Page:Popular Science Monthly Volume 25.djvu/214

204, will be collected in the cold receiver. The operation becomes complicated and the results perplexing when the mixture consists of substances capable of being modified by the degrees of heat applied. In such cases the analysis must be carried on at a lower temperature, and the operator must depend upon solvents, the effects of which are different on different bodies. This method has been tried on coal by M. Commines de Marcilly, who employed boiling liquids or their vapors in open and in closed vessels, and in Papin's digester, by the aid of which he obtained a stronger pressure than that of the atmosphere. Acids and alkalies had no action, but neutral liquids, such as ether, benzine, sulphuret of carbon, and chloroform, were evidently colored by the coal. The experiments deserve to be carried further.

Coal-tar, the liquid product which is formed when coal is roasted in a close vessel, appears as a thick, black paste, giving no hint of the richness of the substances which may in their turn be formed and separated from it. The first product, water saturated with ammonia, passes over when the liquid is heated to between 175° and 192° Fahr. for twenty or thirty hours. Then a fractional distillation is performed, under which the light oils are separated at below 266° Fahr.; the medium oils at between 266° and 392°; and the heavy oils at between 392° and 678°; while a thick residue is left in the retort. Our study is with the oils.

The first two classes of oils are again distilled in a large alembic heated by steam under high pressure; first is collected for the medium oils all that passes between 266° and 392°. That which passes at below 266° is mixed with light oils, while the products passing at above 392° are mingled with heavy oils. The light oils are next purified in a similar manner. The latter products are known in commerce as naphtha-oils, and are chiefly carburets of hydrogen. The eighteen or twenty of them which have been distinguished form a series, in which the proportion of carbon to hydrogen increases regularly. Those least rich in carbon are gaseous; then come the liquid hydrocarbons, and last the solid compounds. We select the liquid distillates for further operations. The first step is to rid the product of the gases that may still be dissolved in it, and the alkaline or acid impurities it may contain—foreign matters which give to the naphtha a repulsive odor. They are separated by washing successively with water, which removes some of them, sulphuric acid, which acts on the alkalies, and caustic soda for the removal of acids and what excess of sulphuric acid may remain. The naphtha is then subjected to a fourth distillation, and benzine is obtained at a temperature of between 184° and 240°.

Before proceeding with the history of this valuable substance we will mention that the medium oils are treated with sulphuric acid and soda in the same way as the light oils, except that, as they are richer in alkalies and acids, they have to be treated with stronger proportions