In reply it may be said that it is a mineral. It is black or brown in color, solid, heavy, and amorphous. The specific gravity of the average Pennsylvania anthracite is about 1.6, and of the bituminous coal about 1.4. There are four varieties of mineral coal, namely: anthracite, bituminous, lignite or brown coal, and cannel coal. To this list it would not be improper to add peat, since it partakes of most of the characteristics of mineral coal, and would doubtless develop into such coal if the process of transformation were allowed to continue undisturbed. The principal element contained in each of these different kinds of coal is carbon. An analysis of an average piece of Pennsylvania anthracite would show the following chemical composition:—
| Fixed carbon | 86.4 |
| Ash | 6.2 |
| Water | 3.7 |
| Volatile matter | 3.1 |
| Sulphur | .6 |
| —— | |
| Total | 100 |
The composition of the bituminous coals of Pennsylvania, as represented by the gas coal of Westmoreland County, is shown by analysis to be as follows:—
| Fixed carbon | 55. |
| Volatile matter | 37.5 |
| Ash | 5.4 |
| Water | 1.4 |
| Sulphur | .7 |
| —— | |
| Total | 100 |
An analysis of coal from the Pittsburgh region would show its percentage of carbon to be from 58 to 64, and of volatile matter and ash to be proportionately less.
There is no strict line of demarcation between the anthracite and the bituminous coals. They are classed generally, according to the amount of carbon and volatile matter contained in them, as:—
Hard-dry Anthracites,
Semi-Anthracite,
Semi-Bituminous,
Bituminous.
Coals of the first class contain from 91 to 98 per cent, of carbon, and of the second class from 85 to 90 per cent. The volatile matter in the third class is usually less than 18 per cent., and in the fourth class more than 18 per cent. of its composition.