The different kind of retorts of which a description has been given in the preceding pages, were originally heated by means of flues passing under and over them. The retorts were placed horizontally and fixed in brick-work. One fire-place at the extremity of the mouth of the retort where the coals are introduced, and whence the coke is withdrawn, was allotted to every two retorts in the series. At the commencement of the new art of procuring light the quantity of fuel as before stated, necessary to decompose a given quantity of coal, amounted to from thirty to thirty-six per cent of the coal decomposed; that is to say, it required from thirty to thirty-six parts of fuel to decompose one hundred parts of coal. This quantity has been much lessened by a better mode of setting the retorts, and it is now the general opinion that the operation of decomposing coal, by means of cylindrical, parallelopipedal, or semi-cylindrical retorts, must be considered as well conducted when one hundred parts of coal are decomposed by twenty or twenty-five parts of fuel. This appears to be the minimum quantity of fuel, that can be employed for the complete decomposition of coal by means of these retorts, and with the least deterioration of the distillatory vessel. The following statement will exhibit what has been done in this branch of art. _Report on a course of Operations, made with sets of 66, of 30, of 116, and of 64 retorts, worked on the Flue Plan._ In order to determine the relative value of the best method of setting cast-iron retorts, it was deemed necessary to ascertain whether three retorts might not be heated, instead of two, as before stated, by one fire-place and branching flues. To determine this the following processes were carried into effect. _Process I._ Sixty-six cast-iron cylindrical retorts, of the usual size, namely, six and a half feet long, (exclusive of the mouth-piece) and one foot in diameter, internal dimensions, where set on the plan of three retorts to one fire-place, at the Westminster gas-work station, and a series of 30 similar retorts were erected at another station belonging to the same company, at the East end of London. The experiments were pursued with every degree of justice in the detail, the retorts were kept in action day and night for upwards of four months, and the results noted down with exactness. The final reports from the two establishments were found to concur in showing that nothing was to be gained by this method over that previously in use. The time occupied for the distillatory process was not abridged. The consumption of fuel was greater--no larger quantity of gas was obtained from the quantity of coal carbonized. The produce with regard to coke was in the usual ratio, and the retorts were destroyed in about one third less time than when only two were heated by one fire-place. _Process II._