(2) That this carbon formed part of the plants which grew during the Carboniferous period. (3) That the carbon thus accumulated was supplied to the plants by the carbon dioxide existing in the atmosphere at that time. (4) That the separation of the carbon from the oxygen was effected in the presence of chlorophyll, by means of the solar energy transmitted to the earth during the Carboniferous period. We thus arrive at the interesting conclusion, that the heat which we get from coal is sunlight in another form. For every pound of coal that we now burn, and for every unit of heat or work that we get from it, an equivalent quantity of sunlight was converted into the latent energy of chemical separation during the time that the coal plant grew. This energy has remained stored up in the earth ever since, and reappears in the form of heat when we cause the coal to undergo combustion. It is related that George Stephenson when asked what force drove his locomotive, replied that it was "bottled-up sunshine," and we now see that he was much nearer the truth in making this answer than he could have been aware of at the time. Before passing on to the consideration of the different products which we get from coal, it will be desirable to discuss a little more fully the nature of the change which occurs during the transformation of wood into coal. Pure woody fibre consists of a substance known to chemists as cellulose, which contains fifty per cent. of carbon, the remainder of the compound being made up of hydrogen and oxygen. It is thus obvious that during the fossilization of the wood some of the other constituents are lost, and the percentage of carbon by this means raised. We can trace this change from wood, through peat, lignite, and the different varieties of coal up to graphite, which is nearly pure carbon. It is in fact possible to construct a series showing the conversion of wood into coal, this series comprising the varieties given in the table on p. 23, as well as younger and older vegetable deposits. The series will be-- I. Woody fibre (cellulose). II. Peat from Dartmoor. III. Lignite, or brown coal, an imperfectly carbonized vegetable deposit of more recent geological age than true coal. IV. Average bituminous coal. V. Cannel coal from Wigan. VI. Anthracite from Wales. VII. Graphite, the oldest carbonaceous mineral. The percentage of the chief elements in the members of this series is-- Carbon. Hydrogen. Oxygen. I. 50.0 6.0 44.0 II. 54.0 5.2 28.2 III. 66.3 5.6 22.8 IV. 77.0 5.0 11.2 V. 81.2 5.6 7.9 VI. 90.1 3.2 2.5 VII. 94-99.5, the remainder being ash.