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LETTERS TO THE EDITOR 603 be expected from the uncertainties in both the theory and the experiment.Work has been done on a number of other materials including MnO, Cr 2 0 3 , and ZnFe 2 0 4 and is being continued with a view to studying the dependence of the energy distribution on the temperature, the angle of scattering, and the degree of magnetic order. An account of this work will be submitted to the Canadian Journal of Physics. Energy distributions of neutrons scattered by vanadium metal under several different conditions have been measured by the author with results in agreement with the theory. This work is being continued with a view to obtaining the frequency distribution of the normal modes in vanadium metal.

The lattice expansion in neutron-irradiated LiF was investigated in order to establish either Frenkel or Schottky defects as the cause of the expansion. For uniformly irradiated crystals, it was shown that x-ray lattice parameter and density changes are equivalent to within 6 percent. It follows that Schottky defects cannot account for more than 6 percent of the expansion and that Frenkel defects are predominant. The annealing of a nonuniformly irradiated crystal indicates a recovery process of order higher than one.