In combustion in ordinary fires, the _water_ of the fuel is a source of waste, since it consumes heat in acquiring the state of vapor. This is well seen in the comparison of the same kind of peat in different states of dryness. Thus, in the table of Gysser, (page 97) Weber's condensed peat, containing 10 _per cent._ of moisture, surpasses in heating effect that containing 25 _per cent._ of moisture, by nearly one-half. The _oxygen_ is a source of waste, for heat as developed from fuel, is chiefly a result of the chemical union of atmospheric or free oxygen, with the carbon and hydrogen of the combustible. The oxygen of the fuel, being already combined with carbon and hydrogen, not only cannot itself contribute to the generation of heat, but neutralizes the heating effect of those portions of the carbon and hydrogen of the fuel with which it remains in combination. The quantity of heating effect thus destroyed, cannot, however, be calculated with certainty, because physical changes, viz: the conversion of solids into gases, not to speak of secondary chemical transformations, whose influence cannot be estimated, enter into the computation. _Nitrogen_ and ash are practically indifferent in the burning process, and simply impair the heating value of fuel in as far as they occupy space in it and make a portion of its weight, to the exclusion of combustible matter. Again, as regards density, peat is, in general, considerably inferior to anthracite. The best uncondensed peat has a specific gravity of 0.90. Condensed peat usually does not exceed 1.1. Sometimes it is made of sp. gr. 1.3. Assertions to the effect of its acquiring a density of 1.8, can hardly be credited of pure peat, though a considerable admixture of sand or clay might give such a result. The comparative heating power of fuels is ascertained by burning them in an apparatus, so constructed, that the heat generated shall expend itself in evaporating or raising the temperature of a known quantity of water. _The amount of heat that will raise the temperature of one gramme of water, one degree of the centigrade thermometer, is agreed upon as the unit of heat._[16] In the complete combustion of carbon in the form of charcoal or gas-coal, there are developed 8060 units of heat. In the combustion of one gramme of hydrogen gas, 34,210 units of heat are generated. The heating effect of hydrogen is therefore 4.2 times greater than that of carbon. It was long supposed that the heating effect of compound combustibles could be calculated from their elementary composition. This view is proved to be erroneous, and direct experiment is the only satisfactory means of getting at the truth in this respect.