298 CHEMICAL DISCOVERY AND INVENTION 



these theories, and it is probable that it will be some time before 

 the formation of the higher terms of the paraffin series (paraffin 

 wax, ozokerite, etc.), and the differences of constitution between 

 the American and Russian oils are fully explained. 



CHAPTER XX 



COAL-TAR 



IT seems a far cry from the black, sticky, and stinking liquid 

 coal-tar to the brilliant or delicate colours adorning a lady's 

 dress, to the perfume of the Tonquin bean, or the little white 

 tabloid of aspirin or phenacetin which affords relief from pain. 

 But the black tar is the chemical ancestor of these and many 

 other valuable products unknown to our grandfathers, and the 

 business of extracting from it the intermediate substances which 

 are more directly the parents of the dyes and other things is a 

 matter of great national importance. 



When coal is heated strongly in a closed vessel, so that it 

 does not burn, it yields four chief products, viz. : 



Gas. 



Watery ammoniacal liquor. 



Tar. 



Coke. 



At the gas works the first of these is the primary object of the 

 manufacture, while the liquor and tar are spoken of as residuals, 

 together with the coke which is left behind in the retorts. But 

 in connection, specially, with the production of iron and steel the 

 purpose in view in heating coal is not so much the production 

 of gas as of the residual coke. The process is carried out in 

 coke " ovens," and down to comparatively recent times the coke 

 alone was preserved, all the gas and other volatile products 

 given off were burned to waste. This, however, is no longer the 

 case, and since the more general recognition of the value of the 

 by-products many of the coke ovens of the present day are 

 constructed so as to provide for collecting and so utilising what 

 was formerly wasted. 



The accompanying diagram shows in section the construction 

 of what is known as a beehive oven. A number of these are 

 always placed side by side with the object of economising heat 



