If energy is indestructible, we have a right to inquire in the next place from whence the coal has derived this enormous store. A consideration of the origin of coal, and of its chemical composition, will enable this question to be answered. The origin of coal has already been discussed. Chemically considered, it consists chiefly of carbon together with smaller quantities of hydrogen, oxygen, and nitrogen, and a certain amount of mineral matter which is left as ash when the coal is burnt. The following average analyses of different varieties will give an idea of its chemical composition:-- ------------------------------------------------------------------- Variety of Coal. | Carbon. | Hydrogen. | Oxygen. | Nitrogen. | Ash. -------------------+---------+-----------+---------+-----------+----- S. Staffordshire | 73.4 | 5.0 | 11.7 | 1.7 | 2.3 Newcastle (Caking) | 80.0 | 5.3 | 10.7 | 2.2 | 1.7 Cannel (Wigan) | 81.2 | 5.6 | 7.9 | 2.1 | 2.5 Anthracite (Welsh) | 90.1 | 3.2 | 2.5 | 0.8 | 1.6 --------------------------------------------------------------------- There are in addition to these constituents small quantities of sulphur and a certain variable amount of water (5 to 10 per cent.) in all coals, but the elements which most concern us are those heading the respective columns. From the foregoing analyses, which express the percentage composition, it will be seen that carbon is by far the most important constituent of coal. Carbon is a chemical element which is found in a crystalline form in nature as the diamond, and which forms a most important constituent of all living matter, whether animal or vegetable. Woody fibre contains a large quantity of this element, and the carbon of coal is thus accounted for; it was accumulated during the growth of the plants of the Carboniferous period.