PURIFICATION.

187

The muriate of lime is superior in activity to the sulphate, and the mode of using it forthe purification of coal-gas, as patented by Mr. Laming, possesses great advantages. Butwhatever the plan adopted, the theory and practice of the application of the salts of lime areequally satisfactory. The ammonia combined with carbonic acid is arrested by the limesalts, and a double decomposition ensues, attended with the production of two new sub-stances, neither of which has any affinity for the illuminating matters of the gas. The natureof this decomposition may be very easily shown. Thus, if we suppose muriate of lime tohave been employed, we shall have—

In the gas I Carbonic "^.-Muriatic acid 1 for , he if ; substanceSi(. Ammonia,— Lime . . )

which, by simple interchange of elements, will form muriate of ammonia and carbonate oflime,—two salts devoid of action upon every form of hydro-carbon.

Foul lime, as it is removed from the purifier, consists of an admixture of several salts.Those only that need to be practically considered are the carbonate and hydro-sulphate oflime, and the hydro-sulphate of ammonia, which constitute an important part of the totalbulk. The analysis of these principal compounds shows that more than half the mass con-sists of carbonate of lime, nearly one-fourth of hydro-sulphate of lime, about one-fifth ishydrate of lime unacted on, and the residue contains the cyanogen salts, and the siliceousand other impurities of the limestone, with a variable proportion of hydro-sulphate of am-monia retained in the mass mechanically. With such a mixture it is not surprising that anoffensive odour is emitted on exposure to the atmosphere; for, in the first place, the hydro-sulphate of ammonia must pass into the air by a simple process of diffusion, whilst thehydro-sulphate of lime, being acted on by the carbonic acid of the atmosphere, will give riseto the formation of carbonate of lime and a copious evolution of sulphuretted hydrogen gas.It is in the prevention of this evolution of sulphuretted hydrogen that the process of purifi-cation by oxide of iron owes its chief recommendation; for the affinity of oxide of iron forsulphuretted hydrogen is greater than for carbonic acid. Hence the sulphuret of iron whenremoved from the purifiers and exposed to the air emits no odour, whilst the sulphuret oflime is readily decomposed and produces an abominable smell.

The analysis of impure gas-lime depends upon very simple chemical properties. Thehydrate of lime and the sulphuret of calcium are readily soluble in a solution of muriate ofammonia slightly warmed ; but the carbonate of lime is not soluble, to an appreciable de-gree, in that menstruum. If therefore we weigh 100 grains of the impure lime, and digestit with a solution of muriate of ammonia in a closed vessel for a few minutes at a tempe-rature of 80°Fahr., we dissolve the hydrate of lime and sulphuret of calcium, but leave thecarbonate of lime at the bottom. If we then pour into the clear solution a quantity ofmuriate of copper, previously supersaturated with ammonia, a black precipitate will form,and this collected in a filter and washed, first with diluted vinegar, then with water, andlastly dried at a heat of not less than 212°, the weight of it, when divided by three, willshow the amount of sulphur present in the lime; and since 16 of sulphur represent 28 oflime, the weight of the last combined with the sulphur is easily estimated by multiplying

2 b 2