sulphide of ammonium by hydrated oxide of iron, passing the gases through water and then burning them up in the furnace fire, running the pitch into a closed vessel to cool, and lastly, where creasote oil is used as a fuel, burning the oil in furnaces specially constructed for that purpose.

(210) The Manufacture of Carbolic Acid.

The carbolic oil obtained from the distillation of coal tar is treated in an iron tank with caustic soda, which dissolves out the carbolic and cresylic acids, and leaves "tar oils " unacted upon. The caustic soda solution is separated from the "tar oils" by syphoning, and then the alkaline liquid is supersaturated with sulphuric acid; thereupon the crude carbolic and cresylic acids separate, and the acids are ladled off and distilled. The distillation is fractional and is carried to dryness, the residue in the retort consisting of a light coke which subsequently is used as a fuel.

Nuisance is liable to be caused by the odour of carbolic acid, by the escape of offensive gases towards the end of the distillation,

g

FIG. 46.

and by the removal of the coke from the retorts. The general odour of carbolic acid is much lessened by receiving the condensed liquids into covered vessels. The offensive gases are dealt with at Mr. Lowe's works at Reddish in the following way :-The

pipe from the worm condenser a divides into an ascending and a descending branch; the descending branch, b, carries the condensed liquids and is trapped by a syphon at i, while the ascending pipe delivers gases into a 6-inch main pipe, c, which runs along the whole range of stills. From the main a pipe conveys the gases to a small worm condenser, d, and from this to a stoneware bottle, e, which receives any liquid matter condensing; a pipe from e is supplied with a steam exhauster jet, f which causes a greater or less vacuum in the gas main, and drives the gases onwards through water or milk of lime, contained in a cask, g, whereby sulphuretted hydrogen is arrested, and from this washer a pipe conveys any gases not arrested into the ash-pit of a fire. This arrangement of Mr. Lowe's seems generally applicable to offensive distillations, and is highly ingenious.

Nuisance occasioned by the removal of coke from the retorts is said not to occur, provided the distillation is complete, and the coke only removed when cold. There ought to be no difficulty in passing the products of combustion through scrubbers of coke through which water trickles.

(211) The Manufacture of Alkali.

The materials used are common salt, sulphuric acid, limestone and coal. The salt is decomposed by sulphuric acid, assisted by heat. The reaction here is a simple case of double decomposition, sulphate of soda, is formed and hydrochloric acid escapes. The crude sulphate of soda, termed technically "salt cake," is mixed with crushed limestone, or chalk and coal, and heated strongly, the ultimate result being a mixture of sodic carbonate, calcium sulphide and unburnt carbon; the latter gives the mass a black colour, hence it is termed technically "black ash." The black ash is treated with water which dissolves out the sodic carbonate; the residue is known as (6 tank waste."

The manufacture has been the subject of special legislation, the more particularly with reference to the acid fumes or gases which are necessarily produced. These are defined by the Act to be sulphuric acid, sulphurous acid, nitric acid, or other noxious oxides of nitrogen, sulphuretted hydrogen and chlorine. At the present time great improvements in the apparatus for "scrubbing" or

washing the issuing gases have generally reduced the nuisance from the acid gases, and the fact that special inspectors are appointed to see that the requirements of the Act are observed rather take the matter out of the hands of the medical officer of health and his subordinates.

What, however, will directly concern the sanitary chief, is nuisance arising from tank waste. These form mounds, embankments, and even small hills of an artificial earth consisting of various compounds of sulphur and calcium, the oldest layers being oxidized to a greater or less degree to sulphite and sulphate of calcium, while the more recent layers have a large proportion of sulphide, and if from any cause moistened are liable to give forth sulphuretted hydrogen.

The chief source of waste heap nuisance is the soluble matter, which is "a sulphuretted compound of calcium of indefinite composition, but which is mainly composed of sulphide of calcium, partly converted by oxidation into hyposulphite of calcium, and holding in solution with it a considerable but indefinite quantity of sulphur." The solution of the sulphur matter occurs from being washed by the rain, from being lixiviated by springs, or, as at the St. Rollox works, the heaps being situated on the banks of a tidal river, and being washed by its waters as it rises and falls.

In recent years successful attempts have been made for the utilization of the waste, the sulphur contained in them being extracted by appropriate chemical treatment.

In Mond's process, air is blown through the fresh tank waste, and by this means the sulphur compounds oxidized to sulphite and sulphate; these soluble compounds are lixiviated out by means of water, the aqueous solution is then treated by steam and hydrochloric acid, sulphur is precipitated and allowed to subside, it is afterwards collected, and forms a very pure kind of sulphur. The process is not productive of nuisance in theory, but in practice there is no doubt that sulphuretted hydrogen is not unfrequently evolved.

Mactear's process in use at the St. Rollox works is as follows: The sulphur heaps, which are situated in this instance on ground permeated with springs, supply a large and regular amount of liquid holding in solution calcic sulphite and hyposulphite. Hydrochloric acid is added, and both salts decomposed; in the

decomposition of the hyposulphite, sulphur is thrown down and sulphurous acid evolved; in the decomposition of the sulphide sulphuretted hydrogen is evolved, but this gas coming in contact with the sulphurous acid is immediately decomposed, and sulphur falls, so that if there is always a sufficiency of the hyposulphite to evolve enough sulphurous acid to decompose all the sulphuretted hydrogen, there is no nuisance, and the whole of the sulphur of the sulphide is recovered. If there is not a sufficiency in the liquid itself the requisite quantity of the hyposulphite is added.

(212) The Manufacture of Picric Acid and the Manufacture of Aniline and Aniline Colours.

The different processes in use for the manufacture of picric acid, of nitro-benzole, of arsenic acid, of magenta and of the aniline dyes generally, are all liable to permit the escape of nitrous acid or other acid fumes. Since however the escape of any acid is a distinct loss to the manufacturer, in all well-managed works they are condensed as much as possible.

The best method to obviate nuisance seems to be to pass the gases through water, and lastly through fire.

A heavy odour of essence of myrbane often pervades aniline works, this is due to small leakages.

(213) The Manufacture of Sulphuric Acid.

Sulphuric acid is made by burning either sulphur, cupreous pyrites, mundic, sulphur holding oxide of iron from the gas works, or from sulphuretted hydrogen. The sulphurous acid produced by the burning of these forms of sulphur or sulphides is oxidized into sulphuric anhydride in large lead chambers by means of the higher oxides of nitrogen, set free by the action of sulphuric acid on sodic nitrate. The weak acid is subsequently concentrated in platinum stills and in glass retorts.

Nuisance may be caused by the escape of sulphurous acid gases and by the higher oxides of nitrogen. The gases may escape from (a) the chimney; (b) leakages in the burners and escape in the opening of the feeding doors; (c) leakage from the lead chambers arising from defective joints; (d) concentration of the chamber acid;

e) evolution of acid fumes from accidental breaking of a glass retort. Escapes of gas by the chimney will not occur in large works where the Gay Lussac and Glover towers are used. The Glover tower is essentially a coke tower, and the principle of its action is that when strong sulphuric acid charged with nitrous acid is mixed with either water or weak acid, most of the nitrogen compounds are given off. The gases from the burners stream up this coke tower and meet two streams of acid, viz., weak chamber acid and acid charged with nitrous fumes from the Gay Lussac tower; the reaction mentioned above takes place, and the nitrosulphuric acid is almost completely denitrated. The gases pass on into the chambers, the denitrated acid passes into the Gay Lussac tower, which is a leaden scrubber packed with coke; the strong denitrated acid being forced up to the top of this tower, trickles down through the coke, absorbing any nitrous fumes, and thus the acid is turned into nitro-sulphuric acid, the destination of which is as before to the Glover tower. Hence there is a perpetual circulation, and theoretically the same nitrogen could be used over and over again; leakages and some other chemical changes prevent the entire realization of this. It is however clear that where these towers are used no nitrous fumes should escape from the chimneys.

The other sources of nuisance are more or less remediable by proper fittings, suitable stoking, suitable covers, condensing arrangements and obvious precautions.

(214) The Manufacture of Salt.

This manufacture consists in the evaporation of brine which has been raised from salt mines. The chief salt mines are in Cheshire and Worcestershire. The brine is first pumped into reservoirs to deposit and then run into large shallow evaporating pans. Each pan is heated either directly by a fire placed underneath it or by a system of flues. The flues from the fires of the salt pans usually terminate in a number of low chimneys from 30 to 60 feet in height, the reason assigned for not replacing the short chimneys by one tall shaft, is the danger to the tall shaft from the subsidence of the land, which as is well known is constantly taking place in salt districts.

Salt works are productive of two kinds of nuisance, viz., the