The radioactive fallout particles were white, opaque, irregularly shaped grains, Some of them were fluffy and very fragile while others appeared hard and dense. They varied in size from about 25 pt to 1 or 2 mm. in diameter, X-ray diffraction analyses showed that they were composed primarily of caleium hydroxide and calcium carbonate (calcite structure). Other compounds occurring in minor amounts were calcium oxide, calcium carbonate (aragonite structure), sodium chloride and magnesium carbonate triehydrate, Tentatively identified as present in several particles were calcium nitrate tetra-hydrate and calcium sulfate dihydrate and hemihydrate,. Studying the thin sections of the fallout particles with the petrographic microscope gave a detailed picture of the distribution of the hydroxide and carbonate. Most of the particles were composed largely of calcium hydroxide in the central part with an outer layer of calcium carponate. The thickness of the outer layer of carbonate varied froma few microns up to about 1004. While the areas of the two compounds were distinct, the transition between them was sufficiently gradual to indicate that the outer carbonate layer had been formed by the carbonation of the calcium hydroxide, Occasionally, a particle was found with an inner core of unaltered calcium carbonate and an outer zone of calcium hydroxide. A few particles were found with cores of calcium oxide the outer layer of which had been hydrated to calcium hydroxide. Some radioactive particles consisting of unaltered coral grains were found, 43.2 Distribution of Activity By making radioautogranhs of the thin sections with Eastman NTB stripping film, a knowledge of the distribution of the radioactivity within the particles was obtained. In practically all of the thin sections studied, the radioactivity was located on the exterior of the particle. The activity was most intense on the surface and diminished fairly abruptly to very low levels at depths of 50 to 1504. The graded appearance of the boundary suggested that the penstration of the activity into the particle was by solution and deposition, The distribution of the activity was independent of the compositional structure of the particle. In a few instances, the radioactivity was distributed ir-egularly in patches throughout the particles. In these cases, the particles themselves usually had spherical or spheroidal shapes as contrasted with the angular shapes of the particles in which the activity was found on the exterior, 4.3.3 Solubility Four samples of fallout material from Shot 1 were leached in water for varying times. An initial separation was then made into soluble and insoluble fractions by filtering the suspensions through millipore filters. According to the manufacturer's description, millipore filters are completely retentive for particles one-half micron and larger in diameter. The filtrate was then subdivided into ionic and colloidal fractions by treatment in an electrodialysis cell. The results are tabulated below: 97