Chap. XX.
123?
"When coal gas contains 5 per cent, of these gases, every 100 cube feet will require 1]pound of lime, or about 7 ' g in weight of the coal subjected to decomposition, with which asmuch water must be mixed as will make it into 1 cubic foot of the cream. The purifiershould have a capacity sufficient to hold ;} of a cubic foot for every 100 feet of gas thatpasses through it in one operation, and the contents should be changed every C hours: 20,000cubic feet would require 114 feet cube of cream of lime; so that a cylindrical vessel inwhich it could stand to the height of 3 feet must be 7 feet in diameter.
The purifier is generally formed of three vessels within each other, air-tight, and of acylindrical form, with an upright shaft in the axis, to which is attached a number of arms orpaddles for the purpose of stirring up and agitating the mixture, and preventing the limefrom settling at the bottom : as the gas enters at the lower part of the vessel, it passes in air-bubbles through a succession of plates around the edges, where a space is left for the pur-pose, and in its passage becomes exposed to the action of the cream of lime: in this processall the sulphuretted hydrogen is taken up ; this is tested hv allowing a portion to escapethrough a cock, and pass over a piece of paper previously dipped in a solution of sugarof lead, which will become brown if any sulphuretted hydrogen remain : when this is thecase, it must be again submitted to the cream of lime.
At large gasworks it has been found necessary to have three purifiers, so connectedtogether that the second is placed rather higher than the first, and the other a little aboveit, so that the discharge-tube of the highest vessel, which is a little below the top, entersinto the upper portion of that which stands lower ; by this means the milk of lime inthe uppermost vessel is made to rise above its ordinary level, flow over into the second,and then into the third, from whence it is drawn off by the eduetion-pipe ; the gas isintroduced into the first vessel, and passed successively through the others into thegasometer : when the milk of lime is in sufficient quantity to allow the gas in its pro-gression to pass over an entire new surface, it becomes thoroughly purified: itspurity may be tested, after it has passed through the vessel containing the lime, by meansof a siphon-tube with a stopcock ; this, inserted in the cover of the purifier, may have oneend in contact with a solution of acetate of lead, which will become turbid, when it isnecessary to renew lime in the machine. It is very desirable that gas should he renderedperfectly pure, and that no sulphuretted hydrogen should remain; it has been recom-mended to make use of pyrolignite of lead for the purpose ; the only objection is itscost : coal-gas contains also sulphuret of carbon, which gives out sulphurous acid whenconsumed, and which it is exceedingly difficult to prevent.
Various other methods have been tried to render gas pure : one was putting a smallquantity of water to the lime, and mixing it to such a consistence that it could bespread into cakes over perforated iron plates, through which it was allowed to pass;another was to pass it through red-hot pipes, in which was previously placed black oxideof iron, iron filings, &t\, or other matter capable of oxidation, which separated the sul-phuretted hydrogen and carbonic acid from the carburetted hydrogen. By the former planthe necessity of evaporating the waste liquor is avoided, and the product presumed to hebetter, retaining the whole of its olefiant gas, from which the whiteness of the flame is saidto arise, whereas this latter, being slightly soluble in water, loses some of this quality inits passage through the milk of lime: it is beneficial to submit the gas in the purifierto the pressure of a column of water about 2 feet in height, and an Archimedean screw hasin some cases been used for the purpose.
Gasometers are now made of a cylindrical form, as this figure is the most capaciousfor the quantity of metal used; its height ought to be equal to its radius, to comply withthis condition : sheet-iron is usually employed for its construction, and the cylindricalform is strengthened by the introduction on the inside of cross iron rods, both at its top andbottom ; the top is like a very fiat cone, the rods radiating from the centre to the outerrim. The bottom rim is cast-iron, held to the upper by perpendicular rods bolted throughthem; oblique and other rods are introduced in large gasometers, when additional strengthis required. The gas-holder lias on the upper rim several rings, placed at the ends ofthe perpendicular rods, to which rings the chains are fixed, which pass over the pulleys,to balance it in its rise ; this is aided and performed most effectually by attaching tothe other end of the chain a weight acting as a counterpoise.
Around the gasometer are three perpendicular iron columns, placed triangularly, overwhich the chains pass, to mark the rising or falling of the gas-holder; a greaternumber of columns are necessary when the diameter is considerable, to keep it steady.The gas-holder is made to descend into a cistern of water, constructed either of brickor cast-iron: as the gas-holder sinks, it of course loses a proportion of its weightequal to the quantity of water it displaces; so that when entirely immersed, as it iswhen nearly empty, it exercises little or no pressure, but when entirely out of the tankor water cistern its pressure is considerable ; to render the escape of the gas equal, whichis important, it is necessary so to adjust the balance chain, that throughout its motion itshall be equal to half the weight which the gas-holder loses by immersion.
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