GAS ‘ 93 nearly that of the surrounding atmosphere. The first con- trivaiices employed for the purpose of condensation were all constructed on the supposition that the object would be best attained by causing the gas to travel through a great extent of pipes surrounded by cold water, and winding -tliroiigh it like the worm of a still, or ascending upwards and downwards in a circuitous manner. An improvement on this form of condenser, and one now in general use, is represented in fig. 8. It consists of a series of upright _ _ gzg ._- if-7-1411: - —- un2.::.r— __..'-,—.._ Fig. 8. pipes connected in pairs at the top by semicircular pipes e, e, and terminating at the bottom in a trough X Y con- taining water, and divided by means of partitions in such a way that, as the gas enters the trough from one pipe, it passes up the next pipe and down into the next partition, and so on to the end of the condenser. The cooling power of this air condenser, as it is called, is sometimes assisted by allowing cold water to trickle over the outer surface of the pipes. Annulur tubes for condensing are also used, in which the gas is exposed to a much greater cooling surface, and in some large works the condensers are cooled by a current of water. In passing through the pipes the gas is considerably reduced in temperature, and the tar and am- moiiiacal liquor condense, the tar siibsiding to the bottom of the troughs, and theammoniacal liquorfloatin g on thesiirface. In course of time the water in the trough is entirely displaced by these two gaseous products, and as they accumulate they pass off into the tar-tank, from which either liquor can be removed by means of a pump adapted to the purpose. The New York Gas Lighting Company employ a multitubular condenser, consisting of two sets of eight boxes, each con- taining 100 tubes 3 inches diameter by 15 feet long. Through each set of tubes, up one and down another, the gas travels, cooled by an external stream of water, while it traverses the 240 feet of piping in the condenser. The practice of condensation and separation of tarry matter by rapid cooling is condemned by Mr Bowditch and many eminent authorities, on the ground that thereby a proportion of light hydrocarbons are thrown down with the heavier deposit, which on another method of treatment would form part of the permanent gas and materially enrich its quality. A system of treating gas has accordingly been introduced by Messrs Aitken 6: Young, in which the gas, kept at a high temperature, is carried from the retorts into an apparatus termed an analyser, which consists of an enclosed series of trays and chambers arranged in vertical series, in principle like a Coffey still, the lower portion of which is artificially heated. In action the analyser separates the heavier carbonaceous part of the tarry matter in the lower part or chambers, and as the gas gradually ascends from one tray or tier to another, it is at once cool- ing and depositing increasingly lighter fluids, while it is meeting and being subjected to the purifying action of the light hydrocarbons already deposited. Thus on entering the analyser it meets, at a high temperature, heavy tar deposits, and it passes out of the apparatus cooled down to nearly atmospheric temperature after being in contact with the lightest fluid hydrocarbons. E.rlzcmstion.—To the subsequent progress of the gas con- siderable obstructions are interposed in connexion with its further purification and storing in the gas-holders, and the result of which would be that, were it not artificially pro- pelled, there would be a pressure in the retort equal to the amount of the resistance the gas meets with in its onward pro- gress. The relief of this back pressure not only improves the quality of the gas, but also increases its amount by about 10 per cent. Among the numerous methods of exhaustion which have been proposed since the operation was first introduced in 1839, there are several rotary exhausters, hav- ing more or less of a fan action, and recently an apparatus on the principle of a Gifi'ard’s injector has been intro- duced, chiefly in Continental works. A most eflicient form is found in the piston exhauster, a kind of pumping engine with slide valves, which exhausts the gas in both the upward and the downward strokes of its piston. The action of the exhauster is controlled by a governor, which passes back a proportion of the gas when the apparatus is working too fast for the rate of production in the retorts ; and “pass by” valves are arranged to carry the gas onward without passing through the exhauster should it cease to work from accident or any other cause. Pm-2;ficalion.—Tlie operations embraced under this head have for their object the removal from the gas of am- monia, sulphuretted hydrogen, and carbonic acid as the main impurities, with smaller proportions of other sulphuric and of cyanogen compounds. The agencies adopted are partly mechanical and partly chemical, the separation of the ammonia being first effected in the “ scrubber,” from which the gas passes on to complete its purification in the “ purifiers.” In early times the purify- ing was performed in a single operation by the use of milk of lime in the wet purifier, a form of apparatus still in use where wet purifying is permissible. The Wet [’ur{fier.—This apparatus was supplied with a cream of lime and water, but, although it was a most etficient purifying agent, the ammonia now of so much value was lost by its use, and the “ blue billy,” as the saturated liquid holding the impurities was termed, created an intol- erable nuisance, and could be in no harmless way got rid of. Except in small works, wet purifying is not now practised. T/ze Sc-rubber.—The object sought in an ordinary scrubber is to cause a large amount of gas to come in con- tact with the smallest possible qiiantity of water, so as at
once to dissolve out ammoniacal gases, which are exceedingly