204 [February, 1S45, 



certain limits) will be the proportion of fixed carbon.* The estimation of heat- 

 ing powers, therefore, from the quantity of fixed carbon which coals contain, if 

 not wholly erroneous in principle, must be liable to considerable uncertainty in 

 practice. 



Many highly bituminous coals contain more than 5 per cent, of materials 

 convertible into ammoniacal liquor by simple distillation without contact of air. 

 This is proved on the largest 6cale in the manufacture of illuminating gas. 

 That proportion, therefore, is not only unavailable for heating purposes, but it 

 also abstracts from the really combustible materials of the fuel, all the heat, sen- 

 sible and latent, which the vaporized ammoniacal products receive during com- 

 bustion. 



The proper water of combustion, namely, that derived from the hydrogen in 

 excess, and oxygen of the atmosphere, must in every instance where heat is ap- 

 plied to evaporate water above the boiling point, as in all ordinary steam boilers, 

 be likewise incapable of giving up its latent, as well as much of its sensible 

 heat. 



The average specific gravity of the 6ix varieties of bituminous coals assayed 

 is 1.31, — that of water at 60° being unity. Admitting the hydrogen in its solid 

 state to have a density of only 1.25, it must in passing into the state, first of 

 gaseous hydrogen, and then into that of watery vapour, (still having the same 

 bulk as the hydrogen,) undergo an enlargement to 2117 times its original bulk. 

 This volume is farther increased according to the usual law of gaseous expan- 

 sion, by whatever heat above boiling point is left in the vapour, when it passes 

 away from the surface to be heated. In a well constructed evaporative appa- 

 ratus producing steam of 6 pounds pressure, in which the circuit traversed by 

 the gases after passing the grate, and before reaching the chimney, was 121 feet, 

 the temperature was generally about 100° above the boiling point; and the 

 watery vapour, being of course surcharged with heat, possessed 2431 times the 

 bulk which it had in the solid state and at 60 degrees of temperature. 



By the experiments of Dulong, (Comptes Rendus, torn. 7,-j-) one gram "of 

 pure carbon developes, in burning, heat enough to raise the temperature of 7170 

 grams of water, 1° centigrade, or 12906 grams 1° Fahrenheit. This latter 

 number is, therefore, used as a co-efficient, by which to multiply the numbers 

 in the 12th column of the following table to obtain those of the 15th. By the 

 same authority, 1 gram of gaseous hydrogen gives heat sufficient to raise 62,535 

 grams of water 1° Fah. 



The average excess of hydrogen for the six varieties of coal tried by evapora- 

 tion, as deduced from columns 13 and 14 of the table, is 4.636 per cent, which, 

 calculated after the manner of the European chemists, ought to possess an eva- 

 porative power of 2.814. This would raise the average of the 15th column 

 from 10.700 to 13.514, as the calculated evaporative power of the unit of com- 

 bustible matter, showing the calculated to be 26.3 per cent, higher than the ex- 

 perimental effect. 



* See Proceedings of the Acad, of Nat. Sciences, Vol. 2, pages 9 — 10. 

 -j- See also Poclet, Traite de la Chaleur, Tom. I, p. 50. 



