Atomic Energy Commission, the universities, or several of the industries . By way of illustration of the increase in knowledge that has resulted, the number of known isotopes has doubled from around 500 in 1939 to somewhat over 1,000 at the present time. The increase of course has occurred princi pally among the radioactive isotopes, though the number of stable isotopes has risen from 261 to 283 in this period, while the radioactive species have risen from 220 to somewhat over 730. Nine new elements are reported in the newly discovered isotopes: five transuranic (neptunium, plutonium, ameri cium, curium and berkIium) and technetium (element 43), prometheum (element 61), astatine (element 85), and francium (element 87). The dis covery of these isotopes has of course made possible a considerable eludication of the chemical properties of these missing elements. The discovery of the fission process (1) and the ensuing intense investi gation of its characteristics, particularly for U236, revealed that about 250 isotopes, largely radioactive, are produced. The development of the chain reacting pile provided a source of these isotopes of previously undreamed of magnitude, tens and hundreds of megacuries being the equilibrium amounts developed in a single pile for the fission products of highest yield. This new factor required the immediate development of a new branch of radiochemistry, called devoted to the techniques of han dling hundreds, thousands, or even millions of curies of radioactivity, It also led to an acceleration of the researches in radiation chemistry because of the iJJ?portance of radiation decomposition effects on the st ructural materials of the pile itself and on the solutions and chemicals used in the separation of the plutonium produced in the uranium metal in the piles (this subject is treated elsewhere in this volume: cf. article on radiation chemistry), The branch of radiochemistry known as hot atom chemistry, which is concerned with the chemical effects of nuclear transformations on those atoms under going the transformations, as distinguished from the radiation chemistry