_Prep._ 1. From GAS-LIQUOR:——The crude ammoniacal liquor of the gas-works is, either at once, or after distillation,[48] neutralised with hydrochloric or sulphuric acid, the choice being given to the one which is the cheaper and more accessible at the place where the works are situated. When hydrochloric acid is employed, the SATURATION is usually effected by allowing the acid to flow from a large wooden vessel or tank lined with lead or gutta percha into a large underground reservoir or tank containing the ammoniacal liquor, and having an exit-tube passing into the chimney or shaft of the steam-engine, to carry off the sulphuretted hydrogen and other offensive gases liberated during the mixture. Sometimes the gas-liquor is accumulated in enormous covered wooden tuns, capable of holding from 10,000 to 20,000 gallons, or more; and the acid is added by raising the gutta-percha carboys containing it by means of cranes, and then thoroughly mixing it with the liquor by means of powerful ‘agitators,’ whilst the offensive fumes are either passed off as before, or made to traverse the fire of the steam-engine before entering the chimney-shaft. The quantity of acid employed to effect saturation must, of course, depend on the ammoniacal strength of the gas-liquor operated on. The usual proportions are 1-1/2 to 2 _lbs._ of the former, to each gal. of the latter; but in all cases sufficient should be added to impart a very faint acid reaction to the mixture. This last having been effected, the saline solution, now containing hydrochlorate of ammonia, is, after repose, ready to be pumped or run off into the evaporators. [Footnote 48: This is now generally conducted in a large wrought-iron boiler, connected with a rude modification of Coffey’s still; the object being to obtain the liquor freer from tar and more concentrated.] The EVAPORATION of the crude saline solution is usually carried on in large square or rectangular cast-iron vats, of very moderate depth, and capable of holding from 1000 to 1500 gallons, or more. These are encased in brickwork, and are heated by a furnace, of which the flues pass in a sinuous course beneath the lining of brickwork on which the vats or pans rest. During the concentration of the liquid, the tar, &c., which separates and floats on the surface, and which thus seriously impedes evaporation, is, from time to time, removed by skimming. As soon as the sp. gr. reaches 1·25, any excess of acid in the solution is exactly neutralised with a little fresh ammoniacal liquor; by which any waste of acid is prevented, at the same time that any ferric salt present, and which would contaminate the ultimate product, is precipitated as sesquioxide. After settling for a short time, the hot liquor is ready to be transferred to the crystallisers.