much direct information about transuranic elements. Thus, the present work was undertaken to correct certain of these deficiencies, and provide further information. This paper is a progress report of what has been accomplished to date, primarily with material published before the early sixties. Consequently, most cratering shots have not as yet been considered in detail. The properties to be reported here are density, color, and specific activity. The first two properties are rather nonspecific and are dealt with rather briefly. The specific activity, however, is dealt with in some detail. Its general behavior as a function of particle size will be sketched, though not all relevant data have as yet been incorporated. Completion of the project is planned within the next three months. PHENOMENOLOGY As a first step towards our attempt to bring some order to the mass of data on radioactive fallout particles, we are presenting here a brief review of those early processes which are relevant to the generation of cloud particles. We focus first on the distinction between worldwide fallout, intermediate fallout, and close-in or local fallout and their relationship to various kinds of tests. The different types of fallout mentioned are qualitatively distinguished by their distance of occurrence from the location of the test. Local fallout is present in the immediate environment of the shot point, such as within the boundaries of the Nevada Test Site. Intermediate fallout is found beyond the region of occurrence of local fallout, perhaps for several hundred miles or more. Worldwide fallout occurs beyond the region of intermediate fallout. The occurrence and the relative magnitude of these different kinds of fallout depend on the particle sizes present in the cloud and on the altitude at which the cloud stabilizes, as the main mechanism for the appearance of fallout on the surface of the earth is gravitational settling through the atmosphere. Thus, the mean particle size of the fallout decreases with increasing distance from the shot point. This is not necessarily true anymore for worldwide fallout because the particle sizes are sufficiently small for these particles to remain suspended, and to be carried to the surface by moving air parcels and surface-air exchange mechanisms other than deposition by simple gravitational settling. For bursts occurring at the surface of soil or rock and above, the mean size of the cloud particles generally decreases with increasing height of burst, scaled to some reference yield such as 1 kiloton or 1 megaton. Thus, a surface burst will yield considerable local fallout, whereas a free-air burst yields virtually no local fallout. Tower bursts are intermediate between surface and airbursts; balloon shots approach freeair bursts with regard to deposition of fallout. 550