The ammoniacal liquor which has been passing over during distillation of the coal, and which has been collecting in the hydraulic main and in other parts of the gas-making apparatus, is set aside to be treated to a variety of chemical reactions, in order to wrench from it its useful constituents. Amongst these, of course, _ammonia_ stands in the first rank, the others being comparatively unimportant. In order to obtain this, the liquor is first of all neutralised by being treated with a quantity of acid, which converts the principal constituent of the liquor, viz., carbonate of ammonia (smelling salts), into either sulphate of ammonia, or chloride of ammonia, familiarly known as sal-ammoniac, according as sulphuric acid or hydrochloric acid is the acid used. Thus carbonate of ammonia with sulphuric acid will give sulphate of ammonia, but carbonate of ammonia with hydrochloric acid will give sal-ammoniac (chloride of ammonia). By a further treatment of these with lime, or, as it is chemically known, oxide of calcium, ammonia is set free, whilst chloride of lime (the well-known disinfectant), or sulphate of lime (gypsum, or "plaster of Paris" ), is the result. Thus: Sulphate of ammonia + lime = plaster of Paris + ammonia. or, Sal-ammoniac + lime = chloride of lime + ammonia. Ammonia itself is a most powerful gas, and acts rapidly upon the eyes. It has a stimulating effect upon the nerves. It is not a chemical element, being composed of three parts of hydrogen by weight to one of nitrogen, both of which elements alone are very harmless, and, the latter indeed, very necessary to human life. Ammonia is fatal to life, producing great irritation of the lungs. It has also been called "hartshorn," being obtained by destructive distillation of horn and bone. The name "ammonia" is said to have been derived from the fact that it was first obtained by the Arabs near the temple of Jupiter Ammon, in Lybia, North Africa, from the excrement of camels, in the form of sal-ammoniac. There are always traces of it in the atmosphere, especially in the vicinity of large towns and manufactories where large quantities of coal are burned. Coke, if properly prepared, should consist of pure carbon. Good coal should yield as much as 80 per cent. of coke, but owing to the unsatisfactory manner of its production, this proportion is seldom yielded, whilst the coke which is familiar to householders, being the residue left in the retorts after gas-making, usually contains so large a proportion of sulphur as to make its combustion almost offensive. No doubt the result of its unsatisfactory preparation has been that it has failed to make its way into households as it should have done, but there is also another objection to its use, namely, the fact that, owing to the quantity of oxygen required in its combustion, it gives rise to feelings of suffocation where insufficient ventilation of the room is provided.