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Microscopic examination of (NH,),Cr,O, crystallites, previously decomposed to measured extents, has given strong evidence that reaction proceeds through the intervention of a melt. This reaction has previously been accepted as occurring in the solid phase and the use of higher magnifications than were applied previously revealed the progressive development of foam-like zones of active reaction. These structures are termed 'fusion nuclei'. We conclude that an initial, and probably reversible, dissociation of the reactant