Pierre Curie (1859-1906), Marie Sklodowska Curie (1867-1934), and G. Bémont

On a new, strongly radioactive substance, contained in pitchblende[1]

note by M. P. Curie, Mme. P. Curie, and M. G. Bémont, presented by M. Becquerel

Comptes Rendus 127, 1215-7 (1898) translated and reprinted in Henry A. Boorse and Lloyd Motz, eds., The World of the Atom, vol. 1 (New York: Basic Books, 1966)

Two of us have shown that by purely chemical processes one can extract from pitchblende a strongly radioactive substance. This substance is closely related to bismuth in its analytical properties. We have stated the opinion that pitchblende may possibly contain a new element for which we have proposed the name polonium.[2]

The investigations which we are now following are in accord with the first results obtained, but in the course of these researches we have found a second substance strongly radioactive and entirely different in its chemical properties from the first. In fact, polonium is precipitated out of acid solutions by hydrogen sulphide, its salts are soluble in acids, and water precipitates them from these solutions; polonium is completely precipitated by ammonia.

The new radioactive substance that we have just found has all the chemical aspects of nearly pure barium: It is precipitated neither by hydrogen sulphide, nor ammonium sulphide, nor by ammonia; the sulphate is insoluble in acids and water; the carbonate is insoluble in water; the chloride, very soluble in water, is insoluble in concentrated hydrochloric acid and in alcohol. Finally this substance shows the easily recognized spectrum of barium.

We believe nevertheless that this substance, although constituted for the greater part by barium, contains in addition a new element which gives it its radioactivity and which moreover is very close to barium in its chemical properties. The following are the reasons which argue in favour of this view:

1. Barium and its compounds are not ordinarily radioactive but one of us has shown that radioactivity appears to be an atomic property, persisting in all the chemical and physical states of the material.[3] From this point of view, if the radioactivity of our substance is not due to barium, it must be attributed to another element.

2. The first preparations which have been obtained, in the form of the hydrated chloride, have a radioactivity sixty times stronger than that of metallic uranium (the radioactive intensity being evaluated by the conductivity of the air in our plate apparatus). In dissolving these chlorides in water and in precipitating a part with alcohol, the part precipitated is much more active than the part remaining in solution. Based on this fact, one ought to be able to effect a series of fractionations, securing an activity nine hundred times greater than that of uranium. We have been stopped by the lack of material but, from the progress of the work, it is anticipated that the activity would have increased much more if we had been able to continue. These facts may be understood in terms of the presence of a radioactive element whose chloride is less soluble in a water solution of alcohol than that of barium.

3. M. Demarçay has examined the spectrum of our material so obligingly that we cannot thank him enough. These results of his examination are set forth in a special note following ours. M. Demarçay has found in the spectrum a line which does not appear to belong to any known element. This line, hardly visible with the chloride sixty times more active than uranium, is considerably stronger when the chloride is enriched by fractionation to nine hundred times that of uranium. The intensity of this line thus increases at the same time as the radioactivity and this we think is a very weighty reason for attributing the radioactive part of our substance to it.

The various reasons which we have just enumerated lead us to believe that the new radioactive substance contains a new element to which we propose to give the name radium.

We have determined the atomic weight of our active barium by titrating chlorine in the anhydrous chloride. We have found values which differ very slightly from those obtained in a parallel manner with inactive barium chloride; however, the values for the active barium are always a little greater but the difference is of the order of magnitude of the experimental errors. The new radioactive substance certainly contains a large proportion of barium, despite the fact that the radioactivity is considerable. The radioactivity of radium ought therefore to be enormous.

Uranium, thorium, polonium, radium, and their compounds make the air a conductor of electricity and expose photographic plates. From these two points of view, polonium and radium are considerably more active than uranium and thorium. On photographic plates one obtains good images with radium and polonium in a half-minute exposure; it takes several hours to obtain the same results with uranium or thorium.

The rays emitted by the compounds of polonium and radium make barium platinocyanide fluorescent. Their action from this point of view is analogous to that of Roentgen rays, but considerably weaker.[4] To make the experiment, one places on the active substance a very thin leaf of aluminum, upon which a thin film of barium platinocyanide is spread; in the dark, the platinocyanide appears weakly luminous in front of the active substance.

Thus one contructs a source of light, a very weak one to tell the truth, but one that functions without a source of energy. There is a contradiction, or an apparent one at the very least, with Carnot's principle.

Uranium and thorium produce no light under these conditions, their activity being probably too weak.[5]

[1]This work was done at the Municipal School of Industrial Physics and Chemistry. --original note

[2]M. P. Curie and Mme. P. Curie, Comptes Rendus, vol. 127, p. 175. --original note

[3]Mme. P. Curie, Comptes Rendus, vol. 126, p. 1101. --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

[5]Here we wish to thank M. Suess, a Corresponding member of the Institute, Professor at the University of Vienna. Thanks to his gracious intervention, we received from the Austrian government a shipment, at no charge, of 100 kg of a residue from the processing of Joachimsthal pitchblende that no longer contained uranium, but did contain polonium and radium. This shipment greatly assisted our research. --original note

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