Abstract
The author reviews the present state of knowledge of the liquid and solid lubricant boundary layer. The function of the boundary layer is to replace the oxide-layer component of friction by one of still lower value. The structure of the oil boundary layer is discussed, and the effects of heat and sliding on this structure, particularly on the orientation of the lubricant molecules. Solid lubricants of the layer lattice type depend for their functioning on a large ratio of surface area to particle mass, and having on their surfaces firmly adhering liquid or gaseous adsorbed layers.