As to the calorific properties of coal, it is well known that the theoretic quantity of heat that should be chemically liberated by the consumption of a given quantity of that fuel is more than ten times as much as that which is ordinarily obtained, even by well-constructed steam-boilers. For a pound of coal to evaporate eight pounds of water may be taken as a very favorable average. In domestic consumption there is nothing approaching to this economy of heat. A considerable quantity of unburned carbon passes up the chimney in the form of smoke; and probably three fourths of the heat actually liberated by combustion is carried off in the same manner by the draught.
According to the experiments made by Mr. Vernon Harcourt, before referred to as quoted by Mr. D. K. Clark in his "Manual of Rules, Tables, and Data," a pound of gas, with a volume of thirty cubic feet, will evaporate thirty pounds of water from 212°, or 21·4 pounds from 62°. This gas is reckoned at 9,000 cubic feet to the ton of coal; so that the evaporation (of one pound of water by one cubic foot of gas) is effected by the quantity of the latter derived from almost exactly a quarter of a pound of coal. At the more ordinary allowance of 10,000 cubic feet per ton, ·224 pound of coal yields a foot of gas. It has to be borne in mind that only about thirty-six per cent, of the coal ordinarily used for gas is volatilized in the process of distillation. Of the coke, which is the chief residual product, from fifty-eight to ninety-three per cent, is carbon; sulphur and other impurities going to make up the rest of the bulk. There are produced on the average thirteen and one-half hundred weight of coke, and ten gallons of tar, from a ton of coals, besides the gas. The calorific and luminiferous values of these residual products are thus much greater than that of the gas itself. But a better use can be made of tar than to burn it; and we have considered the value of these products as absorbed by the cost of the process of making gas.
With these qualifications, the calorific effect of the gas produced from a pound of coal is about half that which would be produced by the burning of a pound of coal under a well-constructed boiler, where of course both coke and tar are consumed together with the gas. But in cases of domestic consumption the economy in the use of gas will be immense. There is no waste, no smoke. Instead of seventy-five per cent, of the heat going up the chimney, nearly all will be directly utilized. There is no loss of heat in lighting the fire; none in cooling when the work is done; no labor in the carriage of coal to the furnace; none in the removal of the ashes. Bearing: in mind all these sources of economy, the domestic use of gas for heating purposes is so advantageous that it is extraordinary that the introduction of so clean, cheap, and manageable a source of comfort should make such slow progress in England. In America the improvement is more rapid and more general.
