In conducting the decomposition of coal, the evolution of the gas is far from being, with regard to quantity, uniform during different periods of the distillatory process. The formation of the gas is more rapid in the beginning of the process, and gradually slackens as the operation proceeds. The gas also differs in its chemical constitution, at different periods of the process; although in the case of large supplies, this difference is of little consequence after the gas is purified in the usual manner. The former consideration, however, has given rise to various modes of operating, of which it will be proper to take some notice. It must be obvious, that in proportion as the mass of coal in the retort becomes carbonized or converted into coke, the exterior surface becomes a gradually increasing obstacle to the action of the heat upon the interior or central part of the coal remaining to be decomposed. The heat required on that account must be more intense, and kept up to no purpose, and the extrication of gas becomes slower and slower, as the operation proceeds. The loss occasioned by this rapid diminution of the means employed, is serious in every point of view, in regard as well to the quantity of fuel used and time wasted, but it is unavoidable in the operation of decomposing coal in masses or layers from 5 to 10 inches in thickness, and must be a great drawback on the value of the gas-light discovery. The loss of fuel, it is obvious, must be just in proportion to the quantity of carbonised matter, or coke, which is kept hot to no purpose, awaiting the decomposition of that portion of coal, which it is the very means of protecting from becoming decomposed. A striking exemplification of this statement will be seen in the following table, exhibiting the result of the progressive produce of coal gas, obtainable, in a given time, by means of cylindrical and parallelopipedal retorts. _Experiment with one cylindrical Retort, containing two bushels of coal._ Hours of the Quantity of Gas distillatory process produced. First hour 115 cubic feet Second ditto 81 ditto Third ditto 78 ditto Fourth ditto 70 ditto Fifth ditto 66 ditto Sixth ditto 55 ditto Seventh ditto 49 ditto Eighth ditto 42 ditto --- 555 cubic feet. The quantity of gas is at the rate of ten thousand cubic feet to the chaldron (27 cwt.) of coal. _Experiment[14] with eighteen cylindrical Retorts, containing one chaldron of coal._ Hours of the Quantity of gas distillatory process produced.