The first, and perhaps, most important portion of the apparatus used in gas-making is the series of retorts into which the coal is placed, and from which, by the application of heat, the various volatile products distil over.  These retorts are huge cast-iron vessels, encased in strong brick-work, usually five in a group, and beneath which a large furnace is kept going until the process is complete.  Each retort has an iron exit pipe affixed to it, through which the gases generated by the furnace are carried off.  The exit pipes all empty themselves into what is known as the hydraulic main, a long horizontal cylinder, and in this the gas begins to deposit a portion of its impurities.  The immediate products of distillation are, after steam and air, gas, tar, ammoniacal liquor, sulphur in various forms, and coke, the last being left behind in the retort.  In the hydraulic main some of the tar and ammoniacal liquor already begin to be deposited.  The gas passes on to the condenser, which consists of a number of U-shaped pipes.  Here the impurities are still further condensed out, and are collected in the tar-pit whilst the gas proceeds, still further lightened of its impurities.  It may be mentioned that the temperature of the gas in the condenser is reduced to about 60 deg.  F., but below this some of the most valuable of the illuminants of coal-gas would commence to be deposited in liquid form, and care has to be taken to prevent a greater lowering of temperature.  A mechanical contrivance known as the exhauster is next used, by which the gas is, amongst other things, helped forward in its onward movement through the apparatus.  The gas then passes to the washers or scrubbers, a series of tall towers, from which water is allowed to fall as a fine spray, and by means of which large quantities of ammonia, sulphuretted hydrogen, carbonic acid and oxide, and cyanogen compounds, are removed.  In the scrubber the water used in keeping the coke, with which it is filled, damp, absorbs these compounds, and the union of the ammonia with certain of them takes place, resulting in the formation of carbonate of ammonia (smelling salts), sulphide and sulphocyanide of ammonia.

[Illustration:  FIG. 35.—­Filling Retorts by Machinery.]

[Illustration:  FIG. 36.—­CONDENSERS.]

Hitherto the purification of the gas has been brought about by mechanical means, but the gas now enters the “purifier,” in which it undergoes a further cleansing, but this time by chemical means.

[Illustration:  FIG. 37.]

The agent used is either lime or hydrated oxide of iron, and by their means the gas is robbed of its carbonic acid and the greater part of its sulphur compounds.  The process is then considered complete, and the gas passes on into the water chamber over which the gas-holder is reared, and in which it rises through the water, forcing the huge cylinder upward according to the pressure it exerts.