PURIFICATION OF GAS. 167 



Purification of Cras. — All the sulphuretted hydrogen may 

 be removed from coal-gas by the washers and lime purifier, 

 but a perfect decomposition may also be effected (according 

 to Croll, Lond. Journ. 1849) by passing the gas through a 

 solution of sulphurous acid, whereby water is formed and sul- 

 phur deposited. The excess of sulphurous acid is removed 

 by washers and the dry lime purifier. 



Lanning's process (Gh. Gaz. viii.), which has been success- 

 fully carried out at the Chartered Company's works, is said 

 to remove from illuminating gas every trace of ammoniacal 

 and sulphuretted impurity. The principal agent employed is 

 the carbonic acid of the gas, assisted by a mixture of oxide 

 of iron and chloride of calcium. The latter is made by pre- 

 cipitating solution of chloride of iron with lime or chalk, and 

 adding sawdust to the mass to render it permeable. The pre- 

 cipitated iron becomes peroxidized by the atmosphere during 

 the progress of preparation. In its transit through this mix- 

 ture, the gas loses its impurities in the following manner. 

 The chloride of calcium contained in it acts by its hygroscopic 

 property as an absorbent or solvent, and thus promotes the 

 contact of the foul matters with the disinfecting material. 

 The peroxide of iron takes the sulphur of the hydrosulphuret 

 of ammonia and becomes sesquisulphuret, at the same time 

 surrendering its oxygen to the eliminated hydrogen to form 

 water. The ammonia set free immediately unites with the 

 carbonic acid as carbonate, and this latter salt exchanges bases 

 with the muriate of lime as fast as it is produced. A portion 

 of it, however, forms sulphate with the spontaneously gene- 

 rated sulphuric acid. 



The mixture may be repeatedly regenerated by exposure to 

 air, and thus made serviceable for new operations. When it 

 becomes surcharged with ammoniacal salt, the latter must be 

 removed by washing with water. In the original mixture, the 

 lime-salt was a chloride, whereas after usage it becomes sul- 

 phate ; thus, the sesquisulphuret of iron in contact with air 

 changes into sulphate by the absorption of oxygen, and this 

 sulphate, reacting upon the carbonate of lime thrown down 



