Que 0018933 of this nuclide to the number of atoms of a reference substance (usually Mo?%) in the sample divided by the sameratio for the products of thermal neutron fission, Atoms which do not separate from the reference sub- stance have R-values appropriate for the type of detonation, while en- richment or depletion are manifested by positive or negative deviations from the characteristic curve, Knowledge of the causes and mechanism of fractionation is still largely incomplete at the present time. One effect that seems to be indi- cated by the available data may occur in the isobaric chains near mass numbers 90 and 140 which contain rare gas nuclides as prominent chain . members, These have half-lives and independent fission yields such that they comprise a considerable fraction of the total chain yield during the period when the environmental material is condensing. If the rare gas atoms which collide with the liquid drops of environmental material are not held by strong forces, as appears probable, the particles formed at this stage will be depleted in the nuclide chains in question. A variety of types of particles have been observed in the local fallout at previous test series (References 8,9,10,11, 12, 13}. For land surface shots inthe Pacific they have been mainly of three kinds: irregular grains, spherical solids and fragile agglomerated flakes. The grains were not, in general, uniform throughout, but consisted of layers or shells of calcium SAN BRUNO FRE 17