up less activity because many of the short-lived isotopes have decayed, and with the growth of the cloud, fewer of those isotopes remaining can be contacted by any one particle. The particles, as they cool below 1,700° C, are still able to pick up some activity on the outer surfaces. Krypton-89, krypton-90, and xenon-140, which are present during the formation of the fireball and are precursors for strontium-89, strontium90, and barium-140, have very little tendency to be incorporated uniformly in the particles during the early stage of formation. These roble gases, when associated with a particle, are deposited unevenly on ‘the surface layers and distributed along with relatively large deposits of inactive debris which were drawn toward the fireball too late to form fused radioactive particles. Tests made on sample particles from Nevada shots indicate that even in the case of fused particles, the radioactivity tends to concentrate toward the outside of the particle. The shape of the Nevada particles is essentially spherical and they may differ markedly one from the other in physical appearance. Some are black and ferromagnetic; others resemble glass beads; some appear to be glass beads that have been fractured, but all appear under the microscope to be different from the inactive soil collected in the same fall-out. The particles which have been collected in the South- west Pacific, although different in chemical composition from those collected in Nevada, follow essentially the same general pattern. The aize of these particles ranges from 0.02 micron to 1,000 microns. In the case of Pacific Proving Ground shots, calcium carbonate in the ground is heated to a very high temperature and is fused and vaporized, generally resulting in particles so fine that they are relatively unimportant as vehicles for immediate fall-out. However, a portion of the soil passes into the fireball and is heated to a temperature of about 2000° to 2700° C, which is sufficient to decompose the calcium carbonate to calcium oxide and also to soften the particle or to even melt it at ‘the higher temperature given.o/ When such particles become 6/ Adams, C. E., The Nature of Individual Radioactive Particles; II. Fall-out Particles from M-Shot, Operation IVY, USNRDL-408, July 1953, 14