Some ovens on this principle have, as we have already mentioned, been erected at the Gas Works at Abergavenny, and it is stated, that, after being kept constantly at work for two years, they were not apparently the worse for wear, whilst the charges for repairs had not reached twenty shillings each per annum. Another advantage is, that as these ovens contain a charge equal to about six full sized iron retorts, and require to be charged but once in twenty-four hours, there is not only a saving in the first cost of erection, and in the annual wear and tear, but in all the daily labour consequent upon the old process, in which much time, as well as labour, is usually expended in the drawing off the charge, and in recharging.

The next improvement, and which indeed constitutes a principal feature in Mr. Broadmeadow's invention, is that of his patent application of an exhausting cylinder, or other apparatus, to exhaust the gas from the condenser, thereby causing a partial vacuum, and enabling the gas to flow from the ovens as fast as it is generated. By means of this exhausting apparatus a portion of atmospheric air, equal to about one-eighth part of the entire quantity of gas, is admitted into the gasometer, when the oxygen of the atmosphere mixing with the suphuretted hydrogen precipitates the sulphur, and gives to the lighted gas a greater degree of brilliancy. This mode of purifying gas is said, by the patentee, to be so efficient, that, when the coal used is of good quality, no other purifying process is required. As the admission of too great a quantity of atmospheric air would prove injurious, the requisite speed at which the exhauster should be worked is shown by a water gauge.

We shall now proceed to describe Mr. Ibbetson's patent process of preparing inflammable gas by decomposing water, in conjunction with coal, in a furnace of a peculiar construction. The following engraving represents a vertical section of the apparatus employed. In the central compartment at a is an iron door and frame, opening above the fire-place for supplying the fuel thereto; immediately under the arched top of the fire-place is a small aperture b, for the admission of the air requisite for the combustion of the fuel; there is another small door, (shown by dots at c,) for the purpose of lighting the fire; d is the ash-pit, e e e is the flue which descends, and then takes the course pointed out by the direction of the arrows to the chimney, thus enveloping the decomposing chamber, which occupies the space between the flues and the central furnace. The coals or other substances to be decomposed are introduced through an iron door h; this door, as well as the two other doors, 11 (shown by dotted lines), for extracting the coke, are kept closed air tight, by luting, during the process of distillation; and for clearing out the ashes under the gratings, there are apertures at i i, fitted also with doors, and kept closed like the last mentioned, whilst the decomposition is going forward within.

The steam is introduced at two places in the decomposing chamber; one at f by a pipe of retort earth, from whence it ascends among the ignited coke, passing round the chamber in the direction shown by the arrows; the other at k, where a tube of retort earth is extended across the chambers horizontally, the steam escaping from it through numerous small holes at the bottoms and sides. The gases and vapours produced by these combined circumstances make their exit by a pipe at o. By this apparatus, the patentee also professes to decompose tar and oil along with the coal; in which case, these fluids would he introduced on the right hand side, (opposite to k,) through tubes, regulating the quantity by means of stop-cocks, which quantity should of course never be more than will become decomposed, whilst circulating through the burning coal, without reaching the bottom of that side where it enters. The patentee observes, that the coals should be broken into pieces not exceeding the size of walnuts, before they are put into the decomposing chamber; and that the charges should be made from time to time by fresh layers of an inch and a half in thickness, after the previous charge has become red hot.

We have already stated that Mr. Murdoch experimented upon wood and peat, as well as pit coal, in order to ascertain the qualities and quantities of illuminating gas furnished by each. Shortly after the introduction of gas lighting into public use, experiments were made upon various other substances, with the same view; as coal tar, pitch, pine knots, and sawdust; and in 1815 Mr. John Taylor obtained a patent for an apparatus for the purpose of preparing inflammable gas from any kind of animal, vegetable, or mineral oil, fat, bitumen, or resin, which can be rendered fluid by heat. The process may be briefly described as follows: the liquid material is allowed to flow in a very minute stream into a retort heated to redness, and containing a quantity of coke, hard broken bricks, or other porous and refractory substances; here the oil is speedily converted into a very fine and brilliant gas, which is conducted to a close vessel surrounded by water, in order to condense any oil which may have come over, not decomposed, or in the state of vapour, which oil is returned to the retort by a very simple arrangement.

From the condenser the gas enters a vessel containing the liquid from which the gas is prepared, by which a further condensation of any condensible vapour which the gas may contain is effected, and the gas then passes into the gasometer.

Mr. Philip Taylor some time afterwards obtained a patent for an improved apparatus for obtaining gas from various substances in a liquid state, a section of which apparatus is exhibited in the following engraving. When the liquid matter introduced into the retort is of easy decomposition, one distillation of it through a single retort will be in general sufficient to separate the gas in a tolerably pure state; when they are, however, such as to require a longer process, the gaseous products of the first distillation are passed into a second retort, to complete the separation of the contaminating matter. As many as ten retorts similar to those shown in the engraving may be conveniently arranged over one furnace. In the case of a single distillation it is therefore to be understood that these are all supplied at once with the fluid matter, and the product of each retort respectively is carried from them direct to the gas holder; but when a second distillation is necessary, the ten retorts before mentioned would be employed as five pair, and the diagram is drawn so as to show the operation either by a single or by a pair of retorts, which we shall now explain.