The results before detailed, with regard to the mode of setting cylindrical retorts suggested the propriety of an entire change in the mode of applying the heat, and this was at length fully carried into effect by the adoption of ovens, or air furnaces, in which the retorts are equally exposed to the action of heat on all sides. Mr. Rackhouse has the merit of having first carried into effect this method, since generally known by the name of the _oven plan_. The first experiments with these ovens were made on only one retort, exposed in an oven to air intensely heated; but they were afterwards repeated on two, three, four, and five retorts, successively. The retorts suffered the action of heat thus applied, exceedingly well; their deterioration was uniform, and the quantity of fuel required to work them, was found always to be in a direct ratio to the number of retorts employed. These experiments were carried on for upwards of nine months, and it was found, that with five retorts in one oven, so that the heated air could act upon all of them equally, without the flame being directed forcibly upon them, this plan had a decided advantage, in point of economy, over every other method previously adopted. Each oven, containing five retorts, is heated by means of three fire-places, and although it is true that the number of retorts is less by one, than what could have been heated by three fire-places, on the original plan of two retorts to one fire, yet still this method has been found to be far more productive. The front wall of the oven may be readily taken down so that a retort, when damaged, may be withdrawn, and replaced without materially disturbing the rest. The oven plan of applying heat has been found equally advantageous for parallelopipedal and semi-cylindrical retorts.[13] [13] The only gas-light establishment of great extent in the metropolis, at which parallelopipedal retorts are still in use, is the _South London Gas Works_. But it is solely owing to the very peculiar care and economy with which all the details of this establishment are conducted, under the immediate superintendence of a few active, skilful and scientific proprietors, that they are able to compensate for the loss, which in all ordinary cases is inseparable from the employment of vessels of that description. _Description of the Retort Oven._ Fig. 1, plate IV., represents a transverse section of one of the retort ovens now in action at the Westminster Chartered Gas-Light Company’s Works; similar ovens are likewise in use at the City of London Chartered Gas-Light Works, and in many other provincial gas establishments. Fig. 2, plate IV., exhibits a longitudinal section, and fig. 1, plate V. shows the front elevation of the oven, built about ten feet above the ground, upon piers or arches, which saves brick-work and allows a stage or platform to be erected in front of the fire-places of the ovens. See fig. 2, plate IV.