fifty cubic feet of gas; and are equal to three hundred and fifty, divided by twenty, which last is equivalent to one pound of tallow, making one hundred and twelve pounds of cannel-coal, equal to 350/20 = 17½lbs. of tallow. Further, one hundred and twelve pounds of cannel-coal, divided by seventeen and a half of tallow make six and four-tenths of cannel-coal, equal to one pound of tallow. With regard to Newcastle coals[17], it may be stated that one chaldron of Wall’s-End coal may be made to produce in the large way upwards of 11,000 cubic feet of crude gas; which, when properly purified, diminishes to nearly 10,000 cubic feet. [17] One chaldron of Newcastle coal weighs from 2850 to upwards of 2978lb. The production of carburetted hydrogen, both with regard to quantity and quality from the same kind of coal depends much upon the degree of temperature employed in the distillatory process. If the tar and oil produced during the evolution of the gas in its nascent state, be made to come in contact with the sides of the red hot retorts, or if it be made to pass through an iron cylinder or other vessel heated red hot, a large portion becomes decomposed into carburetted hydrogen gas and olifiant gas, and thus a much larger quantity of gas is produced than would be obtained without such precaution from the same quantity of coal.[18] [18] One pound of coal-tar produces 15 cubic feet of carburetted hidrogen abounding in olifiant gas. The distillation of the coal, (if gas be the chief object) should therefore not be carried on too rapidly. Most of the retorts used in the large way, are calculated for containing about one hundred weight of coal, and in general, when previously heated, produce from two and one-half to three cubic feet of gas, in four hours for each pound of coal they contain; but when the layer of coals in them does not exceed four inches in depth, three and one-half to four feet of gas may be obtained in the same time.