Chap. XX.

GAS LIGHTING.

1235

Bicarburotted hydrogen - -Olefiant gas -

Carbonic acid -Azote - -

The whole combined having a specific gravity of *650:hours, it may be thus stated: —.

Bicarburetted hydrogen -

Olefiant gas -

Carbonic oxide -Hydrogen -

Azote ------

82 *513*

3-21 -2

when the retort has been heated

56

11 -

21-3

47

The specific gravity of the compound being *5.

At the end of 10 hours it contains 20 of carburetted hydrogen, 10 of carbonic oxide, COof hydrogen, 10 of azote, and the specific gravity is only 0-345.

In the first hour of distillation one-fifth of the whole quantity is given out, and the re-mainder in tolerably equal quantities for each successive hour, until the whole, whichamounts to about fifty-four parts out of the hundred, is obtained: it is consequently advisableto commence the operation by making the retort cherry-red; the best gas being then pro-duced, when a portion of the tar is converted, instead of being distilled over. At thecommencement of gas-lighting it required half as much coal to heat a retort as it wascharged with ; now five retorts are heated in one oven by a single furnace, and one-thirdthe contents of the retorts is sufficient for the purpose: 65 per cent, of coke remains in theretort after distillation, and its volume is increased in the proportion of 4 to 3: it is nowpreferred for heating the retorts, and its power may be estimated at 65 per cent, of thecoals consumed. When the retorts have been some time in use, it is found that theirexternal capacity is diminished, in consequence of an incrustation of carl>onaceous matterwithin them : this has sometimes been removed by throwing in jets of heated atmosphericair with considerable force, when the retort is heated to redness; it has been suggestedthat this might be effected by common air, if applied in sufficient quantities, which wouldburn away the extraneous matter.

The Hydraulic Main is a strong cast-iron pipe, usually about 12 inches in diameter, of asufficient length to receive all the perpendicular pipes that convey the gas generated in theseveral retorts : this main is made perfectly horizontal, and supported on the brickworkwhich covers the ovens. The gas leaves the retort by means of an upright conductingiron pipe, about 3 inches in diameter, and 4 or 5 feet in length, fixed at one end into themouth-piece and mounting perpendicularly, curving at the top, before it is allowed to de-scend and join the hydraulic main ; through this pipe the impure gas passes.

The hydraulic main contains a certain quantity of cold water, for the purpose of con-densing the several substances which constitute the first-formed gas ; one end of the main isfirmly closed by means of a flange;the other has a similar contrivance,and a little above its diameteris attached the pipe through whichthe gas passes -. at the end is asemicircular piece of iron, whichmaintains the water at a uniformheight.

The upright pipes, which arebent at the top, and convey the gasinto the hydraulic main, dip 2 inchesor more into the water, obligingthe gas to flow through a portion I942 *

of it before it rises into the upper half of the main and passes off. The tar depositedin the water from this process is drawn off by means of pipes, whilst the greater portionof the ammonia, from its strong affinity for water, combines with it.

The Condenser , which receives the gas from the hydraulic main for the purpose ofcooling it, is a large cistern or tank constantly immersed in cold water ; its form may beeither square or circular; but it must be so arranged, that when the gas has once enteredit may be obliged to pass over a considerable cold surface before it can escape : it is usuallycovered with wrought-iron plates pierced with holes, into which are inserted a number oftubes; these are 4 inches in diameter, and attached by flanges and screws, connected inpairs at top, by means of curved or saddle tubes. The chest or vessel which is socovered is divided by vertical iron partitions, as many in number as there arc pairs oftubes; and as these vertical plates do not touch the bottom of the chest, the gas is obliged,or rather made, to pass up and down this series of partitions and pipes, or from one

4 k 2

{P

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HYDRAULIC MAIN.