In all places where the average price of coal, equal in quality to (Bewick and Craister’s Walls End) Newcastle coal, or any other species of coal, capable of producing from fifteen to eighteen thousand feet of gas from one chaldron, is not less than £ 2. 8s. the chaldron (27 Cwt.) or upwards, and where coke can be sold at the average price of £. 1 the chaldron, the horizontal rotary retort should be worked at such a temperature, that when viewed through the sight hole, it shall appear of a bright cherry redness, and at which it produces from 15,000 to 16,000 cubic feet of gas, from a chaldron of coal.
But in all other places where coal of the same quality to (Bewick and Craister’s Walls End) Newcastle coal, may be purchased at £. 1 8s. the chaldron, or at a less price, it will be more advantageous to the manufacturer, to work the horizontal rotary retort, at a lower temperature, so as to produce only at the rate of thirteen or fourteen thousand cubic feet of gas from the chaldron of coal. In the latter case the manufacturer expends coal in order to save his retort, whereas in the former case he economizes the fuel, as productive of a gain more than commensurate for the waste of the retort.
When the supply of gas required, happens at any time to be less than the retort when in action is capable of furnishing, the fire must then be kept low, but the retort should never be suffered to acquire a lower temperature, than that of a dull red heat visible by day-light.
PART VIII.
Purifying Apparatus, or Lime Machine.
Coal gas, even as obtained from the best species of coal, must be rendered pure before it is fit for the purpose of illumination. The gas in its crude state always contains a portion of sulphuretted hydrogen and carbonic acid; and when burnt, although its illuminating power is greater in an impure than in a pure state, it produces an oppressive and suffocating odour, which is speedily perceptible in confined places. The gaseous product evolved during its combustion, blackens paint and tarnishes metallic bodies; an impure gas besides strongly acts upon the copper branch pipes through which it is conveyed.
To obviate these defects the sulphuretted hydrogen and carbonic acid which are the cause of them must be removed, and to effect this, no method more economical and efficacious, has as yet been discovered, than to bring the gas confined under a pressure equal to a column of water, not less than eight or ten inches in height, into contact with quick-lime, diffused through water. Other means have been tried, but all of them have failed to be sufficiently efficacious or economical on a large scale.