GAS WORKS, PLYMOUTH. 47 



the gas, are inverted ; see the engraving. But it 

 should be observed here, that these bent pipes do not 

 terminate at the top of the hydraulic, to which they 

 are connected by means of flanges, strongly bolted on, 

 but pass through the top and dip into the included 

 liquid, about two inches below the surface. This sim- 

 ple but admirable contrivance acts, as a seal, to prevent 

 the return of the gas, when the lids are removed from 

 the mouths of the retorts, for the purpose of discharg- 

 ing and recharging : hence its name the hydraulic 

 . main. 



We must now take leave of our drawing, and the 

 retort house ; the remaining part of this paper will be 

 merely descriptive. 



A pipe rises from the right hand end of the hydrau- 

 lic main, shewn in the print as if cut off: this pipe 

 conveys the gas away to other parts of the works, to 

 pass through other processes : the first of these is the 

 condenser, as it is usually called in gas establishments ; 

 but it is more properly a refrigerator ; the use of which 

 is to reduce the temperature of the gas, and to con- 

 dense the tar into a liquid, which comes over with the 

 gas in an aeriform state. This condenser or refrigerator 

 is simply a cistern of cold water, through which a 

 series of pipes is passed, similar in principle to a dis- 

 tiller's worm. The gas being now sufficiently cooled, 

 and the tar deposited in a chamber prepared for it at 

 the bottom of the condenser, it flows thence to the 

 purifier. This is an iron vessel, shaped like a very 

 large drum, closed at the top and bottom ; it is char- 

 ged with a strong solution of lime water, about the 

 consistency of thick cream, through which the gas is 

 forced, in actual contact with the solution. This 

 process is of vital importance in all gas establishments. 

 It is known that two distinct gases are liberated by 

 the decomposition of pit coal, namely, carburetted, and 

 sulphuretted, hydrogen gas. The first of these is the 

 gas required for illuminating purposes, in as pure a 

 state as it is possible to obtain it ; consequently, if the 

 least particle of the sulphuret be suffered to go over, 



