found in nature as a small constituent of the natural decay of actinium.

The last of the original 92 elements to be discovered was element 87, francium. It was identified in 1939 by French scientist Marguerite Perey.

Children have a game in which they pile blocks up to see how high they can go before they topple over. In medieval times, petty rulers in their Italian states vied with one another to see who could build the tallest tower. Some beautiful results of this game still remain in Florence, Siena, and other Italian hill cities. Currently, Americans vie in a similar way with the wheelbase and overall length of their cars. After 1934, the game among scientists took the form of seeing who could extend the length of the periodic system of the elements; as with medieval towers, it was Italy that again began with the most enthusiasm and activity under the leadership of Enrico Fermi.

Merely adding neutrons would not be enough; that would make only a heavier isotope of the already known heaviest elements, uranium. However, if the incoming neutron caused some rearrangement within the nucleus and if it were accompanied by expulsion of electrons, that would make a new element. Trials by Fermi and his co-workers with various elements led to unmistakeable evidence of the expulsion of electrons (beta activity) with at least four different rates of decay (half-lives). Claims were advanced for the creation of elements 93 and 94 and possibly further (the transuranium elements, Table I). Much difficulty was experienced, however, in proving that the activity really was due to the formation of elements 93 and 94. As more people became interested and extended the scope of the experiments, the picture became more confused rather than clarified. Careful studies soon showed that the activities did not decay logarithmically—which means that they were caused by mixtures, not individual pure substances—and the original four activities reported by Fermi grew to at least nine.

As a matter of fact, the way out of the difficulty had been indicated soon after Fermi's original announcement. Dr. Ida Noddack pointed out that no one had searched among the products of Fermi's experiment for elements lighter than lead, but no one paid any attention to her suggestion at the time. The matter was finally cleared up by Dr. Otto Hahn and F. Strassmann. They were able to show that instead of uranium having small pieces like helium nuclei, fast electrons, and super-hard x-rays, knocked off as expected, the atom had split into two roughly equal pieces, together with some excess neutrons. This process is called nuclear fission. The two large pieces were unstable and decayed further with the loss of electrons, hence the β activity. This process is so complicated that there are not, as originally reported, only four half-lives, but at least 200 different varieties of at least 35 different elements. The discovery of fission attended by the release of enormous amounts of energy led to feverish activity on the part of physicists and chemists everywhere in the world. In June 1940, McMillan and Abelson presented definite proof that element 93 had been found in uranium penetrated by neutrons during deuteron bombardment in the cyclotron at the University of California Radiation Laboratory.

The California scientists called the newly discovered element neptunium, because it lies beyond the element uranium just as the planet Neptune lies beyond Uranus. The particular isotope formed in those first experiments was ₉₃Np²³⁹; this is read neptunium having a nuclear charge of 93 and an atomic mass number of 239. It has a half-life of 2.3 days, during which it gives up another electron (β particle) and becomes element 94, or plutonium (so called after Pluto, the next planet beyond Neptune). This particular form of plutonium (₉₄Pu²³⁹) has such a long