, 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.

The data of Karmarsch, Brix, and Gysser, already given, were obtained by
the experimental method. They were, however, made mostly on a small
scale, and, in some cases, without due regard to the peculiar
requirements of the different kinds of fuel, as regards fire space,
draught, etc. They can only be regarded as approximations to the truth,
and have simply a comparative value, which is, however, sufficient for
ordinary purposes.

The general results of the investigations hitherto made on all the
common kinds of fuel, are given in the subjoined statement. The
comparison is made in units of heat, and refers to equal weights of the
materials experimented with.

HEATING POWER OF DIFFERENT KINDS OF FUEL.

Air-dry Wood 2800
" Peat 2500 3000
Perfectly dry Wood 3600
" " Peat 3000 4000
Air-dry Lignite or Brown Coal 3300 4200
Perfectly dry Lignite or Brown Coal 4000 5000
Bituminous Coal 3800 7000
Anthracite 7500
Wood Charcoal 6300 7500
Coke