Dr. Trotter's recent theory of devolatilization of coal seams is criticized on structural grounds and the “square law” suggested is shown to be no improvement on many other expressions of varied types. The data are best represented both for South Wales and the Kent Coalfield by Hilt's law. The influence of depth of burial on coal vegetation is discussed; loss of volatile matter is probably promoted by temperatures, but is almost certainly retarded by high pressures. The Hilt rate is controlled by the chemical-physical factors and may be influenced also by varying rates of sedimentation during the accumulation of the Coal Measures.