then done once more, to confirm that the intended particle had indeed Photomicrographs were prepared of each particle for the been mounted. record, but also to accompany the mounts for recognition by analysts. OBSERVATIONS AND RESULTS When the alpha emitter is contained in a small particle of a few millimicrons diameter or less, the tracks in the nuclear emulsion are radially oriented and appear to have emanated from a well-defined central point or very small area. However, when the alpha emitter is randomly distributed over the surface or throughout the volume of the particle, the orientation of the tracks is more random, although the general appearance of a star is maintained. Therefore, if one observes the track star generated by a large particle, or in a small particle attached to or incorporated in the large particle, one can determine the manner in which the alpha-emitter is incorporated into the observed particle. The application of these inferences to our observations on the isolated particles yields the result that, with the exception of the particles isolated from samples 16910 and 16911 and a few particles from samples 17153 and 17154, the plutonium is present in small particles attached to large ones, presumably soil. Generally, there was little difference between the particles from the two locations near a given detonation point. The exception was the pair of samples 16910 and 16911. The particles in 16910 contained a large amount of plutonium compared to the particles in samples from other Slides, and were easily recognized and isolated. Only one such particle was isolated from sample 16911. A number of other particles with plutonium were initially located on the slides, but these particles could not be isolated. They may have been very small particles attached to large particles, becoming separated from the host particles during manipulation, or they were very fragile and broke up when attempts were made to isolate them. Selected isolated particles are shown in Figure 2. The results of our measurements are listed in Table 1. The plutonium concentrations in the particles have been calculated as though the plutonium is distributed homogeneously throughout these particles. This fiction may have some validity if one wants to consider inhalation hazards, but is useless if one wants to search for size dependence of the concentration. Such a search was made with the data from sample 16910 but no correlation was found, although the particles from this sample were probably homogeneous. It has been our experience, however, that the variability of fission product and heavy element concentrations in particles of any given size generally extends over one or more orders of magnitude, so that 9 or 10 particles are not sufficient to establish the presence or absence of a concentration-particle size with any degree of confidence. 540