The recommendations of Dr. Ballard with regard to the remedy of these nuisances are briefly as follows :—

To burn up the uncondensed gases, or to carry them up a tall chimney shaft.

The nuisance arising from an escape from Bowers's apparatus, in clearing the exit pipes, may be obviated by stopping the feed rollers for five minutes before the opening of the pipes; under these circumstances no escape of vapour takes place.

The nuisance caused in drawing the charge may be lessened by hastening the process. The charge from a cylinder should be drawn into boxes fitted with covers, in which after luting down it may be left to cool, or which may be used to convey it to the underground brick chambers technically called "extinguishers." But in the large square ovens, the process of drawing the charge is necessarily a long one; here the most feasible plan seems to be to construct the "extinguisher" as close as possible to the oven, so that the charge may be directly raked out into it; if in addition to these precautions the ovens and shoots opened into a closed building ventilated by means of a wide pipe leading to a pipe. flue or tall chimney shaft with good draught, such an arrangement would minimise the danger of external nuisance.

When the charge is drawn into boxes, there should be no delay in luting down the covers. The boxes should not be carried into the open air until the luting is finished. The buildings into which the ovens open should be closed, and arrangements made for ventilating them into a fire or tall chimney.

(206) Nuisances depending chiefly on the Production
of Acrolein.

Acrolein is a product of the decomposition of glycerin ; hence whenever a fat or oil is heated to a decomposition temperature it is produced. Acrolein is a light, volatile liquid of low specific gravity, boiling-point about 120° F. It is extremely irritating to the eyes, throat, and to the mucous membranes of the bronchial tubes. It is readily combustible.

Acrolein vapours are given off during the bleaching of palm oil, the boiling of linseed oil (which is in point of fact decomposing rather than boiling), the manufacture of oil varnishes, the manufac

ture of leather cloth, enamelled table covers, the enamelling of leather, the manufacture of floor cloth and linoleum, the recovering oil from oily clothes and shoddy, the distillation of palm oil, cotton oil, "foots" and other kinds of grease.

In all these processes, at some stage or other, oils and oily matters are heated to a high temperature, and as a consequence irritating fumes are given off.

The principle to be adopted is to either condense the fumes by conveying them through pipes where they can be washed by a spray of water, or to lead the tube into a furnace fire so as to burn the fumes up. In certain cases it seems to be necessary to boil acrolein-producing substances in an open vessel, and in that case the adoption of the principle of Heywood's and Lloyd's arrangement for collecting and condensing vapours from varnish-making may be adopted. The apparatus is as follows: A is the boiling pot; on the top of this is fitted a funnel-shaped cover, B; from this cover a pipe, C, is led, which pipe is attached to an extracting fan D, and the fan

M

FIG. 45.

drives the vapours to a series of condensing tubes E, to a fire, or to a high chimney shaft as occasion requires. The consequence of this arrangement is that there is always a strong downcurrent through B into A to supply air rushing through C, and no vapour can escape into the air except by the channel provided for it.

(207) The Manufacture of Coal Gas.

Coal is submitted to destructive distillation in a close retort, volatile and gaseous matters are expelled, and a carbonaceous residue left behind. A brief description of the outlines of the process is as follows:-The coal is placed in a retort, and the retort heated to dull redness. The volatile substances in the ordinary manufacture pass up a pipe called "the ascension pipe," the upper end of which is curved downwards so as to discharge near the floor of a large horizontal pipe called "the hydraulic main"; in this pipe water

and tarry matters condense to a thick liquid which forms at the end of the ascension pipe a trap or liquid seal (In some places, as at the Fulham works, the hydraulic main is dispensed with.) The gas at a high temperature streams from the hydraulic main into a series of cooling pipes called condensers; the pipes are simply exposed to the air, and it is found not advisable to cool the gas below 60° or 65° F. The condensers free the gas from a good deal of watery vapour, from tar, and from as much ammonia as the deposited water is capable of absorbing; the condensed substances fall into a space below, and are received into the tar well. In works of any size, the gas now passes to the exhauster or air pump as crude coal gas; containing as impurities, carbonic acid, ammonia, sulphuretted hydrogen, bisulphide of carbon, some cyanogen compounds, and small quantities of tarry matters, with aqueous vapour. The more special purifying processes now commence. First the gas passes to a scrubber," i.e. a tower or cylindrical vessel of considerable size in which by means of ingeniously arranged trays or other contrivances the gas is made to pass through water, and is thus deprived of all ammonia, besides some proportion of the other impurities already detailed. "scrubbed " gas next passes into the lime purifiers-that is, boxes on which are trays of hydrate of lime. The lime combines with the carbonic acid gas to form carbonate of lime, and with sulphuretted hydrogen to form calcic sulphide. When all the lime is turned into calcic carbonate and sulphide, and the gas still streams in, then the carbonic acid gas decomposes the calcic sulphide, and sulphuretted hydrogen is given off, until the whole of the lime becomes carbonate of lime. Hence lime alone cannot be conveniently used to cleanse the gas of sulphuretted hydrogen. The gas freed from carbonic acid gas is next passed into the oxide of iron purifier; here it is deprived of the sulphuretted hydrogen, which combines with the iron, forming iron sulphide. Where by the conditions under which the company is governed, the gas must be purified from carbon disulphide, it is got rid of by interposing a small purifier filled with calcic sulphide between the lime purifier and the oxide of iron; with this substance bisulphide of carbon forms a sulpho-carbonate.

The

The process described is not followed in all the works, some one or other detail varying; thus ammonia which has escaped the

scrubber is sometimes absorbed in sawdust alone, or sawdust mixed with sulphuric acid. Instead of oxide of iron, sulphate or chloride of iron is sometimes used.

There is some nuisance in the drawing of the retorts, an operation permitting of the escape of a certain amount of gases, and attendant with the production of offensive steam, the red-hot coke being raked out into water; but this seldom travels beyond the works. The chief source of nuisance is the emptying of the lime purifiers, and the conveyance of the spent lime to its destination. Nuisance from the lime purifiers is much aggravated by improper working. In small country places the writer has seen purifiers changed so seldom that practically crude coal gas was delivered to the consumers. Directly the purifier gets at all foul from sulphur compounds it should be emptied. Dr. Ballard, in his report on effluvium nuisances, has described in great detail the method in use at the Fulham works, where, despite the large scale of the works, by carefully covering up with sacking those portions of the foul lime in a purifier not in actual process of being dug out, and having special covers for every part of the subsequent operations-covers, that is, for the trucks, and for the barges into which the spent lime is conveyed-and by previously watering the foul lime in the purifier to prevent the rising of dust, this usually offensive operation is done with but little odour.

In one case, in which the writer was a witness, a gas-works was a considerable source of nuisance to a watering-place, from defects in the construction of the works themselves; in particular the scrubber had only a wooden top; through this wooden top abundance of gas streamed, blackening lead paper. Sometimes there is nuisance from the revivification of the spent oxide, defects in the scrubbing process allowing ammonia compounds to reach the iron oxide; in such cases, as the oxide always heats in the revivification, much ammoniacal vapour may be given off.

(208) The Manufacture of Sulphate of Ammonia and of Sal Ammoniac.

The ammoniacal liquor from the gas-works is mixed with lime, distilled, and the ammonia received into either sulphuric or hydrochloric acid until saturated, and the salt crystallized out.

Nuisance may be caused during the transference of the liquor from the gas-works, from the storage of the liquor, or during the manufacturing process. The main cause of complaint is the escape of sulphuretted hydrogen. The methods in use for abatement of this nuisance are mainly two: (1) burning the fumes, (2) absorbing them by chemical agents. The sulphuretted hydrogen and other matters are burnt by carrying them either into a special furnace or into the same furnace which distils the liquor; in either case, the vapours, consisting of sulphurous acid and other gases, should be carried into a tall chimney shaft; otherwise one nuisance will only be replaced by another. The absorption of SH, is effected by lime, or by hydrated oxide of iron mixed with sawdust. The latter is preferable.

(209) Distillation of Tar.

Coal tar, mixed with more or less ammoniacal liquor, is distilled in suitable stills. The products are (1) "light oils," floating in water, from which benzole and solvent naphtha are obtained; (2) "heavy oils," or creasote oils, sinking in water, and containing cresylic and carbolic acids; (3) anthracene oil, still heavier than creasote oils; and lastly, (4) pitch.

The sources of nuisance, according to Dr. Ballard, are as follows:

1. The reception of the tar in uncovered barges, and its transference to the tanks or storage receptacles at the works; the offensive odour is chiefly from sulphide of ammonium.

2. The escape into the atmosphere of offensive distillation products. These come off mostly towards the end of the distillation. 3. The escape of a more or less dense white vapour when hot pitch is run off from the still or pitch oven into the pitch bay, or a similar vapour from the pitch bay or tubs before they cool.

4. The escape of offensive vapours from the pitch oven.

5. The combustion of creasote oil as fuel for the stills, where the arrangements for its proper combustion are incomplete.

All these nuisances have been dealt with successfully at properly designed and conducted works, e.g. like those of Messrs. Burt, Boulton, and Haywood, Silvertown. The main points are transference of the crude material in air-tight vessels, absorption of