306 CHEMICAL DISCOVERY AND INVENTION 



exceedingly complex mixture of upwards of two hundred dif- 

 ferent chemical compounds. These may be divided into hydro- 

 carbons, of which the most important are benzene, toluene, 

 xylen^, naphthalene, anthracene, etc., and compounds contain- 

 ing oxygen, of which the most important are phenol, commonly 

 called carbolic acid, and other substances of that class. A small 

 proportion of basic substances containing nitrogen, and others 

 containing sulphur are also present, but are of minor importance 

 and at this point may be neglected. 



On distillation 1 ton of average gas works tar will produce 

 approximately : 



Ammoniacal Liquor ... 5 gallons 



Crude Naphtha . . . . 5-6 



Light Oils 26-0 



Creosote Oils . . . . . 17-0 ,, 



Anthracene Oils .... 38-0 ,, 



Pitch 12 cwt. 



The general arrangement of the stills is shown in the accom- 

 panying photograph taken in the works of Messrs. Major and Co. 

 of Hull, Fig 102. The importance of coal-tar and of economy in its 

 manipulation has led to invention of many modifications in the 

 form of the stills and the process of distillation. One of the most 

 important of these is the continuous process introduced by Messrs. 

 Hird Chambers and Hammond of Huddersfield. 



The principle here applied is very simple, although to the non- 

 expert reader the details may look complicated. If crude tar 

 is run into a heated still it gives off first its most volatile con- 

 stituents, which may be passed into a condenser and collected. 

 The residue in the still may be allowed to flow into a second still 

 alongside of the first, and being heated to a higher temperature 

 the portion distilling away consists of liquids having a higher 

 boiling point. Similarly a third still may be arranged so as to 

 receive the residue from the second as it parts with its vaporis- 

 able constituents. A series of three stills is found to be sufficient, 

 and the final residue runs into the pitch-cooler. The process is 

 made automatic by placing the crude tar tank at a higher level 

 than the stills so that it flows by gravitation, and on the way it 

 is warmed by making it pass through the vessels in which are 

 coiled pipes conveying the hot vapours from the stills to the 

 condensers. All this will become clear by consulting the diagram 



