Berthelot's law, also known as the law of mixtures, states that when two substances are mixed, the resulting temperature change is proportional to the heat capacities of the individual substances. This principle is often applied in thermodynamics to predict how mixtures will behave when they undergo temperature changes. The law is named after the French chemist Marcellin Berthelot, who contributed significantly to the field of thermochemistry. It helps in understanding how different materials interact thermally, which is essential in various applications, including chemical engineering and material science.