Some time after Becquerel's discovery, Mme. Curie made a systematic examination by the electric method of a large number of chemical elements and their compounds to test whether they possessed the "radioactive" property of uranium. Only one other element, thorium, was found to show this effect to a degree com parable with that of uranium—a result independently observed by Schmidt. Mme. Curie examined the activity of the various com pounds of uraniurri and found that their radioactivity was an atomic property, i.e., the activity was proportional to the amount of the element uranium present, and was independent of its com bination with other substances. In testing the activity of the minerals containing uranium, Mme. Curie found that the activity was always four to five times as great as that to be expected from their content of uranium. If the radioactivity were an atomic phenomenon, this could only be explained by the presence in these minerals of another substance more active than uranium itself.
Relying on this hypothesis, Mme. Curie made a chemical examina tion of uranium minerals in order to try to separate this new radioactive substance. In these experiments, the Austrian Govern ment generously provided Mme. Curie with a ton of the residues from the State manufactory of uranium at Joachimstahl, Bo hemia. At that place there are extensive deposits of pitchblende or uraninite which are mined for the uranium. After separation of the latter, the residues are three to five times as radioactive weight for weight as the uranium. From this residue Mme. Curie separated a substance far more radioactive than uranium, which she called polonium in honour of the country of her birth. This substance is usually removed with bismuth in the mineral, but by special methods can be partly separated from it. A further exami nation revealed the presence of a second radioactive substance which is normally separated with the barium, to which the name "radium" was given. This name was happily chosen, for in the pure state radium bromide has a very great activity—about two million times as great as an equal weight of uranium. By means of successive fractionations of the chloride, the radium was gradu ally concentrated, until finally the radium was obtained so that the barium lines showed very faintly. The atomic weight was found by Mme. Curie to be 226. Radium was found to give a characteristic spark spectrum of bright lines analogous in many respects to the spectra of the alkaline earths. Giesel in the early days took an active part in the preparation of pure radium com pounds, and was the first to place preparations of pure radium bromide on the market. He found that the separation of radium from the barium mixed with it proceeded much more rapidly if the crystallizations were carried out using the bromide instead of the chloride. He states that six to eight crystallizations are suf ficient for an almost complete separation. From the chemical point of view radium possesses all the characteristic properties of a new element. It has a definite atomic weight, a well-marked and characteristic spectrum, and distinct chemical properties. Its
comparative ease of separation and great activity has attracted much attention to this substance, although we shall see that very similar radioactive properties are possessed by a large number of distinct substances.
In addition to polonium and radium, a number of other radio active substances have been separated from uranium and thorium minerals which have an activity comparable with that of radium, although they have not been isolated in a pure state. Amongst these may be mentioned actinium, discovered by Debierne and Giesel, ionium, discovered by Boltwood and radiothorium and mesothorium discovered by Hahn.
Besides these substances in which the duration of the activity is measured by years a large number of radioactive bodies have been found which have a transient activity lasting for only a few minutes or hours. It will be seen that from the scientific point of view these short lived radioactive substances are just as im portant as those which have a much longer life.