Pierre Curie (1859-1906) and Marie Sklodowska Curie (1867-1934)

On a New Radioactive Substance Contained in Pitchblende[1]

note by M. P. Curie and Mme. S. Curie, presented by M. Becquerel

Comptes Rendus 127, 175-8 (1898) translated and reprinted in Henry A. Boorse and Lloyd Motz, eds., The World of the Atom, Vol. 1 (New York: Basic Books, 1966)

Certain minerals containing uranium and thorium (pitchblende, chalcolite, uranite) are very active from the point of view of the emission of Becquerel rays. In a previous paper, one of us has shown that their activity is even greater than that of uranium and thorium, and has expressed the opinion that this effect was attributable to some other very active substance included in small amounts in these minerals.[2]

The study of uranium and thorium compounds has shown in fact that the property of emitting rays which make the air conducting and which affect photographic plates, is a specific property of uranium and thorium that occurs in all compounds of these metals, being weaker in proportion as the active metal in the compound is diminished. The physical state of the substances appears to have an entirely secondary importance. Various experiments have shown that the state of mixture of these substances seems to act only to vary the proportions of the active bodies and the absorption produced by the inert substances. Certain causes (such as the presence of impurities) which have so great an effect on the phosphorescence or fluorescence are here entirely without effect. It is therefore very probable that if certain minerals are more active than uranium and thorium, it is because they contain a substance more active than these metals.

We have sought to isolate this substance in pitchblende and experiment has just confirmed the preceding conjectures.

Our chemical researches have been guided constantly by a check of the radiant activity of the separated products in each operation. Each product was placed on one of the plates of a condenser and the conductivity acquired by the air was measured with the aid of an electrometer and a piezoelectric quartz, as in the work cited above. One has thus not only an indication but a number which gives a measure of the strength of the product in the active substance.

The pitchblende which we have analysed was approximately two and a half times more active than uranium in our plate apparatus. We have treated it with acids and have treated the solutions obtained with hydrogen sulfide. Uranium and thorium remain in solution. We have verified the following facts:

The precipitated sulphides contain a very active substance together with lead, bismuth, copper, arsenic, and antimony. This substance is completely insoluble in the ammonium sulphide which separates it from arsenic and antimony. The sulphides insoluble in ammonium sulphide being dissolved in nitric acid, the active substance may be partially separated from lead by sulphuric acid. On washing lead sulfate with dilute sulphuric acid, most of the active substance entrained with the lead sulphate is dissolved.

The active substance present in solution with bismuth and copper is precipitated completely by ammonia which separates it from copper. Finally the active substance remains with bismuth.

We have not yet found any exact procedure for separating the active substance from bismuth by a wet method. We have, however, effected incomplete separations as judged by the following facts:

When the sulphides are dissolved by nitric acid, the least soluble portions are the least active. In the precipitation of the salts from water the first portions precipitated are by far the most active. We have observed that on heating pitchblende one obtains by sublimation some very active products. This observation led us to a separation process based on the difference in volatility between the active sulphide and bismuth sulphide. The sulphides are heated in vacuum to about 700° in a tube of Bohemian glass. The active sulphide is deposited in the form of a black coating in those regions of the tube which are at 250° to 300°, while the bismuth sulphide stays in the hotter parts.

More and more active products are obtained by repetition of these different operations. Finally we obtained a substance whose activity is about four hundred times greater than that of uranium. We have sought again among the known substances to determine if this is the most active. We have examined compounds of almost all the elementary substances; thanks to the kindness of several chemists we have had samples of the rarest substances. Uranium and thorium only are naturally active, perhaps tantalum may be very feebly so.

We believe therefore that the substance which we have removed from pitchblende contains a metal not yet reported close to bismuth in its analytical properties. If the existence of this new metal is confirmed, we propose to call it polonium from the name of the country of origin of one of us.

M. Demarçay has been kind enough to examine the spectrum of the substance which we studied. He was not able to distinguish any characteristic line apart from those ascribable to impurities. This fact is not favourable to the idea of the existence of a new metal. However, M. Demarçay called our attention to the fact that uranium, thorium, and tantalum exhibit spectra formed of innumerable very fine lines difficult to resolve.[3,4]

Allow us to note that if the existence of a new element is confirmed, this discovery will be uniquely attributable to the new method of detection that Becquerel rays provide.

[1]This work was done at the Municipal School of Industrial Physics and Chemistry. We particularly thank M. Bémont, head of chemical operations, for his advice and the assistance he willingly provided. --original note

[2]Mme. P. Curie, Comptes Rendus, vol. 126, p. 1101. --original note

[3]The peculiarity of these three spectra is described in the fine work of M. Demarçay, Electric Spectra (1895). --original note

[4]The excerpt in Boorse and Motz ends here. The remainder of the paper was translated by Carmen Giunta, as were the original footnotes.--CJG

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