The heat-units employed in thermochemistry have varied from time to time.
Other branches of this subject are treated in the articles Chemical Action; Energetics; Solution; Alloys; Thermochemistry.
Combustion is a familiar example of the transformation of chemical energy into heat and light; the quantitative measures of heat evolution or absorption (heat of combustion or combination), and the deductions therefrom, are treated in the article Thermochemistry.
Reference should be made to the articles Chemical Action, Thermochemistry and Solutions, for the theory of the strength or avidity of acids.
Thus the conversion of yellow into red phosphorus evolves about one-sixth of the heat of combustion of the latter in oxygen, and so the knowledge of which variety of phosphorus has been employed is of essential importance in the thermochemistry of that element.