P. AND M. S. CURIE

Polonium

Certain minerals containing uranium and thorium (pitch blende, chalcolite, uranite) are very active in emitting Becquerel rays. One of us has already shown that their activity is greater than that of uranium and thorium, and has expressed the opinion that this effect arises from some other very active substance contained in these minerals in small quantity.

The study of the compounds of uranium and of thorium has shown, in fact, that the property of emitting rays which make air conducting and which act on photographic plates, is a specific property of uranium and of thorium, which appears in all the compounds of these metals, being so much the more feeble as the proportion of the active metal in the compound is itself less. The physical state of the substances seems to be of altogether secondary importance. Various experiments have shown that if the substances are mixed with others their condition seems to have no effect except as it varies the proportion of the active body and the absorption produced by the inert substance. Certain causes, such as the presence of impurities, which have so great an effect on phosphorescence or fluorescence, are here altogether without effect. 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 attempted to isolate this substance in pitch blende, and the experiment has confirmed our expectations.

Our chemical investigations have been guided by the tests made of the radiating activity of the products which were separated in each operation. Each product was placed on the plates of a condenser, and the conductibility acquired by the air was measured by an electrometer and a piezoelectric quartz, as in the work already referred to. We thus have not only an indication but a number which gives some measure of the richness of the product in the active substance.

The pitch blende that we analyzed was about two and a half times more active than uranium in our apparatus.

By carrying on these different operations we obtained products which were more and more active. Finally, we obtained a substance whose activity is about 400 times greater than that of uranium.

We have attempted to discover among bodies which are already known if there are any which are radioactive. We have examined compounds of almost all the simple bodies; thanks to the kindness of several chemists we have received specimens of very rare substances. Uranium and thorium were the only ones which were evidently active, tantalum perhaps is very feebly so.

We believe, therefore, that the substance which we removed from pitch blende contains a metal which has not yet been known, similar to bismuth in its chemical properties. If the existence of this new metal is confirmed, we propose to call it polonium, after the name of the native country of one of us.

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