In chemistry, Hess’ law or “rule” states that the heats of reaction in successive reactions must be added. The rule was determined in 1840 by Swiss-born Russian chemist Germain Hess. The rule is used to determine values of reaction heats that are difficult to determine directly. After the 1876 work, On the Equilibrium of Heterogeneous Substances, of American engineer Willard Gibbs, Hess’ rule became a corollary of that work. [1]

References
1. Muller, Ingo. (2007). A History of Thermodynamics - the Doctrine of Energy and Entropy (pg. 154-55). New York: Springer.

External links
● Hess’s law – Wikipedia.