The discovery of radioactivity by Becquerel in 1896 (touched off by Roentgen's discovery of x rays the year before) gave an even more sensitive method of detecting the presence or absence of certain kinds of matter. It is well known that Pierre and Marie Curie used this new-found radioactivity to identify the new elements polonium and radium. Compounds of these new elements were obtained by patient fractional recrystallization of their salts. The "explanation" of radioactivity led to the discovery of isotopes by Rutherford and Soddy in 1914, and with this discovery a revision of our idea of elements became necessary. Since Boyle, it had been assumed that all atoms of the individual elements were identical and unlike any others, and could not be changed into anything simpler. Now it became evident that the atoms of radioactive elements were constantly changing into other elements, thereby releasing very large amounts of energy, and that many different forms of the same element (lead was the first studied) were possible. We now think of an element as a form of matter in which all atoms have the same nuclear charge. The human mind has always sought order and simplification of the external world; in chemistry the fruitful classifications were Dobereiner's Triads (1829), Newland's law of octaves (1865), and Mendeleev's periodic law (1869). The chart expressing this periodic law seemed to indicate the maximum extent of the elements and gave good hints "where to look for" and "the probable properties of" the remaining ones (see Fig. 2). By 1925, all but four of the slots in the 92-place file had been filled. The vacancies were at 43, 61, 85, and 87. [Illustration: Fig. 2. Periodic chart of the elements (1963)] Workers using traditional analytical techniques continued to search for these elements, but their efforts were foredoomed to failure. None of the nuclei of the isotopes of elements 43, 61, 85, and 87 are stable; hence weighable quantities of them do not exist in nature, and new techniques had to be developed before we could really say we had "discovered" them. In 1919, Rutherford accomplished scientifically what medieval alchemists had failed to do with "magic" experiments and other less sophisticated techniques. It wasn't gold (the goal of the alchemists) he found but something more valuable with even greater potential for good and evil: a method of transmuting one element into another. By bombarding nitrogen nuclei with alpha particles from radium, he found that nitrogen was changed into oxygen. The process for radioactive transmutation is somewhat like a common chemical reaction. An alpha particle, which has the same charge (+2) and atomic mass (4) as a helium nucleus, penetrates the repulsive forces of the nitrogen nucleus and deposits one proton and one neutron; this changes the nitrogen atom into an oxygen atom. The reaction is written {7}N<14> + {2}He<4> --> {1}H<1> + {8}O<17>.