, M. Curie and G. B~mont: Comptes Rendus Acad. Sciences 127 (1898) 1215.

[5] M. Curie: Revue G6n6rale des Sciences 10 (1898) 41.

[6] G. C. Schmidt: Verh. Dtsch. Phys. Ges. Berlin 17 (1898) 14.

[7] G. C. Schmidt: Comptes Rendus Acad. Sciences 126 (1898) 1264.

[8] E. Curie: Madame Curie, Gallimard, Paris, 1938.

[9] R. Reid: Marie Curie derriere la Hgende, Seuil, Paris, 1979.

[10] E. Demarcay: Comptes Rendus Acad. Sciences 127 (1898) 1218

28 Czech. J. Phys. 49/$1 (1999)

the laboratory notebooks of pierre and marie curie and the ... · pierre and marie curie, polonium...

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