In 1896 Becquerel discovered that uranium and its compounds emit these new rays continuously. Schmidt and Mme. Curie then found almost simultaneously that thorium compounds are also radioactive. The radiations emitted by thorium compounds are comparable in intensity with those from the compounds of uranium. Radioactivity is an atomic property that accompanies the atoms of uranium and thorium wherever they are found; in a compound or a mixture its intensity is proportional to the amount of the metal present.
New Radioactive Substances.—Mme. Curie, in 1898, tried to learn whether there were among the elements then known any others possessing radioactivity; she could not find a single substance giving any considerable radiation, and concluded that the radioactive properties of the elements are at least 100 times more feeble than those of uranium and thorium. She found, on the other hand, that certain minerals containing uranium (pitchblende, chalcolite, and carnotite) are more active than metallic uranium; the activity of these minerals could not, then, be due solely to uranium or to other known elements. This discovery was fertile with new results. Mme. Curie and I showed, in an investigation carried on together, that pitchblende contains new radioactive substances, and we supposed that these substances contained new chemical elements.
There are known with certainty three new strongly radioactive substances: polonium, which was found in the bismuth obtained from the uranium minerals; radium, found with barium from the same source, and actinium, which was discovered by Debierne among the rare earths extracted from the same minerals. These three substances are present only in infinitesimal quantities in the uranium minerals, and all three possess a radioactivity about a million times greater than that of uranium or thorium.
Recently Giesel and Hofmann announced the presence of a fourth strongly radioactive substance in the uranium minerals, which had properties closely resembling those of lead; from the publications that have appeared up to this time I have not been able to form an opinion as to the nature of this substance.
It may be asked whether radioactivity is a general property of matter. This question cannot be regarded as actually settled. The investigations of Mme. Curie have proved that the different known substances do not possess an atomic radioactivity one-hundredth as great as that of uranium or thorium. On the other hand, certain chemical reactions may cause the formation of ions, conductors of electricity, without the active substance giving any evidence of atomic radioactivity. Thus white phosphorus by its oxidation renders the surrounding air a conductor of electricity, while red phosphorus and the phosphates are not at all radioactive.
Some old experiments by Russell, Colson and Tengyel showed that certain substances act upon the photographic plate at a distance. It is possible that this phenomenon is partly due to radioactivity, but it is not certain. Recent work by MacTennan and Burton, Strutt, and Lester Cooke, leads to the supposition that radioactivity belongs to all bodies to an extremely slight extent. The identity of these very feeble phenomena with atomic radioactivity can not yet be regarded as proved.
Radium.—Of all the strongly radioactive substances radium is the only one that has been proved to be a new element. Radium possesses a characteristic spectrum, the discovery and first study of which we owe to Demarçay, and which has since been studied by Runge and Precht and by Crookes. Radium is an element belonging after barium in the series of alkaline earths. Its atomic weight as determined by Mme. Curie is 225.
At present radium is obtained from a residue left on extracting uranium from pitchblende. This residue contains 2 or 3 decigrams of radium in a ton. At first 10 or 15 kilograms of radiferous barium salt are extracted from a ton of the residue, and from this the radium is prepared by fractional crystallization (with the chloride or bromide), the crystals that form from the solution being richer in radium than the salt that remains in the liquid.
The radiant activity of the radium salt can be measured at different times from the beginning of crystallization until it is finally dried in the air-bath. It is found that the activity has a certain initial value, then it increases as a function of the time, at first rapidly, then more and more slowly; it approaches asymptotically a limiting value which is about five times as great as the initial activity. The activity then remains invariable for some years if the salt be left as it is.
Polonium.—Polonium is, on the contrary, a substance that slowly loses its radioactivity from the moment when it is separated from the uranium mineral containing it. After some years the radioactivity of polonium almost completely disappears. Hence it acts like an unstable substance. It has not yet been shown that polonium is a new element distinct from ordinary bismuth.