204 THE POPULAR SCIENCE MONTHLY, 



ature, 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 me- 

 dium 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 be- 

 low 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 con- 

 tain — foreign matters which give to the naphtha a repulsive odor. 

 They are separated by washing successively with water, which re- 

 moves 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 distilla- 

 tion, 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 



