EXPLOSIONS IN COAL MINES. 211 



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 con- 

 sumption 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, be- 

 fore 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 advan- 

 tageous 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. 



