from the median. The examination of samples by electron microscope shows the maximum number to occur below 0.1 micron in diameter, with particle sizes as low as 0.01 micron being present. The particle size of 0.9 micron is theorized to contain most of the radioactivity from an air burst. The particle size of 0.2 to 8 microns is the range of par- ticle size which has been found to be an inhalation hazard, inasmuch as such particles can be inhaled into the lungs and retained. The radio- activity from these particles can exert its influence on the lung where the particles lodge if insoluble, or may be transported to other tissues if the particles are in a soluble form. Unlike an air burst with its fine particles, when an atomic weapon is detonated on or near the ground so that the fireball intercepts the surface, molten earth is drawn into the cloud and is present when the condensation phase of the oxidized atoms of the bomb debris occwrs. The size, shape and distribution of the particles formed are influenced by the chemical composition and the original particle sizes of the earth which is fused. The two test sites normally used by the United States for atomic bomb testing have widely different soils; at the Nevada Test Site silicates predominate, while at the Pacific Proving Ground the land surface is almost entirely calcium carbonate. _ When the earth contains 510,, as it does at the Nevada Test Site, molten silicate is drawn up into the elouad/ and swept around in typical toroidal motion. The molten silicate most significant for particle formation is that portion which is swept up with the fireball and only reaches temperatures of 2,000° to 2,800° C. It is theorized that these hedted particles are swept around at the top of the stem or in the cloud in an atmosphere of fission products, plutonium and bomb case fragments. It is further hypothesized that condensation or agglomera- tion results in the bomb debris being deposited uniformly in the center of the molten silica. As the particles cool, they tend to pick 5/ Tompkins, R. C., Krey, P. W., Radiochemical Studies on Size-graded Fall-out and Filter Samples from Operation JANGLE. Radiological Payision, U. S. Army Chemical Corps, August 1952, SECRET Restricted 13