CHAMBERS'S INFORMATION FOR THE PEOPLE. 



all observations being corrected to these generally 

 recognised standards. As a general rule, the 

 parrot coals which yield the greatest quantity 

 of gas, yield also gas of the highest illuminating 

 power. 



As the value of coal-gas depends on the pro- 

 portion of olefiant gas and heavy hydro-carbons 

 which it contains, great attention is required in 

 heating the retorts ; if their temperature be too 

 low, the tar and liquor are increased in quan- 

 tity, and the gas diminished in quantity. If the 

 temperature be too high, the olefiant gas is 

 decomposed, and light carburetted hydrogen 

 formed. 



While different parts of the apparatus necessary 

 for producing, purifying, storing, and sending out 

 the gas are capable of many variations in size, 

 form, and construction, the order in which they 

 come into use is almost invariable. First, there 

 are the retorts, ascension and dip pipes, and the 

 hydraulic main; then the tar-well and. condenser, 

 the exhauster, the washer or scrubber, the purifier, 

 the station-meter, the gas-holder, and the governor 

 the parts printed in italics being indispensable. 

 Besides the above, valves of various forms and 

 water-traps are employed. The annexed wood- 

 engraving shews an arrangement common in small 

 gas-works. 



Retorts, &c. Tar-welL Condenser. Washer. Dry-lime Purifier. 



Fig. 13. ELEVATION OF GAS-WORKS. 



The retorts are generally made of fire-clay ; cast- 

 iron retorts, once common, are now seldom to be 

 met with. They are made D-shaped, cylindrical, 

 kidney-shaped, and elliptical. The sizes most 

 common are from 6 to 9 feet in length, and from 

 12 to 20 inches in diameter. The retort is built 

 horizontally into an arched oven, in such a manner 

 as to be equally heated throughout from a furnace 

 beneath. From one to seven retorts, and some- 

 times a greater number, are set in the same oven. 

 The mouth-piece of the retort is of cast-iron, 

 and projects outwards from the front wall of the 

 oven sufficiently far to admit between the mouth 

 and the front of the oven an opening, to which the 

 ascension-pipe is connected for conveying the gas 

 to the hydraulic main. When the coal to be dis- 

 tilled is introduced into the retort, the mouth is 

 closed tight with a lid. 



The hydraulic main is a large horizontal pipe 

 made of thick plate or cast-iron. It is first about 

 half-filled with water, which in the course of a 

 short time is entirely displaced by the liquid 

 product of distillation. The dip-pipes, which are 

 the continuation of the ascension-pipes, dip into 

 the liquid through which the gas bubbles up into 

 the upper portion of the hydraulic main. The gas 

 and liquid come off at the end of the hydraulic 

 main, and flow together till they reach the tar-well, 

 into which the liquid, by its greater gravity, falls. 

 The liquid consists of tar and ammoniacal water. 

 These are withdrawn from the tar-well, and 

 become the raw material from which other pro- 

 ducts are manufactured. From the tar, naphtha, 



494 



pitch-oil, pitch, and coke, together with some of the 

 most beautiful dye-stuffs and delicate essences, 

 are obtained ; and from the water, salts of ammonia, 

 so valuable in agriculture and the arts, are 

 prepared. 



The simplest form of condenser consists of a 

 series of upright pipes, connected in pairs at the 

 top. These are erected upon a horizontal chest, 

 the top of which has an opening into the bottom 

 of each upright pipe. Immediately under each 

 arch pipe a plate descends from the top of the 

 chest, and dips into the liquid with which the chest 

 is partly filled. In this way the gas is pre- 

 vented from passing through the chest horizontally, 

 and compelled to traverse the whole series of 

 pipes, the extensive surfaces of which serve to 

 radiate the heat from the cooling gas into the 

 atmosphere, and bring it down to about 50, at 

 which temperature it is found that the gas, in 

 passing out of the condenser, has left entrapped 

 therein the great bulk of condensable vapours 

 with which it was charged in its journey from the 

 retorts these consisting of light tarry matters, 

 ammoniacal compounds, tar-oils, and water. 



The exhauster, when used, is a species of pump, 

 and serves the purpose of relieving the retorts of 

 the resistance or pressure, created in the passing 

 of the gas through the apparatus, and in raising 

 the gas-holders. The use of the exhauster is 

 attended with other important advantages. 



At this stage of the process, the liquid products 

 have been separated from the gaseous. A portion 

 of the ammonia and the sulphuretted hydrogen 



