202 THE POPULAR SCIENCE MONTHLY. 



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 com- 

 binations 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 mole- 

 cule 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 ben- 

 zine. 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 syn- 

 thesis, 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 naphtha- 

 line, a solid hydrocarbon, which crystallizes in thin lamellce 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 anthra- 

 cene 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 ex- 

 ceeding 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 hydro- 

 gen in it is replaced with chlorine, loses hydrogen and yields anthra- 

 cene. 



