THE SOURCE OF LIGHT, HEAT, COLOUR, ETC. 153 



are bituminous ; whilst, for obtaining the various products by destruc- 

 tive distillation, the cannels, or semi-bituminous varieties, are most 

 esteemed, from the large proportion of volatile matter they contain ; and 

 the anthracites, or non-bituminous coals, are useful as fuel for steam- ships, 

 &c, also in the manufacture of iron, where the intense draught required 

 stimulates the combustion, and developes the intense heat which, from their 

 greater proportion of carbou, they possess naturally. 



The manufacturing industries connected with coal exclusively are not 

 at present very extensive, although, from the great attention now directed 

 to the hydro-carbons, many more will doubtless spring into existence. Gas 

 is manufactured from coal pretty generally throughout the United King- 

 dom, and a large quantity of the bituminous coals are consumed annually 

 in this way. The manufacture of gas in its minute details has been so 

 often described, that 1 shall content myself with the briefest possible notice 

 of it. The coal is introduced into clay retorts surrounded with fire, and as 

 soon as the temperature is sufficiently high, the gas begins to disengage 

 itself, and enters a receiver, where it is condensed, and afterwards it is 

 purified by various processes. Besides the gas, there are other products of 

 this manufacture, as coke and coal-tar, in about the following proportions : 



Gas i 



Coke f 



Coal-tar •§■ 



The coke is the principal fuel employed in heating the retorts for the pro- 

 duction of the gas, and the coal-tar is the base of a separate industry of the 

 most attractive character. This coal-tar is of the consistence of honey, 

 densely black, and disagreeable in smell, and by distillation can be sepa- 

 rated' into no less than fifty or more different products, each possessing its 

 specific characteristic, and many of them already proved to be of inestimable 

 value in the arts. A few of these I enumerate in the following table : 



Napthaline 



C 20 



H 8 





Sp. gr. 



1-048 



Paraffine - 



C 2() 



H 21 





Sp. gr. 



•87 



Carbolic acid 



C 12 



H 5 



0, HO 



Sp. gr. 



1-062 



Creasote - 



C 26 



Hie 



0, 



Sp. gr. 



1-040 



Naphtha - 



- c 6 



H 6 





Sp. gr. 



•765 



Benzole 



C 12 



H 6 





Sp. gr. 



0-85 



Aniline 



C 12 



H 7 



N 



Sp. gr. 



1-020 



Coke-making, for foundry and locomotive purposes, is a further indus- 

 try connected with coal by no means contemptible. It is effected by means 

 of closed ovens, where the volatile products are separated and exhausted 

 without destruction of the carbon, which remains unconsumed in the form 

 of coke. The non-bituminous coals are best suited for this manufacture. 



Another deeply interesting industry connected with coal is that carried 

 on for obtaining various volatile products by the destructive distillation of 

 Cannel, Boghead mineral, &c, at low temperatures. When the tempe- 



