198 SECONDARY PRODUCTS. 



SECONDARY PRODUCTS. 



COKE. 



THE most valuable of the secondary products of a gas establishment is coke. 

 The best kind is obtained from coal when carbonized in large masses, in ovens 

 constructed on purpose. In a gas manufactory, the production of coke being 

 of minor importance to the formation of good gas, it is generally of an in- 

 ferior quality to that made in coke ovens, where it is the primary, and indeed 

 sole object for which the coal is carbonized. But gas-coke is excellent for 

 many purposes in the arts and manufactures, producing as clear a fire as that 

 of the first quality, though it is neither so lasting or so free from slag : for 

 domestic use, however, it is unobjectionable, and may be burnt both in the 

 drawing-room and kitchen with ceconomy and comfort. 



The distinguishing characters of good coke are, first, a clean, granular frac- 

 ture in any direction, with a pearly lustre, inclining to that exhibited by cast 

 iron. Secondly, density, or close proximity of its particles, which adhere 

 together in masses, and specific gravity of T 10, or rather higher. Thirdly, 

 when exposed to a white heat it consumes entirely away, without leaving 

 either slag or ashes. 



It is invariably the case that the quality of the coke is inversely as that of 

 the gas. The manufacturer must not expect to produce both of the best qua- 

 lity. The process by which the best gas is made generally leaves the coke 

 light, spongy and friable, although an increase of quantity is gained ; for the 

 simple reason, that the degree of heat and other circumstances required to 

 form perfect coke must be entirely changed when gas of a high specific gravity 

 is to be obtained. Thus large masses of coal exposed to a red heat in close 

 vessels are acted upon by slow degrees, the external portions preventing heat 

 from penetrating into the interior until most of the bituminous portions are 

 given off in condensable vapour, or as charcoal and free hydrogen ; the after 

 products being light carburetted hydrogen, carbonic oxide and carbonic acid 

 gases. The residue is a carbon of a dense granular composition. 



