This note gives an account of some experiments carried out in still air on smouldering fires resulting from small sources of ignition buried in heaps of dust of up to 85 cm in depth. The heaps were either formed from flat-topped conical moulds or contained in lidless boxes, and the smouldering was initiated at the bottom and propagated upwards. The results have shown that a small source of ignition when buried in a heap of dust can initiate and propagate smouldering upwards through the heap for many days without visible signs of combustion; an 85 em. depth of mixed wood sawdust, for example, was found to take over 12 days for the smouldering to penetrate to the surface. The time for the smouldering to penetrate varies approximately as the square of the depth of dust; the rate of smouldering is consequently inversely proportional to the depth and is, in general, appreciably slower than when under open conditions, and for large heaps may become very slow. When the smouldering penetrated the top surface of a heap the smouldering was observed to spread out over the surface and the smoke produced inflamed above the smouldering region, even in still air. Larger but transient flames occurred as a result of disturbing the smouldering heap and allowing the core ef carbonized dust to fall in fine suspension through the air. Some practical aspects of this work are briefly discussed.