0018933 1,-2.1 Formation and Nature of Fallout Particles, When a surface burst is detonated, great quantities of the adjacent environment are swept up and mixed with the incandescent air in the fireball, There is sufficient thermal energy in the hot gas to completely vaporize all the material in the immediate vicinity, but the flow of heat into a massive object, such as a shot tower, shield or coral rock, will be comparatively slow even with a high temperature gradient. Consequently, the interior portions of large structures in the neighborhood may not receive enough heat to evaporate and will be melted only, Later, when the fireball has risen above the surface, the material carried into it by the vertical air currents around ground zero will not be heated to the melting point. As a result, the fire- ball in its later stages will contain the environmental components as a& mixture of solid particles, molten drops and vapor, The extraneous ma- terial in the Pacific shots will consist of coral and ocean water salts plus the components of the device, shield, and tower or barge. The preponderance of oxygen and of the environmental material in the fireball is of outstanding importance in the formation of the fallout particles. As the hot air cools through the range 3500-1000 K®,, it be- comes saturated with respect to the vaporized constituents and they con(Reference 3) dense out as an aggregate of liquid drop a most of which are very small (References 4 and 5). These are mixed with the larger drops formed by SAN BRUNO FRE 14