Comparison with later determinations have established their substantial accuracy.  The general conclusion arrived at is thus stated: 

“As a rule there is an equality between the heat disengaged or absorbed in the acts, respectively, of chemical combination or decomposition of the same elements, so that the heat evolved during the combination of two simple or com-pound substances is equal to the heat absorbed at the time of their chemical segregation.”

       TABLE I.—­SUBSTANCES ENTERING INTO THE COMPOSITION OF FUEL.

-----------------------+-------------+-----------+-----

--------------+
|                         | Heat evolved in   |
|     Symbol and Atomic   |the Combustion of  |
|           Weight.       |  1 lb. of Fuel.   |
+------------+------------+--------+----------+
|            |            |        |In Pounds |
|            |            |  In    | of Water |
|            |            |British |Evaporated|
|   Before   |    After   |Thermal | from and |
| Combustion | Combustion | Units. | at 212 deg.. |
+------------+------------+--------+----------+
Hydrogen burned        | H        1 | H2O     18 | 62,032 |  64.21   |
in oxygen.           |            |            |        |          |
-----------------------+------------+------------+--------+-

---------+
Carbon burned to       | C       12 | CO      28 |  4,451 |   4.61   |
carbonic oxide.      |            |            |        |          |
-----------------------+------------+------------+--------+-

---------+
Carbon burned to       | C       12 | CO2     44 | 14,544 |  15.06   |
carbonic acid.       |            |            |        |          |
-----------------------+------------+------------+--------+-

---------+
Carbonic oxide burned  | CO      28 | CO2     44 |  4,326 |   4.48   |
to carbonic acid.    |            |            |        |          |
-----------------------+------------+------------+--------+-

---------+
Olefiant gas (ethylene)| C2H4    28 | 2CO2   124 | 21,343 |  22.09   |
burnt in oxygen.     |            | 2H2O       |        |          |
-----------------------+------------+------------+--------+-

---------+
Marsh gas (methane)    | CH4     16 | 2CO2    80 | 23,513 |  24.34   |
burnt in oxygen.       |            | 2H2O       |        |          |
-----------------------+------------+------------+--------+-

---------+

Composition of air—­

by volume 0.788 N + 0.197 O + 0.001 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