GAS, COAL 



581 



It is shown in section in fig. 1040. Tho axle A is reduced at each end, and passes 

 into two cylindrical boxes bored to a larger diameter than the axle at those parts ; 

 and in the annular space between the axle and the box antifriction rollers are intro- 



1040 



duced, their diameter being equal to the width of the annular space ; the box at one 

 end is fitted with a stuffing box, through which the axle passes for the application of 

 the driving power. Upon motion being given to the axle, the sliding pistons B B are 

 carried with it. These sliding pistons are furnished at their ends with cylindrical 

 pins, which project and fit into cylindrical holes bored in the guide blocks c c, which 

 fit into annular recesses D in the end plates, and keep the slides in contact with the 

 cylinder. The slides are fitted with metallic packing to allow of wear. The axle 

 continuing to revolve, as one slide reaches the outlet and ceases to exhaust, the 

 other comes into action, and the exhaustion is unceasing. Thus the pressure upon 

 the retorts (which is indicated by a gauge) is reduced to about half an inch of water. 



III. APPABATUS TJSED IN THE PUBIFICATION OF COAL-GAS. 



The Purifier. One form of this apparatus, represented at x,fig. 1026 (p. 573), is 

 composed of a cylindrical iron vessel, with an air-tight cover screwed upon it, through 

 which the cylinder n is also fixed air-tight. The bottom of this cylinder spreads out 

 like the brim of a hat, forming a horizontal circular partition, which is pierced with 

 holes. Through a stuffing box, in the cover of this interior cylinder, the vertical axis 

 of the agitator passes, which is turned by wheel-and -pinion work, in order to stir up the 

 lime from the bottom of the water in the purifier. The vessel o, serves for introducing 

 fresh milk of lime, as also for letting it off by a stopcock when it has become too foul 

 for further use. 



The quantity of lime should be proportioned to the quantity of sulphuretted hydro- 

 gen and carbonic acid contained in the gas. Supposing that in good coal-gas there is 

 6 per cent, of these gases, about one pound and a half of lime will be requisite for 

 every hundred cubic feet of coal-gas generated, which amounts to nearly one- 

 sixteenth of the weight of coal subjected to decomposition. This quantity of lime 

 mixed with the proper quantity of water will form about a cubic foot of milk of 

 lime. Consequently, the capacity of the purifier, that is, of the interior space filled 

 with liquid, may be taken at four-sevenths of a cubic foot for every hundred cubic 

 feet of gas passing through it in one operation ; or for 175 cubic feet of gas, one cubic 



