PURIFIERS. 121 



ammonia, and a part of the sulphuretted hydrogen, producing at the same time 

 muriate of ammonia. He introduces a solution of the muriate of zinc into 

 a vessel, upon the same construction as a wet-lime purifier ; on admitting 

 the gas double decomposition ensues ; an insoluble sulphuret of zinc and a 

 solution of muriate of ammonia are produced. The gas must be further puri- 

 fied with lime, in the usual way. 



Lime for the purpose of purifying coal-gas should be free from foreign 

 matter. That which slackens the quickest, and produces the greatest heat 

 during the operation, is the best. When dissolved in diluted muriatic acid it 

 should not effervesce, and should, when perfectly pure, leave no insoluble 

 residue. 



The purest lime is obtained from lias-limestone, and the lower oolite. The 

 former contains 90 per cent, of carbonate of lime, has a brownish tinge, is 

 perfectly non-effervescent when properly burnt, and slackens readily. The latter 

 is white, with the same properties, but when dissolved in acid leaves a larger 

 residue. Dorking and Mersham lime, obtained from chalk, are also esteemed, 

 particularly in the neighbourhood of London. Magnesian limestone is in- 

 ferior. 



