196

PURIFICATION.

goes first to the purifier which has been longest in use, and last to the solution most recently intro-duced. This process separates ammonia, converting it into hydrochlorate, which is run oft undercover to barges for conveyance away. It was stated at the Great Central Works that from one-fourth to one-fifth of the sulphuretted hydrogen is also thus removed.

“ ‘Gypsum material;’ sulphate of lime. This salt acts upon the gas by a double decomposition ;the lime combines with carbonic acid, while its sulphuric acid unites with the ammonia. Laming’soxide of iron, prepared by precipitation of the sulphate of iron by lime, necessarily contains this salt,and acts in a similar manner upon the gas. In the Chartered Gas Works it is the means relied uponfor separation of ammonia.

“Oxide of iron. The preparation used varies at different Works. An oxide prepared by precipi-tation (as Laming’s, previously referred to) is mostly in use. At the Western Gas-works the oxideis prepared on the premises, by adding some of the amtnoniacal liquor to crude green copperas. Theoxide is usually mixed with sawdust, with a view to extension of surface, and to porosity. It isintroduced upon four or five metal screens into air-tight boxes. Where, as at the Chartered GasWorks, the oxide is mainly relied upon for the purification, the gas is made to traverse three suchboxes. Where other means are added, one oxide purifier is commonly considered sufficient; and insome works, where the use of dry lime is conjoined, the upper screens contain dry lime, and thelower, through which the gas first passes, oxide of iron, both being in the same purifier. The effectof the oxide is to separate sulphuretted hydrogen, a black sulphuret of iron being formed. The oxideis changed at intervals, varying in different works from 36 hours to 10 days. "When removed from thepurifiers, it is ‘ revivified ’ by exposure to the action of the air, the oxygen of which decomposes thesulphuret, throws down the sulphur, and re-converts the iron into peroxide. It is then damped, andagain used. The duration of the exposure necessary for revivification varies, with the age of theoxide and the length of time it has been in the purifier, from 124 hours to 14 days. It resumes eachtime its red colour. The process of revivification is capable of very frequent repetition. The sameoxide may thus be in use for many months, in fact until the sulphur precipitated accumulates insuch quantity as to prevent the revivification of the oxide. Dr. R. D. Thomson has found the sul-phur reach 45 per cent.

“Cream of lime ; ‘ wet lime.’ The apparatus used for this mode of purification mostly resemblesthat described when the muriate of manganese was mentioned. In the Phoenix Works (Yauxhall),—a model of the wet lime process,—the gas is made, by an arrangement of plates and bells withinthe purifier, to bubble three times through the wet lime contained in one purifier; the lime beingprevented from settling by agitation effected by machinery. When the cream of lime is saturated,(viz. about once in twenty-four hours), it is usually run off into pits, where the lime may settle; theplasterv matter which deposits being used for luting, sometimes alone, sometimes when mixed, forthe sake of an improved consistence, with refuse dry lime. The cream of lime separates carbonicacid, sulphuretted hydrogen, and ammonia.

“ The ‘ dry lime,’ or hydrate of lime, is exposed on screens in a box to the action of the gas, inthe same manner as the oxide of iron. It serves to remove carbonic acid, sulphur, and cyanogencompounds; and, when other effectual means of separating ammonia have not preceded, becomesstrongly charged with ammonia. When removed from the purifiers (usually every four or five days),it is commonly placed in a heap in the yard, and used for luting.”

Since the expiration of the term of Mr. Hills’s patent, oxide of iron has been used forpurification in nearly all the gas-works of London ; generally with the addition of lime inthe sieves of the last purifier, though in some instances without any.

By none of the processes of purification we have described, is the bisulphide of carbon