The ammonia water of gas works contains from 2-3 per cent. of ammonia, some of which can be recovered on boiling, but some is in a non-volatile form, and to be recovered the compound must be decomposed. The volatile compounds are principally ammonium carbonate and, to a less extent, ammonium sulphide and cyanide; the non-volatile compounds are ammonium sulphocyanide, ammonium chloride, sulphate, thiosulphate, &c. Other noteworthy substances in ammonia water are pyridine, pyrrol, phenols, hydrocarbons, and tarry compounds. Decomposition of the non-volatile compounds is effected by lime. Hence the ammonia water is distilled first alone, and then with lime. The distillate is passed into sulphuric acid, ammonium sulphate being formed. Distillation apparatus constructed on the principle usual in rectifying spirit is used, so that continuous action is secured; the ammonia water flows into the apparatus continuously and is freed of the volatile compounds by the steam. At a later stage milk of lime is added, which liberates the ammonia from the nonvolatile compounds. Of the ammonium salts there require mention: _Ammonium sulphate_ ((NH₄)₂SO₄), which serves for the production of other ammonium salts. It is usually centrifugalised out from the sulphuric acid tank previously described. _Ammonium chloride_ (sal-ammoniac, NH₄Cl) is formed by bringing the ammonia fumes given off in the process described in contact with hydrochloric acid vapour. The crude salt so obtained is recrystallised or sublimed. _Ammonium phosphate_ ((NH₄)₂HPO₄) is made in an analogous manner by leading ammonia into phosphoric acid. It is useful as an artificial manure. _Ammonium carbonate_ is made either by bringing together ammonia vapour and carbonic acid or by subliming ammonium sulphate with calcium carbonate. It is very volatile. The thick vapour is collected and purified in leaden chambers. [Illustration: FIG. 22.—Preparation of Ammonia. Column Apparatus of Feldman (_after Ost_) A, B, C Columns; D Saturator; (a) Settling tank and regulator for flow of ammonia; (b) Economiser; (f) Milk of lime; (g) Pump] _Caustic ammonia_ is prepared either from gas liquor or, more usually, from ammonium sulphate by distillation with caustic alkali in a continuous apparatus. USE OF AMMONIA.—Ammonia is used in laundries and bleaching works in dyeing and wool washing. It is used especially in making ammonium salts, in the preparation of soda by the Solvay process (see Soda Manufacture), and in making ice artificially. It is used also in the preparation of indigo, in lacquers and colours, and the extraction of chloride of silver, &c.