SECONDARY PRODUCTS. 199 



If coal is immediately decomposed, the coke formed is spongy and friable, 

 owing to the quick distillation of the expelled gases ; the bulk is increased 

 and the density destroyed, but the volatile products are rich in gas of a high 

 illuminating power ; and it is to effect this latter object the gas manufacturer 

 must more particularly confine his operations. In other words, he must be a 

 gas, and not a coke manufacturer. 



The average quantities of coke obtained from 27 cwt. of Newcastle coal 

 distilled in cylindrical and D-retorts, set on the plan shown in Plates I. and 

 II., are as follow : 



bushels. cwts. qrs. Ibs. 



Berwick's Wall's-end - 47| = 18 2 8 



Heaton Main 45^ = 18 1 4 



Primrose Main 46 = 18 1 25 



Felling Main 44 = 18 3 24 



Pearith's Wall's-end 46 = 18 2 



Eden Main - 45 18 1 14 



Russell's Wall's-end 45 = 18 1 



We may take the average production of coke from a ton of Newcastle coal 

 to be 33 bushels, or 13^ cwt. The quantity of coke required to heat the retorts 

 for the distillation of a ton of coal I have already stated to be about 12^ 

 bushels ; consequently, if the fuel used was entirely coke, we should have 

 33 12 J = 20^ bushels for sale ; but when tar is substituted, the quantity 

 for sale will be proportionably increased. 



COAL-TAR. 



WHEN the manufacture of gas from coal was in its infancy, great advantages 

 were expected to be derived from the coal-tar which distilled over with the 

 inflammable gas. It was considered to be a substance possessing even supe- 

 rior properties to the vegetable tar for the preservation of timber, and other 

 perishable materials exposed to the influences of the weather. 



In the year 1665, a German chemist proposed to distil coal for the sole 

 purpose of obtaining this tar, and in 1781 the Earl of Dundonald took out a 

 patent for collecting the tar which appeared during the formation of coke. 

 Neither scheme answered. After a few years' trial, coal-tar as a substitute 

 for vegetable tar fell into disuse. It was tried, I believe, in the navy, and 



