so much energy is expended in the decomposition of the hydrogen and
iodine molecules and in the conversion of the iodine into the gaseous
condition, that the heat which it may be supposed is developed by the
combination of the hydrogen and iodine atoms is insufficient to balance
the expenditure, and the final result is therefore negative; hence it
is necessary in forming hydriodic acid from its elements to apply heat
continuously.
These compounds also afford examples of the fact that, generally
speaking, those compounds are most readily formed, and are most stable,
in the formation of which the most heat is developed. Thus, chlorine
enters into reaction with hydrogen, and removes hydrogen from
hydrogenized bodies, far more readily than bromine; and hydrochloric
acid is a far more stable substance than hydrobromic acid, hydriodic
acid being greatly inferior even to hydrobromic acid in stability.
Compounds formed with the evolution of heat are termed exothermic, while
those formed with an absorption are termed endothermic. Explosives are
the commonest examples of endothermic compounds.
When two substances which by their action upon each other develop much
heat enter into reaction, the reaction is usually complete without the
employment of an excess of either; for example, when a mixture of
hydrogen and oxygen, in the proportions to form water--