Diffraction patterns of low energy electrons produced by a (111) face of a crystal of nickel have been accelerated sufficiently to show on a fluorescent screen, where they can be photographed. Various gases at low controlled pressures have been admitted to a clean surface, and the arrangements of atoms or molecules upon the surface deduced from their diffraction patterns. Molecules of CO are adsorbed at low pressures as monolayers. They form a single two-dimensional crystal having a spacing between molecules double that between surface nickel atoms; these molecules are completely removed from the surface by heating to 175°c. Experiments with nitrogen have produced a polycrystalline monolayer having the same structure as the CO crystal. (Because of low sticking probability adequate experiments to prove that the layer is really nitrogen are difficult and have not yet been satisfactorily completed. A new type of experimental equipment seems to be required which is now being constructed.) Crystal growth in this monolayer is induced by heating to 125°c; heating above 200°c results in transformation into one or the other of two polymorphic forms either of which is more stable than the original structure; heating to 300°c results in melting the monolayer without removing it, the crystalline structure returning upon cooling. Oxygen can be adsorbed in a layer several atoms or molecules thick; this shows no structure until annealed at 125°c after which the structure is that of the most complex of the polymorphic forms assigned to nitrogen; this complex structure is polycrystalline with very small individual crystals; its diffraction pattern is unchanged when the oxygen is reduced to a single monolayer by heating. No diffraction pattern has been obtained from hydrogen, although the presence of hydrogen is proved by rise in pressure when the crystal is subsequently heated.