In the above table the increase of carbon and the decrease of oxygen is well brought out; the hydrogen also on the whole decreases, although with some irregularity. The exact course of the chemical change which occurs during the passage of wood into coal is at present involved in obscurity. The oxygen may be eliminated in the form of water or of carbon dioxide or both; some of the carbon is got rid of in the form of marsh gas, a compound of carbon and hydrogen, which forms the chief constituent of the dangerous "fire-damp" of coal mines. Marsh gas is an inflammable gas which becomes explosive when mixed with air and ignited; it often escapes with great violence during the working of coal seams, the jets blowing out from the coal or underclay with a rushing noise, indicative of the high pressure under which the hydrocarbon gas has accumulated. These jets of escaping gas are known amongst miners as "blowers." If the air of a mine contains a sufficient quantity of the gas, and a flame accidentally fires the mixture, there results one of those disastrous explosions with which the history of coal mining has unfortunately only made us too familiar. From the account of coal which has thus far been rendered, it will be seen that as a source of mechanical power, we are far from using it as economically as could be desired; and when we look at our open grates with clouds of unburnt carbon particles escaping up the chimney, and so constructed that only a small fraction of the total heat warms our rooms, it will be seen that the tale of waste is still more deplorable. But we are at present rather concerned with what we actually do get from coal than with what we ought to get from it, and here, when we come to deal with the various material products, we shall have a better account to present. If instead of heating coal in contact with air and allowing it to burn, we heat it in a closed vessel, such as a retort, it undergoes decomposition with the formation of various gaseous, liquid, and solid products. This process of heating an organic compound in a closed vessel without access of air and collecting the products, is called destructive distillation. The tobacco-pipe experiment of our boyhood is our first practical introduction to the destructive distillation of coal. We put some powdered coal into the bowl of the pipe, plaster up the opening with clay and then insert the bowl in a fire, allowing the stem to project from between the bars of the grate. In a few minutes a stream of gas issues from the orifice of the stem; on applying a light it burns with a luminous flame, and we have made coal-gas on a small scale.