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Specifies a method for determining the swelling properties of hard coal when heated under standard conditions in a dilatometer. In performing the method a pencil made of powdered coal formed under pressure is inserted into a precisely calibrated narrow tube and topped by a calibrated steel rod which slides in the bore of the tube. The whole is heated at a constant and definite rate. By making regular readings of the displacement of the rod as a function of the temperature and expressing the displacements observed as percentages of the original length of the penicl a curve is plotted.