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THE paramagnetic resonance of picrylaminocarbazyl (PAC) and Wurster's blue perchlorate (WB) at very low temperatures (1. S-4 K) has been studied by Edelstein and Mande1, l and they have suggested that these solid free radicals become ferromagnetic at sufficiently low temperatures. A recent single-crystal resonance study2 of WB indicates that the elementary electronic excitations in this substance are triplet excitons, and that the resonance studied by Edelstein and Mandel in WB is very probably due to an impurity. However, the work of Edelstein and Mandel still raises the question of ferromagnetic interaction in PAC and in solid free radicals in general. Present experimental evidence2-s indicates that the paramagnetic susceptibilities of most crystalline aromatic free radicals approach zero as T~ O, when allowance is made for the impurity spins that are almost always present in such crystals. It is reasonable to …