224 METALLOIDS. 



How is it pu- 555. PURIFICATION. The gas as it rises, 



rifled f contains ammonia and sulphuretted hydro- 



gen, two impurities which it is desirable 



to remove. The first may be stopped 



in its passage, by a loose wad of moist- 



ened paper ; the last, by a similar wad, 



moistened with solution of sugar of lead. 



The papers having been introduced, the 



pipe-stem is fitted to the tube with a pa- 



per stopper, and the tube heated over the 



alcohol flame, with the help of a blow-pipe. When 



the coal has become red hot, the gas will pass off in 



sufficient quantity to be ignited, at the extremity of 



the tube. 



How ore the ' ^ ^ conclusion of the process, 



impurities the upper wad contained in the tube, will 

 be found blackened by the sulphuretted 

 hydrogen which it has retained. On removing the 

 second one, it will be found to smell of ammonia. The 

 presence of this body may also be shown, by the fumes 

 which it yields with muriatic acid. 



557. ARRANGEMENTS IN GAS WORKS. 



Describe the 



process in gas The process in gas works is essentially the 

 same, as that above described. The coal 

 is heated in iron retorts. The tar collects in pipes lead- 

 ing from it. Carbonate of ammonia is washed out by 

 a jet of water, which plays in an enlargement of the 

 pipe. Lastly, sulphuretted hydrogen is removed by 

 the retentive power of a metallic base, lime being gen- 

 erally substituted for lead. 



