CHAP. IL] THE ART OF WARMING. 355 



always the vegetable kingdom, whether that of to-day or of ancient 

 geological periods, that supplies the heat developed in the various 

 apparatus we have passed under review. The gas that is also used 

 as fuel, is but one of the ingredients of coal, and this simple, fact 

 proves why heating by gas is dearer than that by coal from which 

 the gas is extracted. 



We will now compare the heating powers of these different sub- 

 stances, according to the experiments of physicists. By the number 

 of calories is meant the number of units of heat developed by the 

 complete combustion of a kilogramme of each substance. 



Ratios of 

 Fuels. Calories. heating powers. 



Coal 8,000 1-00 



Anthracite . '..../ 7,500 0'95 



Coke . . ' < . . *. . . . 7,000 0'90 



Lignite 6,500 0'80 



Charcoal .."/. '. . v . . 6,000 075 



Peat ...... i v . . . 5,000 0'60 



Dry wood . ... . . . 4,000 0'50 



Wood (20 per cent, of water) . 3,000 . . . . . . . . 0"38 



Gas . '. . . . 10,000 . . t 1-25 



Gas has the greatest heating power ; next come coal and mineral 

 fuels, then turf and wood, and moist wood is lowest of all. 



We have seen further to what extent the heating apparatus, even 

 the most improved, utilize the heat of combustion. The greatest part 

 of the heat goes up the chimney. 



If we would know the amount of heat lost annually in smoke we 

 may take the consumption of fuel in Paris for example. The Annuaire 

 du Bureau des Longitudes for 1872, gives us this consumption for the 

 year 1869 : 



Wood of various sorts 994,057 steres. 



Charcoal .';.. 4,902,315 hectolitres. 



Coal, coke, turf 682,011,827 kilogrammes. 



If we take a number less than the mean value of these various 

 fuels the total value cannot be estimated at less than sixty-nine 

 millions of francs. Twenty-five millions for wood, twenty-four 

 millions for coal, and twenty millions for charcoal. But this last fuel 

 is not used at all for heating purposes, and a considerable proportion 

 of the coal is used for industrial purposes. The loss of heat is not 



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