When we consider the before mentioned varieties of coal in an economical point of view, as fuel to be used in the gas-light process, for heating the retorts, it appears from a series of experiments that have been made under my direction, that the second class of coal comprehending those varieties which contain a larger quantity of carbon than bitumen (p. 45,) afford the most economical fuel, they act less on the grate bars, and fire bricks of the furnace than those varieties which take fire easily and burn briskly with a strong blaze. A mixture of Welch Stone coal, and Newcastle coal forms an excellent economical fuel, where an intense glowing fire is required. PART IV. _Form and dimensions of the Retorts originally employed for manufacturing Coal Gas._ The proper mode of constructing the retorts in which the coal is distilled, and the art of applying them form an object of primary importance in every gas-light establishment. According as the manufacture is conducted in these respects with a due regard to physical principles, depends the quantity of gas which can be obtained in any given time, from any given quantity of coal, the consumption of fuel requisite for the production of that quantity of gas, the degree of deterioration to which the distillatory vessel is subjected, the quality in some measure, of the gas itself; and, as the ultimate result of all these circumstances, the cheapness at which the gas light can be furnished to the consumer. The essential influence of these various particulars on the value of the art of lighting with coal gas, has led to much assiduous enquiry to ascertain that sort of construction and mode of operation in respect to each of them which may be most advantageous. And in no branch of the new art of procuring light, has a greater variety of plans of improvement been submitted to the several directing boards of gas works, or more labour and expence been incurred in experiments conducted on a large scale, to ascertain the relative merits of these plans. Nor is there any part of the gas-light process in which a greater number of material alterations have been put in practice. In the earlier periods of lighting with coal gas the retorts employed at some of the gas-light establishments in the metropolis, were hollow cast-iron cones from six to seven feet in length. The greatest diameter of the cone which formed the mouth of the retort, measured from twelve to fifteen inches, and its smallest diameter at the vertex from nine to ten inches. At other gas works the form of the retort was a parallelopiped from six to seven feet long, the horizontal, and vertical sides were respectively to each other, as 20 to 15 inches. The angles of these retorts were slightly rounded. Fig. 16, plate V. exhibits a vertical section of this retort.