Coals are classified upon the ratios which the volatile materials or hydro-carbons bear to fixed carbon. Anthracite coal is low in volatile material and high in fixed carbon. Soft coals are high in volatile matter and low in carbon. The percentage of coal in volatile hydrocarbons runs as follows:

The remaining components of the coals are absorbed moisture, fixed carbon and ash. The ash in a first class coal should not be above 8 per cent.