IV. On the dependence of radiation on temperature

Sir Isaac Newton held that the radiation of heat from a hot body, increased in arithmetical ratio with the difference of temperature between it and the surrounding bodies. This law forms a rough approximation to the truth over a very limited range of temperature. MM. Dulong and Petit carried out an elaborate experimental research on the rate of cooling of hot bodies by radiation, extending to somewhat higher temperatures, and deduced from their observations the empirical formula- Rate of cooling =m (l·0077)‘(l·0077T-t -l). Here T is the temperature of the hot body in degrees Centigrade,t the temperature of the surrounding matter, and m is a constant depending on the nature of the radiating body. This formula agrees very fairly with experimental results for ordinary temperatures, but like Newton’s law, it has been shown that it cannot be applied for a wider range.