It has been found by recent experiments on this subject, that the loss in weight, due to a slow oxidation and to the disengagement of gases which form the richest part of bituminous coal, may equal one third of the original weight. The heating power in such coal was lowered to 47 per cent of its former capacity. The same coal exposed to the air, but in a closed receptacle, did not lose more than 25 per cent of gas and 10 per cent of heating power. Bituminous coals alter most rapidly. This shows the disadvantage of damp cellars, and of leaving coal uncovered for long periods and subject to bad weather.