On a new radioactive substance contained in pitchblende (1898, pages 175-178, Sur une substance nouvelle radio-active, contenue dans la pechblende, Marie Curie's Polonium paper)

Some 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 work, one of us showed that their activity is even greater than that of uranium and thorium, and expressed the opinion that this effect was due to some other very active substance contained in small quantities in these minerals.

The study of compounds of uranium and thorium had shown, in fact, that the property of emitting rays which make air conductive and which act on photographic plates is a specific property of uranium and thorium which is found in all compounds of these metals, all the more weakened as the proportion of active metal in the compound is itself lower. The physical state of the substances seems to have a completely secondary importance. Various experiments have shown that the state of mixture of the substances seems to act only by varying the proportion of active bodies and the absorption produced by the inert substances. Certain causes (such as the presence of impurities) which act so powerfully on phosphorescence or fluorescence are therefore here completely without action. It therefore becomes 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 experience has confirmed the above predictions.

Our chemical research has been constantly guided by the control of the radiant activity of the products separated at each operation. Each product is placed on one of the plates of a condenser, and the conductivity acquired by the air is measured using an electrometer and a piezoelectric quartz, as in the work cited above. We thus have not only an indication but a number which accounts for the richness of the product in active substance.

• the new elemnt is next to bismuth. If you stop now and look at a periodic table, you will see that bismuth is exactly one element before polonium
• "simple bodies" is their cute terminology for atoms, whose existence at the time not yet full aknowledged Section "History of the atomic theory"
• we call it polonium

By carrying out these various operations, we obtain increasingly active products. Finally, we obtained a substance whose activity is about 400 times greater than that of uranium.

We have sought, among the bodies currently known, to see if there are any active ones. We have examined compounds of almost all simple bodies; thanks to the great kindness of several chemists, we have had samples of the rarest substances. Uranium and thorium are the only ones clearly active, tantalum is perhaps very weakly so.

We therefore believe that the substance that we have removed from the pitchblende contains a metal not yet reported, close to bismuth by 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.

Mr. Demarçay was kind enough to examine the spectrum of the body that we are studying. He was unable to distinguish any characteristic line apart from those due to impurities. This fact does not support the idea of ​​the existence of a new metal. However, Mr. Demarçay pointed out to us that uranium, thorium and tantalum offer particular spectra, formed of innumerable lines, very fine, difficult to perceive.

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.