In the previous reading, from Chap. 18 of Maxwell’s Theory of Heat, we focused on the transport of heat by conduction and (to a lesser extent) convection. In the former case, heat is transported between stationary bodies which are in thermal contact—like a hot lead ball buried in cool sand. In the latter case, a warm substance moves into a cooler region, bringing heat with it—like an ascending hot-air balloon. A third mode of heat transport, which has only briefly been mentioned, consists of bodies exchanging heat across empty space in the form of radiation. This phenomenon is illustrated by the heating of a cool thermometer suspended in a sealed warm vacuum chamber, or by the heating of the earth across vast space by the sun. In fact, all bodies emit heat radiation, whether a red-hot rod of iron, a warm human body or a cold dust cloud in a distant nebula. In the reading selection that follows Maxwell describes the nature of radiant heat, and how it depends not only on the temperature of the body, but also on the characteristics of the body’s surface. Although these phenomena had been studied extensively by many scientists up to the time of Maxwell, no satisfactory theory had been provided to accurately explain them. Within 30 years of Maxwell’s publication, Max Planck’s careful examination of the color and intensity of radiation emitted by black-bodies—those which appear black when cold—led to the birth of modern quantum theory.