Unmelted calcium oxide particles can logically be expected not to have been in the hotter portions of the fireball. They then retain both their irregular shape and their porous structure. tionation effects result in Frac- relatively nore of the radionuclides that are volatile or whose progenators are volatile depositing on these cooler, and consequently unmeited, particles than on the hotter melted particles. On the other hand the spherical particles of calcium oxide are formed from melted calcium oxide in the hotter portions of the fireball. These then lose the porous structure of unmelted particles, with the result that hydration in the particles of melted origin proceeds at a much slower rate. - These spherical particles can logi- nally be expected te contain mere radicactive debris ef teth structural material and fission products since more of these vapors would have been in contact with the molten calcium oxide. Particle Sizes Fallout particles range in size from particles smaller than fine sand, i.e., approximately 100 microns in diameter, in the more distant portions of the fallout area to pieces about the size of marbles, i.e., roughly 1 cm in diameter, close to the point of the explosion, 1964, p. 41). (Glasstone, For ground surface bursts, the distribution in the size of the fallout particles is lognormal (normal or Gaussian distribution law with the logarithm of the particle diameter as the variable) with mass medians in the order of 100 microns and with logarithmic standard