Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/105

GAS 95 2 inches above the surface of the water. The cover which fits into the cylinder is 4 feet 3 inches in diameter, and is divided into five parts, the first of which, 1, fits over the_ inlet pipe a, and over eitlicr of the inlet pipes leading to the pui'ifiei's. I‘lie partitions 2, Fig. 12. 3, and 5 fit cacli over an inlet and an outlet pipe, while one parti- tion, 4, fits over one outlet pipe from one purifier, and over the pipe 71, which leads to the gas-liolder. In fig. 12 the arrangement is such as to open a communication between the inlet pipe a and the purifier A. N ow supposing the gas to have passed from the scrubber into the centre of the cylinder, its only means of escape is to pass down the pipe b into the purifier A, where it ascends through the layers of lime, and passing over the top of a dividing plate, descends and escapes from the bottom of the purifier by the pipe 0 back to the cylinder. Here its only means of escape is by the pipe d, which conducts it to the purifier B, in which it ascends and descends as before, returning by the pipe c to the cylinder, whence it proceeds by the pipe f into the purifier C, then along the pipe g, which is shut off from communication with any pipe except /1, by which it is eonveyed away to the gas-holder. By this arraiigenient the three purifiers A B C are being worked, while a fourth purifier D is being emptied and recharged with lime. Vheii it is found, on testing the gas, that the lime is unfit for its office, the purifier A is thrown out of work, and D is brought in. 'l‘lie frame is then shifted so as to bring the triangular division 1 over (7,, by which means B C D will be the working purifiers, and A will be thrown out of use. In this way, by shifting the frame round its eentre over each of the four outlet pipes, any three of the purifiers can be brought into action. The “oxide” method of purifying the gas, originally introduced by M. Laining, and shortly afterwards patented by Mr Hills, is now largely used in ordinary gas-works. It is based upon the property of the hydrated oxide of iron to decompose sulphuretted hydrogen, a portion of the sulphur forming a sulphide with the iron. Quicklime is also used to separate carbonic acid, an.d the oxide of iron is mixed with sawdust or ciiiders (breeze) for the purpose of increas- ing the surfaces of contact, and this mixture is placed in the purifiers. When a sufficient quantity of gas has passed through it, the purifiers are opened, and the mixture is exposed to the air, under which new condition it combines with oxygen, and again becomes fitted for use in the puri- fiers. The chemical changes which occur in these opera- tions are thus stated. The mixture of hydrated oxide of iron, &c., absorbs sulphuretted hydrogen, forming ferrous sulphide and water, and liberating sulphur, thus :— F8303 + 3H._,S = ‘7FeS + S + 3H2O. The ferrous sulphide, by exposure to the air, absorbs oxygen, and its sulphur is separated in an uncombined form, ‘.2FeS + 30 = FezO3 + 2S. The mixed material can be again employed in the purifica- tion of the gas, an.d the process may be repeated until the accumulation of sulphur mechanically impairs the absorbent powers of the mixture. The sulphocyanogen which accom- panies the gas is retained by the oxide of iron, and gradually accumulates in the mixture. For the separation of the carbonic acid, which is iinaffected by this treatment, the gas next passes on to a dry lime purifier. The gas is now ready for use, and it is passed on through the station meter to register the amount made and stored in the gas-holders. At this stage it may be interest- ing to compare the composition of the gas as it exists at different stages of the manufacture, as these show the result of the successive purifying processes. Taking 1000 cubic feet, the figures are—- From Purifiers. From -From Condenser. Scrubber. Iron. Lime. Hydrogen .................. .. 380 380 380 380 lI-arsli gas ................. .. 390 388 403 394 Carbonic oxide ........... .. 72 71 39 30 Heavy hydrocarbons .... .. 42 46 46 43 Nitrogen .................. . . 48 50 79 100 Oxygen ..................... .. 3 5 5 6 Carbonic acid ............. .. 40 39 33 4 Sulpliuretted hydrogen... 15 15 3 Ammonia ................. .. 10 5 1000 999 988 957 STORING AND DISTRIBUTION. The G'as-kolder.——This, which is frequently designated the gasometer, though incorrectly, since it does not in any way measure gas, but simply stores it for consumption, con- sists of two portions—the “tank” T (fig. 13) an.d the “holder” G. The tank is a cylindrical pit, surrounding a central core, which is usually covered with concrete c at top, an.d has its sides built of masonry or brick-work, p,‘ b. The tank is water-tight, and is filled to a high level with water, above which project two tubes mm, one being the inlet and the other the supply pipe which leads to the main governor. _ Formerly gas-holders were made of heavy plate iron, strengthened by angle-iron and stays, and of so great a F10. 13.—Sectioii of Gas-holder. weight as to require a complex system of equilibrium chains and counterbalancing weights to relieve the gas from the great pressure to which it would otherwise be subjected.

They are now made so light that they require to be 10358-1