PART VIII.

Purifying Apparatus, or Lime Machine.

Coal gas, even as obtained from the best speciesof coal, most be rendered pure before it is fit fortbe purpose of illumination. The gas in its crudestate always contains a portion of sulphuretted hy-drogen and carbonic acid ; and when burnt, althoughits illuminating power is greater in an impure thanin a pure state, it produces an oppressive and suffo-cating odour, which is speedily perceptible in con-fined places. The gaseous product evolved duringits combustion, blackens paint and tarnishes metallicbodies ; an impure gas besides strongly acts uponthe copper branch pipes through which it isconveyed.