Joseph Black distinguished between the quantity of heat in a body and its intensity, or temperature, which was introduced in Chap. 1. This chapter introduces calorimetry, the measurement of the quantity of heat, and the contemporary theory of heat, the caloric theory of heat. Development of calorimetry and the caloric theory was the beginning of the quantitative study of heat, and the important stepping stone toward the mechanical theory of heat, the idea that heat is a form of energy.