spongy coke was a kind of skeleton of coal intimately united with more complex substances, and that coal is a mixture of pure carbon and combined carbon? No; coal, as a whole, is a mass of substances composed of combinations of carbon with other bodies. These combinations are modified by heat. The tarry liquids and the gases do not exist in the coal, but are formed as the temperature rises in the retort. Coke is left, because in the changes that are made carbon is in excess. The coal-tar is not separated from the coke, but is made in the retort, and the bodies we find in it are results of combinations that are brought about between the substances which existed in the coal.
M. Berthelot heats to a dull red heat the gas acetylene, the molecule of which is composed of four atoms of carbon and two atoms of hydrogen. At the end of the operation the acetylene is condensed and is changed into a liquid, benzine, which is composed of twelve atoms of carbon and six of hydrogen. Three molecules of acetylene have been in some way welded together to furnish a molecule of benzine. We have seen acetylene condensed and combined as it were with itself. It also combines with hydrogen and forms olefiant gas, or ethylene. The latter unites with the benzine and gives, by synthesis, a liquid hydrocarbon, styrolene, identical with the styrolene which is produced by the styrax or Oriental liquidamber. Finally, from the union of the styrolene and the olefiant gas results naphthaline, a solid hydrocarbon, which crystallizes in thin lamellæ and abounds in coal-tar. Anthracene is one of the most valuable of the hydrocarbon extracts of coal-tar. It evidently did not exist as anthracene in the coal, but has been formed during the distillation, a solid, crystalline body, by the combination and condensation of gases.
So, when coal is heated to a very high temperature, the substances that are disengaged in a gaseous form do not always remain in that state. Heat is not always a cause of the dissolution of bodies and of the dispersion of their elements. When exposed to a temperature exceeding 1,000° C. (1,800° Fahr.) these gases condense; their molecules draw together; and they form, after a few changes, combinations richer in carbon, and consequently less volatile. We had gases, but, when our apparatus has had time to cool, we shall find liquids, even crystals. In other cases, dissociation is effected by heat. Carbonic acid, one of the most common and stable compounds in the world, the final resultant of all combustion, loses its oxygen under excessive heat, and becomes an oxidizing agent. In this way good authorities explain the production of phenic, acetic, and cresylic acids, as hydrocarbons oxidized by the oxygen of carbonic acid. The hydrocarbons may also be dissociated. A liquid hydrocarbon analogous to benzine, toluene, takes hydrogen and leaves a deposit of anthracene. Formene, or marsh-gas, a hydrocarbon which produces chloroform when the hydrogen in it is replaced with chlorine, loses hydrogen and yields anthracene.
