THE gain of energy of cold neutrons on being scattered by crystalline materials has been investigated with a beam of lead-filtered neutrons from the Harwell pile. The maximum energy of the neutrons in the beam was 0.0026 eV. and the mean energy was 0.0017 eV. The experimental arrangement is shown in the accompanying diagram. The counter and sample were carefully shielded from stray neutrons, and the sample was enclosed in a vacuum chamber to eliminate air scattering. The angle of scattering was 30 ± 5°. The transmission of a lithium fluoride plate (0.14 gm./cm.2) was measured in positions 1 and 2 for different scattering samples. The transmission of the plate had been measured as a function of energy with a mechanical spectrometer, so that a mean energy of the beam could be determined from these measurements. From the measurement in position 1 the mean energy of the neutrons which are scattered into the counter is determined before they are scattered, and from the measurement in position 2 the mean energy is determined after scattering. The results are shown in the accompanying table.