574 



GAS, COAL 



risen to a certain level, they overflow into the tar-pit, as shown in tho figure, to be 

 drawn off by the stopcock, as occasion may require. 



The refrigerated gas is now conducted into tho purifier E, which wo will suppose to 

 bo filled with milk of lime, made by mixing 1 part of slaked lime with 25 parts of 

 water. The gas, as it enters by tho pipe I, depresses tho water in the wide cylinder 

 72, thence passes under tho perforated disc in the under part of that cylinder, and 

 rising up through innumerable small holes is distributed throughout tho lime-liquid 

 in the vessel m. By contact with tho lime on this extended surface, tho gas is 

 stripped of its sulphuretted hydrogen and carbonic acid, which are condensed into 

 the sulphide of calcium and carbonate of lime ; it now enters tho gas-holder r in ft 

 purified state, through tho pipe p t, and occupies tho space q. The gas-holder 

 pressing with a small unbalanced force over the counter-weight, , expels it through 

 the main u , in communication with tho pipes of distribution through tho buildings 

 or streets to bo illuminated. 



Such are the chief and essential parts of the apparatus used in every gas-work for 

 the generation, refrigeration, purification, and storage of coal-gas. The construction 

 and mode of working these separate portions of the apparatus vary much, however, 

 in different works, and it will therefore be necessary here to enter more minutely into 

 the details of the four departments of the manufacture just enumerated. 



I. APPARATUS USED IN THE GENERATION OF COAL-GAS. 



Retorts. The use of this portion of the apparatus is to expose the coal to a high 

 temperature, to exclude atmospheric air, and to deliver the gaseous and vaporous pro- 

 ducts of distillation into the refrigeratory portion of the apparatus. The materials 

 composing the retorts should therefore possess tho following properties : 1st, high 

 conducting power for heat ; 2nd, rigidity and indestructibility at a high temperature ; 

 and 3rd, impermeability to gaseous matter. The materials hitherto used in the con- 

 struction of retorts aro cast iron, wrought iron, and earthenware ; but none of those 

 materials possess the above qualifications in the high degree that could be wished. 

 Thus cast iron, though a good conductor of heat, is not perfectly rigid and inde- 

 structible. At high temperatures it becomes slightly viscous, and at the same time 

 undergoes rapid oxidation. "Wrought iron is a still better conductor of heat, but its 

 qualities of indestructibility and rigidity are even lower than those of cast iron ; 

 whilst earthenware, though rigid, and indestructible by oxidation, is a very bad 



1029 



1030 



1031 



1032 



conductor of heat, and is moreover very liable to crack from changes of tempera- 

 ture. Very various forms of retort have been employed at different times in order to 

 secure as far as possible, the conditions just enumerated. 



1033 ( j Cast-iron Retorts. Tho chief forms of 



the cast-iron retorts are : First, the 

 cylindrical,/^. 1029, used in tho Man- 

 chester Gas Works, 1 2 inches diameter, 



an ? ? to / eefc l n second, the 

 elliptical, 18 inches by 12 inches, by 



1034 



