108 PURIFIERS. 



PURIFIERS. 



THE gas, when it leaves the retorts, retains its impurities, and in this state is 

 quite unfit for the illumination of private houses, or even public thorough- 

 fares. The impurities are bituminous vapour, ammoniacal gas, essential oil, and 

 sulphuretted hydrogen ; the processes adopted for removing these are partly 

 mechanical and partly chemical. The first operation is the condensation of 

 the volatile portions, which is effected at different places in different ways. 

 The condensers generally adopted either consist of a series of pipes arranged 

 in the manner of a distiller's worm, or of a number of chambers contained in 

 a tank and surrounded by cold water ; at the lowest points of these vessels 

 siphons are attached, sealed by dipping them into tar to a sufficient depth to 

 prevent the gas from escaping, and through them the condensed bituminous 

 and ammoniacal vapours pass away to the cistern constructed to receive them 

 in the forms of tar and ammoniacal liquor. 



The same tank serves to contain both, the difference of their 'specific 

 gravities keeping them separate ; the ammoniacal liquor, being the lightest, 

 swims on the surface of the tar. The tank is generally sunk below the sur- 

 face of the ground ; the respective heights of the two fluids are registered by 

 floats and gauges, and, when found necessary, are pumped out. If there be no 

 sale for the tar, it is burned beneath the retorts, and the ammoniacal liquor is 

 either evaporated in the cast-iron pans placed under the furnaces for that 

 purpose, manufactured into the carbonate and muriate of ammonia, or used 

 as manure. The simplest and best condenser is formed of a few upright 

 pipes, as shown in Plate IX. at S S. Their number and length being regu- 

 lated by the quantity of gas required to pass through them, ten feet run of 

 pipe for every 10,000 feet of gas is ample ; in height they may be equal to 

 that of the wall of the retort-house, for the convenience of placing a tank on 

 the roof to supply them with water. In this instance the tank was placed 

 against the chimney, thus allowing a greater length of pipe ; at the bottom of 



