The gases and vapors may be divided into three classes. First, those separated by the lime purifier, viz. carbonic, hydro sulphuric, and hydrocyanic acids and am monia. Second, those separated by water or in the alum, or green vitriol puri fier, viz. ammonia (and hydrocyanic acid by green vitriol). Third, those which pass on to the gasometer, viz. trace of naptha vapor, trace of vapor of sulphuret carbon, nitrogen, hydrogen, carbonic oxide, light carburetted hydrogen, and olefiant gas.
These numerous substances are not af forded in the same relative proportions, at the same periods of the distillation. On the first application of the heat to coal, steam, along.with the air of the retort, comes off; as the heat approaches red ness, tar is disengaged, but only a small proportion of gas below a red heat; and such gas has a feeble illuminating power ; when the retort is heated bright red, the evolution of gas is at its maximum, but tar is still produced, though slowly. At a white heat, carried on for two hours, the tar is small in proportion, that of the gas is still large, but decreasing. At length, the as ceases to be given off. Mr. Peck eon's table, showing the relative amount of gas given off' from one chaldron of Newcastle coal at different periods of the process, illustrates this— The composition and illuminating power of gas produced at different periods of the process, vary considerably. The gas evolved before the retort is red hot, con tains a great deal of carbonic oxide, hence its feeble illuminating power ; that pro duced at a bright red heat contains a larger portion of olefiant gas and vapors of hydro-carbons, than what is formed at a higher or lower temperature. As dis tillation advances the temperature in creases ; the proportion of illuminating gas decreases, while that of carbonic oxide and hydrogen increases in proportion. The density of the gas is also in some de gree in proportion to its illuminating power, but decreases as the heat ad vances. Dr. Henry's table, here annex ed, shows the nature of the gas evolved from Cannel coal, at different periods of the process :— No. 1, 2, and 3 were produced during the first hour, 4 at the commencement of the 6th hour, and No. 5 10 hours from be ginning.
After those various liquid and gase ous substances are obtained, they have to be separated so as to isolate the illu minating gases : the first step is to pass the whole of the volatile matters through the condenser. The warm gases which issue [have a tendency to condense and stop up the tubes] are conducted to the coolers or condensers, which are of various construction. Ordinarily it con sists of an iron chest filled with water, and having a false bottom ; a series of tubes connected by saddle joints are in the box. The lower part of this is di vided into cells in which the fluids col lect until they reach the level of a draw ing-off tube, and they are thus separated —other condensers consist of a high per pendicular tank, with a system of zig-zag gas tubes, over which an uninterrupted shower of water rains from above. On leaving the condenser the whole of the gases are still retained ; several of those are useless, as carbonic oxide and hydro gen, which burn with a very slight evo lution of light, and only tend to dilute the gas: others, on the contrary, are det rimental, as ammonia, combined with carbonic, muriatic, and sulphuric and sulphurous acids and sulphuretted hydro gen. The purification of the gas only removes the latter class, not the former.
The lime purifier consists of a cham ber containing milk of lime, stirred up with water, and agitated with a stirrer : the gas is passed through it in a very fine stream of bubbles. This removes the carbonic acid and sulphuretted hydro gen, but appears to diminish the illumi nating power of the gases. To separate the ammonia a solution of alum is some times used. Protosulphate of iron has been used for the same purpose. Dilute sulphuric acid removes the ammonia much more rapidly, forming a sulphate of ammonia, which is produced in gas works in large quantity, and is sold either to farmers as manure, or to manufacturers for the formation of other salts of sin monia. Washing the gas with water, will by itself separate the ammonia. Mallet proposes to transmit the gas through two purifiers, one a solution of green vitriol or sulphate of manganese, and the other one of milk of lime. This is the most profitable and least laborious plan.