CO2 + 0.014 H2O
----------------------------------------------------
by weight 0.771 N + 0.218 O + 0.009 CO2 + 0.017 H2O

This law is, however, subject to some apparent exceptions. Carbon
burned in protoxide of nitrogen, or laughing gas, N_{2}O, produces
about 38 per cent. more heat than the same substance burned in pure
oxygen, notwithstanding that the work of decomposing the protoxide of
nitrogen has to be performed. In marsh gas, or methane, CH_{4}, again,
the energy of combustion is considerably less than that due to the
burning of its carbon and hydrogen separately. These exceptions
probably arise from the circumstance that the energy of chemical
action is absorbed to a greater or less degree in effecting molecular
changes, as, for example, the combustion of 1 pound of nitrogen to
form protoxide of nitrogen results in the absorption of 1,157 units of
heat. Berthelot states, as one of the fundamental principles of
thermochemistry, "that the quantity of heat evolved is the measure of
the sum of the chemical and physical work accomplished in the
reaction"; and such a law will no doubt account for the phenomena
above noted. The equivalent heat of combustion of the compounds we
have practically to deal with has been experimentally determined, and
therefore constitutes a secure basis on which to establish
calculations of the caloric value of fuel; and in doing so, with
respect to substances composed of carbon, hydrogen, and oxygen, it is
convenient to reduce the hydrogen to its heat-producing equivalent of
carbon. The heat of combustion of hydrogen being 62,032 units, that of
carbon 14,544 units, it follows that 4.265 times the weight of
hydrogen will represent an equivalent amount of carbon. With respect
to the oxygen, it is found that it exists in combination with the
hydrogen in the form of water, and, being combined already, abstracts
its combining equivalent of hydrogen from the efficient ingredients of
the fuel; and hence hydrogen, to the extent of 1/8 of the weight of
the oxygen, must be deducted. The general formula then becomes:

Heat of combustion = 14,544 {C + 4.265 (H-(O/8))},

and water evaporated from and at 212 deg., taking 966 units as the heat
necessary to evaporate 1 pound of water,

lb. evaporated = 15.06 {C + 4.265 (H-(O/8))},

carbon, hydrogen, and oxygen being taken at their weight per cent. in
the fuel. Strictly speaking, marsh gas should be separately
determined. It often h