1 1 gram of SO 2 for every million mL of air (parts per million by mass per volume). 2  Write ppm in terms of cubic meters. 3  There are a million cubic centimeters in a cubic meter (1 cm 3 = (10 -2) 3 m 3 = 10 -6 m 3) so one part per million of SO 2 by volume in air is actually 1 cm 3 SO 2 / 1 m 3 of air. 1 gram of SO 2 for every million grams of air (parts per million by mass); 2  1 mL of SO 2 for every million mL of air (parts per million by volume); 3  1 gram of SO 2 for every million mL of air (parts per million by mass per volume). 4  Write ppm in terms of cubic meters.
Mining is classified by the method needed to reach the coal seam. When the coal is found close to the Earth's crust and taking away the overlying layers of material is not too expensive, surface mining is used to remove the top layers of materials and expose the coal.If coal is found in layers far from the surface, underground mining is the preferred technique.Vertical or slanted holes (shafts) are cut down to the mining area underground for ventilation for the workers and for transporting the miners, equipment, and coal.ining is classified by the method needed to reach the coal seam. When the coal is found close to the Earth's crust and taking away the overlying layers of material is not too expensive, surface mining is used to remove the top layers of materials and expose the coal.
The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Lignite, or brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. 2  Jet, a compact form of lignite, is sometimes polished and has been used as an ornamental stone since the Upper Palaeolithic.