of dyer’s madder, which was known as alizarin, would also be obtained therefrom.  Hitherto this had been obtained from the root of the madder-plant, but by dint of careful and well-reasoned research, it was obtained by Dr Groebe, from a solid crystalline coal-tar product, known as anthracene, (C_{12}H_{14}).  This artificial alizarin yields colours which are purer than those of natural madder, and being derived from what was originally regarded as a waste product, its cost of production is considerably cheaper.

We have endeavoured thus far to deal with (1) gas, and (2) tar, the two principal products in the distillation of coal.  We have yet to say a few words concerning the useful ammoniacal liquor, and the final residue in the retorts, i.e., coke.

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.