Before considering the quality of these coals, I will, for better understanding, make some prefatory statements in regard to the character of coals generally. Charcoal has greater purity than mineral coals usually have, because there is nothing in the charcoal except what naturally belongs to the woody matter. Mineral coal, however, having been buried in water, mud, and sand, must, almost of necessity, have some mixture of foreign matter, either slate, which is simply hardened mud; silica, which may have been derived from sand; iron and sulphur, some of which may have been in the wood, but most of it, probably, introduced in solutions; to which should be added, unexpelled oxygen, which is not only useless as fuel, but which combines with a portion of the contained hydrogen, and forms water in the substance of the coal. The proportion of ash in coals of the same class is usually determined by the amount of slate in the coal, in addition to the mineral matter which belonged originally to the vegetable material from which the coal was formed. In the pure state, the proportion of ash increases as the transformation of woody fibre goes on from peat to anthracite. [Sidenote: The chemical changes in coal beds.] It is worth while to note what are the changes which take place in the vegetable matter during the process. It may be described in a word as a progressive loss of oxygen, and by this loss the coal becomes richer, for the reason just given. The deoxidizing process is carried on by means of chemical changes in the substance of the coaly matter. The oxygen combining with a certain proportion of the carbon, forms carbon di-oxide, or carbonic acid gas; and a certain other portion, combining with hydrogen, forms water. Both of these are volatile in their character, and gradually escape. Another loss is effected by the combination of hydrogen and carbon, forming marsh gas. We have deadly proof that these combinations are in progress in all coal mines by the occurrence of "choke-damp" and "fire-damp," which are the miners' names for these gases. [Sidenote: Deficient nomenclature.]