GAS, COAL 



foot of liquor. After every operation, that is, after every five or six hours, the puriiirr 

 must bo filled afresh. Suppose that in the course of one operation 20,000 cubic 



feet of gas passes through the machine, this should be able to contain .' =114 



175 



cubic feet of milk of lime ; whence its diameter should be 7 feet, and the height of the 

 liquid 3 feet. If the capacity of the vessel bo less, the lime-milk must bo more fre- 

 quently changed. 



In some gas-works the purifier has the following construction, whereby an uninter- 

 rupted influx and efflux of milk of lime takes place. Three single purifiers are so 

 connected together that the second vessel stands higher than the first, and the third 

 than the second ; so that the discharge-tube of the superior vessel, placed somewhat 

 below its cover, enters into the upper part of the next lower vessel ; consequently, 

 should the milk of lime in the third and uppermost vessel rise above its ordinary 

 level, it will flow over into the second, and thence in the same way into the first ; 

 from which it is let off by the eduction pipe. A tube introduces the gas from the con- 

 denser into the first vessel ; another tube does the same thing for the second vessel, &c. ; 

 and the tube of the third vessel conducts the gas into the gasometer. Into the third 

 vessel, milk of lime is constantly made to flow from a cistern upon a higher level. 

 By this arrangement, the gas passing through the several vessels in proportion as it 

 is purified, comes progressively into contact with purer milk of lime, whereby its puri- 

 fication becomes more complete. The agitator c, provided with two stirring paddles, 

 is kept in continual rotation. The pressure which the gas has here to overcome is 

 naturally three times as great as with a single purifier of like depth. 



Fig. 1027 (p. 573) is a simple form of purifier, which has been found to answer well 

 in practice. Through the cover of the vessel A B, the wide cylinder erf is inserted, 

 having its lower end pierced with numerous holes. Concentric with that cylinder is the 

 narrower one sz, bound above with the flange a b, but open at top and bottom. The under 

 edge g h of this cylinder descends a few inches below the end c d of the outer one. About 

 the middle of the vessel the perforated shelf m n is placed. The shaft of the agitator /, 

 passes through a stuffing-box upon the top of the vessel. The gas-pipe g, proceeding 

 from the condenser, enters through the flange a b in the outer cylinder, while the gas- 

 pipe h goes from the cover to the gasholder. A stopcock upon the side, whose orifice 

 of discharge is somewhat higher than the under edge of the outer cylinder, serves to 

 draw off the milk of lime. As the gas enters through the pipe g into the space between 

 the two cylinders, it displaces the liquor till it arrives at the holes in the under edge of 

 the outer cylinder, through which, as well as under the edge, it flows, and then passes 

 up through the apertures of the shelf m n into the milk-of-lime chamber ; the level of 

 which is shown by the dotted line. The stirrer, I, should be turned by wheel-work, 

 though it is here shown as put in motion by a winch handle. 



1041 



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B p "IlX VP F? n o n n ^/ 



In order to judge of the degree of purity of the gas after its transmission through 

 the lime machine, a slender syphon tube provided with a stopcock may have the one 

 end inserted in its cover, and the other dipped into a vessel containing a solution of 

 acetate of lead. Whenever the solution has been rendered turbid bv the precipitation 

 of black sulphuret of lead, it should 1 >e renewed. The saturated and fetid milk of lime 



